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

Anatomical and immunohistochemical identification of catecholaminergic neurones in brain slice preparations used in electrophysiology

The physiological characteristics of central neural populations are being increasingly explored in slice preparations. A major challenge of this approach is to correlate the physiological properties of individual neurones or groups of neurones with their anatomical and chemical properties in order to gain key insights into their functional identities. The present study describes a method for determining the precise topographical position and the immunohistochemical characteristics of neurones in brain slice preparations that are used frequently in electrophysiological investigations. Thick horizontal slices of rat brainstem were re-cut using a method that provided thin sections that were always in the same plane as the parent slice and that were of suitable thickness for immunohistochemistry. Catecholaminergic neurones in these co-planar (horizontal) sections were stained using antisera to tyrosine hydroxylase, the rate-limiting enzyme for catecholamine synthesis. To identify individual catecholamine neurones in the co-planar sections, we constructed a reference atlas of the distribution of catecholamine neurones in the horizontal plane of the rat brain. The combined use of the horizontal atlas and of immunohistochemical techniques in co-planar sections of horizontal slices enables the determination of several key properties: (1) whether a neurone is TH-positive, (2) its precise topographical position and (3) its content of neuropeptides and other immunohistochemical markers. Thus our study offers a readily feasible method for correlative anatomy and immunohistochemistry of physiologically identified catecholaminergic neurones in brain slices.

Inhibition of the ATP-sensitive potassium channel in the guinea pig substantia nigra by BMS 181100 is not mediated by a sigma-binding site

Quantitative autoradiography of brain tissue has revealed a high density of binding sites for the K-ATP channel antagonists, the sulphonylureas, and for sigma-ligands in the substantia nigra (SN). In view of the high density of the two binding sites in the SN the possibility has been investigated that the K-ATP channel and the sigma-binding site are functionally linked. The K-ATP channel-mediated membrane hyperpolarisation and decrease in input resistance produced by hypoxia and by the metabolic inhibitor, cyanide, in rostral substantia nigra pars compacta neurons are antagonised by the sigma-ligand BMS 181100. In addition, BMS 181100 antagonises activation of the K-ATP channel by diazoxide; cromakalim is found to be without effect in these neurons. Antagonism of the cyanide-induced hyperpolarisation is dose dependent and is observed at concentrations of the drug which have no observable effect on the resting membrane properties of the neurons. By contrast, the nonselective sigma ligands 1,3-di-O-tolylguanidine (10 microM) and (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine (100 microM), and the selective sigma 1-ligand (+)-pentazocine (5-10 microM) have no effect on the cyanide-induced hyperpolarisation. 5-HT (50-100 microM) and the selective 5-HT1A receptor agonist 8-OH-DPAT (50 microM) also fail to antagonise the cyanide-induced hyperpolarisation. The antagonism of the cyanide-induced hyperpolarisation by BMS 181100 persists in the presence of tetrodotoxin (1 microM) and in the presence of high concentrations of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, but not under conditions of reduced calcium (0.1-0.2 mM) and raised magnesium (6 mM) concentrations, which block synaptic transmission. It is concluded that in substantia nigra phasic neurons the sigma-binding site does not regulate activation of the ATP-sensitive channel. However, BMS 181100 antagonises K-ATP channel activation in these neurons independently of sigma-binding sites and 5-HT receptors. This action of BMS 181100 is TTX insensitive and Ca2+ dependent.

Electrophysiology of the neurons in the area of the enkephalinergic magnocellular dorsal nucleus of the guinea-pig hypothalamus, studied by intracellular and whole-cell recordings

The electrophysiological characteristics of 103 hypothalamic neurons in the area of the guinea-pig enkephalinergic magnocellular dorsal nucleus were studied in a thick slice preparation with sharp microelectrodes (63 neurons) and patch pipettes for whole-cell recordings (40 neurons). Of the sampled cells, 79.6% displayed tetrodotoxin-resistant, calcium-dependent slow-depolarizing potentials when the membrane potential was hyperpolarized to approximately -70 mV (type I neurons). Half of them showed robust slow depolarizing potentials, generating bursts of fast action potentials. In the remaining neurons, the slow-depolarizing potentials did not cause burst-firing action potentials but triggered single action potentials. The other class of neurons (20.4% of the sample: type II neurons) did not exhibit calcium-dependent slow-depolarizing potentials. Resting potential, input resistance and the membrane time constant did not distinguish among the two classes of neurons. Current-voltage relationships were heterogeneous. A transient outward rectification was observed in the two classes. This was not totally blocked by 2 mM 4-aminopyridine but was abolished when using perfusion with cobalt instead of calcium. Input resistance and the time constant were higher when measured in the whole-cell mode but the other electrical parameters and the sampling of the recorded neurons were strikingly similar between the two methods of recording. Intracellular staining of 22 neurons retrogradely labelled from the lateral septum allowed confirmation of their location within the magnocellular dorsal nucleus. The study indicates that the electrical properties of these neurons did not differ from those of neurons found throughout the area explored. It also indicates the presence of distinct electrophysiological types of cells in the magnocellular dorsal nucleus, although the nucleus is composed of a single type of enkephalinergic neuron. It provides a basis for the study of the regulation of activity of the neurons at the origin of an enkephalinergic tractus which is involved in neuroendocrine, psychoneuroendocrine and immune processes.

Metabolic inhibition and low internal ATP activate K-ATP channels in rat dopaminergic substantia nigra neurones

The patch-clamp technique was used to study whole-cell currents of acutely dissociated rat substantia nigra (SN) neurones. In perforated-patch current-clamp recordings, inhibition of mitochondrial metabolism by rotenone (5 microM) produced a hyperpolarisation and inhibited electrical activity. These effects were reversed by the sulphonylureas tolbutamide (0.5 mM) or glibenclamide (0.5 microM). Under voltage-clamp conditions, rotenone induced a time- and voltage-independent K+ current which was selectively blocked by sulphonylureas. The glibenclamide-sensitive current reversed at -81.7 +/- 2.7 mV (n = 5) and showed marked inward rectification. Intracellular dialysis with 0.3 mM adenosine 5'-triphosphate (ATP), but not 2 mM or 5 mM ATP, in standard whole-cell recordings also resulted in activation of a sulphonylurea-sensitive K+ current with similar properties (reversal potential, -81.9 +/- 2.5 mV, n = 5). The close similarity in the properties of the ATP-sensitive K+ current observed in whole-cell recordings and the K+ current activated by metabolic inhibition in perforated-patch recordings suggest that they both result from activation of the same type of ATP-sensitive K+ channel. Sulphonylureas had no effect on electrical activity or membrane currents in the absence of rotenone in perforated-patch recordings, or in cells dialysed with 5 mM ATP, indicating that in SN neurones these drugs are selective for the ATP-sensitive K+ current.

Glutamatergic and cholinergic inputs from the pedunculopontine tegmental nucleus to dopamine neurons in the substantia nigra pars compacta

Postsynaptic responses of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) to stimulation of the pedunculopontine tegmental nuclei (PPN) were studied in in vitro slice preparations in the rat. The recorded neurons were intracellularly injected with biocytin and also identified as DA neurons by an immunocytochemical technique. PPN stimulation induced in DA neurons monosynaptic excitatory postsynaptic potentials (EPSPs) that consisted of early transient and slow components. An application of anti-glutamatergic agents (1 mM kynurenic acid and/or 30 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)) in the bathing media partially suppressed the EPSPs, indicating that PPN inputs to SNc DA neurons are glutamatergic and non-glutamatergic. Anti-glutamatergic resistant EPSPs were suppressed by applications of anti-cholinergic agents such as atropine, mecamylamine, and pirenzepine. These data indicate a convergence of glutamatergic and cholinergic excitatory inputs from the PPN to SNc DA neurons and that both nicotinic and muscarinic receptors are involved in the cholinergic transmission.

Determinants of neuronal firing pattern in the guinea-pig subthalamic nucleus: an in vivo and in vitro comparison

To ascertain the extent to which neuronal firing pattern in the subthalamic nucleus (STN) is determined by afferent inputs, a comparison was made between STN neurons recorded in vivo and in vitro (a largely denervated preparation). In vivo, the majority of cells exhibited an irregular firing pattern, although some showed evidence of burst firing. In contrast, all cells had a regular firing pattern in vitro. Electrical stimulation of the striatopallidal complex in vivo induced a short latency inhibition in STN neurons, followed by a burst of spikes. These effects could be reproduced in vitro; hyperpolarising pulses gave rist to a slow depolarising potential upon termination, which was accompanied by a burst of action potentials. Hence, the evidence suggests that afferents play an important role in determining the firing pattern of STN neurons. However, the cells also possess intrinsic membrane properties which allow inputs to trigger either single spikes or bursts.

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.

Cholinergic stimulation of rostral and caudal substantia nigra pars compacta produces opposite effects on circling behavior and striatal dopamine release measured by brain microdialysis

Turning in circles is among the behaviors elicited by unilateral cholinergic stimulation of the substantia nigra. Recent studies have shown that microinjection of cholinergic agonists into the substantia nigra pars compacta increases dopamine release and turnover in the striatum of anesthetized rats [Hernández-López et al. (1992) Brain. Res. 598, 114-120; Blaha and Winn (1993) J. Neurosci, 13, 1035-1044]. In this study, the relationship between circling behavior and striatal dopamine release following cholinergic stimulation of the substantia nigra pars compacta neurons was assessed by brain microdialysis in awake rats. The results indicate that cholinergic stimulation of the substantia nigra pars compacta with the mixed nicotinic-muscarinic cholinergic agonist carbachol modulates striatal dopamine release, and this effect is accompanied by circling behavior and stereotypies. Microinjection of carbachol (109 nmol) in the caudal portions of the substantia nigra pars compacta induced contralateral circling associated with an increase of dopamine release in neostriatum. On the contrary, ipsilateral circling and reduction of striatal dopamine release was elicited when the same dose of the drug was applied in the rostral portions of the substantia nigra pars compacta. The above findings are in accordance with recent electrophysiological studies suggesting the existence of sub-populations of nigrostriatal dopaminergic neurons, and indicate that the substantia nigra pars compacta is functionally compartmentalized. We conclude that the cholinergic input to the substantia nigra pars compacta could modulate the motor behavior through regulating the firing rate of nigrostriatal dopaminergic neurons and dopamine release in the neostriatum.

Direct monitoring of dopamine and 5-HT release in substantia nigra and ventral tegmental area in vitro

Fast-scan cyclic voltammetry with carbon fibre microelectrodes was used to detect endogenous dopamine (DA) and 5-hydroxytryptamine (5-HT) release from three distinct regions of guinea-pig mid-brain in vitro: rostral and caudal substantia nigra (SN) and the ventral tegmental area (VTA). Previous electrophysiological studies have demonstrated that cells of the caudal SN and the VTA have similar characteristics, whereas cells in the rostral SN have distinctly different properties. In the present study, we confirmed that each region has tyrosine hydroxylase-positive neurons and determined, using high-performance liquid chromatography, that DA levels were similar in rostral and caudal SN, but lower in SN than in VTA. In each region, application of veratrine, which was shown by intracellular recordings to have a reversible depolarising action, evoked a signal attributable to DA and distinguishable from that of 5-HT. Release signals were monitored every 250 ms with a spatial resolution of less than 50 microns.l DA release was calcium-dependent and was not detectable in a catecholamine-poor area such as the cerebellum, or in mid-brain tissue pre-treated with reserpine. Within the normal mid-brain, the amount of DA released was correlated with tissue content in that it was higher in the VTA than in either region of SN. It is concluded that DA released from somato-dendritic parts of mid-brain neurons exhibits site-specific variation. This is the first report of direct monitoring of DA and 5-HT release from these regions with in situ electrodes and demonstrates the utility of fast-scan cyclic voltammetry to investigate the mechanisms and possible non-classical functions of somato-dendritic DA release.

Calcium spike underlying rhythmic firing in dopaminergic neurons of the rat substantia nigra

In order to study a possible mechanism for rhythmic firing of dopaminergic (DA) neurons, intracellular recordings were obtained from 56 rhythmically firing DA neurons in the rat substantia nigra compacta (SNc), using in vitro slice preparations. In the presence of TTX, spontaneous oscillation of the membrane potential was induced in SNc DA neurons when the membrane potential was depolarized more positive from -60 to -40 mV. Each oscillation wave was characterized by a pacemaker-like slow depolarization (PLSD) followed by a relatively prompt repolarization. As the DC depolarization was increased from -60 to -40 mV, the oscillation frequency increased from 0.5 to 5 Hz, but the amplitude of the wave decreased. Of 17 neurons tested in the presence of TTX, the maximum amplitudes of the oscillation varied from 10-15 mV in 8 neurons and were less than 5 mV in 9 neurons. In those 9 neurons, an application of TEA greatly enhanced (up to 15 mV) the amplitude of oscillation. The oscillation ceased when the membrane was hyperpolarized more negative than -60 mV. At the membrane potential more negative than -60 mV in the presence of TTX an injection of a depolarizing current pulse could evoke PLSD which was an all-or-nothing regenerative spike potential. The rate of rise of the PLSD changed depending on the intensity of injected current pulses but their amplitude remained constant. Its time-to-peak was slow (up to 1400 ms), while the decay time was relatively brief (< 500 ms). The threshold membrane potential for evoking PLSD was -53.7 +/- 3.2 mV (n = 10). This was higher than the previously reputed threshold for low threshold Ca2+ spike (LTS) (< -60 mV) and lower than that for high threshold Ca2+ spike (HTS) (> -35 mV) in SNc DA neurons. Even at a holding potential of -45 mV, a depolarizing current pulse could trigger PLSD while LTS was completely inactivated. Cd2+ (0.4 mM) abolished the oscillation and PLSD without marked effects on the LTS (n = 6). A low Ca2+ and high Mg2+ Ringer's solution also abolished the oscillation and PLSD (n = 4). An intracellular injection of EGTA markedly prolonged the decay time course of PLSD characterized by a slow and a relatively fast falling phase (n = 5). This would suggest an involvement of Ca(2+)-dependent K+ conductance and/or Ca2+ dependent inactivation of Ca2+ conductance during repolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

A whole cell patch-clamp study on the pacemaker potential in dopaminergic neurons of rat substantia nigra compacta

A whole-cell patch-clamp recording was obtained from dopamine (DA) neurons (n = 68) in the substantia nigra compacta (SNc) in in vitro slice preparations in order to study the underlying current for pacemaker-like slow depolarization (PLSD) which was considered as a basis for rhythmic firing of DA neurons. SNc DA neurons were identified immunohistochemically after recording. Results demonstrated that: (1) Under current clamped condition in the presence of TTX, DA neurons (n = 5) displayed the oscillation of membrane potential with high threshold spikes. An application of a hyperpolarizing and depolarizing current pulse (at the membrane potential where oscillation was no longer seen) induced a prominent anomalous rectification and pacemaker-like slow depolarization (PLSD), respectively. (2) Under voltage-clamped conditions in the presence of TTX, a command pulse positive to -50 mV from a holding potential of -80 mV induced a persistent Ca2+ current which was usually preceded by either a transient K+ (n = 7) or a transient Ca2+ (n = 4) current recorded with a patch pipette containing potassium gluconate (145 mM). (3) When outward currents were suppressed by 140 mM CsCl and 10 mM EGTA intercellularly applied through the patch pipette, a command pulse positive to -50 to -40 mV induced either a persistent Ca2+ current alone (n = 4) or a persistent Ca2+ current preceded by a transient Ca2+ current (n = 11). (4) The threshold for activation of the persistent Ca2+ current (Ip) was around -60 to -55 mV. The amplitude of Ip produced by a command pulse stepped to -50 mV from a holding potential of -80 mV was -78 +/- 42 pA (n = 23). (5) The threshold for activation of transient Ca2+ current (IT) was around -70 to -65 mV and inactivated completely at -70 to -65 mV (n = 11). The peak amplitude of IT evoked at -60 to -55 mV from a holding potential of more negative than -80 mV was 489 +/- 170 pA (n = 11). (6) The decay time constant of IT was 28 +/- 12 ms at -60 mV (n = 8) and that of IP was 2.35 +/- 1.37 s at -50 mV (n = 11) when recorded with a pipette containing 10 mM EGTA and 140 mM CsCl. (7) The decay of IP was apparently accelerated by decreasing the concentration of EGTA in the pipette solution from 10 to 1 mM.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

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.