Excitability to β-endorphin, morphine and catecholamines of the vas deferens of the rat at different stages of development

The effect of β-endorphin and morphine on neuromuscular twitching of the vas deferens was examined in rats of 10-120 days of age. The maximal tension developed by the smooth muscle was markedly dependent on the age of the rat and on the kind of stimulus applied. The potency of β-endorphin to inhibit the electrically induced twitching or of catecholamines to induce muscle contraction was not significantly altered by development of the animal. The excitatory action of morphine showed significant variations in activity reaching maximal sensitivity at about two months of age. Age or the onset of puberty do not significantly modify the sensitivity of the vas deferens to β-endorphin; the peptide may play the role of modulating adrenergic transmission.