| Abstract: | On their 1st experience with milk, fetal rats express a stretch response that is similar to the postnatal behavior exhibited by infant rats at the nipple. Fetuses also possess a functional opioid system that is activated by prenatal milk exposure. The opioid receptor antagonist naloxone and the specific kappa antagonist nor-binaltorphimine blocked the stretch response and prevented the increase in rearlimb activity that is typically induced by milk. The mu antagonist β-funaltrexamine blocked the stretch while permitting the expression of rearlimb activity. The kappa agonist U50,488 promoted rearlimb activity in the absence of milk, whereas the mu agonist {d}-Ala–2,NMe-Phe–4,Gly–5-ol]-enkephalin (DAMGO) exerted little influence on fetal behavior. Fetuses pretreated with U50,488 stretched to nonmilk stimuli (saline or lemon), but fetuses pretreated with DAMGO did not. Opioid activation is part of a chain of events that culminates in the fetal stretch response and may be important in promoting milk ingestion during the newborn's 1st suckling episode. (PsycINFO Database Record (c) 2010 APA, all rights reserved) |
