Basal Forebrain Cholinergic System Is Involved in Rapid Nerve Growth Factor (NGF)-Induced Plasticity in the Barrel Cortex of Adult Rats

Departments of 1 Neurobiology and Behavior, 2 Biomedical Engineering, and the 3 Center for the Neurobiology of Learning and Memory, University of California, Irvine, California, 92697-4550 Submitted 21 May 2003; accepted in final form 15 September 2003 We have previously reported that topical application of nerve growth factor (NGF) to the barrel cortex of an adult rat rapidly augmented a whisker functional representation (WFR) by increasing its area and height within minutes after NGF application. In addition, we found that TrkA, the high-affinity NGF receptor, was only found on fibers projecting into the barrel cortex. Here we use a combination of techniques including chronic intrinsic signal optical imaging, neuronal fiber tracking and immunohistological techniques, to test the hypothesis that NGF-induced rapid cortical plasticity is mediated by the cortical projections of the basal forebrain cholinergic system (BFCS). Our studies localize the source of the cells in the BFCS that project to a single WFR and also demonstrate that TrkA-immunoreactive fibers in the cortex are also cholinergic and likely arise from the BFCS. In addition, by selectively lesioning the BFCS cortical fibers with the immunotoxin 192 IgG-saporin, we show that NGF-induced WFR-cortical plasticity is eliminated. These results, taken together with our previously reported imaging results that demonstrated that agonists of the cholinergic system (particularly nicotine) showed transient NGF-like augmentations of a WFR, implicate the BFCS cortical projections as necessary for NGF's rapid plasticity in the adult rat somatosensory cortex. Address for reprint requests and other correspondence: R. D. Frostig, Dept. of Neurobiology and Behavior, University of California, Irvine CA, 92697-4550 (E-mail: rfrostig{at}uci.edu ).