Stimulus contingencies that permit classical conditioning of opioid activity in the rat fetus.

Abstract

In a prenatal model of classical conditioning, rat fetuses received presentations of an artificial nipple (conditioned stimulus; CS) paired with milk (unconditioned stimulus). Infusion of milk promotes activity in the kappa opioid system of the fetus, but after 2, 3, or 6 pairings with the artificial nipple, milk evoked both kappa and mu opioid activity. The nipple CS has no effect on opioid activity, but after pairing with milk evoked a mu opioid response. Conditioned mu opioid activity was evident in 60% of subjects tested after 1 paired conditioning trial. Significantly more fetal subjects (90%) exhibited conditioned opioid activity if preexposed to the nipple twice before conditioning. CS preexposure altered behavior during the conditioning trial, with preexposed fetuses showing more pronounced responses to milk infusion. Exposure to familiar stimuli facilitates classical conditioning of physiological responses, including opioid activity, during the first suckling episode.

Classical conditioning in the rat fetus. III. Retention, extinction, and re‐activation of the conditioned response (CR)

“…Classical conditioned fetuses show fewer oral grasp responses, compared to control fetuses, during a test trial when they are re-exposed to the artificial nipple (CS). These findings replicate earlier reports of one-trial classical conditioning in the E20 rat fetus involving formation of an association between stimuli normally present immediately after birth in the context of suckling (Petrov et al, 2000;Robinson & Smotherman, 1997;Smotherman, 2002b).…”

“…When reexposed to the artificial nipple CS after conditioning and observed in the test of oral grasping, fetuses show fewer oral grasp responses compared to control fetuses exposed to the milk (US) and artificial nipple (CS) in an unpaired fashion. These findings replicate earlier reports of one-trial classical conditioning in the E20 rat fetus (Petrov, Varlinskaya, & Smotherman, 2000;Robinson & Smotherman, 1997;Smotherman, 2002b). The classically conditioned reduction in oral grasping is evident when the test of oral grasping is administered 18 min after conditioning, but not when it is administered 21 to 30 min after conditioning (Experiments 1a and 1b).…”

Emergence of long‐term memory for conditioned aversion in the rat fetus

“…CS and US of various natures, for example, orotactile stimulation as CS (Robinson & Smotherman, 1997;Varlinskaya et al, 1997) and various substances as US such as lemon, dynorphin, and milk (Smotherman & Robinson, 1991;Varlinskaya, Petrov, & Smotherman, 1996;Varlinskaya et al, 1997) have been used successfully. In general, stimuli acting on gustatory and/or olfactory sensitivity have been used as the CS (Mickley et al, 2000;Smotherman, 1982a;Smotherman & Robinson, 1985, 1991 and agents inducing a malaise as the US (Mickley et al, 1995;Mickley et al, 2000;Smotherman, 1982a;Smotherman & Robinson, 1985;Stickrod, Kimble, & Smotherman, 1982a).…”

Endogenous opioids and their role in odor preference acquisition and consolidation following odor–shock conditioning in infant rats

“…Similarly, intraoral infusion of milk activates endogenous opioids (Blass & Fitzgerald, 1988;Kehoe, 1988). Other work shows that m receptors are involved in suckling responses (Petrov et al, 1998) and that nipple-milk conditioning involves endogenous opioids (Robinson, Arnold, Spear, & Smotherman, 1993;Robinson & Smotherman, 1997;Smotherman & Robinson, 1992). Overall, these studies demonstrate a role of endogenous opioids in normal mother±infant attachment.…”