Divergent axon collaterals of rat locus coeruleus neurons: demonstration by a fluorescent double labeling technique

The distribution of the noradrenaline-containing neurons of the rat locus coeruleus has been investigated with retrograde labeling techniques using two different fluorescent tracers. Injections were placed in the prefrontal cortex, the striatum, the thalamus, the hippocampus, the cerebellar cortex and the lumbar spinal cord. No evidence for locus coeruleus projections to the striatum was found. Injections in the cortex, thalamus and hippocampus revealed not only ipsilateral but also contralateral labeling of cells in the locus coeruleus. Following unilateral or bilateral homo- or heterotopic injections of the two tracers several cells of the locus coeruleus were double labeled. Combined injections of the two fluorophores in any of these forebrain areas and in the spinal cord also produced double labeled cells. The majority of double labeled cells was located in an area between the ventral and the dorsal parts of the locus coeruleus. These results indicate that individual neurons of the locus coeruleus have the possibility to influence adrenergic receptors at remote areas in the central nervous system.