Identification of branching paraventricular neurons of the hypothalamus that project to the rostroventrolateral medulla and spinal cord.

The paraventricular nucleus of the hypothalamus has efferent connections to autonomic nuclei known to ultimately regulate cardiovascular function. Studies have revealed projections to the sympathetic preganglionic neurons of the spinal cord and presympathetic motor neurons of the rostral ventrolateral medulla. This study set out to establish whether the same neurons in the paraventricular nucleus innervate both these regions. In rats the fluorescent neuroanatomical tracers FluoroGold, Fast Blue or Dextran tetramethyl rhodamine were injected into either the rostral ventrolateral medulla or T2 region of the spinal cord. After a suitable survival period (five to seven days) three populations of neurons could be identified in the paraventricular nucleus, double-labelled neurons and single-labelled neurons resulting from the injections into the spinal cord or injections into the rostral ventrolateral medulla. The neurons were of similar size regardless of the dye content. Most neurons were found in the parvocellular subdivision of the mid rostral paraventricular nucleus. The number of labelled neurons decreased in the caudal sections. This study provides an anatomical basis for three means of influence that the paraventricular nucleus can have on sympathetic activity; a hierarchical in series projection via the rostral ventrolateral medulla; a projection running in parallel with this but bypassing the rostroventrolateral medulla; and a branching population innervating neurons in both the rostral ventrolateral medulla and spinal cord. The paraventricular nucleus of the hypothalamus is an important brain area concerned with maintaining cardiovascular homeostasis. This anatomical study has not only provided confirmatory evidence that direct projections arising from the paraventricular hypothalamic nucleus do project to the rostral ventrolateral medulla and spinal cord, regions known to influence cardiovascular regulation. The study has identified a branching projection originating in the paraventricular nucleus of the hypothalamus that projects to both the rostral ventrolateral medulla and the spinal cord. Thus the paraventricular nucleus of the hypothalamus has three pathways in which to influence cardiovascular homeostasis.