Inhibition of N-methyl-D-aspartate- and kainate-evoked noradrenaline release in human cerebral cortex slices by ethanol

Serotonin and beyond-a tribute to Manfred Göthert (1939-2019)

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

Serotonin discovery and stepwise disclosure of 5-HT receptor complexity over four decades. Part II. Some contributions of Manfred Göthert

About 40% of the papers within the scientific oeuvre of Manfred Göthert (1939-2019) were dedicated to serotonin (5-hydroxytryptamine, 5-HT). He was not only the witness of the gradual definition of the fourteen 5-HT receptor subtypes but also was involved directly by identifying 5-HT_1B, 5-HT_1D and 5-HT₃ receptors. Moreover, he identified presynaptic 5-HT receptors on central and/or peripheral serotoninergic, noradrenergic and/or cholinergic neurones. Two inhibitory (5-HT_1B, 5-HT_1D) and two facilitatory (5-HT₃, 5-HT₄) receptors were found, the 5-HT_1B receptor representing a possible target for antidepressant drugs. Ten years earlier than electrophysiologists, he identified ligand-gated receptors like the 5-HT₃ and the nicotinic acetylcholine (nACh) receptor as targets of halothane. Simultaneously with, but independent of, other authors he found that ethanol allosterically inhibits N-methyl-D-aspartate (NMDA) receptors, which are affected at an even lower concentration than 5-HT₃ and nACh receptors. The latter two receptors were shown to be subject to allosteric inhibition also by cannabinoids via a mechanism unrelated to cannabinoid CB₁ or CB₂ receptors; cannabinoid inhibition of 5-HT₃ receptors may represent a new target for the treatment of neuropathic pain.

Human presynaptic receptors

Presynaptic receptors are sites at which transmitters, locally formed mediators or hormones inhibit or facilitate the release of a given transmitter from its axon terminals. The interest in the identification of presynaptic receptors has faded in recent years and it may therefore be justified to give an overview of their occurrence in the autonomic and central nervous system; this review will focus on presynaptic receptors in human tissues. Autoreceptors are presynaptic receptors at which a given transmitter restrains its further release, though in some instances may also increase its release. Inhibitory autoreceptors represent a typical example of a negative feedback; they are tonically activated by the respective endogenous transmitter and/or are constitutively active. Autoreceptors also play a role under pathophysiological conditions, e.g. by limiting the massive noradrenaline release occurring during congestive heart failure. They can be used for therapeutic purposes; e.g., the α₂-adrenoceptor antagonist mirtazapine is used as an antidepressant and the inverse histamine H₃ receptor agonist pitolisant has been marketed as a new drug for the treatment of narcolepsy in 2016. Heteroreceptors are presynaptic receptors at which transmitters from adjacent neurons, locally formed mediators (e.g. endocannabinoids) or hormones (e.g. adrenaline) can inhibit or facilitate transmitter release; they may be subject to an endogenous tone. The constipating effect of the sympathetic nervous system or of the antihypertensive drug clonidine is related to the activation of inhibitory α₂-adrenoceptors on postganglionic parasympathetic neurons. Part of the stimulating effect of adrenaline on the sympathetic nervous system during stress is related to its facilitatory effect on noradrenaline release via β₂-adrenoceptors.

Functional Pharmacology in Human Brain

Most neurological and psychiatric disorders involve selective or preferential impairments of neurotransmitter systems. Therefore, studies of functional transmitter pathophysiology in human brain are of unique importance in view of the development of effective, mechanism-based, therapeutic modalities. It is well known that central nervous system functional proteins, including receptors, transporters, ion channels, and enzymes, can exhibit high heterogeneity in terms of structure, function, and pharmacological profile. If the existence of types and subtypes of functional proteins amplifies the possibility of developing selective drugs, such heterogeneity certainly increases the likelihood of interspecies differences. It is therefore essential, before choosing animal models to be used in preclinical pharmacology experimentation, to establish whether functionally corresponding proteins in men and animals also display identical pharmacological profiles. Because of evidence that scaffolding proteins, trafficking between plasma membrane and intracellular pools, phosphorylation and allosteric modulators can affect the function of receptors and transporters, experiments with human clones expressed in host cells where the environment of native receptors is rarely reproduced should be interpreted with caution. Thus, the use of neurosurgically removed fresh human brain tissue samples in which receptors, transporters, ion channels, and enzymes essentially retain their natural environment represents a unique experimental approach to enlarge our understanding of human brain processes and to help in the choice of appropriate animal models. Using this experimental approach, many human brain functional proteins, in particular transmitter receptors, have been characterized in terms of localization, function, and pharmacological properties.

Ethanol Modulation of Excitatory and Inhibitory Synaptic Transmission in Rat and Monkey Dentate Granule Neurons

BACKGROUND

The physiological mechanisms underlying the behavioral and cognitive effects of ethanol are not fully understood. However, there is now compelling evidence that ethanol acts, at least in part, by modulating the function of a small group of proteins that mediate excitatory and inhibitory synaptic transmission. For example, intoxicating concentrations of ethanol have been shown to enhance GABAergic synaptic inhibition and depress glutamatergic excitatory neurotransmission in a number of brain regions. Because all of these electrophysiological studies have been performed in rodent brain slice or neuronal culture preparations, direct evidence that ethanol exerts similar effects on synaptic transmission in the primate central nervous system is lacking.

METHODS

We have therefore developed methods to perform patch-clamp electrophysiological recordings from neurons in acutely prepared monkey (Macaca fascicularis) hippocampal slices. We have used these methods to compare the acute effects of ethanol on excitatory and inhibitory synaptic transmission in rat and monkey dentate granule neurons.

RESULTS

Under our recording conditions, ethanol significantly potentiated gamma-aminobutyric acid type A inhibitory postsynaptic currents in both rat and monkey neurons. In addition, ethanol significantly inhibited NMDA, but not AMPA, excitatory postsynaptic currents in dentate granule neurons from both species. Notably, no significant differences were observed in any of the pharmacological properties of inhibitory or excitatory synaptic responses recorded from rat and monkey neurons.

CONCLUSIONS

These data suggest that the differences in the behavioral effects of ethanol that have been observed between rats and higher-order mammals, such as monkeys and humans, may not reflect differences in the sensitivity of some of the major synaptic sites of ethanol action. Moreover, our results provide empirical evidence for the use of rodent brain slice preparations in elucidating synaptic mechanisms of ethanol action in the primate central nervous system.

Ethanol sensitivity of recombinant human N-methyl-D-aspartate receptors

In this study, the ethanol sensitivity of human N-methyl-D-aspartate (NMDA) receptors stably expressed in L(tk-) cells, or transiently expressed in HEK 293 cells and Xenopus oocytes was determined. NMDA receptor function was measured using fura-2 calcium imaging for L(tk-) cells, whole cell voltage-clamp for HEK 293 cells, and two-electrode voltage clamp for oocytes. Ethanol inhibited NMDA receptor function in all three expression system, but was less potent for receptors expressed in L(tk-) cells. NMDA receptors composed of NR1a/2B subunits were inhibited to a greater extent by ethanol than NR1a/2A receptors when expressed in L(tk-) cells and HEK 293 cells, but not in oocytes. These results suggest that the method of receptor expression and assay system used may influence the degree of ethanol inhibition of recombinant NMDA receptors.

Effects of moderate alcohol consumption on the central nervous system

The concept of moderate consumption of ethanol (beverage alcohol) has evolved over time from considering this level of intake to be nonintoxicating and noninjurious, to encompassing levels defined as "statistically" normal in particular populations, and the public health-driven concepts that define moderate drinking as the level corresponding to the lowest overall rate of morbidity or mortality in a population. The various approaches to defining moderate consumption of ethanol provide for a range of intakes that can result in blood ethanol concentrations ranging from 5 to 6 mg/dl, to levels of over 90 mg/dl (i.e., approximately 20 mM). This review summarizes available information regarding the effects of moderate consumption of ethanol on the adult and the developing nervous systems. The metabolism of ethanol in the human is reviewed to allow for proper appreciation of the important variables that interact to influence the level of exposure of the brain to ethanol once ethanol is orally consumed. At the neurochemical level, the moderate consumption of ethanol selectively affects the function of GABA, glutamatergic, serotonergic, dopaminergic, cholinergic, and opioid neuronal systems. Ethanol can affect these systems directly, and/or the interactions between and among these systems become important in the expression of ethanol's actions. The behavioral consequences of ethanol's actions on brain neurochemistry, and the neurochemical effects themselves, are very much dose- and time-related, and the collage of ethanol's actions can change significantly even on the rising and falling phases of the blood ethanol curve. The behavioral effects of moderate ethanol intake can encompass events that the human or other animal can perceive as reinforcing through either positive (e.g., pleasurable, activating) or negative (e.g., anxiolysis, stress reduction) reinforcement mechanisms. Genetic factors and gender play an important role in the metabolism and behavioral actions of ethanol, and doses of ethanol producing pleasurable feelings, activation, and reduction of anxiety in some humans/animals can have aversive, sedative, or no effect in others. Research on the cognitive effects of acute and chronic moderate intake of ethanol is reviewed, and although a number of studies have noted a measurable diminution in neuropsychologic parameters in habitual consumers of moderate amounts of ethanol, others have not found such changes. Recent studies have also noted some positive effects of moderate ethanol consumption on cognitive performance in the aging human. The moderate consumption of ethanol by pregnant women can have significant consequences on the developing nervous system of the fetus. Consumption of ethanol during pregnancy at levels considered to be in the moderate range can generate fetal alcohol effects (behavioral, cognitive anomalies) in the offspring. A number of factors--including gestational period, the periodicity of the mother's drinking, genetic factors, etc.--play important roles in determining the effect of ethanol on the developing central nervous system. A series of recommendations for future research endeavors, at all levels, is included with this review as part of the assessment of the effects of moderate ethanol consumption on the central nervous system.

Tolerance to inhibition by ethanol of N-methyl-D-aspartate-induced depolarization in rat locus coeruleus neurons in vitro

Intracellular recordings were made in a pontine slice preparation of the rat brain containing the nucleus locus coeruleus. The pressure application of N-methyl-D-aspartate (NMDA) produced reproducible depolarizations of stable amplitude. Superfusion with ethanol (100 mM) for 15 min inhibited the depolarizing response to NMDA: the effect of ethanol was rapidly reversed on washout. When the superfusion time of ethanol (100 mM) was increased to 60 min, its inhibitory effect disappeared after 50 to 60 min. Moreover, after the subsequent washout of ethanol a withdrawal-like increase in the sensitivity to NMDA became evident. Hence, adaptive mechanisms of locus coeruleus neurons during the long-time contact with ethanol may be modelled in an in vitro system.

Inhibitory effect of ethanol on the 5-hydroxytryptamine-induced Bezold-Jarisch reflex--involvement of peripheral 5-HT3 receptors

The influence of ethanol (0.5, 1.0 and 2.0 g/kg i.p.) on the Bezold-Jarisch reflex in urethane-anaesthetized rats was studied. 5-Hydroxytryptamine (serotonin; 5-HT; 1, 3, 10 and 30 micrograms/kg i.v.) and capsaicin (1, 3 and 10 micrograms/kg i.v.) reflexly decreased heart rate in a dose-dependent manner. The 5-HT3 receptor antagonist ondansetron 10 micrograms/kg i.v. abolished the 5-HT- but not the capsaicin-stimulated bradycardia, indicating that 5-HT and capsaicin acted via different trigger mechanisms (5-HT3 receptor-dependent and -independent, respectively). Ethanol at 1.0 and 2.0 g/kg i.p. inhibited in a dose-dependent manner (by 20-45%) the 5-HT- but not the capsaicin-stimulated decrease in heart rate. Our results demonstrate that the inhibitory effect of ethanol on the 5-HT3 receptor-mediated Bezold-Jarisch reflex may be related to the direct effect of ethanol on 5-HT3 receptors on sensory vagal nerves in the heart.

The effects of thiol reduction and oxidation on the inhibition of NMDA-stimulated neurotransmitter release by ethanol

The N-methyl-D-aspartate (NMDA) receptor is sensitive to inhibition by pharmacologically relevant concentrations of ethanol and may be an important target for the actions of ethanol in the brain. The mechanisms responsible for ethanol's action are unknown but may involve perturbation of one of the multiple regulatory sites that have been identified on the receptor. Previous studies have shown that thiol reducing and oxidizing agents selectively after the activity of the NMDA receptor via a redox modulatory site. In this study, these agents were tested for their ability to modify the inhibitory actions of ethanol on NMDA-stimulated neurotransmitter release from rat brain slices and the potentiation of NMDA-stimulated release by the glycine agonist D-serine. Treatment of hippocampal slices with the thiol reducing agent dithiothreitol (DTT) significantly potentiated (> 2-fold) the NMDA-stimulated release of tritiated norepinephrine ([3H]NE) from rat hippocampal slices. Slices treated with the thiol oxidizing agent dithionitrobenzoic acid (DTNB) had slightly lower (10-15%) levels of NMDA-stimulated neurotransmitter release. Ethanol (25-200 mM) dose-dependently inhibited the NMDA-stimulated release of [3H]NE from control slices with a calculated IC50 value of 73.1 mM. The inhibitory potency for ethanol was increased following DTNB treatment (IC50 of 40.9 mM) while DTT treatment slightly reduced ethanol's potency (IC50 of 85.6 mM). The ability of D-serine to augment NMDA-stimulated neurotransmitter release was tested in the presence and absence of ethanol following treatment of slices with either DTT or DTNB to examine any potential interaction between these modulatory sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential control of glucoregulatory hormone response and glucose metabolism by NMDA and kainate

The aim of the present study was to elucidate the effect of kainate and N-methyl-D-aspartate (NMDA), two different excitatory amino acid (EAA) agonists, on glucoregulatory hormone production and whole body glucose metabolism. Rates of hepatic glucose production (HGP) and peripheral glucose utilization (GU) were assessed in overnight fasted, catheterized, conscious rats using [3-3H]glucose. At the highest dose of kainate examined (16 mg/kg), glucose levels increased 97% after 1 h; thereafter, glucose fell towards basal values but was still elevated 25% at the end of the 3 h experiment. This hyperglycemia resulted from a rapid increase in HGP that exceeded an increased rate of GU. Both HGP and GU were elevated 86% throughout the final 2 h of the experiment. NMDA induced changes in glucose flux that were qualitatively similar, yet of smaller magnitude and of shorter duration, than those produced by kainate. Kainate-induced increases in glucose metabolism were associated with an early transient hyperinsulinemia followed by a period of insulinopenia, and sustained increases in the plasma concentrations of glucagon, corticosterone, epinephrine and norepinephrine. In contrast, sustained increases in glucagon and catecholamines, as well as the late hypoinsulinemia were not detected in NMDA-treated rats. Adrenergic blockade attenuated the kainate- but not the NMDA-induced increase in glucose metabolism. These results indicate that EAA agonists that bind preferentially to different receptor subtypes produce qualitatively similar changes in glucose metabolism. Whereas the increased HGP in kainate-injected rats was associated with sustained elevations in glucagon, catecholamines and corticosterone, NMDA only transiently elevated circulating glucocorticoid levels, suggesting a different mechanism of action. These data, support the involvement of EAA in various aspects of glucoregulation.

Functional studies of neurotransmitter receptors in human brain

Understanding synaptic transmission in the human brain is of the uppermost importance due to the involvement of neurotransmitters in several neurological and psychiatric disorders. Studies of animal pharmacology and of molecular biology are revealing that transmitter receptors are highly heterogeneous. It is therefore essential, also in view of using animal models in the development of therapeutically useful drugs, to establish if functionally corresponding receptors in men and animals also display identical pharmacological profiles. Using human brain tissue samples removed during neurosurgery and monitoring transmitter release as a functional response, a number of neurotransmitter receptors have been identified, localized and pharmacologically characterized as types and subtypes.

Ethanol dose-dependently attenuates NMDA-mediated thermal hyperalgesia in the rat

Recent observations using acute and persistent pain models have suggested that activation of the N-methyl-D-aspartate (NMDA) receptor is required for mechanisms that underly the development and maintenance of thermal hyperalgesia. The present results document that both NMDA-mediated thermal hyperalgesia produced after acute intrathecal NMDA administration and NMDA-mediated thermal hyperalgesia produced in a model of neuropathic pain are dose-dependently and reversibly attenuated by intrathecal administration of ethanol (0.5-1.0%; total dose, 106-213 nmol, i.t.). This is consistent with recent reports that ethanol may function as a selective NMDA receptor antagonist at low concentrations and further extends the evidence that thermal hyperalgesia is mediated by NMDA receptors.