Morphological diversity of nitric oxide synthesising neurons in mammalian cerebral cortex

Neocortical neurons that utilise nitric oxide (NO) differ in morphology in different mammalian species. In the present study we examine these differences in the neocortex of mouse, rat, guinea-pig, rabbit, cat and monkey using histochemistry for nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and immunocytochemistry for nitric oxide synthase (NOS), gamma amino-butyric acid (GABA), calbindin (CB), parvalbumin (PV) and calretinin (CR). NO neurons are non-pyramidal and can be divided into two distinct types, both of which react for NOS and NADPH-d. Type I neurons have a relatively large soma with heavy reaction product filling even the fine processes. They occur in all species, mainly near the border between the cortex and white matter, with fewer in the cortex, mostly in the superficial layers (II-IV). Type II cells are more numerous, smaller, and lighter in reactivity. They are in all species examined here except rodents, and in all cortical layers, but mainly layers II-IV. Most intracortical and some subcortical Type I neurons express GABA. A few intracortical Type I cells contain CB. All Type II cells express GABA and most also CB. Neither Type I nor Type II cells stain for PV or CR. We conclude that there is a tendency for a reduction of Type I cells, and increase of Type II, in mammalian neocortex with phylogeny.