Autoradiographic localization of GABA_B receptors in rat cerebellum

Abstract

γ-Aminobutyric acid (GABA) is a major inhibitory neuro-transmitter in the mammalian cerebellum^1–4. Evidence for the role of GABA in this brain region stems not only from neuro-pharmacological studies but also from in vitro and ex vivo ligand binding studies with ³H-GABA or ³H-muscimol^5–10. The cerebellar concentration of GABA binding sites is higher than that elsewhere^11,12 even though it contains less endogenous GABA than many other brain regions¹³. Using an autoradiographic technique, Kuhar and co-workers¹⁴ have recently demonstrated that these binding sites in the rat cerebellum (detected in Tris-citrate buffer) are located primarily in the granule cell layer, a finding which supports the observations of Candy and Martin¹⁵ and Olsen and Mikoshiba⁸ that ³H-GABA binding is four to six times greater in homogenates of the granule cell layer than in the molecular layer. The GABA sites detected in these studies were bicuculline sensitive and exhibited a high affinity for muscimol. We would therefore classify them as GABA_A sites because our recent observations have indicated the presence of other GABA sites (GABA_B sites) in the central nervous system which are insensitive to bicuculline^16–18. These sites have a high affinity for β-p-chlorophenyl GABA (baclofen), the GABA analogue which is devoid of activity at GABA_A sites. We describe here the autoradiographic localization of these GABA_B sites within the cerebellum and show that, unlike GABA^A sites, they are confined almost exclusively to the molecular layer.