Differential expression of pre- and postsynaptic GABAB receptors in rat substantia nigra pars reticulata neurones

doi:10.1016/S0014-2999(98)00194-0

European Journal of Pharmacology

Volume 349, Issues 2–3, 22 May 1998, Pages 187-197

https://doi.org/10.1016/S0014-2999(98)00194-0 Get rights and content

Abstract

Whole-cell recordings were made from substantia nigra pars reticulata in rat midbrain slices to study the functional expression of pre- and postsynaptic GABA_B receptors in GABA output neurones. Baclofen (up to 300 μM) dose-dependently activated a weak current which was insensitive to tetrodotoxin and Ca²⁺-free solution but blocked by Ba²⁺ and 2-OH-saclofen. The maximum current activated by baclofen (30 μM) was 43.0±4.5 pA (n=27), representing only 23% of that in dopamine neurones. Baclofen (1–30 μM) also reduced the frequency of the GABA_A receptor-mediated miniature inhibitory postsynaptic currents while the distribution of their amplitudes was unaffected. This presynaptic effect of baclofen, prominent at a concentration as low as 1 μM, was sensitive to 2-OH-saclofen and occluded by Cd²⁺, but was unaffected by Ba²⁺. The results suggest a predominant role of the presynaptic GABA_B receptors in substantia nigra pars reticulata. The relative abundance of pre- and postsynaptic GABA_B receptor subtypes in this brain region may also be important in mediating the anticonvulsant effect of baclofen in rats.

Introduction

The substantia nigra pars reticulata is one of the output centers of the basal ganglia circuit. It contains a high concentration of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) (Ottersen and Storm-Mathisen, 1984). This observation suggests an important function of GABA-mediated neurotransmission in this area of the brain. In fact, GABAergic projections from striatum to substantia nigra pars reticulata, and from pars reticulata to thalamus, superior colliculus and tegmentum, are important in controlling normal voluntary movement (Chevalier and Deniau, 1990; Parent and Hazrati, 1995). The nigrotectal projection has also been found to play a role in controlling the propagation of epileptic seizures (Gale, 1989; Depaulis et al., 1994).

While the role of GABA_A receptors in substantia nigra pars reticulata has been studied extensively (Martin et al., 1978; Grace and Bunney, 1979; Reisine et al., 1979; Wurpel et al., 1988; Sakamoto and Hikosaka, 1989; Marksteiner et al., 1995), the importance of GABA_B receptors in this area has gained attention slowly. Previous studies do suggest a role of GABA_B receptors in both normal and pathological conditions. For example, it has been shown that intranigral administration of baclofen, the prototypic GABA_B receptor agonist, induces turning behaviour in rats (Waddington, 1977; Kaakkola, 1980). It has also been demonstrated that intranigral injection of baclofen suppresses flurothyl seizures in rats in an age-dependent manner (Sperber et al., 1989). It is effective in young (16 days old) rats but is ineffective in adult rats. This finding shows good parallel with the fact that the density of GABA_B receptor binding sites in substantia nigra is greater in young than adult rats (Garrant et al., 1992). It is therefore tempting to hypothesize that the age-dependency of the effect of baclofen on convulsion may be the consequence of changes in expression of GABA_B receptors. However, GABA_B receptors may exist pre- or postsynaptically. Since activation of the presynaptic receptors is likely to lead to a decrease of GABA release and therefore disinhibition of substantia nigra pars reticulata neurones while activation of the postsynaptic receptors would lead to a direct inhibition of these cells, it is necessary to have a knowledge of the relative abundance of these two types of GABA_B receptors.

In the present study, we make use of the advantage of the patch-clamp technique in resolving the pre- vs. postsynaptic actions of drugs on neurones. We quantified the postsynaptic response of substantia nigra pars reticulata neurones to baclofen, and compared with that of dopamine neurones in the same area. In addition, we tested the existence of presynaptic GABA_B autoreceptors on the inhibitory nerve terminals by examining the effect of baclofen on the GABA_A receptor-mediated miniature inhibitory postsynaptic currents. The results confirm the existence of pre- and postsynaptic GABA_B responses in substantia nigra pars reticulata neurones. Furthermore, the data indicate that postsynaptic GABA_B receptors are only weakly expressed, suggesting the dominance of presynaptic GABA_B receptors in mediating effects of GABA_B receptor agonists and antagonists in substantia nigra pars reticulata of young rats.

Section snippets

In vitro slice preparation

With approval of our Animal Research Ethics Committee, Sprague–Dawley rats aged 2 to 3 weeks were used for the preparation of acute brain slices. The animals were deeply anaesthetized (i.p. injection of 40 mg/kg of sodium pentobarbital) and then sacrificed by decapitation. The brains were rapidly removed and immediately placed in ice-cold artificial cerebrospinal fluid of the following composition (in mM): NaCl 125, KCl 2.0, MgSO₄ 1.2, CaCl₂ 2.5, KH₂PO₄ 1.2, glucose 11, and NaHCO₃ 26, which was

Identification of GABA and dopamine neurones

It is known that substantia nigra pars reticulata contains mainly GABA neurones but also some dopamine neurones (e.g., Yung et al., 1991). The latter are generally believed to be neurones displaced from the dorsal pars compacta. In the present study, the distinction between dopamine and non-dopamine, putative GABA neurones was based on the well-documented electrophysiological characteristics of dopamine cells (Grace and Onn, 1989; Yung et al., 1991; Häusser et al., 1995). Briefly, dopamine

Postsynaptic GABA_B receptors are weakly expressed in pars reticulata

Baclofen even at a saturating concentration (≥30 μM) can only activate a small outward current in substantia nigra pars reticulata GABA neurones at their resting membrane potentials. Nevertheless, the EC₅₀ obtained in the present study (4.5 μM and 9.2 μM for GABA and dopamine neurones respectively) is comparable to the value of 3 μM reported in a recent study on dissociated hippocampal CA3 neurones using the more active (−)-baclofen (Sodickson and Bean, 1996). The properties of the

Acknowledgements

The authors wish to thank Dr. G.L. Ye for her excellent technical assistance. W.H. Yung also wishes to thank Prof. B. Sakmann and Dr. M. Häusser for allowing him to learn the slice–patching technique in their laboratory. This work was supported by grants awarded to W.H.Yung by the Research Grants Council of Hong Kong (CUHK 227/94M and CUHK 4234/97M).

References (44)

N.G Bowery et al.
GABA_A and GABA_B receptor site distribution in the rat central nervous system
Neuroscience
(1987)
D.C.M Chu et al.
Distribution and kinetics of GABA_B binding sites in rat central nervous system: a quantitative autoradiographic study
Neuroscience
(1990)
A Depaulis et al.
Endogenous control of epilepsy: the nigral inhibitory system
Prog. Neurobiol.
(1994)
B Floran et al.
Presynaptic modulation of the release of GABA by GABA_A receptors in pars compacta and by GABA_B receptors in pars reticulata of the rat substantia nigra
Eur. J. Pharmacol.
(1988)
M.T Giralt et al.
GABA terminal autoreceptors in pars compacta and in pars reticulata of the rat substantia nigra are GABA_B
Eur. J. Pharmacol.
(1990)
A.A Grace et al.
Paradoxical GABA excitation of nigral dopaminergic cells: indirect mediation through reticulata inhibitory neurons
Eur. J. Pharmacol.
(1979)
M Häusser et al.
Axonal initiation and active dendritic propagation of action potentials in substantia nigra neurons
Neuron
(1995)
G.E Martin et al.
Contralateral turning evoked by the intranigral microinjection of muscimol and other GABA agonists
Brain Res.
(1978)
U Misgeld et al.
A physiological role for GABA_B receptors and the effects of baclofen in the mammalian central nervous system
Prog. Neurobiol.
(1995)
C Nitsch et al.
Immunocytochemical demonstration of GABAergic synaptic connections in the rat substantia nigra after different lesions of the striatonigral projection
Brain Res.
(1988)

A Parent et al.

Functional anatomy of the basal ganglia: I. The cortico-basal ganglia–thalamo–cortical loop

Brain Res. Rev.

(1995)

T.D Reisine et al.

The localization of receptor binding sites in the substantia nigra and striatum of the rat

Brain Res.

(1979)

C.E Rick et al.

Rat substantia nigra pars reticulata neurones are tonically inhibited via GABA_A, but not GABA_B receptors in vitro

Brain Res.

(1994)

M Sakamoto et al.

Eye movements induced by microinjection of GABA agonist in the rat substantia nigra pars reticulata

Neurosci. Res.

(1989)

Y Smith et al.

Neurons of the substantia nigra reticulata receive a dense GABA-containing input from the globus pallidus in the rat

Brain Res.

(1989)

E.F Sperber et al.

Evidence for the involvement of nigral GABA_B receptors in seizures of rat pups

Dev. Brain Res.

(1989)

G.J Stuart et al.

Mechanisms of presynaptic inhibition studied using paired-pulse facilitation

Neurosci. Lett.

(1991)

P Vincent et al.

Neighboring cerebeller Purkinje cells communicate via retrograde inhibition of common presynaptic interneurons

Neuron

(1993)

J.N Wurpel et al.

Age-related changes of muscimol binding in the substantia nigra

Brain Res.

(1988)

G.L Ye et al.

Pre-synaptic effect of the ATP-sensitive potassium channel opener diazoxide on rat substantia nigra pars reticulata neurons

Brain Res.

(1997)

G Chevalier et al.

Disinhibition as a basic process in the expression of striatal functions

Trends Neurosci.

(1990)

V.A Doze et al.

Calcium channel involvement in GABA_B receptor-mediated inhibition of GABA release in area CA1 of the rat hippocampus

J. Neurophysiol.

(1995)

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Differential expression of pre- and postsynaptic GABAB receptors in rat substantia nigra pars reticulata neurones

Abstract

Introduction

Section snippets

In vitro slice preparation

Identification of GABA and dopamine neurones

Postsynaptic GABAB receptors are weakly expressed in pars reticulata

Acknowledgements

Neuroscience

Neuroscience

Prog. Neurobiol.

Eur. J. Pharmacol.

Eur. J. Pharmacol.

Eur. J. Pharmacol.

Neuron

Brain Res.

Prog. Neurobiol.

Brain Res.

Brain Res. Rev.

Brain Res.

Brain Res.

Neurosci. Res.

Brain Res.

Dev. Brain Res.

Neurosci. Lett.

Neuron

Brain Res.

Brain Res.

Disinhibition as a basic process in the expression of striatal functions

Trends Neurosci.

Calcium channel involvement in GABAB receptor-mediated inhibition of GABA release in area CA1 of the rat hippocampus

J. Neurophysiol.

Differential expression of pre- and postsynaptic GABA_B receptors in rat substantia nigra pars reticulata neurones

Postsynaptic GABA_B receptors are weakly expressed in pars reticulata

Calcium channel involvement in GABA_B receptor-mediated inhibition of GABA release in area CA1 of the rat hippocampus