Effects of methylphenidate on working memory in pigeons

Behavioral pharmacology of the odor span task: Effects of flunitrazepam, ketamine, methamphetamine and methylphenidate

The Odor Span Task is an incrementing non-matching-to-sample procedure that permits the study of behavior under the control of multiple stimuli. Rats are exposed to a series of odor stimuli and selection of new stimuli is reinforced. Successful performance thus requires remembering which stimuli have previously been presented during a given session. This procedure has been frequently used in neurobiological studies as a rodent model of working memory; however, only a few studies have examined the effects of drugs on performance in this task. The present experiments explored the behavioral pharmacology of a modified version of the Odor Span Task by determining the effects of stimulant drugs methylphenidate and methamphetamine, NMDA antagonist ketamine, and positive GABA_A modulator flunitrazepam. All four drugs produced dose-dependent impairment of performances on the Odor Span Task, but for methylphenidate and methamphetamine, these occurred only at doses that had similar effects on performance of a simple odor discrimination. Generally, these disruptions were based on omission of responding at the effective doses. The effects of ketamine and flunitrazepam were more selective in some rats. That is, some rats tested under flunitrazepam and ketamine showed decreases in accuracy on the Odor Span Task at doses that did not affect simple discrimination performance. These selective effects indicate disruption of within-session stimulus control. Overall, these findings support the potential of the Odor Span Task as a baseline for the behavioral pharmacological analysis of remembering.

Dopamine and norepinephrine receptors participate in methylphenidate enhancement of in vivo hippocampal synaptic plasticity

Attention-deficit hyperactive disorder (ADHD) is the most commonly studied and diagnosed psychiatric disorder in children. Methylphenidate (MPH, e.g., Ritalin) has been used to treat ADHD for over 50 years. It is the most commonly prescribed treatment for ADHD, and in the past decade it was the drug most commonly prescribed to teenagers. In addition, MPH has become one of the most widely abused drugs on college campuses. In this study, we examined the effects of MPH on hippocampal synaptic plasticity, which serves as a measurable quantification of memory mechanisms. Field potentials were recorded with permanently implanted electrodes in freely-moving mice to quantify MPH modulation of perforant path synaptic transmission onto granule cells of the dentate gyrus. Our hypothesis was that MPH affects hippocampal synaptic plasticity underlying learning because MPH boosts catecholamine signaling by blocking the dopamine and norepinephrine transporters (DAT and NET respectively). In vitro hippocampal slice experiments indicated MPH enhances perforant path plasticity, and this MPH enhancement arose from action via D1-type dopamine receptors and β-type adrenergic receptors. Similarly, MPH boosted in vivo initiation of long-term potentiation (LTP). While there was an effect via both dopamine and adrenergic receptors in vivo, LTP induction was more dependent on the MPH-induced action via D1-type dopamine receptors. Under biologically reasonable experimental conditions, MPH enhances hippocampal synaptic plasticity via catecholamine receptors.

Reconsidering the concept of behavioral mechanisms of drug action

A half-century of research in behavioral pharmacology leaves little doubt that behavior-environment contingencies can determine the behavioral effects of drugs. Unfortunately, a coherent behavior-analytic framework within which to characterize the myriad ways in which contingencies interact with drugs, and to predict effects of a given drug under a given set of conditions, still has not developed. Some behavioral pharmacologists have suggested the concept of behavioral mechanisms of drug action as a foundation for such a framework. The notion of behavioral mechanisms, however, does not seem to have been fully embraced by behavioral pharmacologists. It is suggested here that one reason for this is that the concept itself has not been sufficiently clarified (i.e., stimulus control over use of the phrase is not sufficiently precise). Furthermore, early behavioral pharmacologists may not have possessed an adequate set of analytic tools to develop a viable framework based upon behavior mechanisms. In the first part of this paper, the notion of behavioral mechanisms of drug action is explored, and the sort of data that might provide evidence of a behavioral mechanism is considered. In the second part, it is suggested that the increased availability of quantitative models in behavior analysis may help provide the tools needed for elucidating behavioral mechanisms of drug action. Some examples of how these models have been, and could be used are provided.

The effects of L-amphetamine sulfate on cognition in MS patients: results of a randomized controlled trial

Defects in processing speed and memory are common in multiple sclerosis (MS) patients. In other populations, amphetamines have been shown to enhance cognition, but their use is limited by adverse behavioral effects. The L-isomer may have equivalent cognition enhancement with less adverse effects due to decreased potency in subcortical areas. The aim of this study was to assess the safety and efficacy of L-amphetamine sulfate in the treatment of cognitive dysfunction in MS. This was a 2:1 randomized, placebo-controlled, double-blind trial, involving 33 MS clinics across the USA. One hundred and fifty-one clinically definite MS patients with documented cognitive dysfunction who were relapse free for >or=90 days, with an Expanded Disability Status Scale (EDSS) <or=6.5, and with no other medical/psychiatric condition that may cause psychological dysfunction were randomized to 30 mg of oral L-amphetamine sulfate or placebo for 29 days, including a dose escalation period. A history of cardiac disease, uncontrolled hypertension or electrocardiograph abnormalities resulted in exclusion. The primary outcomes were the Subject Global Assessment of Change and Symbol Digit Modalities Test (SDMT). Secondary outcomes were the results from the California Verbal Learning Test, second edition (CVLT2), Brief Visual Memory Test-Revised (BVMTR), and Paced Auditory Serial Addition Test (PASAT). One hundred and thirty-six subjects completed the study. No differences were found at baseline in demographics or in the results of the neuropsychological tests. After treatment, the active group performed significantly better for total learning (P = 0.041) and delayed recall (P < 0.01) on the BVMTR, and for delayed recall (P = 0.012) on the CVLT2. Five patients (four from the treatment group, one placebo) withdrew due to intolerable adverse events. L-amphetamine sulfate was associated with improved learning and memory and was well tolerated in this study. However, because the positive findings were observed on secondary outcome measures, the study requires replication before L: -amphetamine sulfate can be recommended for the treatment of cognitive impairment in MS.

Methylphenidate modulates activity within cognitive neural networks of patients with post-stroke major depression: A placebo-controlled fMRI study

BACKGROUND

Methylphenidate (MP) is a dopamine- and noradrenaline-enhancing agent beneficial for post-stroke depression (PSD) and stroke recovery due to its therapeutic effects on cognition, motivation, and mood; however, the neural mechanisms underlying its clinical effects remain unknown. This study used functional magnetic resonance imaging (f MRI) to investigate the effect of MP on brain activity in response to cognitive tasks in patients with PSD.

METHODS

Nine stroke outpatients with DSM IV defined major depression underwent fMRI during two cognitive tasks (2-back and serial subtraction) on four occasions, on the first and third day of a three-day treatment of MP and placebo. Nine healthy control (HC) subjects matched for age and sex scanned during a single session served as normative data for comparison. The main outcome measure was cognitive task-dependent brain activity.

RESULTS

For the 2-back task, left prefrontal, right parietal, posterior cingulate, and temporal and bilateral cerebellar regions exhibited significantly greater activity during the MP condition relative to placebo. Less activity was detected in rostral prefrontal and left parietal regions. For serial subtraction, greater activity was detected in medial prefrontal, biparietal, bitemporal, posterior cingulate, and bilateral cerebellar regions, as well as thalamus, putamen, and insula. Further, underactivation observed during the placebo condition relative to HC improved or reversed during MP treatment. No significant differences in behavioral measures were found between MP and placebo conditions or between patients and HC.

CONCLUSIONS

Short-term MP treatment may improve and normalize activity in cognitive neuronal networks in patients with PSD.

Seasonal variation in pigeon body weight and delayed matching-to-sample performance

The weights of 5 pigeons with free access to food, monitored over 3 calendar years in the laboratory, were found to fluctuate with season. All pigeons were at their heaviest in the winter and were lightest in the summer. Five different pigeons performed a standard delayed matching-to-sample task for 44 weeks from January to November. Their weights were held at 85% of their summer free-feeding weights, making their predicted deprivation level higher in the winter relative to predicted winter free-feeding weights. Slopes of forgetting functions fit to weekly response totals for each pigeon were shallower in winter, showing an improvement in accuracy with longer delays. Thus, delayed matching-to-sample performance may have been affected by the practice of maintaining the pigeons at a constant body weight throughout the calendar year.