Colocalization of GABA and tyrosine hydroxylase immunoreactivities in the axons innervating the neurointermediate lobe of the rat pituitary: an ultrastructural immunogold study.

Distribution of the gamma-aminobutyric acid (GABA)ergic and dopaminergic innervations was studied in the rat neurointermediate lobe using antibodies against GABA and tyrosine hydroxylase. In light microscopy, immunoperoxidase staining revealed similar distribution patterns of the axons reacting with both antibodies. Diffusely scattered in both lobes, they were more concentrated along the marginal zone of the neural lobe. Application of a double, recto-verso, immunogold labelling method in electron microscopy revealed systematic colocalization of GABA and tyrosine hydroxylase (TH) immunoreactivities in the axons innervating the intermediate lobe; in the neural lobe, almost all GABA-immunoreactive axons were also labelled for TH. Thus, GABA and dopamine, hitherto reported to occur in distinct axons, in fact colocalize in the axonal systems which innervate the pituitary neurointermediate lobe. These observations suggest possible interactions (pre- or postsynaptic) of both transmitters in the functional regulation of the intermediate and neural lobes.