Neurotransmitters are the molecules that neurons use to communicate with each other and with the other cell types of the nervous system, such as glial cells and cells of the vasculature. The best characterized actions of neurotransmitters are the alterations in excitability that they elicit in other neurons. These changes in neuronal firing rate are due to the opening or closing of transmembrane channels selectively permeable to given ionic species. We have however recently demonstrated that certain neurotransmitters can regulate energy metabolism within discrete regions of the central nervous system. In particular we have observed that Vasoactive Intestinal Peptide stimulates glycogenolysis in the cerebral cortex. This action is also exerted by the monoamines noradrenaline, serotonin and histamine. Studies in primary cultures indicate that the glycogenolysis elicited by neurotransmitters may take place in astrocytes, which are glial cells and where glycogen is predominantly stored in the nervous system. These observations suggest that the primary function of certain neuronal circuits may be to regulate the availability of energy substrates within discrete neuronal ensembles.