The control of firing pattern in nigral dopamine neurons: single spike firing

Dopamine (DA) neurons have been recorded in vivo in four states of activity: hyperpolarized, nonfiring; single spike firing; burst firing; and depolarization inactivation. Nonfiring DA neurons can be made to fire by iontophoretic application of the excitatory substances glutamate and cholecystokinin, or by depolarizing current injection. Spontaneously active DA cells typically fire in a slow (3 to 8 Hz) irregular pattern. In vivo intracellular recordings revealed that this pattern is sustained by the alternation of two currents: a spontaneously occurring slow depolarization (13 +/- 3 mV amplitude, 78 +/- 40 msec duration) which brings the membrane potential of the DA cell to spike threshold (-42 mV), and an afterhyperpolarization mediated by a calcium-activated potassium conductance (IK(Ca)). The slow depolarization is a pacemaker-like conductance, with a rate of rise proportional to the membrane potential. The regular pacemaker pattern of the spontaneously occurring slow depolarization is interrupted by the IK(Ca) which appears to be triggered by calcium entry during the action potential. Thus, intracellular injection of the calcium chelator EGTA will cause DA cells to fire in a regular, pacemaker pattern. The IK(Ca) is observed after single spikes and trains of spikes with the amplitude of the afterhyperpolarization being proportional to the number of spikes in a train. Both the afterhyperpolarization and the firing accommodation observed during depolarizing current injection can be blocked by intracellular injection of the calcium chelator EGTA.     

Morphology and electrophysiological properties of immunocytochemically identified rat dopamine neurons recorded in vitro

In vitro intracellular recordings were made from neurons in the rat midbrain slice. Two neuronal types could be distinguished in dopamine-containing (DA) midbrain regions based on electrophysiological criteria. One neuron type exhibited short duration action potentials (less than 1.5 msec), could fire at high frequencies (greater than 10 Hz), and exhibited either phasic or burst firing patterns. This neuron did not exhibit tyrosine hydroxylase immunoreactivity. A second neuronal type exhibited a unique set of electrophysiological properties, which included (1) a spontaneous pacemaker-like depolarizing potential, (2) a highly regular firing pattern, (3) long duration (greater than 2 msec) action potentials, and (4) a high (i.e., depolarized) spike threshold. This neuron was consistently double labeled using intracellular staining and immunocytochemical localization of the catecholamine-specific enzyme tyrosine hydroxylase, and thus represented the DA neuronal type. Midbrain DA neurons stained with Lucifer yellow could be separated into 3 classes based on their location and morphology: (1) fusiform neurons with laterally projecting dendrites in the dorsal substantia nigra zona compacta region, (2) multipolar cells with laterally and ventrally projecting dendrites in the ventral substantia nigra zona compacta, and (3) neurons with fusiform and multipolar somata and radially projecting dendrites in the ventral tegmental area. The dendrites also exhibited spine-like protrusions and ended with specialized forked processes. Spontaneously firing DA cells recorded in vitro had a number of distinguishing electrophysiological characteristics in common with those of DA neurons recorded in vivo, such as the presence of a slow depolarizing potential driving spike activity and a characteristic depolarized spike threshold (approximately-36 mV). However, in contrast to that found in vivo, the DA cells characterized here exhibited substantially higher input resistances and fired spontaneously in a very regular pacemaker pattern. Burst firing was not observed. Spike activity was apparently dependent on 4 depolarizing events: (1) a voltage-dependent TTX-sensitive slow depolarization, (2) a cobalt-sensitive low threshold depolarization that was activated during the rebound from brief membrane hyperpolarizations, (3) high threshold dendritic calcium spikes which gave rise to the spike afterhyperpolarization, and (4) a high threshold initial segment sodium spike. These depolarizations were modulated by several processes, including a 4-aminopyridine-insensitive delayed repolarization, an instantaneous and time-dependent anomalous rectifier, and an afterhyperpolarization. Although low threshold depolarizations and rebound action potentials could be triggered by the membrane repolarization following small membrane hyperpolarizations, comparatively larger hyperpolarizations attenuated this rebound activation, thereby suppressing anodal break excitation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Identification and characterization of striatal cell subtypes using in vivo intracellular recording in rats. I. Basic physiology and response to corticostriatal fiber stimulation

The electrophysiological characteristics of two subtypes of striatal neurons, identified by their distinct patterns of response to paired impulse stimulation of corticostriatal afferents, were compared using in vivo intracellular recordings in rats. As observed in previous extracellular recording studies, the majority of neurons (73%) were found to be of the Type II class, with the remaining cells exhibiting the Type I response patern. For all cells, cortical stimulation elicited 5–30 mV EPSPs at latencies ranging from 2.0–5.3 msec; Increasing the stimulating current intensity caused a progressive increase in the amplitude of the evoked EPSPs without altering their latencies, suggesting that the EPSPs are monosynaptically mediated. Both the average amplitude and duration of the evoked EPSPs at spike threshold in Type I neurons (9.8 ± 1.7 mV, 11.8 ± 2.8 msec; mean ± SEM) were significantly smaller than those of Type II cells (20.3 ± 1.4 mV, 22.7 ± 2.1 msec). Although the average latency to the onset of the EPSP was similar for both cell classes (Type I cells: 2.3 ± 0.3 msec; Type II cells: 2.2 ± 0.2 msec), the EPSPs in Type I cells reached peak amplitude and the spikes were triggered at significantly longer latencies than in the Type II cells (peak I: 11.2 ± 2.5 msec vs. II: 7.6 ± 0.7 msec; spike I: 8.0 ± 1.2 msec vs. II: 5.7 ± 0.4 msec). Striatal neurons had a comparatively hyperpolarized resting membrane potential (?70.2 ± 2.1 mV) and had an average input resistance of 35.4 ± 7.6 MΩ. Overall, striatal neurons exhibited low levels of spontaneous activity (0.6 ± 0.7 Hz) with 50% of the neurons being quiescent. Type I cells exhibited significantly higher firing rates (3.2 ± 0.8 Hz) than Type II cells (0.8 ± 0.3 Hz), although their resting membrane potentials were not significantly different. Spontaneously occurring spikes had an average amplitude of 72.7 ± 3.4 mV and spike thresholds of ?50.1 ± 1.5 mV. Irregularly occurring depolarizing plateau potentials, which typically gave rise to spike discharge, were frequently observed in both spontaneously firing and quiescent neurons. A small proportion of the cells recorded (3/55) exhibited a Type I response pattern but had unique physiological characteristics that were similar to those described by others as arising from large, aspiny striatal neurons. The present study shows that these two physiologically distinct neuron types appear to be similar in terms of their passive membrane properties (e.g., resting membrane potentials, input resistance, etc.) and firing characteristics, despite their unique patterns of response to corticostriatal stimulation. Therefore, the source of the distinct paired impulse response profiles of these neurons is more likely to arise from differences in their afferent drive than from a heterogeneity in their membrane properties. © 1994 Wiley-Liss, Inc.     

Evidence for the functional compartmentalization of spike generating regions of rat midbrain dopamine neurons recorded in vitro

The regulation of spike generation in rat midbrain dopamine (DA) neurons was investigated using in vitro intracellular recordings. DA neurons fired long (greater than 1.8 ms) action potentials that exhibited comparatively depolarized spike thresholds (approx. -35 to -45 mV). Depolarization of the DA neuron increased the duration and the threshold of subsequent action potentials. The action potential was composed of two distinct components, a fast (0.8-1.5 ms duration) initial segment (IS) spike which triggered a slow (1.5-3 ms duration) somatodendritic (SD) component. Cobalt application (2 mM) blocked the SD spike component and revealed fast TTX-sensitive spikes. These fast spikes were also observed in untreated neurons following large hyperpolarizing pulses, and showed consistent changes in threshold and amplitude during membrane depolarization. Administration of 4-aminopyridine decreased the threshold of this TTX-sensitive spike, whereas tetraethylammonium (TEA) had no effect. When the fast spike was blocked by TTX, depolarization was ineffective in triggering further spike activity. However, after the administration of TEA (but not 4-AP), high threshold cobalt-sensitive spike activity could be triggered by relatively small depolarizations. TEA increased the duration of the SD portion of the action potential without altering the action potential threshold. The effect of 4-AP on spike threshold and the increase in SD spike duration caused by TEA were similar in nature to the changes in action-potential waveforms produced by polarizing the DA neuron membrane. Drawing from evidence gathered here and in previous in vivo studies, the properties of the TTX-sensitive fast spike are consistent with those of the IS spike component of the action potential, whereas the SD component is similar in nature to the high threshold calcium spike. One hypothesis that can be drawn from these studies is that dendritic and axonal spiking regions may exist in different functional subcompartments of the DA neuron, and may be independently modulated by pharmacologically distinct conductances. Under these conditions, synaptic influences could exist to modulate dendritic excitability and thus regulate putative dendritic spike-dependent functions, such as neuronal activity state, electrical coupling, and dendritic DA synthesis and release.     

Cellular mechanisms contributing to response variability of cortical neurons in vivo.

Cortical neurons recorded in vivo exhibit highly variable responses to the repeated presentation of the same stimulus. To further understand the cellular mechanisms underlying this phenomenon, we performed intracellular recordings from neurons in cat striate cortex in vivo and examined the relationships between spontaneous activity and visually evoked responses. Activity was assessed on a trial-by-trial basis by measuring the membrane potential (Vm) fluctuations and spike activity during brief epochs immediately before and after the onset of an evoked response. We found that the response magnitude, expressed as a change in Vm relative to baseline, was linearly correlated with the preceding spontaneous Vm. This correlation was enhanced when the cells were hyperpolarized to reduce the activation of voltage-gated conductances. The output of the cells, expressed as spike counts and latencies, was only moderately correlated with fluctuations in the preceding spontaneous Vm. Spike-triggered averaging of Vm revealed that visually evoked action potentials arise from transient depolarizations having a rise time of approximately 10 msec. Consistent with this, evoked spike count was found to be linearly correlated with the magnitude of Vm fluctuations in the gamma (20-70 Hz) frequency band. We also found that the threshold of visually evoked action potentials varied over a range of approximately 10 mV. Examination of simultaneously recorded intracellular and extracellular activity revealed a correlation between Vm depolarization and spike discharges in adjacent cells. Together these results demonstrate that response variability is attributable largely to coherent fluctuations in cortical activity preceding the onset of a stimulus, but also to variations in action potential threshold and the magnitude of high-frequency fluctuations evoked by the stimulus.     

Evidence for the functional compartmentalization of spike generating regions of rat mdbrain dopamine neurons recorded in vitro

The regulation of spike generation in rat midbrain dopamine (DA) neurons was investigated using in vitro intracellular recordings. DA neurons fired long (greater than 1.8 ms) action potentials that exhibited comparatively depolarized spike thresholds (approx. −35 to −45 mV). Depolarization of the DA neuron increased the duration and the threshold of subsequent action potentials. The action potential was composed of two distinct components, a fast (0.8–1.5 ms duration) initial segment (IS) spike which triggered a slow (1.5–3 ms duration) somatodendritic (SD) component. Cobalt application (2 mM) blocked the SD spike component and revealed fast TTX-sensitive spikes. These fast spikes were also observed in untreated neurons following large hyperpolarizing pulses, and showed consistent changes in threshold and amplitude during membrane depolarization. Administration of 4-aminopyridine decreased the threshold of this TTX-sensitive spike, whereas tetraethylammonium (TEA) had no effect. When the fast spike was blocked by TTX, depolarization was ineffective in triggering further spike activity. However, after the administration of TEA (but not 4-AP), high threshold cobalt-sensitive spike activity could be triggered by relatively small depolarizations. TEA increased the duration of the SD portion of the action potential without altering the action potential threshold. The effect of 4-AP on spike threshold and the increase in SD spike duration caused by TEA were similar in nature to the changes in action-potential waveforms produced by polarizing the DA neuron membrane. Drawing from evidence gathered here and in previous in vivo studies, the properties of the TTX-sensitive fast spike are consistent with those of the IS spike component of the action potential, whereas the SD component is similar in nature to the high threshold calcium spike. One hypothesis that can be drawn from these studies is that dendritic and axonal spiking regions may exist in different functional subcompartments of the DA neuron, and may be independently modulated by pharmacologically distinct conductances. Under these conditions, synaptic influences could exist to modulate dendritic excitability and thus regulate putative dendritic spike-dependent functions, such as neuronal activity state, electrical coupling, and dendritic DA synthesis and release.     

Biophysical properties of scorpion alpha-toxin-sensitive background sodium channel contributing to the pacemaker activity in insect neurosecretory cells (DUM neurons)

A scorpion alpha-toxin-sensitive background sodium channel was characterized in short-term cultured adult cockroach dorsal unpaired median (DUM) neurons using the cell-attached patch-clamp configuration. Under control conditions, spontaneous sodium currents were recorded at different steady-state holding potentials, including the range of normal resting membrane potential. At -50 mV, the sodium current was observed as unclustered, single openings. For potentials more negative than -70 mV, investigated patches contained large unitary current steps appearing generally in bursts. These background channels were blocked by tetrodotoxin (TTX, 100 nm), and replacing sodium with TMA-Cl led to a complete loss of channel activity. The current-voltage relationship has a slope conductance of 36 pS. At -50 mV, the mean open time constant was 0.22 +/- 0.05 ms (n = 5). The curve of the open probability versus holding potentials was bell-shaped, with its maximum (0.008 +/- 0.004; n = 5) at -50 mV. LqhalphaIT (10-8 m) altered the background channel activity in a time-dependent manner. At -50 mV, the channel activity appeared in bursts. The linear current-voltage relationship of the LqhalphaIT-modified sodium current determined for the first three well-resolved open states gave three conductance levels: 34, 69 and 104 pS, and reversed at the same extrapolated reversal potential (+52 mV). LqhalphaIT increased the open probability but did not affect either the bell-shaped voltage dependence or the open time constant. Mammal toxin AaHII induced very similar effects on background sodium channels but at a concentration 100 x higher than LqhalphaIT. At 10-7 m, LqhalphaIT produced longer silence periods interrupted by bursts of increased channel activity. Whole-cell experiments suggested that background sodium channels can provide the depolarizing drive for DUM neurons essential to maintain beating pacemaker activity, and revealed that 10-7 m LqhalphaIT transformed a beating pacemaker activity into a rhythmic bursting.     

Electrical membrane properties of rat subthalamic neurons in an in vitro slice preparation

The electrical membrane properties of subthalamic (STH) neurons and their response characteristics to stimulation of the internal capsule (IC) were studied in an in vitro slice preparation. Most STH neurons recorded exhibited spontaneous repetitive firing. The input resistance of STH neurons was 146 +/- 48 M omega and showed both an anomalous and a delayed rectification when the membrane was hyperpolarized or depolarized by current injections. In neurons with the membrane potential less negative than 65 mV, depolarizing current pulses generated repetitive firing with the maximum frequency of up to 500 Hz. Two types of tetrodotoxin (TTX)-resistant cobalt-sensitive potentials, slow depolarizing potential and slow action potential, were observed in STH neurons. The slow depolarizing potential had a long duration (over 500 ms in some cases) and was able to trigger repetitive firing. The slow action potential had a duration of about 30 ms and triggered a burst of firing. The slow action potential was seen only when the neurons were hyperpolarized to more negative than 65 mV by a current injection. Electrical stimulation of IC evoked monosynaptic inhibitory postsynaptic potentials (IPSPs) in most of the neurons examined. The polarity of IPSPs was reversed in the depolarizing direction by intracellular injection of Cl-. Bath application of bicuculline markedly suppressed IPSPs and unmasked monosynaptic excitatory postsynaptic potentials (EPSPs). The EPSP was able to trigger a slow depolarization with repetitive firing or a slow action potential with burst of firing when the neuron was hyperpolarized by a continuous current injection. The results demonstrated that STH neurons in an in vitro preparation have spontaneous discharges, high input resistance, capability to generate high-frequency firing, and Ca potentials. The pattern of responses of STH neurons to synaptic inputs is dependent on their membrane potentials.     

Endogenous pacemaker potentials develop into paroxysmal depolarization shifts (PDSs) with application of an epileptogenic drug

Well-known invertebrate ganglia (buccal ganglia of Helix pomatia, abdominal ganglia of Aplysia californica) were used to study the contribution of synaptic potentials, central pattern generators, and endogenously generated neuronal potentials to the development of epileptiform activity. Epileptiform activity which was induced with application of pentylenetetrazol (1 to 100 mM) or etomidate (0.12 to 1.0 mM) consisted of paroxysmal depolarization shifts (PDSs) recorded simultaneously from several identified neurons with sharp microelectrodes. With application of an epileptogenic drug, endogenous pacemaker potentials develop into PDSs. With increasing concentration of the drug, (i) amplitude of pacemaker-depolarizations and (ii) delay of pacemaker-repolarization increased progressively finally resulting in PDSs. Additionally, the activation characterists of currents shifted from between -50 and -40 mV (pacemaker potentials, control conditions) to between -100 and -40 mV (PDS, epileptic conditions). Only neurons which generated pacemaker potentials under control conditions could generate PDSs under epileptic conditions. Chemical synaptic inputs triggered or blocked pacemaker potentials as well as PDSs. Activities induced from central pattern generators were identified with simultaneous recordings from several identified neurons. The central pattern generators could trigger or block pacemaker potentials as well as PDSs. Results demonstrate that, in the used model nervous systems, pacemaker potentials which are generated by the single neurons are the physiologic basis of epileptic activity.     

Voltage-dependent neuromodulation of Na+ channels by D1-like dopamine receptors in rat hippocampal neurons.

Activation of D1-like dopamine (DA) receptors reduces peak Na+ current in acutely isolated hippocampal neurons through phosphorylation of the alpha subunit of the Na+ channel by cAMP-dependent protein kinase (PKA). Here we report that neuromodulation of Na+ currents by DA receptors via PKA is voltage-dependent in the range of -110 to -70 mV and is also sensitive to concurrent activation of protein kinase C (PKC). Depolarization enhanced the ability of D1-like DA receptors to reduce peak Na+ currents via the PKA pathway. Similar voltage-dependent modulation was observed when PKA was activated directly with the membrane-permeant PKA activator DCl-cBIMPS (cBIMPS; 20 microM), indicating that the membrane potential dependence occurs downstream of PKA. PKA activation caused only a small (-2.9 mV) shift in the voltage dependence of steady-state inactivation and had no effect on slow inactivation or on the rates of entry into the fast or slow inactivated states, suggesting that another mechanism is responsible for coupling of membrane potential changes to PKA modulation. Activation of PKC with a low concentration of the membrane-permeant diacylglycerol analog oleylacetyl glycerol also potentiated modulation by SKF 81297 or cBIMPS, and these effects were most striking at hyperpolarized membrane potentials where PKA modulation was not stimulated by membrane depolarization. Thus, activation of D1-like DA receptors causes a strong reduction in Na+ current via the PKA pathway, but it is effective primarily when it is combined with depolarization or activation of PKC. The convergence of these three distinct signaling modalities on the Na+ channel provides an intriguing mechanism for integration of information from multiple signaling pathways in the hippocampus and CNS.     

Two cell types in rat substantia nigra zona compacta distinguished by membrane properties and the actions of dopamine and opioids

Intracellular recordings were made from 475 rat substantia nigra zona compacta neurons in vitro. The region from which recordings were made was rich in catecholamine fluorescence. Two groups of neuron, termed principal neurons (95% of the total) and secondary neurons (5% of the total) were clearly distinguishable according to one or more of the following 4 electrophysiological properties. Secondary neurons (23 cells) (1) fired spontaneous action potentials at frequencies greater than 10 Hz, or were quiescent (30%); (2) had action potentials less than 1 msec in duration; (3) did not show time-dependent inward rectification with step hyperpolarization; and (4) had slope conductances of about 4 nS (between -75 and -90 mV). In contrast, principal neurons (1) fired spontaneous action potentials in the range 1-8 Hz, or were quiescent (33%); (2) had action potentials greater than 1 msec in duration; (3) showed pronounced time-dependent inward rectification; and (4) had steady-state membrane slope conductances of around 22 nS (between -75 and -90 mV). Secondary cells were not affected by dopamine but were hyperpolarized by baclofen, GABA, and the mu opioid receptor agonist Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO). On the other hand, dopamine and baclofen inhibited firing and/or hyperpolarized all principal cells tested, but mu or delta opioid receptor agonists had no effect. The properties of these 2 cell types broadly correspond with those described by electrophysiological studies in vivo, in which case the majority, or principal, cells are believed to be dopaminergic.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inward Rectification (Ih) in Immunocytochemically-ldentified Vasopressin and Oxytocin Neurons of Guinea-Pig Supraoptic Nucleus

Intracellular recordings of magnocellular neurons from the supraoptic nucleus of guinea-pigs were made with KCI/K citrate- and biocytin-filled electrodes. Fifty of 99 cells exhibited a time-dependent inward rectification (TDR). The TDR was activated during hyperpolarizing current pulses to membrane potentials more hyperpolarized than −75 mV. In voltage-clamp recordings, an inward current appeared at voltage steps more hyperpolarized than −75 mV, with properties similar to the slow inward rectifier (I_h) described in other tissues. The I_h was blocked by 2 mM CsCI. BaCI₂ (100 to 500 μM) did not block the I_h. Immunocytochemical identification of the recorded cells revealed that both vasopressin (AVP)- and oxytocin (OT)- containing neurons exhibited an I_h.     

The performance of synapses that convey discrete graded potentials in an insect visual pathway.

Synapses from nonspiking neurons transmit small graded changes in potential, but variability in their postsynaptic potential amplitudes has not been extensively studied. At synapses where the presynaptic signal is an all-or-none spike, the probabilistic manner of neurotransmitter release causes variation in the amplitudes of postsynaptic potentials. I have measured the reliability of the operation of synapses that convey small graded potentials between pairs of identified large, second-order neurons in the locust ocellar system. IPSPs are mediated by small rebound spikes, which are graded in amplitude, in the presynaptic neuron. A transfer curve plotting amplitudes of spikes against amplitudes of IPSPs has a characteristic S shape with a linear central portion where IPSP amplitude is between -0.2 and -0.6 as large as spike amplitude but shows appreciable scatter. Approximately half of the scatter is attributable to background noise, most of which originates in photoreceptors and persists in darkness. The remaining noise is intrinsic to the synapse itself and is usually 0.3-0.7 mV in amplitude. It limits the resolution with which two spike amplitudes can be distinguished from one another to approximately 2 mV and, because the linear part of the transfer curve occupies approximately 10 mV in spike amplitudes, limits the number of discrete signal levels that can be conveyed across the synapse to approximately five. The amplitude of the noise is constant throughout the synaptic operating range, which means it is unlikely that presynaptic membrane potential controls transmitter release by setting a single probability level for quantal release.     

Reticulospinal pacemaker neurons of the rat rostral ventrolateral medulla with putative sympathoexcitatory function: an intracellular study in vitro

Extra- and intracellular recordings of tonically active neurons were obtained in slices of the rat rostral ventrolateral medulla maintained at 31 degrees C. The predominant type consisted of cells with a regular non-bursting discharge rate of 9 +/- 0.3 spikes/s (mean +/- S.E.M., n = 84). Intracellular recordings revealed that these neurons (n = 43) exhibited typical pacemaker potentials reset after a single spike, and an input resistance of 138 +/- 10 M omega (n = 21). No excitatory postsynaptic potentials were detected even during hyperpolarization (5-10 mV) which invariably resulted in silencing the cells (n = 28). Eighteen cells were injected intracellularly with Lucifer yellow, and the tissue was subsequently processed for the immunohistochemical detection of the adrenergic marker phenylethanol-amine N-methyltransferase (PNMT). None of the 12 dye-marked cells recovered exhibited any PNMT-like immunoreactivity, but all were surrounded by numerous adrenergic neurons. In 7 rats subjected to intraspinal injections (T3) of rhodamine-tagged microbeads, 4 out of 9 pacemaker cells marked intracellularly with Lucifer yellow were found labeled with the retrograde marker. It is concluded that the rostral ventrolateral medulla contains non-adrenergic reticulospinal cells with intrinsic pacemaker properties. These neurons probably represent a group of sympathoexcitatory cells on which the basal sympathetic tone depends.     

In vitro effects of acetyl-DL-leucine (tanganil) on central vestibular neurons and vestibulo-ocular networks of the guinea-pig

For 40 years, the amino acid acetyl-DL-leucine (or isoleucine/Tanganil) has been used in clinical practice to reduce the imbalance and autonomic signs associated with acute vertigo crises. In animal models, acetyl-DL-leucine was shown to accelerate vestibular compensation following unilateral labyrinthectomy, while having only minor effects on normal vestibular function. However, the underlying mechanisms are unknown. In this study, the effect of acetyl-DL-leucine on the activity of central vestibular neurons of the medial vestibular nucleus (MVN) and/or the overall activity of vestibular-related networks was electrophysiologically measured in brainstem slices and in the isolated, in vitro whole brain (IWB) of guinea-pig. Only moderate effects were obtained in normal animals, where both excitatory and inhibitory actions of acetyl-DL-leucine were obtained. However, intracellular recordings from MVN neurons revealed that the nature of the response depended on the resting membrane potential. The neurons excited by acetyl-DL-leucine were significantly hyperpolarized compared to nonsensitive cells, whereas the neurons inhibited by this compound tended to display higher than normal membrane potentials. In accordance with these data, acetyl-DL-leucine reduced the prominent asymmetry characterizing the vestibular-related networks of IWBs taken from previously labyrinthectomized animals, by decreasing the activity of the abnormally depolarized neurons on the hyperactive side. Altogether, our results suggest that acetyl-DL-leucine might act mainly on abnormally hyperpolarized and/or depolarized MVN neurons, by bringing back their membrane potential towards a mean value of -65 to -60 mV. Since in animal models, acute vestibular disorders are associated with asymmetrical spontaneous activities of MVN neurons, this study suggests how acetyl-DL-leucine may reduce acute, vestibular-related imbalances in humans.     

Membrane properties and response to opioids of identified dopamine neurons in the guinea pig hypothalamus

The electrophysiological properties and opioid responsiveness of the dopamine-containing neurons in the arcuate nucleus of the guinea pig hypothalamus were examined. Dopamine-containing neurons, identified immunocytochemically by the presence of tyrosine hydroxylase, had a mean length-to-width profile of 14.9 +/- 4.4 x 11.5 +/- 3.1 microns (N = 14). The Na+ action potential of these neurons was of short duration, and induction of repetitive firing (20-50 Hz) caused an afterhyperpolarization of 6-9 mV in amplitude, with a decay half-time of approximately 1.5 sec. Dopamine-containing cells exhibited a low threshold spike, which induced 1-4 Na+ action potentials. This potential had a threshold close to -65 mV, could not be induced without prior hyperpolarization and was not sensitive to TTX. Dopamine-containing neurons also exhibited a time- and voltage-dependent inward current at potentials negative to -70 mV, and Cs+ blocked this conductance. The mu-opioid agonist Tyr-D-Ala-Gly-mePhe-Gly-ol hyperpolarized (14 +/- 3 mV) dopamine neurons via induction of an outward current (93 +/- 44 pA near the resting membrane potential) which had a reversal potential similar to that expected for a selective potassium conductance. TTX (1 microM) did not block the opioid effects. These results show that dopamine neurons of the arcuate nucleus differ in their intrinsic conductances and their responsiveness to opioids from other CNS dopaminergic neurons. Furthermore, opioid activation of a potassium conductance resulted in a direct hyperpolarization of dopamine neurons of the arcuate nucleus, and we suggest that this mechanism may underlie the effects of opioids on dopamine-mediated prolactin release.     

In vivo intracellular characteristics of inferior colliculus neurons in guinea pigs

Intracellular in vivo recordings of physiologically identified inferior colliculus central nucleus (ICc) auditory neurons (n=71) were carried out in anesthetized guinea pigs. The neuronal membrane characteristics are described showing mainly quantitative differences with a previous report [Nelson, P.G. and Erulkar, S.D., J. Neurophysiol., 26 (1963) 908–923]. The spontaneous spike activity was consistent with the discharge pattern of most extracellularly recorded units. The action potentials showed different spike durations, short and long, and some of them exhibited hyperpolarizing post-potentials. There were also differences in firing rate. The ICc neurons exhibited irregular activity producing spike trains as well as long silent periods (without spikes). Intracellular current injection revealed membrane potential adaptation and shifts that outlasted the electrical stimuli by 20–30 ms. Both evoked synaptic potentials and the spike activity in response to click and tone-burst stimulation were analyzed. Depolarizing-hyperpolarizing synaptic potentials were found in response to contralateral and binaural sound stimulation that far outlasted the stimulus (up to 90 ms). When ipsilaterally stimulated, inhibitory responses and no-responses were also recorded. Although few cells were studied, a similar phenomenon was observed using tone-burst stimulation; moreover, a good correlation was obtained between membrane potential shifts and the triggered spikes (input–output relationship). These in vivo results demonstrate the synaptic activity underlying many of the extracellularly recorded discharge patterns. The data are consistent with the known multi-synaptic ascending pathway by which signals arrive at the ICc as well as the descending corticofugal input that may contribute to the generation of long duration post-synaptic potentials.     

Hyperpolarizing action of enkephalin on neurons in the dorsal motor nucleus of the vagus, in vitro

Intracellular recordings were made from neurons of the dorsomotor vagal nucleus (DMV) in slices of rat medulla oblongata. [D-Ala2, D-Leu5]-enkephalin (DADLE), applied by perfusion (0.01-3 microM) or droplets, dose-dependently hyperpolarized 85% of the DMV neurons tested. The hyperpolarization, associated with a decrease in membrane resistance, persisted after elimination of synaptic activity by perfusion with Ca2(+)-free/high-Mg2+ solution or with 1 microM TTX solution. The opioid antagonist, naloxone, reversibly inhibited DADLE-induced hyperpolarization. The hyperpolarization depended on extracellular K+ concentration and reversed at about -90 mV. DADLE also decreased Ca2(+)-dependent spike duration and after-hyperpolarization (AHP). DAGO (a selective mu-receptor agonist), but not DPLPE (a selective delta-receptor agonist), mimicked DADLE's effects on membrane potential, Ca2(+)-dependent spike duration, and AHP. It is concluded that DADLE, through postsynaptic mu-type opioid receptors, hyperpolarized DMV neurons by increasing K+ conductance, which may have an inhibitory effect on DMV output. DADLE-induced decrease of spike duration and AHP was also mediated by mu-receptors and could have additional effects on functions of the DMV neuron by virtue of reduction in Ca2+ entry.     

Modulation of neuronal activity by EGCG

This study aims to investigate whether (-)-epigallocatechin-3-gallate (EGCG) affects neuronal activity of acutely isolated rat medial vestibular nuclear neurons in whole-cell configuration patch-clamp experiments. EGCG (0.5 and 1 muM) lowered the spontaneous firing rate and hyperpolarized the membrane potential of medial vestibular nuclear neurons. However, it did not change the amplitude of afterhyperpolarization or the spike width of the action potential. A second application of EGCG with the same concentration elicited lesser responses. These results suggest that EGCG decreases neuronal activity by affecting potassium currents which are responsible for membrane potentials.     

Sleep-related variations of membrane potential in the lateral geniculate body relay neurons of the cat

Membrane potential of lateral geniculate body relay neurons was monitored in chronic cats during the sleep-waking cycle. Neurons were tonically depolarized throughout paradoxical (P) sleep and the maximal level of polarization occurred during slow (S) sleep (mean difference of membrane potential between S and P sleep: + 10.2 +/- 1.3 mV, n = 6, range: 8-12 mV). Some features of the spontaneous activity of S and P sleep are briefly discussed in relation to the level of membrane potential. In particular it is suggested that the phasic depolarizations underlying the bursts of action potentials during S sleep, and which are reproduced retinal cell axons impinging upon the hyperpolarized membrane.