Enkephalinergic and catecholaminergic neurons constitute separate populations in the rat Kölliker-Fuse/A7 region.

Using a double-labeling immunohistochemical and in situ hybridization technique for the simultaneous detection of tyrosine hydroxylase and preproenkephalin mRNA, we demonstrate that catecholaminergic and enkephalinergic neurons in the Kölliker-Fuse nucleus (K-F)/A7 region in the dorsolateral pons 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 catecholaminergic neurons, but enkephalinergic neurons are also interspersed among the catecholaminergic neurons. Taken together with previous demonstrations 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 catecholaminergic neurons.