Endogenous L-dopa in the rat dorsal vagal complex: an immunocytochemical study by light and electron microscopy

The aim of this work was to examine L-DOPA immunoreactivity (L-DOPA-IR) in the dorsal vagal complex (DVC) of the rat medulla oblongata containing A2/C2 catecholaminergic cell groups, in order to further evaluate the previously proposed hypothesis that various pools of endogenous L-DOPA could be immunocytochemically demonstrated in the mammalian brain. For this purpose, L-DOPA-IR was studied in DVC in comparison with both some other catecholaminergic areas and dopamine immunoreactivity (DA-IR) on adjacent sections of the same brain, by using specific antibodies against glutaraldehyde conjugated L-DOPA and DA. Also, the first preliminary observations of L-DOPA-IR in DVC neurons at the ultrastructural level are reported. The following main results were obtained: (1) bright, intense and homogeneous L-DOPA staining was found in perikarya and proximal neuronal processes situated within the rostrocaudal extension of the DVC; (2) this staining pattern was readily distinct from weak and heterogeneous DA staining; (3) an inverse L-DOPA/DA staining pattern ratio was identified between the DVC and the mesencephalon; (4) L-DOPA-IR at electron microscopic level was roughly similar to that previously observed for DA-IR in mesencephalic cells and their presumptive projections. Although some discrepancies were noticed between L-DOPA staining and data from the literature on tyrosine hydroxylase labeling, our results could not invalidate the hypothesis that, among high L-DOPA/DA ratio containing neurons, some cells in the DVC may contain only L-DOPA.