Oliver Adolph

Atomoxetine acts as an NMDA receptor blocker in clinically relevant concentrations

Background and purpose:  There is increasing evidence that not only the monoaminergic but also the glutamatergic system is involved in the pathophysiology of attention‐deficit hyperactivity disorder (ADHD). Hyperactivity of glutamate metabolism might be causally related to a hypoactive state in the dopaminergic system. Atomoxetine, a selective noradrenaline reuptake inhibitor, is the first non‐stimulant approved for the treatment of this disorder. Here we have evaluated the effects of atomoxetine on glutamate receptors in vitro.

Rapid increase of glial glutamate uptake via blockade of the protein kinase A pathway

Glutamate is the main excitatory neurotransmitter in the vertebrate central nervous system. Removal of this transmitter from the synaptic cleft by glial and neuronal transporter systems plays an important role in terminating glutamatergicneurotransmission. The effects of different activators and blockers of PKA and PKC on glutamate uptake were studied in primary glial cells cultivated from the rat cortex using the patch‐clamp recording technique and immunocytochemical methods. GF 109203X enhances glutamate‐induced membrane currents in a concentration‐ and time‐dependent manner. After pre‐application for 40 s the maximal transport capacity was increased by 30–80%. The estimated Km‐value of the transport system did not change after drug application and the enhanced glutamate uptake was reversible within a few minutes upon washout. Activators and blockers of the PKC pathway did not affect glutamate uptake, whereas H89, a selective blocker of PKA, mimicked the effects of GF 109203X, indicating involvement of the protein kinase A pathway. The GF 109203X‐induced increase in transport capacity is likely to be mediated by GLAST since the GLT‐1 selective blocker dihydrokainate was unable to block basal or stimulated glutamate uptake. Furthermore, the increase in transport activity may well be based on an increase in cell surface expression of the transporter protein since preincubation with cytochalasin‐B, a protein that blocks actin polymerization, almost completely abolished the effect of GF 109203X and H89. These results indicate that GF 109203X and H89 enhance glial glutamate uptake via blockade of the PKA. The described effect may affect glutamatergic neurotransmission by reducing the glutamate concentration in the synaptic cleft.

Xenon-induced changes in CNS sensitization to pain.

Electrophysiological investigations of the spinal cord in animals have shown that pain sensitizes the central nervous system via glutamate receptor dependent long-term potentiation (LTP) related to an enhancement of pain perception. To expand these findings, we used functional magnetic resonance (fMRI), blood oxygen level dependent (BOLD) and perfusion imaging in combination with repeated electrical stimulation in humans. Specifically we monitored modulation of somatosensory processing during inhibition of excitatory transmission by ocular application of the glutamate receptor antagonist xenon. BOLD responses upon secondary stimulation increased in mid insular and in primary/secondary sensory cortices under placebo and decreased under xenon treatments. Xenon-induced decreases in regional perfusion were confined to stimulation responsive brain regions and correlated with time courses of xenon concentrations in the cranial blood. Moreover, effects of xenon on behavioral, fMRI and perfusion data scaled with stimulus intensity. The dependence of pain sensitization on sufficient pre-activation reflects a multistage process which is characteristic for glutamate receptor related processes of LTP. This study demonstrates how LTP related processes known from the cellular level can be investigated at the brain systems level.

Etomidate reduces glutamate uptake in rat cultured glial cells: involvement of PKA.

Glutamate is the main excitatory neurotransmitter in the vertebrate CNS. Removal of the transmitter from the synaptic cleft by glial and neuronal glutamate transporters (GLTs) has an important function in terminating glutamatergic neurotransmission and neurological disorders. Five distinct excitatory amino‐acid transporters have been characterized, among which the glial transporters excitatory amino‐acid transporter 1 (EAAT1) (glutamate aspartate transporter) and EAAT2 (GLT1) are most important for the removal of extracellular glutamate. The purpose of this study was to describe the effect of the commonly used anaesthetic etomidate on glutamate uptake in cultures of glial cells.

Promethazine inhibits NMDA-induced currents - new pharmacological aspects of an old drug.

BACKGROUND AND PURPOSE The phenothiazine derivative promethazine was first introduced into clinical practice as an antiallergic drug owing to its H1-receptor antagonizing properties. Nowadays, promethazine is primarily used as a sedative and/or as an antiemetic. The spectrum of clinically relevant effects is mediated by different molecular targets. Since glutamate is the predominant excitatory transmitter in the vertebrate brain and involved in alertness control, pain processing, and neurotoxicity we tested the hypothesis that promethazine interacts with excitatory ionotropic glutamate receptors. EXPERIMENTAL APPROACH Electrophysiological experiments were performed by means of the patch-clamp technique at glutamate receptors heterologously expressed in human TsA cells. KEY RESULTS Promethazine selectively inhibited NMDA receptors whereas AMPA- and kainate receptors were hardly affected. Inhibition of NMDA-induced membrane currents occurred in a reversible manner with a half-maximal effect at around 20 μM promethazine. The inhibition occurred in a non-competitive manner as it did neither vary with the glutamate nor the glycine concentration. Analysis of the underlying mechanism revealed only a weak dependency on receptor usage, pH value (pH 6.8-7.8), and membrane potential (zδ = 0.44 ± 0.04 according to the Woodhull-model). In line with the latter finding, promethazine did not interact with the Mg(2+) binding site. However, the displacement of promethazine by 9-aminoacridine indicates that promethazine may interact with the channel pore more externally in relation to the Mg(2+) binding site. CONCLUSION AND IMPLICATIONS Promethazine inhibits NMDA-mediated membrane currents in a reversible and concentration-dependent manner. The results presented here provide evidence that the NMDA receptor antagonism may contribute to clinically relevant effects of promethazine like sedation, analgesia or neuroprotection.

Evaluation of Homobivalent Carbolines as Designed Multiple Ligands for the Treatment of Neurodegenerative Disorders.

Neurodegenerative diseases represent a challenge for biomedical research due to their high prevalence and lack of mechanism-based treatments. Because of the complex pathology of neurodegenerative disorders, multifunctional drugs have been increasingly recognized as potential treatments. We identified homobivalent γ-carbolinium salts as potent inihitors of both cholinesterases, N-methyl-D-aspartate receptors, and monoamine oxidases. Homobivalent γ-carbolines displayed similar structure-activity relationships on all tested targets and may present promising designed multiple ligands for the treatment of neurodegenerative disorders.

Intranasal Application of Xenon Reduces Opioid Requirement and Postoperative Pain in Patients Undergoing Major Abdominal Surgery A Randomized Controlled Trial

Background:Both central sensitization after peripheral tissue injury and the development of opioid tolerance involve activation of N-methyl-d-aspartate (NMDA) receptors. At subanesthetic doses the NMDA receptor antagonist xenon suppresses pain-evoked sensitization of pain-processing areas in the central nervous system. Although numerous studies describe the effect of NMDA receptor antagonists on postoperative pain, clinical studies elucidating their intraoperative analgesic potency when applied in a low dosage are still largely missing. Methods:To analyze the analgesic effect of low-dose xenon using new application methods, the authors tested nasally applied xenon as an add-on treatment for analgesia in 40 patients undergoing abdominal hysterectomy. Within a randomized double-blind placebo-controlled study design, intraoperative and postoperative requirement of opioids as well as postoperative subjective experiences of pain were measured as primary outcome variables. Results:Intranasal application of xenon significantly reduced intraoperative opioid requirement (mean difference [MD] −2.0 &mgr;g/min; 95% CI [CI95]−0.53 to −3.51, Bonferroni correction adjusted P value [pcorr]= 0.028) without relevant side effects and significantly reduced postoperative pain (MD −1.34 points on an 11-point rating scale; CI95 −0.60 to −2.09, pcorr = 0.002). However, postoperative morphine consumption (MD −8.8 &mgr;g/min; CI95 1.2 to −18.8, pcorr = 0.24) was not significantly reduced in this study. Conclusions:Low-dose xenon significantly reduces intraoperative analgesic use and postoperative pain perception. Because NMDA receptor antagonists suppress central sensitization, prevent the development of opioid tolerance, and reduce postoperative pain, the intraoperative usage of NMDA receptor antagonists such as xenon is suggested to improve effectiveness of pain management within a concept of multimodal analgesia.

Evidence That Xenon Does Not Produce Open Channel Blockade of the NMDA Receptor

Previous studies had not excluded the possibility that the mechanism by which Xenon (Xe) blocks N-methyl-D-aspartate (NMDA) receptors might be that of an open-channel blocker. We tested this possibility on mutant NMDA receptors carrying an alanine (A) to cysteine (C) mutation located within the SYTANLAAF-motif of the third transmembrane region (TM3). This mutation was shown to yield constitutively open ion channels after modification with a thiol-modifying reagent. We expressed such mutant channels in Neuro2A cells and recorded glutamate (50 microM)-induced currents in the whole cell recording mode. Although Xe (3.5 mM) blocked the currents through the wild-type receptor NR1-1a/NR2A and NR1-1a/NR2B by approximately 40% and those through the mutant receptors NR1-1a/NR2A(A650C) or NR1-1a/NR2B(A651C) by approximately 30%, it was unable to block the currents through the methane thiosulfonate etyhlammonium-modified mutant receptors. On the other hand, established open-channel blockers of the NMDA receptor such as MK-801 (1 microM) or Mg ions (Mg(2+); 1 mM) were able to block these permanently open channels. These results suggest that Xe does not act as a classical open-channel blocker at the NMDA receptor.

Glucose modulates food-related salience coding of midbrain neurons in humans.

Although early rat studies demonstrated that administration of glucose diminishes dopaminergic midbrain activity, evidence in humans has been lacking so far. In the present functional magnetic resonance imaging study, glucose was intravenously infused in healthy human male participants while seeing images depicting low‐caloric food (LC), high‐caloric food (HC), and non‐food (NF) during a food/NF discrimination task. Analysis of brain activation focused on the ventral tegmental area (VTA) as the origin of the mesolimbic system involved in salience coding. Under unmodulated fasting baseline conditions, VTA activation was greater during HC compared with LC food cues. Subsequent to infusion of glucose, this difference in VTA activation as a function of caloric load leveled off and even reversed. In a control group not receiving glucose, VTA activation during HC relative to LC cues remained stable throughout the course of the experiment. Similar treatment‐specific patterns of brain activation were observed for the hypothalamus. The present findings show for the first time in humans that glucose infusion modulates salience coding mediated by the VTA. Hum Brain Mapp 37:4376–4384, 2016.

Surgical treatment of melanoma in pregnancy: a practical guideline.

A tumor primarily requiring surgical treatment, newly diagnosed or preexisting melanoma during pregnancy is a clinical rarity. In such cases, the surgeon faces the challenge of having to decide on the appropriate therapeutic course of action. Based on our clinical experience and a review of the literature, we herein provide a guideline on how to practically deal with this rare clinical conundrum. In our experience, pregnant melanoma patients require thorough counseling with respect to their therapeutic options. They naturally tend to put their unborn child first, and are hesitant to consent to necessary surgery despite a potentially life‐threatening diagnosis. It is therefore crucial to clearly inform these patients that – based on existing medical experience – pregnancy by itself is no reason to hold off on any type of necessary melanoma surgery. However, various parameters such as preoperative imaging procedures, positioning on the operating table, monitoring, anesthesia, and perioperative medication require certain adjustments in order to comply with this special situation.