Hydrogen sulfide as a neuromodulator.

Hydrogen sulfide (H2S) is a well-known toxic gas with the smell of rotten eggs. Since the first description of the toxicity of H2S in 1713, most studies about H2S have been devoted to its toxic effects. Recently, H2S has been proposed as a physiologically active messenger. Three groups discovered that the brain contains relatively high concentrations of endogenous H2S. This discovery accelerated the identification of an H2S-producing enzyme, cystathionine beta-synthase (CBS) in the brain. In addition to the well-known regulators for CBS, S-adenosyl-L-methionine (SAM) and pyridoxal-5'-phosphate, it was recently found that Ca2+/calmodulin-mediated pathways are involved in the regulation of CBS activity. H2S is produced in response to neuronal excitation, and alters hippocampal long-term potentiation (LTP), a synaptic model for memory. can also regulate the release of corticotropin-releasing hormone (CRH) from hypothalamus. Another H2S producing enzyme, cystathionine gamma-lyase (CSE), has been identified in smooth muscle, and H2S relaxes smooth muscle in synergy with nitric oxide (NO). Recent progress in the study of H2S as a novel neuromodulator/transmitter in the brain is briefly reviewed.