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.