Presynaptic modulation of the release of GABA by GABAA receptors in pars compacta and by GABAB receptors in pars reticulata of the rat substantia nigra

The effect of GABA agonists and antagonists on K⁺-stimulated [³H]GABA release was studied to assess how presynaptic GABA receptors modulate GABA release. The release was affected in a quite different manner in the pars compacta and in the pars reticulata. Muscimol markedly inhibited the release from the pars compacta but had no effect on the release from the pars reticulata. Baclofen inhibited the release from the pars reticulata without affecting the release from the pars compacta. Bicuculline itself facilitated the release from the pars compacta but inhibited the release from the pars reticulata. Picrotoxin facilitated the release from the pars compacta and had no effect in the pars reticulata. The results suggest that the release of GABA from GABAergic terminals in the substantia nigra of the rat brain is modulated by GABA_A autoreceptors in the pars compacta and by GABA_B receptors in the pars reticulata.     

Role of pars compacta of the substantia nigra in circling behavior

The significance of pars compacta of the substantia nigra in the mediation of ci circling behavior was assessed by placing unilateral radio-frequency lesions in that neural region, in adjoining tegmental areas, or in the median raphe and adjacent reticular formation; additional ablations were made by infusing 8 μg/4 μl 6-hydroxydopamine (6-OHDA) into the ventral mesencephalon. Rotational behavior was recorded in a photocell apparatus during 15 min sessions. Precise and virtually complete lesions of pars compacta, as evidenced by histological and histochemical analyses, resulted in spontaneous contralateral circling for a postoperative period of 5 days. More vigorous and sustained contralateral rotations were observed in animals with lesions in the region of the median raphe. Other thermal or 6-OHDA mesencephalic lesions did not produce spontaneous circling. On postoperative days 9–23, the animals were administered in random sequence d-amphetamine (1.5 and 5.0 mg/kg) and apomorphine (1.0 mg/kg). At least 48 hr separated each drug test. Amphetamine (1) facilitated contralateral turning in animals with damage to the median raphe and adjacent regions but induced ipsilateral circling in animals sustaining variable degrees of pars compacta damage as a result of midbrain 6-OHDA injections, (2) produced ipsilateral turning in rats with lesions involving portions of pars compacta in addition to the medial lemniscus (at 1.5 but not 5.0 mg/kg drug), pars reticulata (at 5.0 but not 1.5 mg/kg drug), or the ventromedial tegmentum and, in some cases, subthalamic regions (at 5.0 but not 1.5 mg/kg drug), and (3) had no effect upon circling in animals with discrete pars compacta lesions in a dose of 1.5 mg/kg but produced ipsiversive circling after 5.0 mg/kg. Apomorphine did not elicit turning in animals with relatively precise lesions of pars compacta but did produce turning in experimental groups that sustained simultaneous damage to pars compacta and additional systems including portions of the ventromedial tegmentum, pars reticulata of the substantia nigra, or medial lemniscus. The contralateral turning induced by apomorphine in rats with 6-OHDA or radio-frequency lesions of the ventral mesencephalon suggests that mechanisms mediating this drug-induced behavior, although previously attributed almost exclusively to dysfunctions in nigral function, probably include systems involving extra-nigral regions as well. Coupling this consideration with the finding that lesions in the median raphe and contiguous areas elicited more intense circling than nigral ablations suggests that the role of pars compacta in rotational behavior may be less significant than previously believed.     

Presynaptic GABA autoreceptors on GABAergic nerve endings of the rat substantia nigra.

The release of 3H-GABA evoked by exposure to 30 mM potassium during 1 min was found to calcium independent in the rat occipital cortex and calcium dependent in the substantia nigra. Exposure to either muscimol 1 microM or GABA 1 microM inhibited the potassium-evoked release of 3H-GABA from the substantia nigra but not from the occipital cortex. The inhibitory effect of muscimol 1 microM on the potassium-evoked release of 3H-GABA from the substantia nigra was significantly antagonized by picrotoxin 10 microM. Exposure to either 10 or 100 microM pricrotoxin alone did not affect the potassium-evoked release of 3H-GABA. These results are compatible with the presence of a negative feedback mechanism in bagaergic nerve terminals of the rat substantia nigra which is mediated by presynaptic GABA autoreceptors. In addition, these results emphasize the importance of the calcium-dependent nature of the release of the neurotransmitter for demonstrating the modulation of transmitter release through presynaptic receptors.     

Zetidoline blocks dopamine autoreceptors in the rat substantia nigra

Zetidoline, applied microiontophoretically onto neurones in the substantia nigra, zona compacta of anaesthetized rats, reversibly blocked the depressant effect of iontophoretically-applied dopamine. The inhibitory actions of gamma-aminobutyric acid (GABA) and glycine and the excitatory action of glutamate were unaffected by the antagonist. Zetidoline applied by bath perfusion, was similarly an antagonist of the inhibitory effect of bath applications of dopamine on neurones in the zona compacta of slices of substantia nigra; the pA2 for zetidoline in this preparation was 7.0.     

Release of endogenous GABA from the substantia nigra is not controlled by GABA autoreceptors

Summary The characteristics of the release of GABA from slices of the rat substantia nigra, elicited by electrical stimulation at frequencies of 0.5–48 Hz and by elevated K⁺ concentrations ranging from 15–35 mmol/l, was studied. Comparisons were made with cortical slices where the data were not available from previous studies.No GABA release could be evoked from rat nigral slices by electrical stimulation between 0.5 and 4 Hz, in contrast to cortical slices, in which this pool is sensitive towards inhibition by (–)-baclofen. Also, comparatively less GABA release could be evoked from nigral than from cortical slices by K⁺ concentrations between 15 and 25 mmol/l. While (–)-baclofen at 10 mol/l inhibited release caused by 15 mol/l K⁺ in cortical, it did not in nigral slices. GABA release caused by higher frequencies (8–48 Hz) or 30 mmol/l K⁺ concentrations was Ca²⁺-dependent and in the former case also tetrodotoxin-sensitive. It had similar characteristics as in cortical slices and was insensitive towards (–)-baclofen, muscimol and bicuculline. Even more markedly than in the cortex, 30 mmol/l K⁺ released greater amounts of GABA than electrical stimulation at 24 Hz of a similar duration, suggesting the existence of one or several additional pool(s) of lesser excitability.Since the majority of gabaergic nerve endings in the nigra belong to striato- and pallidonigral projection neurons and those in the cortex probably exclusively to various types of interneurons, it seems that (a) one or several of the latter release GABA at low frequencies in a baclofen-sensitive manner and are absent or rare in the s. nigra, and (b) the striato- and pallidonigral projection neurons are not controlled by presynaptic autoreceptors of the GABA_A or GABA_B type, because neither GABA release elicited by electrical stimulation nor by 30 mmol/l K⁺ was affected by agents interfering with these types of receptors.Send offprint requests to P. C. Waldmeier at the above address     

Nicotine modulates GABAergic transmission to dopaminergic neurons in substantia nigra pars compacta

Aim： Dopaminergic neurons in the substantia nigra pars compacta （SNc） play important roles in motor control and drug addiction. As the major afferent, GABAergic innervation controls the activity of SNc dopaminergic neurons. Although it is clear that nicotine modulates SNc dopaminergic neurons by activating subtypes of somatodendritic nicotinic acetylcholine receptors （nAChRs）, the detailed mechanisms of this activation remain to be addressed. Methods： In the current study, we recorded GABAA receptor-mediated spontaneous inhibitory postsynaptic currents （sIP- SCs） from dissociated SNc dopaminergic neurons that were obtained using an enzyme-free procedure. These neurons preserved some functional terminals after isolation, including those that release GABA. Results： We found that both extra- and intra-cellular calcium modulates sIPSCs in these neurons. Furthermore, both nicotine and endogenous acetylcholine enhance the frequency of sIPSCs. Moreover, endogenous acetylcholine tonically facilitates sIPSC frequency, primarily by activating the α4β2 nAChRs on the GABAergic terminals. Conclusion： Nicotine facilitates GABA release onto SNc dopaminergic neurons mainly via the activation of presynaptic α4β2 nAChRs.     

Tolbutamide- and diazoxide-sensitive K+ channel in neurons of substantia nigra pars reticulata

Summary Single-channel K⁺ currents were recorded in cell-attached patches from slices of rat substantia nigra. On the somata of neurons in the caudal half of the substantia nigra pars reticulata a K⁺ selective channel with a unitary conductance of 71 pS (154 mmol/l K⁺ in pipette filling solution) was identified. The channel was activated both by application of diazoxide (300 mol/l) and by energy-depleting conditions (200 mol/l cyanide) and was reversibly blocked by tolbutamide (0.1–1 mmol/l). It is concluded that neurons in the substantia nigra pars reticulata of the rat contain a typical ATP-sensitive K⁺ channel the activity of which can be modulated by diazoxide and sulfonylureas.Correspondence to: C. Schwanstecher at the above address     

Paraquat inhibits postsynaptic AMPA receptors on dopaminergic neurons in the substantia nigra pars compacta

Parkinson's disease (PD) is a neurodegenerative disease that mainly affects dopaminergic (DA-ergic) neurons in the substantia nigra pars compacta (SNc). Glutamate modulates neuronal excitability, and a high concentration of glutamatergic receptors is found on DA-ergic neurons in the SNc. Paraquat (PQ) is a putative causative agent for PD. Its effects on synaptic glutamate transmission in SNc DA-ergic neurons were evaluated using whole-cell voltage-clamp recording in brain slices from 7- to 14-day-old Wistar rats. In the presence of bicuculline (BIC), strychnine, and dl-aminophosphonovaleric acid, PQ reversibly suppressed AMPA receptor-mediated evoked excitatory postsynaptic currents (eEPSCs) in a concentration-dependent manner (P < 0.05). In the presence of tetrodotoxin (1 μM), PQ (50 μM) significantly reduced the amplitudes, but not the frequencies, of miniature EPSCs in the SNc, suggesting PQ inhibited eEPSCs through a postsynaptic mechanism. Exogenous application of AMPA to induce AMPA-mediated inward currents excluded involvement of a presynaptic response. The AMPA-induced currents in the SNc were significantly reduced by PQ (50 μM) to 74% of control levels (P < 0.05), supporting that PQ acts on postsynaptic AMPA receptors. No effect of PQ on eEPSCs was seen in the LD thalamic nucleus and hippocampus, showing PQ specifically inhibited DA-ergic neurons in the SNc. Our results demonstrate a novel mechanism of action of PQ on glutamate-gated postsynaptic AMPA receptors in SNc DA-ergic neurons. This effect may attenuate the excitability and function of DA-ergic neurons in the SNc, which may contribute to the pathogenesis of PD.     

Regulation of substantia nigra pars reticulata neuronal activity by excitatory amino acids

Midbrain non-dopaminergic neurons of the substantia nigra pars reticulata play an important role in the basal ganglia circuitry. The regulation of their electrical activity by excitatory amino acid (EAA) inputs was investigated using in vivo electrophysiological methods in chloral hydrate-anaesthetized rats. We first determined the subtypes of EAA receptors present on reticulata neurons, using microiontophoretic application of selective agonists: kainic acid (KA), (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), N-methyl-D-aspartic acid (NMDA), and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD). Each agonist activated reticulata neurons and the apparent rank order of efficacy was: KA> or =AMPA=NMDA>trans-ACPD. Using pressure or iontophoretic microejections of ionotropic and metabotropic receptor antagonists, we then investigated EAA receptor subtypes involved in the spontaneous firing rate of reticulata neurons. Kynurenic acid and (+/-)-2-amino-5-phosphonopentanoic acid (AP-5) markedly decreased the spontaneous firing rate of reticulata neurons, while 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was much less effective. The metabotropic receptor antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) failed to affect the spontaneous electrical activity. In contrast to CNQX, microapplications of AP-5 sometimes produced total inhibition. This powerful effect may reflect the potential importance of NMDA receptors in regulating the activity of some reticulata neurons. These results indicate that both functional ionotropic (NMDA and non-NMDA) and metabotropic EAA receptors are present on non-dopaminergic substantia nigra pars reticulata neurons. Moreover, in the anaesthetized animal, the spontaneous firing rate of these neurons, mediated by EAA inputs, seems mainly due to the tonic activation of ionotropic, but not metabotropic, receptors.     

Modulation of synaptic GABAA receptor function by zolpidem in substantia nigra pars reticulata

AIM: The substantia nigra pars reticulata (SNr) constitutes one of the output centers of the basal ganglia, and its abnormal activity is believed to contribute to some basal ganglia motor disorders. Different lines of evidence revealed a major contribution of GABAA receptor-mediated synaptic inhibition in controlling the activity of SNr. The benzodiazepine binding site within the GABAA receptor is a modulation site of significant clinical interest. A high density of benzodiazepine binding sites has been reported in the rat SNr. In the present study, we investigate the effects of activating benzodiazepine binding sites in the SNr. METHODS: Whole-cell patch-clamp recordings and motor behavior were applied. RESULTS: Superfusion of zolpidem, a benzodiazepine binding agonist, at 100 nmol/L significantly prolonged the decay time of GABAA receptor-mediated postsynaptic currents. The prolongation on decay time induced by zolpidem was sensitive to the benzodiazepine antagonist flumazenil, confirming the specificity on the benzodiazepine site. Zolpidem at 1 micromol/L exerted a stronger prolongation on the decay time. A further experiment was performed on behaving rats. A unilateral microinjection of zolpidem into the rat SNr caused a robust contralateral rotation, which was significantly different from that of control animals receiving the vehicle injection. CONCLUSION: The present in vitro and in vivo findings that zolpidem significantly potentiated GABA currents and thus inhibited the activity of the SNr provide a rationale for further investigations into its potential in the treatment of basal ganglia disorders.     

Analysis of the effect of cannabinoids on GABAergic neurotransmission in the substantia nigra pars reticulata

The CB(1) cannabinoid receptor is widely distributed in the central nervous system. The substantia nigra pars reticulata (SNR) belongs to the brain regions with the highest density of CB(1) receptors. According to anatomical studies, most of the CB(1) receptors in the SNR are localized on terminals of striatonigral GABAergic neurons. The aim of the present study was to clarify the function of these receptors.Electrophysiological properties of SNR neurons were studied in brain slices with the patch-clamp technique. Inhibitory postsynaptic currents (IPSCs) were elicited in parasagittal slices by electrical stimulation in the internal capsule. The mixed CB(1)/CB(2) cannabinoid receptor agonist WIN55212-2 (1 microM and 10 microM) concentration dependently decreased the amplitude of IPSCs. CP55940, another mixed CB(1)/CB(2) cannabinoid receptor agonist, also lowered IPSC amplitude. Superfused alone, the CB(1)-selective antagonist SR141716A (1 microM) increased the amplitude of IPSCs. In interaction experiments, SR141716A (1 microM) prevented the inhibition produced by WIN55212-2 (1 microM). WIN55212-2 (1 microM) had no effect on GABAergic currents elicited by ejection of muscimol (1 mM) to the surface of the slices. WIN55212-2 (10 microM) did not influence the frequency and amplitude of spontaneously occurring IPSCs (sIPSCs) and the firing rate of SNR neurons.The results show that activation of CB(1) cannabinoid receptors inhibits GABAergic neurotransmission in the SNR. The likely mechanism is presynaptic inhibition of GABA release, since cannabinoids had no effects on currents evoked by direct stimulation of GABA(A) receptors by muscimol and on the amplitude of sIPSCs. The enhancement of IPSCs by the cannabinoid antagonist probably reflects continuous inhibition of GABAergic neurotransmission by an endogenous cannabinoid. SNR neurons receive GABAergic input from three sources: from the corpus striatum, the globus pallidus and from neighbouring SNR neurons. The observed inhibition of GABAergic neurotransmission was probably due to depression of the transmission between striatonigral axons and SNR neurons. No direct actions of cannabinoids on SNR neurons were observed in addition to this synaptic effect.     

Responses of rat substantia nigra compacta neurones to L-DOPA.

1. The electrophysiological actions of L-DOPA were studied on substantia nigra dopamine-containing neurones by use of intracellular recordings, current and voltage clamp, in vitro. 2. L-DOPA (3-300 microM), applied by superfusion, decreased the spontaneous firing activity of the neurones, hyperpolarized the membrane potential and produced an outward current. 3. These effects of L-DOPA were graded and were antagonized by sulpiride, a D2-receptor antagonist. 4. The L-DOPA activated membrane hyperpolarization persisted in slices exposed to tetrodotoxin while it was markedly reduced or abolished in solutions with no calcium plus cobalt (2 mM). 5. In slices treated with carbidopa, an inhibitor of the aromatic amino acid decarboxylase enzyme, the actions of L-DOPA were greatly reduced. 6. We propose that L-DOPA inhibits the firing of substantia nigra zona compacta neurones by being converted to dopamine which is then released by these cells.     

Effects of methamphetamine on dopamine cells in the substantia nigra pars compacta and the ventral tegmental area

The ability of methamphetamine to inhibit the firing rate of dopamine cells in the substantia nigra pars compacta (SNC) and the ventral tegmental area (VTA) was studied. Methamphetamine reduced the firing rates of the dopamine cells in a dose-dependent manner in the SNC and the VTA. The doses of methamphetamine required to produce a 50% inhibition of firing rate in the SNC and the VTA were 0.37 mg/kg and 0.28 mg/kg, respectively.     

Characterization of extracellular GABA in the substantia nigra reticulata by means of brain microdialysis

Summary Brain microdialysis was used to characterize extracellular gamma-aminobutyric acid (GABA) in the substantia nigra reticulata (SNR) of freely moving rats. The extracellular GABA in the SNR was characterized using acutely implanted probes (4–8 h after surgery; day 1) and chronically implanted probes (24 h after surgery; day 2).3-Mercaptopropionic acid, a glutamic acid decarboxylase inhibitor, was used to identify GABA. This drug induced an immediate decrease in the extracellular GABA levels to 40% of basal values, suggesting that the detected GABA is, at least in part, newly synthesized.The basal levels of extracellular GABA measured either on day 1 or day 2 were not affected by infusion of micromolar amounts of tetrodotoxin. Therefore, a direct coupling between GABA dialysate concentrations and nerve-impulse flow does not seem to exist. Infusion of the GABA uptake inhibitor nipecotic acid (0.5 mmol/l) resulted in a 4-fold increase in the dialysate levels of GABA lasting at least for 3 h on both days. K⁺ stimulation (60 mmol/l) increased extracellular GABA levels in the SNR to 450% of basal values. This effect again did not differ significantly on day 1 and day 2.The origin of the extracellular GABA in the SNR, as recorded by microdialysis under the two experimental conditions, is discussed.Send offprint requests to W. Timmerman at the above address     

Presynaptic modulation of spontaneous inhibitory postsynaptic currents by gamma-hydroxybutyrate in the substantia nigra pars compacta.

The regulation of GABA release from the inhibitory input to dopamine cells in the substantia nigra pars compacta (SNc) plays a key role in different reward-related behaviors. Gamma-hydroxybutyrate (GHB) has therapeutical properties in various psychiatric disorders, especially in alcohol abuse. GHB is also used as a drug of abuse, which induces sedation and euphoria. Using whole-cell patch-clamp recordings, we studied the effects of GHB on GABA release in the SNc by recording spontaneous inhibitory postsynaptic currents (sIPSCs) in brain slices of 21- to 25-day-old rats. We found that GHB depressed the frequency and amplitude of sIPSCs, while the frequency and the amplitude of miniature inhibitory postsynaptic currents (mIPSCs), recorded in the presence of TTX, were not affected. However, in the presence of high extracellular potassium (15 mM), which increases the contribution of voltage-dependent calcium channels, GHB induced a reduction in the frequency of the mIPSCs without any effect on their amplitude. All of these effects were GABA(B)-independent and they were blocked by the GHB receptor antagonist NCS-382. The present results indicate that GHB inhibits spontaneous inhibitory synaptic transmission recorded from dopaminergic neurons in the SNc likely by reducing voltage-dependent calcium influx involved in presynaptic GABA release.     

The participation of substantia nigra zona compacta and zona reticulata neurons in morphine suppression of caudate spontaneous neuronal activities in the rat

Spontaneous, unitary activity from the caudate nucleus, substantia nigra zona reticulata and zona compacta were recorded extracellularly from lightly anesthetized rats. Intravenous injection of morphine (5 mg/kg) produced a depression of both caudate and reticulata discharges with a concomitant increase in the compacta activities. These effects were reversed by naloxone (1 mg/kg, i.v.). Microinjection of morphine (25, 50, and 100 μg) directly into the substantia nigra zona compacta induced a naloxone-reversible depression of caudate activity. Direct nigral application of strychnine (5 μg), a glycine antagonist, resulted in a moderate decrease in caudate activity and an increase in compacta discharges, but no appreciable effect on the reticulata neurons. Morphine (5 mg/kg, i.v.) further enhanced the strychnine effects on both caudate and compacta cells, while depressing the reticulata neurons. These results suggest that morphine may suppress caudate spontaneous activity via an activation of the nigrostriatal neurons and an inactivation of a novel striatonigral pathway. The latter may incorporate the reticulata neurons, which may exert a tonic inhibition on the compacta cells, possibly using glycine as the putative neurotransmitter.     

GABA(B) receptor activation inhibits dopamine D1 receptor-mediated facilitation of [(3)H]GABA release in substantia nigra pars reticulata

GABA(B) receptors inhibit and dopamine D1 receptors stimulate the release of GABA from striatal terminals in the pars reticulata of the substantia nigra. Here we have studied the interaction between both classes of receptors by exploring the effect of GABA(B) receptors upon the stimulation of depolarization-induced [(3)H]GABA release induced by the activation of D1 receptors in slices of the pars reticulata of the rat substantia nigra. The activation of GABA(B) receptors with baclofen (100 microM) inhibited by 48+/-8% the evoked [(3)H]GABA release in normal slices but did not modify the release in slices from reserpine-treated rats, indicating that the inhibition was dependent on endogenous dopamine. The inhibitory effect of baclofen was also abolished by the D1 receptor antagonist SCH 23390 (1 microM), indicating a D1 receptor-dependence of the baclofen inhibition. Baclofen dose-dependently inhibited (IC(50)=3.6 microM) the stimulation of release induced by the D1 agonist SKF 38393 (1 microM). Baclofen also blocked the stimulation of release induced by forskolin but not that induced by 8-Br-cAMP, indicating that the inhibitory effect was exerted before cAMP synthesis. N-ethylmaleimide (NEM), a selective inactivator of PTX-sensitive G-proteins, abolished the baclofen inhibition of the SKF 38393-induced stimulation of the release without affecting the stimulation induced by the D1 agonist, suggesting that the baclofen effect was mediated by Galpha(i/o) proteins. These results might have relevance in the control motor disorders associated with D1 receptor supersensitivity.     

Immunohistochemical evidence for localization of NMDAR1 receptor subunit on dopaminergic neurons of the rat substantia nigra, pars compacta

Non-fluorescent, double-labeling techniques were used in order to investigate whether NMDAR1 receptor subunits are localized on dopaminergic (i.e. tyrosine hydroxylase-positive) neurons of the rat substantia nigra, pars compacta. It has been found that NMDAR1 receptor subunits are highly abundant in the pars compacta neurons and their dendritic processes. It was also found that vast majority, if not all, of pars compacta neurons which are positive for the presence of NMDAR1 receptor subunits are dopaminergic ones. It is concluded that if NMDAR1 receptor subunits, an indispensable element of functional NMDA receptor ion channel complex, is co-assembled with other subunits of NMDA receptor ion channel complex, NMDA receptors might directly control the activity of dopaminergic neurons.     

Role of muscarinic cholinergic mechanisms in the substantia nigra pars reticulata in mediating muscular rigidity in rats

Summary Bethanechol chloride (5–25 g), when injected into the substantia nigra pars reticulata (SNR) of rats, produced muscular rigidity in a dose-dependent way, and in addition, catalepsy and ipsilateral posture. The effects of bethanechol in the dose of 25 g were prevented by coadministration of 10 g scopolamine hydrochloride. Injections of 25 g betanechol or 10 g scopolamine into the reticulata only slightly affected the muscular rigiditiy produced by 15 mg/kg i.p. morphine hydrochloride. The results suggest that muscarinic cholinergic mechanisms in the substantia nigra pars reticulata, although effective by themselves, affect by expression of at least one striatal functional alteration, the muscular rigidity, in a less effective way than GABAergic or endogenous opioid mechanisms do.     

Effects of chronic diazepam exposure on GABA sensitivity and on benzodiazepine potentiation of GABA-mediated responses of substantia nigra pars reticulata neurons of rats

This study examined the effects of chronic diazepam treatment on GABA sensitivity of substantia nigra pars reticulata neurons and on the ability of benzodiazepines to enhance GABAergic responses of these neurons in rats. Chronic diazepam exposure failed to significantly alter the sensitivity of reticulata neurons to microiontophoretically applied GABA. However, following chronic diazepam treatment for 1 day, or 1, 3 or 7-11 weeks, reticulata neurons showed tolerance to additional systemically or iontophoretically applied benzodiazepines and displayed an increased firing rate following injection of Ro 15-1788. These changes were not apparent 24 h after cessation of chronic treatment. Thus, tolerance to the effects of benzodiazepines on reticulata neurons appeared to develop after a single day of diazepam exposure and to dissipate by 24 h after cessation of treatment. When compared to our previous studies on dorsal raphe neurons, these results demonstrate regional differences in neuronal responses to chronic diazepam exposure, which may help elucidate neural systems which are involved in tolerance to the various functional aspects of benzodiazepines.