Phasic activation of the locus coeruleus enhances responses of primary sensory cortical neurons to peripheral receptive field stimulation

Abstract

In the present study we examined the effects of phasic activation of the nucleus locus coeruleus (LC) on transmission of somatosensory information to the rat cerebral cortex. The rationale for this investigation was based on earlier findings that local microiontophoretic application of the putative LC transmitter, norepinephrine (NE), had facilitating actions on cortical neuronal responses to excitatory and inhibitory synaptic stimuli and more recent microdialysis experiments that have demonstrated increases in cortical levels of NE following phasic or tonic activation of LC. Glass micropipets were used to record the extracellular activity of single neurons in the somatosensory cortex of halothane-anesthetized rats. Somatosensory afferent pathways were activated by threshold level mechanical stimulation of the glabrous skin on the contralateral forepaw. Poststimulus time histograms were used to quantitate cortical neuronal responses before and at various time intervals after preconditioning burst activation of the ipsilateral LC. Excitatory 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 micron) or within LC and were pharmacologically specific in that they were not consistently observed in animals where the LC-NE system had been disrupted by 6-OHDA pretreatment. Overall, these data suggest that following phasic activation of the LC efferent system, the efficacy of signal transmission through sensory networks in mammalian brain is enhanced.