Fast and Furious: Identifying the Target Engagement and Mechanisms Underlying the Rapid Behavioral and Synaptic Actions of MRK-016, A Negative Allosteric Modulator of Αlpha5-Containing GABAARs, and the Commonalities with Other Fast-Acting Antidepressant Compounds

Abstract

Major depressive disorder (MDD) is a common, yet debilitating psychiatric disorder characterized by chronic low mood, lack of energy and is comorbid with suicidal ideation. Most pharmacotherapies currently used to treat depression inhibits the reuptake of monoamine neurotransmitters such as serotonin. These compounds have a significant number of off-target effects and require chronic dosing for many weeks before symptoms are relieved. Worryingly, half of patients treated with selective serotonin reuptake inhibitors will remain refractory to treatment. Ketamine has rapid-acting antidepressant properties in treatment-resistant individuals, but its broad antagonism of NMDARs complicates its widespread clinical use. However, ketamine’s induction of glutamatergic transmission to strengthen stress-weakened synapses provides a translatable mechanism to relieve symptoms of MDD. Negative allosteric modulators of α5-containing GABAA receptors (GABA-NAMs) have ketamine-like antidepressant properties in rodents, but their target engagement remains unconfirmed. I first hypothesized that GABA-NAMs act through the benzodiazepine site of the GABAAR. Chronically stressed male mice demonstrated diminished sucrose and female urine preference, and a reduction in hippocampal TA-CA1 synaptic strength. The GABA-NAM MRK-016 restored hedonic behavior and AMPA:NMDA ratios, but this was prevented by pretreatment with the benzodiazepine site antagonist flumazenil. I then asked if α5-containing GABAARs were necessary to generate increases in EEG gamma power, a hallmark of rapid-antidepressant efficacy and activity-dependent plasticity. Wildtype, but not α5 KO mice generated gamma power over the PFC following MRK-016 administration, while ketamine promoted gamma power in both genotypes. Concordantly, MRK-016 restored hedonic behaviors and TA-CA1 AMPA:NMDA ratios only in wildtype, but not in α5 KO mice. Additionally, I provide preliminary evidence that overexpression of the human gabra-5 gene in α5 KO mice rescues MRK-016’s ability to generate gamma power. Together, I conclude the rapid-acting antidepressant-like activity of MRK-016 is mediated at an α5-containing benzodiazepine site on the GABAAR. I also describe additional experiments that elucidate mechanism of other rapid-acting antidepressant compounds, including ketamine, (2R,6R)-hydroxynorketamine, and psilocybin. The actions of these putative antidepressant compounds ultimately converge on promoting excitatory, AMPAR-mediated glutamatergic transmission. Modulation of the balance between excitatory and inhibitory neurotransmission within emotional- and reward-processing pathways provides a novel substrate to understand and treat psychiatric disorders.