Interactions between vasopressin and mu-opioid systems in the rat fetus.

Administration of arginine-vasopressin (AVP) into the cisterna magna (IC injection) of the E20 rat fetus increases motor activity and promotes expression of rare patterns of behavior including mouthing, licking, and facial wiping. These effects are mediated by V1 receptors in the brain stem and spinal cord. In this study, AVP-induced changes in motor behavior were measured to characterize interactions within the AVP system and between the AVP and mu-opioid systems in the fetal rat. Injection of AVP into the brain hemispheres (IH injection) diminished the effects of an IC injection of AVP. AVP effects were potentiated by blockade of hemispheric V1 receptors, suggesting that hemispheric V1 receptors inhibit V1 receptor-containing neurons in the brain stem and spinal cord. Intracisternal injection of the mu agonist DAGO suppressed the effects of AVP whereas blockade of mu-opioid receptors in the brain stem and spinal cord with CTOP and activation of mu receptors in the hemispheres with DAGO potentiated the behavioral effects of AVP. The behavioral effects of AVP are mediated by V1 receptors in the brain stem and spinal cord and may be under the inhibitory control of a mu-opioid system localized at the same level of the brain. Facilitation of AVP effects following IH injection of DAGO may involve an inhibition of the inhibitory effects of V1 receptor-containing neurons located in the hemispheres. Interactions between mu-opioid and AVP systems in the caudal and rostral portions of the fetal brain may be based on a common principle.