Rapid desensitization of the TRH receptor and persistent desensitization of its constitutively active mutant

1 We studied rapid desensitization of the thyrotropin-releasing hormone receptor (TRH-R) or the m1-muscarinic receptor (m1-R) to a short challenge of threshold TRH concentration and persistent desensitization due to constitutive activity of a mutant TRH-R. 2 Xenopus oocytes expressing TRH-Rs and/or m1-Rs were challenged for 15 s with threshold concentrations of TRH ([TRH]) and then immediately with supraoptimal [TRH] or acetylcholine ([ACh]). The threshold challenge caused desensitization of 50-57% of responses to subsequent supraoptimal stimulation with TRH or ACh. 3 The homologous desensitization was reversible within 60 s after removal of the agonist. 4 The protein kinase C (PKC) inhibitor, chelerythrine, inhibited the control responses by 30-40%, without affecting the desensitized responses. Chelerythrine or the phosphatase inhibitor, okadaic acid, had little effect on the kinetics of resensitization, indicating limited involvement of PKC. 5 In oocytes coexpressing wild type TRH-Rs or m1-Rs with a constitutively active TRH-R mutant (C335Stop TRH-R), a persistent desensitization (33-57%) of the responses to TRH or ACh was observed. Additionally, there was a complete loss of the rapid desensitization induced by threshold [TRH]. 6 Chlorodiazepoxide (CDE), a competitive binding antagonist of TRH-Rs and an inverse agonist of C335Stop TRH-Rs, abolished the persistent desensitization induced by C335Stop TRH-Rs and enabled the rapid desensitization, conferring the wild type phenotype on C335Stop TRH-Rs. Chelerythrine had qualitatively the same effect as CDE. 7 In conclusion, unlike the rapid desensitization, the persistent desensitization caused by the constitutively active C335Stop TRH-Rs is largely mediated by PKC. It abrogates, however, the rapid desensitization, suggesting a common mechanistic step(s).