Identification of an efferent projection from the paraventricular nucleus of the hypothalamus terminating close to spinally projecting rostral ventrolateral medullary neurons.

The paraventricular nucleus of the hypothalamus is increasingly being viewed as an important site for cardiovascular integration because of its connections to regions in the brain and spinal cord which are known to be important in cardiovascular control. Like the vasomotor neurons of the rostral ventrolateral medulla, descending axons from paraventricular neurons can be identified that form synapses on sympathetic preganglionic neurons in the thoracic spinal cord. The purpose of this study was to determine whether paraventricular axons project to the rostral ventrolateral medulla and whether they are closely apposed to reticulospinal neurons in this region. Descending paraventricular axons were labelled with biotin dextran amine, while rostral ventrolateral medullary neurons were retrogradely labelled from the spinal cord with wheatgerm agglutinin conjugated to horseradish peroxidase. This revealed, within the rostral ventrolateral medulla, paraventricular axon and terminal varicosities closely apposed to and apparently contiguous with retrogradely labelled spinally projecting neurons. Thus our study at the light microscopical level has shown the potential for the paraventricular nucleus to directly influence rostral ventrolateral reticulospinal neurons. We suggest these connections, if confirmed by electron microscopy, could be one means by which activation of paraventricular neurons elicits alterations in blood pressure.