Opioid receptors (μ, κ, and δ) are distributed throughout the brain in both infants and adults. Opiate receptors are present in 14-day-old fetuses and begin reaching adult levels by the 3rd postnatal week (Clendeninn, Petraitis, & Simon, 1976). μ and κ receptors are present at birth while δ receptors are not present until the 2nd postnatal week (Kornblum, Hurlbut, & Leslie, 1987; Petrillo, Tavani, Verotta, Robson, & Kosterlitz, 1987; Spain, Roth, & Coscia, 1985). μ receptors are morphine and enkephalin selective, mediating nociception and reward. δ receptors are enkephalin selective, mediating affective behaviors and are found primarily in the limbic system while κ receptors are dynorphine selective and mediate less-addicting analgesia as well as affective behaviors (reviewed in Kehoe, 1988; Tseng, 1995).

Pairings of morphine with saccharine or odor show that the endogenous opioid system is present and functional in pups as early as 5 days old, and these pairings produce a conditioned preference to the conditioned stimulus (Kehoe & Blass, 1986). In postnatal Day 5 pups, pairings of low doses of morphine with odor produce odor preferences while high doses produce odor aversions (Randall, Kraemer, Dose, Carbary, & Bardo, 1992). Injections of morphine into the ventral tegmental area (an area associated with the adult reward pathway) paired with an odor in pups as young as postnatal Day 4 are sufficient for an odor preference (Barr & Rossi, 1992). Intraoral infusions of sucrose paired with odor produce an odor preference in 6-day-old pups, and both the acquisition and expression of the odor preference is naltrexone reversible, suggesting the role of endogenous opioids in odor preference formation (Shide & Blass, 1991). Similarly, intraoral infusion of milk activates endogenous opioids (Blass & Fitzgerald, 1988; Kehoe, 1988). Other work shows that μ 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.

Other labs have investigated the role of specific opioid receptors in conditioning using other models, such as nipple-milk conditioning in fetuses. The unconditioned response to milk involves κ receptors (Smotherman & Robinson, 1992). Additionally, conditioning with milk and an artificial nipple in the rat fetus promotes κ and conditioned μ receptor activity (Robinson et al., 1993; Robinson & Smotherman, 1997). Classical conditioning of responses to an artificial nipple in the rat fetus employs both caudal κ receptor activity and rostral μ receptor activity (Petrov, Varlinskaya, & Smotherman, 2000). The role of opioids in conditioning in adults has been studied as well. Fanselow (1979) examined the effect of naloxone, another nonspecific opioid antagonist, on signaled-shock-preference conditioning to one side of a shuttlebox. Preference tests revealed that subjects that received naloxone during training did not show a subsequent side preference. Similarly, Foo and Helmstetter (2000) suggested the role of μ opioid receptors in mediating conditioned responses to shock in adult rats. Overall, results from these studies suggest a role of opioid receptors in behavioral conditioning and that multiple opioid receptors mediate the conditioning.

Since naltrexone is a nonspecific opioid antagonist, naltrexone may act at different receptors during the processes of acquisition (learning) and consolidation, offering a plausible explanation for the results from Experiments 2 and 3. For example, naltrexone may have blocked active κ receptors during the learning process, thus naltrexone-paired pups did not show conditioned behavioral activation as did the saline controls. Also, when given naltrexone after acquisition (using the example κ receptors were unaffected during the training), naltrexone may have then affected conditioned μ receptors during the process of consolidation of an odor preference. If μ receptors are involved in consolidating the memory of a stimulus as being rewarding, then blocking these receptors during consolidation would affect the number of choices made towards the odor when tested. If blocking these receptors during consolidation alters cellular activity in reward or aversion areas, the consolidation could be altered from a preference to aversion.

Further evidence for this interpretation was shown by Carr, Kutchukhidze, and Park (1999); systemic injections of naltrexone in adults induced c-fos immunoreactivity, which indicates increased cellular activity, in the extended amygdala. These areas—bed nucleus of the stria terminalis (BSTLD), nucleus accumbens shell (NACshell), and the central nucleus of the amygdala (ceA) —mediate motivation and reward. Carr et al. (1999) also showed that κ receptor blockade is responsible for the immunoreactivity in the BSTLD and CeA while μ receptor blockade is responsible for the increased activity in the NACshell. They concluded that the CeA might be under inhibitory control of both μ and κ receptors. These results suggest that blockade of receptors in particular areas of the reward pathway is responsible for our results. Cellular activation by naltrexone may alter perception of stimuli, which Park and Carr (1998) hypothesized to account for aversion and even suppression of the positive responses towards stimuli. Blocking κ receptors is not aversive (Carr, Papadouka, & Wolinsky, 1993; Leventhal, Kirkham, Cole, & Bodnar, 1995), thus if κ receptors participate in acquisition, this may explain why pups demonstrated neither an aversion nor preference in Experiment 2. If blockade of μ receptors is aversive (Shippenberg, 1993), and these receptors are involved in the consolidation of a conditioned odor preference, then this may explain why subjects demonstrated an aversion in Experiment 3; naltrexone altered opioid secretion in areas necessary for reward, thus altering the perception of the conditioned stimulus. Taken together, our results as well as results from the aforementioned studies suggest that odor conditioning involves several opioid receptors, and how and when developmentally they are altered may offer insight into the role of this neurotransmitter system in mother–infant attachment.