PHASIC ACTIVATION OF THE LOCUS-COERULEUS ENHANCES RESPONSES OF PRIMARY SENSORY CORTICAL-NEURONS TO PERIPHERAL RECEPTIVE-FIELD STIMULATION

In the present study we examined the effects of phasic activation of t he nucleus locus coeruleus (LC) on transmission of somatosensory infor mation to the rat cerebral cortex. The rationale for this investigatio n was based on earlier findings that local microiontophoretic applicat ion of the putative LC transmitter, norepinephrine (NE), had facilitat ing actions on cortical neuronal responses to excitatory and inhibitor y synaptic stimuli and more recent microdialysis experiments that have demonstrated increases in cortical levels of NE following phasic or t onic activation of LC. Glass micropipets were used to record the extra cellular activity of single neurons in the somatosensory cortex of hal othane-anesthetized rats. Somatosensory afferent pathways were activat ed by threshold level mechanical stimulation of the glabrous skin on t he contralateral forepaw. Poststimulus time histograms were used to qu antitate cortical neuronal responses before and at various time interv als after preconditioning burst activation of the ipsilateral LC. Exci tatory and postexcitatory inhibitory responses to forepaw stimulation were enhanced when preceded by phasic activation of LC at conditioning intervals of 200-500 ms. These effects were anatomically specific in that they were only observed upon stimulation of brainstem sites close to (> 150 mu m) or within LC and were pharmacologically specific in t hat they were not consistently observed in animals where the LC-NE sys tem had been disrupted by 6-OHDA pretreatment. Overall, these data sug gest that following phasic activation of the LC efferent system, the e fficacy of signal transmission through sensory networks in mammalian b rain is enhanced. (C) 1998 Elsevier Science B.V.