5,7-Dihydroxytryptamine lesions of the ascending 5-hydroxytryptamine pathways: habituation, motor activity and agonistic behavior

Rats were injected stereotactically in mesencephalon with 5,7-dihydroxytryptamine (5,7-DHT) in the medial 5-hydroxytryptamine (5-HT) pathway (n = 8) and in the medial plus the lateral 5-HT pathways (n = 7) or injected with vehicle (n = 8), or sham-operated (n = 8). The 5,7-DHT lesions reduced the in vitro 3H-5-HT uptake in the hypothalamus and the cortex cerebri to 27-51% of control values, 3H-noradrenaline uptake was not significantly changed. 5,7-DHT lesions of the medial, and of the medial plus the lateral, 5-HT induced mouse killing behavior and increased number of boxing positions in the shock elicited fighting test. Both lesions also reduced the rate of habituation to touch, but only the lesion of the medial plus the lateral 5-HT pathway significantly reduced the rate of habituation to acoustic stimulation. Activity in the home cage was not significantly changed by the lesions. It was concluded that selective chemical lesions of the ascending 5-HT pathways result in prolonged habituation of the orienting response and increase in particular components of agonistic behavior. The increase in locomotor activity observed after electrolytic lesions of nucleus raphe medianus seems not to be due only to lesion of the 5-HT neurons ascending from this nucleus.