Putative endogenous ligands for the benzodiazepine receptor.

The recent discovery of pharmacologically relevant, high affinity, stereospecific binding sites for the benzodiazepines in the central nervous system (CNS) has rekindled investigations concerning the mechanism of action of these drugs. It has become increasingly clear that elucidation of benzodiazepine action will provide new and important insights into the neurochemical substances of seizure activity, centrally mediated muscle relaxation and anxiety, three major actions of this class of drugs.The existence of a functional receptor for the benzodiazepines, compounds not present $\text{in vivo}$, suggests that endogenous substances exist that serve as natural substrates for this receptor. Furthermore, the characterization of endogenous benzodiazepine receptor ligands affords an opportunity to determine the neurochemical mechanisms underlying the pharmacologic and behavioral effects manifested by the benzodiazepines.Using receptor binding methodology to assay tissue extracts for [³H] diazepam binding inhibitory activity, putative endogenous ligands for the benzodiazepine receptor have been isolated and identified as the purine nucleosides. Compounds such as inosine and hypoxanthine exhibit competitive inhibition of [³H] diazepam binding. The low affinity purinergic inhibition of diazepam binding is consistent with their $\text{in vivo}$ concentrations. Distinct structure-activity relationships exist for the purines with subtle structural alterations having marked effects on diazepam binding inhibitory potency. The methylxanthine stimulants, caffeine, theophylline, and theobromine, also competitively inhibit diazepam binding, suggesting that some of their actions may be mediated by the benzodiazepine receptor.The purines also have “benzodiazepine-like” pharmacologic properties, since they have been shown to antagonize pentylenetetrazol induced seizures in mice in a dose dependent manner. Neurophysiologic studies have also shown that iontophoresis of inosine on cultured mouse primary neurons produce neurotransmitter like effects. Furthermore, these effects are similar to those observed with flurazepam, a finding that provides additional evidence for the “benzodiazepine-like” properties of the purines.The preliminary studies outlined below indicate that the purines are good candidates as putative endogenous ligands for the benzodiazepine receptor and provide a foundation for future studies that concern the homeostatic mediation of seizure activity and anxiety.