Alteration of the characteristics of learned taste aversion by manipulation of serotonin levels in the rat.

Electrolytic lesions placed in the dorsal and median raphe nuclei of the rat brain deplete hypothalamic and telencephalic serotonin. These lesions also enhance the learned suppression of saccharin consumption which results from pairing the ingestion of a saccharin solution with the injection of a toxic drug. Pretreatment of rats with raphe lesions with the serotonin precursor DL-5-hydroxy-tryptophan (5HTP) immediately prior to the conditioning trial blocks the learning of the aversion to saccharin. In normal rats, 5HTP pretreatment also attenuates the suppressive effects of conditioning on saccharin drinking. These results differ from the findings of previous research using the flinch-jump technique. When sensitivity to shock is measured, 5HTP pretreatment in rats with forebrain serotonin depletion has been reported to restore both serotonin levels and behavior to normal. No behavioral effects are observed in normal animals. Possible explanations for the differential effects obtained in the two paradigms are discussed.