Peripheral cholecystokinin type A receptors mediate oxytocin secretion in vivo

Cholecystokinin-octapeptide (CCK8) administered intraperitoneally (i.p.) in rats induces a rapid elevation in serum oxytocin (OT). The receptor subtype mediating this action of CCK was investigated with selective CCK-A and CCK-B receptor agonists and antagonists. CCK8 and A-71623, a potent CCK-A selective agonist, were similar in efficacy and potency for stimulating OT secretion. Both compounds at 10 nmol/kg elicited approximately one-half the response of 100 nmol/kg, which elevated serum OT to approx. 20 to 30-fold above basal level. The potent CCK-B selective agonist, A-63387, at doses up to 100 nmol/kg did not increase serum OT. MK-329, a CCK-A receptor selective antagonist, at a dose of 20 nmol/kg fully inhibited the action of 20 nmol/kg CCK8, while 100 nmol/kg of (R)L-365,260, a CCK-B selective antagonist, had no effect on the CCK8 response. These results, together with previous lesion studies, suggest that vagal CCK-A receptors in the periphery mediate the activation of the oxytocinergic pathway in vivo.