Although the distribution of cholinergic cells is remarkably similar across the vertebrate
species, no data are available on more primitive species, such as cartilaginous fishes.
To extend the evolutionary analysis of the cholinergic systems, we studied the distribution
of cholinergic neurons in the brain and rostral spinal cord of Scyliorhinus canicula
by immunocytochemistry using an antibody against the enzyme choline acetyltransferase
(ChAT). Western blot analysis of brain extracts of dogfish, sturgeon, trout, and rat
showed that this antibody recognized similar bands in the four species. Putative cholinergic
neurons were observed in most brain regions, including the telencephalon, diencephalon,
cerebellum, and brainstem. In the retrobulbar region and superficial dorsal pallium
of the telencephalon, numerous small pallial cells were ChAT-like immunoreactive.
In addition, tufted cells of the olfactory bulb and some cells in the lateral pallium
showed faint immunoreactivity. In the preoptic-hypothalamic region, ChAT-immunoreactive
(ChAT-ir) cells were found in the preoptic nucleus, the vascular organ of the terminal
lamina, and a small population in the caudal tuber. In the epithalamus, the pineal
photoreceptors were intensely positive. Many cells of the habenula were faintly ChAT-ir,
but the neuropil of the interpeduncular nucleus showed intense ChAT immunoreactivity.
In the pretectal region, ChAT-ir cells were observed only in the superficial pretectal
nucleus. In the brainstem, the somatomotor and branchiomotor nuclei, the octavolateral
efferent nucleus, and a cell group just rostral to the Edinger-Westphal (EW) nucleus
contained ChAT-ir neurons. In addition, the trigeminal mesencephalic nucleus, the
nucleus G of the isthmus, some locus coeruleus cells, and some cell populations of
the vestibular nuclei and of the electroreceptive nucleus of the octavolateral region
exhibited ChAT immunoreactivity. In the reticular areas of the brainstem, the nucleus
of the medial longitudinal fascicle, many reticular neurons of the rhombencephalon,
and cells of the nucleus of the lateral funiculus were immunoreactive to this antibody.
In the cerebellum, Golgi cells of the granule cell layer and some cells of the cerebellar
nucleus were also ChAT-ir. In the rostral spinal cord, ChAT immunoreactivity was observed
in cells of the motor column, the dorsal horn, the marginal nucleus (a putative stretch-receptor
organ), and in interstitial cells of the ventral funiculus. These results demonstrate
for the first time that cholinergic neurons are distributed widely in the central
nervous system of elasmobranchs and that their cholinergic systems have evolved several
characteristics that are unique to this group.