Glutamate, Learning, and Working Memory

Impairments in plasticity and working memory in schizophrenia have been hypothesized to reflect dysfunction at the N-methyl-D-aspartate glutamate receptor (NMDAR). However, the specific mechanisms through which the NMDAR is involved in working memory versus plasticity differ. Towards gaining a deeper understanding of how NMDAR signaling relates to individual cognitive functions in healthy adults and patients with schizophrenia, the investigators used a single dose of d-cycloserine (DCS) as an experimental probe to examine the effects of enhancing NMDAR signaling on plasticity versus working memory in healthy adults and individuals with schizophrenia.

Background: Cognitive impairments in schizophrenia, such as deficits in plasticity and working memory, have been hypothesized to reflect dysfunction at the N-methyl-D-aspartate glutamate receptor (NMDAR). However, given that divergent properties of the NMDAR underlie its roles in plasticity versus working memory and that various aspects of NMDAR function are abnormal in schizophrenia, examining the effects of DCS in both healthy and patient populations is crucial.

Methods: The investigators used a single dose of the partial NMDAR agonist, d-cycloserine (DCS) to probe the effects of enhancing NMDAR signaling on working memory and plasticity. Working memory was assessed using a spatial n-back task. Plasticity was assessed using two learning tasks, the weather prediction task and information integration task, and an EEG paradigm that assesses changes in visual evoked potential amplitude following high frequency visual stimulation. Sixty-five healthy adults and forty-five schizophrenia patients were randomized to receive 100 mg acute DCS (healthy adult n = 32; schizophrenia n = 24) or placebo (healthy adult n = 33; schizophrenia n = 21).

Eligibility Criteria:
Inclusion criteria for healthy subjects:

1. between the ages of 18 and 30 years
2. comfortable reading in English
3. normal visual acuity or corrected vision
4. normal or corrected hearing.

Exclusion criteria for healthy subjects:

1. history or seizures or neurologic diseases
2. currently prescribed medication for any psychiatric conditions
3. any medical condition affecting fine motor movement of the hands
4. pregnancy or suspected pregnancy
5. use of recreational drugs or drugs taken not as prescribed in the past month
6. having a full scale intelligence quotient (IQ) \< 70, as assessed by the Wechsler Abbreviated Scale of Intelligence (WASI)
7. having consumed alcohol in the 24 hours prior to the first lab visit
8. known allergy to any antibiotics.

Inclusion criteria for patients with schizophrenia:

1. between the ages of 18 and 50 years
2. comfortable reading in English
3. normal visual acuity or corrected vision
4. normal or corrected hearing
5. meets criteria for Diagnostic and Statistical Manual of Mental Disorders 4th edition (DSM-IV) diagnosis of schizophrenia.

Exclusion criteria for patients with schizophrenia:

1. history or seizures or neurologic diseases
2. currently prescribed Clozapine or medications contraindicated for DCS
3. any medical condition affecting fine motor movement of the hands
4. pregnancy or suspected pregnancy
5. history of traumatic brain injury requiring hospitalization for 2 or more days
6. IQ \< 70, as assessed by the WASI
7. having consumed drugs other than as prescribed in the 48 hour prior to the testing visit or having consumed alcohol in the 24 hours prior to the testing visit
8. known allergy to any antibiotics
9. current alcohol or substance dependence