W Zieglgansberger G Rammes R Spanagel W Danysz and Ch Parsons

1. INTRODUCTION

The taurine analog acamprosate (calcium acetylhomotaurinate) has received considerable attention in Europe for its ability to prevent relapse in abstained alcoholics [(1); Chapter 28] and has been suggested to act by reducing craving associated with conditioned withdrawal (2,3).

2. ANTICRAVING AND ANTIRELAPSE PROPERTIES OF ACAMPROSATE

Novel aspects of addictive behavior to alcohol (craving, relapse, and sensitization processes) are uncovered by a new animal model of long-term, free-choice, alcohol self-administration followed by alcohol-deprivation phases. After several months of voluntary alcohol consumption, the drug-taking behavior following a deprivation (withdrawal) phase is characterized by increased alcohol intake and preference. During this so-called alcohol-deprivation effect (relapselike behavior) rats exhibit a high motivation for alcohol (4). This behavior is interpreted as craving, and this model has been used to investigate the potential of new anticraving agents (5). In this model,acamprosate (50-200 mg/kg, ip) administered twice daily during the alcohol-deprivation phase dose-dependently reduced the subsequent alcohol deprivation effect (6). The effects of acamprosate on drinking behavior under operant conditions, both during normal training conditions (i.e., at baseline) and during the alcohol-deprivation effect, were also studied (7). Under baseline conditions, acamprosate reduced operant responding in long-term alcohol-drinking rats. At maximal acamprosate levels in the blood and the brain, however, the agent reduced alcohol consumption more effectively during the alcohol-deprivation effect than during baseline drinking. Because the intensity of the alcohol-deprivation effect can serve as a measure of craving, these findings suggest that acamprosate indeed has anticraving properties. Acute administration of acamprosate (400 mg/kg) reduced oral ethanol consumption under limited access where rats were trained to respond for ethanol (10% w/v) or water in a two-lever free-choice operant paradigm (8). Repeated administration of lower daily doses of acamprosate (100 and 200 mg/kg) selectively blocked increased ethanol consumption typically observed in the same model after an imposed abstinece period (8). In a model in which ethanol administration was repeatedly paired with plus-maze exposure, the opioid antagonist naltrexone had no significant effect on alcohol-conditioned abstinence behavior in the plus-maze, but acamprosate reduced the incidence of stretched-attend postures (9). This difference was attributed to effects of acamprosate on conditioned negative reinforcement, whereas naltrexone is thought to have effects on positive reinforcement for ethanol.

From: Contemporary Clinical Neuroscience: Glutamate and Addiction Edited by: Barbara H. Herman et al. © Humana Press Inc., Totowa, NJ

The mechanism of action of acamprosate in the central nervous system (CNS) is still unclear. A better understanding of this is important to increase our knowledge of the fundamental processes governing alcohol abuse that would, in turn, allow the development of better drugs to prevent relapse in weaned alcoholics. Although early studies indirectly suggested an action at GABA receptors (10-13), more recent data suggest that acamprosate rather or also interacts with the N-methyl-d-aspartate (NMDA) subclass of ionotropic glutamate receptor. These Ca2+-permeable channels have, in turn, been implicated in the induction of alcohol dependence.

3. THE GLUTAMATERGIC/NMDA RECEPTOR SYSTEM

N-Methyl-d-aspartate receptors consist of tetrameric and heteromeric subunit assemblies that have different physiological and pharmacological properties and are differentially distributed throughout the CNS (14-19). So far, three major subunit families designated NR1, NR2, and NR3 have been cloned. Functional receptors in the mammalian CNS are almost certainly only formed by combination of NR1 and NR2 subunits, which express the glycine and glutamate recognition sites, respectively (20,21). NR3 subunits seem to inhibit receptor function and are expressed at higher levels during development (22).

Alternative splicing generates eight isoforms for the NR1 subfamily (23-25). The variants arise from splicing at three exons; one encodes a 21-amino acid insert in the N-terminal domain (N1, exon 5) and two encode adjacent sequences of 37 and 38 amino acids in the C-terminal domain (C1, exon 21, and C2, exon 22). NR1 variants are sometimes denoted by the presence or absence of these three alternatively spliced exons (from N to C1 to C2); NR1m has all three exons, NR1000 has none, and NR1i00 has only the N-terminal exon (23). The variants from NR1000 to NR1111 are alternatively denoted as NMDAR1 -4a, -2a, -3a, -1a, -4b, -2b, -3b, and -1b. The NR2 subfamily consists of four individual sub-units, NR2A to NR2D (14-19). Various heteromeric NMDA receptor channels formed by combinations of NR1 and NR2 subunits are known to differ in gating properties, magnesium sensitivity, and pharmacological profile (e.g., see Table 1 of ref. 15). Only the heteromeric assembly of NR1 and NR2B subunits for instance are potentiated in a glycine-independent manner by the polyamines spermine and spermidine and are selectively blocked by ifenprodil and related compounds. In situ hybridization has revealed overlapping but different expression for NR2 mRNA; for example, NR2A mRNA is distributed ubiquitously like NR1 with highest densities occuring in hippocampal regions and NR2B is expressed predominantly in the forebrain but not in the cerebellum, where NR2C predominates.

Glycine is a coagonist at NMDA receptors at a strychnine-insensitive recognition site (glycineB) and its presence at moderate nanomolar concentrations is a prerequisite for channel activation by glutamate or NMDA (26-28) (for review, see ref. 16). Physiological concentrations of glycine reduce one form of relatively rapid NMDA receptor desensitization (29-31). Recently, it has been suggested that d-serine may be more important than glycine as an endogenous coagonist at NMDA receptors in the telencephalon and developing cerebellum (32).

The endogenous polyamines spermine and spermidine have multiple effects on the activity of NMDA receptors. These include an increase in the magnitude of NMDA-induced whole-cell currents seen in the presence of saturating concentrations of glycine, an increase in glycine affinity, a decrease in glutamate affinity, and voltage-dependent inhibition at higher concentrations (33,34). Glycine-inde-pendent stimulation requires the presence of NR1 variants that lack an amino-terminal insert such as NR1a (NR1011) but not NR1b (NR1111). The stimulatory effect is also controlled by NR2 subunits in heteromeric complexes—it is observed at heteromeric NR1a/NR2B receptors but not at heteromeric NR1a/NR2A or NR1a/NR2C receptors (35,36). Glycine-dependent stimulation is mediated via an increase in glycine affinity and probably involves a second binding site, as it is also seen at NR1a/NR2A receptors (36,37). Spermine also induces a small decrease in the affinity of NR1a/NR2B but not NR1a/2A receptors for NMDA and glutamate (37). The voltage-dependent inhibitory effect of higher concentrations of spermine is similar for NR1A/NR2A and NR1A/NR2B receptors but is apparently absent at NR1A/NR2C receptors (36). This effect seems to be mediated at the Mg2+ channel site. Endogenous polyamines could therefore act as a bidirectional gain control of NMDA receptors, by dampening toxic chronic activation by low concentrations of glutamate—through changes in glutamate affinity and voltage-dependent blockade—but enhancing transient synaptic responses to millimolar concentrations of glutamate (33).

The polyamine modulatory site of the NMDA receptor was discovered in the late 1980s and then ifenprodil and its analog eliprodil were found to block NMDA receptors in a spermine-sensitive manner and proposed to be polymaine antagonists (38-40). Initial patch-clamp evidence for NMDA receptor-subtype selectivity of ifenprodil and eliprodil was provided by Legendre and Westbrook (41), followed by conclusive evidence for NR2B subtype selectivity by Williams (42).

4. ACTION OF ALCOHOL ON THE GLUTAMATERGIC/NMDA RECEPTOR SYSTEM

Ethanol can be seen as an NMDA receptor antagonist (43-45) at concentrations reached in the brains of alcohol abusers. There is some in vitro evidence that the effects of ethanol may be related to selective actions at NR2B receptors (46,47). NR2B selective actions are supported by the finding that chronic ethanol treatment of cultured cortical neurons has been reported to increase NR2B mRNA (48) and receptor expression (49-51). Moreover, chronic exposure of cultured cortical neurons or NR2B-transfected HEK-293 cells to ethanol increases sensitivity to NMDA measured by Ca2+ influx and neurotoxicity and causes a 10-fold increased sensitivity to the blockade by ifenprodil (49). In vivo data based on comparing sensitivity to ifenprodil and alcohol also suggest that both interact with the same subtype of NMDA receptors [i.e., containing the NR2B subunit (47)]. In contrast, Mirshahi and Woodward (45) reported that ethanol is somewhat more potent against NR2A receptors in Xenopus oocytes. Furthermore, no selective change in NR2B was observed in two other studies on chronic exposure of cultures to ethanol (52-53). It should also be noted that, apart from interactions with both NMDA and GABA receptors, at low millimolar concentrations (reached in the brain during intoxication), actions on nicotinic receptor micromolar levels were also detected recently (IC50 = 89 |M) (54).

In rats, at 9 h after withdrawal from chronic treatment with ethanol, an increase in NR2A and NR2B but no change in NR1 was seen (55). A recent postmortem study on the brains of alcoholics showed a modest (but significant) increased binding for [3H]glutamate and [3H]CGP-39653—compet-itive NMDA receptor antagonists (56). In humans with a history of alcohol abuse, an increase in immunoreactivity toward AMPA GluR2 and GluR3 subunits was also found (57).

It has been shown that upon withdrawal from ethanol in dependent rats, an increase in glutamate release is seen in the striatum that temporally corresponds to the duration of withdrawal syndrome (hyperactivity, treading, shakes, jerks, twitches) (58). (+)MK-801 normalized both effects, whereas diazepam only affected the behavioral aspects. Treatment of mice with ethanol increases fast a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-mediated excitatory postsynapthic currents (EPSPs) in the CA1 region in hippocampal slices ex vivo at 4-6 h but not 2 h after withdrawal (59). Under similar conditions, Ca2+ channel activity and NMDA receptor-mediated EPSPs were reported to be potentiated in the hippocampus (60). NMDA receptor antagonists (MK-801 and CGP-39551) inhibit the development of tolerance to ethanol (measured as a decrease of sleeping time) even if given 120 min after the daily alcohol injection (61). This indicates that NMDA receptor activation is probably involved in plastic changes following chronic ethanol-induced receptor adaptation. Consistent with the role of the NMDA receptor in alcohol tolerance is the finding that the partial agonist d-cycloserince enhances tolerance to the motor-impairing effect of ethanol in rats (62).

In the alcohol "craving"/relapse model discussed above for acamprosate, memantine given via osmotic mini-pumps at a dose leading to steady-state serum levels similar to those seen in clinical practice also greatly inhibited alcohol consumption during the relapse phase (63). This suggests that The blockade of NMDA receptors inhibits some aspects of alcohol dependence. There are several possibilities as to how NMDA receptor antagonists exert this effect: (1) produce alcohol-like effects because memantine and other NMDA receptor antagonists show partial or full generalization to the ethanol cue in rats trained to discriminate ethanol (64,65); (2) block recognition of the alcohol cue; (3) inhibit association of environmental cues with alcohol use (4) directly interfere with the reinforcing properties of ethanol. Another study examined the effects of the NMDA receptor antagonist AP5 on alcohol preference. Following intracerebroventral (icv) infusion of AP5, ethanol preference was reduced in untrained but not trained rats (66). This indicates that NMDA receptor antagonists may interact with the association but not the recognition of the alcohol cue or inhibition of its hedonic effects.

There is also increasing evidence that NMDA receptors may participate in the execution of pathological changes in Wernicke-Korsakoff syndrome. In animals, thiamine deficiency produces a pattern on neuronal damage resembling that found in Wernicke-Korsakoff syndrome (e.g., in the thalamus). In such animals, an increase in extracellular glutamate in the brain is observed and NMDA receptor antagonists prevent both neurodegeneration and the increase in glutamate (67). In a goat version of this model, there is a decrease in the NMDA receptors in the motor cortex, probably reflecting damage to neurons rich in this receptor type (68). Interestingly, in traumatic brain injury in rats, alcohol exerts potent neuroprotective potential at moderate doses (1-2.5 g/kg), whereas at higher doses, aggravation of injury was observed (69). This later effect is possibly related to ethanol-induced hemodynamic and respiratory depression.

5. INTERACTION OF ACAMPROSATE WITH THE GLUTAMATERGIC/NMDA RECEPTOR SYSTEM

Acamprosate has been reported to bind to a specific, spermidine-sensitive site (Kd of 120 |M and a Bmax of 450 pmol/mg of protein) and modulate NMDA receptor function by acting as a partial coago-nist. Thus, low concentrations enhanced functional [3H]-MK-801 binding (under nonequilibrium conditions) when receptor activity was low (i.e., in the absence of added agonists), whereas higher concentrations (>100 |M) were inhibitory under high levels of receptor activation (i.e., in the presence of 100 |M glutamate and 30 |M glycine) (70). Interestingly, only the inhibitory effects of acamprosate were seen in rats made dependent on alcohol following 10 d of ethanol inhalation (71). Similar effects were seen in rats that had received 400 mg/kg/d of acamprosate in their drinking water with or without concurrent ethanol inhalation for 10 d (71). These results suggest that the inhibitory effects of acam-prosate are more important for its therapeutic effects. However, a recent patch-clamp study showed no effect of acamprosate (0.1-300 |M) on NMDA- or glutamate-induced currents in primary cultured cerebellar granule cells under control conditions or in the presence of spermine and no modification of the potency of ethanol as an NMDA receptor antagonist in these cells (72). However, this study did report reversal of polyamine potentiation in a subset of cultured striatal neurons, although, again, no influence on the potency of ethanol was seen in these cells. Similarly, another recent study reported extremely weak effects of acamprosate on NMDA receptor in cultured hippocampal neurons and NR1a/2A and NR1a/2B receptors expressed in Xenopus oocytes or HEK-293 cells (all IC50's > 100 | M) and no increase in potency of acamprosate following in vitro exposure of neurons to ethanol for 48 h (73). This study was also unable to show any interaction of acamprosate with the polyamine site or influence on agonist affinity. However, in this same study, acamprosate produced similar increases in NR1 and NR2B receptor expression in vivo to those seen following acute treatment with (+)MK-801 and memantine, indicating that acamprosate may produce changes in the CNS that are similar to those seen following NMDA receptor antagonists and that these changes may, in turn, underlie the effects of both kinds of drugs in the treatment of alcohol abuse.

The nucleus accumbens (NAc) is a brain region thought to mediate ethanol reinforcement. Acam-prosate (300 | M) has been reported to selectively increase the NMDA receptor-mediated component of EPSCs recorded from NAc core neurons in vitro with no effect on resting membrane potential or the AMPA receptor-mediated component (74). In the same preparation, acamprosate had little effect on postsynaptic GABAa receptors (i.e., monosynaptic IPSCs) but significantly decreased paired-pulse inhibition (PPI) in the presence of D-APV and CNQX. This latter finding was taken to imply that acamprosate may concomitantly enhance NMDA receptor-mediated excitatory transmission and disin-hibit NAc core neurons by blocking presynaptic GABAB receptors (74). A similar selective enhancement of NMDA receptor-mediated transmission was reported for the Schaffer collateral input onto CA1 neurons recorded in hippocampal slices (75). In this study, the effects of acamprosate seemed to be mediated directly via actions at postsynaptic NMDA receptors because acamprosate (100-1000 |M) dramatically increased inward current responses in most CA1 neurons to exogenous NMDA applied in the presence of TTX to block synaptic transmission. In contrast, higher concentrations of acamprosate (100-1000 |M) inhibited both inhibitory and excitatory transmission in the neocortex in vitro and blocked responses to iontophoretic excitatory amino acids both in vitro and in vivo (76).

Cotreatment with acamprosate (400 mg/kg/d) for 4 wk. has also been reported to block the withdrawal-induced increase in glutamate increase in the nucleus accumbens microdialysate in rats made dependent on ethanol by inhalation (77). One of the known behavioral actions of accamprosate is to decrease hypermotility during alcohol withdrawal. It therefore, seems plausible that glutamate release in the nucleus accumbens could underlie hyperexcitability during ethanol withdrawal. More recent results from the same group indicate that this effect could be secondary due to an increase in the levels of taurine or GABA (78,79). Although acamprosate (200 mg/kg, ip) caused an increase in c-Fos expression in the hippocampus (CA1) and the cerebellum in drug-naive animals, the same dose of acamprosate reduced elevated c-Fos mRNA levels in these structures following 24 h of ethanol withdrawal in alcohol-dependent rats or ip administration of the convulsant pentylenetetrazole (80). This finding also supports the notion that acamprosate elicits its preventive effect on relapse by reducing the hyperexcitability of central neurons during withdrawal, following long-term ethanol consumption.

6. EFFECTS OF ACAMPROSATE ON MORPHINE DEPENDENCE

Acamprosate attenuated the expression of sensitized locomotor activity and dopamine release in the nucleus accumbens following daily injection of morphine (10 mg/kg, sc) for 14 d; however, it did not have any consistent effect on either iv heroin self-administration during the maintenance phase or the relapse to heroin seeking in a drug-free state induced by priming injections of heroin or a foot-shock stressor after a 5- to 8-d period of extinction (81). This, in turn, implies that acamprosate may not have similar beneficial effects to NMDA receptor antagonists in other forms of drug-seeking behavior such as heroin or cocaine abuse (see ref. 15). This is supported by the finding that although acamprosate has similar effects to NMDA receptor antagonists in animal models of alcohol abuse, it does not cross-discriminate for the high-affinity uncompetitive NMDA receptor antagonist (+)MK-801 and, in contrast to this agent, also does not cross-discriminate for the ethanol cue (6,82,87). Moreover, it is important to note that acamprosate lacked both reinforcing properties and discriminative stimulus properties similar to d-amphetamine or pentobarbital in rhesus monkeys, suggesting that it has little or no abuse potential in it's own regard (83,86). Similarly, acamprosate (170 and 320 mg/kg, ip) did not cross-discriminate for morphine or amphetamine in rats (84); however, these data are questionable because the sodium salt of acamprosate was used in this study, which barely crosses the blood-brain barrier. In contrast, acquisition of conditioned place aversion by naloxone 5-6 d after the subcutaneous implantation of a 75-mg morphine pellet was completely inhibited by the pretreatment with acamprosate (200 mg/kg, ip) prior to conditioning, indicating that ethanol and opiates share similar properties in the neuronal mechanisms of conditioned withdrawal and craving (85).

7. CONCLUSIONS AND PERSPECTIVES

In summary, recent studies have shown that acamprosate primarily interacts with the glutamatergic system. Chronic alcohol use leads to an upregulation of this system (i.e., enhanced glutamate release, less glutamate reuptake, and alterations at NMDA receptors). Although acamprosate probably has not direct antagonistic effect at NMDA receptors, it counteracts alcohol-induced alterations in the glutamatergic system. It is important to note, however, that not all patients benefit from acamprosate. The task for the future will be to identify, prior to medication, those who will respond to acamprosate and those who will not respond to this treatment. Interdisciplinary research in animals and humans has shown that relapse for alcohol involves multiple pathways with different neurobiological mechanisms. The first pathway may trigger relapse as a result of the mood-enhancing and appetitive effects of alcohol intake (alcohol-associated positive-mood states). The second pathway may solicit relapse by negative motivational states, including conditioned withdrawal and stress (alcohol-associated negative-mood states). Because acamprosate interacts primarily with the glutamatergic system and might affect alcohol-associated negative-mood states, it is hypothesized that acamprosate is most effective among individuals with alcohol-associated negative-mood states. In this respect, preclinical studies will be useful, and for this purpose, reinstatement of alcohol-seeking behavior in long-term alcohol-experienced rats will be studied—a procedure recognized as a model of craving and relapse. Reinstatement will be tested in response to different classes of craving and relapse inducing stimuli. In particular, alcohol-associated positive- and negative-mood states will be induced and acamprosate treatment will be matched to specific relapse-inducing stimuli. This procedure will allow us to identify responders to acamprosate treatment. This knowledge can subsequently be transferred to the human situation and will lead to better treatment success.

Beat The Battle With The Bottle

Beat The Battle With The Bottle

Alcoholism is something that can't be formed in easy terms. Alcoholism as a whole refers to the circumstance whereby there's an obsession in man to keep ingesting beverages with alcohol content which is injurious to health. The circumstance of alcoholism doesn't let the person addicted have any command over ingestion despite being cognizant of the damaging consequences ensuing from it.

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