Fos expression in serotonergic midbrain neurons projecting to the paraventricular nucleus of hypothalamus after noxious stimulation of the stomach: A triple labeling study in the rat

Horseradish peroxidase (HRP) was stereotaxically injected into the paraventricular nucleus of the hypothalamus (PVH) and visceral noxious stimulation was given by an administration of formalin into the stomach. The brain sections were subjected to HRP histochemistry and immunostained with Fos and serotonin antibodies. The periaqueductal gray (FAG) and dorsal raphe nucleus (DR) contained single-labeled (Fos, 5HT, HRP), double-labeled (Fos/HRP, Fos/5HT, HRP/5HT) and triple-labeled (HRP/Fos/5HT) neurons. Triple-labeled neurons were mainly observed in the DR and in discrete areas of the FAG ipsilateral to side of the HRP injection. These findings suggest that a population of serotonergic midbrain neurons receive nociceptive visceral information and project to the PVH. (C) 1997 Elsevier Science Ireland Ltd.