CHOLINE ACETYLTRANSFERASE-IMMUNOREACTIVE PATCHES OVERLAP SPECIFIC EFFERENT CELL GROUPS IN THE CAT SUPERIOR COLLICULUS

Fibers containing acetylcholine (ACh) form distinct patches in the dor sal intermediate gray layer (IGL) of the cat superior colliculus (SC). Although these patches are known to overlap several afferent projecti ons to SC, it is not known whether they are associated with specific p ostsynaptic cell groups. We have examined the relationship of these AC h fiber patches to specific efferent cell groups by combining retrogra de transport of horseradish peroxidase (HRP) with choline acetyltransf erase (ChAT) immunocytochemistry. Successful HRP injections were made into the predorsal bundle (PB), the tecto-pontine-bulbar pathway (TPB) and the cuneiform region (CFR), the inferior olive (IO), the dorsolat eral pontine gray nucleus (PGD), and the pedunculopontine- tegmental n ucleus (PPTN). The distribution of HRP-labeled neurons which project t o these targets was mapped by a computer-based microscope plotter. Dis tinct clusters of HRP-labeled neurons in the IGL were seen after three injections into the mesencephalic reticular formation that involved t he caudal TPB and cuneiform region (CFR), and after one injection into the medial accessory nucleus of IO. As many as seven clusters of labe led neurons were found in some sections through the caudal one-half of SC after the TPB/CFR injections. Each cluster consisted of 3-20 cells , all of which were small to medium in size. In sections also tested f or ChAT, the cell clusters in the TPB/CFR cases were found to overlap precisely the ACh patches in the IGL. In addition, SC neurons projecti ng to the IO formed clusters above the ChAT patches and in the interme diate white layer (IWL) of SC. None of the other HRP injections produc ed any obvious cell clusters in the deep layers of SC. These results a re the first to show that specific cell groups, distinguished by size and projection site, form clusters that match the patch-like innervati on of cholinergic afferents to SC. This modular organization may corre spond to saccade-related cells that have also been reported to be orga nized into clusters in the cat SC. (C) 1993 Wiley-Liss, Inc.