Extranuclear axon collaterals of paraventricular neurons in the rat hypothalamus: intracellular staining, immunocytochemistry and electrophysiology.

Recent studies have suggested that some paraventricular nucleus (PVN) neurons projected to more than one target and, thereby, perhaps coordinate some aspects of seemingly diverse functions. We have systematically investigated the existence, location, hormonal contents and functional integrity of some axon collaterals arising from PVN neurons. This was done using intracellular injections of the fluorescent dye, Lucifer Yellow, extracellular ejections of horseradish peroxidase (HRP), immunocytochemistry with antisera directed against vasopressin (VP) and oxytocin (OX) and electrophysiological analysis of synaptic activation of perifornical neurons in response to electrical stimulation of the PVN in hypothalamic slices. Each of the three morphological techniques revealed clear axon collaterals, arising in the lateral hypothalamus and generally ventrolateral to the PVN. Most branching axons appeared to have a small number of branch points, and many collaterals appeared to terminate near their parent axon. Electrical stimulation of the PVN was found to activate synaptically perifornical neurons located in the areas where the other methods revealed collaterals. Stimulation outside of the nucleus was ineffective unless current intensities were increased 10-30-fold over those applied to the PVN. We conclude that many PVN neurons, at least some of these containing OX and other VP, give rise to axons that branch in the perifornical and more ventral lateral hypothalamus, and that some of their collaterals probably terminate on neurons close to the PVN.