Our finding that gabapentin acutely and robustly increases visual cortex GABA concentrations in most healthy subjects is consistent with two smaller studies, one conducted in healthy controls scanned at 4.1T (Kuzniecky et al, 2002
) and one conducted in individuals with epilepsy scanned at 2.1T (Petroff et al, 1996b
). Similar to findings from Petroff et al, (1996a
), we detected, on average, a 0.84
mM increase in GABA concentration with acute gabapentin administration, the magnitude of increase being inversely correlated with baseline GABA concentrations (Petroff et al, 2000
). Although this study was conducted in healthy controls, these data are relevant as previous studies have found little correlation between antiepileptic (gabapentin, topirimate, and vigabatrin) dose and seizure control (Petroff et al, 2006
). In contrast, the larger the increase in GABA concentration with acute administration of vigabatrin, a GABA transaminase inhibitor, the greater the seizure control in epileptic subjects (Petroff et al, 1999
). Whether the same relationship exists between seizure control and gabapentin induced increase in GABA concentrations has not been studied.
Not surprisingly, there appears to be a ceiling with respect to how high GABA concentrations can be pushed with antiepileptic administration and at which there is no additional benefit with respect to seizure control (Petroff et al, 1996a
). In our study, the highest GABA concentration obtained with gabapentin administration was 1.88
mM, representing a 79% increase from baseline. In other studies of healthy subjects and those with epilepsy, GABA concentrations were as high as 2.3 and 2.7
mM, respectively, within hours of ingesting gabapentin 1200
mg (Kuzniecky et al, 2002
; Petroff et al, 2000
). The differences in final post-drug levels between our study and others is likely due to differences in baseline GABA levels, ours being lower, and variations in GABA concentrations reported from studies conducted a different magnetic field strengths.
Another important finding is that MEGA-PRESS GABA-editing methods can be successfully applied to the acquisition of GABA spectra with sufficient SNR to allow for reliable quantification of GABA concentration. With an average <6% difference in GABA concentrations both within day and between day, we are capable of detecting smaller differences between groups (Epperson et al, 2002
; Sanacora et al, 1999
) or with pharmacologic and non-pharmacologic interventions than those previously observed (Sanacora et al, 2002
Although these data are compelling, there are several limitations in the generalizability of our findings. The most obvious is that we did not include women in this study. Our group has previously demonstrated menstrual cycle phase differences in GABA concentrations in healthy women (Epperson et al, 2002
) and for this reason we chose to focus on males as they would be expected to have less day-to-day variation in GABA concentrations (Epperson et al, 2006
). We could have chosen to study all women in the same menstrual cycle phase. However, the early to mid-follicular phase would have been the most ideal as ovarian hormones are consistently low, but this is the time when healthy women tend to have their highest GABA concentrations (Epperson et al, 2002
), perhaps limiting our detection of drug-induced changes in GABA concentration. Sex difference in GABA response to AEDs has not been previously explored and deserves full investigation with women being studied in both follicular and luteal phases of the menstrual cycle.
Our initial in vivo
quantification using the LC model (O'Gorman et al, 2011
; Provencher, 1993
) for the reproducibility studies were inconsistent. In addition, the LC model phantom basis for 7T is not readily available. Hence, our data processing relies on custom-developed processing packages. With this approach, we have obtained consistent quantification of metabolites from within day scans of control subjects.
The detection of GABA resonance using the editing method can be complicated by overlap from macromolecules. However at 7T, the editing pulse becomes more selective because of the increased frequency separation between GABA and macromolecules. Based on the previous study on macromolecular contamination at 7T (Terpstra et al, 2002
), we estimate that macromolecules contribute about 15 to 30% to the estimated GABA concentration in our study. However, macromolecule contamination remains a challenge for accurate quantification of brain metabolites using short echo-time spectroscopy. Methods currently under development include using double inversion recovery for metabolite-nulling to obtain macromolecular spectra, which can be used for subtraction or as prior knowledge for fitting (Kassem and Bartha, 2003
; Mader et al, 2002
). Further validation may be needed to test efficiency of macromolecule removal using these methods.
Although gabapentin administration resulted in an increase in GABA concentrations in all subjects, the glutamate response was considerably more variable and there was no overall mean difference in glutamate concentrations between pre- and post-drug administration. However, we cannot rule out that chronic doses of gabapentin would have had a consistent effect on glutamate concentrations as an acute dose failed to alter glutamate concentrations while twice daily dosing for 8 days led to a decrease in glutamate in a rodent model (Leach et al, 1997
). It is possible that healthy individuals have a narrow homeostatic range for GABA concentrations as reflected by the <6% difference in within day and between day values, whereas there is a relatively larger range in glutamate concentration across and between days that is physiologic given the subjects included in this study were free from psychiatric and substance abuse disorders. One potential source of variability between scans conducted on the same day is caffeine withdrawal. Subjects are fasting between scans in a given day and acute caffeine administration increases glutamate concentrations in some brain regions (Solinas et al, 2002
). Glutamate acquisition occurred at the end of each scan session and the quality of the data could have been affected by subject fatigue with having to remain motionless. However, we lost data from only two subjects because of motion artifact.
Finally, what do these data suggest about the relationship between GABA and glutamate in response to treatment with this novel antiepileptic drug? With acute gabapentin dosing, the increase in GABA is predicated on the baseline GABA concentration. Whether chronic gabapentin treatment, which is required for its therapeutic effect, has a similar relationship with baseline GABA concentration is not known. Rodent studies would suggest that gabapentin enhances GAD activity at clinically relevant drug levels, whereas much higher drug levels than are typically obtained in the clinical setting are required to inhibit GABA-T (Silverman et al, 1991
; Taylor et al, 1992
). Both drug effects would be expected to lead to an overall increase in GABA concentration similar to that observed in this study. However, gabapentin also reduces calcium influx into glutamatergic terminals contributing to a decrease in glutamate release (Fink et al, 2000
), However, chronic treatment would be necessary most likely to observe an overall change in GABA concentration because of decreased release of glutamate. Moreover, inhibition of calcium influx leads to reduced cytosolic calcium and overall neuronal excitability (Fink et al, 2000
), suggesting gabapentin's antiepileptic mechanism of action could have little to do with acute increases in GABA concentration. Investigation of acute and chronic effects of gabapentin on both GABA and glutamate, particularly in brain regions typically implicated in seizure disorders, would be required to further examine gabapentin's anti-seizure effects in human subjects.
In conclusion, with the largest group to date, we demonstrated GABA-enhancing effects of gabapentin administration in healthy subjects. Glutamate concentrations measured during the same scan did not show a characteristic pattern of response to drug administration and were consistent with findings when no drug was administered. Whether each individual has a tightly controlled homeostatic GABA concentration and relatively greater variability in glutamate concentrations from day-to-day needs to be confirmed in a larger sample. Regardless, these data suggest that future studies focusing on glutamate concentrations in human subjects should account for day-to-day variability and include at least two baseline measurements for each subject. The same does not seem to be true for GABA concentrations, at least not in the visual cortex. Finally, 1H-MRS conducted at 7T provides an exquisitely sensitive tool for quantification of GABA and glutamate in human subjects at baseline and with pharmacologic manipulation.