Intracellular Ca2+-dependent and Ca2+-independent responses of rat brain serotonin receptors transplanted to Xenopus oocytes.

Xenopus oocytes injected with messenger RNA extracted from rat brain are induced to acquire a variety of neurotransmitter receptors and voltage-operated membrane channels. Activation of the receptors to serotonin, acetylcholine (muscarinic) and glutamate elicits oscillatory membrane currents carried by chloride ions. These currents are not abolished by removing external calcium, but are completely abolished after EGTA is injected into the oocytes to chelate intracellular calcium. A smooth current response to serotonin remained in EGTA-loaded oocytes, indicating that this response does not require intracellular calcium. In contrast to the oscillatory chloride currents, the chloride currents activated by GABA or glycine are not abolished by intracellular injection of EGTA. Thus, there appear to be two classes of chloride channels one of which requires intracellular calcium to open.