Racemic Ketamine Attenuates Anterior Cingulate Cortex GABA Deficits Among Remitters in Treatment-Resistant Depression: A Pilot Study

Chronic stress and deficits in gamma-aminobutyric acid (GABA) neurotransmission have been implicated in the pathophysiology of major depressive disorder (MDD) as evidenced by: a meta-analysis reporting decreased brain [occipital cortex, anterior cingulate cortex (ACC), prefrontal cortex] GABA levels in MDD (Schur et al., 2016), a significant inverse correlation between lower cerebrospinal fluid GABA and increase anxiety symptom severity in MDD, and a significant inverse correlation between low GABA and higher brain glutamate (Kantrowitz et al., 2021). It has been proposed that a low GABAergic tone in MDD drives glutamate elevation (Sanacora et al., 2003) which makes both low GABA and increased glutamate targets for pharmacotherapy.

The ACC plays a key role in the emotional and cognitive processing of MDD and is a target brain region for depression treatment. Our pilot study, showed an increase in peak ACC GABA levels, during the ketamine infusion, was associated with next day remission and correlated to the degree of clinical improvement (Singh et al., 2021).

Earlier studies have shown that antidepressants, transcranial magnetic stimulation (TMS), and electroconvulsive treatment (ECT) can all attenuate or normalize central GABA deficits (Sanacora et al., 2003; Sanacora et al., 2002). In this post-hoc analysis of our preliminary pilot study, we investigated the hypothesis that TRD patients who remitted with a single ketamine infusion had a baseline GABA deficit which improved with intravenous (IV) ketamine infusion therapy. We utilized novel dynamic sliding-window functional magnetic resonance spectroscopy (fMRS) technique to measure the ACC GABA levels before and at the end of an IV ketamine infusion in patients with TRD.

Methods:

This was an open-label feasibility trial that enrolled 12 adults (18-65 years) with treatment-resistant moderately-severe/severe (PHQ-9 score ≥ 15) MDD (defined as failure to respond to two adequate trials of antidepressive treatments, including pharmacotherapy with antidepressants, ECT, or TMS) and who received a single IV ketamine infusion (0.5 mg/kg, infused over 40 minutes). Exclusion criteria included: any additional psychiatric disorder(s) other than anxiety disorder; taking any medication(s) known to affect glutamate (riluzole, carbamazepine) or GABA (zaleplon, zolpidem, zopiclone, valproate, gabapentin, pregabalin, tiagabine, and vigabatrin); ongoing prescription of > 4 mg lorazepam equivalents; any current substance use disorder (excluding nicotine and caffeine); any neurologic disease or history of traumatic brain disorder; pregnancy; contraindication to MRI; and not received ketamine treatment for depression within the prior 2 months. The study details are published earlier (Singh et al., 2021).

Ketamine infusions were performed with the patient in the MRI scanner. After a structural MRI was completed, the fMRS sequence was started. At exactly 13 minutes into the fMRS scan, 0.5 mg/kg of ketamine hydrochloride in 50 cc normal saline was administered intravenously over 40 min duration to finish exactly as the fMRS sequence finished. Five patients were excluded due to excessive patient motion (3) and scanner/IV pump failure (2). We used an fMRS scan at 3T using the MEGA-PRESS sequence to measure the ACC GABA and glutamate+glutamine (Glx) levels before and during the 40-min infusion. We used creatine (Cr) as a normalization reference for the relative quantification of GABA and used GABA/Cr as an outcome measure as well.

Depression symptoms were measured using Montgomery–Asberg Depression Rating Scale (MADRS) at baseline (before the start of ketamine infusion) and 24 hours after the infusion. Remission was defined as MADRS score ≤9 at 24 hours post-infusion.

Two-sample t tests and 95% confidence intervals (CI) were used to compare differences in the ACC GABA level, Cr level, and GABA/Cr level between the remitters and non-remitters at baseline and at the end of infusion. Shapiro-Wilk normality test was used to assess the normality. For the endpoint time, our hypothesis was that the measures were equivalent between groups.

Results:

Seven subjects (4 F, 3 M), middle age (mean±SD, 45±11.7 years), had moderate depression (mean MADRS score ±SD at baseline of 23.1±SD3.6) and 6/7 (86%) had comorbid anxiety disorder. Three subjects remitted at 24 hours (43% remission rate). All the patients tolerated ketamine well.

At baseline, significantly lower mean ACC GABA were observed in remitters compared to non-remitters (mean 0.0015 vs 0.0027, p=0.02). GABA/Cr showed similar findings. This GABA deficit appears to have improved by the end of the infusion. At 40 minutes/end of the infusion, the ACC GABA levels were similar between the remitters and non-remitters (0.0031 vs 0.0027, 95% CI −0.0017 to 0.0032, p=0.49). Similar findings were seen for GABA/Cr ratio. There was no significant difference in the Glx, and Glx/Cr at baseline/endpoint.

Discussion:

This preliminary pilot study suggests a reduction of ACC GABA deficit may be a potential mechanism of action of ketamine. GABA is a major inhibitory neurotransmitter and extensive alteration of cortical GABAergic inhibitory circuit has been shown with chronic stress and depression leading to neuronal excitability. Thus, reducing GABA deficits could be a potential pathway to reduce neuronal excitability. Supporting this hypothesis would be prior studies which have indicated lower GABA levels in ACC in MDD but not remitted MDD (Schur et al., 2016). These data would suggest that ketamine actively engages central ACC GABA, and, if replicated, may represent a potential biomarker for ketamine treatment response (Singh et al., 2021). This is consistent with preclinical evidence highlighting countering GABAergic deficits with antidepressant therapies.

Our study lacked a healthy control group to truly assess drug treatment normalization of GABA, thus, limiting our ability to compare the GABAergic deficits with euthymic individuals or knowing if GABA deficit is a state or a trait marker of MDD. These important questions need to be investigated further in a prospective manner. We did not observe a baseline glutamate elevation in our study, this could be due to the small sample size. Ketamine use is expanding unabated without a clear understanding of its mechanism of action. To identify a biomarker for ketamine response is a priority and will provide greater precision in selecting patients for ketamine treatments. These feasibility data provide further impetus to investigate the role of central GABAergic signaling as a central biomarker of ketamine response.

In conclusion, this pilot study suggests ketamine reduces GABA deficits among TRD patients who remitted with ketamine infusion. Our findings need to be replicated in a larger sample size study.

Data Availability:

The datasets generated during and/or analyzed during the current study are not publicly available but are available from the corresponding author on reasonable request.