Glutamate concentration in whole saliva and taste responses to monosodium glutamate in humans

It is universally accepted that saliva plays an important role in taste sensations. However, interactions between constituents of whole saliva and the five basic taste modalities are still poorly understood. The aim of the present study was to evaluate possible relationship between endogenous glutamate (Glu) levels in whole saliva and taste responses to a prototypic umami substance, monosodium glutamate (MSG; 0.03-10.0%). Rated intensity and pleasantness of MSG taste was studied in healthy volunteers divided into a high glutamate (HG) in saliva (HG; n = 19) and low glutamate in saliva (LG; n = 18) group based on the median split level of salivary Glu. The HG and LG group did not differ in terms of electrogustometric thresholds, rated intensity of the MSG samples and pleasantness of distilled water and the lower MSG concentrations (0.03-1.0%). Perceived intensity of water taste was significantly (P < 0.05) higher in the LG subjects. The LG group rated the higher MSG concentrations (3.0-10.0%) as more unpleasant (P < 0.01). The difference remained significant after controlling for a between-group difference in age. The present results suggest that individual differences in salivary Glu levels may alter hedonic responses to suprathreshold MSG concentrations.

Taste responses in patients with Parkinson's disease

OBJECTIVE

Preclinical studies indicate that dopaminergic transmission in the basal ganglia may be involved in processing of both pleasant and unpleasant stimuli. Given this, the aim of the present study was to assess taste responses to sweet, bitter, sour, and salty substances in patients with Parkinson's disease (PD).

METHODS

Rated intensity and pleasantness of filter paper discs soaked in sucrose (10-60%), quinine (0.025-0.5%), citric acid (0.25-4.0%), or sodium chloride (1.25-20%) solutions was evaluated in 30 patients with PD and in 33 healthy controls. Paper discs soaked in deionised water served as control stimuli. In addition, reactivity to 100 ml samples of chocolate and vanilla milk was assessed in both groups. Taste detection thresholds were assessed by means of electrogustometry. Sociodemographic and neuropsychiatric data, including cigarette smoking, alcohol consumption, tea and coffee drinking, depressive symptoms, and cognitive functioning were collected.

RESULTS

In general, perceived intensity, pleasantness, and identification of the sucrose, quinine, citric acid, or sodium chloride samples did not differ between the PD patients and controls. Intensity ratings of the filter papers soaked in 0.025% quinine were significantly higher in the PD patients compared with the control group. No inter-group differences were found in taste responses to chocolate and vanilla milk. Electrogustometric thresholds were significantly (p = 0.001) more sensitive in the PD patients.

CONCLUSIONS

PD is not associated with any major alterations in responses to pleasant or unpleasant taste stimuli. Patients with PD may present enhanced taste acuity in terms of electrogustometric threshold.

Effects of morphine on gene expression in the rat amygdala

Influence of morphine self-administration on gene expression in the rat amygdala was studied using rat genome DNA arrays U34A from Affymetrix. Animals were trained to self-administer morphine, each having two 'yoked' control animals, receiving passive injections of either morphine or saline. After 40 sessions of self-administration, amygdalae were removed, total RNA was isolated and used to prepare probes for Genechip arrays. The treatment was found to significantly change abundance of 29 transcripts. Analysis by means of reverse transcription real-time PCR showed significant changes in abundance of five transcripts: gamma protein kinase C (PKC), upstream binding factor 2 (UBF2), lysozyme, noggin and heat shock protein 70 (hsp70). After 30 days of forced abstinence from morphine self-administration, abundance of hsp70 and lysozyme returned to basal levels. Changes in abundance of UBF2 persisted, and abundance of three additional genes, namely nuclear factor I/A, gamma1 subunit of GABAA receptor and the neuronal calcium sensor 1, changed. Additionally, acute as well as chronic intraperitoneal morphine administration changed the abundance of PKC gamma, gamma1 subunit of GABAA and hsp70 genes.

Lack of ifenprodil anxiolytic activity after its multiple treatment in chronically ethanol-treated rats

AIMS

The purpose of this study was to assess the anxiolytic activity of ifenprodil in Warsaw high-preferring (WHP) and low-preferring (WLP) rats after chronic ethanol treatment.

METHODS

WHP and WLP animals, their paired-ethanol-naive groups and control Wistar rats were treated with ifenprodil (1.0 mg/kg, intraperitoneally) for 21 consecutive days. Anxiolytic activity was evaluated by using the two-compartment exploratory test. In addition, the locomotor activity paradigm was also assessed.

RESULTS

Ifenprodil did not affect this paradigm in all investigated groups. The ethanol treatment led to lowering of anxiolytic scores in WHP rats. Multiple ifenprodil administration showed an anxiogenic-like activity in both WHP- and WLP-ethanol-treated groups.

CONCLUSIONS

Our results suggest that, under some conditions, the role of ifenprodil in the treatment of alcoholism may be insufficient to support its use.

Novelty-seeking behaviour and operant oral ethanol self-administration in Wistar rats

The aim of the present study was to investigate the relationship between novelty-seeking behaviour and operant oral ethanol self-administration in Wistar rats. The open field and novel object test was used to assess novelty-seeking. Ethanol self-administration was initiated in an operant procedure where ethanol was introduced in the presence of sucrose. Eighteen out of 32 rats were successfully initiated to lever-press for 8% (v/v) ethanol. None of the parameters assessed in the open field (horizontal activity, rearings) or novel object test (number of contacts with an object, exploration time) differed between the initiated and non-initiated subjects. In addition, correlational analysis revealed that response to novelty did not predict individual differences in ethanol intake in the initiated rats. These results suggest that there is no relationship between novelty-seeking and operant ethanol self-administration in Wistar rats.

Taste responses in alcohol-dependent men

The aim of the present study was to compare taste responses to sweet, bitter, sour and salty solutions in male alcoholics and control subjects. The groups did not differ in terms of rated intensity or pleasantness of sucrose (1-30%), quinine (0.001-0.005%), citric acid (0.02-0.1%) and sodium chloride (0.18-0.9%) solutions. The proportion of sweet-likers was also similar in both groups.

Changes in ethanol preference by rats treated with gamma1 and gamma2 GABA(A) receptor subunit antisense oligodeoxynucleotides

Micro-injections (10 nmol/day over 5 days) of antisense oligodeoxynucleotides (aODNs) to gamma-aminobutyric acid A (GABA(A)) receptor alpha1 and gamma2 subunits reduce the mRNA for these subunits in rat brain. In this study, the effects of alpha1 and gamma2 subunit aODNs on rat alcohol preference were investigated. Reduction of the alpha1 subunit mRNA decreased, whereas reduction of the gamma2 subunit mRNA increased, ethanol intake in rats.

Neonatal treatment with 5,7-dihydroxytryptamine induces decrease in alcohol drinking in adult animals

It has long been suggested that serotonin (5-HT) neurotransmitter system activity is associated with ethanol (ETOH) intake and dependence. The authors studied the effects of neonatal 5,7-dihydroxytryptamine (5,7-DHT) lesions on voluntary alcohol drinking in adult Wistar rats. At 3 days after birth animals were pretreated with desipramine (DMI) and then given a bilateral injection of 5,7-DHT into lateral ventricles. Afterwards, the rats were kept under standard laboratory conditions until at least 2 months of age following which they were tested. 5,7-DHT induced a marked and permanent decrease in brain 5-HT content, measured in the prefrontal cortex, hippocampus and striatum, but did not modify noradrenaline content in these structures. Lesioned animals, both males and females displayed lower preference for ETOH than sham-lesioned animals. Total fluid intake was significantly higher in 5,7-DHT-lesioned than sham-lesioned rats. A significant decrease in body weight was observed in 5,7-DHT-treated rats. This effect was not caused by a significant change in food intake. Both groups showed high preference for a 0.1% saccharin. In conclusion, the present results demonstrated that neonatal treatment with 5,7-DHT evoked long-lasting neurochemical changes and reduction of ETOH intake in adult rats. Neonatally 5,7-DHT-treated rats may be considered as a suitable model in further research on the relationship between the function of central 5-HT system and alcohol intake and dependence.

Effects of a novel uncompetitive NMDA receptor antagonist, MRZ 2/579 on ethanol self-administration and ethanol withdrawal seizures in the rat

It has been repeatedly reported that NMDA receptors may contribute to ethanol-induced discriminative stimulus effects and withdrawal syndrome. However, the role of NMDA receptors in the reinforcing properties of ethanol remains unclear. The aim of the present study was to evaluate effects of the novel low-affinity, uncompetitive NMDA receptor antagonist, 1-amino-1,3,3,5,5-pentamethyl-cyclohexane hydrochloride (MRZ 2/579), on ethanol self-administration and ethanol withdrawal-associated seizures in rats. Both an operant (lever pressing for ethanol) and non-operant two-bottle choice setups were employed to initiate ethanol self-administration. In another procedure, forced treatment with high doses (9--15 g/kg/day) was used to induce physical dependence on ethanol. MRZ 2/579 delivered chronically by osmotic minipumps (9.6 mg/day, s.c.) did not alter either operant or non-operant ethanol drinking behaviour in a maintenance phase of ethanol self-administration. In contrast, repeated daily injections of the drug (5 mg/kg, i.p.) led to a progressive decrease in operant responding for ethanol. MRZ 2/579 (0.5--7.5 mg/kg, i.p.) and another low-affinity NMDA receptor antagonist, memantine (1--10 mg/kg, i.p.) dose-dependently suppressed ethanol withdrawal seizures with efficacies comparable with that of a standard benzodiazepine derivative, diazepam. The results of the present study indicate that: (i) intermittent administration of MRZ 2/579 may lead to a gradual decrease of operant responding for ethanol; and (ii) the group of low-affinity uncompetitive NMDA receptor antagonists may be an interesting alternative to benzodiazepines in the treatment of alcohol withdrawal.

Taste responses in sons of male alcoholics

The aim of the present study was to compare taste responses (intensity and pleasantness/unpleasantness) to sweet, bitter, sour, and salty solutions in sons of male alcoholics (SOMAs) and control subjects with no family history of alcoholism. In addition, responses to Coca-Cola flavour were evaluated in both groups. Unpleasantness of salty solutions was significantly enhanced and intensity of sour solutions tended to be higher in the SOMAs. There were no other differences between the groups. Thus, contrary to previous suggestions, genetically determined vulnerability to alcohol dependence may not be associated with altered responses to sweet substances. The present findings would rather suggest that increased aversive responses to salt taste may predict future development of alcohol dependence.

Animal model of ethanol abuse: rats selectively bred for high and low voluntary alcohol intake

The selectively bred alcohol-preferring and alcohol-non-preferring lines of rats have been used to study the biology of alcohol abuse and dependence. In our laboratory new lines of Wistar rats have been selectively outbread for 7 years and 19 generations for high and low ethanol intake (WHP--Warsaw High Preferring) and WLP--Warsaw Low Preferring respectively). After the first selection procedure, the highest scoring females and males were used initiate upward selection, while the lowest scoring pairs were used to initiate downward selection. Mated pairs were housed in breeding cages, pups were allowed to nurse for 3 weeks before weaning, then the pups of each litter were culled to the same-sex cage and allowed to mature until they were subjected to the selection procedure. In order to determine the alcohol intake and preference, the rats were individually housed in wire cages containing two graduated drinking tubes mounted at the front. During the entire investigation, the subjects had free access to standard lab chow (Bacutil, Poland). Ethanol solution was prepared from 95% stock ethanol and tap water. The animals were presented with 10% ethanol solution and water (two-bottle choice test). The drinking tubes were rotated daily to prevent position preference. Alcohol intake was calculated as average g/kg/day (absolute ethanol) while alcohol preference (in %) was calculated as the amount of alcohol consumed/total fluid x 100. Our results (17-19 generations) have shown that mean alcohol intake in WHP rats was higher than 5.0 g/kg/24 h ethanol, while WLP rats generally consumed less than 2.0 g/kg/24 h ethanol. Our results also showed that the total fluid intake in WHP rats slightly but not significantly higher as compared with WLP rats. Maximal ethanol consumption (in both lines) occurred during the natural dark phase three bungs (19.00-20.00 hrs, 23.00-02.00 hrs and 04.00-05.00 hrs). Interestingly, the intakes of high concentrations of sucrose and saccharin solutions were significantly higher in WHP than in WLP rats. Furthermore, the WHP rats reduced their alcohol and water intakes in the presence of 10% sucrose solution. Thus, it appears that high consumption of sweets may be a neurobiological factor promoting increased ethanol intake by WHP rats.

[Ototoxic mechanism of aminoglycoside antibiotics--role of glutaminergic NMDA receptors]

Recent studies have indicated that glutamatergic NMDA receptors in the cochlea may be involved in ototoxic effects of aminoglycosides in animal subjects. Aminoglycoside antibiotics enhance the function of NMDA receptors by interaction with a polyamine modulatory site. Accordingly, high doses of aminoglycosides may increase calcium entry through the NMDA receptor-associated channel and promote degeneration of hair cells and cochlear nerve fibers. In line with the above, a polyamine site antagonist, ifenprodil as well as a high-affinity channel blocker, dizocilpine (MK-801) attenuates ototoxic effects of aminoglycosides in rats. Notably, ifenprodil as well as low-affinity channel blockers (e.g. memantine and amantadine) may be safely used in humans. Taken together, the above findings seem to open new avenues of research on selective pharmacotherapy of aminoglycosides-induced ototoxicity in humans.

An opioid receptor antagonist, naltrexone, does not alter taste and smell responses in humans

Several studies have shown that an opioid receptor antagonist, naltrexone, decreases palatable food consumption. Naltrexone has also been reported to reduce ethanol intake in alcohol-preferring rodents and human alcoholics. The aim of the present study was to assess the effects of naltrexone on taste and smell responses in healthy male volunteers. Naltrexone did not alter intensity and pleasantness of sucrose, quinine, citric acid, sodium chloride, and ethanol taste. Similarly, ratings of olfactory stimuli (orange extract and ethanol) and Coca-Cola flavor were not influenced by the opioid antagonist. Our findings may indicate that: (i) naltrexone exerts marginal, if any, effects on gustatory and olfactory responses in humans; (ii) the drug does not alter orosensory responses to ethanol.

Bitter and sweet components of ethanol taste in humans

This study examined taste descriptions elicited by ethanol and by other tastants in humans. All subjects described 10% ethanol as bitter and approximately 30% of the subjects described it as sweet and/or sour. Highly significant correlations were found between sweetness of some sucrose solutions (0.6-1%) and intensity of the taste of ethanol. In another experiment, quinine (bitter) solutions were rated as similar to 10% ethanol taste and this effect was potentiated by the addition of sucrose. In contrast, citric acid (sour) tended to decrease similarity ratings when added to the quinine solutions. Taken together, these findings suggest that: (1) in humans ethanol tastes both bitter and sweet; and (2) the relationship between sucrose and ethanol intakes previously found in animals and humans may result, at least partially, from similar taste responses elicited by sucrose and ethanol.

[Perspectives of therapy of Alzheimer's disease]

Alzheimer's disease is the most common cause of memory disruption in elderly people. The main pathogenic factor of the disease is beta-amyloid protein, which may cause toxic damage of neurones. Other suggested pathogenic factors include an inflammatory process around the senile plaques, apoptosis and necrotic death of neurones, and, in consequence, changes in functioning of neurotransmitter systems. In this article the authors present the main directions in pharmacotherapy of Alzheimer's disease: causal therapy, which prevents the neurodegenerative changes and slows down the pathogenetic process, and symptomatic therapy. The aim of symptomatic therapy is to reduce memory disruption and psychiatric symptoms associated with the disease. Positive influence on cognitive processes is exerted by cholinergic drugs, e.g. the actually used inhibitors of acetylcholinesterase (rivastigmine, donepezil), the nootropic agents (piracetam, nefiracetam) and extracts of Gingko biloba. For treatment of the disease accompanying psychiatric symptoms (anxiety, depression, hallucinations, sleepness) the drugs with minimal influence on cognitive processes are recommended. Attempts at causal therapy are focussed on searching for the substances that can prevent the formation and toxicity of beta-amyloid (droloksifen, estrogens, agonists of muscarinic receptors M1), the cytotoxic influence of excitatory aminoacids (memantine, lamotrigine), calcium (nimodipine) and free radicals (selegiline, alpha-tocoferol), and the development of inflammatory process (non-steroidal antiinflammatory drugs). The new target of research is correction of deficits of nerve growth factor and neurotransmitters by intracerebral implantation of modified fibroblasts. Another way is prevention of the formation of amyloid plaques using appropriate antisense oligonucleotides.

Reinstatement of ethanol seeking in rats: behavioral analysis

The reinstatement model has been repeatedly used to study relapse to heroin- or cocaine-seeking behaviour in rats. The aim of the present study was to evaluate basic behavioral parameters of cue-induced reinstatement of ethanol seeking in a within-session paradigm. Rats were trained to respond for ethanol in an oral self-administration procedure where each lever press resulted in presentation of 0.1 ml of 8% ethanol from a liquid dipper. In the reinstatement paradigm operant behaviour was first extinguished for 20 or 60 min by switching the dipper off. Then, ethanol-associated stimuli were noncontingently delivered and reinstatement of responding was assessed. Deliveries of the empty dipper, i.e., visual/auditory cues only, did not result in any reinstatement. In contrast, 15 random presentations of the dipper containing either ethanol (4-8%; v/v) or water significantly reinstated ethanol seeking. In a control self-administration experiment responding dropped to nonsignificant levels when water was substituted for ethanol. The magnitude of reinstatement did not depend on the duration of the extinction phase. These results seem to indicate that in the present paradigm reinstatement of ethanol seeking is driven by a compound stimulus including the visual/auditory cues and some nonspecific sensory properties of liquid available in the dipper.

Operant responding for ethanol in rats with a long-term history of free-choice ethanol drinking

Wistar rats were allowed to drink ethanol in a two-bottle (water vs 2-8% v/v ethanol) and then in a three-bottle choice paradigm (water vs 8% ethanol vs 16% ethanol, v/v). After 7 months of free access to alcohol, the subjects were trained to respond for 8% ethanol in an operant procedure. No relationship was found between prior alcohol drinking and lever pressing for ethanol.

Development of alcohol deprivation effect in rats: lack of correlation with saccharin drinking and locomotor activity

The present study addressed the relationship between the parameters of saccharin drinking behaviour and locomotor activity in an open field environment and long-term alcohol self-administration. In a 22-day initiation phase, male Wistar rats were presented with increasing concentrations of ethanol (2-8%, v/v) in a choice with water. The rats were then given the choice between water and two ethanol solutions (8 and 16%). Every 28 days, ethanol was withdrawn for 5 days. The ethanol intake and the transient increase in ethanol consumption after each of six deprivation episodes (alcohol deprivation effect) was monitored and correlated with parameters of the subsequent saccharin drinking and open field tests. The total ethanol intake (g/kg/24 h) as well as the consumption of 16% ethanol were stable over time. However, the magnitude of the alcohol deprivation effect increased with the repeated deprivation episodes. None of the parameters measured in the open field or the saccharin drinking tests correlated with either ethanol consumption or the alcohol deprivation effect. These results suggest that (1) repeated episodes of ethanol deprivation may increase the magnitude of the alcohol deprivation effect, (2) neither saccharin drinking nor locomotor activity correlates with long-term ethanol drinking behaviour in rats.

The role of drug-paired stimuli in extinction and reinstatement of ethanol-seeking behaviour in the rat

Male Wistar rats were trained to respond for ethanol (30 min/day) in an oral self-administration procedure. A single lever press resulted in presentation of 0.1 ml of 8% ethanol from a liquid dipper. When responding for ethanol stabilised, reinstatement sessions started. In the 30-min reinstatement session, lever pressing was first extinguished for 20 min by switching the dipper off. Then, different kinds of stimuli were non-contingently delivered and reinstatement of lever pressing was assessed. Fifteen random (random time = 15 s) presentations of the dipper containing 8% ethanol potently reinstated ethanol-seeking. The reinstatement of lever pressing was immediate and most responses were emitted during the time needed for the first five presentations to occur. Presentations of the empty dipper or delivery of a non-specific stimulus (high-amplitude tone) did not produce any reinstatement. These results indicate that non-contingent presentations of the ethanol-associated stimulus complex may reinstate operant behaviour previously reinforced with ethanol.

Ethanol-reinforced behaviour in the rat: effects of naltrexone

It has been repeatedly reported that endogenous opioid pathways play an important role in ethanol drinking behaviour. In line with these findings, a non-selective opioid receptor antagonist, naltrexone, seems to reduce relapse rates in detoxified alcoholics. The aim of the present study was to evaluate the effects of naltrexone on (i) ethanol self-administration; (ii) extinction of responding for ethanol; (iii) reinstatement of ethanol-seeking induced by non-contingent presentations of ethanol-associated stimuli. Male Wistar rats were trained to lever-press for 8% ethanol in an operant procedure where ethanol was introduced in the presence of sucrose. The selectivity of naltrexone's actions was assessed by studying its effects on water-reinforced behaviour in separate control experiments. Acute injections of naltrexone (1 or 3 mg/kg) did not alter ethanol self-administration. Repeated treatment with naltrexone (3 mg/kg, before three consecutive self-administration sessions) progressively reduced ethanol intake. In the extinction procedure, acute administration of 3 mg/kg naltrexone suppressed responding previously reinforced with ethanol. Similarly, naltrexone (1-3 mg/kg) potently and dose-dependently inhibited reinstatement of ethanol-seeking produced by non-contingent deliveries of the liquid dipper filled with 8% ethanol. In the control experiments, lower doses of naltrexone (1-3 mg/kg) did not exert any effect on either reinforced or non-reinforced (extinction) lever-pressing for water. These results indicate that: (i) subchronic treatment with naltrexone leads to progressive reduction of ethanol self-administration; (ii) single doses of naltrexone may increase extinction and attenuate cue-induced reinstatement of ethanol-reinforced behaviour.

Effects of N-methyl-D-aspartate receptor antagonists on reinforced and nonreinforced responding for ethanol in rats

Results of several recent studies indicate that the discriminative stimulus effects of ethanol are related, at least partially, to ethanol-induced decrease in the N-Methyl-D-aspartate (NMDA) receptor function. The role of NMDA receptors in ethanol reinforcement remains still unclear. The aim of the present study was to evaluate the effects of two novel NMDA receptor antagonists in rats lever pressing for 8% ethanol in the oral self-administration procedure. In addition, the effects of the drugs on intensity of nonreinforced responding for ethanol (i.e., "experimental craving") were examined in the extinction procedure. To assess selectivity of the drugs' actions the same range of doses was tested in rats lever pressing for water (control experiments). A low-affinity, uncompetitive NMDA receptor antagonist, MRZ 2/579 (2.5-7.5 mg/kg) selectively and dose-dependently decreased ethanol self-administration. This compound exerted also selective effects on nonreinforced responding for ethanol with lower dose (2.5 mg/kg) increasing and higher dose (5 mg/kg) suppressing operant behavior in the extinction procedure. MRZ 2/579 (5 mg/kg) did not alter open field activity when given in combination with either saline or ethanol (0.5-1 g/kg). In contrast, a glycineB site antagonist, MRZ 2/576 (2.5-7.5 mg/kg) did not produce any selective effects on either reinforced or nonreinforced lever pressing for ethanol. The present results suggest that MRZ 2/579 may selectively suppress both ethanol self-administration and experimental ethanol craving.

Discriminative stimulus properties of ethanol in rats: studies on the role of nitric oxide

The present study examined the role of the L-arginine-nitric oxide pathway in mediation of the ethanol interoceptive (discriminative) cue. Adult male Wistar rats (n = 16) were trained to discriminate ethanol (1 g/kg, 10% v/v) from saline under a fixed-ratio 10 (FR10) schedule of sweetened milk reinforcement. A nonselective nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME; 10-540 mg/kg) did not substitute for ethanol. Similarly, a relatively selective neuronal NOS inhibitor, 7-nitroindazole (7-NI; 10-80 mg/kg), did not mimic the ethanol cue. However, both L-NAME and 7-NI produced significant reduction in the rate of operant responding. A nitric oxide precursor, L-arginine (100-500 mg/kg) neither substituted for nor antagonize the ethanol stimulus. Taken together, these results suggest that the L-arginine-nitric oxide pathway is not involved in mediation of the discriminative stimulus effects of ethanol in the rat.

Alcohol intake and brain [3H]muscimol binding sites in alcohol-preferring and non-preferring rats

The present study has employed in vitro autoradiography to determine the distribution and density of [3H]muscimol binding sites in the brains of alcohol high-preferring line of rats (WHP) and alcohol low-preferring line of rats (WLP). While the density of [3H]muscimol binding was found to be similar in the frontal cortex, caudate-putamen, nucleus accumbens, lateral and medial septum, the density of [3H]muscimol binding was lower in cingulate cortex of alcohol low-preferring rats as compared to alcohol high preferring rats. Moreover, the density of muscimol binding sites within this area was positively correlated with the intensity of ethanol consumption.

Ethanol discrimination in the rat: lack of modulation by restraint stress and memantine

There is a large body of experimental evidence that both stress and N-methyl-D-aspartate (NMDA) receptor antagonists may alter acute behavioural effects of ethanol. Notably, an uncompetitive, low-affinity NMDA receptor antagonist, memantine, has been recently claimed to possess anti-craving properties in rats with a long-term history of ethanol consumption. The aim of the present study was to assess the effects of restraint stress and memantine on the dose-response curve of ethanol discrimination. Rats were trained to discriminate 1 g/kg ethanol from saline in the two-lever drug discrimination procedure. When ethanol discrimination was acquired, the subjects were exposed to 30-min sessions of acute restraint stress, and different doses of ethanol (0.25, 0.5 or 1 g/kg) or saline were administered. In subsequent experiments the effects of memantine (2.25 or 4.5 mg/kg) on the cueing effects of ethanol were tested. Neither the stress sessions nor memantine influenced the ethanol discrimination dose-response curve. Moreover, the stress did not alter the rate of responding. However, both doses of memantine tended to increase the rate of responding when given in combination with lower doses of ethanol (0.25-0.5 g/kg). In contrast, 4.5 mg/kg memantine decreased the response rate when combined with 1 g/kg ethanol. These results suggest that: (1) pre-exposure to acute restraint stress or memantine does not affect the dose-response curve of ethanol discrimination; (2) memantine given in combination with low doses of ethanol may stimulate operant behaviour in the food-reinforced drug discrimination procedure.

Involvement of nicotinic acetylcholine receptors in the regulation of alcohol drinking in Wistar rats

The aim of the present study was to determine if nicotinic acetylcholine receptors (nAChRs) might be involved in the regulation of alcohol intake by Wistar rats. A non-selective nAChR agonist, nicotine, and a non-competitive nAChR antagonist, mecamylamine, were tested in alcohol-preferring Wistar rats maintained on a limited access (4 h/24 h) to ethanol (10%, v/v). In addition, the effects of nicotine and mecamylamine on intake of standard laboratory chow were studied in a separate control experiment. Nicotine (0.1-0.6 mg/kg, s.c.) decreased ethanol consumption, but had no effect on food intake. In contrast, mecamylamine (1-3 mg/kg, s.c.) did not alter ethanol drinking even at the dose (3 mg/kg) which significantly decreased food intake. These results suggest that activation of nAChRs may selectively reduce ethanol consumption in outbred Wistar rats.

Discriminative stimulus effects of ethanol: neuropharmacological characterization

Generally, compounds discriminated by animals possess psychotropic effects in animals and humans. As with many other drugs of abuse, strength of the ethanol discriminative stimulus is dose related. The majority of studies show that doses close to 1.0 g/kg are close to the minimum at which the discrimination can be learned easily. Substitution studies suggest that anxiolytic, sedative, atactic, and myorelaxant effects of ethanol all play an important role in the formation of its intercoeptive stimulus. Low doses of ethanol produce more excitatory cues, similar to amphetamine-like subjective stimuli, whereas higher doses produce rather sedative/hypnotic stimuli similar to those elicited by barbiturates. Substitution studies have shown that the complete substitution for ethanol may be exerted by certain GABA-mimetic drugs acting through different sites within the GABA(A)-benzodiazepine receptor complex (e.g., diazepam, pentobarbital, certain neurosteroids), gamma-hydroxybutyrate, and antagonists of the glutamate NMDA receptor. Among the NMDA receptor antagonists both noncompetitive (e.g., dizocilpine) and competitive antagonists (e.g., CGP 40116) are capable of substituting for ethanol. Further, some antagonists of strychnine-insensitive glycine modulatory sites among the NMDA receptor complex (e.g., L-701,324) dose-dependently substitute for the ethanol discriminative stimulus. On the other hand, neither GABA-benzodiazepine antagonists nor NMDA receptor agonists produce contradictory effects (i.e., reduce the ethanol discriminative stimulus). There is influence of a particular training dose of ethanol on the substitution pattern of different compounds. For example, 5-HT(1B/2C) agonists substitute for intermediate (1.0 g/kg) but not higher (2.0 g/kg) ethanol training doses. Discrimination studies with ethanol and drugs acting on NMDA and GABA receptors consistently indicate asymmetrical generalization. For example, ethanol is able to generalize to barbiturates and benzodiazepines, but neither the benzodiazepine nor barbiturate response generalizes to ethanol. Only a few drugs are able to antagonize, at least to some extent, the discriminative stimulus of ethanol (e.g., partial inverse GABA-benzodiazepine receptor antagonist Ro 15-4513 and the opioid antagonist naloxone). The ethanol stimulus effect may be increased (i.e., stronger recognition) by N-cholinergic drugs (nicotine), dopaminergic drugs (apomorphine), and 5-HT3 receptor agonists (m-chlorophenylbiguanide). Thus, the ethanol stimulus is composed of the several components, with the NMDA receptor and GABA(A) receptor complex being of particular importance. This suggests that a drug mixture may be more capable of substituting for ethanol (or block its stimulus) than a single compound. The ability of drugs to substitute for the ethanol discriminative stimulus is frequently, although not preclusively, associated with the reduction of voluntary ethanol consumption. The examples of positive correlation are gamma-hydroxybutyrate, possibly memantine and certain serotonergic drugs such as fluoxetine. However, it remains uncertain to what extent the discriminative stimulus of ethanol can be seen as relevant in the understanding of the complex mechanisms of dependence.

Ethanol-reinforced behaviour in the rat: effects of uncompetitive NMDA receptor antagonist, memantine

Ethanol has been reported to alter NMDA receptor-mediated biochemical and electrophysiological responses in vitro. The aim of the present study was to evaluate the effects of an uncompetitive NMDA receptor antagonist memantine, in animal models of alcoholism. Male Wistar rats were trained to drink 8% ethanol in a free-choice, limited access procedure. A separate group of animals was trained to lever press for 8% ethanol in an operant procedure where ethanol was introduced in the presence of sucrose. The selectivity of memantine's actions was assessed by studying its effects on food or water consumption in separate control experiments. Memantine (4.5-24 mg/kg) significantly, but not dose dependently, affected ethanol drinking in the limited access procedure. However, only 6 mg/kg memantine selectively decreased ethanol drinking. Memantine did not alter ethanol intake in rats trained to lever press for ethanol in the operant procedure. Only 9 mg/kg memantine reduced operant responding in the extinction procedure in the rats trained to lever press for ethanol. The same dose of memantine significantly reduced the operant behaviour of rats trained to respond for water. These results indicate that: (i) single doses of memantine only moderately and not dose dependently reduce alcohol drinking in the limited access procedure; (ii) memantine produces non-selective effects on operant behaviour in rats trained to lever press for ethanol in an oral self-administration procedure.

Studies on the role of nicotinic acetylcholine receptors in the discriminative and aversive stimulus properties of ethanol in the rat

The role of the nicotinic acetylcholine receptor (nAChR) in the discriminative and aversive stimulus effects of ethanol was studied in rats. In the operant drug discrimination procedure the rats were trained to discriminate between 1.0 g/kg ethanol and saline under the FR10 schedule of sweetened milk reinforcement. Neither the nAChR agonist, nicotine (0.1-0.6 mg/kg) nor the nAChR antagonist, mecamylamine (3.0-6.0 mg/kg) substituted for the ethanol stimulus. Moreover, mecamylamine (0.5-6.0 mg/kg) did not antagonise the ethanol stimulus. The cross-familiarisation conditioned taste aversion procedure was used as an alternative method to study stimulus resemblance between ethanol and nicotine. Six daily injections of nicotine (0.6 mg/kg) significantly decreased a subsequent ethanol-induced taste aversion conditioning. The aversive stimulus effects of ethanol were investigated with the conditioned taste aversion (CTA) paradigm. Mecamylamine (1.0-3.0 mg/kg) did not attenuate an ethanol-induced CTA. These results suggest that: (1) nAChRs are not primarily involved in the discriminative stimulus effects of ethanol when studied with the operant drug discrimination test; (2) nAChRs are not critically involved in the ethanol-induced CTA.

Discrimination of ethanol in rats: effects of nicotine, diazepam, CGP 40116, and 1-(m-chlorophenyl)-biguanide

The drug discrimination paradigm was used to evaluate the role of certain ligand-gated ion channels in the discriminative stimulus properties of ethanol. Rats were trained to discriminate ethanol (1.0 g/kg) from saline vehicle under the FR10 schedule of sweetened milk reinforcement. The discrimination of lower ethanol doses was enhanced by either the GABA(A) receptor positive modulator, diazepam (0.5 mg/kg), or nicotinic acetylcholine receptor agonist, nicotine (0.3 mg/kg). Neither diazepam nor nicotine produced any effect on the rate of responding. Both the NMDA receptor competitive antagonist, CGP 40116 (0.5 mg/kg) and the 5-HT) receptor agonist, 1-(m-chlorophenyl)-biguanide (5.0 mg/kg) enhanced the cueing properties of lower ethanol doses, but these effects were associated with a significant reduction in the response rate. The ethanol-like stimulus effects produced by diazepam or CGP 40116 were not influenced by 0.3 mg/kg nicotine. In contrast, CGP 40116 moderately enhanced the ethanol-like stimulus effects of diazepam. The present results show that: 1) pretreatment with nicotine, diazepam, CGP 40116 or 1-(m-chlorophenyl)-biguanide enhance the ethanol discrimination; 2) neither the GABA(A) nor the NMDA receptor complex alone is critically involved in the nicotine-induced enhancement of the ethanol discrimination; 3) NMDA receptor competitive antagonist and GABAergic benzodiazepine derivative may produce moderate additive effects in rats trained to discriminate ethanol.

Prior repeated exposure to a 5-HT3 receptor agonist does not alter the ethanol-induced conditioned taste aversion in rats

Several reports have indicated that the brain serotonergic 5-HT3 receptors are involved in at least some central effects of ethanol in rats. However, using an operant drug discrimination procedure, we have shown that these receptors are not primarily involved in the discriminative stimulus effects of ethanol. The aim of the present study was to further elucidate the role of 5-HT3 receptors in the formation of the ethanol-cueing effects in rats. To this purpose, a crossfamiliarization conditioned taste aversion (CF-CTA) procedure was used. Four daily injections of 1.5 g/kg ethanol (10% v/v) resulted in a significant attenuation of the subsequent ethanol-induced CTA. In contrast, four daily injections of the 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide (mCPBG; 50 microg per rat, i.c.v.) did not alter the subsequent ethanol-induced CTA. The 50 microg dose of mCPBG produced a marked CTA in a control experiment. These results taken together with some previous findings from our laboratory suggest that the brain 5-HT3 receptors do not play any crucial role in the mediation of the discriminative stimulus effects of ethanol.

Saccharin drinking rather than open field behaviour predicts initial ethanol acceptance in Wistar rats

This study examined the relationship between saccharin drinking, open field behaviour and ethanol drinking in Wistar rats. Correlational analysis revealed that both absolute saccharin drinking and an increase in total fluid intake in the presence of saccharin positively correlated with the initial acceptance of increasing ethanol concentrations in a two-bottle choice situation (2-8% v/v ethanol vs water). This relationship disappeared, however, during further weeks of ethanol drinking when ethanol was available in a three-bottle choice situation (8% ethanol vs 16% ethanol vs water). In contrast, none of the behavioural parameters measured in the open field test (forward locomotion, rearings, central entries, time in central area) correlated with subsequent ethanol consumption. These results indicate that saccharin drinking, rather than open field parameters, may predict subsequent ethanol intake during the initial period of exposure to low ethanol concentrations.

Prior exposure to MK-801 sensitizes rats to ethanol-induced conditioned taste aversion

Pretreatment with an uncompetitive NMDA receptor antagonist, dizocilpine [(+)MK-801; six daily injections of 0.1 or 0.2 mg/kg, i.p.] significantly enhanced subsequent 1.5 g/kg ethanol-induced conditioned taste aversion (CTA). In a control experiment, dizocilpine (0.05-.2 mg/kg) produced only a marginal CTA. Thus, pre-exposure to low, non-aversive doses of MK-801 may sensitize rats to the aversive stimulus effects of ethanol.

Study on the role of glycine, strychnine-insensitive receptors (glycineB sites) in the discriminative stimulus effects of ethanol in the rat

Several recent studies indicate that both competitive and noncompetitive NMDA receptor antagonists substitute for ethanol in a drug discrimination procedure. In the present study we examined compounds from another class of NMDA receptor antagonists--glycine, strychnine-insensitive, receptor (glycineB site) antagonists in rats trained to discriminate between i.p.-administered 1.0 g/kg ethanol (10% v/v) and saline. When the animals met the discriminative criteria, substitution tests were conducted with the noncompetitive NMDA receptor antagonist, memantine (3.0-12.0 mg/kg, i.p.) and selective, glycineB site antagonists--L-701,324 (0.3-3.0 mg/kg, i.p.) and MRZ 2/576 (0.1-10.0 mg/kg, i.p.). Memantine completely substituted for ethanol at the dose of 6.0 mg/kg, which significantly suppressed the rate of responding, L-701,324 substituted for ethanol at the dose of 3.0 mg/kg, which only tended to decrease the response rate. MRZ 2/576 produced maximal ethanol-appropriate responding (50%) at the dose of 5.0 mg/kg, which did not affect the rate of responding. Glycine (200-800 mg/kg, i.p.) did not antagonize the ethanol stimulus. These results indicate that glycine, strychnine-insensitive, site antagonists may induce some ethanol-like stimulus effects in the rat.

Interactions of ethanol with nicotine, dizocilpine, CGP 40116, and 1-(m-chlorophenyl)-biguanide in rats

The present study examined the effect of ethanol (0.25-1.0 g/kg, I.P.) alone and in combination with drugs affecting different ligand-gated ion channels on a horizontal locomotor activity of male Wistar rats. None of the drugs given alone affected the locomotor activity. Similarly, combining ethanol either with nicotine (0.1 or 0.6 mg/kg, S.C.) or the competitive NMDA receptor antagonist, CGP 40116 (0.5 mg/kg, I.P.) did not result in any significant changes in ambulation. On the other hand, a significant hyperadditive interaction between ethanol (0.5 or 1.0 g/kg) and the uncompetitive NMDA receptor antagonist, dizocilpine (0.1 mg/kg, I.P.) was found. Thus, a combined administration of ethanol and dizocilpine produced a marked stimulation of the locomotor activity. Combining 1.0 g/kg ethanol with the 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide (5.0 mg/kg, I.P.) tended to produce locomotor stimulation. Our results suggest the existence of interaction between ethanol and the NMDA receptor complex in mediation of locomotor stimulation. Alternatively, a common neurotransmitter system (other than glutamatergic) mediate central stimulatory effects of ethanol and dizocilpine. A possible role of dopamine in this interaction is being discussed.

Novel systemically active antagonists of the glycine site of the N-methyl-D-aspartate receptor: electrophysiological, biochemical and behavioral characterization

A series of novel tricyclic pyrido-phthalazine-dione derivatives was tested for antagonistic effects at the strychnine-insensitive modulatory site of the N-methyl-D-aspartate (NMDA) receptor (glycineB). All compounds displaced [3H]MDL-105,519 binding to rat cortical membranes with IC50 values of between 90 nM and 3.6 microM. In patch-clamp experiments, steady-state inward current responses of cultured hippocampal neurons to NMDA (200 microM, glycine 1 microM) were antagonized by these same compounds with IC50 values of 0.14 to 13.8 microM. The antagonism observed was typical for glycineB antagonists, i.e., they induced desensitization and their effects were not use or voltage dependent. Moreover, increasing concentrations of glycine were able to decrease their apparent potency. Much higher concentrations (>100 microM) were required to antagonize alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced currents. They were potent, systemically active NMDA receptor antagonists in vivo against responses of single neurons in the rat spinal cord to microelectrophoretic application of NMDA with ID50 values in the low milligram per kilogram i.v. range. They also inhibited pentylenetetrazol-, NMDA- and maximal electroshock-induced convulsions in mice with ED50 values ranging from 8 to 100 mg/kg i.p. The duration of anticonvulsive action was rather short but was prolonged by the organic acid transport inhibitor probenecid (200 mg/kg). The agents tested represent a novel class of systemically active glycineB antagonists with greatly improved bioavailability.

Discriminative stimulus properties of ethanol in the rat: differential effects of selective and nonselective benzodiazepine receptor agonists

Rats were trained to discriminate between ethanol (1.0 g/kg; 10% v/v) and saline under a fixed ratio 10 schedule of sweetened milk reinforcement. Both diazepam [nonselective, full benzodiazepine (BZ) receptors agonist] and bretazenil (nonselective, partial BZ receptor agonist) produced dose-dependent ethanol-appropriate responding (>75%). Neither diazepam nor bretazenil affected the response rate at the doses producing maximal generalisation from ethanol. In contrast, zolpidem (full BZ1 receptor agonist) and abecarnil (full BZ1/full or partial BZ2 receptor agonist) produced only moderate (<50%) ethanol-appropriate responding when tested up to doses that markedly decreased the overall response rate. These results suggest that: 1) there are no major differences between full and partial, nonselective BZ receptor agonists in their ability to substitute for 1.0 g/kg dose of ethanol; 2) stimulation of BZ1 receptors alone is not sufficient to produce ethanol-like discriminative stimulus effects in the rat.

5,7-dihydroxytryptamine lesion does not affect ethanol-induced conditioned taste and place aversion in rats

The effect of the lesion of central serotonergic neurons by 5,7-dihydroxytryptamine (5,7-DHT), on ethanol-induced taste and place aversion conditioning was studied in male Wistar rats. Control biochemical analysis revealed that 5,7-DHT (250 micrograms per rat, free base, i.c.v.) produced marked and selective depletion of serotonin (5-HT) in the hippocampal formation and the limbic forebrain complex. Ethanol-induced (1.5 g/kg, i.p.) conditioned taste aversion (CTA) to saccharin solution was unaffected by the lesion of central serotonergic neurons. The 5,7-DHT-lesioned and sham-lesioned rats showed comparable ethanol-induced CTA even 30 days after the last ethanol injection. Similarly, ethanol-induced (1.5 g/kg, i.p.) conditioned place aversion (CPA) was unaffected by 5,7-DHT administration. These results suggest that central serotonergic pathways are not primarily involved in the aversive effects of high ethanol doses in rats.

Nitric oxide donors antagonize N-nitro-L-arginine and haloperidol catalepsy: potential implication for the treatment of Parkinsonism?

Nitric oxide (NO) synthase inhibitor, N-nitro-L-arginine (NNLA) produced dose-dependent, long-lasting catalepsy in rats, the effect being attenuated by NO donors L-arginine and molsidomine. Catalepsy induced by haloperidol (0.4 mg/kg i.p.), D2 receptor antagonist, was reduced dose-dependently by molsidomine (10.0-100.0 mg/kg) and by L-arginine at a dose of 100.0 mg/kg. Low, non-cataleptic doses of NNLA (0.1 mg/kg) and haloperidol (0.1 mg/kg) given in combination produced a marked and long-lasting catalepsy. The results suggest that NO plays a role in NNLA-induced catalepsy as well as in catalepsy elicited by haloperidol.

Effects of 5-HT3 receptor agonists on voluntary ethanol intake in rats maintained on a limited access procedure

Recent evidence from a variety of laboratory studies suggests that the central effects of ethanol (EtOH) are mediated by serotonin 5-HT3 receptors. Notably, EtOH is able to potentiate 5-HT action on 5-HT3 ionophore, and 5-HT3 antagonists are known to reduce certain effects of EtOH. In the present study, we evaluated the effects of two agonists of 5-HT3 receptors, 2-methyl-5-HT (2-Me-5-HT) and m-chlorophenylbiguanide (m-CPBG) that were microinjected i.c.v. and into the nucleus accumbens (NAC) on EtOH intake in Wistar rats with high EtOH preference. 2-Me-5-HT given i.c.v. (1 and 10 microg per rat) and into the NAC (bilaterally 1 and 10 microg per site) significantly reduced EtOH intake in the limited access paradigm (2h session). On the other hand m-CPBG was inactive after intra-NAC administration. It is concluded that central 5-HT3 receptors are involved in the regulation of EtOH consumption.

Discriminative stimulus effects of ethanol: lack of antagonism with N-methyl-D-aspartate and D-cycloserine

Several drug discrimination studies reported that both competitive and uncompetitive NMDA receptor antagonists substituted for ethanol stimulus in rats. In the present study we examined if compounds that act as agonists at the NMDA receptor complex, D-cycloserine (a partial agonist at the glycine positive modulatory site) and N-methyl-D-aspartate (an agonist at the glutamate binding site), could antagonize the discriminative stimulus effects of ethanol. Rats were trained to discriminate between IP administered 1.0 g/kg of ethanol (10% v/v) and saline under a sweetened milk-reinforced fixed ratio 10 (FR10) schedule of reinforcement. When the animals met the discriminative criteria, antagonism tests were conducted with D-cycloserine (0.3-10.0 mg/kg, IP) and N-methyl-D-aspartate (15.0-60.0 mg/kg, IP). Neither D-cycloserine nor N-methyl-D-aspartate antagonized the ethanol-mediated discriminative stimulus effects. In addition, D-cycloserine (3.0-300.0 mg/kg, IP) did not substitute for ethanol. These results indicate that at least certain agonists at the NMDA receptor complex do not attenuate the ethanol interoceptive cue in the rat.

Discriminative stimulus properties of ethanol in the rat: effects of neurosteroids and picrotoxin

Tetrahydrodeoxycorticosterone (5beta-THDOC; 1.3-12.0 mg/kg), a neurosteroid enhancing the GABA(A) receptor-associated chloride conductance, produced predominantly ethanol-appropriate responding (> 80%) in rats trained to discriminate 1.0 g/kg ethanol from saline. However, neither picrotoxin (0.25-1.5 mg/kg), nor dehydroepiandrosterone sulfate (0.01-100.0 mg/kg), a neurosteroid acting as a GABA(A) receptor antagonist, attenuated the stimulus effects of ethanol. These results indicate that: (1) at least certain neurosteroids may produce subjective states similar to these induced by ethanol; (2) blockade of the GABA(A) receptor-associated channel does not eliminate the ethanol interoceptive cue in rats.

5-HT3 receptor antagonist, tropisetron, does not influence ethanol-induced conditioned taste aversion and conditioned place aversion

Numerous works have demonstrated an interaction between 5-HT3 receptor antagonists and some of the effects of ethanol (EtOH) using biochemical, electrophysiological, and behavioral techniques. Thus 5-HT3 antagonists are capable of reducing EtOH-induced release of dopamine in the nucleus accumbens, EtOH-induced hyperlocomotion, and voluntary EtOH consumption in laboratory animals. In addition to its rewarding effect, EtOH possesses aversive properties as demonstrated in the conditioned taste aversion (CTA) and conditioned place aversion (CPA) paradigms. The role of 5-HT3 receptors in aversive effects of EtOH remains, however, unknown. We decided to study the effect of 5-HT3 antagonist, tropisetron, on aversive properties of EtOH (1.5 g/kg i.p.) in rats using the CTA and CPA models. In addition, effect of tropisetron on morphine (Mf)-induced CTA (10.0 mg/kg SC) was investigated. Tropisetron (0.001-0.5 mg/kg) did not influence CTA produced by EtOH and Mf. When given alone, it failed to produce any taste conditioning. Furthermore, tropisetron did not modify CPA induced by EtOH. Our results suggest that 5-HT3 receptors are not involved in aversive effects of acute doses of EtOH.

Low dose of ethanol induces conditioned place preference in rats after repeated exposures to ethanol or saline injections

Using the place conditioning paradigm (biased design), we have shown that five conditioning sessions with ethanol (0.5 or 1.0 g/kg i.p.) did not result in place conditioning response. In contrast, rats that received 20 injections of ethanol (0.5 g/kg) or saline, before the conditioning procedure, showed significant place preference to the compartment paired with 0.5 g/kg ethanol (but not 1.0 g/kg).

Competitive NMDA receptor antagonist, CGP 40116, substitutes for the discriminative stimulus effects of ethanol

A drug discrimination procedure was used to compare the ability of competitive (CGP 37849, D,L-(E)-2-amino-4-methyl-5-phosphono-3-pentanoate; CGP 40116, D-(E)-2-amino-4-methyl-5-phosphono-3-pentanoate) and non-competitive (dizocilpine) NMDA receptor antagonists to substitute for ethanol in rats trained to discriminate between a 1.0 g/kg dose of ethanol (i.p.) and saline. Dizocilpine (0.1-0.3 mg/kg) substituted partially for ethanol at doses that markedly reduced the rate of responding. CGP 37849 (1.25-5.0 mg/kg) substituted partially for ethanol and suppressed the response rate. CGP 40116 (0.5-2.5 mg/kg), and active D-stereoisomer of CGP 37849, completely substituted (88%) for ethanol, and caused only moderate suppression of the response rate.

Studies on the role of 5-HT3 receptors in the mediation of the ethanol interoceptive cue

The drug discrimination test was used to evaluate the role of 5-HT3 receptors in the mediation of the stimulus properties of ethanol in rats trained to discriminate between ethanol (1.0 g/kg, 10% v/v, i.p.) and saline vehicle. Rats trained to discriminate between a lower dose of ethanol (0.5 g/kg i.p.) failed to attain discrimination criteria after 20 weeks (100 sessions) of training. None of the doses of 5-HT3 receptor antagonists (0.001, 0.01, 0.1, 1.0, 10.0 mg/kg of tropisetron or ondansetron) administered i.p. 30 min before ethanol, antagonized the discriminative stimulus properties of ethanol. Furthermore, none of the centrally (1, 10, 35 micrograms per rat) or i.p. (0.1, 1.0, 2.5, 5.0, 10.0 mg/kg) administered doses of 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide, could replace the ethanol discriminative cue. These results suggest that 5-HT3 receptors are not primarily involved in the mediation of the stimulus properties of ethanol.

5-HT3 receptors and central effects of ethanol

This review discusses recent experimental findings in serotonin-3 (5-HT3) receptors and ethanol (EtOH) research. The role of these receptors in central effects of EtOH including the effects of 5-HT3 agonists and antagonists on EtOH intake, rewarding and aversive properties of EtOH and EtOH interoceptive cue is reviewed. It is now recognized that 5-HT3 receptors while involved in EtOH intake, EtOH tolerance and withdrawal and rewarding mechanisms do not play an important role in aversive effects of EtOH and EtOH discriminative stimulus. 5-HT3 receptors involved in EtOH consumption are supposedly located on dopaminergic neurons in the nucleus accumbens.

Comparison of the potency, kinetics and voltage-dependency of a series of uncompetitive NMDA receptor antagonists in vitro with anticonvulsive and motor impairment activity in vivo

The amino-adamantane derivatives memantine (1-amino-3,5-dimethyladamantane) and amantadine (1-amino-adamantane) are relatively low affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists which have been used clinically in the treatment of dementia and Parkinson's disease respectively for several years without serious side effects. The aim of this study was to test whether memantine, amantadine and other low affinity uncompetitive NMDA receptor antagonists also have better therapeutic indices than high affinity antagonists in preclinical models of epilepsy by assessing the potency, kinetics and voltage-dependency of open channel blockade for a series antagonists in vitro and comparing these effects to anticonvulsive and motor impairment activity in vivo. The compounds tested were memantine, amantadine, 14 other amino-adamantanes, (+)-MK-801, ketamine, dextrorphan, dextromethorphan and phencyclidine. The offset kinetics of open-channel blockade assessed with whole cell patch clamp recordings from cultured superior colliculus neurones were highly correlated to potency i.e. the less potent antagonists showed faster unblocking kinetics (Koff, r = 0.904). Although, onset kinetics as assessed by Kon were not correlated to potency (r = 0.023), tau on estimated at IC50 is perhaps a more meaningful measure of onset kinetics at equieffective concentrations and was also well correlated to potency (r = -0.863). All amino-adamantanes tested were strongly voltage-dependent. There was also a good correlation between the in vitro potencies of uncompetitive NMDA receptor antagonists assessed with patch clamp recordings and displacement of equilibrium [3H](+)-MK-801 binding and their in vivo activity against maximal electroshock (MES) and pentylenetetrazol (PTZ) induced tonic convulsions and NMDA-induced lethality in mice. Memantine and four other amino-adamantanes with somewhat lower potency and faster blocking kinetics had better therapeutic indices (ED50 rotarod and traction reflex over ED50 in MES-induced convulsions; TI = 2-4) than substances with higher affinity such as ketamine, dextrorphan and (+)-MK-801 (TI < 2). However, amantadine and several other amino-adamantanes with lower potency than memantine actually had poorer therapeutic indices (TI < or = 0.5) which may have been due to additional actions at other ion channels or receptors at the doses necessary to protect against seizures. In fact, ED50 in the MES test was negatively-correlated to therapeutic indices (traction r = -0.790, rotarod r = -0.797) i.e. the less potent uncompetitive antagonists had worse therapeutic indices. The data from the present study do not lend support to the idea that low affinity, open channel NMDA receptor blockers are also effective in models of epilepsy at doses having little effect on physiological processes. It should be stressed that these data do not contradict the known therapeutic safety of memantine and amantadine in dementia and Parkinson's disease respectively. Thus the good clinical profile of memantine in dementia has been attributed not only to its fast blocking/unblocking kinetics but also to its strong voltage-dependency. These biophysical properties may allow therapeutically-relevant concentrations to block chronic, low level pathological activation of NMDA receptors whilst leaving their synaptic activation intact. Precisely these properties may also underlie the poor therapeutic indices seen in the present study on antiepileptic activity due to the synaptic nature of both seizures and normal glutamatergic transmission.