DIFFERENTIAL CHANGES IN CHOLINERGIC MARKERS FROM SELECTED BRAIN-REGIONS AFTER SPECIFIC IMMUNOLESION OF THE RAT CHOLINERGIC BASAL FOREBRAIN SYSTEM

The aim of this study was to characterize the effects of cortical chol inergic denervation on cholinergic parameters in the cerebral cortex a nd basal forebrain using a novel immunotoxin (conjugate of the monoclo nal antibody 192IgG against the low-affinity nerve growth factor recep tor armed with cytotoxin saporin) to efficiently and selectively lesio n cholinergic neurons in rat basal forebrain. Seven days following an intracerebroventricular injection of the cholinergic immunotoxin 192Ig G-saporin the binding levels of nicotinic and M(1)- and M(2)-muscarini c acetylcholine receptors (mAChR), high-affinity choline uptake sites, as well as the m1-m4 mAChR mRNA were determined in coronal brain sect ions by both receptor autoradiography and in situ hybridization, and q uantified by image analysis. Hemicholinium-3 binding to high-affinity choline uptake sites was decreased by up to 45% in all cortical region s and in the hippocampus after a single injection of the immunotoxin c ompared to controls. In contrast, M(1)-mAChR sites were increased over the corresponding control value in the anterior parts of cingulate, f rontal, and piriform cortex by about 20%, in the hindlimb/forelimb are as (18%), in the parietal cortex (35%), in the occipital cortex area 2 (17%), as well as in the temporal cortex (25%) following immunolesion . M(2)-mAChR levels were found to be significantly increased in the po sterior part of the parietal cortex area 1 (by about 22%) and in the o ccipital cortex area 2 (20%) only. With respect to laminar cortical lo calization, M(2)-mAChRs and choline uptake sites were altered in all c ortical layers, whereas M(1)-mAChRs were preferentially affected in th e upper cortical layers by the immunolesion. The increase in M(1)-mACh R binding in the temporal and occipital cortex as a consequence of the immunolesion was complemented by an increase in the amount of m1 and m3 mAChR mRNA of about 20% in these regions. The elevated levels of M( 2)mAChR sites in the occipital and temporal cortex following immunoles ion were accomplanied by an increase in the m4 (by 25%) but not m2 mAC hR mRNA. There was no effect of the immunolesion on the m1-m4 mAChR mR NA in frontal cortical regions, in the basal forebrain, however, immun olesioning caused about a 40% decrease in the level of m2 mAChR mRNA i n the medial and lateral septum as well as in the vertical and horizon tal limb of the diagonal band, whereas M(1)- and M(2)-mAChR binding an d the levels of m1, m3, and m4 mAChR mRNA were not affected by the imm unolesion in any of the basal forebrain nuclei studied. Seven days aft er a single dose of the 192IgG-saporin immunotoxin there was no change in the level of cortical nicotinic acetylcholine receptor sites in an y of the regions studied compared to corresponding controls. The regio n-specific changes in the level of M(1)- and M(2)-mAChRs, as well as c orresponding receptor gene expression and the lack of effects on corti cal nicotinic receptors, may be part of an adaptive mechanism in respo nse to cholinergic degeneration. These data further support the useful ness of the 192IgG-saporin conjugate as an appropriate tool to produce cortical cholinergic dysfunction. (C) 1995 Wiley-Liss, Inc.