Reticulata Neurons Research Articles

Electrophysiological investigation of adenosine trisphosphate-sensitive potassium channels in the rat substantia nigra pars reticulata

Adenosine trisphosphate-sensitive potassium (K-ATP) channels in the substantia nigra pars reticulata were studied in rat brain slices using whole-cell patch clamp recording. Substantia nigra pars reticulata neurons were identified as such by their spontaneous action potential firing at mean rate of 15.3 Hz, virtual absence of hyperpolarization-activated inward current I h, and unresponsiveness to dopamine (30 μM), quinpirole (10 μM) and [Met]enkephalin (10 μM). Intracellular dialysis with Mg 2+-ATP-free pipette solutions caused a slowly developing membrane hyperpolarization (13±4 mV), accompanied by a cessation of action potential firing, or an outward current (79±30 pA at around −60 mV), which were reversed by the sulphonylurea K-ATP channel blockers tolbutamide (100 μM) and glibenclamide (3 μM). When Mg 2+-ATP (2 mM) was included in the recording pipette no membrane hyperpolarization or outward current was observed. Neither the sulphonylureas nor the potassium channel activator lemakalim (200 μM) altered membrane potential, firing rate or holding current under these recording conditions. The outward current induced by dialysis with Mg 2+-ATP-free solutions reversed polarity negative to −94±9 mV (9 cells), close to the estimated K + equilibrium potential (−105 mV) for the conditions used, and was associated with a conductance increase that was blocked by Ba 2+ (100 μM). The current blocked by the sulphonylureas had a similar reversal potential (−97±7 mV; 13 cells), and both currents were voltage independent over the range −50 to −100 mV with slope conductance of approximately 2.0 nS. Outward synaptic currents were evoked by single shock electrical stimulation, in the presence of glutamate receptor antagonists, at a holding potential of −50 mV. These synaptic currents were blocked by bicuculline (10 μM) and reversed polarity at around −65 mV, close to the Cl − equilibrium potential, and were thus mediated by GABA A receptors. They were reversibly depressed by 37±14% in lemakalim (200 μM) in 6/12 cells tested, an effect that was partially reversed by tolbutamide (200 μM). It is concluded that functional K-ATP channels are present both presynaptically and postsynaptically in the substantia nigra pars reticulata. Postsynaptic K-ATP channels may control excitability in conditions where intracellular ATP is reduced, whereas presynaptic K-ATP channels, sensitive to the potassium channel activator lemakalim, can modulate the release of GABA, which probably arises from fibres of extranigral origin. Pharmacological differences between these two sites could be exploited to treat epilepsies, dyskinesias and akinesia.

Responses of substantia nigra pars reticulata neurons to intrastriatal D1 and D2 dopaminergic agonist injections in the rat

Extracellular neuronal recordings were performed in the substantia nigra pars reticulata (SNr) of normal and 6-hydroxydopamine (6-OHDA)-lesioned rats during intrastriatal D1 and D2 dopaminergic-agonist injections. Three types of responses were observed, inhibition, excitation and/or regularization of neuronal discharge patterns. In normal rats, variable responses were observed after D2 injections and a predominance of inhibition after D1 injections. In lesioned rats, the percentage of neurons inhibited and that of neurons regularized increased following D2 injections. During sequential intrastriatal D1 and D2 agonist injections, a response to both dopaminergic agents was observed in 22.5% and 55.5% of SNr neurons in normal and lesioned rats, respectively. Our data suggests that the physiological relevance of direct and indirect pathways convergence upon a given SNr target neuron depends on their respective synaptic weight, the influence of surrounding SNr neurons and the state of dopamine depletion.

(−)-Stepholidine acts as A D 1 partial agonist on firing activity of substantia nigra pars reticulata neurons in 6-hydroxydopamine-lesioned rats

(−)-Stepholidine (SPD), a novel dopamine (DA) D, and/or D 2 receptor antagonist in normosensitive animals, shows agonistic effects on D 1 receptors in rotational behavior of 6-hydroxydopamine (6-OHDA)-lesioned rats. To further characterize the pharmacological properties of SPD, we investigated the effects of SPD on firing activity of substantia nigra pars reticulata (SNR) neurons in different sensitive models. In control rats, the selective D 1 agonist SKF38393 (4 mg/kg, i.v.) induced inconsistent changes (i.e. increase, decrease or no change) in firing of SNR neurons. These effects were completely antagonized by SPD (i.v.), regardless of the changes induced by SKF38393. SPD (4 mg/kg), per se, increased firing by 30.9 ± 14.4%. In reserpinized rats, SKF38393 also induced SPD-reversible inconsistent changes as in control rats. Nevertheless, SPD per se produced no alteration in firing of SNR neurons. In 6-OHDA-lesioned rats, 5 6 SNR neurons were inhibited by SKF38393. The inhibition was completely abolished by Sch23390, a selective D 1 antagonist (0.5–2 mg/kg), but partially reversed by SPD (1–16 mg/). Moreover, SPD (4 mg/kg) itself caused SNR increased or decreased neuron firing, and these effects were completely reversed by Sch23390 (0.5–2 mg/kg) in 8 12 neurons recorded. These results suggest that SPD acts as a partial agonist to D 1 receptors in 6-OHDA-lesioned rats, but as an antagonist to D 1, receptors in normal and reserpinized rats.

Total number of neurons in the neostriatal, pallidal, subthalamic, and substantia nigral nuclei of the rat basal ganglia: a stereological study using the cavalieri and optical disector methods.

The total number of neurons within six subdivisions of the rat basal ganglia was estimated using unbiased stereological counting methods and systematic random sampling techniques. Six young adult rats were perfuse-fixed, their right cerebral hemispheres were embedded in glycolmethacrylate, and a complete set of serial 40-mu m sections was cut through each hemisphere. After a random start, a systematic subset (e.g., every tenth) of these sections was used to estimate the total volume of each subdivision using Cavalieri's method. The same set of sampled sections was used to estimate the number of neurons in a known subvolume (i.e., the Nv) by the optical disector method. The product of the total volume and the Nv by these methods yields an unbiased estimate of the total number of neurons. It was found that the right basal ganglia consisted, on average, of 2.79 million neostriatal or caudate-putamen neurons (with a coefficient of variation of 0.07), 46,000 external globus pallidus neurons (0.11), 3,200 entopeduncular/internal globus pallidus neurons (0.10), 13,600 subthalamic neurons (0.10), 7,200 substantial nigra pars compacta neurons (0.15), and 26,300 substantia nigra pars reticulata neurons (0.07).

Substantia Nigra Pars Reticulata Neurons in Parkinson's Disease

We have examined by immunohistochemistry the parvalbumin-containing neurons of the substantia nigra in patients with Parkinson's disease and in age-matched controls. Parvalbumin, a calcium binding protein, is involved in buffering intracellular calcium and in this study was localized within the majority of non-pigmented neurons of the human pars reticulata. Previous studies have shown that the parvalbumin-immunoreactive pars reticulata neurons are GABAergic and project to the motor thalamus and tectum. Their increased output, due to the loss of dopaminergic inhibition in Parkinson's disease, decreases cortical activation via thalamic pathways, causing parkinsonian symptoms. In Parkinson's disease there was a significant loss of parvalbumin-immunoreactivity from these neurons, though there was no evidence of actual cell loss. This loss of parvalbumin-immunoreactivity was detected only in those cases with end-stage Parkinson's disease.

Prefrontal cortex inputs of the nucleus accumbens-nigro-thalamic circuit.

The functional organization of the cortico-nucleus accumbens-substantia nigra pars reticulata circuit was investigated in the rat using combined anatomical and electrophysiological approaches. The nucleus accumbens neurons which project to the substantia nigra pars reticulata are located in a circumscribed region of the core immediately adjacent and extending dorsally to the anterior commissure. As shown by retrograde and anterograde transports of wheatgerm agglutinin conjugated to horseradish peroxidase, the region of the nucleus accumbens related to the substantia nigra was found to receive bilateral inputs from restricted areas of the medial and lateral prefrontal cortex, i.e., prelimbic/medial orbital and dorsal agranular insular areas. The electrical stimulation of these medial and lateral prefrontal cortical areas induced excitatory responses in nucleus accumbens neurons projecting to the dorsomedial substantia nigra pars reticulata. Interestingly, an important proportion (61%) of the nucleus accumbens-nigral cells responding to the stimulation of the lateral prefrontal cortex were also excited by the stimulation of the medial prefrontal cortex, demonstrating the existence of a convergent influence of these cortical areas on single nucleus accumbens cells. Furthermore, the present data also show that the stimulation of the medial prefrontal cortex results in a powerful inhibition of the tonic firing of the substantia nigra pars reticulata neurons. In conclusion, this study reveals the existence of a functional link between the prefrontal cortex (prelimbic/medial orbital and agranular insular areas) and the nucleus accumbens neurons which innervate the dorsomedial region of the substantia nigra pars reticulata. Since the dorsomedial region of substantia nigra pars reticulata is known to project to subfields of the mediodorsal and ventromedial thalamic nuclei related to the prefrontal cortex, the present data further demonstrate the existence of a prefrontal-nucleus accumbens-thalamo-cortical circuit involving the substantia nigra pars reticulata.

Electrophysiological changes in substantia nigra after striatal infarction

The GABAergic efferent pathway from the striatum exerts inhibitory control on the substantia nigra pars reticulata (SNR) neurones. We studied sequential changes in spontaneous single-unit activities in the ipsilateral SNR 1 h, 1 day, 7 days and 14 days after striatal infarction induced by middle cerebral artery occlusion (MCAO) in rats. Compared with a sham-operated group, there was no change in the firing rate 1 h after MCAO, and one day after MCAO the mean firing rate decreased. The firing rate of SNR neurones at 7 and 14 days after MCAO was significantly reduced compared with the sham-operated group. Only two neurones (sham-operated group and 1 h after MCAO group) fired with high frequency. Histological examination revealed degeneration of the ipsilateral SNR 7 and 14 days after MCAO. Our results indicated that SNR neuronal degeneration accompanied by striatal ischaemia does not simply depend on hyperexcitation due to the activation of a disinhibition mechanism.

Electrophysiological changes in substantia nigra after striatal infarction

The GABAergic efferent pathway from the striatum exerts inhibitory control on the substantia nigra pars reticulata (SNR) neurones. We studied sequential changes in spontaneous single-unit activities in the ipsilateral SNR 1 h, 1 day, 7 days and 14 days after striatal infarction induced by middle cerebral artery occlusion (MCAO) in rats. Compared with a sham-operated group, there was no change in the firing rate 1 h after MCAO, and one day after MCAO the mean firing rate decreased. The firing rate of SNR neurones at 7 and 14 days after MCAO was significantly reduced compared with the sham-operated group. Only two neurones (sham-operated group and 1 h after MCAO group) fired with high frequency. Histological examination revealed degeneration of the ipsilateral SNR 7 and 14 days after MCAO. Our results indicated that SNR neuronal degeneration accompanied by striatal ischaemia does not simply depend on hyperexcitation due to the activation of a disinhibition mechanism.

G protein-coupled inward rectifier modulated by dopamine agonists in cultured substantia nigra neurons.

Dopamine release from dopaminergic neurons in the substantia nigra plays an important role in regulating their activity. In the present experiments the whole-cell clamp technique was used to elucidate the modulatory role of dopamine on K+ conductance in substantia nigra neurons in culture. In pars compacta neurons held near the resting potential, dopamine and quinpirole (a D2 receptor agonist) increased membrane conductance. This response was blocked by S(-)-sulpiride, a D2 receptor antagonist. The current-voltage relation of the quinpirole-induced current showed an inward rectification with a reversal potential that approximately equaled the K+ equilibrium potential. Intracellular application of a non-hydrolysable GTP analogue, guanosine 5'-O-(3-thiotriphosphate; 0.1-0.5 mM) into pars compacta neurons produced an initial spontaneous increase in an inwardly rectifying K+ conductance. Quinpirole markedly accelerated this conductance increase. In cells loaded with a poorly metabolized GDP analogue, guanosine 5'-O-(2-thiodiphosphate), the quinpirole response was diminished. Treatment of pars compacta neurons with pertussis toxin abolished the D2 response. Intracellular application of cyclic AMP (1 mM) neither mimicked nor occluded the D2 response. These results indicate that D2 receptor agonists produce neuronal inhibition by activating an inwardly rectifying K+ current, that this D2 agonist effect is mediated by a pertussis toxin-sensitive G protein, and that cyclic AMP does not mediate the D2 response. Unexpectedly, in cells loaded with guanosine 5'-O-(3-thiotriphosphate) after the inwardly rectifying K+ conductance spontaneously reached a maximum, it started to decline slowly. In cells loaded with guanosine 5'-O-(2-thiodiphosphate), the quinpirole response became diminished at first, but then the membrane conductance slowly increased, together with an increase in the quinpirole response. These results were explained by an interplay of two G proteins, one producing an increase and the other producing a decrease in the K+ conductance. In some pars reticulata neurons, R(+)-SKF-38393 (a D1 receptor agonist) reduced an inwardly rectifying K+ conductance; this would excite the neuron under current clamp conditions. The present study shows that cultured substantia nigra neurons produce functional responses to dopamine agonists. Using these neurons, the ionic mechanisms and signal transduction mechanisms of the responses were elucidated in a more critical manner than in previous studies.

Excitation of rat substantia nigra pars reticulata neurons by 5-hydroxytryptaminein vitro: Evidence for a direct action mediated by 5-hydroxytryptamine2C receptors

Single-unit extracellular and whole-cell patch clamp recording were used to study the actions of exogenously applied 5-hydroxytryptamine on substantia nigra pars reticulata neurons in parasaggital slices of rat midbrain. Seventy-six per cent of substantia nigra pars reticulata cells (254/334) recorded extracellularly were excited by 5-hydroxytryptamine (EC50 = 9.56 microM); in the remainder, inhibitions (13.5%), biphasic responses (4.2%) or lack of response (6.3%) were observed. Using whole-cell patch recording, 5-hydroxytryptamine (10 microM) caused either an inward current (9/9 cells) or a depolarization (3/3 cells) at membrane potentials in the range -50 to -90 mV, which was resistant to tetrodotoxin (4/4 cells), indicating that the predominant, excitatory action of 5-hydroxytryptamine was due to a direct action on substantia nigra pars reticulata neurons. The 5-hydroxytryptamine excitation (recorded extracellularly) was reduced to 24 +/- 6% of control values by methysergide (0.1 microM) and to 17 +/- 5% of control by ketanserin (10 microM), but was unaffected by the 5-hydroxytryptamine antagonists spiperone (0.1 microM), yohimbine (0.1 microM), pindolol (1 microM), GR113808A (1 microM) or ICS 205930 (10 microM). In addition, the 5-hydroxytryptamine excitation was mimicked by the 5-hydroxytryptamine2C receptor--preferring agonist alpha-methyl 5-hydroxytryptamine (10 microM), but the agonists CP93, 129 (0.1-1 microM) and (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (0.1-1 microM) were without effect. Taken together, this pharmacology indicated involvement of the 5-hydroxytryptamine2C receptor in the 5-hydroxytryptamine excitation, while other candidate receptors known to be present in rat substantia nigra pars reticulata (5-hydroxytryptamine1B, 5-hydroxytryptamine2A and 5-hydroxytryptamine4) could be excluded from consideration. While in accord with current information on the location of 5-hydroxytryptamine receptor subtypes in substantia nigra pars reticulata, and the consequence of activation of neuronal 5-hydroxytryptamine2C receptors, these results contrast with data from in vivo experiments which suggest that the net effect of 5-hydroxytryptamine is to inhibit substantia nigra pars reticulata neurons. The reason for this apparent discrepancy may lie in detailed consideration of the microcircuitry of the substantia nigra pars reticulata. This may lead to a re-evaluation of the influence of 5-hydroxytryptamine on this basal ganglia output relay nucleus, and its role in motor control and the gating of generalized seizure activity.

Reduction in firing rate of substantia nigra pars reticulata neurons by valproate: influence of different types of anesthesia in rats

Nondopaminergic, presumably GABAergic neurons in the substantia nigra pars reticulata (SNR) are thought to function as a gating mechanism for seizure propagation. Systemic administration of anticonvulsant doses of the antiepileptic drug valproate (VPA) has previously been reported to inhibit the firing of nondopaminergic SNR neurons in anesthetized but not in awake, paralyzed and locally anesthetized rats, suggesting that the findings in anesthetized rats were due to an interaction between VPA and the general anesthetic used. In the present study, we determined the influence of different anesthetic measures on the effect of an anticonvulsant dose of VPA (100 mg/kg) on extracellularly recorded spontaneous single unit activity of nondopaminergic SNR neurons in rats. Rats were anesthetized by continuous infusion of the general anesthetic chloral hydrate, the dissociative anesthetic ketamine or the narcotic opioid fentanyl, or were only locally anesthetized and paralyzed. VPA significantly reduced SNR firing in all groups with a time course that matched its anticonvulsant time course in rodents. However, VPA's inhibitory effect on SNR firing was significantly less marked under anesthesia with chloral hydrate than in any of the other groups, indicating that this anesthetic suppresses the action of VPA, which may be related to an interaction with GABA-related processes in the SNR. The closest approximation to the effect of VPA in awake rats was obtained under anesthesia with ketamine, while VPA's inhibitory action on SNR neuronal firing seemed to be enhanced in the fentanyl group, which exhibited the highest baseline firing rates of all groups. Determination of VPA in the SN showed that the difference in VPA's inhibitory effect on SNR neurons was not secondary to differences in local drug concentrations. The data demonstrate that VPA is capable of significantly slowing the spontaneous activity of nondopaminergic SNR neurons, but that the magnitude of this effect depends on the anesthetic measures used. In view of the presumed role of SNR neurons in seizure propagation and the finding that VPA consistently inhibits these neurons at an anticonvulsant dose, the present data suggest that suppression of spontaneous SNR neuronal firing may be an important mechanism through which VPA exerts its anticonvulsant properties.

Reduction of apomorphine-induced rotational behaviour by subthalamic lesion in 6-OHDA lesioned rats is associated with a normalization of firing rate and discharge pattern of pars reticulata neurons.

The effect of subthalamic nucleus (STh) lesion on apomorphine-induced rotational behaviour and unit activity of substantia nigra pars reticulata (SNr) neurons was studied in normal, sham-control and unilateral 6-OHDA-lesioned rats [SN pars compacta (SNc)-lesioned]. In the latter, contraversive rotational behaviour was greatly reduced by an additional ipsilateral STh lesion. A moderate ipsiversive rotation was observed in rats with a single STh lesion. Concurrently, SN unit extracellular recordings were performed in age-matched normal rats, sham-controls for both lesions, STh-lesioned rats, SNc-lesioned rats, and SNc-lesioned rats with an ipsilateral STh lesion (SNc+STh-lesioned). Pars reticulata neurons had a higher mean firing rate in SNc-lesioned rats than in control rats. Furthermore, 68% of SNr neurons in SNc-lesioned rats had a tonic discharge pattern (against 92.3% in control rats) and 32% a mixed or bursting pattern. After STh lesion, a clear decrease in SNr firing rate was observed in SNc-lesioned rats. Moreover, STh lesion improved interspike interval regularity and decreased the occurrence of bursting patterns. In rats with a single STh lesion, the firing rate was no different from that of the sham-controls but the discharge pattern was more regular. These data show that STh lesion decreased apomorphine-induced rotational behaviour in dopamine-depleted animals. This effect could be related to the suppression of the excitatory effect of STh efferents on the SNr neurons. STh lesion both counterbalanced the increased activity of SNr neurons and regularized their discharge pattern.

Effects of a cannabinoid on spontaneous and evoked neuronal activity in the substantia nigra pars reticulata

Single unit electrophysiology was used to explore the role of cannabinoid receptors in the substantia nigra pars reticulata. Intravenous and intraperitoneal injections of the potent and selective synthetic cannabinoid ( R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthalenyl) methanone (WIN 55,212-2) produced modest but significant increases in the spontaneous firing rate of neurons in the substantia nigra pars reticulata. In a second set of experiments, WIN 55,212-2 (up to 1.0 mg/kg i.v.) antagonized the inhibition of firing produced in the substantia nigra pars reticulata by electrical stimulation of the striatum. The pharmacological specificity of this effect was demonstrated using the inactive enantiomer WIN 55,212-3. The possibility that WIN 55,212-2 exerts its effects by regulating γ-aminobutyric acid (GABA) release from striatonigral fibers was suggested by the observation that bicuculline (up to 0.5 mg/kg i.v.) reversed the effect of striatal stimulation. It thus appears that cannabinoid receptors on striatonigral neuron terminals may regulate movement by disinhibiting the activity of substantia nigra pars reticulata neurons, perhaps by inhibiting the release of GABA into the substantia nigra pars reticulata.

GABA and GABAA receptor changes in the substantia nigra of the rat following quinolinic acid lesions in the striatum closely resemble Huntington's disease

GABA and GABAA receptors have been studied in the substantia nigra of the rat following quinolinic acid lesions in the striatum. The regional distribution of GABA and GABAA receptors was investigated using immunohistochemical techniques with monoclonal antibodies to GABA and to the beta 2.3 subtypes of the GABAA receptor complex. The distribution, density and cellular localization of GABAA receptors were studied using quantitative receptor autoradiography and 6-hydroxydopamine-induced degeneration of dopaminergic pars compacta neurons. The subunit configuration of GABAA receptors was investigated using in situ hybridization histochemistry and subunit subtype-specific oligonucleotide probes. The results showed that in the normal substantia nigra GABA and GABAA receptors were mainly localized within the pars reticulata. GABAA receptors were mainly of the BZI variety, had a subunit subtype configuration that included alpha 1 and beta 2.3 subtypes, and showed a rostrocaudal gradient in the density of receptors; the density of receptors in the caudal third was 56% higher than that in the rostral third of the pars reticulata. Following quinolinic acid-induced degeneration of the striatonigral pathway, there was a marked loss of GABA immunoreactivity and a 59% increase in the density of GABAA receptors in the substantia nigra pars reticulata. There was a corresponding regional topography in the pattern of loss of GABA immunoreactivity and in the pattern of increase in GABAA receptors in the pars reticulata; the topography varied with the size and placement of the lesion in the striatum and correlated with the known topographical organization of the striatonigral projection. The quantitative autoradiographic results showed that following quinolinic acid lesions in the striatum: (i) the greatest increase in the density of GABAA receptors occurred in the middle third (91% increase) of the pars reticulata; (ii) the receptors were mainly of the GABAA/BZI variety; and (iii) 6-hydroxydopamine-induced degeneration of the dopaminergic pars compacta neurons did not significantly affect the density of receptors, indicating that the increased receptor binding was mainly localized on non-dopaminergic pars reticulata neurons. The immunohistochemical and in situ hybridization studies showed that, as in the normal substantia nigra, GABAA receptors in the substantia nigra pars reticulata on the lesioned side contained the alpha 1 and beta 2.3 GABAA receptor subtypes; the alpha 1 and beta 2.3 subtypes (but not the alpha 2) were increased after quinolinic acid lesions.(ABSTRACT TRUNCATED AT 400 WORDS)

Rat substantia nigra pars reticulata neurones are tonically inhibited via GABA A, but not GABA B, receptors in vitro

Extracellular single unit recordings were made from substantia nigra pars reticulata (SNr) neurones in slices of rat brain. Cells fired spontaneous action potentials at 11.4 ± 0.8Hz. The GABA A receptor agonist isoguvacine (1–10 μM) reduced firing rate in a concentration-dependent manner [50% of maximal inhibition (IC 50) with 3.2 μM], as did the GABA B agonist baclofen (0.3–10 μM; IC 50 1.4 μM). The GABA A antagonist bicuculline (30 μM) not only blocked the action of isoguvacine, but also increased the basal firing rate to 187.5 ± 12.6% of control. The GABA B antagonist CGP 55845A (0.1 μM), while blocking the inhibitory action of baclofen, was without effect on spontaneous firing rate, as was strychnine (10 μM), the antagonist of glycine and taurine, and also Met-enkephalin (10 μM). Tiagabine (50 μM), the blocker of GABA uptake, caused an inhibition of firing which could be reversed with bicuculline (30 μM) but not CGP 55845A (1 μM). We conclude that the firing rate of SNr neurones is under tonic inhibition by GABA in vitro, which can be relieved by antagonists of GABA A, but not GABA B receptors, and enhanced by blockade of GABA reuptake. The source of this GABA tone is likely to be from recurrent axon collaterals of SNr neurones themselves.

Synaptic connections between pars compacta and pars reticulata neurones: electrophysiological evidence for functional modules within the substantia nigra

Intracellular recordings were performed in vitro from both pars compacta and pars reticulata neurones of the substantia nigra, and their postsynaptic responses to electrical stimulation within the nucleus were analysed. Intracellular staining by biocytin was used to reveal the morphology and location of the recorded neurone and its position and distance to the stimulating electrode. Inhibitory postsynaptic potentials in pars compacta neurones were evoked exclusively from a specific region of stimulation within the pars reticulata; this field could be mapped out as a cone-shaped region surrounding the apical dendrite of the cell and perpendicular to the plane of the pars compacta. Furthermore, hemitransection, prior to the experiments which eliminated the most likely source of extrinsic inhibition (the GABAergic striatonigral pathway) affected neither the generation of inhibitory postsynaptic potentials nor its topographic pattern during pars reticulata stimulation. In contrast to the response of dopaminergic pars compacta neurones, pars reticulata neurones responded to stimulation over wide areas of substantia nigra, without any clear site-specific selectivity. It is concluded that within the substantia nigra, dopaminergic neurones are arranged in functional modules such that most inhibition is derived from a highly local circuit with the collaterals of adjacent pars reticulata cells. This module is intrinsic to the substantia nigra and may represent the basic functional unit of the nucleus.

Occurrence of heat shock response in deafferented neurons in the substantia nigra of rats

The substantia nigra is innervated by massive inhibitory GABAergic projections from the striatum and globus pallidus, deafferentation of which is supposed to lead to anterograde trans-synaptic degeneration of the nigral neurons. An immunohistochemical method was used to examine the induction of 72,000 mol. wt heat shock protein in the substantia nigra following cerebral hemitransection or transient middle cerebral artery occlusion. At three and four days post-transection, strong immunoreactivity for 72,000 mol. wt heat shock protein was found in the ipsilateral substantia nigra pars reticulata. Light microscopic observation revealed a number of pars reticulata neurons showing strong immunoreactivity for 72,000 mol. wt heat shock protein in their perikarya and proximal processes. In addition, Golgi-like stained neurons with dystrophic features were occasionally observed in the ipsilateral substantia nigra pars reticulata. The immunoreactivity for 72,000 mol. wt heat shock protein in the ipsilateral pars reticulata gradually declined and almost disappeared by 15 days after transection. No apparent induction of 72,000 mol. wt heat shock protein was found in the substantia nigra pars compacta throughout the time period examined. Massive striatal ischémie injury produced by transient middle cerebral artery occlusion also induced expression of 72,000 mol. wt heat shock protein in the pars reticulata neurons three and four days postoperatively. These findings suggest that deafferentation of the striatal or striatopallidal inputs per se is a harmful stress for the substantia nigra pars reticulata neurons, inducing 72,000 mol. wt heat shock protein synthesis. The present data may contribute to our understanding of the molecular basis of the pathomechanism of the transneuronal regression of substantia nigra pars reticulata neurons, which may occur after removal of inhibitory GABAergic inputs.

D1 agonist-induced excitation of substantia nigra pars reticulata neurons: mediation by D1 receptors on striatonigral terminals via a pertussis toxin-sensitive coupling pathway

Iontophoresis of dopamine or the D1 agonist SKF 38393 has been shown to elicit current-dependent increases in the firing of rat substantia nigra pars reticulata neurons, suggesting a discrete physiological role for the D1 dopamine receptor population in the substantia nigra. The effects of SKF 38393 differed from those of dopamine, however, in that the D1 agonist also augmented inhibitory responses to applied GABA, whereas dopamine and D2-like agonists were previously found to attenuate responses to GABA. The present studies involved various manipulations of the nigral D1 receptors in order to examine the pharmacological specificity, receptor localization, and second messenger coupling underlying the D1 agonist response. The excitatory and GABA-potentiating effects of SKF 38393 were found to be attributable to D1 receptor stimulation, rather than a nonspecific action, since (1) the effect was mimicked by iontophoresis of A-68930, a D1 agonist of a different structural class than SKF 38393, and (2) the response to SKF 38393 was prevented by intranigral injection of the receptor inactivator N-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ; 50 nmol/0.5 microliter) 1 d before, or the D1 antagonist SCH 23390 (1 microgram/microliter) 1 hr before electrophysiological testing. Additional studies revealed that the involved D1 receptors were located presynaptically on striatonigral terminals. For instance, in rats given ipsilateral striatal kainic acid lesions 1 week earlier, application of SKF 38393 failed to elicit the usual increases in cell firing, but loss of the response was observed only among the group of pars reticulata neurons that were shown to be unresponsive to striatal stimulation (i.e., those whose striatonigral inputs had been terminated by the lesion). Finally, to examine the second messenger coupling characteristics of the involved D1 receptors, several membrane-permeable analogs of cAMP were tested iontophoretically in place of SKF 38393. Surprisingly, none of these compounds gave a pattern of response typical of the D1 agonist, raising questions about the involvement of cAMP. Even more suggestive of an unconventional D1 coupling pathway, the excitatory and GABA-potentiating effects of applied SKF 38393 were completely abolished by prior intranigral injection of the G(i)/G(o) protein inactivator, pertussis toxin. Collectively, these results suggest that stimulation of D1 receptors on striatonigral terminals causes an excitation of substantia nigra pars reticulata neurons with an exaggerated responsiveness to GABA, and the effects appear to be mediated by a pertussis toxin-sensitive (i.e., a non-G-like) G-protein and possibly a second messenger other than cAMP.

Increased expression of growth-associated protein GAP-43/B-50 following cerebral hemitransection or striatal ischemic injury in the substantia nigra of adult rats

The substantia nigra receives massive inhibitory γ-aminobutyric acid (GABA)-ergic inputs, deafferentation of which is supposed to lead to anterograde transsynaptic regression of the nigral neurons. An immunohistochemical technique was used to examine growth-associated protein GAP-43 expression following cerebral hemitransection or transient middle cerebral artery occlusion (MCAO) in the substantia nigra of adult rats. GAP-43 expression was transiently elevated in the substantia nigra pars reticulata (SNr) neurons, but not in the pars compacta neurons at 3 and 4 days post-hemitransection. Massive striatal ischemic injury produced by transient MCAO also caused an increase in GAP-43 synthesis in the SNr neurons at 3 and 4 days after operation. The present findings raise the possibility that deafferentation of the GABAergic inputs leading to transneuronal regression of the SNr neurons is responsible for the elevated expression of GAP-43 in the substantia nigra of adult rats.

Polymodal sensory and motor convergence in substantia nigra neurons of the awake monkey

Responses of substantia nigra (SN) pars compacta (SNc) and reticulata (SNr) neurons to sensory stimulation and movement were investigated in awake Macaca fascicularis. Concern was assigned to polysensory and sensory / motor convergence on the same cell, which were found in 12% and 9% of the neurons, respectively. Others were modality specific (35%) or unresponsive (44%). The convergence lends support to the notion that SN plays an important role in sensory-motor integration.