Roles of protein kinases in neurotransmitter responses in Xenopus oocytes injected with rat brain mRNA

Microinjection of rat brain mRNA in Xenopus oocytes induced acetylcholine, neuroteisin, serotonin, and glutamate receptors in the cells. These receptors stimulate an intracellular reaction pathway, including G‐protein activation, inositol trisphosphate (IP₃) formation, and Ca²⁺‐dependent CI⁻ channels. In the present study, we examined the roles of several protein kinases in these responses by means of inhibitors and activators of these kinases. Isoquinolinesulfonamides, inhibitors of protein kinases, caused no current responses and affected no receptor‐mediated responses when injected into the oocytes at low doses (30–50 pmol), which inhibit cyclic nucleotide‐dependent kinases or kinase C specifically, but abolished the receptor‐mediated responses at a higher dose (300 pmol), which inhibit most protein kinases nonspecifically. Calmodulin inhibitors blocked the receptor‐mediated responses strongly. Activation of cyclic nucleotide‐dependent kinases or kinase C by injection of cAMP (or cGMP) or perfusion with phorbol esters caused no direct current responses but suppressed receptor‐mediated responses. Current responses triggered by IP₃ injection were not suppressed by these treatments. These results suggest that cAMP‐ (or cGMP‐)dependent kinases or kinase C may not be involved in the pathway directly but may modulate it by inhibiting the initial part of the pathway (receptors, G‐proteins, and/or phospholipase C), and they suggest that calmodulin may most likely be involved in the activation of Ca²⁺‐dependent CI⁻ channels.