A comparison between the in vivo and in vitro activity of five potent and competitive NMDA antagonists

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Abstract
Phosphonate analogues of glutamate have been tested and compared as N-methyl-D-asparate (NMDA) antagonists in electrophysiological and binding experiments. The compounds tested were three established. NMDA antagonists: D-2-amino-5-phosphonopentanoate (D-AP5), DL-2-amino-7-phosphonoheptanoate (DL-AP7), 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), and two novel putative NMDA antagonists: 3-(2-carboxypiperidin-4yl)propyl-1-phosphonate (CPPP) and 3-(2-carboxy-piperidin-4-yl)methyl-1-phosphonate (CPMP). When administered electrophoretically to rat spinal neurones in vivo, these compounds were found to be selective NDA antagonists with little effect on excitations evoked by quisqualate and kainate. CPMP and CPPP were approximately equipotent with CPP and about 5 times more potent than D-AP5. Following systemic administration, 2-5mg kg-1 i.v. of CPP, CPMP and CPPP reduced NMDA-evoked excitations by 70-100% whereas 50-100 mgkg-1 of D-AP5 and DL-AP7 produced a similar effect. The onset of the effects required 20-30 min and lasted more than six hours. On bath application to cortical wedges, the IC50 values (.mu.M) for antagonism of 40 .mu.M NMDA were: CPP, 0.64 .+-. 0.06 (mean .+-. s.e.mean; n > 4); CPMP, 1.65 .+-. 0.13; CPPP 0.89 .+-. 0.09; D-AP5, 3.7 .+-. 0.32; DL-AP7, 11.1 7U 2.1; and DL-AP4 and DL-AP6 were inactive at 100 .mu.M. In binding studies with [3H]-CPP, the Ki values (nM) were: CPP, 446 .+-. 150 (mean 7U s.e.mean; n .gtoreq. 3); CPMP, 183 .+-. 74 and CPPP, [ .+-. 13 whereas against NMDA (10 .mu.M)-stimulated [3H]-TCP (thienylcyclohexylpoperidine) binding the IC50 values (.mu.M) for CPMP and CPPP respectively were 5.6 .+-. 2.7 and 4.5 .+-. 2.2. Systemic administration of CPPP and CPMP, at doses sufficient to antagonize NMDA, also reduced cardiovascular responses to 5-hydroxytryptamine (Bezold-Jarisch reflex). This illustrates a role for NMDA receptors in central cardiovascular control. The results indicate the systemic doses of piperidine and piperazine analogues of D-AP5 which may be used for assessing the role of NMDA receptors in central synaptic function.