Leucine-enkephalin-like immunoreactive fibers in the medial preoptic area of the rat: their distribution and origin.

We studied the distribution of fibers with leucine-enkephalin-like immunoreactivity (L-ENKI) in the medial preoptic area (MPO) of the rat, and the origins of such fibers, using indirect immunofluorescence and a combination of a retrograde tracer with immunocytochemistry that we have developed. These fibers were very dense throughout the rostro-caudal part of the MPO. The distribution was uneven with the highest density in the lateral part. Destruction of the arcuate nucleus, which contains a group of L-ENKI neurons, resulted in the marked reduction of these fibers in the ipsilateral MPO, suggesting that most of these fibers originate in this nucleus. This was also suggested by the fact that injection of biotin-wheat germ agglutinin into the MPO labelled many neurons in the arcuate nucleus ipsilaterally. Simultaneous staining with antiserum showed that some of these neurons are L-ENKI.