On the location of gamma-aminobutyrate and benzodiazepine receptors in the cerebellum of the normal C3H and Lurcher mutant mouse.

Binding of gamma-aminobutyrate and benzodiazepine receptor ligands has been studied in the cerebellum of adult normal (C3H) and Lurcher mutant mice. The adult mutant has lost all Purkinje cells and more than 90% of the granule cells in the cerebellar cortex. When compared with their normal littermates Lurcher mice displayed large decreases in the number of high-affinity binding sites for [3H]muscimol, a synaptic gamma-aminobutyrate receptor ligand, in washed cerebellar homogenates. This observation was consistent with the extensive loss of gamma-aminobutyrate receptive Purkinje and granule cells from the Lurcher cerebellum. However, specific binding of the benzodiazepine-receptor ligand [3H]flunitrazepam to Lurcher cerebellum remained unchanged. Indeed quantitative autoradiography, employing [3H]flunitrazepam as a photoaffinity label, showed no significant differences in the density of labelling between Lurcher and normal littermate mice in any region of the cerebellum. These benzodiazepine binding sites in washed homogenates or tissue sections displayed a gamma-aminobutyrate-induced enhancement of [3H]flunitrazepam binding which occurred to the same extent in both Lurcher and normal cerebellum, a facilitatory effect which could be blocked by the addition of bicuculline methobromide. Our results suggest that a large proportion of the high-affinity, specific benzodiazepine binding sites in mouse cerebellum are not coupled to the synaptic gamma-aminobutyrate receptors thought to be labelled by high affinity [3H]muscimol binding. Further, that benzodiazepine binding sites do not appear to be enriched on either the soma or dendrites of Purkinje cells, as has been suggested from previous studies. Investigations at the electron microscope level are now required to elucidate the cellular location of benzodiazepine binding sites in the cerebellar cortex and to examine whether or not they are likely to be exposed to gamma-aminobutyrate in vivo.