Neurophysiological maturation of cat substantia nigra neurons: evidence from in vitro studies

Differential Modulation of Ventral Tegmental Area Circuits by the Nociceptin/Orphanin FQ System

The neuropeptide nociceptin/orphanin FQ (N/OFQ) can be released by stressors and is associated with disorders of emotion regulation and reward processing. N/OFQ and its receptor, NOP, are enriched in dopaminergic pathways, and intra-ventricular agonist delivery decreases dopamine levels in the dorsal striatum, nucleus accumbens (NAc), and ventral tegmental area (VTA). We used whole-cell electrophysiology in acute rat midbrain slices to investigate synaptic actions of N/OFQ. N/OFQ was primarily inhibitory, causing outward currents in both immunocytochemically identified dopaminergic (tyrosine hydroxylase positive (TH(+))) and non-dopaminergic (TH(-)) VTA neurons; effect at 1 μm: 20 ± 4 pA. Surprisingly, this effect was mediated by augmentation of postsynaptic GABAAR currents, unlike the substantia nigra pars compacta (SNc), where the N/OFQ-induced outward currents were K+ channel dependent. A smaller population, 17% of all VTA neurons, responded to low concentrations of N/OFQ with inward currents (10 nm: -11 ± 2 pA). Following 100 nm N/OFQ, the response to a second N/OFQ application was markedly diminished in VTA neurons (14 ± 10% of first response) but not in SNc neurons (90 ± 20% of first response). N/OFQ generated outward currents in medial prefrontal cortex (mPFC)-projecting VTA neurons, but inward currents in a subset of posterior anterior cingulate cortex (pACC)-projecting VTA neurons. While N/OFQ inhibited NAc-projecting VTA cell bodies, it had little effect on electrically or optogenetically evoked terminal dopamine release in the NAc measured ex vivo with fast scan cyclic voltammetry (FSCV). These results extend our understanding of the N/OFQ system in brainstem circuits implicated in many neurobehavioral disorders.

Electrophysiological characteristics of dopamine neurons: a 35-year update

This chapter consists of four sections. The first section provides a general description of the electrophysiological characteristics of dopamine (DA) neurons in both the substantia nigra and ventral tegmental area. Emphasis is placed on the differences between DA and neighboring non-DA neurons. The second section discusses the ionic mechanisms underlying the generation of action potential in DA cells. Evidence is provided to suggest that these mechanisms differ not only between DA and non-DA neurons but also between DA cells located in different areas, with different projection sites and at different developmental stages. Some of the differences may play a critical role in the vulnerability of a DA neuron to cell death. The third section describes the firing patterns of DA cells. Data are presented to show that the current "80/160 ms" criteria for burst identification need to be revised and that the burst firing, originally described by Bunney et al., can be described as slow oscillations in firing rate. In the ventral tegmental area, the slow oscillations are, at least partially, derived from the prefrontal cortex and part of prefrontal information is transferred to DA cells indirectly through inhibitory neurons. The final section focuses on the feedback regulation of DA cells. New evidence suggests that DA autoreceptors are coupled to multiple effectors, and both D1 and D2-like receptors are involved in long-loop feedback control of DA neurons. Because of the presence of multiple feedback and nonfeedback pathways, the effect of a drug on a DA neuron can be far more complex than an inhibition or excitation. A better understanding of the intrinsic properties of DA neurons and their regulation by afferent input will, in time, help to point to the way to more effective and safer treatments for disorders including schizophrenia, drug addiction, and Parkinson's disease.

Electrical synapses in basal ganglia

The basal ganglia (BG) provide a major integrative system of the forebrain involved in the organization of goal-directed behaviour. Pathological alteration of BG function leads to major motor and cognitive impairments such as observed in Parkinson's disease. Recent advances in BG research stress the role of neural oscillations and synchronization in the normal and pathological function of BG. As demonstrated in several brain structures, these patterns of neural activity can emerge from electrically coupled neuronal networks. This review aims at addressing the presence, functionality and putative role of electrical synapses in BG, with a particular emphasis on the striatum and the substantia nigra pars compacta (SNc), two main BG nuclei in which the existence and functional properties of neuronal coupling are best documented.

Connexin mRNA expression in single dopaminergic neurons of substantia nigra pars compacta

Dopaminergic neurons of the substantia nigra pars compacta play a major role in goal-directed behavior and reinforcement learning. The study of their local interactions has revealed that they are connected by electrical synapses. Connexins, the molecular substrate of electrical synapses, constitute a multigenic family of 20 proteins in rodents. The permeability and regulation properties of electrical synapses depend on their connexin composition. Therefore, the knowledge of the molecular composition of electrical synapses is fundamental to the understanding of their specific functions. We have investigated the connexin mRNA expression pattern of dopaminergic neurons by single-cell RT-PCR analysis, during two periods in which dopaminergic neurons are electrically coupled in vitro (P7-P10 and P17-P21). Our results show that dopaminergic neurons express mRNAs of various connexins (Cx26, Cx30, Cx31.1, Cx32, Cx36 and Cx43) in a developmentally regulated manner. Furthermore, we have observed that dopaminergic neurons display different connexin expression patterns, and that multiple connexins can be expressed in a single dopaminergic neuron. These observations underline the importance of electrical coupling in the development of dopaminergic neurons and raise the question of the existence of functionally distinct electrically coupled networks in the substantia nigra pars compacta.

Functional properties and synaptic integration of genetically labelled dopaminergic neurons in intrastriatal grafts

Intrastriatal grafts of fetal ventral mesencephalic tissue, rich in dopaminergic neurons, can reverse symptoms in Parkinson's disease. For development of effective cell replacement therapy, other sources of dopaminergic neurons, e.g. derived from stem cells, are needed. However, the electrophysiological properties grafted cells need to have in order to induce substantial functional recovery are poorly defined. It has not been possible to prospectively identify and record from dopaminergic neurons in fetal transplants. Here we used transgenic mice expressing green fluorescent protein under control of the rat tyrosine hydroxylase promoter for whole-cell patch-clamp recordings of endogenous and grafted dopaminergic neurons. We transplanted ventral mesencephalic tissue from E12.5 transgenic mice into striatum of neonatal rats with or without lesions of the nigrostriatal dopamine system. The transplanted cells exhibited intrinsic electrophysiological properties typical of substantia nigra dopaminergic neurons, i.e. broad action potentials, inward rectifying currents with characteristic 'sag', and spontaneous action potentials. The grafted dopaminergic neurons also received functional excitatory and inhibitory synaptic inputs from the host brain, as shown by the presence of both spontaneous and stimulation-evoked excitatory and inhibitory postsynaptic currents. Occurrence of spontaneous excitatory and inhibitory currents was lower, and of spontaneous action potentials was higher, in neurons placed in the dopamine-depleted striatum than of those in the intact striatum. Our findings define specific electrophysiological characteristics of transplanted fetal dopaminergic neurons, and we provide the first direct evidence of functional synaptic integration of these neurons into host neural circuitries.

Dendritic projections and dye-coupling in dopaminergic neurons of the substantia nigra examined in horizontal brain slices from young rats

We examined the rostro-caudal dendritic spread of striatally projecting dopaminergic neurons of the Substantia Nigra pars compacta (SNc) and investigated the presence of dye-coupling after labeling these cells with a mixture of lucifer yellow (LY) and neurobiotin (NB) or with LY alone. Whole cell recordings were made from horizontal brain slices (400 microm) obtained from P5-P20 rats. SNc neurons retrogradely labeled with Fluoro-Gold and located in the region containing tyrosine hydroxylase-immunoreactive cells displayed Ih current and other properties characteristic of SNc neurons. To prevent extracellular leakage, dyes were introduced into patch pipettes after the establishment of whole cell configuration, and cells were filled under visual control. In contrast to previous studies conducted in coronal sections that identified dendritic projections of SNc neurons mainly in the medio-lateral and ventral directions, almost all neurons labeled in our study (53/54) additionally displayed a large rostro-caudal dendritic span (649 +/- 219 microm). Dye-coupling between SNc neurons was not observed under basal conditions, in the presence of gap junction "openers" (forskolin, trimethylamine), or after neurons were filled with LY using sharp intracellular microelectrodes. As a "positive control," dye-coupling was demonstrated in four hippocampal dentate gyrus neurons that were filled using the same patch pipette technique. In addition, none of the tested SNc cells (n = 12) showed expression of connexin 36 (the "neuronal" connexin) when tested with single-cell RT-PCR. In conclusion, this study revealed extensive rostro-caudal dendritic projections of SNc neurons. Under our in vitro conditions, no evidence was found for dye-coupling among these neurons.

Changes in the kinetics of dopamine release and uptake have differential effects on the spatial distribution of extracellular dopamine concentration in rat striatum

The objective of this study was to examine whether the limited diffusion distance of dopamine in rat striatum produces spatial heterogeneity in the extracellular dopamine concentration on a dimensional scale of a few micrometers. Such heterogeneity would be significant because it would imply that the concentration of dopamine at a given receptor depends on the receptor's ultrastructural location. Spatially resolved measurements of extracellular dopamine were performed in the striatum of chloral hydrate-anesthetized rats with carbon fiber microdisk electrodes. Dopamine was monitored during electrical stimulation of the nigrostriatal pathway before and after administration of drugs that selectively affect the kinetics of evoked dopamine release and dopamine uptake. The effects of nomifensine (20 mg/kg), L-DOPA (250 mg/kg), and alpha-methyl-p-tyrosine (250 mg/kg) on the amplitude of the stimulation responses were examined. The outcome of these experiments was compared with predictions derived from a mathematical model that combines diffusion with the kinetics of release and uptake. The results demonstrate that the extracellular dopamine concentration is spatially heterogeneous on a micrometer scale and that changing the kinetics of dopamine release and uptake has different effects on this spatial distribution. The impact of these results on brain neurochemistry is considered.

Electrophysiological and immunocytochemical characterization of GABA and dopamine neurons in the substantia nigra of the rat

Neurons in the substantia nigra pars reticulata and pars compacta of the rat were studied using a combination of intracellular electrophysiological recording in in vitro and subsequent immunocytochemical double and triple labelling techniques. The neurons recorded in the pars reticulata were identified as either GABA or dopamine neurons: neurons were considered to be GABA neurons if they were immunopositive for glutamate decarboxylase, whereas those neurons which were immunopositive for tyrosine hydroxylase were considered to be dopaminergic. The GABA neurons had short duration action potentials (0.45+/-0.03 ms halfwidth), no apparent rectifying currents, no low threshold calcium spikes, were spontaneously active (7.4+/-3.7 Hz), and could maintain high firing rates. The dopamine neurons had long duration action potentials (1.49+/-0.10 ms), displayed both anomalous inward and transient outward rectifying currents, and more than half (12/17 neurons) displayed a low threshold calcium spike. Their spontaneous firing rate was lower than that of the GABA neurons (2.3+/-1.0 Hz), and they displayed strong frequency adaptation. Morphological reconstruction of neurobiotin-filled neurons revealed that the pars reticulata GABA neurons had more extensive local dendritic arborization than the dopamine neurons from either the pars reticulata or the pars compacta. All of the neurons recorded from the pars compacta were dopamine neurons; they were found not to be different either electrophysiologically or morphologically from pars reticulata dopamine neurons. The electrophysiology of the GABA neurons suggests that input activity is translated linearly to spike frequency. These GABA neurons probably represent the projection neurons of the pars reticulata, and it is thus likely that this basal ganglia output is frequency coded. The close similarity between the dopamine neurons in the pars compacta, which give rise to the nigrostriatal pathway, and those in the pars reticulata supports the notion that the dopamine neurons in these two regions are part of the same neuronal population.

Development of electrical membrane properties and discharge characteristics of superior olivary complex neurons in fetal and postnatal rats

Although hearing onset occurs relatively late during ontogeny of rats [around postnatal day (P) 12], anatomical brainstem connections are formed much earlier and are present before birth, indicating that a substantial amount of maturation occurs without acoustic input. Electrical activity is thought to influence neuronal development, but the physiological properties of auditory brainstem neurons during perinatal maturation are barely known. The present study focuses on the development of electrophysiological membrane properties of neurons in the rat's superior olivary complex (SOC), the first binaural station in the mammalian auditory brainstem. In in vitro slice preparations, intracellular recordings were obtained from 115 SOC cells from embryonic day (E) 18 to P17, and cells were morphologically identified by intracellular injection of biocytin or neurobiotin. By E18, i.e. 4 days before birth, SOC neurons were capable of generating Na(+)-dependent action potentials. Several passive and active membrane properties, including the resting potential, spike threshold and spike amplitude, did not change with development. In contrast, input resistance, time constant and spike duration decreased significantly, and maximal spike frequency increased significantly during the age period sampled. Our results show that rat SOC neurons display mature as well as immature electrical membrane properties during the same developmental period when anatomical connections are refined and when the soma-dendritic morphology develops. We conclude, therefore, that their membrane properties represent adequate physiological adaptations to the immature auditory brainstem microcircuits and that they form a basis upon which the development of these microcircuits is shaped.

Electrophysiological characterization of dopaminergic and non-dopaminergic neurones in organotypic slice cultures of the rat ventral mesencephalon

The aim of the present study was to characterize electrophysiologically neurones in organotypic cultures of the rat ventral mesencephalon and to compare these results with results published for the same neurones in other types of preparation. Intracellular recordings were obtained in 3- to 8-week-old organotypic slice cultures of the ventral mesencephalon prepared from new-born rats. Dopaminergic neurones were distinguished from non-dopaminergic neurones by staining with the autofluorescent serotonin analogue 5,7-dihydroxytryptamine and briefly viewing the preparation with short exposures to ultraviolet (UV) light (365 nm). Short exposures to UV light did not affect the electrophysiological properties. There were no significant differences between dopaminergic and non-dopaminergic neurones with regard to resting membrane potential or action potential threshold and amplitude, and in both types of neurone spontaneous burst activity and glutamatergic excitatory postsynaptic potentials were seen. There were differences in the following parameters, which can be used to distinguish between the two types of neurone. Dopaminergic neurones had broad action potentials (2-9 ms), high input resistance (mean 81 M omega), were silent or fired spontaneously at a low frequency (0-9 Hz), and no spontaneous GABAA-ergic inhibitory postsynaptic potentials or inward rectification were present. In contrast, non-dopaminergic neurones had fast action potentials (0.6-3.2 ms), low input resistance (mean 32 M omega), were silent or fired spontaneously at relatively high firing frequency (0-28 Hz), and sometimes inhibitory postsynaptic potentials and inward rectification were seen. In the presence of 1 microM tetrodotoxin and 10 mM tetraethylammonium, Ca2+ spikes could be evoked in both dopaminergic and non-dopaminergic neurones. Dopaminergic neurones in 3- to 8-week-old organotypic slice cultures have a number of distinguishing electrophysiological characteristics similar to those recorded in other types of acute or cultured preparations. However, some intrinsic regulatory mechanisms, namely the slow oscillatory potentials, inward rectification and the K+ current, IA, seem to be missing in the cultured neurones.

Postnatal changes in the distribution and morphology of rat substantia nigra dopaminergic neurons

Significant changes in the neurophysiology and neuropharmacology of nigral dopaminergic neurons take place in the first postnatal month. In order to correlate these changes with the postnatal development of dopaminergic neuron morphology and substantia nigra cytoarchitecture, brains from Sprague-Dawley rat pups of age postnatal days 1, 7, 14, 21 and 28 and adult rats were sectioned and processed for tyrosine hydroxylase immunocytochemistry. At postnatal day 1, pars compacta and pars reticulata were not clearly delineated; tyrosine hydroxylase positive neurons and a dense plexus of fibers were scattered throughout the substantia nigra. By day 7 the density of tyrosine hydroxylase positive neurons decreased markedly in ventral substantia nigra, and a dopaminergic pars compacta and a non-dopaminergic pars reticulata could be more clearly distinguished. By day 14 the substantia nigra appeared essentially as it does in the adult. Cell counts during development revealed that the number of tyrosine hydroxylase positive neurons/section in both pars compacta and pars reticulata decreased significantly from postnatal day 1 to postnatal day 14, while those in pars lateralis did not change. Tyrosine hydroxylase-positive somatic size increased modestly but significantly from postnatal day 1 to day 14 as did the diameter of the proximal and distal dendrites. However, even at day 1, the morphology of tyrosine hydroxylase positive neurons appeared essentially the same as in adults. Dendritic arborizations were well developed. The dendrites were non-varicose and modestly branched, with some of the longer ventrally directed dendrites passing through pars reticulata into the crus cerebri.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurophysiological, pharmacological and morphological properties of human caudate neurons recorded in vitro

Tissue samples from the caudate nucleus were obtained from eight children (eight to 172 months of age) who underwent hemispherectomies for the relief of intractable seizures. Neurophysiological, pharmacological and morphological properties of caudate neurons were characterized by intracellular recordings in an in vitro slice preparation. These properties were compared with those of tissue obtained from animal studies. Electrophysiological properties of human caudate neurons that were similar to those of cat caudate and rat neostriatal cells included resting membrane potential, input resistance, action potential rise time, fall time, duration and action potential afterhyperpolarization amplitude, as well as the general characteristics of locally evoked synaptic responses. Properties that were different included action potential amplitudes and time-constants. Human caudate neurons also displayed responses similar to those of cat caudate or rat neostriatal cells to manipulation of excitatory amino acid receptor systems and to dopamine application. Kynurenic acid, a broad-spectrum excitatory amino acid receptor antagonist, decreased the amplitude of evoked synaptic responses, indicating that they were partially mediated by excitatory amino acids. In Mg2+ free Ringer's solution, the amplitudes and durations of postsynaptic responses were increased and bursts of action potentials were induced. These effects were mediated by activation of N-methyl-D-aspartate receptors since they were blocked by 2-amino-5-phosphonovalerate, a specific N-methyl-D-aspartate-receptor antagonist. Iontophoretic application of N-methyl-D-aspartate also induced membrane oscillations and bursts in almost all caudate neurons. Dopamine decreased the amplitude of postsynaptic responses, an effect antagonized by domperidone, a selective D2 dopamine receptor antagonist. Developmentally, the greatest change was an increase in action potential amplitude, although input resistance decreased and action potential afterhyperpolarization amplitude increased. Postsynaptic responses were similar across age. All but one of the caudate neurons identified by intracellular injection of biocytin or Lucifer Yellow were medium-sized spiny cells. These experiments show that human caudate neurons display a number of electrophysiological properties similar to rat neostriatal or cat caudate neurons recorded in brain slices. Furthermore, few electrophysiological parameters changed significantly over the age period examined suggesting that the human caudate at eight months displays many of the neuronal functions of the more mature caudate nucleus.

Topographic heterogeneity of substantia nigra neurons: diversity in intrinsic membrane properties and synaptic inputs

The passive and active membrane properties of substantia nigra neurons were recorded in vitro at various locations throughout its anterior-posterior extent and their responses to extracellular electrical stimulation within the pars reticulata were analysed. One class of nigral pars compacta cell showed the well-established electrophysiological characteristics of mesencephalic dopaminergic neurons, i.e. spontaneous discharge in a very rhythmic, pacemaker fashion without bursting activity and with broad action potentials. However, these neurons could be subdivided further according to differences in electrophysiological properties which correlated with their position within the substantia nigra. Thus, neurons recorded from the anterior part of the substantia nigra, at the level of the mammilary bodies displayed a significantly higher firing rate and shorter action potential than those located in posterior slices at the level of the accessory optic tract. The location of the cell was also a critical factor in its response to stimulation of the pars reticulata: in anterior slices only 45.5% of the cells responded with inhibitory postsynaptic potentials to stimulation, while in posterior slices inhibitory postsynaptic potentials occurred in 85.7% of the neurons (n = 44). In addition, anteriorly located neurons were more sensitive to direct electrical stimulation than posteriorly located cells and they also exhibited excitatory postsynaptic potentials (33%) on pars reticulata stimulation. However, the actual properties of inhibitory postsynaptic potentials were essentially the same in these neurons irrespective of whether they were located either in the anterior or posterior part of the nigra: reversal potentials of inhibitory postsynaptic potentials were found at two distinct potentials indicating involvement of both GABAA and GABAB receptors. This deduction is also supported by additional pharmacological findings: application of the GABAA antagonist, bicuculline methiodide and/or GABAB antagonist, 2-hydroxysaclofen blocked both the inhibitory postsynaptic potentials and the cessation of spontaneous firing activity of the cells to stimulation of the pars reticulata. The other type of pars compacta neuron recorded discharges phasically and was located exclusively in the anterior pole of the substantia nigra. These cells showed a wide range of spontaneous firing activity, a non-rhythmic, irregular pattern of firing, a shorter action potential width and the presence of a low-threshold calcium conductance. These "phasic" neurons also differed greatly from other compacta neurons in their response to pars reticulata stimulation: spontaneous activity of these cells was not inhibited nor did they show inhibitory postsynaptic potentials. Instead, the majority was preferentially activated by direct stimulation of the dendrites, although excitatory postsynaptic potentials could also be evoked.(ABSTRACT TRUNCATED AT 400 WORDS)

A model for the functioning of the striatum

A model is presented for the operation of the striatum. The model posits that the basal ganglia are responsible for driving smooth transitions of state for an organism. We propose that this is accomplished through the computation of a potential function within the striatum on which a gradient descent is performed toward the goal state. The model suggests that various somatotopic regions of the striatum correspond to state spaces, each of which pertains to a different aspect of the organism. This paper discusses this model only in the context of motor control, i.e., egomotion and limb movement. The model appears to account for a variety of experimental results, and for some unusual properties of the striatum.

Electrophysiological localization of distinct calcium potentials at selective somatodendritic sites in the substantia nigra

The dendrites of dopaminergic neurons in the substantia nigra play a pivotal role in the neurochemical homeostasis of the nucleus. It is conceivable therefore that the cell body and dendrites of these nigral neurons possess distinct and independent electro-responsive features. By means of differential polarization through applied electric fields, the cell body and dendrites have been activated in effective isolation during intracellular recordings from pars compacta neurons in the substantia nigra in vitro. In one class of neurons, which discharge in a "phasic" fashion and are located in the rostral substantia nigra, the dendrites are shown to be the origin of classic low-threshold and high-threshold type calcium potentials: indeed the high-threshold conductance appears to be exclusively dendritic. By contrast, in a second, more caudally located cell type, which discharges rhythmically, a high-threshold calcium spike is located principally in the cell body. The differential localization of these calcium conductances in sub-populations of neurons is likely to determine the functions for the calcium responses in each type of neuron, and moreover highlight the dendrites as dynamic and selective components in the physiology of the substantia nigra. The presence, for example, of the high-threshold calcium conductance in the dendrites of only one class of neuron suggests that this sub-population plays a prominent role in non-classical phenomena of dendritic release of a variety of chemical mediators.

Differential actions of acetylcholinesterase on the soma and dendrites of dopaminergic substantia nigra neurons in vitro

In the substantia nigra, acetylcholinesterase (AChE) has non-cholinergic action on dopaminergic neurons. The subset of neurons particularly sensitive to AChE are characterized by functionally active apical dendrites extending into the pars reticulata and generating a powerful calcium conductance. This study thus attempted to establish directly the importance of these dendrites regarding the action of AChE. Segregation of the pars compacta from the pars reticulata did not affect the AChE-induced hyperpolarization on this sub-set of dopaminergic neurons. However, the ionic basis of the hyperpolarization was related to the integrity of the neurons: AChE caused an opening of potassium channels in intact cells. On the other hand when the pars reticulata containing apical dendrites was removed, an action of AChE involving the closure of calcium/sodium channels was revealed. The results demonstrate that the net effect of AChE need not be related to any particular segment of the dopaminergic neurons, whereas the nature of the mechanism underlying that effect depends on the presence, or otherwise, of the apical dendrites.

Sub-populations of pars compacta neurons in the substantia nigra: the significance of qualitatively and quantitatively distinct conductances

In the substantia nigra pars compacta neurons can be classified in two sub-populations. In this study the distinguishing criteria have been the presence of four distinct calcium-dependent potentials, two each generated selectively and exclusively in each cell type. One class of cells, found in the more caudal pars compacta, displays calcium-mediated, slow oscillatory potentials which occur spontaneously and generate long-duration afterhyperpolarizations. A second, much faster calcium spike can be evoked after the blockade of sodium and potassium channels. This spike has a high generation threshold and is followed by a fast afterhyperpolarization. The other group of neurons is distributed principally in the rostral substantia nigra, at the level of the mammilary bodies. In these cells, a low-threshold calcium spike is generated that (i) inactivates at depolarized potentials, (ii) has no active negative phase, and (iii) causes burst firing action potentials. In all these three respects, this transient differs from the slow oscillatory potential in the more caudal group of neurons. In addition, a short-duration calcium-dependent potential can be evoked at a high threshold. Both the low- and high-threshold spikes of the rostral cells are attenuated in experiments where dendrites have been sectioned prior to the recording. The membrane properties of the caudal cell group, including the fast calcium spike, are unaffected by dendritic sectioning. It is suggested that in the guinea-pig the calcium conductances in the caudal neurons operate in or near the cell body and might play a large (though not necessarily exclusive) role in regulating autorhythmicity. In the more rostral cells, the characteristics of their particular calcium conductances which seem to be located more distally would prompt a mediating function in the secretion, and subsequent action, of neuroactive substances from dendrites.

Neurophysiological maturation of cat caudate neurons: evidence from in vitro studies

The membrane properties and synaptic physiology of developing cat caudate (Cd) nucleus neurons were studied in in vitro slice preparations. Recordings were obtained from 98 cells in kittens from fetal day (F) 56 to postnatal day (P) 90. With increasing age, the following maturational changes occurred; resting membrane potentials became more negative, action potential rise times decreased, action potential amplitudes increased, and action potential durations and input resistances decreased. The frequency of occurrence of afterhyperpolarizations and of anomalous rectification increased with age. The primary response to local extracellular stimulation was a depolarization usually accompanied by an action potential. Evoked hyperpolarizing responses were seen after P28 but only occurred if the membrane was depolarized by intracellular current injection. Cells identified by intracellular injection of Lucifer yellow were primarily medium-sized spiny neurons although it was not always possible to determine the cell type in slices from animals less than P5. Somatic diameter, dendritic length, and spine density increased with age. Dye-coupling occurred in slices less than P20. Its frequency decreased with age. These results show that Cd neurons undergo significant maturation during late prenatal and early postnatal periods. In contrast, substantia nigra neurons mature more rapidly and should be capable of influencing the less mature Cd neurons during development.(ABSTRACT TRUNCATED AT 250 WORDS)