A quantitative analysis of the shaking behavior induced in rats by beta-endorphin and [D-Ala2, Met5]enkephalinamide

beta-Endorphin (5-80 microgram) or [D-Ala2,Met5]enkephalinamide (DALA) (5-40 microgram) was administered intracerebroventricularly to rats. With both opioid peptides, there was no direct relationship between log dose and mean number of wet-dog shakes (WDS) that occurred during the following 15 min. When the results were analyzed quantitatively, the dose of DALA that caused 50% of the rats to shake at least twice was 8.6 microgram (4.9-15 microgram). beta-Endorphin had such poor efficacy that an ED 50 could not be obtained. Morphine (1 and 5 mg/kg, s.c.) antagonized shaking caused by the optimal dose of DALA (20 microgram). Naloxone (0.1-10 mg/kg, s.c.) attenuated both DALA- and beta-endorphin-induced WDS in a dose-related manner. This latter result differentiates shaking associated with opioid peptides from that caused by thyrotropin releasing hormone (TRH), another endogenous stimulant of WDS in rats. There was no cross-tolerance between RX 336-M (7,8-dihydro-5',6'-dimethylcyclohex-5'-eno-1',2',8',14 codeinone), a novel shake inducing agent, and beta-endorphin. This finding again differentiates beta-endorphin-induced shaking from that caused by TRH and also from that associated with several exogenous stimulants of WDS.