Modulation of human motor cortex excitability by the cholinesterase inhibitor rivastigmine

Effect of cholinergic treatment depends on cholinergic integrity in early Alzheimer's disease

In early Alzheimer's disease, which initially presents with progressive loss of short-term memory, neurodegeneration especially affects cholinergic neurons of the basal forebrain. Pharmacotherapy of Alzheimer's disease therefore often targets the cholinergic system. In contrast, cholinergic pharmacotherapy of mild cognitive impairment is debated since its efficacy to date remains controversial. We here investigated the relationship between cholinergic treatment effects and the integrity of the cholinergic system in mild cognitive impairment due to Alzheimer's disease. Fourteen patients with high likelihood of mild cognitive impairment due to Alzheimer's disease and 16 age-matched cognitively normal adults performed an episodic memory task during functional magnetic resonance imaging under three conditions: (i) without pharmacotherapy; (ii) with placebo; and (iii) with a single dose of rivastigmine (3 mg). Cortical acetylcholinesterase activity was measured using PET with the tracer 11C-N-methyl-4-piperidyl acetate (MP4A). Cortical acetylcholinesterase activity was significantly decreased in patients relative to controls, especially in the lateral temporal lobes. Without pharmacotherapy, mild cognitive impairment was associated with less memory-related neural activation in the fusiform gyrus and impaired deactivation in the posterior cingulate cortex, relative to controls. These differences were attenuated under cholinergic stimulation with rivastigmine: patients showed increased neural activation in the right fusiform gyrus but enhanced deactivation of the posterior cingulate cortex under rivastigmine, compared to placebo. Conversely, controls showed reduced activation of the fusiform gyrus and reduced deactivation of the posterior cingulate under rivastigmine, compared to placebo. In both groups, the change in neural activation in response to rivastigmine was negatively associated with local acetylcholinesterase activity. At the behavioural level, an analysis of covariance revealed a significant group × treatment interaction in episodic memory performance when accounting for hippocampal grey matter atrophy and function. Our results indicate that rivastigmine differentially affects memory-related neural activity in patients with mild cognitive impairment and cognitively normal, age-matched adults, depending on acetylcholinesterase activity as a marker for the integrity of the cortical cholinergic system. Furthermore, hippocampal integrity showed an independent association with the response of memory performance to acetylcholinesterase inhibition.

Pharmacological Manipulation of Cortical Inhibition in the Dorsolateral Prefrontal Cortex

Cortical inhibition (CI) occurs largely through GABA receptor-mediated inhibitory neurotransmission, which can be modulated by cholinergic, dopaminergic, and glutamatergic inputs. Transcranial magnetic stimulation (TMS) can be used to index CI through a paradigm known as long-interval CI (LICI). When TMS is combined with electroencephalography (EEG), LICI can index GABA receptor-mediated inhibitory neurotransmission in the dorsolateral prefrontal cortex (DLPFC). We conducted a hypothesis-driven pharmacological study to assess the role of cholinergic, dopaminergic, GABAergic, and glutamatergic neurotransmission on LICI from the DLPFC using TMS-EEG. In this randomized controlled, double-blind crossover within-subject study, 12 healthy participants received five sessions of LICI to the DLPFC in a random order, each preceded by the administration of placebo or one of the four active drugs. LICI was assessed after each drug administration and compared to LICI after placebo. Relative to placebo, baclofen resulted in a significant increase in LICI, while rivastigmine resulted in a significant decrease in LICI. Dextromethorphan and L-DOPA did not result in a significant change in LICI relative to placebo. Our study confirms that LICI in the DLPFC is largely mediated by GABA_B receptor-mediated inhibitory neurotransmission and also suggests that cholinergic modulation decreases LICI in the DLPFC. Such findings may help guide future work examining the neurophysiological impact of these neurotransmitters in healthy and diseased states.

Neonatal pain control and neurologic effects of anesthetics and sedatives in preterm infants

Preclinical and clinical studies have demonstrated the adverse consequences of untreated pain and stress on brain development in the preterm infant. Sucrose has widely been implemented as standard therapy for minor procedural pain. Anesthetics are commonly utilized in preterm infants during major surgery. Pharmacologic agents (benzodiazepines and opioids) have been examined in clinical trials of preterm infants requiring invasive mechanical ventilation. Controversy exists regarding the safety and long-term impact of these interventions. Ongoing multidisciplinary research will help define the impact of these agents and identify potential alternative therapies.

Modulation of human motor cortex excitability by valproate

RATIONALE

Valproate is widely used in the treatment of epilepsy, bipolar disorder, and chronic pain disorders, but its exact mechanisms of action is still incompletely understood.

OBJECTIVES

Here we used transcranial magnetic stimulation to explore effects of a single dose of 800 mg valproate on motor cortex excitability in healthy volunteers.

METHODS

Motor threshold, peripheral maximum M-wave, cortical silent period short intracortical inhibition, intracortical facilitation, and motor evoked potential recruitment were assessed before and 1.5 h after the administration of valproate in 15 (eight male, seven female) healthy volunteers.

RESULTS

None of the measures of cortical excitability were found to be altered significantly after valproate.

CONCLUSION

These results are in line with previous findings of unaffected intracortical excitability after a single dose of valproate, suggesting that valproate's immediate in vivo actions do not resemble the effects of classic GABAergic compounds.

Considerations for using sucrose to reduce procedural pain in preterm infants

Preterm and critically ill newborns admitted to a NICU undergo repeated skin-breaking procedures that are necessary for their survival. Sucrose is rapidly becoming the accepted clinical standard nonpharmacologic intervention for managing acute procedural pain for these infants. Although shown to be safe in single doses, only 4 studies have evaluated the effects of repeated doses of sucrose over relatively short periods of time. None has examined the use of sucrose throughout the NICU stay, and only 1 study evaluated the neurodevelopmental outcomes after repeated doses of sucrose. In that study, infants born at <31 weeks' gestational age and exposed to >10 doses per day in the first week of life were more likely to show poorer attention and motor development in the early months after discharge from the NICU. Results of studies in animal models have suggested that the mechanism of action of sucrose is through opioid pathways; however, in human infants, little has been done to examine the physiologic mechanisms involved, and the findings reported thus far have been ambiguous. Drawing from the growing animal literature of research that has examined the effects of chronic sugar exposure, we describe alternative amine and hormone pathways that are common to the processing of sucrose, attention, and motor development. In addition, a review of the latest research to examine the effects of repeated sucrose on pain processing is presented. These 2 literatures each can inform the other and can provide an impetus to initiate research to examine not only the mechanisms involved in the calming mechanisms of sucrose but also in the long-term neurodevelopmental effects of repeated sucrose in those infants born extremely preterm or critically ill.

Modulation of human motor cortex excitability by quetiapine

RATIONALE

Quetiapine is increasingly used for the treatment of patients with psychosis and bipolar disorder. However, the neurobiological mechanisms, which may account for the favourable risk/benefit profile of this drug, are not entirely understood.

OBJECTIVES

Transcranial magnetic stimulation was used to investigate the effects of acute and repeated administration of quetiapine on cortical excitability in healthy volunteers.

MATERIALS AND METHODS

Within a double-blind, placebo-controlled, randomized cross-over design motor threshold, intracortical inhibition, intracortical facilitation and cortical silent period were studied in 15 healthy volunteers before and after a single dose of placebo and 100 mg quetiapine. Additional measurements were performed after 5 days of daily intake of 100 mg quetiapine.

RESULTS

We observed a significant prolongation of the cortical silent period after a single dose of quetiapine, whereas the placebo had no effects. After repeated administration, there was a trend towards CSP prolongation, which did not reach significance. However, plasma concentrations at this time point were relatively low, as measurements were performed 15 h after the last drug intake. Other parameters of cortical excitability remained unaffected.

CONCLUSIONS

By lengthening CSP without affecting MT, ICI and ICF, quetiapine demonstrates a unique neurophysiological profile which differs distinctively from brain excitability profiles of typical antipsychotics such as haloperidol. Provided that the CSP prolongation reflects the antipsychotic potential of quetiapine, TMS may be developed as a tool to monitor neurobiological effects of quetiapine treatment in schizophrenic patients and to explore the efficacy of other antipsychotic drugs with a similar mode of action.

On the inhibitory affect of some dementia drugs on DNA polymerase Beta activity

Some drugs are routinely prescribed for dementia that sets in either due to normal ageing or due to neurodegenerative disorders. We have studied the effect of three of these drugs, Donepezil hydrochloride, Rivastigmine tartrate and Nootropyl, on the activity of DNA polymerases beta, a crucial enzyme in the base excision repair pathway, the most important mode of DNA repair in brain. All the three drugs inhibited DNA polymerase beta activity to varying degrees although the affects of Donepezil being the least and inconsistent. The drugs preferentially bind to and inhibit the activities of 8 kDa domain of DNA polymerase beta that is known to possess the dRP lyase activity. The function of 31 kDa domain dealing with template driven addition of nucleotides at 3' end of the primer is not adversely affected. The inhibitory action of most widely used dementia drugs on DNA repair potential signifies that pharma sector needs to consider this aspect especially while designing drugs targeted towards brain.