Role of GABA B receptors in intracellular Ca 2+ homeostasis and possible interaction between GABA A and GABA B receptors in regulation of transmitter release in cerebellar granule neurons

The expression of GABA B receptors in cultured mouse cerebellar granule cells was investigated in binding experiments using [ 3 H]( S, R )‐baclofen as well as in functional assessment of the ability of ( R )‐baclofen to interact with depolarization (15–40 mM KCI) coupled changes in intracellular Ca 2+ homeostasis and neurotransmitter release. In the latter case a possible functional coupling between GABA A and GABA B receptors was investigated. The binding studies showed that the granule cells express specific binding sites for ( R )‐baclofen. The number of binding sites could be increased by exposure of the cells to the GABA A receptor agonist THIP (4,5,6,7‐tetrahy‐droisoxazolo[5,4‐ c ]pyridin‐3‐ol) during the culture period. Pretreatment of the neurons with pertussis toxin showed that the GABA B receptors are coupled to G‐proteins. This coupling was, however, less pronounced when the cells had been cultured in the presence of THIP. When 45 Ca 2+ uptake was measured or the intracellular Ca 2+ concentration ([Ca 2+ ] i ) determined using the fluorescent Ca 2+ chelator Fluo‐3 it could be demonstrated that culturing the neurons in THIP influences intracellular Ca 2+ homeostasis. Moreover, this homeostasis was found to be functionally coupled to the GABA B receptors as ( R )‐baclofen inhibited depolarization‐induced increases in 45 Ca 2+ uptake and [Ca 2+ ] i . ( R )‐Baclofen also inhibited K + ‐induced transmitter release from the neurons as monitored by the use of [ 3 H] D ‐aspartate which labels the neurotransmitter pool of glutamate. Using the selective GABA A receptor agonist isoguvacine it could be demonstrated that the GABA B receptors are functionally coupled to GABA A receptors in the neurons leading to a disinhibitory action of GABA B receptor agonists. © 1994 Wiley‐Liss, Inc.