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

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-P₃), 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-P₃ 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-P₃ 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-P₃ caused a dose-dependent, long-lasting inhibition of subsequent Ins1,4,5-P₃ and 5HT responses. Desensitization induced by Ins1,4,5-P₃ affected both 5HT₁ and 5HT₂. Thus, Ins1,4,5-P₃ 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-P₃ response was reversed by PhoOBt₂ suggesting a role for protein kinase C in recovery from desensitization.