Tyrosine hydroxylase-immunoreactive intrinsic neurons in the rat cerebral cortex

Using specific antisera against the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), in combination with the peroxidase-antiperoxidase method and/or the avidin-biotin complex method, we have found a new group of TH immunoreactive (TH-I) neurons in the rat cerebral cortex. Numerous TH-I cells were observed all over the isocortex, that is, frontal, temporal, parietal and occipital regions, and in some parts of the allocortex such as the anterior cingulate cortex, the retrosplenial cortex and anterior part of the insular cortex. In contrast, they were rare in the perirhinal cortex, posterior part of the insular cortex, piriform cortex, entorhinal cortex and hippocampal formation. TH-I cells were situated throughout all cortical layers, but were most concentrated in layer II/III. Although TH-I cells were heterogeneous in shape, the majority were bipolar. All TH-I cells so far examined appeared to be of the nonpyramidal type. The majority of these intrinsic TH-I neurons also contained the GABA-like immunoreactivity and thus could be regarded as a subpopulation of cortical GABAergic neurons.