Inosine, an endogenous ligand of the brain benzodiazepine receptor, antagonizes pentylenetetrazole-evoked seizures.

Abstract

Partially purified extracts of bovine brain were previously found to inhibit competitively the binding of [3HJ-diazepam to-rat brain synaptosomal membranes. The purines inosine and hypoxanthine were subsequently identified as the compounds responsible for this inhibitory activity. Intracerebroventricular administration of inosine to mice of the C3H/ HEN and NIH general purpose strains caused a dose-and time-dependent increase in the latency to clonicotonic seizures produced by intraperitoneal administration of the convulsant pentylenetetrazole. Intracerebroventricular administration of equimolar doses of 2'-deoxyinosine, which is more potent than inosine in inhibiting the binding of [3H1diazepam in vitro, significantly increased pentylenetetrazole-evoked seizure latency. In contrast, both 7-methylinosine and thymidine were ineffective in inhibiting the in vitro binding of [3Hldiazepam and increasing the latency to pentylenetetrazole-induced seizures in vivo. These results suggest that endogenously occurring purines such as inosine exhibit diazepam like effects when administered intracerebroventricularly, and these effects may be related to the interaction of inosine and related compounds with benzodiazepine receptors in the central nervous system.The characterization of saturable, high affinity, and stereospecific benzodiazepine binding sites in the central nervous system has been described in vitro and in vivo (1-7). The good correlations obtained between the binding affinity of a series of benzodiazepines in vitro and their anticonvulsant (4), anxiolytic (8), and muscle relaxant properties (4) strongly suggests that these sites may be receptors mediating the pharmacologic actions of the benzodiazepines. However, neither the regional distribution of benzodiazepine binding sites within the central nervous system nor the screening of a large number of putative neurotransmitter substances (1, 8) has provided evidence that such receptor sites are associated with classical neurotransmitter pathways. These findings have prompted a search for an endogenously occurring compound(s) that binds to the benzodiazepine receptor and functions as a physiological ligand (9, 10).Low molecular weight substances that are heat stable, dialyzable, and competitively inhibit [3H]diazepam binding to synaptosomal membranes in vitro have been isolated from bovine brain (9, 11). These substances have been subsequently identified as the purines inosine and hypoxanthine, and it has been suggested that they may modulate the benzodiazepine receptor in vivo (11). Since the inhibition of pentylenetetrazole (PTZ)-induced seizures has been used as a sensitive measure for assessing benzodiazepinelike activity in vivo, the effects of inosine and related compounds have been examined by using this paradigm. We now report that the intracerebroventricular (ICV) administration of inosine elicits a dose-and time-dependent increase in the latency period between the intraperitoneal injection of PTZ After the recovery period, mice were injected IC...

The Antiepileptic Potential of Nucleosides

“…Thus, benzodiazepine receptor ligands, such as diazepam [8] and likely Ino, may enhance GABA-mediated inhibition and may decrease seizure activity. It has been concluded that antiseizure/anticonvulsant effect of endogenous Ino [58] may correlate with its interaction with inhibitory GABA A receptors (benzodiazepine receptors) [386,390,399,402]. Recently, it was discussed that Ino may also bind to Ado receptors (A 1 , A 2A , A 3 ) [403]; thus, the anticonvulsant effect of Ino may involve adenosinergic mechanisms as well.…”

“…It has been demonstrated that Ino (i) increased the latency to PTZ-induced seizures [118,390], (ii) antagonized caffeine-induced seizures [391], (iii) has a role in the electroshock-induced increase in the threshold to PTZ-induced seizures [387], (iv) had an anticonvulsant effect on QA-induced seizures [392] and (v) raised the threshold of seizures induced by PTZ, bicuculline and picrotoxin [58] (Table 4). Interestingly, a synthetic Hyp derivative, AIT-082 (4-[[3-(1,6-dihydro-6-oxo-9-purin-9-yl)-1-oxopropyl]amino]benzoic acid), may exert its neuroprotective effect against kainic-acid-induced status epilepticus (longer latency, shorter duration) partly via Ino [393].…”

“…Quilonilic acid-induced (intracerebroventricular application) seizures in mice Seizure prevention [392] Pentylenetetrazole-induced (intraperitoneal injection) seizures in mice Increased seizure latency [118,390] Bicuculline-, pentylenetetrazole-and picrotoxin-induced (tail vein infusion and intraperitoneal injection) seizures in mice Increased seizure threshold [58] Ino Caffeine-induced seizures in mice Seizure reduction [391] …”

Urinary and brain beta-carboline-3-carboxylates as potent inhibitors of brain benzodiazepine receptors.

“…[3H]diazepam binding (Kj 0.1 mM) and, except maybe for nicotinamide and inosine (21,26), appropriate in vivo actions have not been demonstrated. When human urine was purified and treated with hot ethanol, appreciable amounts of a potent [3H]diazepam displacing factor (the y-fraction) could be isolated (27).…”

Effect of Inhibition of Purine Enzymes on Benzodiazepine Binding in the Human Brain

“…system does not correspond to the regional distribution of any known neurotransmitter, a search for an endogenously occurring com pound that is bound to the ligand receptor, and which functions as a physiological ligand was made [4,9]. Recently, the naturally oc curring purines, inosine, deoxyinosine and hypoxanthine, have been shown to react with benzodiazepine receptors in brain cells, and have been identified as inhibitors of benzodi azepine binding [9,13,14], Speculations that they may be the endogenous substances that normally occupy the benzodiazepine recep tors have been suggested. Whole brain con centrations of inosine, deoxyinosine and hy poxanthine, which have been estimated to be between 20 and 60 pmol/1 [6], are evenly dis tributed throughout the central nervous sys tem.…”