Deficits in conditioned heart rate and taste aversion in area postrema-lesioned rats

Previous studies have shown that area postrema (AP) lesions cause deficits in conditioned taste aversion in the rat. They also lead to chronically lowered heart rate which can be reversed by the animals' increased appetite for and ingestion of hypertonic saline. Although not previously examined in conditioned taste aversion, changes in autonomic nervous system activity as reflected in heart rate may be an important aspect of conditioning. The present study investigated the effects of AP lesions on heart rate conditioned responses (CRs) and unconditioned responses (UCRs). Two groups of AP lesioned and sham-operated rats, one that did and one that did not drink saline solution to raise heart rate, were studied. Both LiCl and scopolamine, which have opposite effects on heart rate, were the unconditioned stimulus (UCS) agents in two separate studies. In intact rats, LiCl-mediated conditioned taste aversion was associated with decreased conditioned stimulus (CS) intake and decreased heart rate Both effects were blunted by AP lesions, although all rats displayed heart rate UCRs to LiCl. The AP rats that drank saline behaved like intact rats exhibiting both a conditioned taste aversion and conditioned heart rate responses to the CS. Although CS intake decreased, no heart rate CRs developed with scopolamine. Scopolamine-mediated conditioned taste aversion was attenuated in both saline and non-saline drinking AP-lesioned groups. Thus, when conditioned taste aversion was associated with heart rate CRs, the AP lesion-induced deficit was counteracted by saline ingestion. Conversely, when there were no heart rate CRs, conditioned taste aversion was disrupted by the lesion regardless of saline ingestion.