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 periora l responsiveness to nipple-like tactile stimulation in the E20 rat fet us. Blockade of caudal mu opioid receptors by intracisterna magna (IC) injection of the selective mu antagonist drug CTOP reduces appetitive responses directed toward the artificial nipple. In contrast, blockad e of mu opioid receptors in the rostral part of the brain by intracere broventricular (ICV) administration of CTOP increases fetal responsive ness to perioral cutaneous stimulation including oral capture and gras ping of the artificial nipple. This pattern of the results suggests th at there are at least two functionally different neuronal populations of mu opioid receptor-containing neurons that are involved in the regu lation 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-lik e perioral stimulation. These findings suggest the possibility that mu opioid systems may play a functional role in regulating neonatal beha vior at the nipple. (C) 1997 Elsevier Science Inc.