An agonist concentration biased allosteric modulator potentiates NMDA
induced ion influx in neurons
Abstract
Background and purpose: Precisely controlled synaptic glutamate
concentration is essential for normal function of the N-methyl
D-aspartate (NMDA) receptors expressed in the brain. Atypical
fluctuations in synaptic glutamate homeostasis lead to aberrant NMDA
receptor activity that results in pathogenesis of neurological and
psychiatric disorders. Therefore, glutamate concentration dependent NMDA
receptor modulators will be clinically useful agents with less on-target
adverse effects. Experimental approach: Two electrode voltage clamp and
patch clamp electrophysiology techniques were used for pharmacological
characterization. Dynamic Ca2+ and Na+ imaging were performed using
cultured rat brain neurons. MTS cell viability assay was used for to
study neurotoxicity. Key results: Identified a compound (coded as CNS4)
that potentiates NMDA receptor currents based on the glutamate
concentration. This compound increases both glycine and glutamate
potency, and exhibits no voltage dependent effect. Electrophysiology
recordings confirmed agonist concentration dependent changes in peak and
steady state currents. Dynamic Ca2+ and Na+ imaging assays using rat
brain cortical, striatal and cerebellar neurons revealed CNS4 mediated
region specific disproportionate influx of Na+ compared to Ca2+ in
native NMDA receptors. Direct exposure of CNS4 unaltered the viability
of cultured cortical or striatal neurons, neither augmented NMDA induced
neuronal death. Conclusion and implications: CNS4 is novel in chemical
structure, mechanism of action and agonist concentration biased
modulatory effect. This compound or its future analogs will be useful
for the treatment of brain disorders associated with hypoglutamatergic
neurotransmission.