NMDA Receptor‐Mediated Increase in Cyclic GMP in the Rat Cerebellum in vivo is Blocked by Alaproclate and GEA‐857

Abstract

Abstract:The effects of alaproclate and GEA-857 (2-(4-chlorophenyl)-1,1 -dimethylethyl 2-amino-3-methylbutanoate) on the production of cyclic GMP in the rat cerebellum in vivo induced by stimulation of N-methyl-D-aspartate (NMDA) receptors were studied. Alaproclate per se at a dose of 20 mg/kg subcutaneously. did not influence the basal cGMP level. The increase in cGMP induced by harmaline (20 mg/kg subcutaneously) was dose-dependently antagonized by alaproclate ( 5 4 0 mgikg subcutaneously). S-(-)-Naproclate was 2 -5 times more potent than the R-(+)-enantiomer. GEA-857 which in contrast to alaproclate is a very weak 5-HT uptake inhibitor shared the ability of alaproclate to inhibit the effect of harmaline on cGMP accumulation with similar potency to S-(-)-alaproclate. Alaproclate at 15 mg/kg subcutaneously blocked the increase in cGMP in cerebellum caused by NMDA itself at 200 mg/kg subcutaneously. In contrast to alaproclate, the K+ channel antagonist, 4-aminopyridine, 5 mgikg subcutaneously, produced per se an increase in cGMP levels in the rat cerebellum by 300% which was antagonized by the NMDA receptor antagonists, dizocilpine, phencyclidine and (2)-CCP, the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester and by alaproclate. Alaproclate and GEA-857 antagonized seizures induced by NMDA, 200 mgikg subcutaneously at doses similar to those antagonizing the harmaline-and NMDA-induced elevation of cerebellar cGMP Neither alaproclate nor GEA-857 caused any behavioural effects typical for uncompetitive NMDA receptor antagonists except a slight increase in motor activity and sniffing. The effect of alaproclate on the NMDA receptor-mediated increase in cGMP in rat cerebellum in vivo might be due to blockade of the cation channel of the NMDA receptor complex previously observed in in vitro experiments and these compounds seems to belong to the group of low-affinity uncompetitive NMDA receptor antagonists that might have clinical interest.

Evaluation of the phencyclidine-like discriminative stimulus effects of novel NMDA channel blockers in rats

“…It has also been found to displace [ 3 H]MK-801 binding with low affinity and dose dependently inhibit NMDA-induced currents (Wilkinson et al 1994;Svensson et al 1994). In vivo, alaproclate attenuates NMDAreceptor-mediated increases in cGMP and prevents NMDA-induced seizures in rats (Hu and Ross 1997). Orphenadrine, a derivative of the antihistamine diphenhydramine, is marketed for use as a muscle relaxant (Finch 1959;Hoddes 1964) and analgesic (Hunskaar and Donnell 1991) and has also been used for over 30 years in the symptomatic treatment of Parkinson's disease (Doshay and Constable 1957;Mindham et al 1972).…”