Atypical neural messengers

Notions of what constitutes a neurotransmitter have changed markedly with t he advent in the past decade of synaptic molecules, which satisfy key neuro transmitter criteria but differ radically from classical transmitters. Thus , NO and carbon monoxide are neither stored in synaptic vesicles nor releas ed by exocytosis. These gases do not act via traditional receptors on posts ynaptic membranes. In addition, zinc, stored together with glutamate in syn aptic vesicles, appears to act as an 'antagonist' co-transmitter at the NMD A receptor, and although localized exclusively to glia, D-serine fulfills m ost neurotransmitter criteria as an endogenous ligand for the 'glycine' sit e of NMDA receptors.