On the distribution and morpho-functional characteristics of 5-HT-immunoreactive cells in the hypothalamus of fetuses and neonatal rats

Abstract

This study attempted to visualize serotonin (5-HT)-immunoreactive (IR) neurons in the hypothalamus of intact fetuses (E18) and neonatal rats (P9) as well as after their pretreatment with some drugs interfering with the 5-HT metabolism and uptake in the serotoninergic neurons ( l-tryptophan, pargyline, 5-hydroxytryptophan, fluoxetine). The 5-HT-IR cells were not observed in the hypothalamus of normal, untreated fetuses and neonatal rats. However, two large accumulations of 5-HT-IR neurons appeared in the anterolateral hypothalamus and in the dorsomedial nucleus after the subsequent injections of the monoamine oxidase inhibitor, pargyline, and the amino acid precursor of the 5-HT synthesis, l-tryptophan. A significantly less intensive reaction was observed after injections either of the second precursor of the 5-HT synthesis, 5-hydroxytryptophan instead of l-tryptophan, or pargyline only. Immunostaining, provoked by the pargyline and l-tryptophan pretreatment, was completely blocked by the injection of the specific 5-HT uptake inhibitor, fluoxetine. It means that the 5-HT immunostaining of the hypothalamic neurons may be accounted for by their capacity to take up specifically 5-HT from the environment rather than by its intraneuronal synthesis from l-tryptophan. Nevertheless, the 5-HT synthesis from 5-hydroxytryptophan in these cells cannot be excluded. The uptake of extracellular 5-HT into catecholaminergic neurons can be excluded as nomifensine, the specific inhibitor of the uptake to these neurons, did not modify the immunostaining.