DIFFERENTIAL-EFFECTS OF SEROTONIN AND RAPHE GRAFTS IN THE HIPPOCAMPUSAND HYPOTHALAMUS - A COMBINED BEHAVIORAL AND ANATOMICAL STUDY IN THE RAT

Combined with a partial cholinergic deficiency, serotonergic lesions i nduce severe spatial learning deficits. Serotonergic lesions, however, have additional effects, such as reduced body weight and disruption o f thermoregulation, which may be the cause of the observed learning de ficits. Restoration of the serotonergic innervation of the hippocampus by raphe grafts reduces these learning deficits. The effects of the g rafts may result from a direct support of spatial learning but may als o be an indirect result of preventing some of the other effects of ser otonergic lesions. In the present study we used raphe grafts to examin e the selectivity and specificity of the effects of serotonergic lesio ns in the rat, and used the behavioural effects as an indication of su ccessful transplantation in order to examine the fine details of such grafts. Raphe grafts in the hippocampus did not prevent the effects of the lesions on body weight, thermoregulation and exploratory behaviou r but did minimize the effects of the lesions on spatial learning. In contrast, raphe grafts in the hypothalamus reduced the effects of the lesions on thermoregulation but failed to support learning. The grafte d fibres showed termination specificity with the interneurons, which i s typical of the serotonergic innervation of the normal hippocampus. T he results indicate that the serotonergic innervation of the hippocamp us functions locally to support spatial learning. This role of seroton in is independent of its involvement in modulation of body weight, the rmoregulation or exploratory behaviour. The results confirm that the m odes of serotonergic action in the hippocampus include the selective i nnervation of specific interneuron subpopulations.