Spinal cord-projecting vasopressinergic neurons in the rat paraventricularhypothalamus

The paraventricular hypothalamic nucleus (PVH) is a key structure for the m aintenance of homeostasis. Homeostatic regulation includes modulation of si gnaling in the spinal cord. This may be exerted by neurons in the PVH with spinal projections. However, the PVH is not a homogeneous structure, but co nsists of anatomically and functionally distinct subdivisions. In this stud y, we have analyzed the distribution of spinal cord-projecting PVH neurons that express vasopressin, an important neuropeptide in autonomic regulation . Vasopressinergic neurons were identified with a radiolabeled riboprobe co mplementary to vasopressin mRNA combined with immunohistochemical labeling of retrogradely transported cholera toxin subunit b in spinally projecting neurons. More than 40% of the spinally projecting neurons in the PVH of nai ve Sprague-Dawley rats were found to express vasopressin mRNA. The lateral parvocellular subdivision and the ventral part of the medial parvocellular subdivision contained the densest distribution of spinal cord-projecting va sopressin mRNA-expressing neurons. The magnocellular subdivisions displayed large numbers of vasopressin mRNA-expressing neurons, but very few of thos e projected to the spinal card. The dorsal parvocellular subdivision contai ned a large number of spinally projecting neurons, but very few of those ex pressed vasopressin mRNA. These findings show that the PVH gives rise to a major vasopressinergic projection to the spinal cord and that the spinal co rd-projecting vasopressinergic neurons are parceled into anatomically disti nct cell groups. This provides an anatomical basis for a selective activati on of functionally different groups in the PVH as part of a behaviorally ad aptive response, including modulation of autonomic activity and pain proces sing at the spinal level. (C) 1999 Wiley-Liss Inc.