NMDA receptor activity and the transmission of sensory input into motor output in introverts and extraverts

Enhancing Effects of NMDA-Receptor Blockade on Extinction Learning and Related Brain Activation Are Modulated by BMI

A distributed network including prefrontal and hippocampal regions is involved in context-related extinction learning as well as in renewal. Renewal describes the recovery of an extinguished response if the context of extinction differs from the context of recall. Animal studies have demonstrated that prefrontal, but not hippocampal N-methyl-D-aspartate receptor (NMDAR) antagonism disrupted extinction learning and processing of task context. However, human studies of NMDAR in extinction learning are lacking, while NMDAR antagonism yielded contradictory results in other learning tasks. This fMRI study investigated the role of NMDAR for human behavioral and brain activation correlates of extinction and renewal. Healthy volunteers received a single dose of the NMDAR antagonist memantine prior to extinction of previously acquired stimulus-outcome associations presented in either identical or novel contexts. We observed better, and partly faster, extinction learning in participants receiving the NMDAR antagonist compared to placebo. However, memantine did not affect renewal. In both extinction and recall, the memantine group showed a deactivation in extinction-related brain regions, particularly in the prefrontal cortex, while hippocampal activity was increased. This higher hippocampal activation was in turn associated with the participants' body mass index (BMI) and extinction errors. Our results demonstrate potentially dose-related enhancing effects of memantine and highlight involvement of hippocampal NMDAR in context-related extinction learning.

The influence of glutamatergic antagonism on motor variability, and comparison to findings in schizophrenia patients

OBJECTIVE

The primary aim of this explorative study was to investigate the influence of the glutamatergic N-methyl-d-aspartate (NMDA) receptor antagonist memantine on motor activity in healthy subjects. Secondarily, we wanted to compare these data to findings in a sample of schizophrenia patients.

METHODS

The healthy subjects acted as their own controls in an open-within-subject design. Motor activity was recorded with an actigraph worn for 24 h in the drug-free, and steady-state memantine conditions, respectively. Motor activity levels for 1-min intervals were analysed by means of both linear and nonlinear methods. The schizophrenia patients were monitored only once, without memantine manipulation.

RESULTS

The root mean square successive differences (RMSSD) and the RMSSD/SD ratio were increased by memantine, and memantine was also associated with lower autocorrelation (lag 1) but in recordings from the right arm only. These movement patterns partly corresponded to those found in a sample of drug-treated schizophrenia patients. Total activity level, standard deviation (SD) and sample entropy were not significantly different in the memantine versus drug-free condition.

CONCLUSION

The findings suggest a role for the NMDA receptor in the regulation of motor activity in healthy individuals as memantine increased the variability in the motor recordings and the alterations between adjacent motor recordings. It is suggested that the findings may be relevant to the role played by glutamate and the NMDA receptor functioning to the motor disturbances in schizophrenia.

Acetylcholinesterase inhibitors and memantine for neuroenhancement in healthy individuals: a systematic review

The term neuroenhancement refers to improvement in the cognitive, emotional and motivational functions of healthy individuals through, inter alia, the use of drugs. Of known interventions, psychopharmacology provides readily available options, such as the anti-dementia drugs, e.g. acetylcholinesterase inhibitors (donepezil, galantamine, rivastigmine) and memantine. Based on a systematic review we found that expectations about the potential of these drugs exceed their actual effects, as has been demonstrated in randomised controlled trials. Both single and repeated dose trials were included in the systematic review, however repeated dose trials have only been conducted for donepezil. In six small trials lasting 14-42 days, the following results emerged: donepezil improved the retention of training on complex aviation tasks and verbal memory for semantically processed words. In one study episodic memory was improved, whereas in others it remained unaffected by donepezil. In a sleep deprivation trial, donepezil reduced the memory and attention deficits resulting from 24h of sleep deprivation. Two studies reported even transient negative effects. Regarding the safety profile of donepezil, these studies found that it was rather well tolerated. In any case, since large longitudinal studies are not available no conclusions can be drawn. Seven small studies about the effects of a single dose of memantine, and one study with a single dose of rivastigmine have been reported. Again, these studies are not adequate to answer our research question. If, as here and elsewhere suggested, the concept of pharmaceutical neuroenhancement is not to be rejected in principle, the decision of healthy individuals to take drugs for the purpose of neuroenhancement should be based on exhaustive information. At the moment, the research that would support or oppose the use of acetylcholinesterase inhibitors and memantine for neuroenhancement by healthy individuals has not yet been performed.

Impulsiveness as a timing disturbance: neurocognitive abnormalities in attention-deficit hyperactivity disorder during temporal processes and normalization with methylphenidate

We argue that impulsiveness is characterized by compromised timing functions such as premature motor timing, decreased tolerance to delays, poor temporal foresight and steeper temporal discounting. A model illustration for the association between impulsiveness and timing deficits is the impulsiveness disorder of attention-deficit hyperactivity disorder (ADHD). Children with ADHD have deficits in timing processes of several temporal domains and the neural substrates of these compromised timing functions are strikingly similar to the neuropathology of ADHD. We review our published and present novel functional magnetic resonance imaging data to demonstrate that ADHD children show dysfunctions in key timing regions of prefrontal, cingulate, striatal and cerebellar location during temporal processes of several time domains including time discrimination of milliseconds, motor timing to seconds and temporal discounting of longer time intervals. Given that impulsiveness, timing abnormalities and more specifically ADHD have been related to dopamine dysregulation, we tested for and demonstrated a normalization effect of all brain dysfunctions in ADHD children during time discrimination with the dopamine agonist and treatment of choice, methylphenidate. This review together with the new empirical findings demonstrates that neurocognitive dysfunctions in temporal processes are crucial to the impulsiveness disorder of ADHD and provides first evidence for normalization with a dopamine reuptake inhibitor.

Energetical bases of extraversion: Effort, arousal, EEG, and performance

This study investigates an extension of H.J. Eysenck's [Eysenck, H.J., 1967. The Biological Basis of Personality. Charles C. Thomas, Springfield, IL] arousal theory of extraversion, incorporating an effort system as a control system for different aspects of arousal. Extraverts were expected to have lower levels of reticocortical arousal than introverts, to invest more effort, and to have lower task performance in a monotonous vigilance task. In a 40-min vigilance task, participants had to react to the shorter of two 1 kHz tones presented binaurally at an event rate of 200 per 10 min. Spontaneous EEG, event-related potential, and performance data of 40 extremely introverted and 41 extremely extraverted students were available for statistical analysis. A tendency for lower arousal levels of extraverts (alpha 2 band), the expected higher effort investment (P300) and a lower performance (hits) of extraverts were found.

Agentic extraversion modulates the cardiovascular effects of the dopamine D2 agonist bromocriptine

A recent psychobiological theory postulates a dopaminergic basis for the agency facet of extraversion, leading to the prediction that this personality trait modulates the psychophysiological effects of dopaminergic drugs. A single dose of the dopamine D2 receptor agonist bromocriptine reduces blood pressure in healthy volunteers. However, it is currently unknown whether this hypotensive effect of bromocriptine is modulated by agentic extraversion. Therefore, we measured resting cardiovascular activation in groups of healthy male volunteers either high or low in agentic extraversion, either under bromocriptine (1.25 mg) or placebo. Focusing the analyses on activation components derived from 18 cardiovascular variables, we found that bromocriptine reduces alpha-adrenergic activation in the sample as a whole, whereas the effects on beta-adrenergic and cholinergic activation are modulated by agentic extraversion.