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Abstract:
Salient events evoke burst-like responses of noradrenergic (NE) neurons of the Locus Coeruleus (LC) and dopaminergic (DA) neurons of the ventral tegmenta l area (VTA). The associated NE and DA release modulates information processing in the projection targets of LC and VTA. In the rat, terminal fields of both LC-NE and VTA-DA neurons converge in the medial pref rontal cortex (mPFC), a cortical area controlling many cognitive capacities. We investigated the role of LC phasic activation for sensory responses in VTA and mPFC. Under urhetaine anestesia, noxious stimulatio n (foot shock, FS) produces a robust short-latency (~20 ms) excitation of LC-NE neurons. In VTA and mPFC, the firing rate modulation induced by FS was present in ~30% of neurons. We classified FS-ind uced responses of VTA neurons according to latency (early: ~40 ms or late: ~150 ms) and duration (phasic: < 300 ms or sustained: > 300 ms). Similarly, the mPFC single-unit responses differed by latency and/or duration. Supression of LC ongoing and FS-evoked activity by iontophoretic injection of clonidine, an alpha2-adrenergic receptor agonist, reduced responsiveness in both VTA and mPFC. Population of initially ‘non-responsive’ mPFC neurons showed ‘gating-effect’. Spontaneous disc harge of substantial proportion of VTA and mPFC neurons was bidirectionally modulated. These results suggest that depending on the motivation al valence of a salient event, LC phasic activation and associated NE releas e may selectively enhance or supress signalling within different and, possibly competing mesolimbic and meso cortical pathways. The behavioral data supporting this hypothesis will be presented.
