Our results suggest that once-weekly dosing with D-cycloserine for eight weeks may be sufficient to produce improvement compared to placebo in negative symptoms. . Furthermore, because the SANS rating at week 8 was performed prior to the weekly D-cycloserine dose, this finding indicates that intermittent dosing with D-cycloserine may produce persistent alterations in excitatory synapses involved in expression of negative symptoms. Whether the magnitude of the effect is of clinical significance is uncertain, since the CGI did not differ between treatments and the significant difference between groups in SANS scores at week 8 was in part the result of worsening in the placebo group. Because the treatment groups were comparable in all demographic characteristics and on all clinical measures at baseline, as well as in change scores on other symptom ratings, it is most likely that the significant difference in SANS scores at week 8 represents a true drug effect and is not the result of clinical worsening unique to the placebo group. The 16.6% net reduction in SANS total score with D-cycloserine compared to placebo (0.7 effect size) was the result of an 8% improvement in the D-cycloserine group and an 8% worsening in the placebo group and was close to the threshold for statistical significance (p=0.048). While of uncertain significance, the net difference in SANS score change in a previous positive 8-week study of daily dosing with D-cycloserine was also16% (0.8 effect size) although, unlike the current study, negative symptom scores improved in the placebo group (Goff, et al., 1999b
). In addition, preliminary evidence has suggested that the magnitude of D-cycloserine’s effect on negative symptoms might be greater when D-cycloserine is added to first generation antipsychotics, as in the daily dosing trial, than to atypicals (Evins, et al., 2002
, Goff, et al., 1999a
, Goff, et al., 1995
In contrast to the beneficial effect on negative symptoms, we did not find enhancement of cognitive function as measured by a standard cognitive battery at week 8. Previous trials employing daily dosing in schizophrenia also failed to produce cognitive benefit at eight weeks (Goff, et al., 1999b
, Goff, et al., 2005
) although one trial found improvement on Sternberg’s Item Recognition Task after two weeks of daily dosing with D-cycloserine 50mg (Goff, et al., 1995
). However, we observed enhancement of delayed recall on the LMT following the first D-cycloserine dose. It is unclear whether this effect would have persisted with repeated once-weekly dosing. Parnas and colleagues (Parnas, et al., 2005
) found that a single pre-exposure dose of D-cycloserine resulted in partial tolerance to D-cycloserine’s facilitative effect on fear extinction and that 5 pre-exposure doses separated by 48 hours fully attenuated the effect. In contrast, a 28 day interval was sufficient to restore the facilitative effect of D-cycloserine on conditioned fear extinction (Parnas, et al., 2005
). The effect of medication-free intervals shorter than 28 days was not studied. While it is not known whether repeated dosing with full agonists at the glycine site will similarly produce tachyphylaxis, Boje and colleagues (Boje, et al., 1993
) demonstrated that desensitization of NMDA receptors on cultured granule neurons occurred with partial agonists and with the full agonist, glycine. Because effects of D-cycloserine on memory consolidation may underlie the augmentation of cognitive behavioral therapy previously demonstrated in anxiety disorders, the identification of a dosing schedule that avoids tachyphylaxis will be of considerable importance if D-cycloserine is also to be used to enhance psychosocial interventions in schizophrenia.
The significant improvement in delayed but not immediate thematic recall on the LMT following the first administration of D-cycloserine is consistent with animal models in which learning effects of D-cycloserine are first detectable after a 24 hour interval, possibly reflecting the role of NMDA receptors in memory consolidation (Santini, et al., 2001
). Similarly, in patients with acrophobia, anxiety symptoms recorded during desensitization procedures performed 2–4 hours following study drug administration did not differ between the D-cycloserine and placebo groups; therapeutic benefit was observed when anxiety symptoms were assessed 1 week later (Ressler, et al., 2004
). Enhancement of memory consolidation may be of particular relevance to schizophrenia. In a study of procedural memory assessed during training and again after 24 hours, schizophrenia patients exhibited a specific deficit in memory consolidation compared to healthy subjects (Manoach, et al., 2004
). Cognitive tests designed to assess learning after an interval of 24 hours or longer will be important for future studies to test the hypothesis that memory consolidation is impaired in schizophrenia and facilitated by glycine site agonists.
Several factors complicate interpretation of cognitive findings in this trial. The use of a 7-day recall condition with the LMT was added as an exploratory measure to capture memory consolidation effects, but has not previously been studied or validated as an outcome measure. Because the story at week 1 (story B) was different from that at baseline (story A) proactive interference from story A may have influenced recall of story B. The worsening of performance in the placebo group may reflect this effect. Recently, the glycine site full agonist, D-serine, was found to preferentially enhance reversal learning in mice, possibly mediated by long term depression (LTD) of glutamatergic neurotransmission (Duffy, et al., 2007
). Such an effect would be consistent with D-cycloserine’s enhancement of conditioned fear extinction and might have contributed to enhanced learning of story B one week after learning story A. The enhancement of reversal learning by glycine site agonists is expected to reduce perseveration and increase cognitive flexibility (Duffy, et al., 2007
). Moreover, NMDA channel blockade with ketamine increases perseverative errors on the WCST in healthy subjects (Krystal, et al., 1994
). However, we found no difference in perseverative errors on the WCST between treatment groups at week 8 (F (1, 29) = 1.80, p = .19). Similarly, Tsai and colleagues did not find an effect on perseverative errors on the WCST in schizophrenia subjects after six-weeks of daily dosing with D-serine, whereas the number of categories completed increased compared to placebo (Tsai, et al., 1998
). Finally, the absence of cognitive effect at week 8 is based on a standard cognitive battery that did not specifically target memory consolidation effects.
The enhanced delayed recall of verbal memory after week 1 administration suggests that D-cycloserine effects on cognition may not be restricted to facilitation of conditioned fear extinction. In animals, spatial memory and visual recognition tests have also shown enhancement following a single dose of D-cycloserine (Matsuoka and Aigner, 1996
, Quartermain, et al., 1994
) suggesting that a range of cognitive functions may respond in a similar pattern to D-cycloserine. It is not clear, however, that our findings will apply to other clinical populations. The hypothesized dysregulation of NMDA receptors in schizophrenia may result in a pattern of cognitive response to D-cycloserine unique to this illness (Goff and Coyle, 2001
). In addition, antipsychotics have been shown to alter NMDA sensitivity by a variety of mechanisms, including alterations in NMDA receptor density and subunit composition, changes in pharmacologic sensitivity of the glycine site, and in regulation of glutamate release (Goff, et al., 2002
, Millan, 2005
). Similar trials in healthy, unmedicated subjects utilizing tests sensitive to memory consolidation effects will be needed to better characterize D-cycloserine effects on cognition.
The improvement in negative symptoms and absence of improvement in cognition at eight weeks suggest that glutamatergic pharmacologic mechanisms may differ between these two psychopathological domains. It is possible that only negative symptoms and not cognitive deficits are potentially responsive to glycine site agonists, a puzzling finding, given that the full range of negative, positive and cognitive symptoms of schizophrenia are elicited by NMDA antagonists (Krystal, et al., 1994
). Previous trials in which glycine site agonists demonstrated efficacy for negative symptoms clearly displayed a gradual improvement in symptom ratings over time with a plateau after approximately six weeks (Goff, et al., 1999b
, Heresco-Levy, et al., 1999
, Tsai, et al., 1998
). In contrast, we found an effect on memory consolidation after a single dose; memory effects have been shown to attenuate with repeated dosing in animal studies. It appears that the mechanisms underlying potential therapeutic effects on negative and cognitive symptoms may be distinctly different. While negative symptom effects increase over time with repeated dosing, enhancement of memory appears to diminish. It is possible that cognitive effects reflect enhancement of NMDA receptor activation, whereas effects on negative symptoms reflect a compensatory process of reduced activation that corresponds to attenuation of memory facilitation. Alternatively, the effects of glycine site agonists on negative symptoms may be mediated by a different subpopulation of NMDA receptors than those that mediate learning and memory.