LATERALIZATION AND FUNCTIONAL-ORGANIZATION OF THE LOCUS-COERULEUS PROJECTION TO THE TRIGEMINAL SOMATOSENSORY PATHWAY IN RAT

The primary goals of this study were to (1) examine the distribution o f locus coeruleus (LC) neurons, which project to cortical and subcorti cal sites along the trigeminal somatosensory pathway in rats, and (2) determine the extent to which different regions within this ascending sensory system receive collateral projections from the same LC neuron. Long-Evans hooded rats received unilateral pressure injections of dif ferent combinations of retrograde fluorescent tracers into whisker-rel ated regions of primary (SI) and secondary (SII) somatosensory cortice s, the ventrobasal (VB) and posterior group (POm) nuclei of the thalam us, and the principalis nucleus of the trigeminal complex (PrV). Coron al sections (40-100 mu m) through the LC were examined by fluorescence microscopy, and the distribution of retrogradely labeled cells was re corded. The major finding was that whisker-related regions of the cort ex receive efferent projections from neurons concentrated in the cauda l portion of the ipsilateral LC, whereas subcortical trigeminal somato sensory structures receive bilateral input from both LC nuclei. Despit e the bilateral nature of the LC projection to subcortical sites, the majority of LC efferents to VB and POm thalamus originate in the ipsil ateral LC nucleus, whereas projections to PrV originate primarily from the contralateral LC. An additional finding was that a relatively lar ge proportion of LC cells, which project to a single somatosensory str ucture, also send axon collaterals to other relay sites along the same ascending somatosensory pathway. Taken together, these results sugges t that the LC-noradrenergic system maintains a more selective relation ship with functionally related efferent targets than has been previous ly appreciated. (C) 1997 Wiley-Liss, Inc.