Pharmacology, distribution, cellular localization, and development of GABAB binding in rodent cerebellum

Quantitative receptor autoradiography using [3H]GABA under selective conditions was used to characterize the pharmacology, distribution, cellular localization, and development of GABAB binding sites in rodent cerebellum. Pharmacologic analysis of [3H]GABA binding showed that drugs active at GABAB receptors displaced [3H]GABA with the following order of potency: 3-aminopropylphosphonous acid > CGP 35348 = 2-hydroxysaclofen > phaclofen. GTP-gamma-S and GDP-beta-S also diminished potently [3H]GABA binding in a dose-dependent manner. The pattern of [3H]GABA binding to GABAB binding sites was systematically mapped throughout the rat cerebellum. GABAB binding was greatest in the molecular layer and a pattern of parasagittal zonation was observed in the molecular layer of lobules VII-X in adult rats. The cellular localization of GABAB binding was investigated using lesion techniques. Neither methyl azoxymethanol lesions of cerebellar granule cells nor 3-acetylpyridine lesions of climbing fibers resulted in a decrease in [3H]GABA binding. Homozygote stumbler mutant mice, deficient in Purkinje cell dendrites, had a significant decrease in [3H]GABA binding in the molecular layer. These results suggest that the majority of cerebellar molecular layer GABAB binding sites detected by [3H]GABA autoradiography are located on Purkinje cell dendrites. Examination of [3H]GABA binding to GABAB binding sites during development revealed that binding in the molecular layer peaks between postnatal day 14 and postnatal day 28 and then decreases to adult levels. Transient expression of high levels of GABAB binding was observed in the deep cerebellar nuclei, peaking at postnatal day 3 and decreasing to adult levels by postnatal day 21. Our investigation of GABAB pharmacology yielded data in agreement with previously reported results. We have described a parasagittal pattern of GABAB binding in the cerebellar molecular layer and assigned the majority of cerebellar GABAB binding sites to Purkinje cell dendrites. Finally, development studies reveal transient peaks in GABAB binding in the cerebellar molecular layer and deep cerebellar nuclei.