ABSTRACT
There is abundant evidence that fetal rats can modify their behavior and responsiveness to sensory stimuli as a function of experience. Repeated infusion of a lemon odor solution into the fetus 's mouth results in diminished behavioral and cardiac responsiveness (habituation). Pairing a chemosensory infusion with LiCl injection suppresses fetal activity upon representation of the taste/odor cue (conditioned aversion). Pairing a chemosensory stimulus with another treatment that elevates motor activity results in increased activity upon representation of the stimulus (conditioned activation). These findings have confirmed that altricial fetuses exhibit basic forms of learning during the prenatal period. Most recently, experiments have suggested that the endogenous opioid system of the fetus and neonate may playa role in facilitating or interfering with early learning. Intraoral infusion of a small volume of milk, for example, results in elevated activity at the kappa subclass of opioid receptors. Opioid activity produced by pharmacological treatment or sensory manipulation has the effect of reducing fetal responsiveness to cutaneous stimulation of the perioral region; perioral cutaneous responsiveness thus can be used as a behavioral bioassay of opioid activity in the fetus. If milk infusion is paired with a chemosensory stimulus (sucrose) or an oral tactile stimulus (artificial nipple), the fetus will exhibit reduced responsiveness in this bioassay upon reexposure to the sucrose or nipple alone. Although opioids are demonstrably involved in mediating both the unconditioned response
(evoked by milk) and conditioned response (evoked by sucrose or nipple after pairings with milk), the conditioned response involves activity in the mu opioid system, not the kappa system, suggesting that early learning promotes interactions between different opioid systems. The results of learning studies conducted with fetuses, which lack experience with milk or other suckling stimuli, have implications for understanding the sensory determinants and underlying neurobiology of behavior in the late prenatal period and during the first suckling episode after birth.
