HARMALINE COMPETITIVELY INHIBITS [H-3] MK-801 BINDING TO THE NMDA RECEPTOR IN RABBIT BRAIN

Harmaline, a beta-carboline derivative, is known to produce tremor thr ough a direct activation of cells in the inferior olive. However, the receptor(s) through which harmaline acts remains unknown. It was recen tly reported that the tremorogenic actions of harmaline could be block ed by the noncompetitive NMDA channel blocker, MK-801. This study exam ined whether the blockade of harmaline's action, in the rabbit, by MK- 801 was due to a pharmacological antagonism at the MK-801 binding site . This was accomplished by measurement of [H-3]MK-801 binding in membr ane fractions derived from tissue containing the inferior olivary nucl eus and from cerebral cortex. Harmaline completely displaced saturable [H-3]MK-801 binding in both the inferior olive and cortex with appare nt IC50 values of 60 and 170 mu M, respectively. These IC50 values art consistent with the high doses of harmaline required to produce tremo r, e.g., 10-30 mg/kg. Non-linear curve fitting analysis of [H-3]MK-801 saturation experiments indicated that [H-3]MK-801 bound to a single s ite and that harmaline's displacement of [H-3]MK-801 binding to the NM DA receptor was competitive as indicated by a shift in K-d but not in B-max. In addition, a Schild plot gave a slope that was not significan tly different from 1 indicating that harmaline was producing a displac ement of [H-3]MK-801 from its binding site within the NMDA cation chan nel and not through an action at the glutamate or other allosteric sit es on the NMDA receptor. These findings provide in vitro evidence that the competitive blockade of harmaline-induced tremor by MK-801 occurs within the calcium channel coupled to the NMDA receptor. Our hypothes is is that harmaline produces tremor by acting as an inverse agonist a t the MK-801 binding site and thus opening the cation channel. (C) 199 7 Elsevier Science B.V.