The cat neostriatum, relative distribution of cholinergic neurons versus serotonergic fibers

The distribution of choline acetyltransferase (ChAT)-containing neurons and serotonin (5-HT)-containing nerve fibers in the cat neostriatum was invest igated by use of immunohistochemical techniques. Both ChAT- and 5-HT-staini ng techniques were applied to alternate brain sections, thereby allowing a precise comparison of the distribution pattern of ChAT-immunopositive cells (ChAT cells) and 5-HT-immunopositive fibers (5-HT fibers). In the neostria tum, ChAT cells were strongly stained throughout their cell bodies and prox imal (first-order) dendrites. The majority of them were multipolar cells wi th a soma diameter of 20-50 mu m (long axis) x 10-30 mu m (short axis). In the caudate nucleus, ChAT cells were evenly and diffusely distributed excep t for the dorsolateral region of its rostral half, in which latter region t hey were distributed in loosely formed clusters. In the rostral portion of the putamen, the density of ChAT-cell distribution was like that in the med ial region of the caudate nucleus. In contrast, this distribution was more dense in the caudomedial region of the putamen, adjacent to the globus pall idus. 5-HT fibers in the neostriatum were dark-stained, of quite fine diame ter (<0.6 mu m), and they contained small, round varicosities (diameter, us ually 0.5-1.0 mu m, but some >1.0 mu m). Such 5-HT fibers were distributed abundantly throughout the caudate nucleus and putamen. In the rostrocaudal portion of the caudate nucleus, their density was high in its dorsal and ve ntral components, and low in the middle component. Throughout the putamen, 5-HT fibers were distributed homogeneously in the mediolateral and dorsoven tral directions. In the caudal portion of the putamen adjacent to the globu s pallidus, the 5-HT fibers had a higher density while maintaining their ho mogenous distribution pattern. In the two main divisions of the striatum, t he so-called 'patch' (acetylcholinesterase (AChE)-poor) and 'matrix' (AChE- rich) compartments, there was a near-even distribution of 5-HT fibers and t erminals. The above results suggest that the 5-HT-dominated, raphe-striatal pathway is optimally arranged for modulating the activity of both the intr insic and the projection neurons of the neostriatum. (C) 2000 Elsevier Scie nce B.V. All rights reserved.