Short-and long-term desensitization of serotonergic response in Xenopus oocytes injected with brain RNA: roles for inositol 1,4,5-trisphosphate and protein kinase C

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Abstract
In Xenopus oocytes injected with rat brain RNA, serotonin (5HT) and acetylcholine (ACh) evoke membrane responses through a common biochemical cascade that includes activation of phospholipase C, production of inositol 1,4,5-trisphosphate (Ins1,4,5-P 3), release of Ca2+ from intracellular stores, and opening of Ca-dependent Cl− channels. The response is a Cl− current composed of a transient component (5HT1 or ACh1) and a slow, long-lasting component (5HT2 or ACh2). Here we show that only the fast, but not the slow, component of the response is subject to desensitization that follows a previous application of the transmitter. The recovery of 5HT1 from desensitization is biphasic, suggesting the existence of two types of desensitization: short-term desensitization (STD), which lasts for less than 0.5 h; and long-term desensitization (LTD) lasting for up to 4 h. The desensitization between 5HT and ACh is heterologous and long-lasting. We searched for (a) the molecular target and (b) the cause of desensitization.(a) Pre-exposure to 5HT does not reduce the response evoked by intracellular injection of Ca2+ and by Ca2+ influx. Cl− current evoked by intracellular injection of Ins1,4,5-P 3 was reduced shortly after application of 5HT, but fully recovered 30 min later. Thus, the Cl− channel is not a target for desensitization. Neither Ins1,4,5-P 3 receptor nor the Ca2+ store is a target of LTD but they may be the targets of STD. (b) Ca2+ injection did not inhibit the 5HT response, suggesting that Ca2+ is not a sole cause of STD or LTD. An activator of protein kinase C, β-phorbol 12,13-dibutyrate (PhoOBt2), is known to inhibit the 5HT response, but this inhibition had completely subsided 30 min after washout of PhoOBt2. A protein kinase inhibitor H-7 did not prevent LTD. Thus, protein kinase C does not appear to be the cause of LTD, but its role in STD cannot be ruled out at present. Injection of Ins1,4,5-P 3 caused a dose-dependent, long-lasting inhibition of subsequent Ins1,4,5-P 3 and 5HT responses. Desensitization induced by Ins1,4,5-P 3 affected both 5HT1 and 5HT2. Thus, Ins1,4,5-P 3 is a possible cause of STD and LTD, but non-specific effects cannot be ruled out at present. The self-desensitization of Ins1,4,5-P 3 response was reversed by PhoOBt2 suggesting a role for protein kinase C in recovery from desensitization.