Role of nitric oxide in the control of the hypothalamic-pituitary-adrenocortical axis

The gaseous radical nitric oxide (NO) is catalyzed by conversion of L-arginine to L-citrulline by one cytokine inducible form (iNOS), which becomes active only within hours after the inducing event, and by two constitutively expressed forms, endothelial (eNOS) and neuronal (nNOS), which are regulated by the cytosolic concentration of free Ca2+. Brain nNOS is physiologically present in discrete populations of neurons, which are all excited by glutamate via the ionotropic N-methyl-D-aspartate (NMDA) receptor, which controls a Ca2+ channel. After its diffusion into the extraneuronal space, NO may activate in neurons, which as a rule do not stain for NOS, soluble guanylyl cyclase and formation of cGMP as an intracellular messenger. Beyond that, NO is important as a feedback regulator of glutamatergic excitation. NO as a nitrosylating agent enhances disulfide bonding of vicinal sulfhydryl (thiol) groups of the redox modulatory site of the NMDA receptor complex and thereby down-regulates its Ca2+ channel activity. Histochemical studies have revealed the presence of a large number of NOS containing neurons in the magnocellular and parvocellular subdivisions of hypothalamic nuclei. Numerous studies conform to the view that NO participates in the control of many different neurosecretory processes, especially of the corticotropin-releasing hormone (CRH) neurosecretory system. The redox-modulatory site of the NMDA receptor appears, therefore, as a critical structure in the control of the hypothalamic-pituitary-adrenocortical (HPA) axis. Moreover, glucocorticoids augment neuronal excitotoxicity by increasing the expression of glutamate receptors and inhibition of glutamate reuptake. In attempting to explain the many conflicting results obtained in studies with NO, it may be worthwhile to consider that the actual redox-environment of distinct loci of the brain may determine the final function of NO, acting either as a transmitter or neuromodulator or, in the worst case, causing neurodestruction. It seems likely that any kind of stress by altering the ratio of reduced vs oxidized thiols within the central nervous system influences neuronal excitability, with NO working either as an amplifier or as a feedback regulator of neuronal excitation or inhibition, which may alter acutely or chronically, among others, the homeostasis of a given neurosecretory system.