Long-range GABAergic projection neurons in the cat neocortex.

Neocortical gamma-aminobutyric acid (GABA)ergic neurons have been previously described as largely involved in local intracortical circuitry. However, our recent findings in the murine model described select neocortical GABAergic neurons that project to both neighboring and more distant neocortical regions. Here, we investigated whether such GABAergic projection neurons are also found in the cat neocortex. Wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected into the visual, auditory, or somatosensory cortex, in order to label efferent cortical neurons retrogradely and to label axons and terminals orthogradely. Staining for nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), an enzyme involved in nitric oxide synthesis, was employed, and co-localization with WGA-HRP was determined by means of both polarizing and brightfield microscopy. We concluded that neurons double-labeled with WGA-HRP and NADPH-d in a distant region from the WGA-HRP-injection site are GABAergic neurons with long-range projection axons. All double-labeled neurons were found in cortical layers VIa and VIb and in the white matter. Neurons with intense NADPH-d reactivity (type I) were determined to be neuronal nitric oxide synthase (nNOS) positive in all cases. However, weakly NADPH-d-reactive neurons (type II) lacked nNOS immunoreactivity. Moreover, nNOS often co-localized with GABA, neuropeptide-Y, and somatostatin in the cat neocortex. In summary, the GABAergic neurons described here projected in a manner similar to that previously described for neocortical principal neurons, although some unique GABAergic long-range projections were also demonstrated.