On the origin of the serotonergic input to the intermediate lobe of the rat pituitary

Serotonin-containing nerve fibers have been visualized immunocytochemically in the intermediate lobe of the rat pituitary. A 50% depletion of the serotonin level in the intermediate lobe was obtained in our previous experiment in rats with pituitary stalk transection, which may represent the total neuronally derived serotonin there. In the present studies we have attempted to determine the source of these fibers by examining the effect of hypothalamic and midbrain lesions or fiber transections on serotonin levels and serotonin-containing neuronal elements. Complete hypothalamic deafferentation resulted in a significant reduction of immunostained fibers in the intermediate lobe. A 27% fall in the serotonin content (measured by HPLC and electrochemical detection) and a significant disappearance of immunostained fibers were observed after transecting the ascending fibers from the raphe nuclei towards the hypothalamus. The transection combined with the lesioning of the hypothalamic dorsomedial nuclei resulted in a 50% decrease of serotonin level in the intermediate lobe. The present data therefore suggest that serotonin fibers in the intermediate lobe may originate from cells both in the midbrain raphe and hypothalamic dorsomedial nuclei.