Endogenous mu opioid systems and perioral responsiveness in the rat fetus.

Pharmacological manipulation of mu opioid receptors located in rostral and caudal parts of the brain produces distinctive changes in perioral responsiveness to nipple-like tactile stimulation in the E20 rat fetus. Blockade of caudal mu opioid receptors by intracistema magna (I.C.) injection of the selective mu antagonist drug CTOP reduces appetitive responses directed toward the artificial nipple. In contrast, blockade of mu opioid receptors in the rostral part of the brain by intracerebroventricular (I.C.V.) administration of CTOP increases fetal responsiveness to perioral cutaneous stimulation including oral capture and grasping of the artificial nipple. This pattern of the results suggests that there are at least two functionally different neuronal populations of mu opioid receptor-containing neurons that are involved in the regulation of the perioral responsiveness in the E20 rat fetus. The caudal part of this mu opioid system increases perioral responsiveness while the rostral part of the system decreases responsiveness to nipple-like perioral stimulation. These findings suggest the possibility that mu opioid systems may play a functional role in regulating neonatal behavior at the nipple.