Multiple studies of the frontal lobe report no changes in metabolite levels that correlate with medication status (Bustillo et al. 2008
; Bustillo et al. 2009
; Pae et al. 2004
; Szulc et al. 2005
; Szulc et al. 2007
). Bustillo et al. (2002)
reported reductions in frontal NAA levels in patients after a year of haloperidol or quetiapine treatment, but did not differentiate between the antipsychotics or between treatment effects and disease progression. Conversely, Choe et al. (1996)
observed increases in NAA/Cr in 13/34 patients following treatment with any antipsychotic. Bertolino et al. (2001)
reported increased NAA/Cr in the dorsolateral prefrontal cortex in chronically ill patients following treatment regardless of antipsychotic. In one of the few studies where medication changes did not occur during the study, the NAA/Cr ratio increased after 8 weeks of clozapine therapy (Ertugrul et al. 2009
). However, the patients in this study were refractory to other medications, so differences due to disease severity or lingering effects from the previous medications cannot be excluded.
In basal ganglia and thalamus, the majority of papers also find no significant effect of medication on metabolite levels. Of the five studies we found reporting measurements in the basal ganglia (Bertolino et al. 2001
; Bustillo et al. 2001
; Bustillo et al. 2008
; Fannon et al. 2003
; Heimberg et al. 1998
) only Heimberg et al. reported any change, a decrease in Cho/Cr. In the thalamus, three of five papers found no effects of medication (Bertolino et al. 2001
; Bustillo et al. 2009
; Heimberg et al. 1998
). However, Szulc et al. (2007)
reported decreased NAA/Cr in patients treated with typical antipsychotics compared with normal controls, but no differences in NAA/Cr between patients treated with typical or atypical antipsychotics. This may be due to insufficient power, since the data they report show that patients taking atypical antipsychotics had NAA/Cr ratios intermediate between controls and patients on typical medications. The same group reports an increase in NAA/Cr and myoinositol/Cr in the thalamus following at least 4 weeks of stable risperidone therapy (Szulc et al. 2005
We found no effects of medication dose or duration in the right striatum of rats treated with haloperidol or clozapine compared with vehicle over 6 months. These findings are in agreement with our previous study (Lindquist et al. 2000
), where with the same (low) doses of these drugs we found no significant changes in NAA/Cr or Cho/Cr at either 24 hours or after 1 week. We are aware of no other in vivo measurements of these metabolite concentrations in rat brain. Of the other animal studies examining metabolic changes following antipsychotic treatment, only the 6-month ex vivo study by Bustillo, et al. (2006)
is readily comparable to these studies due to the similar techniques (high-resolution magic angle spinning 1H MRS of tissue samples vs. in vivo 1H MRS) and similar haloperidol dose (38 mg/kg/month, approximately 1.3 mg/kg/day); they report no changes in any metabolite in any region studied. The remaining studies utilize tissue extracts. The extraction process might free bound metabolite pools, which would be MRS-invisible either in vivo or in tissue samples. Bustillo, et al. (2004)
used high performance liquid chromatography to examine NAA concentrations in extracts of various brain regions from rats treated with either haloperidol (6 mg/kg/day) or clozapine (70 mg/kg/day) for 6 weeks and found no change in NAA concentration in any region with treatment. In contrast, Harte et al. (2005)
, using a similar ex vivo analytical method but lower overall dose of haloperidol (28.5 mk/kg/3 weeks), found increased NAA only in the striatum following a 6-month exposure. McLoughlin et al. (2009)
, using high resolution proton NMR, report multiple changes in metabolite levels in extracts of prefrontal cortex, dorsal striatum, and hippocampus following a 21-day exposure to haloperidol (1 mg/kg/day) or clozapine (20 mg/kg/day). NAA was increased in frontal cortex and hippocampus for both drugs, but no changes in NAA levels were reported in the striatum. Thus, four of five previous studies also found no evidence that haloperidol or clozapine alter striatal NAA levels. Findings from these studies are summarized in .
Results of Previous Animal Studies1
Several studies have suggested that brain volumes are altered upon antipsychotic treatment (see, for example, Ho et al., (2011
). In a study analogous to this one, Vernon et al. (2010)
found that striatal and hippocampal volumes of rats treated with haloperidol or olanzapine for 8 weeks did not differ from control animals, although striatal volume increased for all animals, which suggests that the striatal volume may have increased in our study. Changes in striatal volume may not have a significant effect on metabolite concentrations. If a volume increase resulted from an increased cell density, then both NAA and Cho potentially would be elevated. If a volume increase resulted from an increased cell size, but not number, then perhaps NAA and Cho would decrease; however, one might also expect increases in Ins levels due to its role as an osmolyte. No such changes were seen in this study.
The significant time-by-treatment effect for Cho/Cr could be related to brain volume or metabolic changes. Antipsychotics may alter lipid metabolism (Thomas and Yao 2007
), increase the number of dendritic spines in the hippocampus (Crichtlow et al. 2006
), and affect myelin content (Bartzokis et al. 2009
), all of which could be involved in brain volume changes and be reflected by alterations in choline levels. On the other hand, disturbances in energy metabolism might result in changes in creatine levels. Several treatment-dependent effects of antipsychotics on energy metabolism have been reported, including reduced activity of creatine kinase (Assis et al. 2007
) and succinate dehydrogenase (Streck et al. 2007
). This evidence suggests that choline and creatine levels could change with antipsychotic treatment. While such changes may not be detected separately, they could possibly be detected through ratio data, as occurred here. Although the time-by-treatment effect for Cho/Cr would not survive corrections for multiple comparisons, it is worthy of further study.
The three studies reporting no effect of antipsychotics on metabolite levels used male Sprague-Dawley rats of similar ages, as did the current study. The two studies that reported that metabolite levels were affected by drug treatment used either Long-Evans rats (gender not reported, Harte et al. 2005
) or male Wistar rats (McLoughlin et al. 2009
). Hence, differences in treatment response due to rat strain cannot be excluded.
The lower drug doses used in this study were chosen to give dopamine D2
receptor occupancies of about 60% (Zhang and Bymaster 1999
), since D2
occupancy levels may be better correlates of response than plasma levels (Kapur et al. 2003
; Pani et al. 2007
). Khan et al. (2003)
found that a 2 mg/kg/day haloperidol dose, as used here, produces clinically relevant plasma levels. For clozapine, both doses used in this study produce clinically relevant occupancy and plasma levels (Kapur et al. 2003
; Khan et al. 2003
). Two dose levels of each drug were used in this study to allow us to examine possible dose-dependent effects on NAA or other metabolite levels. No such dose-dependent effects were observed.
According to Kapur et al. (2000)
, studies examining the chronic effects of antipsychotics need to adjust the dose or dosing schedule to better mimic the duration of D2
occupancy in humans. The half-lives of haloperidol and clozapine in rats are about 1.5 hours, as opposed to 24 and 12 hours, respectively, in humans (Kapur et al. 2003
; Kapur et al. 2000
). Kapur et al. (2000)
suggest using a large single dose or multiple doses to minimize the fluctuations in D2
occupancy to better mimic human dosing. Of the previous studies, only the 6-week study by Bustillo et al. (2004)
used multiple daily (high) doses, which would be expected to minimize fluctuations in receptor occupancy. We chose to use drinking water to attempt to maintain a constant level of D2
occupancy. However, Perez-Costas et al. (2008)
have recently shown that although haloperidol in drinking water achieved the desired D2 receptor occupancy, clozapine at either 20 mg/kg or 40 mg/kg did not. D2 receptor occupancy, however, is probably not the only mediator of antipsychotic action (Richtand et al. 2007
). Hence, the influence of the different routes of administration and the role of D2 receptor occupancy on metabolite levels remains unclear, but could explain the lack of a dose-dependent response in this study.
There are several limitations to this study. The antipsychotic dose levels were fixed, which is not representative of the typical clinical population. No measurements of plasma levels or receptor occupancy were made, so drug underexposure cannot be excluded. Time constraints permitted only one region of normal brain to be studied, so regional effects of these medications were not investigated. Multiple studies indicate that these drugs may have regionally specific effects (Fannon et al. 2003
; McLoughlin et al. 2009
; Szulc et al. 2005
) and there is evidence that there are regionally specific, and possibly opposing, differences in metabolite levels in patients with schizophrenia (Ende et al. 2003
), Finally, the negative findings could be due to insufficient statistical power given the high interindividual variation that we observed in this study. While calculating an effect size for a linear mixed models analysis is difficult due to the interplay of the modeled effects, using the 6-month data in a t-test analysis shows that for NAA, detectable changes would be about 25% of the control value.
The absence of disease in this animal model is another limitation of this study. Lithium has no mood-altering properties when given to normal subjects, despite its usefulness in treating patients with bipolar disease (Stone et al. 2009
). Fannon et al. (2003)
found that NAA/Cr ratios in the hippocampus differed at baseline between drug-naïve and previously medicated patients or controls, but that no group differences existed following 3 months of treatment, which suggests that treatment normalized NAA levels. It is conceivable that antipsychotic drugs normalize aberrant metabolic levels in specific regions of diseased brain, but have insignificant effects in normal brain. Future studies in multiple brain regions using animal models would address these issues.
In conclusion, our results are in agreement with the majority of reports suggesting that antipsychotic drugs do not significantly alter the concentrations of any MRS-measurable metabolite in the striatum. Regional and/or disease-related changes may still occur, however, and should be the focus of further study.