Absence of co-localized glutamic acid decarboxylase and neuropeptides in noradrenergic axons of the rat spinal cord.

Patterns of co-localization of immunoreactivity for dopamine beta-hydroxylase (the synthetic enzyme for noradrenaline) and glutamic acid decarboxylase (the synthetic enzyme for GABA) or each one of six neuropeptides (neuropeptide Y, substance P, met-enkephalin, galanin, dynorphin A and somatostatin) were investigated with dual-colour confocal laser scanning microscopy in axons of cervical, thoracic and lumbar spinal segments of six adult rats. Four regions of the grey matter were studied (laminae I-II, V, IX and X) and, in thoracic segments, the intermediolateral cell column was also examined. The extent of co-localization was estimated by direct assessment of merged pairs of optical sections and by automated image analysis. Significant co-localization was found for neuropeptide Y in axons of the intermediolateral cell column of thoracic segments and in lamina X of cervical and thoracic segments. None of the other peptides or glutamic acid decarboxylase were found to coexist at significant levels with dopamine beta-hydroxylase and hence it is likely that this group of neuropeptides and GABA are not co-transmitters of bulbospinal noradrenergic axons in the rat.