Aversion learning in 5-day-old rats: taste-toxicosis and texture-shock associations.

Five-day-old rats that received a single pairing of a novel saccharin flavor with lithium treatment learned to suppress ingestion of saccharin on subsequent occasions (Experiment 1). This flavor aversion learning was inversely related to the interval between the novel flavor experience and drug administraton, wit h significant aversions being learned with intervals of 0 and 30 min but not with intervals of 60 and 90 min (experiment 2). The conditioning was observed when hypertonic lithium served as the unconditioned stimulus but did not occur with isotonic lithium or hypertonic saline injections (Experiment 3). These results indicate that flavor aversion learning in infant rats is a result of the joint action of two factors--pure drug effects and the somatic pain and irritation produced by the tonicity of the injected agent. The taste aversion learning was specific to the drug-paired flavor, and evidence was presented suggesting that 5-day-old rats display a natural hesitancy to consume novel edibles (neophobia) (Experiment 4). Experiment 5 demonstrated that 5-day-old rats selectively associate tactile stimuli with shock but not with the gastrointestinal consequences of lithium. Implications of the mechanisms involved in flavor aversion learning in infant rats are discussed as they relate to adaptation and food selection in adult animals.