Mechanistic evaluation of tapentadol in reducing the pain perception using in-vivo brain and spinal cord microdialysis in rats.

Role of monoamine neurotransmitters in the modulation of emotional and pain processing in spinal cord and brain regions is not well known. Tapentadol, a norepinephrine reuptake inhibitor with µ-opioid receptor agonistic activity has recently been introduced for the treatment of moderate to severe pain. The objective of the present study was to examine the effects of tapentadol on modulation of monoamines in the prefrontal cortex and dorsal horn using brain microdialysis. Tapentadol was administered intraperitoneally at 4.64-21.5mg/kg to male Wistar rats. Based on these results, 10mg/kg i.p. was chosen for spinal microdialysis in freely moving rats. Tapentadol produced significant and dose-dependent increase in cortical dopamine and norepinephrine levels with mean maximum increase of 600% and 300%, respectively. Treatment had no effect on cortical serotonin levels. In the dorsal horn, serotonin, dopamine and norepinephrine levels were significantly increased with mean maximum increases of 220%, 190% and 280%, respectively. Although the density of dopamine transporter is low in cortex, the increase of dopamine and norepinephrine levels in cortex could be mediated through the inhibition of norepinephrine transporter. In the dorsal horn, increase in norepinephrine levels could be due to inhibition of norepinephrine transporter in the spinal cord. Whereas, activation of opioids receptors in non-spinal regions might be responsible for increase in dopamine and serotonin levels. The results from current investigation suggest that clinical efficacy of tapentadol in neuropathic pain is mediated through the enhanced monoaminergic neurotransmission in the spinal cord and regions involved with emotional processing in brain.

Concurrent administration of atypical antipsychotics and donepezil: drug interaction study in rats.

Psychotic and behavioral symptoms are common in patients with dementia. Thus, it is rational to assume that patients with dementia would gain benefit from combination therapy of an antipsychotic agent and a cognitive enhancer. Antipsychotics are not approved by the US FDA in elderly patients with dementia but their use is still prevalent in other population. In the current study, we investigate the effect of atypical antipsychotics on acetylcholine modulation by donepezil. In addition, the plasma pharmacokinetics on concurrent administration of these drugs was studied. Acetylcholine modulation was carried out in the ventral hippocampus of Sprague-Dawley rats using brain microdialysis technique. In a parallel group of animals, pharmacokinetic parameters were evaluated on administration of donepezil (5.0 mg kg(-1), ip) alone and in combination with olanzapine, clozapine, or quetiapine. Donepezil produced 348% increase in hippocampal acetylcholine levels. Coadministration of olanzapine and donepezil produced 393% increase in extracellular acetylcholine, and the effect was supported by a significantly (p < 0.05) decreased clearance of donepezil in plasma. Whereas, other plasma pharmacokinetic parameters of donepezil "AUC(0-24h), T (1/2) and T (max)" were moderately altered after this combination treatment. Concurrent administrations of clozapine or quetiapine with donepezil produced a non-significant change in acetylcholine levels in comparison to donepezil alone. The plasma pharmacokinetics of donepezil was unaltered. Results from this preclinical investigation indicate that extrapyramidal side effects may precipitate upon coadministration of donepezil with olanzapine. Care must be exercised by physicians and caregivers while administering these two drugs together.

Quantification of acetylcholine, an essential neurotransmitter, in brain microdialysis samples by liquid chromatography mass spectrometry.

Chemical neurotransmission has been the subject of intensive investigations in recent years. Acetylcholine is an essential neurotransmitter in the central nervous system as it has an effect on alertness, memory and learning. Enzymatic hydrolysis of acetylcholine in the synaptic cleft is fast and quickly metabolizes to choline and acetate by acetylcholinesterase. Hence the concentration in the extracellular fluid of the brain is low (0.1-6 nm). Techniques such as microdialysis are routinely employed to measure acetylcholine levels in living brain systems and the microdialysis sample volumes are usually less than 50 microL. In order to develop medicine for the diseases associated with cognitive dysfunction like mild cognitive impairment, Alzheimer's disease, schizophrenia and Parkinson's disease, or to study the mechanism of the illness, it is important to measure the concentration of acetylcholine in the extracellular fluid of the brain. Recently considerable attention has been focused on the development of chromatographic-mass spectrometric techniques to provide more sensitive and accurate quantification of acetylcholine collected from in-vivo brain microdialysis experiments. This review will provide a brief overview of acetylcholine biosynthesis, microdialysis technique and liquid chromatography mass spectrometry, which is being used to quantitate extracellular levels of acetylcholine.