We have investigated the secretion of oxytocin and arginine vasopressin (AVP) during vaginocervical stimulation in the conscious goat and examined the effect of the opioid antagonist naloxone on peptide release to this stimulus. Goats were implanted with guide tubes overlying the cisterna magna under anaesthesia and allowed to recover. Vaginocervical stimulation for 60 s resulted in a marked (P < 0·01) release of oxytocin into the plasma but neither plasma AVP nor cerebrospinal fluid (CSF) concentrations of oxytocin changed significantly.
In a second series of experiments, unoperated goats were infused with saline or naloxone (4 mg bolus + 12 mg/h) in random order on two separate occasions. Infusion of naloxone had no effect on basal plasma concentrations of oxytocin or AVP. There was a marked and significant (P < 0·01) potentiation of oxytocin secretion following vaginocervical stimulation in animals infused with naloxone. Naloxone-infused animals showed a significant (P < 0·01) rise in plasma AVP after stimulation but plasma AVP did not change in the saline-infused controls.
We conclude that vaginocervical stimulation leads to the selective release of oxytocin from the neurohypophysis without affecting concentrations of oxytocin in the CSF. Endogenous opioids inhibit the stimulated secretion of oxytocin and AVP in vivo in response to vaginocervical stimulation in the goat.