Interaction of norepinephrine with cerebrocortical activity evoked by stimulation of somatosensory afferent pathways in the rat

This study investigated the action of norepinephrine (NE) on transmission of information through somatosensory cortical neuronal circuits. In the forelimb region of rat somatosensory cortex (SI), single-unit responses to natural stimulation (foot tap) of afferent pathways were recorded before, during, and after microiontophoretic application of NE. Actions of NE were quantitatively assessed by computer-based analysis of poststimulus time histograms. Overall, NE was found to enhance both excitatory and inhibitory responses generated by the afferent synaptic input. In 78% of the cells tested, low doses of NE differentially suppressed background discharge more than stimulus-bound excitation such that the signal to noise ratio was enhanced approximately twofold. Evoked spiking in 12 cells was quantitatively increased above control values during NE administration. In 82% of the units examined, NE augmented stimulus-bound inhibition and postexcitatory suppression of activity. Potentiation of inhibition was observed in 5 cells at doses of NE which caused little or no depression of spontaneous activity. These observed effects on neuronal responsiveness to afferent synaptic input often persisted for several minutes after termination of NE iontophoresis. Such modulatory actions of NE were demonstrated for cells situated throughout the vertical extent of the cortex. These results suggest that low amounts of NE may facilitate transfer of afferent information within the cerebral cortical circuitry and are consistent with a modulatory role rather than a specific information transfer function for NE.