EVIDENCE FOR OPIOIDERGIC INHIBITION OF OXYTOCIN RELEASE IN PERIPARTURIENT MARES

In the horse mare, the onset of parturition is associated with an incr ease in oxytocin secretion, and it has been suggested that the onset o f parturition may be triggered by endogenous oxytocin release. To test the hypothesis that oxytocin secretion is regulated by endogenous opi oids in the periparturient period, we have 1) characterized oxytocin s ecretion in response to vaginocervical stimulation and 2) determined t he effect of naloxone, an opioid antagonist, on oxytocin secretion ind uced by vaginocervical stimulation in prepartum mares and in postpartu m mares at estrus and diestrus. During the last 2 months of pregnancy, the first diestrus and subsequent estrus post partum, a total of 66 v aginocervical stimulations were performed. Mares were pretreated with naloxone (0.5 mg/kg iv) or saline, administered 20 min before vaginoce rvical stimulation on subsequent days, using a randomized switchback d esign in which mares served as their own controls. Plasma was collecte d from 30 min before until 30 min after stimulation and was analyzed f or oxytocin concentrations. Vaginocervical stimulation resulted in a s ignificant increase in oxytocin secretion in all mares. Between Days 3 0 and 20 prepartum, the total amount of oxytocin secreted (calculated as area under the curve for 0 to 10 min after vaginocervical stimulati on) was significantly greater in naloxone-treated than in saline-treat ed mares. From Day 20 prepartum until parturition, the differences bet ween naloxone and saline-treated mares tended to decrease with approac hing parturition, and were no longer statistically different. Peak pla sma oxytocin concentrations were greater in naloxone-treated mares tha n in saline-treated mares during the entire prepartum period. During t he postpartum period, total amount of oxytocin secreted following vagi nocervical stimulation tended to be greater than during the prepartum period, and stimulated oxytocin secretion was significantly greater in naloxone-treated mares than in saline-treated mares. In conclusion th ese data suggest that endogenous opioids suppress oxytocin secretion p re and post partum. It appears that opioid inhibition is not limited t o the prepartum period, tends to decrease gradually towards parturitio n and is reinstated after foaling.