Vascular changes associated with brain functions are thought to be tightly coupled with neuronal activity through neuronal glucose consumption or the local release of vasoactive agents. In contrast, another view suggests that cortical blood flow is strongly regulated by the nucleus basalis of Meynert (NBM), independently of regional metabolism. Thus, although cortical regional cerebral blood flow (rCBF) variations induced by somatosensory stimulation are strongly linked to neuronal activity, they may also be partly controlled by the NBM. In the present study, cortical rCBF alterations in response to innocuous brushing of the hindlimb (HL) were investigated by laser speckle contrast imaging. The contribution of NBM to these changes was examined after injection of the GABAergic agonist muscimol into the right NBM, allowing comparison of somatosensory-evoked cortical rCBF modifications before and after NBM inactivation. As expected, HL brushing elicited a robust rCBF increase in the contralateral parietal cortex (PC), over the representation of the HL. However, these alterations were decreased, by approximately 40%, in the hemisphere ipsilateral to muscimol inactivation of NBM, whereas vehicle injection did not produce any significant variation. The results demonstrate that cortical rCBF changes induced by somatosensory stimulation are partly regulated by NBM.

The effects of non-noxious skin stimulation on nerve growth factor (NGF) secretion in the parietal cortex were examined in anesthetized rats. Innocuous skin stimulation was delivered to the left hindlimb with a soft-hair brush. Extracellular NGF in the right parietal cortex was collected by microdialysis methods using a protein-permeable probe and was measured using an enzyme-linked immune-sorbent assay. Brushing produced a significant increase in extracellular NGF levels. This NGF response was not observed in rats pretreated with a nicotinic ACh receptor (nAChR) antagonist mecamylamine. We further examined whether brushing could activate the basal forebrain nucleus (nucleus basalis of Meynert, NBM), which is the main source of cholinergic fibers in the cerebral cortex, by means of functional MRI. The blood oxygen level-dependent signal in the right NBM was significantly higher during brushing compared to baseline. The results suggest that non-noxious skin stimulation activates NBM and promotes NGF secretion in the parietal cortex via nAChRs.

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