EFFECTS OF CHOLINERGIC DEPLETION ON EXPERIENCE-DEPENDENT PLASTICITY IN THE CORTEX OF THE RAT

Clinical and functional studies have strongly suggested that acetylcho line input from the nucleus basalis of Meynert is important for the co rtex's adaptive response to experience. The purpose of this study was to investigate the effects of depletion of acetylcholine inputs from n ucleus basalis of Meynert on experience-dependent plasticity in the co rtex of young adult male rats. The posteromedial barrel subfield in th e primary somatosensory cortex was studied. Experience-dependent plast icity was elicited using a whisker-pairing paradigm in which all whisk ers except D-2 and D-3 were trimmed daily. Plasticity within barrel D- 2 of the posteromedial barrel subfield was measured using the electrop hysiological extracellular recording technique. An index of plasticity was determined in two ways: as an increase in the magnitude of evoked activity to stimulation of whisker D-3 and as a bias in the ratio of evoked activity for stimulation of paired whisker D-3 and cut whisker D-1 (D-3/D-1). Whiskers D-2, D-3 and D-1 were stimulated (deflected) b y a Chubbuck electromechanical stimulator. Cholinergic neurons in the nucleus basalis of Meynert were selectively lesioned with an immunotox in, 192 IgG-saporin, injected into the left lateral ventricle. Lesions of cholinergic neurons in the nucleus basalis of Meynert were verifie d using choline acetyltransferase immunocytochemistry and radioenzymat ic assay. Experience-dependent plasticity was significantly reduced in cholinergic-depleted animals. The magnitude of evoked activity to sti mulation of whisker D-2 increased by 16-100% in control animals compar ed with 0-20% in cholinergic-depleted animals. Similarly, compared to a 60-100% increase in the D-3/D-1 ratio of evoked activity for phospha te-buffered saline-injected control animals, cholinergic-depleted rats showed no significant increase in the D-3/D-1 ratio (0-15%) after und ergoing the whisker-pairing paradigm. After whisker trimming, the D-3/ D-1 response ratio in immunotoxin-treated animals was essentially the same as in control animals that had not been subjected to the whisker- pairing paradigm. This study showed that no significant plasticity res ponse was observed in the absence of cholinergic input from the nucleu s basalis of Meynert. The mechanisms of the action of acetylcholine in cortical plasticity are still not known, bur we hypothesize that this type of plasticity is activity dependent and is significantly enhance d in the presence of acetylcholine. (C) 1997 IBRO. Published by Elsevi er Science Ltd.