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 sire for cardiovascular integration because of its connections to regions in the brain and spinal cord which are know n to be important in cardiovascular control. Like the vasomotor neuron s of the rostral ventrolateral medulla, descending axons from paravent ricular neurons can be identified that form synapses on sympathetic pr eganglionic neurons in the thoracic spinal cord. The purpose of this s tudy was to determine whether paraventricular axons project to the ros tral ventrolateral medulla and whether they are closely apposed to ret iculospinal neurons in this region. Descending paraventricular axons w ere labelled with biotin dextran amine, while rostral ventrolateral me dullary neurons were retrogradely labelled from the spinal cord with w heatgerm agglutinin conjugated to horseradish peroxidase. This reveale d, within the rostral ventrolateral medulla, paraventricular axon and terminal varicosities closely apposed to and apparently contiguous wit h retrogradely labelled spinally projecting neurons. Thus our study at the light microscopical level has shown the potential for the paraven tricular nucleus to directly influence rostral ventrolateral reticulos pinal 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. (C) 1998 IBRO. publishe d by Elsevier Science Ltd.