IMMUNOCYTOCHEMICAL EVIDENCE FOR A GLIAL LOCALIZATION OF ARGININE, AND A NEURONAL LOCALIZATION OF CITRULLINE IN THE RAT NEUROHYPOPHYSIS - IMPLICATIONS FOR NITRERGIC TRANSMISSION

Nitric oxide (NO) is used as a neurochemical mediator in the rodent hypothalamo-neurohypophysial system. Nitric oxide synthase uses arginine to form both NO and citrulline. In this study immunocytochemistry was used to determine the distributions of arginine and citrulline in the neurohypophysis. Arginine was localised within glia whilst citrulline was present in the nitrergic neurones. Aspartate, an amino acid involved in the recycling of citrulline back to arginine, was localised only in the glia. These findings suggest that nitrergic transmission may be dependent on a cyclical process (analogous to the glutamate-glutamine cycle) based on the transfer of arginine from dia to neurones and the subsequent return of citrulline from nerve terminals to glia for aspartate-dependent conversion back into arginine.