Bjarke Ebert

Ketobemidone, methadone and pethidine are non-competitive N-methyl-d-aspartate (NMDA) antagonists in the rat cortex and spinal cord

The opiate agonists, ketobemidone, methadone and pethidine, were evaluated as N-methyl-D-aspartate (NMDA) receptor antagonists using the rat cortical wedge preparation and the neonatal rat spinal cord preparation for electrophysiological studies and [3H](RS)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-im ine ([3H]MK-801) binding experiments using rat forebrain homogenates. Ketobemidone, methadone and pethidine were inhibitors of [3H]MK-801 binding with Ki values of 26 microM, 0.85 microM and 47 microM, respectively. In the cortex, 1 mM ketobemidone and 1 mM methadone reduced NMDA responses, but not (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionic acid (AMPA) or kainate responses in an use-dependent manner, whereas 1 mM pethidine was devoid of antagonist activity. In the spinal cord preparation, the activities of ketobemidone and methadone were weaker than in cortex. In contrast, pethidine was equipotent with ketobemidone in the spinal cord. These results suggest that ketobemidone and methadone may be useful therapeutic agents in conditions where a combined opiate agonist and NMDA antagonist treatment is desired.

Escitalopram versus citalopram: the surprising role of the R-enantiomer

RationaleCitalopram is a racemate consisting of a 1:1 mixture of the R(−)- and S(+)-enantiomers. Non-clinical studies show that the serotonin reuptake inhibitory activity of citalopram is attributable to the S-enantiomer, escitalopram. A series of recent non-clinical and clinical studies comparing escitalopram and citalopram to placebo found that equivalent doses of these two drugs, i.e. containing the same amount of the S-enantiomer, showed better effect for escitalopram. These results suggested that the R-citalopram in citalopram inhibits the effect of the S-enantiomer.ObjectiveTo review the pharmacological and non-clinical literature that describes the inhibition of escitalopram by R-citalopram, as well as the implications of this inhibition for the clinical efficacy of escitalopram compared to citalopram.MethodsThe information in this review was gathered from published articles and abstracts.ResultsIn appropriate neurochemical, functional, and behavioural non-clinical experiments, escitalopram shows greater efficacy and faster onset of action than comparable doses of citalopram. The lower efficacy of citalopram in these studies is apparently due to the inhibition of the effect of the S-enantiomer by the R-enantiomer, possibly via an allosteric interaction with the serotonin transporter. Data from randomised clinical trials consistently show better efficacy with escitalopram than with citalopram, including higher rates of response and remission, and faster time to symptom relief.ConclusionThe R-enantiomer present in citalopram counteracts the activity of the S-enantiomer, thereby providing a possible basis for the pharmacological and clinical differences observed between citalopram and escitalopram.

Pharmacological characterization of agonists at δ-containing GABAA receptors : Functional selectivity for extrasynaptic receptors is dependent on the absence of γ2

Several groups have characterized the pharmacology of α4- or α6β3δ-containing GABAA receptors expressed in different cell systems. We have previously demonstrated that the pharmacological profiles of a series of GABAA receptor agonists are highly dependent on the α subunit and little on the β and γ subunits, so to further understand the contribution of the different subunits in the GABAA receptor complex, we characterized a series of full agonists, partial agonists, and antagonists at α4β3, α4β3δ, and α6β3δ receptors expressed in Xenopus oocytes. Little or no difference was seen when the compounds were compared at αβ- and αβδ-containing receptors, whereas a significant reduction in both potency and relative efficacy was observed compared with αβγ-containing receptors described in the literature. These data clearly confirm that the presence of the δ subunit in heterotrimeric receptors is a strong determinant of the increased pharmacological activity of compounds with agonist activity. The very similar agonist pharmacology of αβ- and αβδ-containing receptors, which is significantly different from that of αβγ-containing receptors, shows that whereas the presence of a γ subunit impairs the response to an agonist stimulation of the αβ receptor complex, the δ subunit does not affect this in any way. Taken together, these data are well in line with the idea that α4β3δ may contribute to the pharmacological action of exogenously applied agonists and may explain why systemically active compounds such as gaboxadol and muscimol in vivo appear to act as selective extrasynaptic GABAA agonists.

The δ Subunit of γ-Aminobutyric Acid Type A Receptors Does Not Confer Sensitivity to Low Concentrations of Ethanol

GABAA receptors (GABAARs) are usually formed by α, β, and γ or δ subunits. Recently, δ-containing GABAARs expressed in Xenopus oocytes were found to be sensitive to low concentrations of ethanol (1–3 mM). Our objective was to replicate and extend the study of the effect of ethanol on the function of α4β3δ GABAARs. We independently conducted three studies in two systems: rat and human GABAARs expressed in Xenopus oocytes, studied through two-electrode voltage clamp; and human GABAARs stably expressed in the fibroblast L(tk–) cell line, studied through patch-clamp electrophysiology. In all cases, α4β3δ GABAARs were only sensitive to high concentrations of ethanol (100 mM in oocytes, 300 mM in the cell line). Expression of the δ subunit in oocytes was assessed through the magnitude of the maximal GABA currents and sensitivity to zinc. Of the three rat combinations studied, α4β3 was the most sensitive to ethanol, isoflurane, and 5α-pregnan-3α,21-diol-20-one (THDOC); α4β3δ and α4β3γ2S were very similar in most aspects, but α4β3δ was more sensitive to GABA, THDOC, and lanthanum than α4β3γ2S GABAARs. Ethanol at 30 mM did not affect tonic GABA-mediated currents in dentate gyrus reported to be mediated by GABAARs incorporating α4 and δ subunits. We have not been able to replicate the sensitivity of α4β3δ GABAARs to low concentrations of ethanol in four different laboratories in independent studies. This suggests that as yet unidentified factors may play a critical role in the ethanol effects on δ-containing GABAARs.

Norketamine, the main metabolite of ketamine, is a non-competitive NMDA receptor antagonist in the rat cortex and spinal cord

The enantiomers of the potent non-competitive NMDA receptor antagonist ketamine and its major metabolite, norketamine were evaluated as NMDA receptor antagonists using the rat cortical wedge preparation and the neonatal rat spinal cord preparation, respectively, for electrophysiological studies and [3H](RS)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-im ine ([3H]MK801) in homogenate binding experiments. In agreement with earlier studies (S)-ketamine (Ki 0.3 microM) was found to possess a 5 times higher affinity for the NMDA receptor complex than (R)-ketamine (Ki 1.4 microM). (S)-Norketamine (Ki 1.7 microM) had approximately an 8 times higher affinity than (R)-norketamine (Ki 13 microM) in the inhibition of [3H]MK-801 binding. All compounds inhibited responses to NMDA in the rat cortical wedge preparation and the hemisected neonatal rat spinal cord, being approximately four times more potent in the cortex than in the spinal cord except for (R)-norketamine being only twice as potent. In light of the clinically obtained concentrations of norketamine after oral administration of ketamine, these data strongly suggest that (S)-norketamine may contribute significantly to the clinical activity of (S)-ketamine, especially when given orally.

Opioid analgesics as noncompetitive N-methyl-D-aspartate (NMDA) antagonists.

Much evidence points to the involvement of N-methyl-D-aspartate (NMDA) receptors in the development and maintainance of neuropathic pain. In neuropathic pain, there is generally involved a presumed opioid-insensitive component, which apparently can be blocked by NMDA receptor antagonists. However, in order to obtain complete analgesia, a combination of an NMDA receptor antagonist and an opioid receptor agonist is needed. Recent in vitro data have demonstrated that methadone, ketobemidone, and dextropropoxyphene, in addition to being opioid receptor agonists, also are weak noncompetitive NMDA receptor antagonists. Clinical anecdotes suggest that the NMDA receptor antagonism of these opioids may play a significant role in the pharmacological action of these compounds; however, no clinical studies have been conducted to support this issue. In the present commentary, we discuss evidence for the NMDA receptor antagonism of these compounds and its relevance for clinical pain treatment; an overview of structure-activity relationships for the relevant opioids as noncompetitive NMDA receptor antagonists also is given. It is concluded that although the finding that some opioids are weak noncompetitive NMDA receptor antagonists in vitro has created much attention among clinicians, no clinical studies have been conducted to evaluate the applicability of these compounds in the treatment of neuropathic pain conditions.

Distinct activities of GABA agonists at synaptic- and extrasynaptic-type GABAA receptors

The activation characteristics of synaptic and extrasynaptic GABAA receptors are important for shaping the profile of phasic and tonic inhibition in the central nervous system, which will critically impact on the activity of neuronal networks. Here, we study in isolation the activity of three agonists, GABA, muscimol and 4,5,6,7‐tetrahydoisoxazolo[5,4‐c]pyridin‐3(2H)‐one (THIP), to further understand the activation profiles of α1β3γ2, α4β3γ2 and α4β3δ receptors that typify synaptic‐ and extrasynaptic‐type receptors expressed in the hippocampus and thalamus. The agonists display an order of potency that is invariant between the three receptors, which is reliant mostly on the agonist dissociation constant. At δ subunit‐containing extrasynaptic‐type GABAA receptors, both THIP and muscimol additionally exhibited, to different degrees, superagonist behaviour. By comparing whole‐cell and single channel currents induced by the agonists, we provide a molecular explanation for their different activation profiles. For THIP at high concentrations, the unusual superagonist behaviour on α4β3δ receptors is a consequence of its ability to increase the duration of longer channel openings and their frequency, resulting in longer burst durations. By contrast, for muscimol, moderate superagonist behaviour was caused by reduced desensitisation of the extrasynaptic‐type receptors. The ability to specifically increase the efficacy of receptor activation, by selected exogenous agonists over that obtained with the natural transmitter, may prove to be of therapeutic benefit under circumstances when synaptic inhibition is compromised or dysfunctional.

Lu AA21004, a novel multimodal antidepressant, produces regionally selective increases of multiple neurotransmitters—A rat microdialysis and electrophysiology study

The monoaminergic network, including serotonin (5-HT), norepinephrine (NE), and dopamine (DA) pathways, is highly interconnected and has a well-established role in mood disorders. Preclinical research suggests that 5-HT receptor subtypes, including 5-HT1A, 5-HT1B, 5-HT3, and 5-HT7 receptors as well as the 5-HT transporter (SERT), may have important roles in treating depression. This study evaluated the neuropharmacological profile of Lu AA21004, a novel multimodal antidepressant combining 5-HT3 and 5-HT7 receptor antagonism, 5-HT1B receptor partial agonism, 5-HT1A receptor agonism, and SERT inhibition in recombinant cell lines. Extracellular 5-HT, NE and DA levels were evaluated in the ventral hippocampus (vHC), medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) after acute and subchronic treatment with Lu AA21004 or escitalopram. The acute effects of LuAA21004 on NE and DA neuronal firing were also evaluated in the locus coeruleus (LC) and ventral tegmental area (VTA), respectively. Acute Lu AA21004 dose-dependently increased 5-HT in the vHC, mPFC and NAc. Maximal 5-HT levels in the vHC were higher than those in the mPFC. Furthermore, mPFC 5-HT levels were increased at low SERT occupancy levels. In the vHC and mPFC, but not the NAc, high Lu AA21004 doses increased NE and DA levels. Lu AA21004 slightly decreased LC NE neuronal firing and had no effect on VTA DA firing. Results are discussed in context of occupancy at 5-HT3, 5-HT1B and 5-HT1A receptors and SERT. In conclusion, Lu AA21004, acting via two pharmacological modalities, 5-HT receptor modulation and SERT inhibition, results in a brain region-dependent increase of multiple neurotransmitter concentrations.

Emerging anti-insomnia drugs: tackling sleeplessness and the quality of wake time

Sleep is essential for our physical and mental well being. However, when novel hypnotic drugs are developed, the focus tends to be on the marginal and statistically significant increase in minutes slept during the night instead of the effects on the quality of wakefulness. Recent research on the mechanisms underlying sleep and the control of the sleep–wake cycle has the potential to aid the development of novel hypnotic drugs; however, this potential has not yet been realized. Here, we review the current understanding of how hypnotic drugs act, and discuss how new, more effective drugs and treatment strategies for insomnia might be achieved by taking into consideration the daytime consequences of disrupted sleep.

Antidepressant and anxiolytic potential of the multimodal antidepressant vortioxetine (Lu AA21004) assessed by behavioural and neurogenesis outcomes in mice

Vortioxetine (Lu AA21004) is an investigational novel antidepressant with multimodal activity that functions as a 5-HT3, 5-HT7 and 5-HT(1D) receptor antagonist, 5-HT(1B) receptor partial agonist, 5-HT(1A) receptor agonist and inhibitor of the 5-HT transporter in vitro. Here we explore its anxiolytic and antidepressant potential in adult mice. Vortioxetine was assessed in BalB/cJ@RJ mice using the open-field and forced-swim tests (acute: p.o. 1 h, repeated: daily p.o. 21 days), and in 129S6/SvEvTac mice using the novelty suppressed feeding paradigm (acute: p.o. 1 h, sustained: daily p.o. 14 or 21 days). Fluoxetine and diazepam were controls. Acute and repeated dosing of vortioxetine produced more pronounced anxiolytic- and antidepressant-like activities than fluoxetine. Vortioxetine significantly increased cell proliferation and cell survival and stimulated maturation of immature granule cells in the subgranular zone of the dentate gyrus of the hippocampus after 21 days of treatment. After 14 days, a high dose of vortioxetine increased dendritic length and the number of dendrite intersections, suggesting that vortioxetine accelerates the maturation of immature neurons. Vortioxetine displays an antidepressant and anxiolytic profile following repeated administration associated with increased neurogenesis at several stages. Vortioxetine effects were observed at low levels of 5-HT transporter occupancy, suggesting an alternative mechanism of action to 5-HT reuptake inhibition.