Distribution of choline acetyltransferase (ChAT) immunoreactivity in the brain of the adult trout and tract-tracing observations on the connections of the nuclei of the isthmus

The distribution of cholinergic neurons and fibers was studied in the brain and rostral spinal cord of the brown trout and rainbow trout by using an a ntiserum against the enzyme choline acetyltransferase (ChAT). Cholinergic n eurons were observed in the ventral telencephalon, preoptic region, habenul a, thalamus, hypothalamus, magnocellular superficial pretectal nucleus, opt ic tectum, isthmus, cranial nerve motor nuclei, and spinal cord. In additio n, new cholinergic groups were detected in the vascular organ of the lamina terminalis, the parvocellular and magnocellular parts of the preoptic nucl eus, the anterior tuberal nucleus, and a mesencephalic tegmental nucleus. T he presence of ChAT in the magnocellular neurosecretory system of trout sug gests that acetylcholine is involved in control of hormone release by neuro secretory terminals. In order to characterize the several cholinergic nucle i observed in the isthmus of trout, their projections were studied by appli cation of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorat e (DiI) to selected structures of the brain. The secondary gustatory nucleu s projected mainly to the lateral hypothalamic lobes, whereas the nucleus i sthmi projected to the optic tectum and parvocellular superficial pretectal nucleus, as previously described in other teleost groups. In addition, oth er isthmic cholinergic nuclei of trout may be homologs of the mesopontine s ystem of mammals. We conclude that the cholinergic systems of teleosts show many primitive features that have been preserved during evolution, togethe r with characteristics exclusive to the group. (C) 2000 Wiley-Liss, Inc.