Cerebral blood flow responses to somatosensory stimulation are unaffected by scopolamine in unanesthetized rat

Studies with positron-emission tomography have indicated that muscarinic ac etylcholine receptors may be involved in the mechanism of enhancement of ce rebral blood flow (CBF) by neuronal functional activation. We examined the effects of muscarinic receptor blockade by scopolamine on the local CBF res ponses to vibrissal stimulation in the whisker-to-barrel cortex sensory pat hway in unanesthetized rats. Local CBF was measured by the quantitative aut oradiographic [C-14]iodoantipyrine method. Scopolamine (0.4 or 0.8 mg/kg) w as injected i.v. 30 min before measurement of local CBF; control rats recei ved equivalent volumes of physiological saline. Vibrissae on the left side of the face were stroked continuously throughout the 1-min period of measur ement of CBF. Local CBF was determined bilaterally in four structures of th e pathway, i.e., spinal and principal sensory trigeminal nuclei, ventral po steromedial thalamic nucleus, and barrel field of the sensory cortex, as we ll as in four representative structures unrelated to the pathway. The highe r dose of scopolamine raised baseline CBF in the two trigeminal nuclei, but neither dose diminished the percentage of increases in local CBF because o f vibrissal stimulation in any of the stations of the pathway. These result s do not support involvement of muscarinic receptors in the mechanism of en hancement of local CBF by functional neuronal activation, at least not in t he whisker-barrel cortex sensory pathway in the unanesthetized rat.