Quadrimodal in vivo assay for detection of adverse neuro-behavioral effects: validation using dextromethorphan

Drug discovery notoriously suffers from high attrition rates during clinical trials. A major contributing factor to this issue is the frequent failure to identify adverse neural functional effects of compounds in the CNS at earlier stages of drug development than currently possible. To address this unmet need, the (side-)effects of the widely used antitussive dextromethorphan were assessed employing a neuro-behavioral assay originally developed for evaluation of anesthetics. This high-content in vivo assay provides quadrimodal readouts, reflecting behavioral changes of a weakly electric fish model induced by the compound to be tested. Two of these readouts are proxies of the neural activity of the pacemaker nucleus, an endogenous oscillator that drives the periodic electric organ discharges. Administration of dextromethorphan induced a decrease in the frequency of these discharges, an increase in the generation of amplitude-frequency modulations of this electric behavior, and the production of an abnormal motor behavior, consisting of full-transverse-plane rotations of the fish. The rate of occurrence of each of these behaviors was dose dependent. The changes in discharge frequency and rotation of the fish’s body persisted for several hours and in some individuals even for at least 2 days. We hypothesize that the drug-induced changes in the two electric behaviors are mediated by inhibition of N -methyl-D-aspartate (NMDA) receptors and by activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptors in the pacemaker nucleus. The results of this study serve as proof of concept for the applicability of the neuro-behavioral assay for the evaluation of (side-) effects of drug candidates on neural function in the CNS.