Abstract

The isolated hypothalamo-neurohypophysial system (HNS) of male rats was incubated with putative neurotransmitters in vitro to determine their significance in regulating oxytocin release. Dopamine was found to inhibit the spontaneous release of oxytocin from the HNS in a dose-related manner. The maximum inhibition produced by dopamine (10(-9) M) was significantly different from control values and was blocked by haloperidol (5 x 10(-5) M). Apomorphine (10(-9) M) mimicked the effect of dopamine on this system. The type of receptor involved in the inhibitory effect on oxytocin release is unclear, although a dopaminergic mechanism in vivo. When levodopa plus carbidopa were injected into male rats, the gland content of oxytocin in these animals was significantly increased over control values. When a similar dosage regimen was employed with pregnant rats beginning on day 16 of gestation, levodopa plus carbidopa delayed the average delivery time 12 hr. The results of these studies are also consistent with a dopaminergic mechanism for the regulation of oxytocin release.