Relations between muscimol, quinuclidinyl benzilate and nicotine binding sites in brain after very long treatment with ethanol in rats

Alcohol and the dopamine system

The mesolimbic dopamine pathway plays a major role in drug reinforcement and is likely involved also in the development of drug addiction. Ethanol, like most addictive drugs, acutely activates the mesolimbic dopamine system and releases dopamine, and ethanol-associated stimuli also appear to trigger dopamine release. In addition, chronic exposure to ethanol reduces the baseline function of the mesolimbic dopamine system. The molecular mechanisms underlying ethanol´s interaction with this system remain, however, to be unveiled. Here research on the actions of ethanol in the mesolimbic dopamine system, focusing on the involvement of cystein-loop ligand-gated ion channels, opiate receptors, gastric peptides and acetaldehyde is briefly reviewed. In summary, a great complexity as regards ethanol´s mechanism(s) of action along the mesolimbic dopamine system has been revealed. Consequently, several new targets and possibilities for pharmacotherapies for alcohol use disorder have emerged.

The effect of chronic ethanol consumption on muscarinic receptors in rat brain

Ethanol (15% v/v) was administered in the drinking water to male Wistar rats over period of 3 months. Binding properties of muscarinic receptors were studied in synaptosomes from selected brain areas using [(3)H]quinuclidinyl benzilate and its displacement by the selective antagonist, pirenzepine and the agonist, carbachol. Dissociation constants (K(d)) of all three ligands in the cerebral cortex, hippocampus and striatum of ethanol-treated groups did not differ from those in controls. Density of [(3)H]quinuclidinyl benzilate binding sites in the cortex of ethanol-treated animals was approx. 50% higher than in controls (2.06 +/- 0.2 and 1.32 +/- 0.2 pmol/mg of protein respectively, mean +/- SD, n = 6, P < 0.001). This was largely attributable to an increase in M(1) binding sites as shown by pirenzepine displacement studies. In the hippocampus and striatum binding capacity of muscarinic receptors was not affected by ethanol treatment. Synthesis of acetylcholine in cerebral cortex prisms from ethanol-treated animals was not inhibited under resting conditions, but stimulation of synthesis by high K(+) concentration was significantly altenuated by comparison with controls. These results suggest that chronic ethanol consumption induces changes in cholinergic neurotransmission in selected brain areas.

Voluntary ethanol intake increases extracellular acetylcholine levels in the ventral tegmental area in the rat

AIMS

Concurrent use of ethanol and nicotine (tobacco) is often seen in human beings. In previous animal experiments, we have demonstrated that nicotinic acetylcholine receptors, especially alpha-conotoxin MII and mecamylamine sensitive receptors located in the ventral tegmental area may be involved in the stimulatory, dopamine enhancing, and rewarding effects of ethanol in rodents. Ethanol may exert these effects via direct interaction with nicotinic acetylcholine receptors and/or indirectly via enhancement of extracellular acetylcholine levels in the ventral tegmental area. The present experiments investigated a possible indirect effect of ethanol in stimulating the mesoaccumbal dopamine system.

METHODS

Neurochemical effects of voluntary ethanol intake on extracellular ventral tegmental acetylcholine and accumbal dopamine levels were measured by means of in vivo microdialysis with a two-probe approach in freely moving rats.

RESULTS

Obtained data indicate that voluntary ethanol intake ( approximately 0.7 g/kg/h) leads to an increase of extracellular acetylcholine levels in the ventral tegmental area, and an almost time-locked increase of dopamine levels in the nucleus accumbens. A positive correlation between the ventral tegmental acetylcholine levels and ethanol intake as well as preference was also observed.

CONCLUSION

The present results suggest that voluntary ethanol intake enhances extracellular ventral tegmental acetylcholine that may interact with nicotinic acetylcholine receptors, possibly alpha-conotoxin MII sensitive receptors, localized in the ventral tegmental area that subsequently may stimulate dopamine overflow in the nucleus accumbens.

Chronic infusion of nicotine can increase operant self-administration of alcohol

Effects of nicotine, administered by continuous infusion via osmotic minipumps, were studied on the operant self-administration of alcohol by rats, using a variable interval (15 s) schedule, and measuring the acquisition, maintenance, extinction and reinstatement of responding for alcohol. Doses of nicotine of 0.25, 1.25 and 7.5 mg/kg/24 h had no significant effects on the maintenance of responding for alcohol, but 5 mg/kg/24 h nicotine resulted in a significant increase in responding on the lever delivering the reward when water was substituted for the alcohol, indicating delayed extinction of responding. During infusion of 2.5 mg/kg/24 h nicotine, responding was significantly greater over the "sucrose-fading" training sessions, during acquisition of responding, when mixtures of alcohol and sucrose were provided as reward. When minipumps infusing 2.5 mg/kg/24 h nicotine were implanted after the alcohol responding had been acquired, the responding for alcohol increase during the first week of nicotine infusion, but corresponding nicotine infusion doses of 0.25, 1.25 and 7.5 had no significant effects. The results indicate that nicotine can increase operant responding for alcohol and this is crucially dependent on the dose of nicotine and the time of testing. The results have implications for the frequently encountered dependence on the combination of alcohol and nicotine.

Nicotinic mechanisms involved in the dopamine activating and reinforcing properties of ethanol

Ethanol shares with all major dependence producing drugs the ability to activate brain mesocorticolimbic dopamine neurons, an important part of the brain reward systems. This dopamine activation may be involved in mediating the positive reinforcing effects of ethanol. The mechanisms of action of ethanol in its activation of this dopamine system remain, however, to be elucidated. A selective pharmacological interference with these mechanisms may offer a possibility to reduce the reinforcing properties of ethanol without simultaneously interfering with the reinforcing properties of natural rewards. Ethanol has been shown to directly influence the function of various ligand-gated ion-channels. Several of these are located on or nearby mesocorticolimbic dopamine neurons. One such receptor is the nicotinic acetylcholine receptor (nAChR). The present article reviews a series of investigations aimed at investigating whether nAChRs are involved in the dopamine activating and reinforcing properties of ethanol. To this end acute and chronic behavioral and neurochemical experiments were performed in mice and rats. The results obtained indicate that central nAChRs in the ventral tegmental area are involved in mediating the mesolimbic dopamine activating and reinforcing effects of ethanol. Furthermore, the ethanol-induced activation of these receptors is probably indirect, subsequent to a primary interference of ethanol in the nucleus accumbens. Moreover, subchronic nicotine treatment enhances the reinforcing and dopamine activating properties of ethanol. This long-term effect may, however, derive from autonomic adaptations in response to intermittent blockade of peripheral nAChRs (rather than from intermittent stimulation of central receptors), and appears to be associated with development of a disinhibitory behavior that could involve also other neurotransmitters, e.g. serotonin. Taken together, these findings could provide a neurobiological explanation to the often observed co-abuse of nicotine and ethanol in man. Furthermore, since the behavioral models applied previously have predicted therapeutic drug effects in the clinic, the results suggest that selective blockade of the ventral tegmental nAChRs that are involved in the above effects may provide a new pharmacological alternative in the treatment of alcoholism.

Muscarinic and nicotinic receptor changes in the cortex and thalamus of brains of chronic alcoholics

The cholinergic system was studied in the cortical and thalamic brain tissues obtained at autopsy from 21 chronic alcoholics and 20 controls. The age related decrease in choline acetyltransferase (ChAT) activity observed in the thalamus of control brains was not found in the corresponding brain areas of chronic alcoholics. A significant decrease in the number of muscarinic receptor binding sites was observed with age in the frontal cortex of both controls and chronic alcoholics when analysed with the nonselective muscarinic antagonist [3H]quinuclidinyl benzilate ([3H]QNB). A significant increase in the number of muscarinic receptor binding sites was observed in the thalamus of controls but not in chronic alcoholics. When the subjects were divided into young (19-57) years, and old (59-84 years) chronic alcoholics marked losses in the total number of muscarinic receptors as well as M1 and M2 receptor subtypes were found in the thalamus of the old group of alcoholics compared to age-matched controls. A coupling of muscarinic receptors to G proteins was observed in thalamic tissues from both controls and chronic alcoholics. Guanylyl-imidodiphosphate (Gpp(NH)p) induced a steepening and rightward shift of the carbachol/[3H]QNB displacement curves performed in membrane preparations of the thalamus from both controls and chronic alcoholics. The number of high affinity nicotinic binding sites in the frontal cortex and thalamus did not differ significantly between controls and chronic alcoholics.

Cholinergic mechanisms in physical dependence on barbiturates, ethanol and benzodiazepines

The aim of this review is to summarize the effects of acute and chronic treatment with barbiturates, ethanol and benzodiazepines on cholinergic mechanisms in the brains of experimental animals. A single dose of each of these substances reduces the turnover of ACh in the brain. Long-term treatment has the opposite effect; complicated interactions including decreased content of ACh are induced. Barbiturates have been shown to bind stereospecifically to muscarinic and nicotinic receptors in the brain, but this has not been observed for ethanol or the benzodiazepines. The effects on the cholinergic system are affected by the length of treatment and choice of treatment regimen. No effect on cholinergic parameters, such as muscarinic receptors, in the brain is observed on withdrawal of ethanol or barbiturate treatment when the animals are still tolerant towards the substances. The increase in the number of muscarinic receptors observed in several brain regions on withdrawal is seen as a sign of cholinergic supersensitivity. The number of receptors returns to normal when abstinence convulsions have occurred. The assumption of a cholinergic influence is supported by the finding that atropine, given as a single dose on the day of withdrawal of barbital, can prevent the muscarinic receptor changes. Furthermore, long-term barbital or ethanol treatment can induce permanent persistent changes in the cholinergic system in the brain. Cognitive defects and a significant permanent reduction in the content of ACh can be measured in rats which have had long-term barbital treatment. Similarly, a reduced number of muscarinic receptors has been measured in different brain regions of chronic alcoholics. Accumulating data support the role of the cholinergic system in expressing symptoms of physical dependence on barbiturates, ethanol and benzodiazepines as well as in the permanent long-term effects observed after end of treatment.

Modulation of benzodiazepine agonist and inverse-agonist receptor binding by GABA during ethanol withdrawal

1. The present study examined the capacity of GABA to modulate flunitrazepam and Ro15-4513 binding to putative GABAA receptors. Binding was measured in distinct brain regions both before and during selected periods of withdrawal from ethanol. 2. Rats were fed a nutritionally complete liquid ethanol (4.5% w/v) diet for 4 days and at various times after the last dose of ethanol (0, 12, 24, & 72 hr), rats were sacrificed and extensively washed brain membrane fractions were prepared. 3. Competitive inhibition of 3H-flunitrazepam binding by either flunitrazepam or Ro15-4513 (10(-10)M to 10(-7)M) was performed in the absence and presence of GABA (10(-5)M). In the presence of GABA, the apparent affinity for flunitrazepam was increased approximately 1.7 fold and the apparent affinity for Ro15-4513 was decreased by 1.7 fold. 4. No alteration in the capacity of GABA to modulate flunitrazepam or Ro15-4513 affinity (e.g. GABA-shift) was observed in cortical membrane preparations either 12 or 72 hr following ethanol cessation. 5. Further, no changes in GABA-modulation of flunitrazepam binding was evident 0, 12, 24, or 72 hr after the last ethanol dose in membranes prepared from cortex, hippocampus or cerebellum. 6. Therefore, results from the present study indicate that the capacity of GABA to modulate receptor affinity for benzodiazepine agonists and inverse-agonists in rat cortex, hippocampus or cerebellum is not altered during withdrawal from chronic ethanol.

Reduced behavioral responses to intranigral muscimol following chronic ethanol

Increased biochemical measures of GABA activity are observed after acute administration of ethanol and decreased activity has sometimes been observed after chronic ethanol exposure. Since chronic alterations in neurotransmitter activity may result in changes in receptor function, it is possible that changes in GABA-receptive neurons may accompany chronic ethanol treatment. In the present study we examined the incidence of muscimol-induced motor behaviors in ethanol-naive and chronic ethanol-treated animals. Male Sprague-Dawley rats received bilateral cannula implants into substantia nigra pars reticulata for subsequent administration of muscimol or saline. After recovery from surgery, rats received chronic treatment in ethanol-vapor inhalation chambers for 15 days. Animals were then removed from the chambers and examined 10 hours after removal. Muscimol resulted in a general increase in motility in both control and ethanol-treated animals. Animals withdrawn from chronic ethanol exposure, however, exhibited significantly less muscimol-stimulated, repetitive 9 Hz movements. These results suggest that GABA receptive cells within the substantia nigra or its vicinity may be functionally less responsive to GABAergic stimulation after chronic ethanol administration.

Ethanol potentiates and blocks NMDA-activated single-channel currents in rat hippocampal pyramidal cells

Single-channel currents activated by N-methyl-D-aspartate (NMDA) were characterized using the outside-out patch clamp technique in cultured hippocampal cells from the rat. Several conductance states were observed, and the main one of 47 pS was further analyzed for channel lifetime and frequency. Open times decreased with hyperpolarization of the membrane. In view of recent evidence linking NMDA receptors to central nervous system processes such as learning and memory and ethanol (EtOH) tolerance, the effects of EtOH (0.01-1%, v/v, or congruent to 1.74-174 mM) were studied in this preparation. Two effects of EtOH could be discerned: (i) at low concentrations (1.74-8.65 mM) an increase in the probability of opening (p open) of the NMDA-activated channel currents, without change in the mean channel open time, and (ii) at higher concentrations (86.5-174 mM) a decrease in p open with a concomitant decrease in the mean open time. It is suggested that EtOH, even at rather low concentrations, may affect important brain functions.

Hippocampal and cerebellar beta-adrenergic receptors and adenylate cyclase are differentially altered by chronic ethanol ingestion

Chronic ethanol ingestion by mice resulted in the loss of high-affinity beta-adrenergic agonist binding sites and a significant decrease in activation of adenylate cyclase by guanine nucleotides and beta-adrenergic agonists in the hippocampus, although no significant change was noted in the total number of beta-adrenergic receptors, as defined by the binding of the antagonist [125]iodocyanopindolol. In cerebellum, chronic ethanol ingestion resulted in a 16% decrease in the total concentration of beta-adrenergic receptors and in a decrease in the affinity for agonist of the high-affinity beta-adrenergic agonist binding sites. However, neither the amount of the high-affinity agonist binding sites nor the activation of adenylate cyclase by agonist was affected. The different responses to ethanol in hippocampus and cerebellum may result from quantitative differences in distribution of beta 1- and beta 2-adrenergic receptors in the tested brain areas and/or differential effects of ethanol on stimulatory guanine nucleotide binding protein in these brain areas.

Relation between voluntary ethanol intake in rats and changes in striatal muscarinic binding sites seen after induction of stable ethanol intake by an intermittent ethanol treatment

Male rats were treated with ethanol (2.0 g/kg i.p.) once a week for approximately a year. They also had a choice between an ethanol solution (10%) and water as drinking fluid for 24 h prior to the injection. During the following evaluation period lasting around 40 weeks the rats had a continuous choice between ethanol and water as the drinking fluid. The ethanol concentration was varied with 10% as a standard. At sacrifice, muscarinic binding sites in the brain were determined. During the evaluation period the daily dose of ethanol varied considerably but the intake in each individual was remarkably stable independent of offered ethanol concentration. A highly significant regression was found between voluntary ethanol intake and muscarinic binding sites in the striatum (b = -0.003, r = -0.87, df = 8, P less than 0.005).

Influence of age on effects induced by intermittent ethanol treatment on the ethanol drinking pattern and related neurochemical changes in the rat

Male rats were treated with one ethanol (2.0 g/kg i.p.) or saline injections once a week for 50 weeks. During this treatment period the rats had in addition access to ethanol (10% in drinking fluid) as a choice against water for 24 h prior to the injection. During the following evaluation period, animals had a continuous choice between ethanol and water and the concentration of the ethanol solution increased every 3rd week from 5 to 10, 15 and 25%, with 10% as a reference tested between the other concentrations. The animals were killed after an abstinence of 4 weeks, whereupon the concentrations of noradrenaline (NA), dopamine (DA), serotonin (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) were determined in the frontal cortex. In the remaining cerebral cortex, activity of monoamine oxidase, reuptake of NA and stimulated inositol phospholipid (PI) breakdown was also determined. Muscarinic binding sites were determined in the striatum. During treatment, saline injected rats had a constant voluntary 24 h ethanol intake. There was a decrease in the corresponding intake in the animals given the ethanol injections. The diminishing of the intake was more marked in rats starting treatment at an age of 19.4 weeks when compared to rats starting at an age of 5.4 weeks. In the evaluation period the ethanol intake was fairly constant for all groups. However, the regressions between intake of the reference concentration when plotted against the different tested concentrations were most marked in the group where ethanol injections started at an early age. In the total material there were significant F-values when concentrations of NA, 5-HIAA, 5-HT/5-HIAA in the cortex and muscarinic binding sites in the striatum were tested. Age could not be excluded as a contributing factor, but for muscarinic binding sites in the striatum, concentrations of DA and 5-HIAA in the cortex, and potassium stimulated PI breakdown in the cortex significant regressions with voluntary ethanol intake as dependent variable could be established. Since these intakes are stable, a causal relation with dependence may be involved.

Chronic ethanol or nicotine treatment results in partial cross-tolerance between these agents

Female DBA/2Ibg mice were treated chronically (21 days) with ethanol- or dextrin-containing liquid diets or infused chronically with nicotine (8 mg/kg/h) or saline for 10 days. The responses of these animals to challenge doses of ethanol (2.5 g/kg) or nicotine (1 or 2 mg/kg) were measured using a test battery consisting of respiration rate, acoustic startle response, Y-maze crosses and rears, heart rate and body temperature. Chronic ethanol-treated animals were tolerant to the effects elicited by a challenge dose of ethanol on four of the six measures and were cross-tolerant to nicotine's effects on the acoustic startle test. Chronic nicotine-treated animals were tolerant to nicotine's effects on five of the six measures and cross-tolerant to ethanol's effects on heart rate and body temperature. Thus, partial cross-tolerance between ethanol and nicotine exists. Chronic nicotine treatment resulted in significant increases in L-[3H]-nicotine binding in six of seven brain regions and in alpha-[125I]-bungarotoxin binding in three of seven brain regions. Chronic ethanol treatment failed to alter the binding of either ligand. Therefore, the cross-tolerance that develops between ethanol and nicotine is not totally dependent on alterations in the number of brain nicotinic receptors.

Acute and chronic ethanol treatments alter GABA receptor-operated chloride channels

The effect of ethanol exposure in vitro on the GABA receptor-operated chloride channel was evaluated by monitoring 36Cl- influx in a membrane vesicle suspension (microsacs) prepared from mouse cerebellum. These experiments directly demonstrate ethanol augmentation of muscimol-stimulated chloride flux. DBA/2J mice were made tolerant to and dependent on ethanol by administration of an ethanol containing liquid diet for 7 days. Exposure to physiologically relevant concentrations of ethanol (10-45 mM) in vitro potentiated muscimol stimulation of 36Cl- uptake in control (pair-fed) membranes, but had no effect on cerebellar microsacs from tolerant/dependent mice. Muscimol stimulation of 36Cl- uptake was not different for pair-fed and ethanol-treated mice. Augmentation of muscimol-induced 36Cl- flux by in vitro ethanol was abolished by a single 4 g/kg injection of ethanol. This "acute tolerance" occurred within 5 min and disappeared within 24 hr after ethanol treatment. The reduced sensitivity of ethanol treated (chronic and acute) mice to ethanol potentiation of muscimol stimulated 36Cl- uptake offers a biochemical correlate to the phenomenon of ethanol tolerance. Moreover, the findings suggest that this biochemical tolerance develops rapidly following a single hypnotic dose of ethanol.