DIFFERENTIAL INFLUENCE OF CORTICOSTERONE AND DEXAMETHASONE ON SCHEDULE-INDUCED-POLYDIPSIA IN ADRENALECTOMIZED RATS

Previous studies have shown that adrenalectomy prevents the normal acq uisition of schedule-induced polydipsia (SIP), while corticosterone (C ORT) administration reinstates this behavior in adrenalectomized (ADX) rats. These studies investigated which corticosteroid receptor is res ponsible for mediating CORT effects on SIP. In Experiment I the effect s of dexamethasone (DEX) and CORT on the acquisition of SIP were studi ed. DEX and CORT pellets (respectively 15 mg and 200 mg) were implante d subcutaneously in ADX rats. CORT but not DEX replacement was able to reinstate SIP in ADX rats. Because DEX binds almost exclusively to gl ucocorticoid receptors (GRs), while CORT binds to both GRs and mineral ocorticoid receptors (MRs), results from Experiment I indicated that o ccupancy of GRs alone is not sufficient for SIP acquisition. In Experi ment II CORT pellets of different concentrations (1, 10, 50, 200 mg) w ere implanted in ADX rats in order to determine whether MRs alone, or a combination of GRs and MRs are required for SIP reinstatement. Resul ts from Experiment II showed that the 1 and 10 mg CORT pellets were no t able to reinstate SIP in adrenalectomized rats, while animals implan ted with 50 or 200 mg pellets did exhibit the behavior. These results indicate that occupancy of both MRs and GRs is required for SIP acquis ition.