Conditioned taste aversion and c-fos expression in the rat brainstem after administration of various USs

Conditioned taste aversion (CTA) is a form of association learning established when an animal associates a conditioned stimulus (taste) with a subsequent unconditioned stimulus (US, illness). We have studied the relationship between the efficacy of inducing CTA to 0.1% saccharin and c-fos expression in the lower brain stem following administration of 13 different USs in rats. The effective USs were grouped into abdominal irritants, rewarding drugs and emetic agents. Regardless of the properties of USs, good correlation was detected between the strength of CTA and c-fos expression within the area postrema, caudal and intermediate subdivisions of nucleus tractus solitarius and the external lateral subnucleus of the parabrachial nucleus. Only hypertonic saline was exceptional because of the experimental procedure: it induced strong c-fos expression, but was not an effective US for CTA formation. Different dosages of the emetic LiCl induced CTA and c-fos expression in a dose-dependent manner.