Opioid mediated behavioral effects and learning in the neonatal rat

Abstract

The purpose of this study was to explore the behavioral effects of amniotic fluid (AF) and milk in the newborn rat. Previous research has documented behavioral effects in the fetal and neonatal rat. For example, oral exposure to AF and milk reduces the response to chemosensory stimulation in rat fetuses (Korthank & Robinson, 1998) and newborns (Méndez-Gallardo & Robinson, 2010). In addition, some of the behavioral effects of AF and milk are mediated by the endogenous opioid system in the perinatal rat, including modulation of the facial wiping response (Korthank & Robinson, 1998; Méndez-Gallardo & Robinson, 2010), the stretch response induced by milk in the fetal rat (Smotherman & Robinson, 1992b), and the effect of milk as an unconditioned stimulus (US) during associative learning in the fetal rat (Robinson et al., 1993). Taking into account the literature that suggests similarities between AF and milk, this study aimed to evaluate whether transnatal continuity in the behavioral effects of AF and milk could be found and whether mediation by the endogenous opioid system is the underlying mechanism of these effects. To fulfill this purpose, overall behavioral activation, crawling locomotion, oral responses to an artificial nipple, and associative learning were investigated in the newborn rat. Results showed that, (a) oral exposure to AF resulted in higher levels of behavioral activation than oral exposure to milk, (b) exposure to the odor of AF or milk did not produce significant behavioral activation, although the odor of milk seemed to evoke higher levels of behavioral activity than exposure to the odor of AF, (c) both AF and milk odor elicited crawling locomotion, (d) odor of AF or milk did not promote oral grasping of an artificial nipple, but promoted mouthing responses and distinctive movements of the forepaws, (e) contingent presentations of an artificial nipple as the conditioned stimulus (CS), with AF or milk as the US, promoted mouthing responses during reexposure to the CS, but facial wiping after CS reexposure was not modified as a result of conditioning, and (f) mediation of the opioid system was evident only during hindlimb activity after oral exposure to AF or milk and during mouthing responses to the CS after associative learning. These findings suggest that oral exposure to AF or milk consistently evoke opioid responses in the neonatal rat, but exposure to the odor of AF or milk alone does not. Through postnatal testing and the direct comparison of the behavioral effects of AF (a feature of the prenatal environment) with milk (a feature of the postnatal environment), this study contributes to a better understanding of mechanisms that promote behavioral continuity before and after birth.