Analgesic activity of morphiceptin, beta-casomorphin-4, and deltakephalin in normotensive Wistar-Glaxo and spontaneously hypertensive rats

The effects of intraventricular injection of beta-casomorphin-4, morphiceptin and deltakephalin (DTLET) on hot water tail flick and tail compression responses were investigated in Wistar Albino Glaxo (WAG) and spontaneously hypertensive rats (SHR). The effects of the mu agonist morphiceptin (20 nmol/rat), as assessed by the tail compression test, were significantly greater in SHR rats but did not differ between both strains when measured by tail flick latency. Opioid agonist deltakephalin (2 nmol/rat) in both tests elicited stronger analgesic effects in SHR as compared to WAG and these effects were blocked by naloxone in both tests used. beta-Casomorphin-4 exhibits moderate activity for mu receptors. In the tail flick test peptide (60 nmol/rat) produced an increase in latencies in SHR rats that was significantly greater than was observed in WAG rats. Naloxone pretreatment abolished the analgesic activity of beta-casomorphin-4 solely in the tail compression test in SHR. Analysis of the slopes of the dose-response curves seems to suggest that differences between the activity of these opioid Peptides in SHR and WAG rats are based on a difference in the density and affinity of the subpopulation of the opioid receptors in these strains of rats.