Methylphenidate enhances implicit learning in healthy adults.

BACKGROUND AND PURPOSE: One limiting factor in the development of pharmacological interventions to enhance cognition is the absence of biomarkers that can be used in healthy volunteers to screen novel compounds. Drug discovery has tended to rely heavily on explicit measures of cognition, but these are typically insensitive to cognition-enhancing effects in healthy volunteers. This study investigated whether a novel battery of implicit cognition measures is sensitive to the effects of methylphenidate (Ritalin) in healthy volunteers. EXPERIMENTAL APPROACH: Eighty healthy volunteers were randomised to receive either a single (10 mg) dose of methylphenidate or matched placebo. Participants completed a battery of tasks measuring implicit cognition (location priming, contextual cueing, implicit task switching). The effect of methylphenidate on standard, explicit measures of cognition was also assessed. KEY RESULTS: Methylphenidate enhanced implicit learning on the location priming task and the implicit task-switching task. In line with previous work, we found that these effects were greater in male volunteers. There was no evidence for improved learning in any of the explicit measures. CONCLUSION AND IMPLICATIONS: These results demonstrate that implicit measures of cognition are sensitive to pharmacological interventions in healthy volunteers. As such, implicit cognition measures may be a useful way of screening and tracking cognitive effects of novel agents in experimental medicine studies.

Ketamine and other glutamate receptor modulators for depression in bipolar disorder in adults.

BACKGROUND: There is emerging evidence that glutamatergic system dysfunction might play an important role in the pathophysiology of bipolar depression. This review focuses on the use of glutamate receptor modulators for depression in bipolar disorder. OBJECTIVES: 1. To assess the effects of ketamine and other glutamate receptor modulators in alleviating the acute symptoms of depression in people with bipolar disorder.2. To review the acceptability of ketamine and other glutamate receptor modulators in people with bipolar disorder who are experiencing acute depression symptoms. SEARCH METHODS: We searched the Cochrane Depression, Anxiety and Neurosis Review Group's Specialised Register (CCDANCTR, to 9 January 2015). This register includes relevant randomised controlled trials (RCTs) from: the Cochrane Library (all years), MEDLINE (1950 to date), EMBASE (1974 to date), and PsycINFO (1967 to date). We cross-checked reference lists of relevant papers and systematic reviews. We did not apply any restrictions to date, language or publication status. SELECTION CRITERIA: Randomised controlled trials (RCTs) comparing ketamine, memantine, or other glutamate receptor modulators with other active psychotropic drugs or saline placebo in adults with bipolar depression. DATA COLLECTION AND ANALYSIS: At least two review authors independently selected studies for inclusion, assessed trial quality and extracted data. Primary outcomes for this review were response rate and adverse events. Secondary outcomes included remission rate, depression severity change scores, suicidality, cognition, quality of life, and dropout rate. We contacted study authors for additional information. MAIN RESULTS: Five studies (329 participants) were included in this review. All included studies were placebo-controlled and two-armed, and the glutamate receptor modulators - ketamine (two trials), memantine (two trials), and cytidine (one trial) - were used as add-on drugs to mood stabilisers. The treatment period ranged from a single intravenous administration (all ketamine studies), to repeated administration for memantine and cytidine (8 to 12 weeks, and 12 weeks, respectively). Three of the studies took place in the USA, one in Taiwan, and in one, the location was unclear. The majority (70.5%) of participants were from Taiwan. All participants had a primary diagnosis of bipolar disorder, according to the DSM-IV or DSM-IV-TR, and were in a current depressive phase. The severity of depression was at least moderate in all but one study.Among all glutamate receptor modulators included in this review, only ketamine appeared to be more efficacious than placebo 24 hours after the infusion for the primary outcome, response rate (odds ratio (OR) 11.61, 95% confidence interval (CI) 1.25 to 107.74; P = 0.03; I² = 0%, 2 studies, 33 participants). This evidence was rated as low quality. The statistically significant difference disappeared at three days, but the mean estimate still favoured ketamine (OR 8.24, 95% CI 0.84 to 80.61; 2 studies, 33 participants; very low quality evidence). We found no difference in response between ketamine and placebo at one week (OR 4.00, 95% CI 0.33 to 48.66; P = 0.28, 1 study; 18 participants; very low quality evidence).There was no significant difference between memantine and placebo in response rate one week after treatment (OR 1.08, 95% CI 0.06 to 19.05; P = 0.96, 1 study, 29 participants), two weeks (OR 4.88, 95% CI 0.78 to 30.29; P = 0.09, 1 study, 29 participants), four weeks (OR 5.33, 95% CI 1.02 to 27.76; P = 0.05, 1 study, 29 participants), or at three months (OR, 1.66, 95% CI 0.69 to 4.03; P = 0.26, I² = 36%, 2 studies, 261 participants). These findings were based on very low quality evidence.There was no significant difference between cytidine and placebo in response rate at three months (OR, 1.13, 95% CI 0.30 to 4.24; P = 0.86, 1 study, 35 participants; very low quality evidence).For the secondary outcome of remission, no significant differences were found between ketamine and placebo, nor between memantine and placebo. For the secondary outcome of change scores from baseline on depression scales, ketamine was more effective than placebo at 24 hours (MD -11.81, 95% CI -20.01 to -3.61; P = 0.005, 2 studies, 32 participants) but not at one or two weeks after treatment. There was no difference between memantine and placebo for this outcome.We found no significant differences in terms of adverse events between placebo and ketamine, memantine, or cytidine. There were no differences between ketamine and placebo, memantine and placebo, or cytidine and placebo in total dropouts. No data were available on dropouts due to adverse effects for ketamine or cytidine; but no difference was found between memantine and placebo. AUTHORS' CONCLUSIONS: Reliable conclusions from this review are severely limited by the small amount of data usable for analysis. The body of evidence about glutamate receptor modulators in bipolar disorder is even smaller than that which is available for unipolar depression. Overall, we found limited evidence in favour of a single intravenous dose of ketamine (as add-on therapy to mood stabilisers) over placebo in terms of response rate up to 24 hours; ketamine did not show any better efficacy in terms of remission in bipolar depression. Even though ketamine has the potential to have a rapid and transient antidepressant effect, the efficacy of a single intravenous dose may be limited. Ketamine's psychotomimetic effects could compromise study blinding; this is a particular issue for this review as no included study used an active comparator, and so we cannot rule out the potential bias introduced by inadequate blinding procedures.We did not find conclusive evidence on adverse events with ketamine. To draw more robust conclusions, further RCTs (with adequate blinding) are needed to explore different modes of administration of ketamine and to study different methods of sustaining antidepressant response, such as repeated administrations. There was not enough evidence to draw meaningful conclusions for the remaining two glutamate receptor modulators (memantine and cytidine). This review is limited not only by completeness of evidence, but also by the low to very low quality of the available evidence.