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.

Approach to reduce the non-specific binding in microdialysis.

Measurement of unbound test compound concentrations at the biophase is routinely carried out in the drug discovery. Microdialysis is an established sampling technique for in vivo measurement of endogenous and exogenous compounds and it is commonly used for monitoring true concentrations. Endogenous compounds like neurotransmitters and neuropeptides in the brain are routinely evaluated as a proof of pharmacological activity of test compounds. Although, microdialysis offers several advantages over the conventional techniques for its use in brain pharmacokinetics, the absolute determination of extracellular concentrations of test compound depends on the predictable non-specific binding to the tubing and probe membrane. In the present investigation, we have demonstrated steps to predict non-specific binding and described approaches to reduce while working with compounds having different degree of adsorption properties. Non-specific binding to the tubing was measured in vitro for seven structurally diverse compounds and based on the binding characteristics, changes were adapted in study conditions. In vitro probe extraction efficiency was evaluated by gain and loss, which was further used as a second layer of measurement for non-specific binding. For selected compounds, in vivo probe extraction efficiencies were carried out and brain pharmacokinetics was evaluated in the prefrontal cortex of male Sprague-Dawley rats. Thus, the present approach demonstrates a systematic approach for evaluating and reducing the non-specific binding of test compounds to the microdialysis tubing and probe membranes. The stepwise approach described will strengthen the applicability of microdialysis in brain pharmacokinetics.

Difference in the norepinephrine levels of experimental and non-experimental rats with age in the object recognition task.

In the present study, we investigated the performance of adult and juvenile rats in the Object Recognition Task (ORT). While it is well known that the performance of rat in ORT differs with age, the reason for the difference as well as the underlying neurotransmitter that may have led to these differences were investigated. In the present study, juvenile rats of postnatal day 40-45 (PND 40-45) and adult rats of postnatal day 60+ (PND 60+) were subjected to a two trial ORT. The juvenile rats did not discriminate between the novel object and the familiar object, while the adult rats discriminated the novel from the familiar object. On estimating brain concentrations of norepinephrine (NE), it was observed that the NE level in MTL (medial temporal lobe) of adult experimental rats was significantly higher than the adult non-experimental rats. In juvenile rats, no significant difference was observed in the NE levels of experimental rats in comparison to its non-experimental counterparts. Administration of yohimbine (α(2A) adrenergic receptor antagonist) enhanced the level of NE in juvenile rats and reversed the difference seen with age. From the present study, we conclude that the deficit in memory seen is likely due to the difference in NE levels with task and this can be reversed by yohimbine which enhance NE levels.

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.