The role of neuronal calcium channels in dependence on ethanol and other sedatives/hypnotics

Replacement of current opioid drugs focusing on MOR-related strategies

The scarcity and limited risk/benefit ratio of painkillers available on the market, in addition to the opioid crisis, warrant reflection on new innovation strategies. The pharmacopoeia of analgesics is based on products that are often old and derived from clinical empiricism, with limited efficacy or spectrum of action, or resulting in an unsatisfactory tolerability profile. Although they are reference analgesics for nociceptive pain, opioids are subject to the same criticism. The use of opium as an analgesic is historical. Morphine was synthesized at the beginning of the 19th century. The efficacy of opioids is limited in certain painful contexts and these drugs can induce potentially serious and fatal adverse effects. The current North American opioid crisis, with an ever-rising number of deaths by opioid overdose, is a tragic illustration of this. It is therefore legitimate to develop research into molecules likely to maintain or increase opioid efficacy while improving their tolerability. Several avenues are being explored including targeting of the mu opioid receptor (MOR) splice variants, developing biased agonists or targeting of other receptors such as heteromers with MOR. Ion channels acting as MOR effectors, are also targeted in order to offer compounds without MOR-dependent adverse effects. Another route is to develop opioid analgesics with peripheral action or limited central nervous system (CNS) access. Finally, endogenous opioids used as drugs or compounds that modify the metabolism of endogenous opioids (Dual ENKephalinase Inhibitors) are being developed. The aim of the present review is to present these various targets/strategies with reference to current indications for opioids, concerns about their widespread use, particularly in chronic non-cancer pains, and ways of limiting the risk of opioid abuse and misuse.

Upregulation of L-type Ca(v)1 channels in the development of psychological dependence

Although L-type voltage-dependent Ca2+ channels regulate activity-dependent processes including synaptic plasticity and synapse formation, there are few data on the changes of Ca(v)1 channel expression in psychological dependence. This study investigated the role of L-type Ca(v)1 channel expression in the brain of mouse that was psychologically dependent on methamphetamine (2 mg/kg, subcutaneous injection [s.c.]), cocaine (10 mg/kg, s.c.), and morphine (5 mg/kg, s.c.) with the conditioned place preference paradigm. Intracerebroventricular administration of nifedipine (3, 10, and 30 nmol/mouse) dose-dependently reduced the development of methamphetamine-, cocaine-, and morphine-induced rewarding effect. Under such conditions, protein levels of both Ca(v)1.2 and Ca(v)1.3 in the frontal cortex and the limbic forebrain were significantly increased on methamphetamine-, cocaine-, and morphine-induced psychologically dependent mice. These findings suggest that the upregulation of Ca(v)1.2 and Ca(v)1.3 participated in the development of psychological dependence.

Chronic ethanol intake modifies pyrrolidon carboxypeptidase activity in mouse frontal cortex synaptosomes under resting and K+ -stimulated conditions: role of calcium

Pyrrolidon carboxypeptidase (Pcp) is an omega peptidase that removes pyroglutamyl N-terminal residues of peptides such as thyrotrophin-releasing hormone (TRH), which is one of the neuropeptides that has been localized into many areas of the brain and acts as an endogenous neuromodulator of several parameters related to ethanol (EtOH) consumption. In this study, we analysed the effects of chronic EtOH intake on Pcp activity on mouse frontal cortex synaptosomes and their corresponding supernatant under basal and K+ -stimulated conditions, in presence and absence of calcium (Ca2+) to know the regulation of Pcp on TRH. In basal conditions, chronic EtOH intake significantly decreased synaptosomes Pcp activity but only in absence of Ca2+. However, supernatant Pcp activity is also decreased in presence and absence of calcium. Under K+-stimulated conditions, chronic EtOH intake decreased synaptosomes Pcp activity but only in absence of Ca2+, whereas supernatant Pcp activity was significantly decreased only in presence of Ca2+. The general inhibitory effect of chronic EtOH intake on Pcp activity suggests an inhibition of TRH metabolism and an enhancement of TRH neurotransmitter/neuromodulator functions, which could be related to putative processes of tolerance to EtOH in which TRH has been involved. Our data may also indicate that active peptides and their degrading peptidases are released together to the synaptic cleft to regulate the neurotransmitter/neuromodulator functions of these peptides, through a Ca2+ -dependent mechanism.

Sex differences in the neurotoxic effects of adenosine A1 receptor antagonism during ethanol withdrawal: reversal with an A1 receptor agonist or an NMDA receptor antagonist

BACKGROUND

Neuronal adaptations that occur during chronic ethanol (EtOH) exposure have been observed to sensitize the brain to excitotoxic insult during withdrawal. The adenosine receptor system warrants further examination in this regard, as recent evidence has implicated adenosine receptor involvement in the behavioral effects of both EtOH exposure and withdrawal.

METHODS

The current studies examined effects of adenosine A(1) receptor manipulation on neuronal injury in EtOH-naive and EtOH-withdrawn male and female rat hippocampal slice cultures. EtOH-naive and EtOH pretreated (43.1 to 26.9 mM from days 5 to 15 DIV) cultures were exposed to the A(1) receptor agonist 2-Chloro-N(6)-cyclopentyladenosine (CCPA; 10 nM), the A(1) receptor antagonist 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX;10 nM), or the N-methyl-D-aspartate (NMDA) receptor antagonist D,L,-2-amino-5-phosphovalerate (APV; 20 microM) at 15 days in vitro (DIV). Cytotoxicity was measured in the primary neuronal layers of the dentate gyrus, CA3 and CA1 hippocampal regions by quantification of propidium iodide (PI) fluorescence after 24 hours. Immunohistochemical analysis of A(1) receptor abundance was conducted in EtOH-naive and EtOH pretreated slice cultures at 15 DIV.

RESULTS

Twenty-four hour exposure to DPCPX in EtOH-naive slice cultures did not produced neurotoxicity in any region of slice cultures. Though withdrawal from 10 day EtOH exposure produced no toxicity in either male or female slice cultures, exposure to DPCPX during 24 hours of EtOH withdrawal produced a marked increase in PI uptake in all hippocampal culture subregions in female cultures (to approximately 160% of control values). A significant effect for sex was observed in the CA1 region such that toxicity in females cultures exposed to the A(1) antagonist during withdrawal was greater than that observed in male cultures. These effects of DPCPX in EtOH withdrawn female and male slices were prevented by co-exposure to either the A(1) agonist CCPA or the NMDA receptor antagonist APV for 24 hours. No differences in the abundance of A(1) receptors were observed in male and female EtOH-naive or EtOH pretreated cultures.

CONCLUSIONS

The current findings suggest that the female hippocampus possesses an innate sensitivity to effects of EtOH exposure and withdrawal on neuronal excitability that is independent of hormonal influences. Further, this sex difference is not related to effects of EtOH exposure on A(1) receptor abundance, but likely reflects increased NMDA receptor-mediated signaling downstream of A(1) inhibition in females.

Up-regulation of L-type high voltage-gated calcium channel subunits by sustained exposure to 1,4- and 1,5-benzodiazepines in cerebrocortical neurons

The aim of this study is to examine how sustained exposure to two 1,4-benzodiazepines (BZDs) with different action period, diazepam and brotizolam, and a 1,5-BZD, clobazam, affects L-type high voltage-gated calcium channel (HVCC) functions and its mechanisms using primary cultures of mouse cerebral cortical neurons. The sustained exposure to these three BZDs increased [(45)Ca2+] influx, which was due to the enhanced [(45)Ca2+] entry through L-type HVCCs but not through of Cav2.1 and Cav2.2. Increase in [(3)H]diltiazem binding after the exposure to these three BZDs was due to the increase in the binding sites of [(3)H]diltiazem. Western blot analysis showed increase of Cav1.2 and Cav1.3 in association with the increased expression of alpha2/delta1 subunit. Similar changes in [(3)H]diltiazem binding and L-type HVCC subunit expression were found in the cerebral cortex from mouse with BZD physical dependence. These results indicate that BZDs examined here have the potential to increase L-type HVCC functions mediated via the enhanced expression of not only Cav1.2 and Cav1.3 but also alpha2/delta1 subunit after their sustained exposure, which may participate in the development of physical dependence by these BZDs.

Increase in Expression of α1 and α2/δ1 Subunits of L-Type High Voltage-Gated Calcium Channels After Sustained Ethanol Exposure in Cerebral Cortical Neurons

Previous reports revealed up-regulation of L-type high voltage-gated calcium channels (HVCCs) in mouse brains with ethanol physical dependence. We investigated mechanisms of enhancement of L-type HVCC function using mouse cerebrocortical neurons exposed to 50 mM ethanol for 3 days and the brains of mouse physically dependent on ethanol. Ethanol facilitated 30 mM KCl-stimulated (45)Ca(2+) influx in dose- and duration-dependent manners, which was abolished by nifedipine, an inhibitor specific to L-type HVCCs, but not by inhibitors for other types of HVCCs. Increase in [(3)H]PN200-110 binding to the particulate fractions from the ethanol-treated neurons was due to increased B(max) value with no changes in K(d) value. Western blot analysis showed the increased expression of alpha1C, alpha1D, and alpha2/delta1 subunits with decreased beta4 subunit expression and no changes in expressions of alpha1A, alpha1B, alpha1F, and alpha2 subunits. A similar pattern of the changes in the expression of these subunits of L-type HVCCs were observed in the cerebral cortex from mouse with ethanol physical dependence. These results indicate that sustained ethanol exposure to the neurons induces up-regulation of L-type HVCCs, which is due to increased expressions of alpha1C, alpha1D, and alpha2/delta1 subunits, and produces no alterations in P/Q- and N-type HVCC functions.

Effects of finasteride on chronic and acute ethanol withdrawal severity in the WSP and WSR selected lines

BACKGROUND

The neurosteroid allopregnanolone (ALLO) is a potent positive modulator of gamma-aminobutyric acidA (GABAA) receptors that can modulate ethanol (EtOH) withdrawal. The 5alpha-reductase inhibitor finasteride blocks the formation of ALLO from progesterone and was recently found to reduce certain effects of EtOH. Using the Withdrawal Seizure-Prone (WSP) and Withdrawal Seizure-Resistant (WSR) selected lines, in the present studies we examined the effect of finasteride on acute and chronic EtOH withdrawal severity.

METHODS

In the first two studies, male WSP and WSR mice were exposed to 72-hr EtOH vapor or air and received four injections of finasteride (50 mg/kg intraperitoneal (IP) or vehicle 24 hr before and each day of the vapor exposure. After removal from the inhalation chamber, mice were scored for handling-induced convulsions (HICs) hourly for 12 hr and then again at 24 hr (study 1) or were tested on the elevated plus maze at 24 hr after removal from the inhalation chamber (study 2). In the third experiment, mice were pretreated with finasteride or vehicle 24 hr before an acute dose of EtOH (4 g/kg ip) or saline and then were tested for HICs as in the chronic study.

RESULTS

In both chronic EtOH studies, finasteride pretreatment reduced EtOH withdrawal severity, measured by HICs, and anxiety-related behavior, but only in the WSP selected line. However, finasteride pretreatment also significantly decreased blood EtOH concentration on the initiation of withdrawal in both chronic EtOH studies in WSP and WSR mice. In contrast, pretreatment with finasteride slightly enhanced acute EtOH withdrawal severity in WSP mice, whereas there was no effect of finasteride or EtOH injection on HICs in WSR mice.

CONCLUSIONS

Collectively, these findings indicate that the WSP line is more sensitive than the WSR line to the modulatory effects of finasteride in terms of both chronic and acute EtOH withdrawal severity. The differential effect of finasteride on acute versus chronic EtOH withdrawal severity may result from an indirect effect of finasteride on EtOH pharmacokinetics in the chronic paradigm.

Calcium-Acting Drugs Modulate Expression and Development of Chronic Tolerance to Nicotine-Induced Antinociception in Mice

Initial studies in our laboratory suggested that tolerance to nicotine is thought to involve neuronal adaptation not only at the level of the drug-receptor interaction but at postreceptor events such as calcium-dependent second messengers. The present study was undertaken to investigate the hypothesis that L-type calcium channels and calcium-dependent calmodulin protein kinase II are involved in the development and expression of nicotine tolerance. To that end, the effects of modulation of L-type calcium channels (through the use of inhibitors or activators) as well as calcium-dependent calmodulin protein kinase II inactivation were studied in a mouse model of tolerance where mice were infused with nicotine in minipumps (24 mg/kg/day) for 14 days. In addition, the activity of calcium-dependent calmodulin protein kinase II in the lumbar spinal cord region obtained from nicotine-tolerant mice was measured. Our data showed that chronic administration of L-type calcium channel antagonists nimodipine (1 and 5 mg/kg) and verapamil (10 mg/kg) prevented the development of tolerance to nicotine-induced antinociception. In contrast, chronic exposure of BAYK8644 [(+/-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester], a calcium channel activator, enhanced nicotine's tolerance. Moreover, a significant increase in both dependent and independent calcium-dependent calmodulin protein kinase II activity was seen in the spinal cord in nicotine-tolerant mice. Finally, spinal administration of 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-tyrosyl]-4-phenylpiperazine (KN-62), a calcium-dependent calmodulin protein kinase II antagonist, reduced the expression of tolerance to nicotine-induced antinociception in mice. In conclusion, our data indicate that calcium-dependent mechanisms such as L-type calcium channels and calcium-dependent calmodulin protein kinase II activation are involved in the expression and development of nicotine tolerance.

Increased expression of L-type high voltage-gated calcium channel α1 and α2/δ subunits in mouse brain after chronic nicotine administration

We investigated the effect of chronic nicotine administration on high voltage-gated calcium channels (HVCCs) in the mouse cerebral cortex. The treatment significantly increased expression of alpha1C, alpha1D, alpha1F, and alpha2/delta1 subunits with no changes of beta4 subunit of L-type HVCCs. [(3)H]Diltiazem binding to the particulate fractions increased with increased Bmax value. These results indicate that chronic nicotine treatment up-regulates L-type HVCCs, which is due to increased expression of alpha1 and alpha2/delta1 subunits.

The role of pregnane neurosteroids in ethanol withdrawal: behavioral genetic approaches

Within the last 20 years, rapid nongenomic actions of steroid hormones have been demonstrated to occur via an interaction with ligand-gated ion channels. For example, the pregnane neurosteroid allopregnanolone (ALLOP) is a potent positive modulator of gamma-aminobutyric acid(A) (GABA(A)) receptors. The physiological significance of fluctuations in endogenous ALLOP levels has been investigated with regard to disease states and the effect of therapeutic agents on ALLOP levels. Because the pharmacological profile of ALLOP is similar to that of ethanol (EtOH), the modulatory effect of pregnane neurosteroids on EtOH dependence and withdrawal will be the focus of this review. Data on the effects of chronic EtOH exposure and withdrawal on pregnane neurosteroid levels, biosynthetic enzymes, and changes in neurosteroid sensitivity will be summarized. Results from genetic animal models indicate that seizure-prone animals have a persistent decrease in endogenous ALLOP levels during EtOH withdrawal in conjunction with tolerance to ALLOP's anticonvulsant effect. Manipulation of endogenous ALLOP levels with finasteride also markedly reduced the severity of chronic EtOH withdrawal. Gene mapping studies provide a hint for an interaction between genes for GABA(A) receptor subunits and the biosynthetic enzyme 5alpha-reductase. Overall, the results are suggestive of a relationship between endogenous pregnane neurosteroid levels and behavioral changes in excitability during EtOH withdrawal, consistent with recent findings in humans. While the findings with ALLOP emphasize the therapeutic potential of neurosteroid treatment during EtOH withdrawal, the gene mapping studies suggest that pregnane neurosteroid biosynthesis may represent a target for therapeutic intervention in the treatment of alcohol dependence.

Interaction of the Nicotinic Cholinergic System with Ethanol Withdrawal

The observation that alcohol and nicotine are commonly abused together suggests that the two drugs have common sites of action. In vitro studies indicate that nicotinic acetylcholine receptor (nAChR) function is enhanced by ethanol. Furthermore, some ethanol-related behaviors are associated with a region of mouse chromosome 2 that contains the gene encoding the alpha4 subunit of the nAChR (Chrna4). We have identified a polymorphism in Chrna4 that results in an alanine (A) or threonine (T) residue at position 529 in the second intracellular loop of the protein. Nicotinic receptors expressing the A variant have greater responses to nicotine and ethanol than receptors with the T variant when measured in vitro, but the possible effects of the polymorphism on the severity of ethanol withdrawal have not been assessed. The handling-induced convulsion (HIC) assay is an established method for studying drug withdrawal in vivo. We monitored the HIC responses of mice for 8 h after an injection of ethanol (4 g/kg). A survey of 16 mouse strains, as well as previously published data, indicated an association of the A/T polymorphism with ethanol withdrawal. This association was also found in wild-type animals from an F2 intercross of the A/J (A529-genotype) strain with C57BL/6J (T529-genotype) mice that also lack expression of the beta2 nAChR subunit. Beta2 -/- animals, which do not express alpha4beta2 nAChRs in the brain, exhibited significantly lower HIC responses and no effect of the polymorphism. These results suggest that the nicotinic cholinergic system and the A/T polymorphism modulate ethanol withdrawal.

L-type high voltage-gated calcium channels cause an increase in diazepam binding inhibitor mRNA expression after sustained exposure to ethanol in mouse cerebral cortical neurons

Mechanisms for increase in diazepam binding inhibitor (DBI) mRNA expression after sustained exposure to ethanol (EtOH) were investigated. Increases in 30 mM KCl-induced [45Ca(2+)] influx and DBI mRNA expression after EtOH (50 mM) exposure for 3 days were completely abolished by nifedipine, but not by omega-agatoxin VIA and omega-conotoxin GIVA. These results indicate that EtOH-induced increase in DBI mRNA expression is mediated via increased Ca(2+) entry through up-regulated L-type high voltage-gated calcium channels.

Ethanol suppresses fast potentiation of glycine currents by glutamate

Excitatory (glutamate) and inhibitory (GABA(A) and glycine) receptor/channels coexist in many neurons. To assess effects of ethanol on the interaction of glutamate and glycine receptors, glycine-induced current (I(Gly)) was recorded by a whole-cell patch-clamp technique from neurons freshly dissociated from the ventral tegmental area of rats. A conditioning prepulse of glutamate (1-3 s, 1 mM) significantly and reversibly potentiated responses to a pulse of glycine. This potentiation was increased when extracellular calcium was raised to 12 mM and reduced by including 10 mM 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid in the internal recording medium. It was not affected by 5 microM 1-N,O-bis-(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62), a selective inhibitor of calcium/calmodulin-dependent protein kinase II. In a concentration-response analysis, a conditioning pulse of glutamate significantly lowered the EC(50) for glycine and increased the maximal I(Gly). Kinetic analysis of the currents indicated that glutamate slowed deactivation of glycine-gated chloride channels; therefore, glutamate may increase the affinity of glycine receptors for glycine. When coapplied with glycine, ethanol (10 mM) potentiated I(Gly) in 35% of neurons from the ventral tegmental area. In contrast, when coapplied with glutamate and glycine, ethanol suppressed the glutamate-induced potentiation of I(Gly) in these neurons. This suppression was also observed when ethanol and glycine were coapplied after a glutamate prepulse. A similar effect was observed when ethanol alone did not potentiate I(Gly). These findings suggest that glutamate-induced calcium influx modulates glycine receptors by a mechanism that can be blocked by ethanol.

Up-regulation of L-type voltage-dependent calcium channels after long term exposure to nicotine in cerebral cortical neurons

Effects of long term (72-h) exposure to low concentration (0.1 mum) of nicotine on various types of voltage-dependent Ca(2+) channels (VDCCs) and neuronal nicotinic acetylcholine receptors (nnAChRs) were examined using primary cultures of mouse cerebral cortical neurons. High potassium (30 mm KCl)-stimulated (45)Ca(2+) influx into the neurons increased with increasing the duration of nicotine exposure and its concentrations. The maximal increase of the KCl-stimulated (45)Ca(2+) influx was found 24 h after the initiation of exposure and thereafter maintained up to 72 h. This enhancement of KCl-induced (45)Ca(2+) influx after 72-h exposure to 0.1 mum nicotine was completely abolished by concomitant exposure with mecamylamine, an inhibitor for nnAChRs. Only the component of the KCl-induced (45)Ca(2+) influx observed after long term exposure to nicotine, which was sensitive to nifedipine, an inhibitor of L-type VDCCs, was facilitated, while the (45)Ca(2+) influx through P/Q- and N-type VDCCs showed no changes. Moreover, enhanced immunoreactivity against antibody for the alpha(1C) subunit of L-type VDCCs was recognized, whereas no changes in immunoreactivities against antibodies for alpha(1A) and alpha(1B) subunits of other types of VDCCs were noted. In addition, a Western blot analysis showed an increase of immunoreactivities against antibodies for alpha(1D) and alpha(2)/delta(1), and expression of mRNA for L-type VDCC subunit, alpha(1F), was also enhanced, although beta(4) mRNA expression was not changed. Whole cell patch clamp analysis revealed that the increase of the amplitude of Ba(2+) currents was also recognized in the neurons exposed to nicotine, and nicardipine reduced this increased amplitude to the level of the amplitude detected in nontreated neurons with nicardipine. The up-regulation of alpha(4) and beta(2) subunits, but not the alpha(3) subunit of nnAChRs, was also noted after the nicotine exposure when examining by the Western blot analysis. Taken together, these results indicate that the long term exposure of the neurons to a low concentration of nicotine induces both increased (45)Ca(2+) influx through up-regulated L-type VDCCs and nnAChR up-regulation.

Withdrawal from nicotine facilitates diazepam binding inhibitor mRNA expression in mouse cerebral cortex

Changes in diazepam binding inhibitor (DBI) mRNA expression after withdrawal from nicotine were examined. Withdrawal from nicotine Increased DBI mRNA expression in cerebral cortices derived from nicotine-dependent mice and in the neurons continuously exposed to nicotine (0.1 microM). These results indicate that withdrawal from nicotine after its long-term exposure induces steep increase of DBI mRNA expression as reported previously in ethanol- and morphine-dependent animals.

Chronic ethanol treatment reduces adenylyl cyclase activity in human erythroleukemia cells

Characteristic changes of platelet membrane adenylyl cyclase activity have been described in men with alcoholism. We studied the occurrence of these changes in human erythroleukemia (HEL) cells after chronic ethanol treatment. Chronic treatment of the HEL cell with ethanol (50 or 100 mM) for 48 h resulted in significant reduction of prostaglandin E1-stimulated adenylyl cyclase activity. The acute ethanol (200 mM, 5 min) enhancement of adenylyl cyclase activity was significantly reduced after chronic ethanol treatment. We also observed a reduction in phorbol-12,13-dibutyrate (PDB) enhancement of prostaglandin E1-stimulation after chronic ethanol treatment. Chronic ethanol treatment (50 or 100 mM) reduced the activity of adenylyl cyclase in response to stimulation by acute ethanol to a greater extent than that of after acute PDB. The increase in cAMP formation by ethanol and PDB was only evident when prostaglandin E1 was present and under basal conditions (when no stimulatory agent was present) ethanol up to 200 mM, and PDB up to 1 M, had no significant effect on adenylyl cyclase activity. The reduced capacity of ethanol and/or PDB to stimulate adenylyl cyclase activity after chronic ethanol treatment suggests the involvement of a common denominator in the action of ethanol and PDB.

Differential effects of a dihydropyridine calcium channel antagonist on the components of ethanol tolerance

The dihydropyridine calcium channel antagonist, nimodipine, was found to decrease the extent of tolerance that developed to the ataxic action of ethanol in experimental designs in which the tolerance was not context-specific, when ethanol was given by liquid diet. When ethanol was given by injection, so that cues were present for the effects of ethanol during the chronic treatment, tolerance to the ataxic actions of ethanol was unaffected. Nimodipine, however, decreased the tolerance to the hypothermic actions of ethanol, when the ethanol was given by injection. When the rats were given practice sessions on the motor task while under the influence of the ethanol, during the chronic treatment, nimodipine did not affect tolerance to the ataxic actions of ethanol. When nimodipine was given before the motor task learning and ethanol after the practice sessions, the tolerance to the ataxic effect of ethanol was increased. A similar schedule of drug treatment with the NMDA antagonist CGP37849 given before the practice sessions, and ethanol afterwards, resulted in decreased tolerance to ethanol. It is suggested that these changes in ethanol tolerance may be explained by dual actions of nimodipine in, firstly, decreasing the form of tolerance to ethanol that is not dependent on contextual cues and, secondarily, in increasing the learning of a motor task.

Alcohol and the brain. Pharmacological insights for psychiatrists

BACKGROUND

Alcohol misuse, as well as being a major form of psychiatric morbidity, is also commonly associated with other psychiatric disorders. A greater understanding of the brain mechanisms underlying the adverse effects of alcohol is now possible, thanks to significant research advances made over the past decade.

AIMS

To elucidate for psychiatrists the growing knowledge of the importance of specific neurotransmitter interactions in the effects of alcohol.

METHOD

A survey of the literature, extracting current knowledge of interest to psychiatrists.

RESULTS

There is good evidence that the acute effects of alcohol are mediated through interactions with amino acid neurotransmitters plus parallel changes in amines such as noradrenaline, dopamine and serotonin. Neuroadaptive responses at amino acid receptors probably underlie significant components of the withdrawal syndrome and probably also contribute to neuronal death found in chronic alcoholism.

CONCLUSIONS

An understanding of the pharmacology of alcohol use may lead to greater ability to treat psychiatric consequences of alcoholism, and may also prevent some of the secondary psychiatric comorbidity and later brain damage.

Drug withdrawal convulsions and susceptibility to convulsants after short-term selective breeding for acute ethanol withdrawal

High Alcohol Withdrawal (HAW) and Low Alcohol Withdrawal (LAW) mice were selectively bred from a foundation population of C57BL6/J (B6) x DBA/2J (D2) F2 intercross progeny for display of intense or mild handling-induced withdrawal convulsions, respectively, following a single injection of a hypnotic dose of ethanol (alcohol; 4 g/kg). The HAW line had significantly greater alcohol withdrawal severity scores compared to the LAW line after only a single generation of selection; the magnitude of the line difference was 8-fold by the fourth selected generation. We tested these lines for severity of withdrawal convulsions following the benzodiazepine, diazepam; the gaseous anesthetic, nitrous oxide; the imidazopyridine, zolpidem and the barbiturate, pentobarbital. In all cases, HAW mice had significantly greater withdrawal severity than mice of the LAW line. These results indicate that some genes influencing withdrawal convulsion severity following ethanol also affect withdrawal from other CNS depressants. D2 mice are more sensitive to a variety of convulsants than B6 mice (and have more severe withdrawal convulsions). We, therefore, tested separate groups of mice of both selectively bred lines for threshold sensitivity to pentylenetetrazol (PTZ), N-methyl-D-aspartate (NMDA) and kainic acid (KA). No line differences were detected. These results indicate that genes influencing severity of withdrawal from several depressant drugs are largely different from those affecting susceptibility to GABAergic or glutamatergic convulsants.

Acute ethanol alters calcium signals elicited by glutamate receptor agonists and K+ depolarization in cultured cerebellar Purkinje neurons

The effect of acute ethanol on Ca2+ signals evoked by ionotropic (iGluR) and metabotropic (mGluR) glutamate receptor (GluR) activation and K+ depolarization was examined in cultured rat cerebellar Purkinje neurons to assess the ethanol sensitivity of these Ca2+ signaling pathways. Mature Purkinje neurons approximately 3 weeks in vitro were studied. iGluRs were activated by (RS)-alpha-amino-3-hydroxyl-5 methyl-4-isoxazolepropionic acid (AMPA; 1 and 5 microM) and domoate (5 microM). mGluRs were activated by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 300 microM) and (R,S)-3,5-dihydroxyphenylglycine (DHPG; 200 microM). These agents and K+ (150 mM) were applied from micropipettes by brief (1 s) microperfusion pulses. Ca2+ levels were monitored at 2-3 s intervals during pre- and post-stimulus periods using microscopic digital imaging and the Ca2+ sensitive dye fura-2. iGluR and mGluR agonists and K+ produced abrupt increases in intracellular Ca2+ that slowly recovered to baseline resting levels. Acute exposure to ethanol at 33 mM (150 mg%) and 66 mM (300 mg%) significantly reduced the amplitude of the Ca2+ signals to iGluR agonists and K+ with little or no effect on Ca2+ signals to mGluR agonists. In contrast, acute ethanol at 10 mM (45 mg%) had no effect on the Ca2+ signals to the iGluR agonist AMPA but significantly enhanced the Ca2+ signals to the mGluR agonist DHPG. These results show that ethanol modulates Ca2+ signaling linked to GluR activation in a receptor subtype specific manner, and suggest that Ca2+ signaling pathways linked to GluR activation and membrane depolarization may be important mechanisms by which ethanol alters the transduction of excitatory synaptic signals at glutamatergic synapses and thereby affects intercellular and intracellular communication in the CNS.

Conditioned taste aversion and place preference induced by the calcium channel antagonist nimodipine in rats

It has become clear that various calcium channel antagonists are able to suppress excessive intake of ethanol in rats. With respect to these findings, it has become of interest whether these drugs can act as rewarding and/or aversive stimulus. Therefore, such affective stimulus effects of the L-type calcium channel antagonist nimodipine and its enantiomers were studied in Wistar rats in a series of conditioned taste aversion (CTA; two-bottle choice procedure) and conditioned place preference (CPP; two-compartment procedure) experiments. Racemic nimodipine (0.95-15 mg/kg IP) was found to induce a dose-dependent CTA, 7.5 mg/kg being the lowest effective dose. Subsequent studies with both enantiomers revealed that the CTA effects of nimodipine are completely dependent on the activity of (-)-nimodipine. With (+)-nimodipine (0.25-90 mg/kg IP), none of the doses tested induced a significant CTA, whereas with (-)-nimodipine clear and dose-dependent CTA effects were noted (0.5-30 mg/kg IP). For this enantiomer, the lowest effective dose was 15 mg/kg. In additional CPP experiments, it was confirmed that (+/-)-nimodipine and (-)-nimodipine have affective stimulus properties, whereas (+)-nimodipine was again an ineffective stimulus (dose used for all drugs: 15 mg/kg IP). Interestingly, the affective stimulus effects as measured with CPP of (+/-)- and (-)-nimodipine turned out to be rewarding, as it was found that both drugs produced a significant place preference. It is concluded from these studies that nimodipine possesses intrinsic affective stimulus effects which are rewarding in nature. Furthermore, these stimulus effects are mediated by the activity of the (-)-enantiomer. Possibly, these rewarding effects of nimodipine may play a role in the reported attenuating effects of this drug on voluntary ethanol intake in rats.

Comparison of the stimulus properties of ethanol and the Ca2+ channel antagonist nimodipine in rats

A variety of L-type Ca2+ channel antagonists, including the dihydropyridine derivative nimodipine, have been shown to be effective in reducing ethanol intake and preference in animal models of alcoholism. The behavioral mechanism involved in the anti-alcohol effects of nimodipine are, however, not clear yet. The aim of the present study was to investigate the possibility that the effects of nimodipine on ethanol intake are based on stimulus substitution. Therefore, rats were trained to discriminate ethanol (12.5% w/v, 1000 mg/kg i.p.) from saline in a two-lever food-reinforced drug discrimination procedure (dose range of ethanol tested: 125-1000 mg/kg i.p., ED50 value: 488 mg/kg). In cross-generalization tests with nimodipine (0.15-15 mg/kg i.p.), stimulus substitution was not noted. In addition, a cross-familiarization conditioned taste aversion paradigm was utilized. In rats, 1000 mg/kg i.p. ethanol was used as the reference drug producing a conditioned taste aversion. Effects of preexposure to ethanol (500-1500 mg/kg i.p.) and nimodipine (7.5-30 mg/kg i.p.) on the magnitude of the ethanol-induced conditioned taste aversion were investigated as an index for stimulus similarity between preexposure and reference drug. Preexposure to both ethanol and nimodipine prevented the development of a conditioned taste aversion. Contrary to the drug discrimination results, these latter findings suggest that there may be similarities between the stimulus properties of nimodipine and ethanol. Moreover, the apparent discrepancy between the results obtained in drug discrimination and cross-familiarization conditioned taste aversion suggests that different stimulus properties of ethanol control behavior in both procedures. The finding that, under particular conditions, ethanol and nimodipine appear to share common stimulus properties needs to be further evaluated, as this may be related to the reported anti-alcohol effects of nimodipine and other Ca2+ channel antagonists.

Role of taste aversion in calcium channel inhibitor-induced suppression of saccharin and alcohol drinking in rats

L-type calcium (Ca2+) channel inhibitors suppress drinking of highly preferred solutions of simple carbohydrates, saccharin, or alcohol. The present study was designed to examine whether this decrease in drinking behavior can be explained by the development of consummatory aversion. In the first experiment, the propensity of Ca2+ channel inhibitors to induce conditioned taste aversion (CTA) to 0.1% saccharin was examined using two saccharin/drug injection pairings in saccharin-naive rats. We compared three chemically different drugs: diltiazem, isradipine, and nicardipine. A dose-dependent CTA was observed after both conditioning sessions for all three drugs tested. Interestingly, the lowest dose of nicardipine (i.e., 1.25 mumol/kg), significantly increased saccharin intake. A nonsignificant trend to increase saccharin intake was also observed with the lowest dose of isradipine. We then examined whether nicardipine could similarly induce CTA to a novel taste of alcohol (6%, v/v). The drug failed to produce a significant effect. In the third experiment, we found that nicardipine did not induce CTA (or preference) if the saccharin taste was familar to rats. In the final experiment, the interaction of nicardipine (1.25 and 2.5 mumol/kg) with the ethanol (1.5 g/kg)-induced CTA to saccharin was investigated. The higher dose of nicardipine potentiated the aversive effect of ethanol in the test. Overall, the present results suggest that CTA does not play a major role in Ca2+ channel inhibitor-induced suppression of drinking behavior.

Chronic ethanol consumption affects cholinoceptor- and purinoceptor-mediated contractions of the isolated rat bladder

Isolated bladder strips from 12-week ethanol-fed, pair-fed and control adult male rats were investigated. Contractile responses to carbachol (CCh; 0.1-300 microM) were statistically significantly potentiated in the ethanol-fed group compared to pair-fed and control. Contractions to beta,gamma-methylene ATP (beta,gamma-MeATP; 1-300 microM) were statistically significantly potentiated in the ethanol-fed group at the highest concentration tested (300 microM). Neurogenic contractions (0.5-32 pps) from the ethanol-fed group in the absence of atropine and after desensitisation by alpha,beta-methylene ATP (alpha,beta-MeATP; 3 microM), were significantly potentiated compared to the pair-fed and control groups; in the presence of atropine (1 microM), neurogenic contractions were significantly augmented at the higher frequencies. It is concluded that chronic ethanol treatment affects both cholinoceptor- and purinoceptor-mediated contractions of the rat bladder.

Phosducin-like protein: an ethanol-responsive potential modulator of guanine nucleotide-binding protein function

Acute and chronic exposure to ethanol produces specific changes in several signal transduction cascades. Such alterations in signaling are thought to be a crucial aspect of the central nervous system's adaptive response, which occurs with chronic exposure to ethanol. We have recently identified and isolated several genes whose expression is specifically induced by ethanol in neural cell cultures. The product of one of these genes has extensive sequence homology to phosducin, a phosphoprotein expressed in retina and pineal gland that modulates trimeric guanine nucleotide-binding protein (G protein) function by binding to G-protein beta gamma subunits. We identified from a rat brain cDNA library an isolate encoding the phosducin-like protein (PhLP), which has 41% identity and 65% amino acid homology to phosducin. PhLP cDNA is expressed in all tissues screened by RNA blot-hybridization analysis and shows marked evolutionary conservation on Southern hybridization. We have identified four forms of PhLP cDNA varying only in their 5' ends, probably due to alternative splicing. This 5'-end variation generates two predicted forms of PhLP protein that differ by 79 aa at the NH2 terminus. Treatment of NG108-15 cells for 24 hr with concentrations of ethanol seen in actively drinking alcoholics (25-100 mM) causes up to a 3-fold increase in PhLP mRNA levels. Induction of PhLP by ethanol could account for at least some of the widespread alterations in signal transduction and G-protein function that are known to occur with chronic exposure to ethanol.

Psychopharmacological properties of calcium channel inhibitors

The previous decade has witnessed a major expansion of knowledge of the role played by voltage-sensitive calcium channels in the function of the central nervous system. Significant progress in the field has been made possible with the broadening use of organic calcium channel inhibitors (CCIs, Ca2+ antagonists), until recently considered almost exclusively as peripherally active antianginal and antiarrhythmic drugs. CCIs, however, do penetrate the blood-brain barrier from the periphery. Autoradiographic studies have established a highly heterogeneous distribution of CCI recognition sites within the brain. The existing evidence suggests that CCIs have marked psychotropic properties. The profile of their central activity is unique and spans a wide range of effects. Nevertheless, question regarding potentially confounding potent peripheral effects of these drugs remain. This paper reviews the psychopharmacology of CCIs, concentrating on preclinical data, but including supportive clinical and biochemical evidence as well. It focuses on these drugs' antidepressant, antidopaminergic (neuroleptic-like), anxiolytic and anticonvulsant effects. CCIs may also modify the reinforcing properties of some addictive drugs.