CALCINEURIN INHIBITS DESENSITIZATION OF CLONED RAT 5-HT(1C)RECEPTORS

Functional responses to stimulation of rat 5-HT1C receptors expressed in A9 cells were studied using whole cell voltage clamp recording tech nique. Stimulation of 5-HT1C receptors with serotonin (5-HT) evoked ca lcium-dependent outward currents of 109 pA in cells clamped at -50 mV. Pretreatment with the protein kinase C (PKC) activator phorbol myrist ic acetate (PMA) reduced the 5-HT-induced current amplitude by 46% of the control value. Inclusion of inositol triphosphate (IP3) in the pip ette solution induced an outward current of 84 pA. The IP3-induced res ponse was not affected by 60 min pretreatment with PMA. In the presenc e of the PKC antagonist calphostin C, 60 min treatment with PMA (10(-6 ) mol/1) reduced the 5-HT response only by 8%. In cells preincubated w ith PMA, injection of the calcium/calmodulin dependent serine proteinp hosphatase calcineurin gradually increased the 5-HT-induced responses by 34%. In A9 cells which were incubated 24 h with the 5-HT1C receptor agonist meta chlorophenylpiperazine hydrochloride (mCPP), 5-HT-induce d responses were reduced by 23% of the vehicle pretreated control valu e. Injection of calcineurin in mCPP treated cells enhanced the 5-HT-in duced response by 24%. The results suggest that in A9 cells rat 5-HT1c receptors are desensitized after phosphorylation by PKC. This desensi tization can be counteracted by calcineurin-induced dephosphorylation.