The relationship between the agonist-induced activation and desensitization of the human tachykinin NK2 receptor expressed in Xenopus oocytes

1 Repeated applications of neurokinin A (NKA) to oocytes injected with 25 ng wild-type hNK(2) receptor cRNA caused complete attenuation of second and subsequent NKA-induced responses while analogous experiments using repeated applications of GR64349 and [Nle(10)]NKA (4-10) resulted in no such desensitization. This behaviour has been previously attributed tot he ability of the different ligands to stabilize different active conformations of the receptor that have differing susceptibilities to receptor kinases (Nemeth & Chollet, 1995). 2 However, for Xenopus oocytes injected (into the nucleus) with 10 ng wild-type hNK(2) receptor cDNA, a single 100 nM concentration of any of the three ligands resulted in complete densitization to further concentrations. 3 On the other hand, none of the ligands caused any desenitization in oocytes injected with 0.25 ng wild-type hNK(2) receptor cRNA, even at concentrations up to 10 mu M. 4 The two N-terminally truncated analogues of neurokinin A have a lower efficacy than NKA and it is likely that it is this property which causes the observed differences in desensitization, rather than the formation of alternative active states of the receptor. 5 The peak calcium-dependent chloride current is not a reliable measure of maximal receptor stimulation and efficacy is better measured in this system by studying agonist-induced desensitization. 6 The specific adenylyl cyclase inhibitor SQ22536 can enhance NKA and GR64349-mediated desensitization which suggests that agonist-induced desensitization involves the inhibition of adenylyl cyclase and the subsequent down-regulation of the cyclic AMP-dependent protein kinase, possibly by cross-talk to a second signalling pathway.