Synaptic and intrinsic control of membrane excitability of neostriatal neurons. I. An in vivo analysis

1. The relationship between membrane properties of neostriatal neurons and spontaneous and evoked synaptic potentials was studied with the use of intracellular recordings from anesthetized rats. Most of these neurons showed regular or irregular spontaneous depolarizing potentials that only in a few cases triggered action potentials at resting level. 2. The stimulation of the ipsilateral substantia nigra or of the sensorimotor cortex produced a relatively fast depolarizing post-synaptic potential (EPSP). In some cells this potential was followed by an inhibitory period that appeared as an hyperpolarization when the cell was depolarized from the resting level (inhibitory postsynaptic potential, IPSP). A late and long-lasting depolarization (LD) followed the EPSP or the EPSP-IPSP sequence. 3. Repetitive discharge with little adaptation was observed during direct depolarization. Most of the neurons tested for current-voltage (I-V) relationship showed nonlinearity of the input resistance in the hyperpolarizing direction. Spontaneous and evoked EPSPs were decreased in their amplitude and duration when the membrane potential was held at levels more hyperpolarized than -85 mV because of the strong rectification at these levels of hyperpolarization. 4. Local microiontophoretic application of bicuculline (BIC) or systemic administration of BIC and pentylenetetrazole (PTZ) produced a reduction of the IPSPs. The reduction of the inhibitory transmission caused a strong increase of the LD. The current-evoked firing pattern was not greatly altered. 5. The intracellular application of cesium increased the amplitude and the duration of the spontaneous depolarizations that triggered bursts of action potentials under this condition. Spikes were broadened and the rectification in the hyperpolarization direction was reduced. 6. Iontophoretically applied cadmium strongly depressed the amplitude of the spontaneous and evoked postsynaptic potentials. During cadmium application, nigral stimulation produced constant latency, all-or-none spikes in the absence of any synaptic potential. 7. Repetitive stimulation of the ipsilateral substantia nigra by electrical shocks (5 Hz, 25 s) produced a progressive and reversible decrease of the spontaneous depolarizing potentials (SDPs) and a decrease of the firing rate. In the same cells, when the train of stimulation was delivered in the ipsilateral cortex, a membrane depolarization coupled with an increase of the firing rate was observed. 8. We conclude that although synaptic circuits mediate a phasic inhibition in neostriatum, the low level of spontaneous firing of most neostriatal neurons is mainly because of the effects that membrane properties exert on the spontaneous and the evoked synaptic depolarizations in the striatum.(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.     

Plasticity in the intrinsic excitability of cortical pyramidal neurons.

During learning and development, the level of synaptic input received by cortical neurons may change dramatically. Given a limited range of possible firing rates, how do neurons maintain responsiveness to both small and large synaptic inputs? We demonstrate that in response to changes in activity, cultured cortical pyramidal neurons regulate intrinsic excitability to promote stability in firing. Depriving pyramidal neurons of activity for two days increased sensitivity to current injection by selectively regulating voltage-dependent conductances. This suggests that one mechanism by which neurons maintain sensitivity to different levels of synaptic input is by altering the function relating current to firing rate.     

Synaptic potentials evoked in spiny neurons in rat neostriatal grafts by cortical and thalamic stimulation.

1. Fetal rat striatal primordia were implanted into the neostriatum of adult rats 2 days after kainic acid lesion. Two to 6 mo after transplantation, in vivo intracellular recording and staining were performed to study the responses of spiny neurons in the grafts to the cortical and thalamic stimuli. The physiological characteristics and synaptic responses of 27 cells recorded in the grafts were compared with a sample of 23 neurons recorded from the surrounding host neostriatum in the same animals. Nineteen of the graft neurons and 19 of the host neurons were identified as spiny neurons by intracellular staining with biocytin. The responses of the remaining neurons were the same as those of identified spiny cells. 2. The spontaneous synaptically driven membrane potential shifts and long-lasting responses to afferent stimulation that are characteristic of neostriatal cells in normal animals were greatly reduced or absent in graft neurons. Presumably this reflects the reduction in synaptic input to the grafts and the lack of convergence of inputs from diverse sources. 3. Short-latency synaptic responses to cortical and thalamic stimulation were present and could consist of either excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs). The IPSPs were accompanied by a membrane conductance increase, and their reversal potentials could be altered by injection of chloride ions. Several minutes after impaling the cell, the IPSPs gradually disappeared, and the same stimuli could then evoke EPSPs. The disappearance of the IPSPs was independent of the presence of chloride in the electrodes. Most of the EPSP responses appeared to be monosynaptic but occurred at longer latencies than those seen in host neurons of the same type. 4. In cells not exhibiting IPSPs, or after the IPSP responses disappeared, cortical or thalamic stimulation could evoke slow depolarizing potentials and bursts of action potentials. These could not be evoked by current injection. They could be prevented or delayed by an exaggerated action potential after hyperpolarization that developed in neurons maintained in a depolarized state for several seconds, but could not be prevented by passage of hyperpolarizing current from the recording electrode. 5. The input resistance of graft spiny neurons was higher than that of the host cells, and time constants were longer. Both of these properties appeared to be due to the absence of the strong inward rectification that is usually present at resting membrane potentials in neostriatal neurons.     

Age-induced changes in electrophysiological responses of neostriatal neurons recorded in vitro.

The present studies were undertaken to determine whether the major electrophysiological characteristics of neostriatal neurons are altered during aging. The passive and active membrane properties of 130 neostriatal neurons obtained from young (three to five months, N = 65) and aged (24-26 months, N = 65) Fischer 344 rats were compared using an in vitro slice preparation. The results indicated that in a population of aged neostriatal neurons the majority of the electrophysiological changes that occurred resulted in decreases in cellular excitability. These changes included increased threshold to induce action potentials by intracellular current injection and decreased negativity of membrane potentials at which such action potentials were induced. In addition, there were increases in the amplitude of the action potential afterhyperpolarization and increases in the frequency of occurrence of accommodation when trains of action potentials were induced. These two latter effects can limit the frequency of action potential generation. The thresholds to elicit synaptically evoked depolarizing responses and action potentials were increased. The results also indicated that a number of basic electrophysiological parameters were unchanged by the aging process. These included action potential amplitude, rise time and duration, resting membrane potential, input resistance and time constant. Although thresholds for the induction of synaptic and action potentials by extracellular stimulation were increased, the latency, amplitude and duration of the evoked depolarization remained unchanged. These findings suggest that the ability of neostriatal neurons to integrate spatiotemporal inputs must be severely compromised in this population of aged cells. Furthermore, the present findings, when compared with age-induced electrophysiological alterations in neurons in other brain areas, indicate that age may differentially alter electrophysiological properties of neurons in separate nuclei. Profiles of age-related changes in neurophysiological properties of neurons provide important information that can be related to the contributions of individual neural areas to the behavioral effects of aging.     

Inhibitory control of neostriatal projection neurons by GABAergic interneurons.

The basal ganglia are a highly interconnected network of nuclei essential for the modulation and execution of voluntary behavior. The neostriatum is the principal input and one of the principal controllers of the output of the basal ganglia. Neostriatal projection neurons seem to be dynamically and powerfully controlled by GABAergic inputs, but the source(s) and physiological properties of these inputs remain unclear. Here we use paired whole-cell recordings to show that this inhibition derives from small populations of GABAergic interneurons that are themselves interconnected through functional electrotonic synapses. Inhibitory synaptic potentials generated from single interneurons are sufficiently powerful to delay or entirely block the generation of action potentials in a large number of projection neurons simultaneously.     

Development of inward rectification and control of membrane excitability in mesencephalic v neurons

The present study was performed to assess the postnatal development and functional roles of inward rectifying currents in rat mesencephalic trigeminal (Mes V) neurons, which are involved in the genesis and control of oral-motor activities. Whole cell voltage-clamp recordings obtained from Mes V neurons in brain stem slices identified fast (I(KIR)) and slow (I(h)) inward rectifying currents, which were specifically blocked by BaCl(2) (300-500 microM) or 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD 7288, 10 microM), respectively. The whole cell current density for these channels increased between postnatal days 2 to 12 (P2-P12), and the time courses for I(h) activation and deactivation were each well described by two time constants. Application of ZD 7288 produced membrane hyperpolarization in the majority of cells and prolonged afterhyperpolarization repolarization. Additionally, in the presence of ZD 7288, spike frequency was decreased and adaptation was more pronounced. Interestingly, these neurons exhibited a voltage-dependent membrane resonance (<10 Hz) that was prominent around resting potential and more negative to rest and was blocked by ZD 7288. These results suggest that I(h) contributes to stabilizing resting membrane potential and controlling cell excitability. The presence of I(h) imparts the neuron with the unique property of low-frequency membrane resonance; the ability to discriminate between synaptic inputs based on frequency content.     

Long-term potentiation of intrinsic excitability in LV visual cortical neurons

Neuronal excitability has a large impact on network behavior, and plasticity in intrinsic excitability could serve as an important information storage mechanism. Here we ask whether postsynaptic excitability of layer V pyramidal neurons from primary visual cortex can be rapidly regulated by activity. Whole cell current-clamp recordings were obtained from visual cortical slices, and intrinsic excitability was measured by recording the firing response to small depolarizing test pulses. Inducing neurons to fire at high-frequency (30-40 Hz) in bursts for 5 min in the presence of synaptic blockers increased the firing rate evoked by the test pulse. This long-term potentiation of intrinsic excitability (LTP-IE) lasted for as long as we held the recording (>60 min). LTP-IE was accompanied by a leftward shift in the entire frequency versus current (F-I) curve and a decrease in threshold current and voltage. Passive neuronal properties were unaffected by the induction protocol, indicating that LTP-IE occurred through modification in voltage-gated conductances. Reducing extracellular calcium during the induction protocol, or buffering intracellular calcium with bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid, prevented LTP-IE. Finally, blocking protein kinase A (PKA) activation prevented, whereas pharmacological activation of PKA both mimicked and occluded, LTP-IE. This suggests that LTP-IE occurs through postsynaptic calcium influx and subsequent activation of PKA. Activity-dependent plasticity in intrinsic excitability could greatly expand the computational power of individual neurons.     

Conductances mediating intrinsic theta-frequency membrane potential oscillations in layer II parasubicular neurons

Ionic conductances that generate membrane potential oscillations in neurons of layer II of the parasubiculum were studied using whole cell current-clamp recordings in horizontal slices from the rat brain. Blockade of ionotropic glutamate and GABA synaptic transmission did not reduce the power of the oscillations, indicating that oscillations are not dependent on synaptic inputs. Oscillations were eliminated when cells were hyperpolarized 6-10 mV below spike threshold, indicating that they are mediated by voltage-dependent conductances. Application of TTX completely eliminated oscillations, suggesting that Na(+) currents are required for the generation of the oscillations. Oscillations were not reduced by blocking Ca(2+) currents with Cd(2+) or Ca(2+)-free artificial cerebrospinal fluid, or by blocking K(+) conductances with either 50 microM or 5 mM 4-aminopyridine (4-AP), 30 mM tetraethylammonium (TEA), or Ba(2+)(1-2 mM). Oscillations also persisted during blockade of the muscarinic-dependent K(+) current, I(M), using the selective antagonist XE-991 (10 microM). However, oscillations were significantly attenuated by blocking the hyperpolarization-activated cationic current I(h) with Cs(+) and were almost completely blocked by the more potent I(h) blocker ZD7288 (100 microM). Intrinsic membrane potential oscillations in neurons of layer II of the parasubiculum are therefore likely driven by an interaction between an inward persistent Na(+) current and time-dependent deactivation of I(h). These voltage-dependent conductances provide a mechanism for the generation of membrane potential oscillations that can help support rhythmic network activity within the parasubiculum during theta-related behaviors.     

Oscillatory and intrinsic membrane properties of guinea pig nucleus prepositus hypoglossi neurons in vitro

Numerous models of the oculomotor neuronal integrator located in the prepositus hypoglossi nucleus (PHN) involve both highly tuned recurrent networks and intrinsic neuronal properties; however, there is little experimental evidence for the relative role of these two mechanisms. The experiments reported here show that all PHN neurons (PHNn) show marked phasic behavior, which is highly oscillatory in approximately 25% of the population. The behavior of this subset of PHNn, referred to as type D PHNn, is clearly different from that of the medial vestibular nucleus neurons, which transmit the bulk of head velocity-related sensory vestibular inputs without integrating them. We have investigated the firing and biophysical properties of PHNn and developed data-based realistic neuronal models to quantitatively illustrate that their active conductances can produce the oscillatory behavior. Although some individual type D PHNn are able to show some features of mathematical integration, the lack of robustness of this behavior strongly suggests that additional network interactions, likely involving all types of PHNn, are essential for the neuronal integrator. Furthermore, the relationship between the impulse activity and membrane potential of type D PHNn is highly nonlinear and frequency-dependent, even for relatively small-amplitude responses. These results suggest that some of the synaptic input to type D PHNn is likely to evoke oscillatory responses that will be nonlinearly amplified as the spike discharge rate increases. It would appear that the PHNn have specific intrinsic properties that, in conjunction with network interconnections, enhance the persistent neural activity needed for their function.     

Postnatal rat sympathetic neurons in culture. II. Synaptic transmission by postnatal neurons.

1. It was shown in the preceding paper that postnatally derived rat superior cervical ganglion neurons (SCGN) will grow in dissociated cell culture and form functional synaptic connections with each other. In this report, synaptic transmission by the postnatal SCGN is detailed. 2. Synaptic interactions between SCGN were blocked by the nicotinic cholinergic antagonist hexamathonium (C-6), indicating that acetylcholine was the transmitter substance used by these neurons. This was found to be the case even for neurons taken from 12.5-wk-old animals. 3. In a few cases, the beta-adrenergic blocking agent, propranolol, was found to block synaptic potentials, suggesting that a catecholamine might be involved in the transmission process. The possible mechanisms of this involvement are discussed. 4. SCGN taken from up to 10-wk-old rats were able to form functional synaptic contacts with cocultured skeletal muscle cells. These interactions were sensitive to low external Ca2+ and to 1--2 microM d-tubocurarine (d-TC). 5. It is concluded that even adult SCGN retain a certain amount of neurotransmitter "plasticity" when grown under appropriate culture conditions. From the data on the neuron-neuron and SCGN-skeletal muscle interactions, it is suggested that a matching of presynaptic transmitter with postsynaptic receptor is a sufficient condition for the formation of functional nerve-target interactions.     

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)     

Persistent sodium currents in mesencephalic v neurons participate in burst generation and control of membrane excitability

The functional and biophysical properties of a persistent sodium current (I(NaP)) previously proposed to participate in the generation of subthreshold oscillations and burst discharge in mesencephalic trigeminal sensory neurons (Mes V) were investigated in brain stem slices (rats, p7-p12) using whole cell patch-clamp methods. I(NaP) activated around -76 mV and peaked at -48 mV, with V1/2 of -58.7 mV. Ramp voltage-clamp protocols showed that I(NaP) undergoes time- as well as voltage-dependent inactivation and recovery from inactivation in the range of several seconds (tau(onset) = 2.04 s, tau(recov) = 2.21 s). Riluzole (< or =5 microM) substantially reduced I(NaP), membrane resonance, postinhibitory rebound (PIR), and subthreshold oscillations, and completely blocked bursting, but produced modest effects on the fast transient Na+ current (I(NaT)). Before complete cessation, burst cycle duration was increased substantially, while modest and inconsistent changes in burst duration were observed. The properties of the I(NaT) were obtained and revealed that the amplitude and voltage dependence of the resulting "window current" were not consistent with those of the observed I(NaP) recorded in the same neurons. This suggests an additional mechanism for the origin of I(NaP). A neuronal model was constructed using Hodgkin-Huxley parameters obtained experimentally for Na+ and K+ currents that simulated the experimentally observed membrane resonance, subthreshold oscillations, bursting, and PIR. Alterations in the model g(NaP) parameters indicate that I(NaP) is critical for control of subthreshold and suprathreshold Mes V neuron membrane excitability and burst generation.     

Synaptic responses of guinea pig and rat central amygdala neurons in vitro.

1. To investigate postsynaptic potentials (PSPs), we made intracellular recordings from neurons of the amygdaloid central nucleus in slices from the guinea pig and rat brains maintained in vitro. The results from guinea pigs and rats were very similar. 2. In the presence of bicuculline (20 microM), focal electrical stimulation of the amygdaloid basal nucleus with low intensities elicited short-latency excitatory PSPs (EPSPs) followed by long-latency EPSPs. The short-latency EPSP was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dion (CNQX; 10-20 microM). The long-latency EPSP was preferentially abolished by D,L-2-amino-5-phosphonovaleric acid (D,L-APV; 40 microM) and was augmented by removal of extracellular Mg2+. The compound EPSP reversed at -4 mV, which was close to -1 mV, the reversal potential for pressure-ejected glutamate (Glu). 3. When the intensity of the focal stimulation was increased in the presence of bicuculline (20 microM), CNQX (20 microM), and D,L-APV (50 microM), a second EPSP with a short latency and a prolonged duration could be evoked in approximately 65% of the neurons. The EPSPs were reversibly blocked by d-tubocurarine (50 microM) or hexamethonium (200 microM) but were unaffected by atropine (1 microM) or a 5-hydroxytryptamine type 3 receptor antagonist, ICS-205930 (5-10 microM). In these neurons, acetylcholine (ACh; 1-3 mM) caused a depolarization, associated with a decreased input resistance. 4. In the presence of CNQX (20 microM) and D,L-APV (50 microM), single focal stimulation of the dorsolateral subdivision in the central nucleus with low intensities elicited a depolarizing inhibitory PSP (IPSP). The IPSP was reversibly abolished by bicuculline (20-40 microM). The reversal potential (-63 mV) for the IPSP was similar to the reversal potential (-61 mV) for the response to gamma-aminobutyric acid (GABA) applied by pressure ejection. 5. In the presence of bicuculline (20-40 microM) and CNQX (20 microM), a repetitive focal stimulus with high intensities delivered to the dorsolateral subdivision produced a hyperpolarizing PSP followed by a slow depolarization in most neurons. Of putative inhibitory amino acid transmitters, glycine (Gly; 3 mM) produced only a hyperpolarization, associated with a decrease in input resistance. Strychnine (1-2 microM) reversibly blocked both the Gly hyperpolarization and the synaptically evoked hyperpolarization. The reversal potential of -81 mV for the hyperpolarizing PSP was close to -82 mV for the Gly hyperpolarization. The reversal potential for the Gly response was shifted to less negative values by increasing the external K+ concentration or decreasing the extracellular Cl- concentration.(ABSTRACT TRUNCATED AT 400 WORDS)     

Regenerative potentials in rat neostriatal neurons in an in vitro slice preparation

Summary 1. Regenerative potentials in rat neostriatal neurons were studied using the in vitro slice preparation. Some of the recorded neurons were intracellularly labeled with HRP. All had the morphological characteristics of the medium spiny neuron. 2. Application of TTX (10^–5 g/ml) to the superfusing medium abolished fast action potentials generated by intracellularly injected depolarizing current. Application of TEA prolonged the spike duration by decreasing its repolarizing rate without affecting rising phase. After suppression of K-conductance by TEA, depolarizing current elicited both fast and slow all or none action potentials. 3. Combined treatment with TTX and TEA revealed two types of depolarizing potentials, a slowly rising graded depolarizing potential and slow action potential. Substitution of Ca⁺⁺ with Mg⁺⁺ in the medium diminished the amplitude of these potentials. They were also blocked by application of Co⁺⁺ into the superfusion medium. The duration of slow action potentials were increased (1) with increase in the intensity of current pulse, (2) with decrease in the resting membrane potential, and (3) with increase in the concentration of TEA in the bathing medium. 4. In the normal Ringer solution, local stimulation elicited depolarizing postsynaptic responses (DPSPs). Large DPSPs evoked by strong local stimulation triggered one or two fast action potentials. In some neurons, large DPSPs could trigger both fast and slow action potentials. They were consistently triggered after application of TEA (1 mM) to the medium. 5. When a relatively high concentration of TEA (4 mM) was applied to the Ringer solution, locally evoked DPSPs could trigger only slow action potentials. In double stimulation experiments, a large reduction in the amplitude and the duration of test DPSPs was observed up to about 150 ms interstimulus interval.     

Analysis of the galanin-induced decrease in membrane excitability in mudpuppy parasympathetic neurons.

Previously, we showed that the neuropeptide galanin hyperpolarizes and decreases membrane excitability of mudpuppy parasympathetic neurons [Konopka L. M., McKeon T. W. and Parsons R. L. (1989) J. Physiol. 410, 107-122]. We also demonstrated that membrane excitability remains depressed when the agonist-induced potential change is negated electrotonically. We hypothesized that galanin inhibits the membrane conductances associated with spike generation. However, we cannot rule out the possibility that the decreased excitability is due to a galanin-induced increase in membrane potassium conductance which reduces the effectiveness of subsequent depolarizing stimuli. Therefore, in the present study we tested, with the galanin-induced hyperpolarization negated, whether the galanin-induced increased membrane potassium conductance was responsible for the decreased excitability. The results showed that the galanin-induced decreased excitability was not dependent on the peak amplitude of the galanin-induced hyperpolarization. Furthermore, the decreased excitability occurred in cells in which there was no measurable galanin-induced hyperpolarization. Moreover, in most cells the galanin-induced decrease in input resistance, measured at the peak of the hyperpolarization (3-25 mV), was less than 15% and when the hyperpolarization was negated electronically, the decrease was even less (approximately 2%). These results indicated that when the hyperpolarization was negated, the galanin-induced increase in potassium conductance was not responsible for the decreased excitability. In preparations pretreated with 5 mM tetraethylammonium, galanin decreased excitability which indicated that a galanin-induced decrease in the calcium-dependent potassium current was not necessary for the decreased excitability. Galanin also decreased excitability in preparations exposed to either 1-3 microM tetrodotoxin or 100-200 microM cadmium. Following galanin application, the threshold for initiation of tetrodotoxin-insensitive spikes was shifted to more positive membrane potentials. Galanin also decreased the amplitude and hyperpolarizing afterpotential of barium spikes in the absence of any agonist-induced hyperpolarization. These observations confirmed that galanin decreased the voltage-dependent calcium conductance. In the present study, we showed that when the hyperpolarization was negated, galanin decreased excitability by shifting the threshold for spike generation regardless of whether voltage-dependent sodium or calcium currents were primarily responsible for the depolarizing component of the action potential.     

Dopamine modulates excitability of basolateral amygdala neurons in vitro

The amygdala plays a role in affective behaviors, which are modulated by the dopamine (DA) innervation of the basolateral amygdala complex (BLA). Although in vivo studies indicate that activation of DA receptors alters BLA neuronal activity, it is unclear whether DA exerts direct effects on BLA neurons or whether it acts via indirect effects on BLA afferents. Using whole cell patch-clamp recordings in rat brain slices, we investigated the site and mechanisms through which DA regulates the excitability of BLA neurons. Dopamine enhanced the excitability of BLA projection neurons in response to somatic current injections via a postsynaptic effect. Dopamine D1 receptor activation increased excitability and evoked firing, whereas D2 receptor activation increased input resistance. Current- and voltage-clamp experiments in projection neurons showed that D1 receptor activation enhanced excitability by modulating a 4-aminopyridine- and alpha-dendrotoxin-sensitive, slowly inactivating K+ current. Furthermore, DA and D1 receptor activation increased evoked firing in fast-spiking BLA interneurons. Consistent with a postsynaptic modulation of interneuron excitability, DA also increased the frequency of spontaneous inhibitory postsynaptic currents recorded in projection neurons without changing release of GABA. These data demonstrate that DA exerts direct effects on BLA projection neurons and indirect actions via modulation of interneurons that may work in concert to enhance the neuronal response to large, suprathreshold inputs, while suppressing weaker inputs.     

Morphological correlates of intrinsic electrical excitability in neurons of the deep cerebellar nuclei

To what degree does neuronal morphology determine or correlate with intrinsic electrical properties within a particular class of neuron? This question has been examined using microelectrode recordings and subsequent neurobiotin filling and reconstruction of neurons in the deep cerebellar nuclei (DCN) of brain slices from young rats (P13-16). The neurons reconstructed from these recordings were mostly large and multipolar (17/21 cells) and were likely to represent glutamatergic projection neurons. Within this class, there was considerable variation in intrinsic electrical properties and cellular morphology. Remarkably, in a correlation matrix of 18 electrophysiological and 6 morphological measures, only one morphological characteristic was predictive of intrinsic excitability: neurons with more spines had a significantly slower basal firing rate. To address the possibility that neurons with fewer spines represented a slowly maturing subgroup, recordings and reconstructions were also made from neurons at a younger age (P6-9). While P6-9 neurons were morphologically indistinguishable from P13 to 16 neurons, they were considerably less excitable: P6-9 neurons had a lower spontaneous spiking rate, larger fast AHPs, higher resting membrane potentials, and smaller rebound depolarizations. Thus while the large projection neurons of the DCN are morphologically mature by P6-9, they continue to mature electrophysiologically through P13-16 in a way that renders them more responsive to the burst-and-pause pattern that characterizes Purkinje cell inhibitory synaptic drive.