Origin, distribution, and morphology of galaninergic fibers in the rodent trigeminal system

The neuropeptide galanin (Gal) is found throughout the central nervous syst em. Of particular interest is the fact that Gal is present within the major ity of noradrenergic locus coeruleus (LC) neurons. However very few, if any , Gal-immunoreactive fibers have been identified in many of the major effer ent targets of LC, including sensory neocortex and dorsal thalamus. The goa l of the present study was to examine the Gal fiber innervation of the rode nt trigeminal somatosensory system and its connection to the LC. Our result s show that at least two different morphological profiles of Gal-immunoreac tive fibers are present within relay nuclei along the ascending trigeminal pathway. Numerous small caliber Gal-immunoreactive fibers with bouton-like swellings were noted within the barrel cortex, the ventroposterior medial ( VPM) nucleus, the posterior medial (POm) nucleus, the zona incerta (ZI), th e reticular nucleus (nRT) of the thalamus, and the principal (PrV) and spin al (SpV) nuclei of the trigeminal complex. Immunoreactive fibers were preva lent in, but not restricted to, layer I of the barrel cortex. Within the so matosensory thalamus, the density of Gal-immunoreactive fibers was higher i n POm than in VPM. Laminae I and II of SpV and the nRT and ZI also containe d dense, large-diameter Gal-immunoreactive fibers. These large-diameter Gal -immunoreactive fibers did not co-contain dopamine beta-hydroxylase (DBH). In contrast, virtually every small-caliber Gal-immunoreactive fiber colocal ized with DBH. To determine whether Gal-immunoreactive fibers originated fr om LC, we combined immunohistochemical procedures with fluorescent tracing techniques. After retrograde tracer injections into several trigeminal rela y nuclei, we observed that approximately 50% of the labeled LC neuronal pop ulation was immunoreactive for Gal. Our results suggest an extensive Gal-im munoreactive fiber innervation of the rodent trigeminal system, much of whi ch may originate from LC neurons in the brainstem. (C) 1999 Wiley-Liss, Inc .