Oxytocin, but not arginine-vasopressin neurons project from the hypothalamus to amygdala in human: DiI-based tracing study in postmortem brain.

• Time course of postmortem tracing can be shortened by microinjections of DiI dissolved in ethanol to 2-3 mm/day. • Single oxytocinergic neurons of the supraoptic nucleus project to the medial nuclei of the amygdala. • Vasopressinergic magnocellular perikarya of the paraventricular nucleus project to the mammillary bodies. • Most of the hypothalamic projections to the amygdala are devoid of oxytocin or arginine-vasopressin immunostaining. The hypothalamic neuropeptides oxytocin (OT) and arginine-vasopressin (AVP) are important factors involved in the control of socio-emotional behaviors via their modulation of amygdala functions. Since anatomical pathways of magnocellular projections to limbic structures in the human brain have not been dissected, we infused ethanol-dissolved tracer DiI into three amygdala nuclei - medial, central and lateral nuclei, and into the mammillary bodies of postmortem fixed human brains. With this modification, lipophilic diffusion of DiI occurred much faster than with conventional DiI crystals. After staining of resliced sections with antibodies against OT or AVP, we detected DiI/OT-positive neurons and their axons, specifically in the supraoptic nucleus (SON), but not in other hypothalamic nuclei producing OT or AVP. DiI fluorescence was found in the lateral portion of the paraventricular nucleus (PVN) and in the fornix columns, together with VP- immunoreactivity, only after DiI injections into the mammillary bodies. Our findings indicate that OT and AVP may have distinct neuronal pathways to the limbic system, and they are different from those previously reported in rodents. [ABSTRACT FROM AUTHOR]