Intracellular recording from pectoral fin motoneurons of the stingray Dasyatis sabina, an elasmobranch fish

Intracellular recordings were made from antidromically identified pectoral fin motoneurons in unanesthetized, decerebrate stingrays (Dasyatis sabina). These recordings had the three all-or-none components seen in other vertebrate motoneuron recordings. About 25% of the impalements had resting membrane potentials that were greater than -80 mV, which is larger than those of motoneurons from other vertebrate species. A novel depolarizing afterpotential (DAP) is associated with the isolated action potential occurring at the first node of Ranvier of the axon (M-spike). Occlusion experiments exclude recurrent events as the source of this potential. A capacitive source for the DAP is postulated. Using morphological and passive electrical data on motoneurons from previous studies, calculations of the passive decay of the nodal spike indicate that the membrane resistance of the initial segment is low and nearly equal to that of nodal membrane. The soma-dendritic (SD) spike is followed by a prominent, humped delayed depolarization (DD). The DD is temporally associated with the onset of the action potential produced by the initial segment (IS spike). Sources of the long-lasting period of repolarization recorded with the IS spike, which may underlie the DD, are postulated. The afterhyperpolarization (AHP) of stingray motoneurons tends to be shorter and smaller in amplitude than that of other vertebrate motoneurons. A negligible conductance change was often found during the period following an SD spike. No significant correlation was found between AHP duration and axonal conduction velocity. The input conductance of stingray motoneurons ranged between 1.5 X 10(-7) and 13.3 X 10(-7) S. The relationship between input conductance and axonal conduction velocity was determined from 42 motoneurons. These data were fitted by a power function with an exponent of 1.7, indicating that, in terms of membrane conductance properties, large stingray motoneurons are simply scaled-up versions of the small motoneurons.     

How retinal ganglion cells prevent synaptic noise from reaching the spike output

Synaptic vesicles are released stochastically, and therefore stimuli that increase a neuron's synaptic input might increase noise at its spike output. Indeed this appears true for neurons in primary visual cortex, where spike output variability increases with stimulus contrast. But in retinal ganglion cells, although intracellular recordings (with spikes blocked) showed that stronger stimuli increase membrane fluctuations, extracellular recordings showed that noise at the spike output is constant. Here we show that these seemingly paradoxical findings occur in the same cell and explain why. We made intracellular recordings from ganglion cells, in vitro, and presented periodic stimuli of various contrasts. For each stimulus cycle, we measured the response at the stimulus frequency (F1) for both membrane potential and spikes as well as the spike rate. The membrane and spike F1 response increased with contrast, but noise (SD) in the F1 responses and the spike rate was constant. We also measured membrane fluctuations (with spikes blocked) during the response depolarization and found that they did increase with contrast. However, increases in fluctuation amplitude were small relative to the depolarization (<10% at high contrast). A model based on estimated synaptic convergence, release rates, and membrane properties accounted for the relative magnitudes of fluctuations and depolarization. Furthermore, a cell's peak spike response preceded the peak depolarization, and therefore fluctuation amplitude peaked as the spike response declined. We conclude that two extremely general properties of a neuron, synaptic convergence and spike generation, combine to minimize the effects of membrane fluctuations on spiking.     

Intracellular study of electrophysiological features of primate spinothalamic tract neurons and their responses to afferent inputs

1. Intracellular recordings were made from 43 spinothalamic (STT) neurons in the lumbosacral region of the spinal cord in anesthetized macaque monkeys. The antidromic responses of these neurons to electrical stimulation of the ventral posterior lateral (VPL) nucleus of the thalamus were examined, and orthodromic responses to electrical stimulation of the sural nerve or to mechanical stimulation of hindlimb skin were recorded to study the electrophysiological features of these neurons and their responses to afferent inputs. 2. The resting membrane potential of the neurons ranged from -26 to -70 mV and the antidromic latency from 2.3 to 9.1 ms. Three of the neurons were located in lamina 1 and were recorded so briefly that only antidromic and spontaneous activity could be studied. The rest of the neurons were located in laminae III-V and were of the wide-dynamic-range (WDR) type. 3. The antidromic action potentials recorded in the somas of STT neurons typically showed a fast rising phase and a short initial segment-somadendritic (IS-SD) delay. After repetitive antidromic stimulation, a progressive elongation of the IS-SD delay, widening of the spike, and failure of the SD spike were observed. 4. The afterpotential of the antidromic action potential consisted of a fast afterhyperpolarization (AHPf) and sometimes a delayed depolarization (DD) and a slow afterhyperpolarization (AHPs). The amplitude and the duration of the AHPs were progressively increased when longer trains of stimuli were used. When the membrane potential was hyperpolarized, the amplitude of the AHPs decreased, suggesting an involvement of K+ and/or Cl- ions. However, the AHPs completely disappeared when the strength of stimulation was adjusted to a level just below the threshold for the axon, suggesting that it was unlikely that recurrent inhibition contributed to the AHPs. 5. The background activity of 32 STT neurons was analyzed. The membrane potential at which spikes were triggered in these neurons was around -42 mV. The width and the rise time of the spontaneous spikes were shorter than those of antidromic action potentials, although the maximum rate of rise was similar. The heights of the spontaneous spikes were slightly shorter than those of antidromic action potentials. 6. Three types of background activity have been observed. One type had a very low average spontaneous rate with a bursting firing pattern, consisting of a few spikes superimposed on a depolarization. This type of activity was seen mostly in lamina I neurons. The second type of activity had a moderate to high spontaneous rate with a fairly constant interval between spikes.(ABSTRACT TRUNCATED AT 400 WORDS)     

Membrane properties of the smooth-muscle fibres of the guinea-pig portal vein

The membrane activities and the various characteristic constants of the smooth-muscle membrane of the guinea-pig portal vein were investigated with the micro-electrode technique.1. The mean membrane potential was -37 mV. Spontaneous discharges appeared as regular bursts of short trains of spikes alternating with silent periods, as a mixture of single spikes and bursts of spikes appearing continuously, or as regular spikes with low frequency.2. Spontaneous spikes with overshoot were frequently observed. The maximum rate of rise of the spike was 3.7 V/sec. The shapes of the spikes were classified into three different types, i.e. pace-maker type of spike, monophasic spike and spike with a hump during the falling phase.3. Tetrodotoxin (10(-5) g/ml.) did not influence the patterns of the spontaneous train discharges nor the shape of the spike.4. Extracellularly applied outward current elicited spikes which were either monophasic or had a hump on the falling phase. Inward current elicited break excitation of the spike.5. Current-voltage relations, produced by application of inward current pulses to the tissue and measured at various distances from the stimulating partition, were linear.6. The smooth-muscle membrane of portal vein showed cable-like properties. The mean space constant of the membrane was 0.52 mm; the mean time constant of the membrane calculated from the electrotonic potential was 330 msec.7. Conduction velocity of the spike measured by insertion of two micro-electrodes was 0.58 cm/sec.8. The time constant of the foot of the propagated spike was 27 msec. The time constant of the membrane calculated from the time constant of the foot of the spike and the conduction velocity was 310 msec.9. The membrane properties of longitudinal smooth muscle of the portal vein were discussed in comparison with other veins and various visceral smooth muscles.     

Basic electrophysiological properties of spinal cord motoneurons during old age in the cat

1. The electrophysiological properties of alpha-motoneurons in old cats (14-15 yr) were compared with those of adult cats (1-3 yr). These properties were measured utilizing intracellular recording and stimulating techniques. 2. Unaltered in the old cat motoneurons were the membrane potential, action potential amplitude, and slopes of the initial segment (IS) and soma dendritic (SD) spikes, as well as the duration and amplitude of the action potential's afterhyperpolarization. 3. In contrast, the following changes in the electrophysiological properties of lumbar motoneurons were found in the old cats: a decrease in axonal conduction velocity, a shortening of the IS-SD delay, an increase in input resistance, and a decrease in rheobase. 4. In spite of these considerable changes in motoneuron properties in the old cat, normal correlations between different electrophysiological properties were maintained. The following key relationships, among others, were the same in adult and old cat motoneurons: membrane potential polarization versus action potential amplitude, duration of the afterhyperpolarization versus motor axon conduction velocity, and rheobase versus input conductance. 5. A review of the existing literature reveals that neither chronic spinal cord section nor deafferentation (13, 21) in adult animals produce the changes observed in old cats. Thus we consider it unlikely that a loss of synaptic contacts was responsible for the modifications in electrophysiological properties observed in old cat motoneurons. 6. We conclude that during old age there are significant changes in the soma-dendritic portion of cat motoneurons, as indicated by the modifications found in input resistance, rheobase, and IS-SD delay, as well as significant changes in their axons, as indicated by a decrease in conduction velocity.     

Electrical properties of frog motoneurons in the in situ spinal cord.

Electrical properties of the spinal motoneurons of Rana temporaria and R. esculenta were investigated in the in situ spinal cord at 20-22 degrees C by means of intracellular recording and current injection. Input resistance values depended on the method of measurement in a given cell but were generally inversely related to axon conduction velocity. The membrane-potential response to a subthreshold current pulse was composed of at least two exponentials with mean time constants of 2.5 and 20 ms. The membrance potential reached by the peak of a spike depended on the mode of spike initiation and membrane potential. Preceding a suprathreshold depolarization by a hyperpolarizing pulse could delay and eliminate spike initiation, similar to effects reported in certain invertebrate neurons. Antidromic invasion frequently failed in motoneurons of normal resting potential. Antidromic spike components (m,IS, SD) were similar to those of cat motoneurons. The delayed depolarization and the long afterhyperpolarization following an antidromic spike had many properties in common with the analogous afterpotentials of cat motoneurons. The reversal potential of the short afterhyperpolarization occurring immediately after the spike varied with resting potential and could not be used to determine potassium equilibrium potential. Sustained rhythmic firing could be evoked by continuous synaptic drive or long pulses of injected current. The plot of firing rate versus current strength had a substantial linear region. Both steady firing and adaptation properties varied markedly with motoneuron input resistance.     

Three types of sympathetic preganglionic neurones with different electrophysiological properties are identified by intracellular recordings in the cat

Intracellular recordings were obtained from sympathetic preganglionic neurones (SPN) of the third thoracic segment in cats. Based on differences in their active and passive electrophysiological properties, three different types of SPNs were discerned: Type A neurones had a high resting membrane potential (RMP) (-60 to -86 mV) and a low input resistance (RN) 12-23 M omega). Action potentials of these neurones had a pronounced IS-SD inflexion and a prominent shoulder in their falling phase. Spikes were rarely generated from the on-going synaptic activity. Type B neurones had a lower RMP (-48 to -65 mV) and a higher RN (21-37 M omega). Their action potentials were characterized by an after-depolarization; they showed a slight IS-SD inflexion and a less pronounced shoulder in their falling phase. The after-depolarization was abolished by membrane hyperpolarization in a time dependent way. A hyperpolarization of at least 50 ms duration was required for its abolition. The after-depolarization was also abolished during repetitive discharges. In most of these neurones spikes were generated at irregular intervals and low rates (0.06-4.6 spikes/s) from the synaptic activity. Type C neurones were similar to type B neurones, but their action potentials did not show the after-depolarization. Additionally, spikes were generated at fairly regular intervals and rather high rates (0.8-6.5 spikes/s). The rate of spike repolarization of all neurones was markedly increased by hyperpolarization and decreased by membrane depolarization. Current-voltage curves of some type B and C neurones showed a marked rectification upon membrane hyperpolarization.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sodium and calcium components of the action potential in a developing skeletal muscle cell line.

1. Developmental changes in action potential properties were studied in a clonal rat skeletal muscle cell line. 2. Small action potentials were evoked in mononucleate myoblasts. No spike was seen in Na-free saline. A similar spike was evoked in a medium where all NaCl was replaced by LiCl. No spike was evoked when NaCl was replaced by CsCl. 3. Action potentials overshot zero membrane potential in multinucleate myotubes. The action potential was composed of two components, an initial fast spike and a hump on the falling phase or in some cases a distinct second peak. 4. Teh overshoot of the initial fast spike decreased when the external Na concentration was decreased. 5. In saline with 10 mM-Ca the second component often formed a distinct peak following the initial fast spike. A slow regenerative potential was evoked in Na-free media with a depolarizing current pulse. 6. In saline containing BaCl-2 instead of CaCl-2 there was always a second peak, the overshoot of which changed with external Ba concentration. A slow regenerative potential was evoked in Na-free, Ba-saline. The membrane conductance at the peak of the Ba-action potential was larger than in the resting state. 7. In adult rat skeletal muscle, the shape of the action potential was not changed when Ca was replaced by Ba. No action potential was evoked in Na-free Ba-saline or Ba-saline with tetrodotoxin (3 times 10-7 M). 8. The significance of the Ca component in the developing muscle is discussed.     

Spike potentials and membrane properties of dorsal root ganglion cells in pigeons

Active and passive membrane properties of dorsal root ganglion (DRG)-cells from the intact superfused ganglion of pigeons have been compared with the conduction velocity of their centrifugal axons.About two thirds of the neurones were associated with myelinated axons and classified as A-cells; the remainder were associated with unmyelinated axons and classified as C-cells. Slowly conducting group III A-cells (5–25 m·s^–1) constituted half of the A-cell population. With exception of spike duration, spike parameters and membrane properties did not differ among the A-cells. Spike duration increased with decreasing conduction velocity demonstrating a small plateau (hump) during the fall time in group III neurones. This hump was more distinct in C-cells, resulting in a 2–5 times longer duration of action potentials. Amplitude and duration of afterhyperpolarization (AHP) of C-cells was 2–3 times that of A-cells.Administration of 10 mM CoCl₂ decreased the rate of rise and the overshoot but increased the rate of fall of the action potential in C-cells and group III A-cells, largely abolishing the hump. It is suggested that the hump of the spike potential is largely produced by a Cacurrent and that the resultant increase of intracellular Ca might produce the larger AHP in C-cells, secondary to an increase in K-conductance.     

A decrease in firing threshold observed after induction of the EPSP-spike (E-S) component of long-term potentiation in rat hippocampal slices

Summary Two components of long-term potentiation (LTP) are distinguished with extracellular recording electrodes: a synaptic and an EPSP-Spike (E-S) component. The latter consists of the enhancement produced in the population spike amplitude in excess of that predicted by EPSP potentiation alone. The experiments carried out in this study were designed to investigate intracellular correlates of E-S potentiation and to examine the hypothesis that an increased postsynaptic excitability underlies E-S potentiation. CA1 pyramidal neurons were synaptically activated from stratum radiatum. LTP, defined as a stable increase in the probability of firing to afferent stimulation, was found to be related to a decrease in the intracellular PSP peak amplitude and slope required to fire the cells at a probability of 0.5. These changes were accompanied by a decrease in threshold to direct activation. No significant changes in input resistance or resting potential were recorded. These excitability changes were only observed in cells displaying LTP; they were not related to the potentiation of the synaptic component (PSP amplitude). Our results support the hypothesis that different mechanisms underlie the two components of LTP, and that a reduction in threshold for neuronal discharge accompanies tetanus-induced E-S potentiation. It is suggested that an increase in the ratio of synaptically evoked excitation/inhibition and a reduction in tonic synaptic inhibition through GA-BA_A channels contribute to E-S potentiation.     

Extracellular potassium and trasmitter release at the giant synapse of squid.

1. The effects of changes in extracellular K concentration, [K]0, on synaptic transmission were studied at the squid giant synapse with intracellular recording from the presynaptic terminal and post-synaptic axon. 2. The amplitudes of both the presynaptic spike and the e.p.s.p. varied inversely with [K]0. On the average, a 10 mV change in spike height was accompanied by a 3-1 mV change in e.p.s.p. amplitude. 3. The amplitude of the presynaptic spike after-hyperpolarization (AH) varied inversely with [K]0. On the average, increasing [K]0 resulted in a 20% change in e.p.s.p. amplitude per mV change in presynaptic spike AH. 4. Repetitive antidromic stimulation of the post-synaptic giant axon resulted in an exponential decline in the post-synaptic spike AH, a depolarization of the presynaptic membrane potential and a reduction in the AHs of presynaptic spikes. This suggests that the K which accumulates in the extracellular spaces around the post-synaptic axon also affects the presynaptic terminal. 5. Repetitive antidromic stimulation of the post-synaptic axon resulted in a reduction in the amplitude of e.p.s.p.s. elicted by stimulation of the presynaptic axon. The reduction in e.p.s.p. amplitude relative to the change in presynaptic spike AH was quantitatively close to the change produced by increasing [K]0, suggesting that the reduction in e.p.s.p. amplitude is due to the accumulation of extracellular K at the presynaptic terminal. 6. Repetitive stimulation of the presynaptic axon reduced the amplitudes of the e.p.s.p. and the presynaptic spike AH. On the average, a 1 mV change in presynaptic spike AH was accompanied by a 204% change in e.p.s.p. amplitude, suggesting that K accumulation may only contribute to a small extent, under these conditions, to the depression of transmitter release.     

Passive membrane properties,afterpotentials and repetitive firing of superior colliculus neurons studied in the anesthetized cat

Intracellular recording and staining with HRP were used to characterize cat superior colliculus neurons with identified projection into the tecto-bulbo-spinal tract (TBSNs). TBSNs are large multipolar neurons with heavy stem dendrites. First and second order dendrites bifurcate with an average branch power n of about 3/2. More peripheral branch points have n less than 1.5. Input resistances of TBSNs range from 0.9 to 4.6 M omega. Most TBSNs display 'anomalous rectification'. Based on Rall's steady-state cable equations, input resistances were calculated for 3 TBSNs labelled with HRP. Assuming a specific membrane resistance of 2,300-2,600 omega cm2 the/calculated values agree well with the experimentally determined estimates from another set of non-stained TBSNs. Membrane time constants of TBSNs range from 3.0 to 5.6 ms. The electrotonic length was calculated using the ratio tau 0/tau 1. The respective average value was 1.13. TBSNs respond to orthodromic, antidromic and direct stimulation with action potentials of 60-80 mV, composed of IS- and SD-components. The critical interval for IS-SD-invasion was on average 1.6 ms. Spike decomposition occurs usually at M-level. The postspike conductance increase underlying hyperpolarizing afterpotentials (HAP) decays exponentially, with the time constants tau F = 1.5 ms and tau S = 13 ms. The HAP was equilibrated at membrane potentials of -73 to -90 mV. When tested by antidromic stimuli at varying intervals most TBSNs show very poor "summation" of HAPS. A pronounced depolarizing hump (DD) follows antidromic action potentials. Discharging at short intervals leads to a substantial increase and prolongation of DD. This apparent DD-potentiation is interpreted as a phenomenon secondary to the reduction of hyperpolarizing currents. In response to directly injected currents, TBSNs discharge with frequencies up to 1,100 imp/s. The frequency-current curves of TBSNs are characterized by 3 ranges. The average f-i-slopes of the adapted discharge were 19.2 imp/s/nA and 56.4 imp/s/nA for the 1st and 2nd range, respectively. At intermediate current intensities (2nd range) TBSNs discharge in groups of 2 to 7 action potentials, following each other at intervals of 1.0-2.8 ms. The spike groups are separated by pauses of 3.5-6.3 ms duration. The transition from 1st (low frequency continuous) discharge range to 2nd (grouped) discharge range is related to the appearance of extra-spikes. Extra-spikes are generated from a decreased firing level, from the peak of an enhanced DD.(ABSTRACT TRUNCATED AT 400 WORDS)     

A comparison of extracellular and intracellular recordings from medial septum/diagonal band neurons in vitro.

Firing patterns, action potential characteristics and some active membrane properties of guinea-pig medial septum/diagonal band neurons were studied in an in vitro slice preparation. A comparison was made between several types of cells classified according to either extracellularly recorded (n = 130) or intracellularly recorded (n = 30) electrophysiological characteristics. Using multi-barrel extracellular electrodes, three principal cell types were distinguished: slow rhythmic firing cells (29%), fast rhythmic firing cells (65%) and burst-firing cells (6%). Most slow firing cells could also be distinguished from other cell types by their relatively longer action potential duration and a characteristic cadmium-sensitive "hump" in the repolarization phase of the action potential. These characteristics of slow firing cells matched well with the characteristics of cholinergic, slow afterhyperpolarization cells previously identified with intracellular recordings. The action potential shape, firing rate and firing pattern characteristics of about 60% of extracellularly recorded fast rhythmic firing cells matched those of previously identified non-cholinergic fast afterhyperpolarization cells. The remaining extracellularly recorded, rhythmic firing cells (about 10% of slow firing and 40% of fast firing cells) had a mixture of characteristics which precluded unequivocal classification as to cholinergic or non-cholinergic cell type. Using intracellular recording, the bee venom toxin, apamin, was shown to attenuate the characteristic post spike slow afterhyperpolarization of cholinergic cells and greatly enhanced their firing rate to depolarizing pulses. Apamin often attenuated a smaller and more transient afterhyperpolarization found in identified non-cholinergic cells, but firing rate was increased only slightly. Extracellular recordings from slow and fast rhythmic firing cells in the presence of apamin showed that excitability of slow firing cells was enhanced significantly more than fast firing cells. The apamin data support the hypothesis that extracellularly recorded slow firing cells are cholinergic. We conclude that extracellularly recorded medial septum/diagonal band cells characterized by broad action potentials, slow rhythmic firing under microiontophoresed glutamate and a signature "hump" in the falling phase of the action potential are cholinergic cells. Extracellularly recorded fast rhythmic firing cells with a narrow action potential and no "hump" in the action potential are likely to be non-cholinergic cells. This extracellular electrophysiological "fingerprint" for cholinergic medial septum/diagonal band cells in vitro may now be extended to studies in vivo where controversy remains as to the neurochemical identity of basal forebrain cells involved in control of hippocampal slow rhythmic activity.     

Variations in cyclic adenosine 3′,5′-monophosphate and cyclic guanosine 3′,5′-monophosphate content and efflux from the photosensitive pineal organ of the pike in culture

 The photoreceptor cells of the pike pineal organ transduce 24-h light/dark (LD) information to synchronize the clocks driving the melatonin (MEL) rhythm. In fish, the nocturnal rise in MEL synthesis is associated with an increase in cyclic adenosine 3’,5’-monophosphate (cAMP) production and with Ca²⁺ entry, through voltage-gated channels. Light induces inhibition of MEL synthesis and a depression of cAMP content, as well as closure of Ca²⁺ channels. Cyclic guanosine 3’,5’-monophosphate (GMP) levels also are reduced upon acute illumination but this second messenger of phototransduction does not appear to be directly involved in the control of MEL metabolism. It is not known whether cAMP and/or cGMP are components of the clock machinery. In this study we measured cAMP and cGMP contents (static culture) and release (perifusion culture) using pike pineal organs maintained under LD or DD (constant darkness). Under LD, cAMP levels were low at noon and midnight, and high at dawn and dusk, in organs as well as in perfusates. This pattern was maintained under DD, with a major peak occurring at the beginning of subjective light, and a minor peak at the beginning of subjective darkness; only one peak during the subjective light was seen in the perfusates. Under DD, the MEL rhythm displays only one peak during the subjective night. It is suggested that increases in cAMP might not always be correlated with increases in MEL secretion. Under LD, variations in cGMP content were not statistically significant; however, in the perfusates, the levels were higher during the night than during the day. This suggests that: (1) extrusion participates in the regulation of intracellular levels of cGMP, (2) nocturnal synthesis of cGMP is higher than its catabolism, and (3) synthesis is increased during the day to compensate for the light-induced activation of catabolism. Under DD, the cGMP content and release were higher during the subjective night than during the subjective day, revealing a circadian component in the regulation of cGMP metabolism. This may provide the basis for the generation of membrane-related circadian events including variations in membrane potential, in the opening/closure of voltage-gated channels (e.g. Ca²⁺ channels), or in enzyme activities (adenylyl cyclase, cGMP-dependent phosphodiesterase). Received: 7 June 1996 / Received after revision and accepted: 26 September 1996     

Peculiarities of inhibition in cat auditory cortex neurons evoked by tonal stimuli of various durations

Summary The extra- and intracellular responses of 262 neurons in A1 to tones of best frequency with durations ranging from 10 ms to 1.2 min were studied acute experiments on ketamine-anesthetized cats. Following the generation of action potentials in response to the tone stimulus, inhibition of both the background and the auditory stimulus-evoked spike activity were observed in 91% of the investigated neurons. The duration of this inhibition corresponded to the stimulus duration. For the remaining neurons (9%) an inhibition of the stimulus-evoked spike activity alone was seen, also corresponding to the stimulus duration. Maximal inhibition of the spike activity occurred for the first 100–200 ms of the inhibitory response (the period which equalled the time of development of an IPSP in a cell). During this period of IPSP development, the membrane resistance of the neuron was reduced to 60–90% of its initial value. Varying the duration of the acoustic signal within a range of 10–200 ms was accompanied by a change in the IPSP duration and inhibition of the spike acitivity of the neuron. Whenever the tone lasted more than 200 ms, the membrane potential of the neuron was restored to the resting potential. However, during this period, the responsiveness of the neuron was lower than that initially observed. Measurement of the membrane resistance during the inhibitory pause that was not accompanied by hyperpolarization produced an index with an average 17% lower than the initial value for 87% of the neurons.The data indicate that inhibition of the spike activity in Al neurons evoked by tone stimuli of various durations is due to the appearance of postsynaptic inhibition on their membrane. It is concluded that the time course of the cortical inhibitory input to neurons is the major factor determining variations in duration of the inhibition of response of auditory cortex neurons to an auditory stimulus.     

Calcium-dependent potentials with different sensitivities to calcium agonists and antagonists in guinea-pig hippocampal neurons

Effects of organic Ca channel blockers, Ca channel activators and omega-conotoxin on guinea-pig hippocampal CA1 neurons in vitro preparations were studied with intracellular recording methods. Most of the Ca channel blockers, such as prenylamine, D 600, flunarizine, nifedipine, cinnarizine and nicardipine (0.2-4 microM), raised the threshold for Na-dependent spike generation and decreased the amplitude of the spike afterhyperpolarization. Verapamil (5 microM) and diltiazem (5 microM) did not significantly alter the threshold and amplitude of the Na spike. Action potentials elicited in the presence of either tetrodotoxin (0.5 microM) and tetraethylammonium (20 mM) or tetrodotoxin (0.5 microM) and Ba (1.25 mM) consisted of an initial spike component followed by a long depolarization. Both responses were abolished by addition of Co (2 mM) or Cd (0.25-0.5 mM), or by superfusion with a low Ca (0.25 mM)-high Mg(15 mM) medium, indicating that the potentials resulted from Ca entry. The Ca-dependent slow depolarization was preferentially blocked by most of the organic Ca channel blockers at approximately one-third the concentrations (0.1-2 microM) which were required to shorten the Ca spike. When the cell in a solution containing tetrodotoxin (0.5 microM), Co (2 mM) and 4-aminopyridine (2 mM) was hyperpolarized and then depolarized by passing current pulses across the membrane, a transient depolarizing hump occurred on the decay phase of the electrotonic potential. This transient depolarization was abolished by Co (2 mM), Ni (2 mM) or most of the organic Ca channel blockers (0.2-5 microM). Diltiazem (5 microM) did not significantly change these Ca-dependent potentials. The evoked excitatory postsynaptic potential was very resistant to the Ca channel blockers. Approximately 2-10 times higher concentrations (0.5-3 microM) were necessary to decrease the excitatory postsynaptic potential amplitude than to shorten the Ca spike. On the other hand, the minimal concentrations and order of potencies of the Ca channel blockers for depressing the evoked inhibitory postsynaptic potential and for elevating the threshold for Na spike generation were almost the same. Dihydropyridine Ca channel activators, such as Bay K 8644, CGP 28 392 and YC 170 at low concentrations (0.1-1 microM), decreased the Ca spike, the Ca-dependent slow depolarization and the evoked synaptic potentials, while the substances augmented the Ca-dependent transient depolarization. On the other hand, omega-conotoxin (5 microM) reversibly depressed the Ca spike and slow depolarization to the same degree, without affecting the transient depolarization and the evoked excitatory or inhibitory postsynaptic potentials.(ABSTRACT TRUNCATED AT 400 WORDS)     

Classical conditioning reduces amplitude and duration of calcium-dependent afterhyperpolarization in rabbit hippocampal pyramidal cells

1. The afterhyperpolarization (AHP) that follows action potentials was studied in CA1 hippocampal pyramidal cells from classically conditioned and control rabbits. Measurements of the AHP were obtained with intracellular recordings from CA1 cells within hippocampal slices. 2. The AHP of rabbit CA1 pyramidal cells was found to be accompanied by a conductance increase. The AHP was reduced by bath applications of the calcium channel blockers, cadmium and cobalt, by bath application of the cholinergic agonist, carbamylcholine chloride, and intracellular injection of the calcium chelator, ethylene glycol-bis(B-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). 3. The AHP was markedly reduced in cells from rabbits that were well-trained with the nictitating membrane conditioning procedure, as compared with cells from pseudoconditioned or naive control animals. The difference in AHP amplitudes between conditioned and control groups increased as the number of spikes elicited by the stimulation pulse increased from one to four. Both the duration (measured as the time constant of AHP decay) and amplitude of the AHP were reduced in cells from conditioned animals. 4. The reduced AHP in cells from conditioned animals remained reduced in a medium that contained 0.5 microM tetrodotoxin (TTX) and 5.0 mM tetraethylammonium chloride (TEA); the AHP following calcium spikes was measured under these conditions. Since this medium eliminated synaptic transmission elicited by Schaeffer collateral stimulation, the AHP reduction in pyramidal cells from conditioned animals was not due to a modification in synaptic properties. There were no significant differences in the mean voltage thresholds, amplitudes, or durations of calcium spikes between cells from animals in the three groups. Thus the AHP reduction appears to be due to a modification of a Ca2+ -dependent K+ conductance and was not due to a secondary effect of reductions in calcium conductances underlying the spike. 5. In medium containing TTX and TEA, the amount of injected current required to elicit a calcium spike (current threshold) was significantly greater in cells from conditioned animals than in cells from control animals. This increase in current threshold persisted in 4-aminopyridine (4-AP)-containing medium and so cannot be attributed entirely to conditioning-specific increases in the A-current. 6. The conditioning-specific AHP reduction resulted in increased excitability in cells from conditioned animals versus pseudoconditioned control animals. Cells from conditioned animals fired more spikes to trains of 100-ms depolarizing current pulses than did cells from controls.     

Postsynaptic inhibition underlying spike suppression of secondary vestibular neurons during quick phases of vestibular nystagmus

Synaptic mechanisms of spike suppression of vestibular neurons during quick phases of vestibular nystagmus were investigated by intracellular recording in the rostrolateral part of the cat medial vestibular nucleus. When repetitive spike discharges of vestibular neurons were abruptly suppressed at the quick phase, the membrane potential shifted steeply in the hyperpolarizing direction. After the commissural IPSP was inverted into depolarization by intracellular injection of Cl^? ions, the hyperpolarizing deflection of the membrane potential at the quick phase was also inverted into a depolarizing potential. The results indicate that an abrupt generation of IPSPs in vestibular neurons underlies the quick phase suppression of spike activity in these neurons.     

The action of ouabain on the smooth muscle cells of the guinea-pig's taenia coli

1. The action of 10(-6) g/ml. of ouabain on the membrane potential, membrane activity, tension development and the ion content of the smooth muscle of the guinea-pig's taenia coli has been investigated.2. This ouabain concentration produced a depolarization of the membrane, accompanied by an initial increase of the spike frequency and later a depolarization block. Removal of the ouabain produced a sudden and pronounced hyperpolarization leading to a block of spike discharge.3. Reduction of the external sodium concentration, or an increase of the external calcium concentration, or exposure to 10(-7) g/ml. adrenaline repolarized the cell membrane in the presence of ouabain.4. Ouabain decreased the tension development of the taenia coli.5. Ouabain decreased the intracellular potassium and chloride and increased the intracellular sodium concentration. However, the corresponding changes of the equilibrium potentials were not sufficient to explain the changes of the membrane potential.6. A possible explanation for the action of ouabain is presented suggesting that the drug may decrease the calcium binding properties of the cell membrane.     

Membrane properties of the smooth muscle of guinea-pig ureter

1. The membrane properties of the guinea-pig ureter were studied in physiological Krebs solution by intra- and extracellular stimulating methods.2. The mean membrane potential was 50 mV. Action potentials triggered by external stimulation were composed of repetitive spikes and a plateau phase.3. The effects of intracellular polarization on the membrane activity elicited by extracellular stimulation were observed. Anodal polarization enhanced the amplitude and the maximum rate of rise of the spike while cathodal polarization reduced them. The number of the spikes, the duration and amplitude of the plateau phase were not changed by polarization of any direction.4. The spikes triggered by intracellular stimulation were mostly graded, but repetitive spikes sometimes continued even after cessation of the stimulation. The effective membrane resistance was 15-23 MOmega and the time constant was 2-3 msec.5. Conduction velocity (V), chronaxie, time constant (tau) and space constant (lambda) of the tissue were measured by extracellular stimulation. These values were as follows: V, 3-6 cm/sec; chronaxie, 20-40 msec; tau, 200-300 msec; lambda, 2.5-3 mm. The conduction of excitation might be related to the cable properties of the tissue.6. The relative refractory period measured by extracellular stimulation was as long as 30 sec. During the relative refractory period dissociation of the slow depolarization and the spikes was observed by successive stimuli.7. The plateau phase was prolonged and the frequency of the spontaneous discharges was increased by treatment with Ba(2+). Tetrodotoxin had no effect on spike activity nor on the plateau phase, but Mn(2+) blocked the membrane activity.