Absence of lithium-induced taste aversion after area postrema lesion

Abstract

Anatomical and physiological evidence indicates that the area postrema (AP) lacks a blood-brain barrier6, la and is thus permeable to blood-born substances which do not penetrate most other areas of the brain. The classical experiments of Borison (refs. 5, 7) demonstrated that the AP region contains chemoreceptors which stimulate vomiting in some species in response to certain drugs. This finding suggests that one function of the AP is to detect ingested toxins and trigger their expulsion from the gastrointestinal tract. In addition to producing vomiting, some toxic substances, which are encountered by animals in association with novel foods, may result in formation of learned responses known as conditioned taste aversions (CTAs)3A 1. These learned responses decrease the probability that animals will subsequently ingest particular foods which were previously associated with toxic effects. Berger et al. 4 were the first to demonstrate that the AP plays a critical role in the formation of CTA by some chemical substances. Taste aversions formed by scopolamine methyl nitrate (SMN), a neuroactive substance which does not cross the blood-brain barrier, were shown to be mediated by the AP. On the other hand, the CTA formed by D-amphetamine, a drug which readily penetrates the blood-brain barrier, does not require an intact AP. On the basis of these data, it appears that more than one mechanism may exist for the formation of CTAs. Moreover, these results provide evidence of an important role for the AP region in behavioral, as well as in gastrointestinal, functions. Lithium chloride (LiCI) is a compound which is widely used in CTA studles2,~2, t3. However, the mechanism by which LiCI induces CTAs ts not known. The