Habituation to chemosensory stimuli in the rat fetus: effects of endogenous kappa opioid activity.

On Day 21 of gestation, rat fetuses respond to chemosensory stimuli by expressing stereotypic facial wiping behavior. A series of 4 experiments was conducted to investigate (a) the influence of morphine on fetal responsiveness to a single chemosensory infusion, (b) the effect of naloxone blockade of endogenous opioid activity on diminished fetal responsiveness over a series of chemosensory infusions, (c) the effect of endogenous opioids on the recovery of fetal responsiveness to infusion after various dishabituation procedures, and (d) the influence of selective mu and kappa opioid receptor antagonists on fetal habituation. These experiments confirm that fetuses habituate after a brief series of chemosensory infusions and that dishabituation promoted by presentation of a novel stimulus is facilitated by pharmacological blockade of kappa opioid receptors. Endogenous activity in the kappa opioid system may be functional in modulating the sensory environment around the time of birth.