Serotonin 5-HT1c and acetylcholine M1 receptors activate phosphoinositidase, resulting in an increased formation of IP3 and 1,2 diacylglycerol. In Xenopus oocytes injected with mRNA encoding either of these receptors, Ca2+ released from intracellular stores in response to IP3 then opens Ca(2+)-gated Cl-channels. In the present experiments, oocytes expressing a transcript from a cloned mouse serotonin 5-HT1c receptor were exposed to identical 15-s pulses of agonist, administered 2 min apart; the second current response was two to three times that of the first. However, in those oocytes coinjected with the 5-HT1c receptor transcript and a low molecular weight fraction (0.3-1.5 kb) of rat brain mRNA, the second current response was approximately 50% of the first. Thus, the low molecular weight RNA encodes a protein (or proteins) that causes desensitization. Experiments using fura-2 or a Ca(2+)-free superfusate indicated that desensitization of the 5-HT1c receptor response does not result from a sustained elevation of intracellular Ca2+ level or require the entry of extracellular Ca2+. Photolysis of caged IP3 demonstrated that an increase in IP3 and a subsequent rise in Ca2+ do not produce desensitization of either the IP3 or 5-HT1c peak current responses. Furthermore, in oocytes coinjected with the low molecular weight RNA and a transcript from the rat M1 acetylcholine receptor, the M1 current response was greatly attenuated. Our data suggest that the proteins involved in attenuation of the M1 current response and desensitization of the 5-HT1c current response may be the same.