Behavioral activities of opioid peptides and morphine sulfate in golden hamsters and rats

The behavioral effects of β-endorphin, [D-Ala², D-Leu⁵]-enkephalin and morphine were investigated in golden hamsters and in rats. In golden hamsters, β-endorphin and [D-Ala², D-Leu⁵]-enkephalin induced loss of righting reflex, whereas morphine caused no such effect. Both opiate peptides and morphine caused the inhibition of tail-flick response and catalepsy in rats. β-Endorphin was the most potent, followed by [D-Ala², D-Leu⁵]-enkephalin and then by morphine. The catalepsy induced in rats by [D-Ala², D-Leu⁵]-enkephalin was different from that of β-endorphin and morphine in that it produced catalepsy without muscular rigidity. β-Endorphin and [D-Ala², D-Leu⁵]-enkephalin caused hypothermia in golden hamsters; morphine was less active in altering the body temperature. β-Endorphin caused hypothermia at high doses and hyperthermia at low doses in rats. These heterogenous behavioral responses indicate that multiple types of receptors mediate the effects of opiates in the central nervous system.