Nitric oxide synthase in the brain of the clawed toad Xenopus laevis: Is there a relationship with the visual system?

Nitric oxide (NO), a free radical, has emerged as an intracellular and intercellular messenger molecule with many biological functions, including a role in memory. The neuroanatomical distribution of the enzyme nitric oxide synthase (NOS) is described in the brain and pituitary of Xenopus laevis, using immunohistochemistry with a polyclonal antiserum against human brain NOS, and using the nicotinamide adenine dinucleotide phosphate - diaphorase (NADPH-d) histochemical staining. NOS-containing neurons were found in the telencephalon, the diencephalon, the mesencephalon and the metencephalon, and were especially numerous in the pars lateralis of the amygdala, the lateral and dorsal pallium, the deep periventricular layers of the optic tectum and the locus coeruleus. The distribution of NOS-containing neurons in Xenopus is very similar to the distribution of NOS-immunopositive neurons as reported in several amphibian and reptilian species, and is also very reminiscent of the distribution of targets of the visual input system in amphibians. Therefore, a literature survey of tract tracing studies of the visual system in amphibians was performed, particularly referring to the thalamo-tectal, thalamo-telencephalic and crossed tecto-bulbar pathways. Beside a possible role of NO in the control of background adaptation in Xenopus, the present data, in combination with data reported in literature, suggest that NOergic neurotransmission is involved in the processing of visual information in amphibians.