The role of the medial-external subnucleus of the medial parabrachial nucleus in hypertonic NaCl-induced concurrent and delayed-sequential flavor avoidance learning.

In behavioral as well as neuroanatomical studies, the medial area of the parabrachial complex has been associated with flavor avoidance learning (FAL). Within this medial area the medial external subnucleus (PBNme) has been linked to gustatory-visceral convergence or integration, which is apparently required for flavor-illness learning. Therefore, this area may play an important role in sensorial (gustatory and/or visceral) processing and/or in the gustatory-visceral integrative processes implicated in FAL. The present study used a lesion approach in rats to explore the effects of damage to the medial-external medial parabrachial nucleus (PBNme) in FAL using different FAL paradigms and hypertonic NaCl as the aversive agent. In the first of two experiments, PBNme-lesioned rats were subjected to a hypertonic NaCl-induced concurrent FAL. In this modality, both the gustatory and the visceral stimulation take place simultaneously. In the second experiment, rats with lesions similarly located (see above) and rats with lesions in the external area of the lateral parabrachial nucleus (PBNle) were subjected to a delayed-sequential FAL induced by the same chemical agent. The results show a clear disruptive effect of the hypertonic NaCl-induced concurrent FAL (experiment 1), as well as of NaCl-induced delayed-sequential FAL in animals lesioned in the PBNme (experiment 2). However, animals lesioned in the PBNle do learn the delayed-sequential task. The underlying nature, sensorial or associative, of the deficits showed by PBNme-lesioned animals in the acquisition of each modality of FAL is discussed.