The Role of Protein Kinase A in the Ethanol-Induced Increase in Spontaneous GABA Release Onto Cerebellar Purkinje Neurons

Kelm MK, Criswell HE, Breese GR. The role of protein kinase A in the ethanol-induced increase in spontaneous GABA release onto cerebellar Purkinje neurons. J Neurophysiol 100: 3417-3428, 2008. First published October 22, 2008; doi: 10.1152/jn.90970.2008. Ethanol increases miniature inhibitory postsynaptic current frequency and decreases the paired-pulse ratio, which suggests that ethanol increases both spontaneous and evoked GABA release, respectively. We have shown previously that ethanol increases GABA release at the rat interneuron Purkinje cell synapse and that this ethanol effect involves calcium release from internal stores; however, further exploration of the mechanism responsible for ethanol-enhanced GABA release was needed. We found that a cannabinoid receptor 1 (CB1) agonist, WIN-55212, and a GABA B receptor agonist, baclofen, decreased baseline spontaneous GABA release and prevented ethanol from increasing spontaneous GABA release. The CB1 receptor and GABA B receptor are G alpha i-linked G protein-coupled receptors with common downstream messengers that include adenylate cyclase and protein kinase A (PKA). Adenylate cyclase and PKA antagonists blocked ethanol from increasing spontaneous GABA release, whereas a PKA antagonist limited to the postsynaptic neuron did not block ethanol from increasing spontaneous GABA release. These results suggest that presynaptic PKA plays an essential role in ethanol-enhanced spontaneous GABA release. Similar to ethanol, we found that the mechanism of the cannabinoid-mediated decrease in spontaneous GABA release involves internal calcium stores and PKA. A PKA antagonist decreased baseline spontaneous GABA release. This effect was reduced after incubating the slice with a calcium chelator, BAPTAAM, but was unaffected when BAPTA was limited to the postsynaptic neuron. This suggests that the PKA antagonist is acting through a presynaptic, calcium-dependent mechanism to decrease spontaneous GABA release. Overall, these results suggest that PKA activation is necessary for ethanol to increase spontaneous GABA release.