d-Cycloserine, an NMDA Glutamate Receptor Glycine Site Partial Agonist, Induces Acute Increases in Brain Glutamate Plus Glutamine and GABA Comparable to Ketamine

To the Editor: Ketamine, an N-methyl-d-aspartate glutamate receptor (NMDAR) antagonist, is effective acutely for major depression (1), but development of alternative NMDAR antagonists is limited by incomplete understanding of mechanism and optimal dosage. Based on preclinical and clinical studies (1, 2), we hypothesize that NMDAR antagonism will produce an acute increase in GABA and glutamate plus glutamine (Glx) as an early event in antidepressant action. Using proton magnetic resonance spectroscopy (¹H MRS), we found acute ketamine treatment in depressed patients produced rapid, transient elevations (approximately 40%) of both Glx and GABA levels in the medial prefrontal cortex (1), suggesting that this is an initial step in the antidepressant action cascade for NMDAR antagonists. We used the same methods (1) to assess the effects of high-dose d-cycloserine (1000 mg), an NMDAR glycine site partial agonist, on brain GABA and Glx in healthy subjects. Compared with ketamine, d-cycloserine has a slower (onset within 2–4 weeks) but comparable antidepressant effect (3). d-cycloserine has a dose-dependent biphasic effect on the NMDAR, potentiating function at low dosages (<100 mg), but functioning as a net antagonist at dosages >500 mg (4).

After baseline ¹H MRS scans, six healthy subjects (age=33 years, SD=4 years; four males and two females) were administered 1000 mg of oral d-cycloserine, followed by serial ¹H MRS scans for up to 90 minutes to measure Glx and GABA levels. Areas under the curve computed using the trapezoidal rule yielded large effect sizes for both Glx (69%, SD=48%, d=1.44) and GABA (39%, SD=31%, d=1.26) without inducing overt psychosis (Figure 1, next page). There was an overall main effect on Glx (F_5,22.8=2.9, p=0.034), with an initial peak at approximately 35 minutes postdose (increase of 23%, SD=5%) and a second peak 75–90 minutes postdose (increase of 20%, SD=7%). For GABA, there was a comparable, trend-level main effect (F_5,23.3=2.5, p=0.058; increase of 16%, SD=5%).

d-cycloserine increased both neurotransmitter levels comparably to ketamine, and this may be the initial step in the antidepressant cascade of NMDAR antagonism. Future studies should examine the relationship of these neurotransmitters to the degree of the antidepressant effect of d-cycloserine. Targeting the NMDAR and AMPA receptors may have specific advantages in treatment-resistant depression. The more rapid clinical improvement seen with ketamine may be due to other pharmacological effects that distinguish it from d-cycloserine, and these need to be identified to facilitate development of more rapidly acting antidepressants.