Coexistence of GABA receptors and GABA-modulin in primary cultures of rat cerebellar granule cells.

GABA-modulin (GM), a basic polypeptide purified from rat brain synaptosomes, which is an allosteric inhibitor of GABA recognition sites, has been detected in primary cultures of cerebellar interneurons enriched in granule cells by immunohistochemistry, using a specific antibody raised in rabbit injected with GM purified from rat brain synaptosomes. In these cultures, GM is expressed by the granule cells, which are postsynaptic to GABAergic interneurons, but not by glial cells. In rat cerebellar sections anti-GM antiserum intensely strains the granular cell layer and Purkinje cell dendrites and cell bodies. GM has been purified from the cerebellar granule cell cultures and appears to be identical under biochemical, immunological, and functional criteria to authentic GM purified from rat brain synaptosomes. Granule cell cultures devoid of GABAergic neurons contain the GABA/BZ/Cl- receptor complex; in fact, intact cell monolayers, incubated in physiological buffer at 25 degrees C, express 3H-muscimol and 3H-flunitrazepam binding sites, which are comparable to the sites detected in cell membrane preparations and which modulate each other reciprocally. It is concluded that GM might participate in the supramolecular organization of the GABA receptor complex, perhaps functioning as a modulator of this receptor protein.