PREFERRED TASTES AND OPIOID-MODULATED BEHAVIORS IN NEONATAL RATS

Ten-day-old rats were given various tastants to ingest, independent of mother and suckling. Relative to water, the pups ingested significantly more saccharin (0.5%) and NaCI (2.5%) and less quinine (0.4%). Pretreatment with the opiate antagonist, naltrexone, had no effect on water or quinine intake but significantly reduced that of saccharin and NaCI. Furthermore, the preferred solutions of saccharin and NaCI given intraorally caused a decrease in ultrasonic vocalizations of isolated pups and in a separate experiment caused an analgesic response to heat. Both phenomena were reversible with naltrexone administration, suggesting that preferred tastes elicit an endogenous opioid response significantly affecting behaviours seen during isolation.