Classification of compounds for prevention of NMDLA-induced seizures/mortality, or maximal electroshock and pentylenetetrazol seizures in mice and antagonism of MK801 binding in vitro.

Intravenous injection of N-methyl-D,L-aspartic acid (NMDLA) into mice produces characteristic convulsions followed by death. The present study was designed to determine the degree of blockade of these seizures/mortality by compounds acting at various subsites on the N-methyl-D-aspartic acid (NMDA) receptor complex (competitive and noncompetitive antagonists, as well as inhibitors of the strychnine-insensitive glycine subsite, and Zn++ subsite agonists), and also calcium channel blockers, clinically used anticonvulsants, plus selected compounds with activities or structures similar to specific agents chosen. Activity among compounds was correlated to in vitro potency regarding inhibition of binding of MK801 to the ionic channel subsite associated with the NMDA receptor. Furthermore, all compounds were examined for antiseizure properties with respect to tonic hindlimb extension elicited by maximal electroshock (MES) and clonus induced by pentylenetetrazol (PTZ). Drugs were subsequently classified according to their spectra of efficacy in these tests. The following characteristics emerged: 1) agents active at all 3 NMDA mechanisms (convulsions/mortality/MK801 binding) plus MES and PTZ, were MK801 and CPP [3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid]; 2) active at all the NMDA mechanisms and MES were ketamine and dextromethorphan; 3) active against NMDLA-induced convulsions/mortality, MES and PTZ, but not MK801 binding, were doxepin, desipramine and diazepam; 4) active against NMDLA-induced convulsions/mortality and MES were des-Me-doxepin, flunarizine and remacemide; 5) active against NMDLA-induced convulsions/mortality and PTZ was nisoldipine; 6) active against only NMDLA-induced convulsions/mortality were chlorpheniramine and iproniazid; 7) active in the MES and PTZ tests were phenobarbital, pentobarbital and valproate; 8) active in the MES test alone were phenytoin and carbamazepine; 9) active against PTZ only was ethosuximide; 10) active only in the in vitro MK801 binding assay were HA966, 7-Cl-kynurenate and AP7 (2-amino-7-phosphonoheptanoic acid); and 11) no demonstrable actions had AP4 (2-amino-4-phosphonobutyric acid) and mianserin. In conclusion, inhibition of NMDLA-induced convulsions/mortality in vivo is not necessarily correlated to a noncompetitive displacement of MK801 binding to NMDA receptor sites in vitro, nor is inhibition of NMDA-elicited convulsions/mortality correlated with a specific ability of a compound to inhibit either MES or PTZ seizures.