- NAD is a potent inhibitor of electrical activity in the dentate gyrus of the guinea pig hippocampus. NAD is rapidly degraded by an NADase enzyme present on synaptosomal membranes that we have recently found to be inhibited by nicotinamide mononucleotide. In this report we have characterised the binding sites present on brain membranes for [3H]NAD in the presence of this inhibitor. We have demonstrated two binding sites of KdS 49 nM and 4.26 microM that are modulated by GTP. From structure-activity studies we have shown the binding to be stereospecific for the beta-isomer of NAD requiring the whole of the molecule for full receptor affinity. The binding sites are distinct from those reported for adenosine and their presence has significance for the physiological role of NAD in the mammalian brain.