Behaviorally functional opioid systems in infant rats: I. Evidence for olfactory and gustatory classical conditioning.

In order to characterize behavioral function of opioid systems in neonatal rats, two series of experiments were initiated. In one, the reinforcing properties of exogenous opioids were investigated in 5-day-old pups. Specifically, the infant's ability to associate the novel taste of saccharin, received while suckling, with ip morphine injections was assessed. Rats that received 0.5 ml of saccharin prior to morphine administration ingested considerably more saccharin on Day 10 than did control rats. The second set of experiments was conducted to determine whether rat pups could associate a novel odor with morphine administration. Five days after conditioning, that stimulus was highly preferred by morphine-treated pups compared with saline control pups. Thus positive associations were formed with either a novel taste stimulus experienced while suckling or with an odor experienced during social isolation. Conditioning was cue specific and was retained for at least 5 days. The formation of these associations was blocked with opioid antagonists given prior to conditioning. These data suggest behaviorally functional opioid receptors and raise the possibility of a functional role of the endogenous opioids in motivational processes in infant rats.