ORGANIZATION OF THE VISCERAL SOLITARY TRACT NUCLEUS IN THE FERRET AS DEFINED BY THE DISTRIBUTION OF CHOLINE-ACETYLTRANSFERASE AND NERVE GROWTH-FACTOR RECEPTOR IMMUNOREACTIVITY

The cholinergic innervation of the visceral component of the nucleus of the solitary tract in the ferret was investigated by using choline acetyltransferase immunocytochemistry. The subdivisions of the ferret solitary tract nucleus as defined by Nissl architectonics were found to correspond to most of those previously assigned to the cat solitary tract nucleus. The subnuclei of the ferret solitary tract nucleus were also outlined by using immunohistochemical and histochemical methods to stain for nerve growth factor (NGF) receptor and acetylcholinesterase, respectively. In particular, the gelatinosus and interstitial subnuclei stain intensely for NGF receptor immunoreactivity and for acetylcholinesterase activity. Since abundant NGF receptor immunoreactivity is observed also in the nodose ganglion and in the solitary tract, it was assumed that the gelatinosus and the interstitial subnuclei represent the principal sites of termination of primary visceral afferents. A rich choline acetyltransferase-positive terminal axonal arborisation was located in all of the subdivisions of the solitary tract nucleus but was found to be lacking in the gelatinosus and interstitial subnuclei. A small number of giant choline acetyltransferase-positive axon terminals was seen in the subnucleus gelatinosus but was assumed to be of doubtful functional significance because these terminals derive from only one or two large axons on each side of the brain. The weak cholinergic innervation of the gelatinosus and interstitial subnuclei and the stronger innervation of the other subnuclei suggest that acetylcholine has a more important role in the secondary rather than the primary processing of afferent visceral information. Because the distribution of acetylcholinesterase activity in the nucleus of the solitary tract matches that of the NGF receptor immunoreactivity rather than that of the cholinergic acetyltransferase immunoreactivity, a non-cholinergic function for acetylcholinesterase may dominate in the solitary tract nucleus of the ferret.