Membrane properties and synaptic responses of the guinea pig nucleus accumbens neurons in vitro

1. The membrane properties and synaptic responses of guinea pig nucleus accumbens neurons in vitro were studied with intracellular recording methods. 2. The population of neurons could be divided into groups of low (20-60 M omega, average 46.5 M omega) and high (60-180 M omega, average 96.5 M omega) input resistance. The resting membrane potential in both groups was approximately -70 mV. 3. Other membrane properties were quite similar in both groups. Inward rectification occurred at potentials more negative than -80 mV; this was blocked by Cs+ (2 mM). Membrane potential oscillations were observed at potentials between -65 and -55 mV; these were blocked by tetrodotoxin (TTX, 0.5 microM). Outward rectification occurred at potentials less negative than -45 mV; this was depressed by tetraethylammonium (TEA, 10 mM). 4. Action potentials elicited by small depolarizing current pulses (2-5 ms, 0.3-0.5 nA) were approximately 95 mV in amplitude and 1.0 ms in duration. The afterhyperpolarization following each action potential was less than 30 ms in duration, and no accommodation of action-potential discharge was seen at frequencies up to 40 Hz. The action potentials were reversibly blocked by TTX (0.3 microM). In addition, TTX-insensitive, Ca2+-dependent spikes were evoked by passing larger and more prolonged current pulses (greater than 40 ms, greater than 0.5 nA) across the membrane. 5. Focal electrical stimulation of the slice surface with low intensity (1 ms, less than 10 V) elicited excitatory postsynaptic potentials (EPSPs) in neurons of both high- and low-resistance groups. The reversal potential (+10.2 mV) for the EPSPs was close to the reversal potential (+7.7 mV) of the responses to glutamate applied in the superfusing solution. The N-methyl-D-aspartic acid (NMDA) receptor antagonists, D-alpha-aminoadipic acid (1 mM) and DL-2-amino-5-phosphonovaleric acid (DL-APV, 250 microM), reversibly depressed the EPSP; the glutamate uptake inhibitor, L-aspartic acid-beta-hydroxamate (50 microM), or removal of Mg2+ from the superfusate, augmented the EPSP. 6. When the intensity of the focal stimulus was increased (1 ms, greater than or equal to 10 V), a second larger depolarizing response (duration, 800 ms to 2 s) could be evoked in addition to the smoothly graded EPSP. This was seen only in cells of the high-resistance group (90-130 M omega).(ABSTRACT TRUNCATED AT 400 WORDS)     

Voltage behavior along the irregular dendritic structure of morphologically and physiologically characterized vagal motoneurons in the guinea pig

1. Intracellular recordings from neurons in the dorsal motor nucleus of the vagus (vagal motoneurons, VMs) obtained in the guinea pig brain stem slice preparation were used for both horseradish peroxidase (HRP) labeling of the neurons and for measurements of their input resistance (RN) and time constant (tau 0). Based on the physiological data and on the morphological reconstruction of the labeled cells, detailed steady-state and compartmental models of VM were built and utilized to estimate the range of membrane resistivity, membrane capacitance, and cytoplasm resistivity values (Rm, Cm, and Ri, respectively) and to explore the integrative properties of these cells. 2. VMs are relatively small cells with a simple dendritic structure. Each cell has an average of 5.3 smooth (nonspiny), short (251 microns) dendrites with a low order (2) of branching. The average soma-dendritic surface area of VMs is 9,876 microns 2. 3. Electrically, VMs show remarkably linear membrane properties in the hyperpolarizing direction; they have an average RN of 67 +/- 23 (SD) M omega and a tau 0 of 9.4 +/- 4.1 ms. Several unfavorable experimental conditions precluded the possibility of faithfully recovering ("peeling") the first equalizing time constant (tau 1) and, thereby, of estimating the electrotonic length (Lpeel) of VMs. 4. Reconciling VM morphology with the measured RN and tau 0 through the models, assuming an Ri of 70 omega.cm and a spatially uniform Rm, yielded an Rm estimate of 5,250 omega.cm2 and a Cm of 1.8 microF/cm2. Peeling theoretical transients produced by these models result in an Lpeel of 1.35. Because of marked differences in the length of dendrites within a single cell, this value is larger than the maximal cable length of the dendrites and is twice as long as their average cable length. 5. The morphological and physiological data could be matched indistinguishably well if a possible soma shunt (i.e., Rm, soma less than Rm, dend) was included in the model. Although there is no unique solution for the exact model Rm, a general conclusion regarding the integrative capabilities of VM could be drawn. As long as the model is consistent with the experimental data, the average input resistance at the dendritic terminals (RT) and the steady-state central (AFT----S) and peripheral (AFS----T) attenuation factors are essentially the same in the different models. With Ri = 70 omega.cm, we calculated RT, AFS----T, and AFT----S to be, on the average, 580 M omega, 1.1, and 13, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)     

Candidate premotor neurones of skin reflex pathways to T1 forelimb motoneurones of the cat

This study explored the locations and input output properties of a large population of putative premotor neurones of skin reflex pathways in the cat. These neurones, interneurones excited by forelimb skin afferents and antidromically from the T1 motor nucleus (MN) and/or the lateral funiculus (LF, C8/T1 border), termed antidromic cells, were extracellularly recorded at C6-8. Selection of this site was based on data showing that cells retrogradely HRP labelled from the T1 MN were most numerous in C6-8 and the observation that transection of LF at the C8/T1 border abolished most skinevoked postsynaptic potentials of T1 motoneurones. Antidromic cells were located in laminae IV–V, VI and VII. The latencies of antidromic excitation ranged from 0.4 to 1.8 ms, with a tendency for laminae IV–V cells to show longer latencies than laminae VI and VII cells. Latency of skinevoked excitation ranged from 0.6 ms (IV–V cells), 0.8 ms (VI) and 1.4 ms (VII) to greater than 5 ms. The sum of the ortho and antidromic latencies (estimated central latency) of individual cells explained the central latencies of skinevoked postsynaptic potentials in T1 motoneurones. Skin-evoked firing responses (average of eight to ten cells) were earliest and largest in laminae IV–V antidromic cells, and latest and smallest in lamina VII cells. The antidromic cells also received inputs from muscle afferents and descending tracts. The following three results support the suggestion that the sampled antidromic cells are mostly premotor neurones. (1) Projection to the T1 MN via LF was verified in six laminae IV–VII antidromic cells, as tested with threshold mapping for antidromic excitation. (2) Three skinexcited axons of the middle LF projected to T1 MN, as revealed by intra-axonal staining (HRP). (3) PHA-L injection in laminae I–V of C8 anterogradely labelled terminals in lamina IX and LF axons at T1. It is suggested that last order neurones of skin reflex pathways to T1 motoneurones are widely distributed in laminae IV–VII of C6-8 and consist of a variety of neurones with different locations and input patterns.     

Calcium-dependent potassium conductance in rat supraoptic nucleus neurosecretory neurons

Intracellular recordings of rat supraoptic nucleus neurons were obtained from perfused hypothalamic explants. Individual action potentials were followed by hyperpolarizing afterpotentials (HAPs) having a mean amplitude of -7.4 +/- 0.8 mV (SD). The decay of the HAP was approximated by a single exponential function having a mean time constant of 17.5 +/- 6.1 ms. This considerably exceeded the cell time constant of the same neurons (9.5 +/- 0.8 ms), thus indicating that the ionic conductance underlying the HAP persisted briefly after each spike. The HAP had a reversal potential of -85 mV and was unaffected by intracellular Cl- ionophoresis of during exposure to elevated extracellular concentrations of Mg2+. In contrast, the peak amplitude of the HAP was proportional to the extracellular Ca2+ concentration and could be reversibly eliminated by replacing Ca2+ with Co2+, Mn2+, or EGTA in the perfusion fluid. During depolarizing current pulses, evoked action potential trains demonstrated a progressive increase in interspike intervals associated with a potentiation of successive HAPs. This spike frequency adaptation was reversibly abolished by replacing Ca2+ with Co2+, Mn2+, or EGTA. Bursts of action potentials were followed by a more prolonged afterhyperpolarization (AHP) whose magnitude was proportional to the number of impulses elicited (greater than 20 Hz) during a burst. Current injection revealed that the AHP was associated with a 20-60% decrease in input resistance and showed little voltage dependence in the range of -70 to -120 mV. The reversal potential of the AHP shifted with the extracellular concentration of K+ [( K+]o) with a mean slope of -50 mV/log[K+]o.(ABSTRACT TRUNCATED AT 250 WORDS)     

Spike frequency adaptation and signaling properties of identified neurons in rodent deep spinal dorsal horn

Using whole cell recordings, I analyzed the intrinsic discharge properties for 285 neurons in Rexed's laminae III-V of isolated hamster spinal cord preparations. Neurons were characterized by their responses to step-wise and ramp-hold depolarizing current applied through the recording pipettes. Tonic cells (133/285; 47%) fired repetitively during step-wise current application. Firing decayed linearly (-0.14 to -4.3 imp . s(-1) . s(-1)) or was bimodal, with an initial exponential phase (tau approximately 450 ms) followed by a linear decline (-0.02 to -6.3 imp . s(-1) . s(-1)); discharge frequency was unrelated to current trajectory. Phasic-firing cells (108/285; 38%) responded with a burst discharge having an initial rapid, exponential decrease (tau approximately 30 ms) and subsequent linear decline (-1 to -78 imp . s(-1) . s(-1)). Phasic cells were activated preferentially by fast current ramps (slope, 70 pA/s-2.2 nA/s) with the number and frequency of impulses increasing with current slope. Delayed-firing cells (44/285; 15%), responded to current steps with an accelerating firing following a substantial latent period (0.5-4 s) and discharged during current ramps with slopes less than approximately 100 pA/s. Intracellular staining revealed a significant association between electrophysiological profile and neuronal morphology. A majority of presumed projection cells (22/30; 73%) exhibited tonic firing to step-wise activation. The preponderance of phasic and delayed firing cells, 93% (42/45) and 71% (12/17), respectively, were interneurons with local or intersegmental terminations. Differential sensitivity to static and time-varying components of membrane current suggest differences in neuronal signaling properties that may have important implications for integration of mechanosensory information in the deep spinal dorsal horn.     

Intracellular recording of identified neostriatal patch and matrix spiny cells in a slice preparation preserving cortical inputs

1. The morphology, electrical membrane properties, and corticostriatal excitatory postsynaptic potentials (EPSPs) of two groups of neostriatal projection cells, patch cells, and matrix spiny cells were compared in the rat by the use of an in vitro slice preparation that preserves inputs from medial agranular cortex. Spiny cells were stained intracellularly with biocytin and identified as belonging to the patch (striosomal) compartment or to the matrix by immunohistochemistry for the 28 kD calcium-binding protein calbindin on the same sections. 2. Patch and matrix neurons had very similar axonal and dendritic morphology. Both patch and matrix cells extended their dendrites and local axon collaterals almost exclusively in their respective compartments. Patch cells and most matrix cells had local axon collaterals within or near the parent dendritic domain. However there was a class of matrix cells that extended axon collaterals over a much wider portion of the neostriatum but still restricted to the matrix compartment. 3. Input resistance and membrane time constant were estimated from the membrane response to intracellularly applied current pulses. The average values of matrix cells were and 8.41 ms. The values of patch cells were 31.8 M omega and 8.19 ms and were within the range of those of matrix cells. Both types of cells showed the same kinds of membrane nonlinearities when tested with the use of current pulses. Input resistance and time constant were both strongly affected by a fast anomalous rectification and were thus voltage-dependent, decreasing with membrane polarization. Slow ramplike depolarizing responses were observed in response to depolarizing current steps. 4. Repetitive firing was examined with the use of depolarizing current pulses. In both types of spiny cells, trains of action potentials showed little adaptation of spike frequency and linearly increased with current intensities less than 1 nA. The slopes frequency, calculated from the first and second intervals, were 115.0 and 107.2 Hz/nA, respectively, for matrix cells and 86.0 and 82.8 Hz/nA for patch cells. 5. Stimulation of the medial agranular cortex induced EPSPs in some striatal cells in both compartments. EPSP in matrix cells often showed both short-latency and long-latency components, corresponding to two early components of the response observed in vivo. Some matrix cells, and all patch cells, showed only the longer latency EPSP component. The average latency was 6.3 ms in matrix cells and 9.1 ms in patch cells. The relationship between EPSP amplitude and membrane potential was nonlinear, with EPSP amplitude and duration increasing with decreasing membrane polarization.(ABSTRACT TRUNCATED AT 400 WORDS)     

Electrophysiological and morphological properties of rat abducens motoneurones

Summary The electrical and morphological properties of abducens motoneurones were investigated in the rat with intracellular recordings and intracellular HRP-staining. Motoneurones were identified by their antidromic response to electrical stimulation of the lateral rectus muscle. The antidromic action potential was followed by a delayed depolarization and an after hyperpolarization lasting 20 ms to 45 ms. The whole neurone input resistance (R_N) calculated from I/V curves, was found to lie between 2 M and 15 M with a bimodal distribution (mean values 4.9 M and 12 M). In some cases, anomalous rectification was observed with low current intensities. Prolonged hyperpolarizing current pulses revealed the presence of a time dependant inward rectification and slow rebound depolarization. The intensity/frequency curves suggest the existence of three ranges of discharge. The average intensity frequency slope during the steady state was 43 imp/s/ nA. Eight abducens motoneurones were intracellularly labelled with HRP and fully reconstructed. The soma (23 m to 40 m in diameter) gave off 5 to 7 primary dendrites. The general organization and extension of the dendritic trees depended on the location of the soma within the abducens nucleus. The mean diameter of primary dendrites was 4.17 m with similar average values in all motoneurones. The soma size of abducens motoneurones was not correlated with either the size of the proximal tree or the whole neurone input resistance.     

Physiological properties of primate lumbar motoneurons.

1. Intracellular recordings were obtained from 149 motoneurons innervating triceps surae (n = 109) and more distal muscles (n = 40) in 14 pentobarbital-anesthetized monkeys (Macaca nemestrina). The variables evaluated were resting membrane potential, action potential amplitude, conduction velocity (CV), input resistance (RN), membrane time constant (tau m), electrotonic length (L), whole-cell capacitance (Ctot), long current pulse threshold (rheobase), short current pulse threshold (Ishort), afterhyperpolarization (AHP) maximum amplitude (AHPmax), AHP duration (AHPdur), time to half maximum AHP amplitude (AHP t1/2), depolarization from resting potential to elicit action potential (Vdep), and threshold voltage for action potential discharge (Vthr). 2. Mean values +/- SD for the entire sample of motoneurons are as follows: resting membrane potential -67 +/- 6 mV; action potential amplitude 75 +/- 7 mV; CV 71 +/- 6 m/s; RN 1.0 +/- 0.5 M omega; tau m 4.4 +/- 1.5 ms; L 1.4 +/- 0.2 lambda; Ctot 7.1 +/- 1.8 nF; rheobase 13 +/- 7 nA; Ishort 29 +/- 14 nA; AHPmax 3.5 +/- 1.3 mV; AHPdur 77 +/- 26 ms; AHP t 1/2 21 +/- 7 ms; Vdep 11 +/- 4 mV; and Vthr -56 +/- 5 mV. CV is lower in soleus than in either medial or lateral gastrocnemius motoneurons, and RN is lower and tau m is longer in soleus than in lateral gastrocnemius motoneurons. 3. RN is higher in motoneurons with longer tau m and slower CV. A linear relationship exists between log(CV) and log(1/RN) with a slope of 1.8-2.2 (depending on the action potential amplitude acceptance criteria used), suggesting that membrane resistivity (Rm) does not vary systematically with cell size. 4. Rheobase is higher in motoneurons with lower RN, longer tau m, shorter AHP time course, and higher CV. Ishort and normalized rheobase (i.e., rheobase/Ctot) vary similarly with these motoneuron properties, except that Ishort is independent of tau m and normalized rheobase is independent of CV. 5. Vthr tends to be more depolarized in motoneurons with large Ctot, but the relationship is sufficiently weak so that any systematic variation in Vthr according to cell size probably contributes only minimally to recruitment order. Vthr does not vary systematically with CV, AHP time course, RN, or tau m. 6. Quantitative differences between macaque and cat triceps surae motoneurons are apparent in CV, which is slower in macaque than in cat, and to a lesser extent in tau m and RN, which are lower in macaque than in cat.(ABSTRACT TRUNCATED AT 400 WORDS)     

Perforated patch-clamp analysis of the passive membrane properties of three classes of hippocampal neurons.

1. Perforated patch-clamp recordings were made from the three major classes of hippocampal neurons in conventional in vitro slices prepared from adult guinea pigs. This technique provided experimental estimates of passive membrane properties (input resistance, RN, and membrane time constant, tau m) determined in the absence of the leak conductance associated with microelectrode impalement or the washout of cytoplasmic constituents associated with conventional whole-cell recordings. 2. To facilitate comparison of our data with previous results and to determine the passive membrane properties under conditions as physiological as possible, recordings were made at the resting potential, in physiological saline, and without any added blockers of voltage-dependent conductances. 3. Membrane-potential responses to current steps were analyzed, and four criteria were used to identify voltage responses that were the least affected by activation of voltage-dependent conductances. tau m was estimated from the slowest component (tau 0) of multiexponential fits of responses deemed passive by these criteria. RN was estimated from the slope of the linear region in the hyperpolarizing direction of the voltage-current relation. 4. It was not possible to measure purely passive membrane properties that were completely independent of membrane potential in any of the three classes of hippocampal neurons. Changing the membrane potential by constant current injection resulted in changes in RN and tau 0; subthreshold depolarization produced an increase, and hyperpolarization a decrease, in both RN and tau 0 for all three classes of hippocampal neurons. 5. Each of the three classes of hippocampal neurons also displayed a depolarizing "sag" during larger hyperpolarizing voltage transients. To evaluate the effect of the conductances underlying this sag on passive membrane properties, 2-5 mM Cs+ was added to the physiological saline. Extracellular Cs+ effectively blocked the sag in all three classes of hippocampal neurons, but the effect of Cs+ on RN, tau 0, and the voltage dependence of these parameters was unique for each class of neurons. 6. CA1 pyramidal neurons had an RN of 104 +/- 10 (SE) M omega and tau 0 of 28 +/- 2 ms at a resting potential of -64 +/- 2 mV (n = 12). RN and tau 0 were larger at more depolarized potentials in these neurons, but the addition of Cs+ to the physiological saline reversed this voltage dependence. 7. CA3 pyramidal neurons had an RN of 135 +/- 8 M omega and tau 0 of 66 +/- 4 ms at a resting potential of -64 +/- 1 mV (n = 14).(ABSTRACT TRUNCATED AT 400 WORDS)     

Conductance mechanism responsible for long-term potentiation in monosynaptic and isolated excitatory synaptic inputs to hippocampus

The biophysical mechanisms underlying long-term potentiation (LTP) were investigated in identifiable and monosynaptic excitatory inputs to hippocampal neurons. The results provide the first insights into the conductance changes that are responsible for the expression of LTP. Both current- and voltage-clamp measurements of the mossy fiber synaptic response in pyramidal neurons of region CA3 were made with a single-electrode-clamp system. The excitatory postsynaptic response was pharmacologically isolated by bathing hippocampal slices in saline containing 10 microM picrotoxin, which blocks the synaptic inhibition that normally accompanies the experimentally evoked mossy fiber response. LTP was induced by tetanically stimulating the mossy fiber input for 1 s at 100 Hz. Before and 20 min to 1 h after inducing LTP, we attempted to measure the mean excitatory postsynaptic potential (EPSP) amplitude, intrasomatic current-voltage relationship to a step (RN) or alpha function (AN) current waveform, membrane time constant (tau m), spike threshold (T50), peak excitatory postsynaptic current amplitude (IP), synaptic conductance increase (delta G), and synaptic reversal potential (VR); but adequate assessments of all eight of these were not always obtained for every cell that was studied. The induction of LTP increased the mean (+/- SE) EPSP amplitude form 10.5 +/- 1.4 mV during the control period to 16.8 +/- 2.4 mV after the induction of LTP (n = 14; P less than 0.05). This change was not accompanied by increases in the mean value of RN (63 +/- 11 M omega before and 61 +/- 11 M omega after induction; n = 8; P greater than 0.05); AN, which approximates the effective synaptic input resistance at the soma (10.0 +/- 1.50 M omega before and 10.5 +/- 1.60 M omega after; n = 10; P greater than 0.05); or tau m (22 +/- 2 ms before and 20 +/- 2 ms after; n = 8; P greater than 0.05). There was no significant change in T50, which was also assessed with an alpha function current waveform (1.48 +/- 0.11 nA before and 1.49 +/- 0.10 nA after; n = 6; P greater than 0.05). The mean value of IP increased from 1.1 +/- 0.2 nA during the control period to 1.8 +/- 0.3 nA after inducing LTP (n = 15; P less than 0.05). Similarly, delta G increased from 30 +/- 4 nS before to 47 +/- 4 nS after induction (n = 10; P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Responses of cortical neurons to stimulation of corpus callosum in vitro

1. An in vitro slice preparation of rat cingulate cortex was used to analyze the responses of layer V neurons to electrical stimulation of the corpus callosum (CC). In addition, synaptic termination of callosal afferents with layer V neurons was evaluated electron microscopically to provide a structural basis for interpreting some of the observed response sequences. 2. Layer V neurons had a resting membrane potential (RMP) of 60 +/- 0.68 (SE) mV, an input resistance of 47 +/- 4.74 M omega, a membrane time constant of 4.37 +/- 0.51 ms, an electrotonic length constant of 1.38 +/- 0.25, and produced spontaneous action potentials that were 50 +/- 0.3 mV in amplitude. Intracellular depolarizing current pulses evoked spikes that were sometimes associated with low-amplitude (2-5 mV) depolarizing (5-10 ms in duration) and hyperpolarizing (10-20 ms in duration) afterpotentials. 3. A single stimulus of increasing intensities to the CC produced one of the following response sequences: a) antidromic spike and an excitatory postsynaptic potential (EPSP), which initiated one or more spikes; b) antidromic spike, EPSP-evoked action potentials, and a hyperpolarization, which may have represented an intrinsic cell property or inhibitory synaptic activity; c) EPSP and evoked spikes only; d) high-amplitude EPSP with an all-or-none burst of action potentials. 4. Antidromically activated (AA) neurons always produced EPSPs in response to CC stimulation. When compared with nonantidromically activated neurons, AA cells had a more negative RMP, greater electrotonic length constant (LN), higher ratio of dendritic to somatic conductance (rho), and formed shorter duration, callosal-evoked EPSPs. 5. Neurons in anterior cingulate cortex produced EPSPs of longer duration than did those in posterior cortex (50 +/- 3.57 versus 26 +/- 1.56 ms, respectively). EPSPs in anterior neurons also had a higher maximum amplitude (20.5 +/- 1.0 versus 11.5 +/- 0.79 mV) and longer time to peak (11.6 +/- 2.2 versus 8.2 +/- 0.8 ms). 6. Electron microscopy of Golgi-impregnated neurons following contralateral lesions demonstrated that both pyramidal and nonpyramidal neurons received direct callosal afferents. Synaptic termination of callosal axons with the apical dendritic trees of anterior pyramidal cells was 6 times greater than it was with posterior pyramidal neurons. 7. EPSP shape differences in anterior and posterior neurons may be partially accounted for by the density and distribution of callosal afferents to these two cortices.     

An electrophysiological study of inferior mesenteric ganglion of the dog

Intracellular recordings were made in vitro from 430 sympathetic neurons in the inferior mesenteric ganglion (IMG) of the dog. Ganglion cells had resting membrane potentials between -35 and -70 mV; input resistance (Rin) was approximately 22 M omega. Cell rheobase to depolarizing current was 0.3 nA, and the action potential elicited was 80-100 mV in amplitude followed by an afterhyperpolarization of up to 15 mV in size, which decayed to the resting membrane potential over a range of 50-500 ms. Neurons were classified as either phasic (188 of 280) or tonic (92 of 280) firing cells, depending on their discharge pattern in response to depolarizing current. Two hundred eight of 430 neurons showed continuous electrical activity in the form of spontaneous excitatory postsynaptic potentials, 2-5 mV in amplitude. Continuous electrical activity was unaffected by tetrodotoxin (3 X 10(-6) M) but abolished by hexamethonium (10(-4) M). A small number of cells (21 of 430) adopted a repetitive firing pattern not associated with injury discharge. These cells may have been pacemaking neurons. Stimulation of peripheral and central nerves resulted in multiple synaptic input to ganglion cells. There was marked convergence of excitatory fibers to any one cell. Evoked synaptic potentials were abolished by hexamethonium (10(-4) M). Synaptic input from peripheral and central nerves could not be correlated with location of postganglionic neurons in the IMG. The possibility of neuronal intercommunication and dissemination of central and peripheral commands is discussed.     

Currents under voltage clamp of burst-forming neurons of the cardiac ganglion of the lobster (Homarus americanus)

Crustacean cardiac ganglion neuronal somata, although incapable of generating action potentials, produce regenerative, slow (greater than 200 ms) depolarizing potentials reaching -20 mV (from -50 mV) in response to depolarizing stimuli. These potentials initiate a burst of action potentials in the axon and are thus termed driver potentials. The somata of the anterior-most neurons (cells 1 or 2) were isolated by ligaturing for study of their membrane currents with a two-electrode voltage clamp. Inward current is attributed to Ca2+ by reason of dependence of driver potential amplitude on [Ca2+]0, independence of [Na+]0, resistance to tetrodotoxin, and inhibition by Cd (0.2 mM) and Mn (4 mM). Ca-mediated current (ICa) is present at -40 mV. It is optimally activated by a holding potential (Vh) of -50 to -60 mV and by clamps (command potential, Vc) to -10 mV. Time to peak (10-30 ms) and amplitude are strongly voltage dependent. Maximum tail-current amplitudes observed at -70 to -85 mV are ca. 100 nA. Inward tail peaks may not be resolved by our clamp (settling time, 2 ms). Tails relax with a time constant (tau) of approximately equal to 12 ms (at -70 to -85 mV). ICa exhibits inactivation in double pulse regimes. Recovery has a tau of approximately equal to 0.7 s. Tail current analyses indicate an exponential decline (tau approximately equal to 23 ms at -20 mV) toward a maintained amplitude of inward current tails. Analysis of outward currents indicates the presence of three conductance mechanisms having voltage dependences, time courses, and pharmacology similar to those of early outward current (IA), delayed outward current (IK), and outward current (IC) of molluscan neurons. Analysis of tail currents indicates a reversal potential for each of these near -75 mV, indicating that they are K currents. Early outward current, IA, shows a peak at 5 ms followed by rapid decline. Response to a second clamp given within 0.4 s is reduced; recovery is exponential, with a tau of approximately equal to 200 ms (at Vh = -50 mV). The amplitude of IA tested at 0 mV shows activation or deactivation by subthreshold shifts of Vh. The extent and rate of these changes shows voltage dependence (tau approximately equal to 100-500 ms for subthreshold prepulses). At the normal cell resting potential of -50 mV the amplitude of IA is 25% of that tested from -80 mV.(ABSTRACT TRUNCATED AT 400 WORDS)     

Responses of mitral/tufted cells to orthodromic and antidromic electrical stimulation in the olfactory bulb of the tiger salamander

1. Responses evoked by electrical stimulation of the olfactory nerve and olfactory tracts were analyzed in 46 output cells of the salamander olfactory bulb, in vivo. Labeling of several cells with horseradish peroxidase indicated that they were mitral and/or tufted neurons. The responses contained reproducible sequences of depolarizing and hyperpolarizing potentials, which changed with increases in stimulus intensity. 2. Stimulation of the nerve with intensities subthreshold for evoking spikes in the recorded cell resulted in a small depolarization followed by a period of hyperpolarization, during which spontaneous spikes were suppressed. With suprathreshold stimulus intensities, a single spike or often a burst of spikes was evoked, followed by a complex prolonged hyperpolarization. When full spikes were blocked by injecting hyperpolarizing current through the recording electrode, an excitatory postsynaptic potential (EPSP) with two major components and sometimes a fast prepotential were observed at the beginning of the response. Amplitudes of the EPSP and hyperpolarization increased with graded increases in stimulus intensity. In tests with paired stimulus volleys, spike generation was inhibited for at least 1 s and often for several seconds during the hyperpolarization. 3. Stimulation of the tracts with intensities subthreshold for evoking spikes in the recorded cell resulted in a complex prolonged hyperpolarization. With suprathreshold stimulus intensities, a single spike was evoked, followed by a similar period of hyperpolarization. When full spikes were blocked by injecting hyperpolarizing current through the recording electrode, a small antidromic spike, presumably generated in the axon or initial segment, was often observed. Amplitude of the hyperpolarization increased with graded increases in stimulus intensity. In tests with paired volleys, generation of a full antidromic spike was inhibited for a period that usually began 20-30 ms, following the spike evoked by the conditioning stimulus and lasted 100-500 ms. Full antidromic spikes were evoked prior to the period of inhibition and small antidromic spikes were evoked during the period. 4. The mean latencies of single evoked spikes or the first spikes of bursts decreased from 22 to 17 ms with increases in the intensity of nerve stimulation and from 7 to 6 ms with increases in the intensity of tract stimulation. Only decreases in orthodromic latency were significant at P less than or equal to 0.05, as determined by one-sided t tests between the means of responses subdivided according to response pattern and relative stimulus intensity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Transient and delayed potassium currents in the Retzius cell of the leech, Macrobdella decora

The properties of a quickly inactivating transient K current (IA) and a slowly inactivating delayed K current (IK) were investigated with two-electrode voltage-clamp techniques in the isolated soma of the Retzius cell of the leech, Macrobdella decora. The two currents could be pharmacologically separated according to their different sensitivities to tetraethylammonium ions (TEA) and 4-aminopyridine (4-AP). IA was totally blocked by 3 mM 4-AP but not affected by 25 mM TEA. IK was suppressed almost completely by 25 mM TEA, whereas its peak amplitude only decreased by 10-15% in 3 mM 4-AP. IA was activated at membrane potentials more positive than -35 to -30 mV, whereas the threshold for IK was at more positive potentials of approximately -20 to -15 mV. The activation of IA was rapid with a voltage-dependent time constant [tau m(A)] that varied from 6 to 2 ms for command potentials between -20 and 10 mV (at 22-24 degrees C). The inactivation, which was independent of voltage, was somewhat slower with a time constant (tau A) of approximately 90-110 ms. The time constants for activation [tau m(K)] and the early inactivation phase (tau K) of IK were both voltage dependent. In the range of potential steps from 0 to 30 mV, tau m(K) varied from 12 to 4.5 ms and tau K from 1,500 to 700 ms. The steady-state inactivation of IA varied with holding potential and was complete at potentials more positive than -30 mV. IA was fully available from potentials more negative than -70 mV. IK did not show steady-state inactivation below its threshold of activation. The time course of IA during a maintained depolarization could be reasonably described by the expression IA(t) = IA(infinity) [1-exp(-t/tau m(A))]2 exp(-t/tau A). The time course of activation of IK without allowance for inactivation was approximated by the expression IK(t) = IK(infinity) [1-exp(-t/tau m(K))]2. The reversal potentials and magnitude of both IA and IK were dependent on extra-cellular K concentration, which suggest that a substantial part of the two currents was carried by K ions.     

Membrane properties of interneurons in stratum oriens-alveus of the CA1 region of rat hippocampus in vitro

The membrane properties of interneurons situated near the border of stratum oriens and the alveus of the CA1 region were examined with intracellular recording and staining in rat hippocampal slices in vitro. Cellular staining with Lucifer Yellow indicated that the somata of these interneurons were multipolar and their dendrites projected horizontally along the alveus and vertically toward stratum lacunosum-moleculare. Intrinsic properties (input resistance, action potential amplitude, time constant) and spike after-potentials were typical of non-pyramidal cells. Action potential duration, however, was of relatively medium duration (1.15 ms) and slow afterhyperpolarizations followed depolarization-induced trains of action potentials. Spontaneous activity of interneurons was prominent and of either of two types: single action potentials or high frequency bursts of action potentials. Interneurons displayed marked, voltage- and time-dependent inward rectification and anodal break excitation. Analysis of the slope of the charging function of hyperpolarizing transients, suggested that these interneurons were electrically compact (dendrite to soma conductance ratio, p approximately 2.7; and electrotonic length constant, L approximately 1.1). Characteristically, interneurons sustained high frequency repetitive firing during long depolarizing pulses. The slope of the frequency-current relation was 442 Hz/nA for the first interspike interval and 117 Hz/nA for later intervals (no. 60), suggesting the presence of spike frequency adaptation. Physiologically, these interneurons resembled more closely basket cells of stratum pyramidale than stellate cells of stratum lacunosum-moleculare.     

Synaptic responses of guinea pig cingulate cortical neurons in vitro.

1. Intracellular recordings were made from layer V/VI neurons of the guinea pig anterior cingulate cortex to investigate postsynaptic potentials (PSPs) evoked by electrical stimulation of the subcortical white matter (forceps minor). 2. Four distinct types of PSPs were recorded (at the resting potential) under normal physiological conditions; 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive excitatory postsynaptic potentials (EPSPs) were followed by bicuculline- or picrotoxin-sensitive depolarizing or hyperpolarizing inhibitory postsynaptic potentials (IPSPs), which were further followed by phaclofen-sensitive, long-lasting hyperpolarizing postsynaptic potentials (LPSPs). The average times-to-peak for the EPSP, depolarizing and hyperpolarizing IPSPs, and LPSP were 10, 22, 28, and 146 ms, respectively. 3. In the presence of CNQX and bicuculline, high-intensity electrical stimulation elicited a longer lasting EPSP with a time-to-peak of 21 ms. The amplitude and duration of the EPSP decreased with membrane hyperpolarization and increased with membrane depolarization. The EPSP was reversibly abolished by D,L-2-amino-5-phosphonovaleric acid (D,L-APV). 4. The bicuculline- or picrotoxin-sensitive depolarizing and hyperpolarizing IPSPs and the phaclofen-sensitive LPSP were markedly suppressed by CNQX, suggesting that glutamate (Glu) and/or aspartate nerve terminals project to GABAergic interneurons, and that the GABAergic interneurons are activated mainly by non-N-methyl-D-aspartate (non-NMDA) receptors. 5. In the presence of picrotoxin, the average reversal potential for the compound EPSP was 0 mV, which was similar to that (-6 mV) for the Glu-induced depolarization. In a solution containing D,L-APV at low concentrations, the average reversal potentials for the depolarizing and hyperpolarizing IPSPs and for the early and late components of the gamma-aminobutyric acid (GABA)-induced responses were -62, -72, -70, and -61 mV, respectively. Thus the value for the depolarizing IPSP was similar to that for the late response to GABA, whereas the value for the hyperpolarizing IPSP was almost the same as that for the early response to GABA. The average reversal potential of -90 mV for the LPSP was similar to -93 mV for the baclofen-induced hyperpolarization and to -94 mV for the spike afterhyperpolarization. 6. Application of phaclofen decreased the interspike interval of the spontaneous firing and reversed the increase in the interspike interval after subcortical stimulation. This result indicates that, even in a slice preparation, the anterior cingulate neurons are under tonic inhibitory control exerted by spontaneously active GABAergic interneurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Active membrane properties of rat neostriatal neurons in an in vitro slice preparation

Summary The active membrane properties of rat neostriatal neurons have been studied in an in vitro slice preparation. All the neurons examined had resting membrane potentials of more than 50 mV and generated action potentials with amplitudes exceeding 70 mV. The morphological characteristics of the neurons identified by intracellular labeling with HRP indicated that they were medium spiny neurons. 1. Depolarizing current injection through the recording microelectrode generated slow depolarizing potentials and repetitive action potentials with frequencies ranging from less than 10 Hz to over 300 Hz. Adaptation of action potentials was observed when long duration depolarizing current was injected. 2. Depolarizing current injections revealed that the membrane of the striatal neuron had an anomalous rectification when the membrane potential was depolarized to the resting potential. A possible bases for the anomalous rectification might involve inactivation of K-conductance and slow inward Ca- and/or Na-currents. 3. Local electrical stimulation evoked depolarizing postsynaptic potentials (DPSPs) followed by long-lasting small depolarizations. In a double stimulation test, a potentiation of the test DPSP was observed at interstimulus time interval of up to 80 ms. Post-tetanic potentiation of DPSPs was also seen in these neurons. 4. Tests utilizing depolarizing current injection, intracellular Cl^– injection, and Cl-conductance blocking drugs indicated that the DPSPs were composed of EPSPs and overlapping IPSPs. 5. The nature of the longlasting small depolarization succeeding the DPSPs could not be conclusively determined. However, available data suggest that the slow inward Cacurrent may be responsible for this response. 6. In some neurons, antidromic responses were observed following local stimulation. Spike invasion into the somatic region was blocked by an injection of hyperpolarizing current to the neuron or by synaptic inputs evoked by conditioning local stimulation. These findings may explain the difficulties encountered by previous investigators in obtaining antidromic responses from neostriatal neurons in in vivo preparation.     

Characterization of Ca current underlying burst formation in lobster cardiac ganglion motorneurons

1. The anterior motorneurons of the cardiac ganglion of Homarus americanus were ligated less than 300 microns from the soma. This removes impulse-generating membrane and sites of synaptic input while preserving the ability of the soma to generate the burst-forming potentials termed "driver potentials" regenerative, slow (250-ms duration) depolarizations (to -20 mV) in response to brief, depolarizing stimuli. At stimulus intervals corresponding to rates of bursting observed in spontaneously active, intact ganglia (0.3-1.2/s), driver potential amplitude increases with increasing stimulus interval. 2. A two-electrode voltage clamp was used to characterize inward current observable from the ligated neurons in tetrodotoxin (TTX)-tetraethylammonium (TEA)-containing salines. The amplitude of inward current shows a hyperbolic relation to [Ca]o that is well fitted by a form of the Michaelis-Menten equation. Inward current is maintained but not augmented when Ca2+ is replaced by Ba2+ or Sr2+. It is concluded that the inward current, to be referred to as ICa, is mediated by voltage-dependent Ca channels. 3. Contamination of ICa by early outward current (IA) was evaluated by addition of 4-aminopyridine (4-AP, 4 mM). In the presence of 4-AP, the net inward current is increased and the potential at which maximum ICa occurs is shifted 10 mV more positive. 4. Subtraction of outward currents recorded in Mn2(+)-containing saline from overall currents in the absence of Mn2+ provided another means to separate inward from outward current. I-V curves from such "Mn-subtracted" records show ICa approaches a saturating value for steps to -5 mV and more depolarized. The time to peak ICa is voltage dependent. The largest inward currents (up to 240 nA) and minimal time to peak (4 ms) are observed for steps from holding potentials of -50 to -60 mV. 5. Decline of ICa during depolarized steps observed in Mn-subtracted records represents inactivation rather than development of competing outward current. Inactivation is slow and incomplete; the rate and fractional amount of inactivation are not directly voltage dependent. Nonsubtracted responses to 500-ms depolarizations to potentials evoking little outward current show that an initial rapid decline of ICa (tau approximately 40 ms) is followed at approximately 80 ms by a slower phase of decline (tau approximately 180 ms). With repetitive clamps, the early phase proved labile.(ABSTRACT TRUNCATED AT 400 WORDS)     

Substantia nigra stimulation evoked antidromic responses in rat neostriatum

Summary Electrical stimulation of the substantia nigra of rats elicits a burst of small amplitude waves with a latency of 4–6 ms that may last for 10–15 ms throughout much of the neostriatum. Frontal cortex stimulation also elicits a burst response, which can occlude the substantia nigra response. The substantia nigra evoked burst response was still present after chronic neocortical ablation or thalamic transection or both treatments combined. The response corresponds to the first sharp negative wave of the substantia nigra evoked neostriatal field potential. Single substantia nigra evoked action potentials were recorded in neostriatum with a mean latency of 9.8 ms, ranging from 4–22 ms. These action potentials were considered to be antidromic because 1) they were occluded during appropriate collision intervals by orthodromic action potentials elicited by frontal cortex stimulation. Subthreshold frontal cortex conditioning stimulation did not alter the threshold for activation from substantia nigra. The refractory period for the axon was at least as long as that for the soma and ranged between 0.8–2.0 ms. The antidromic responses failed to follow low frequency stimulation (< 40 Hz for 3000 ms). This failure occurred in the axon between substantia nigra and globus pallidus. The burst response and first sharp negative wave of the field potential probably represent the antidromic activation of the ubiquitous and densely packed medium spiny neostriatal projection neurons. These responses 1) occur at the same latency, 2) respond in the same manner to twin pulse and repetitive stimulation and 3) are occluded by frontal cortex stimulation in the same manner as antidromic action potentials.