Feline islands of Calleja complex: II. Cholinergic and cholinesterasic features

Histochemical analyses demonstrated that the islands of Calleja complex (ICC) in the cat is exceptionally rich in choline acetyltransferase (ChAT) and acetylcholinesterase (AChE). Both enzymes are found in neuropil throughout the complex, as well as in a subset of the satellite neurons accompanying Callejal islands. Lateromedial changes in these cholinergic and cholinesterasic tissue elements were consistent with our previous finding that the feline ICC is cytoarchitecturally divided into five successively more medial types of island-satellite cell ensembles or units. In particular, satellite neurons reactive for ChAT and AChE diminished progressively in size and increased steadily in number from the most lateral to the most medial units. A concomitant increase in neuropil levels of both enzymes suggested that the strong cholinergic innervation of the feline ICC is at least partially derived from satellite cells. This possibility gained further credibility from the additional observation that very fine processes from some ChAT and AChE satellite neurons projected into the terminal-like cholinergic field permeating the granular Callejal islands. The granule cells themselves lacked ChAT and (apart from potentially artifactual cases) AChE, as did adjoining groups of dwarf cells and small pyramidal like neurons. The cholinergic and cholinesterasic satellite neurons were preferentially located above tubercular Callejal islands and in otherwise cell-poor spaces within the isla magna. Such neurons appeared to be isodendritic: they commonly had ovoidal somata with one or two processes lacking enzyme-reactive spines. Depending on the type of ICC unit involved, their mean soma length ranged from 15 to 24 micron, all but the largest of which was distinctly smaller than that of ChAT and AChE cells in striatal or basal nuclear structures. Not all the cholinesterase neurons in the feline ICC are cholinergic, judging from the finding that there are a significantly greater number of satellite neurons containing AChE than ChAT. Three cholinergic features of the feline ICC are especially noteworthy. First, each of the island-satellite cell ensembles in the complex is unified by AChE neuropil often denser than that of adjacent striatal areas. Second, cholinergic neuropil is exceptionally dense in the isla magna and in a subpial band under medial Callejal islands. Third, ChAT neurons in the isla magna are among the smallest cholinergic cells found in the brain.