The aim of this study was to characterize the effects of cortical cholinergic denervation on cholinergic parameters in the cerebral cortex and basal forebrain using a novel immunotoxin (conjugate of the monoclonal antibody 192IgG against the low‐affinity nerve growth factor receptor armed with cytotoxin saporin) to efficiently and selectively lesion cholinergic neurons in rat basal forebrain. Seven days following an intracerebroventricular injection of the cholinergic immunotoxin 192IgG‐saporin the binding levels of nicotinic and M1‐ and M2‐muscarinic acetylcholine receptors (mAChR), high‐affinity choline uptake sites, as well as the m1‐m4 mAChR mRNA were determined in coronal brain sections by both receptor autoradiography and in situ hybridization, and quantified by image analysis. Hemicholinium‐3 binding to high‐affinity choline uptake sites was decreased by up to 45% in all cortical regions and in the hippocampus after a single injection of the immunotoxin compared to controls. In contrast, M1‐mAChR sites were increased over the corresponding control value in the anterior parts of cingulate, frontal, and piriform cortex by about 20%, in the hindlimb/forelimb areas (18%), in the parietal cortex (35%), in the occipital cortex area 2 (17%), as well as in the temporal cortex (25%) following immunolesion. M2‐mAChR levels were found to be significantly increased in the posterior part of the parietal cortex area 1 (by about 22%) and in the occipital cortex area 2 (20%) only. With respect to laminar cortical localization, M2‐mAChRs and choline uptake sites were altered in all cortical layers, whereas M1‐mAChRs were preferentially affected in the upper cortical layers by the immunolesion. The increase in M1‐mAChR 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 M2‐mAChR sites in the occipital and temporal cortex following immunolesion were accomplanied by an increase in the m4 (by 25%) but not m2 mAChR mRNA. There was no effect of the immunolesion on the m1‐m4 mAChR mRNA in frontal cortical regions. In the basal forebrain, however, immunolesioning caused about a 40% decrease in the level of m2 mAChR mRNA in the medial and lateral septum as well as in the vertical and horizontal limb of the diagonal band, whereas M1‐ and M2‐mAChR binding and the levels of m1, m3, and m4 mAChR mRNA were not affected by the immunolesion in any of the basal forebrain nuclei studied. Seven days after a single dose of the 192IgG‐saporin immunotoxin there was no change in the level of cortical nicotinic acetylcholine receptor sites in any of the regions studied compared to corresponding controls. The region‐specific changes in the level of M1‐ and M2‐mAChRs, as well as corresponding receptor gene expression and the lack of effects on cortical nicotinic receptors, may be part of an adaptive mechanism in response to cholinergic degeneration. These data further support the usefulness of the 192IgG‐saporin conjugate as an appropriate tool to produce cortical cholinergic dysfunction. © 1995 Wiley‐Liss, Inc.
- choline uptake site
- image analysis
- in situ hybridization
- muscarinic acetylcholine receptor
- nicotinic receptor
ASJC Scopus subject areas
- Cellular and Molecular Neuroscience