Differential expression of immediate early genes in distinct layers of rat cerebral cortex after selective immunolesion of the forebrain cholinergic system

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Abstract
The aim of this study was to show whether reduction or loss of cortical cholinergic activity results in any particular change in the expression of the proto‐on‐cogenes c‐fos and/or c‐jun. To produce cortical cholinergic hypofunction, the monoclonal antibody, 1921gG, to the low‐affinity nerve growth factor receptor p75NGFR coupled to a cytotoxin, saporin, was used as an efficient and selective immunotoxin for cholinergic neurons in rat basal forebrain. Brain sections of adult rats that had received an intracerebro‐ventricular injection of 4 μg of the 1921gG‐saporin were subjected to in situ hybridization using oligonucleotides to detect c‐fos and c‐jun mRNA. Autoradiographs obtained were evaluated by quantitative image analysis. Seven days following injection of the immunotoxin there was a dramatic loss in acetylcholinesterase staining in frontal, parietal, piriform, temporal, and occipital cortices, hippocampus, and olfactory bulb, but not in the striatum and cerebellum. In situ hybridization revealed a considerable increase in the level of c‐fos mRNA in the lateral septum following the cholinergic lesion, whereas in the medial septum both c‐fos and c‐jun mRNA were elevated. Immunolesioning led to a distinct and specific increase in the level of c‐jun but not c‐fos mRNA in the parietal and occipital cortex that was restricted to cortical layer IV. These data suggest that reduced cortical cholinergic activity differentially regulates expression of c‐fos/c‐jun genes in distinct cortical regions of the rat brain.