An enkephalinergic projection from the hypothalamic paraventricular nucleus to the hypothalamic ventromedial nucleus of the rat: an experimental immunohistochemical study.

The distribution and origins of enkephalin-like immunoreactive (ENK-IR) fibers in the nucleus ventromedialis hypothalami (vm) of the rat were examined using immunohistochemistry. A dense plexus was evenly distributed in the vm with no regional differences. A group of ENK-IR neurons was concentrated in the ventrolateral portion of the vm. The destruction of the hypothalamic paraventricular nucleus (pv) which contained numerous ENK-IR neurons, resulted in a marked decrease in ENK-IR fibers in the vm on the operated side. In addition, the destruction of the magnocellular portion of the pv, while leaving most of parvocellular portion intact, failed to substantially decrease the ENK-IR fibers in the vm, suggesting that these fibers originate from ENK-IR neurons located in the parvocellular portion of the pv. The present study further showed that the axons of these neurons first proceeded laterally to the perifornical area, next ran ventrolaterally to the ventrolateral portion of the anterior hypothalamic nucleus and finally turned medially to the vm.