ENKEPHALINERGIC AND CATECHOLAMINERGIC NEURONS CONSTITUTE SEPARATE POPULATIONS IN THE RAT KOLLIKER-FUSE A7 REGION/

Using a double-labeling immunohistochemical and in situ hybridization technique for the simultaneous detection of tyrosine hydroxylase and p reproenkephalin mRNA, we demonstrate that catecholaminergic and enkeph alinergic neurons in the Kolliker-Fuse nucleus (K-F)/A7 region in the dorsolateral pens constitute separate populations. The enkephalinergic cell group is much larger than the catecholaminergic cell group. Most of the enkephalinergic neurons are located caudal to the catecholamin ergic neurons, but enkephalinergic neurons are also interspersed among the catecholaminergic neurons. Taken together with previous demonstra tions that the enkephalinergic neurons in K-F give rise to descending projections to the ventrolateral medulla and spinal cord, the current observations suggest that the antinociceptive effects that result from electrical stimulation of K-F may be a consequence of the activation of enkephalinergic neurons, either alone or in conjunction with catech olaminergic neurons.