The effect of intracerebral injections of 5,7-dihydroxytryptamine and 6-hydroxydopamine on the serotonin-immunoreactive cell bodies and fibers in the adult rat hypothalamus

The effect of intracerebral injections of 5,7-dihydroxytryptamine (5,7-DHT) and 6-hydroxydopamine (6-OHDA) on the serotonin (5-HT)-immunoreactive (IR) cell bodies¹¹ and fibers in the adult rat hypothalamus was studied with 5-HT immunocytochemistry. In rats pretreated with pargyline and l-tryptophan, 5-HT-IR cells were seen in the ventromedial part of the dorsomedial nucleus (DMN) and 5-HT-IR fibers in all hypothalamic areas. In the ventrolateral part of the DMN the 5-HT-IR fibers were of a much finer type than those seen in other hypothalamic areas. Five days after unilateral injection of 5,7-DHT into the dorsolateral hypothalamus, the 5-HT-IR cells were absent from the DMN, and there was a decrease in the number of 5-HT-IR fibers throughout the hypothalamus ipsilateral to the injection. Contralateral to the injection there was evidence of selective 5-HT fiber degeneration but the 5-HT-IR cells and the group of fine fibers in the ventrolateral DMN remained. Unilateral injection of 6-OHDA into the dorsolateral hypothalamus had no effect on the 5-HT-IR fibers or cell bodies in the hypothalamus. Twelve days after unilateral injection of 5,7-DHT into the rostral midbrain, the majority of 5-HT-IR fibers in the ipsilateral hypothalamus had disappeared. The 5-HT-IR cell bodies in the DMN and the group of fine 5-HT-IR fibers in the ventrolateral DMN remained on both sides of the hypothalamus. These results support our previous finding of a group of 5-HT-IR cell bodies in the ventromedial DMN of the hypothalamus, and suggest that these cells innervate the ventrolateral part of the same nucleus. Evidence that these cells constitute a new 5-HT cell group, B-10, is discussed.