Systemic and intra-accumbens microinjections of naltrexone interfere with tolerance to ethanol in rats.

RATIONALE: Evidence suggests a role for the opioid system in the control of ethanol reinforcement and drinking. Previous findings have shown that naltrexone, an opioid antagonist that decreases ethanol consumption in humans and experimental animals, reduces the acquisition of acute ethanol tolerance in rats. However, there are few data regarding the role of the opioid system in the acquisition of ethanol tolerance, particularly in brain areas involved in the rewarding actions of ethanol. OBJECTIVES: This study investigates the effects of systemic and of intra-accumbens injections of naltrexone on the development of rapid tolerance to ethanol. METHODS: Wistar rats received intraperitoneal injections of naltrexone (0.1-3.0 mg/kg) or microinjections into the core or shell portions of the nucleus accumbens (5-20 microg) before ethanol (2.7 g/kg i.p.). The animals were tested for motor coordination on the tilting plane apparatus. Tolerance was assessed 24 h later by administering the same dose of ethanol to all animals and retesting them on the tilting plane. RESULTS: The second injection of ethanol resulted in less motor incoordination on Day 2, suggesting the development of rapid tolerance. Pretreatment with naltrexone, either i.p. (0.3 and 0.6 mg/kg) or intra-accumbens (5-20 microg), on Day 1, blocked the development of rapid tolerance to the motor-incoordinating effects of ethanol on Day 2 without affecting the motor performance of the animals on Day 1. CONCLUSIONS: The results suggest that the opioid system may be involved in the development of ethanol tolerance, and that the nucleus accumbens may play a role in this phenomenon.

Effect of NMDA antagonists, an NMDA agonist, and serotonin depletion on acute tolerance to ethanol.

The effect of N-methyl-D-aspartate (NMDA) antagonists [dizocilpine, (+)MK-801, and ketamine], an NMDA agonist (D-cycloserine) and of brain serotonin (5-HT) depletion with p-chlorophenylalanine (p-CPA) on acute tolerance to ethanol was examined, using the tolerance model proposed by Radlow [Psychopharmacology 114 (1994) 1-8] and Martin and Moss [Alcohol Clin Exp Res 17 (1993) 211-216]. This model is based on the concept of a linear increase of acute tolerance with time; the rate of acute tolerance development is the slope of the output function that relates blood alcohol concentrations (BACs) and intoxication. Pretreatment with NMDA antagonists inhibited the development of acute tolerance to ethanol, whereas pretreatment with D-cycloserine enhanced it. Depletion of 5-HT by p-CPA also blocked acute tolerance to ethanol. These results on acute tolerance are similar to those previously found on rapid and chronic tolerance to ethanol.

Influence of neurosteroids on the development of rapid tolerance to ethanol in mice.

Our recent study demonstrated that neurosteroids might either facilitate or block chronic tolerance to the incoordinating effects of ethanol. The present study investigated the effects of neurosteroids on the development of rapid tolerance to ethanol-induced motor impairment using the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine [(+)-MK-801] or the gamma-aminobutyric acid (GABA) type A (GABA(A)) receptor agonist muscimol. Male Swiss mice were pretreated with pregnenolone sulfate (0.03 to 0.15 mg/kg) or dehydroepiandrosterone sulfate (0.05 to 0.20 mg/kg) before administration of ethanol (1.9 or 2.25 g/kg) and tested with the rota-rod apparatus. Twenty-four hours later, all animals were re-tested with the rota-rod after receiving the same dose of ethanol. Pretreatment with pregnenolone sulfate or with dehydroepiandrosterone sulfate significantly facilitated the acquisition of tolerance. However, the administration of (+)-MK-801 reversed the stimulatory action of pregnenolone sulfate but did not affect the actions of dehydroepiandrosterone sulfate on ethanol tolerance. Pretreatment with pregnenolone sulfate or dehydroepiandrosterone sulfate prevented the inhibitory action of muscimol on tolerance development. Taken together, our results suggest that neurosteroids may stimulate the development of rapid tolerance to ethanol and that GABA(A) and NMDA receptor systems may be involved in these actions.

Rapid tolerance to Delta(9)-tetrahydrocannabinol and cross-tolerance between ethanol and Delta(9)-tetrahydrocannabinol in mice.

Motor incoordination in the rota-rod test was used to assess the development of rapid tolerance to Delta(9)-tetrahydrocannabinol and rapid cross-tolerance between ethanol and Delta(9)-tetrahydrocannabinol in mice. Further, the influence of the cannabinoid receptor antagonist SR 141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide) on the motor impairment induced by both drugs was examined. Mice were injected on day 1 with equipotent doses of Delta(9)-tetrahydrocannabinol (28 mg/kg, i.p.) and ethanol (2.25 g/kg, i.p.) and tested at 30, 60 and 90 min after the injections. On day 2, control groups received ethanol or Delta(9)-tetrahydrocannabinol, some groups received the same treatment as the day before, while the remaining groups switched the treatment. All groups were tested to evaluate tolerance. The development of rapid tolerance to Delta(9)-tetrahydrocannabinol was observed and pretreatment with ethanol resulted in rapid cross-tolerance to Delta(9)-tetrahydrocannabinol. SR 141716A (2 mg/kg, i.p.) failed to block the development of rapid tolerance to both drugs, ethanol and Delta(9)-tetrahydrocannabinol. These results suggest that Delta(9)-tetrahydrocannabinol, similarly to ethanol, can induce rapid tolerance to motor incoordination in mice. They also support the use of the 2-day protocol as an effective procedure to reduce the length of drug exposure necessary to induce tolerance.

Effects of central and systemic injections of peripheral benzodiazepine receptor ligands on the anxiolytic actions of ethanol in rats.

The influence of peripheral benzodiazepine receptor ligands Ro5-4864 (0.05 or 1.0 mg/kg, i.p.) or PK11195 (0.05 or 1.0 mg/kg, i.p.) on the anxiolytic effect of ethanol (1.2 g/kg; 14% p/v; i.p.) was investigated in rats tested on the elevated plus-maze. Other animals were injected through intrahippocampal administrations of the ligands (0.5 or 1.0 nmol/0.5 &mgr;l) before ethanol (1.2g/kg; 14% p/v; i.p.) and submitted to the elevated plus-maze test. The results showed that the systemic administration of either ligands 24 hours before the ethanol treatment resulted in a reduced anxiolytic effect of this drug. Only PK11195 reversed the effect of ethanol after intrahippocampal injection. These data suggest that peripheral benzodiazepine receptors play a role in ethanol anxiolysis.

GABA(B) receptors play a role in the development of tolerance to ethanol in mice.

RATIONALE: There is evidence that drugs that improve or impair learning can facilitate or block ethanol tolerance, respectively. Since GABA(B) receptors have been shown to be involved in processes related to learning, it is possible that this system could play a role in the development of rapid tolerance to ethanol. OBJECTIVES: The aim of this study was to verify the influence of one GABA(B) agonist and two GABA(B) antagonists on tolerance to the effect of ethanol on motor coordination. METHODS: Male Swiss mice were trained on a continuously accelerating rota-rod device. Animals were pretreated with the GABA(B) agonist (-)-baclofen (3, 5, or 7 mg kg(-1)) or saline, 30 min before ethanol (1.75 g kg(-1)), and were tested 5, 10, and 15 min later on the rota-rod. In another set of experiments, mice were pretreated with the GABA(B) antagonists CGP36742 (1, 3, 10, or 30 mg kg(-1)) or CGP56433 (0.1, 0.3, 1.0, or 3.0 mg kg(-1)), or saline, 30 min before the test under ethanol. Rapid tolerance was evaluated 24 h after the first ethanol injection, by injecting all animals with ethanol and retesting them on the rota-rod. RESULTS: The results showed that (-)-baclofen (5 mg kg(-1)) significantly (ANOVA + Tukey's test) blocked rapid tolerance, whereas CGP36742 (3 and 10 mg kg(-1)) and CGP56433 (0.3, 1, and 3 mg kg(-1)) facilitated rapid tolerance in a dose-dependent way. The blockade of rapid tolerance by (-)-baclofen was antagonized by previous administration of CGP36742 or CGP56433. CONCLUSIONS: The current results suggest that rapid tolerance to ethanol is subjected to inhibition by a GABAergic GABA(B) receptor-mediated system in the mouse.

Dexamethasone reverses the ethanol-induced anxiolytic effect in rats.

The effects of intraperitoneal and intrahippocampal administration of the glucocorticoid dexamethasone were assessed regarding ethanol-induced anxiolysis in the elevated plus-maze in rats. Animals pretreated with systemic injections of dexamethasone (0.5, 1. 0, or 2.0 mg/kg, IP) 15 min before ethanol (1.2 g/kg, 14% w/v, IP) administration showed a significant dose-dependent attenuation of the increased percentage of frequency and time spent on open arms of the maze. However, IP dexamethasone treatment 4 h before the test had no effect. Unilateral intrahippocampal injection of dexamethasone (2 and 20 nmol in 0.5 microl) also significantly attenuated the increased exploration of the open arms induced by ethanol. The results are interpreted in terms of the modulation of the anxiolytic effects of ethanol by glucocorticoids and the possible involvement of hippocampus in this response. The rapid blockade of ethanol induced anxiolysis by dexamethasone strengthens the suggestion that a nongenomic mechanism may underlie this response.

Effect of epipregnanolone and pregnenolone sulfate on chronic tolerance to ethanol.

The aim of the present study was to investigate the influence of neurosteroids on the development of tolerance to ethanol. Male Swiss mice were injected daily with the positive allosteric modulator of the gamma amino butyric acid-A (GABA(A)) receptor epipregnanolone (5beta-pregnan-3beta-ol-20-one; 0.15 mg/kg i.p.) or pregnenolone sulfate (5-pregnen-3beta-ol-20-one sulfate sodium; 0.08 mg/kg i.p.) - considered a negative allosteric modulator of this receptor and/or positive allosteric modulator of the N-methyl-D-aspartate (NMDA) receptor - 30 min before ethanol (2.5 g/kg i.p.). They were tested on the rota-rod apparatus, under continuous acceleration (1rpm/s), at 30, 60 and 90 min after ethanol injections for 5 days. The results showed that tolerance to the motor incoordinating effect of ethanol occurred on the fifth day of treatment when this effect was blocked by pretreatment with epipregnanolone. On the other hand, ethanol tolerance was enhanced by pretreatment with pregnenolone sulfate from the second to the fifth days of treatment. Taken together, our results suggest that neurosteroids can either stimulate or block the development of chronic tolerance to ethanol. Moreover, since neurosteroids can interact with GABA(A) or NMDA receptor systems, our results suggest the involvement of these systems in the actions of neurosteroids upon ethanol tolerance.

Role of nitric oxide-dependent pathways in ethanol-induced anxiolytic effects in rats.

BACKGROUND: Nitric oxide (NO) is a neuromodulator and an intercellular and retrograde messenger that mediates several functions in the central nervous system. The effects of ethanol (EtOH) on neuronal NO-dependent pathways have been the focus of recent research. Most studies have concentrated on the actions of chronic EtOH exposure. In this study, we examined the role of NO-dependent pathways in the acute actions of EtOH. METHODS: We used the elevated plus-maze test to study the role of NO-dependent pathways in the behavior of rats treated with acute EtOH. We tested the effects of 7-nitroindazole, a reversible competitive inhibitor of nitric oxide synthase. We also studied the effects of the cyclic guanylate monophosphate (cGMP) analog, 8-Bromoguanosine cyclic 3',5'-monophosphate sodium salt, and the NO donors S-nitroso-N-acetylpenicillamine or sodium nitroprusside. RESULTS: When injected by either intraperitoneal (6 mg/kg) or intrahippocampal (20 nmol) routes, 7-nitroindazole increased the percentage of open arm entries and time spent in open arms for rats injected with EtOH (1.0 g/kg, ip). This dose of EtOH did not produce an anxiolytic effect when administered alone. Additional experiments were performed with a dose of 1.2 g/kg of EtOH (ip), which produced an anxiolytic effect. Intrahippocampal administration of the cGMP analog, 8-Bromoguanosine cyclic 3',5'-monophosphate sodium salt (40 nmol), or the NO donors S-nitroso-N-acetylpenicillamine (20 or 40 nmol) or sodium nitroprusside (20 or 40 nmol) blocked the anxiolytic effect of this dose of EtOH. CONCLUSIONS: These results indicate that inhibition of NO-dependent pathways enhances, whereas stimulation of these pathways decreases, the efficacy of EtOH to produce anxiolytic effects in rats. We postulate that NO-dependent increases in guanylate cyclase activity and cGMP levels oppose the anxiolytic effects produced by acute EtOH administration.

NMDA-receptor antagonists block the development of rapid tolerance to ethanol in mice.

Several studies have emphasized the role of learning in the development of rapid and chronic tolerances. Recently, it was shown that the NMDA antagonists MK-801(dizocilpine) and ketamine block the development of tolerance to ethanol in rats submitted to tilt-plane apparatus. The present study examines the generality of this inhibition using mice submitted to the rota-rod test. Mice were tested in the rota-rod apparatus at 5, 10 and 15 minutes after intraperitoneal ethanol injections. The first experiment evaluated the time course of acute effects of different doses of ethanol (1.0-2.25 g/kg) in the rota-rod test. In the second experiment, the most effective dose of ethanol to produce rapid tolerance (RT) was determined. Mice were injected on day 1 with ethanol or saline and tested on the rota-rod. After 24 hours, all groups were injected with the same doses of ethanol and tested. The third experiment investigated whether ketamine (1.0-5.0 mg/kg) injected before ethanol on day 1 influenced the development of RT to ethanol. The last experiment compared the actions of the (+) and (-)MK-801 isomers (0.015-0.060 mg/kg) on RT to ethanol. Maximum motor impairment was obtained 5 minutes after ethanol injections. Pretreatment of animals with ketamine (2.5 and 5 mg/kg) or with (+)MK-801 (0.030 and 0.060 mg/kg) significantly blocked the development of RT. The (-)MK-801 isomer did not affect RT, suggesting that the blockade by MK-801 is stereospecific. These results confirm and extend previous studies showing that NMDA receptor antagonists block RT to the motor impairment produced by ethanol in other animals tested in different models.

Ethanol-induced hypothermia in rats is antagonized by dexamethasone.

The effect of dexamethasone on ethanol-induced hypothermia was investigated in 3.5-month old male Wistar rats (N = 10 animals per group). The animals were pretreated with dexamethasone (2.0 mg/kg, i.p.; volume of injection = 1 ml/kg) 15 min before ethanol administration (2.0, 3.0 and 4.0 g/kg, i.p.; 20% w/v) and the colon temperature was monitored with a digital thermometer 30, 60 and 90 min after ethanol administration. Ethanol treatment produced dose-dependent hypothermia throughout the experiment (-1.84 +/- 0.10, -2.79 +/- 0.09 and -3.79 +/- 0.15 degrees C for 2.0, 3.0 and 4.0 g/kg ethanol, respectively, 30 min after ethanol) but only the effects of 2.0 and 3.0 g/kg ethanol were significantly antagonized (-0.57 +/- 0.09 and -1.25 +/- 0.10, respectively, 30 min after ethanol) by pretreatment with dexamethasone (ANOVA, P < 0.05). These results are in agreement with data from the literature on the rapid antagonism by glucocorticoids of other effects of ethanol. The antagonism was obtained after a short period of time, suggesting that the effect of dexamethasone is different from the classical actions of corticosteroids.

Ethanol-induced hypothermia in rats is antagonized by dexamethasone

The effect of dexamethasone on ethanol-induced hypothermia was investigated in 3.5-month old male Wistar rats (N = 10 animals per group). The animals were pretreated with dexamethasone (2.0 mg/kg, ip; volume of injection = 1 ml/kg) 15 min before ethanol administration (2.0, 3.0 and 4.0 g/kg, ip; 20 per cent w/v) and the colon temperature was monitored with a digital thermometer 30, 60 and 90 min after ethanol administration. Ethanol treatment produced dose-dependent hypothermia throughout the experiment (-1.84 ñ 0.10, -2.79 ñ 0.09 and -3.79 ñ 0.l5 degrees Celsius for 2.0, 3.0 and 4.0 g/kg ethanol, respectively, 30 min after ethanol) but only the effects of 2.0 and 3.0 g/kg ethanol were significantly antagonized (-0.57 ñ 0.09 and - 1.25 ñ 0.10, respectively, 30 min after ethanol) by pretreatment with dexamethasone (ANOVA, P<0.05). These results are in agreement with data from the literature on the rapid antagonism by glucocorticoids of other effects of ethanol. The antagonism was obtained after a short period of time, suggesting that the effect of dexamethasone is different from the classical actions of corticosteroids.

D-cycloserine blocks the effects of ethanol and HA-966 in rats tested in the elevated plus-maze.

Previous studies from our laboratory have shown gender-related behavior in rats tested in the elevated plus-maze under the influence of ethanol and other drugs. The present study investigated the effects of pretreatment with the NMDA-receptor partial agonist at the glycine site D-cycloserine (DCS; doses 3 to 9 mg/kg for females; 3 to 12 mg/kg for males, intraperitoneally) on the effects of ethanol (1.2 g/kg, i.p.; 14% w/v) and (+/-)-3-amino-1-hydroxy-2-pyrrolidone (HA-966; 2 or 4 mg/kg, i.p.), an antagonist at the glycine site of the NMDA receptor complex in rats submitted to the elevated plus-maze test. The results showed that DCS, at doses that did not affect the behavior of control animals, significantly (p < 0.05) prevented the increase in the percentage of open-arm entries and the time spent in the open arms of elevated plus-maze test induced by ethanol, exhibiting a U-shaped dose-response curve. Similarly, DCS blocked the anxiolytic effects of HA-966 in animals of both gender. Data confirm our previous results, suggesting that the NMDA-receptor system contributes significantly to the anxiolytic effect of ethanol. Furthermore, the similarity between the blockade by DCS of anxiolysis induced either by ethanol or by HA-966 strengthens the suggestion that ethanol acts on the glycine site of the NMDA receptor complex.

Individual housing from rearing modifies the performance of young rats on the elevated plus-maze apparatus.

Previous studies from our laboratory have demonstrated that male and female rats exhibit a differential pattern of behavior in the elevated plus-maze as a function of age. In the present study, the influence of individual housing conditions on young animals treated with one of two classical anxioselective drugs, diazepam or pentylenetetrazole, was investigated in the elevated plus-maze. In Experiment I, males and females were housed for 30 days after weaning either individually or in groups, and tested in the elevated plus-maze at 60 days of age. In Experiment 2, the effects of diazepam (0.75 or 1.0 mg/kg) or of pentylenetetrazole (20 or 30 mg/kg) on the behavior of isolated or grouped rats were studied at 60 days of age in the elevated plus-maze. The results show that isolated housed animals tested with diazepam at 60 days of age exhibited increased frequency and time spent on the open arms of the apparatus compared to control rats. The effect of diazepam was not observed in grouped animals tested at 60 days of age. Pentylenetetrazole produced a decrease in the frequency and time spent on the open arms. This effect was more prominent in grouped animals. The results suggest that 60-day-old rats deprived of playfighting experience present high basal anxiety levels and also that rearing conditions (isolated or grouped) are able to interact with both anxiolytic and anxiogenic effects of experimental drugs.

N-methyl-D-aspartate receptors, nitric oxide, and ethanol tolerance.

Several experimental models have been used to study tolerance to ethanol. The development of tolerance to the motor incoordinating effect of a single administration of ethanol occurs within 8-24 h after the effect of the first dose has disappeared. This form of tolerance is designated rapid tolerance and seems to involve functional rather than pharmacokinetic mechanisms. Like chronic tolerance, rapid tolerance has been shown to be influenced by processes related to learning and memory. It is known that N-methyl-D-aspartate (NMDA) receptor systems are involved in the expression and maintenance of one form of long-term potentiation (LTP), a synaptic adaptive process which has been suggested to be the cellular basis of memory or associative memory. Considering the similarities between learning and tolerance, the effects of NMDA agonists and antagonists on tolerance to ethanol were investigated. Our studies demonstrated that NMDA antagonists that impair learning, such as dizocilpine or ketamine, inhibit tolerance, while NMDA agonists that improve learning, such as D-cycloserine, increase tolerance. Moreover, the nitric oxide synthase inhibitor L-nitroarginine blocks tolerance to the effects of ethanol. Taken together, these data confirm the involvement of the NMDA system in ethanol tolerance and emphasize the participation of learning in this phenomenon.

N-methyl-D-aspartate receptors, nitric oxide, and ethanol tolerance

Several experimental models have been used to study tolerance to ethanol. The development of tolerance to the motor incoordinating effect of a single administration of ethanol occurs within 8-24 h after the effect of the first dose has disappeared. This form of tolerance is designated rapid tolerance and seems to involve functional rather than pharmacokinetic mechanisms. Like chronic tolerance, rapid tolerance has been shown to be infiuenced by processes related to learning and memory. It is known that N-methyl-D-aspartate (NMDA) receptor systems are involved in the expression and maintenance of one form of long-term potentiation (LTP), a synaptic adaptive process which has been suggested to be the cellular basis of memory or associative memory. Considering the similarities between learning and tolerance, the effects of NMDA agonists and antagonists on tolerance to ethanol were investigated. Our studies demonstrated that NMDA antagonists that impair learning, such as dizocilpine or ketamine, inhibit tolerance, while NMDA agonists that improve learning, such as D-cycloserine, increase tolerance. Moreover, the nitric oxide synthase inhibitor L-nitroarginine blocks tolerance to the effects of ethanol. Taken together, these data confirm the involvement of the NMDA system in ethanol tolerance and emphasize the participation of leaming in this phenomenon.

Influence of age and of pre-treatment with D-cycloserine on the behavior of ethanol-treated rats tested in the elevated plus-maze apparatus.

There is evidence that ethanol is able to influence central functions through the antagonism of the NMDA-receptor system. It has been shown that this system is also involved in the modulation of anxiety-related behavior in rats. Recently, we observed gender- and age-related behavioral influences in rats tested on the elevated plus-maze apparatus The present study was undertaken in order to investigate: (1) the effects of ethanol (0.8, 1.0 or 1.2 g/kg, i.p.) on the behavior of male and female rats tested on the elevated plus-maze at 2, 3, 4 or 5 months of age; (2) the effect of the pre-treatment with D-cycloserine (3.0 or 6.0 mg/kg), an agonist of the glutamate NMDA-receptor system, 30 min before the ethanol (1.2 g/kg) injections, in rats tested in the elevated plus-maze at 2 months or 4 months of age. The results demonstrated that ethanol did not affect the time spent and the frequency of entries on the open arms of the elevated plus-maze in rats tested at 2 months of age, but increased these parameters in older animals. Moreover, the results showed that D-cycloserine, at doses that did not affect the behavior of control animals, antagonized the increased frequency of entries and time spent on open arms produced by ethanol in rats tested at 4 months of age. Our results suggest an age-related influence on the anxiolytic action of ethanol in rats tested in the elevated plus-maze. Moreover, the results suggest that the NMDA-receptor system can be involved in this effect, and strengthens the evidence for the participation of the NMDA-receptor system in anxiety-related behavior.

D-cycloserine enhances rapid tolerance to ethanol motor incoordination.

In a recent study, we showed that D-cycloserine, an agonist at the glycine site of the NMDA receptor, enhances the development of rapid tolerance to ethanol. In the present study, we report that the acquisition of rapid tolerance to the motor incoordination effect of ethanol (tilt-plane test) was increased only when D-cycloserine was injected before, but not after, the intoxicated practice under ethanol. The effect of D-cycloserine on tolerance when this agonist was administered in divided doses before and after test was similar to that obtained when D-cycloserine was injected before test. Higher doses of D-cycloserine did not produce a further enhancement of rapid tolerance. Moreover, when the dose of ethanol on day 1 was large enough to induce rapid tolerance per se, D-cycloserine did not further enhance the tolerance. The enhancement of tolerance by D-cycloserine was antagonized by previous administration of ketamine. The enhancement of ethanol tolerance by D-cycloserine and the antagonism of this effect by ketamine cannot be attributed to changes in pharmacokinetics of ethanol. Taken together, these results confirm the participation of the NMDA receptor system in the development of tolerance to ethanol, and reinforce earlier findings about the involvement of learning in tolerance.

Antinociceptive effects of clebopride in the mouse.

1. The effects of the substituted benzamide clebopride, an orthopramide, on nociception of chemical and thermal stimuli were investigated. 2. Clebopride (0.5, 1.0 and 2.0 mg/kg) promoted significant analgesia in the tail-flick and hot-plate tests and against abdominal constrictions produced by acetic acid or acetylcholine. 3. The analgesic effects of clebopride were not influenced by pretreatment with naltrexone (1-3 mg/kg). 4. The results suggest that clebopride induces analgesia against both thermal and chemical nociceptive stimuli, which is not mediated via opioid mechanisms.

Influence of nitric oxide synthase inhibition on the development of rapid tolerance to ethanol.

We recently reported that the nitric oxide (NO) synthase inhibitor L-nitroarginine (L-NA) blocks the development of rapid tolerance to the motor incoordinating effect of ethanol in the tilt-plane test. To clarify the mechanism of L-NA blockade of tolerance, four additional experiments were carried out using the same test. The first demonstrated that L-NA prevented the development of rapid tolerance to ethanol when injected prior to ethanol either on both Days 1 and 2 or only on Day 1. In the second experiment, tolerance was blocked only when L-NA was injected before but not after behavioral testing on Day 1. In the third, L-NA blocked the enhancement of rapid tolerance to ethanol induced by D-cycloserine (CS), an agonist at the N-methyl-D-aspartate (NMDA) receptor. In the last experiment, L-NA pretreatment did not influence blood ethanol disappearance curves on Day 1, or ethanol concentrations in brain, tail blood or decapitated trunk blood on Day 2. These data argue against state-dependent learning as the basis of the L-NA effect, and confirm and extent our previous observation that NO plays a role in the development of rapid tolerance to ethanol.