A. Chau

Inhibition of nitric oxide synthesis impairs rapid tolerance to ethanol

To examine whether nitric oxide was involved in the development of rapid tolerance to the motor-incoordinating effects of ethanol (tilt-plane test), three experiments were undertaken in a rapid tolerance paradigm in rats. The first experiment tested the effect of the nitric oxide synthase inhibitor, L-nitro-arginine, on the acquisition of ethanol tolerance. The second compared the effects of L-nitro-arginine with those of its inactive isomer, D-nitro-arginine. The third examined whether overload with the substrate L-arginine would prevent the action of L-nitro-arginine on rapid tolerance. The results demonstrated that L-nitro-arginine prevented the development of rapid tolerance, while D-nitro-arginine was ineffective. An excess of the substrate L-arginine reversed the inhibitory action of L-nitro-arginine on tolerance development. These data suggest that nitric oxide may play a role in the development of tolerance to ethanol. The role of nitric oxide in ethanol tolerance may be similar to its role in memory and learning, involving facilitation of transmission in certain NMDA synapses.

Ketamine retards chronic but not acute tolerance to ethanol

Motor impairment (tilt-plane test) was used to investigate whether the noncompetitive N-methyl-D-aspartate (NMDA) antagonist ketamine prevents the development of chronic and acute tolerance to ethanol. Rats were treated with ethanol or saline in the presence and absence of ketamine (separate groups) for 10 days and tested for ethanol tolerance in the absence of ketamine on the fifth and tenth days. In other studies, the effect of ketamine on acute tolerance to ethanol was examined. Rats that received ethanol daily without ketamine showed significant tolerance to ethanol on days 5 and 10, but those receiving ethanol plus ketamine daily showed significantly less tolerance to ethanol. Thus, ketamine interfered with the development of chronic tolerance just as it had been found previously to prevent rapid tolerance. In contrast, ketamine failed to block acute tolerance to ethanol. These results would suggest that the phenomena of acute tolerance and chronic tolerance have differences not previously reported.

Effect of (+)MK-801 and ketamine on rapid tolerance to ethanol

The motor impairment (tilt-plane test) responses to ethanol were significantly reduced on days 2, 3, 4, or 5 in rats receiving ethanol (2.3 and 1.7 g/kg) 24 and 22 h earlier, compared to the control group pretreated with saline. Administration of (+)MK-801, prior to behavioral testing with ethanol on day 1, inhibited the development of tolerance on all these days. Tolerance and the inhibitory effect of (+)MK-801 could no longer be seen if the second injection of ethanol was given on day 7, 8 or 11. Administration of (+)MK-801 on day 1 but after behavioral testing with ethanol did not block the development of rapid tolerance to ethanol on day 2. Administration of another commonly employed NMDA antagonist, i.e., ketamine, prior to ethanol on day 1, also blocked the development of rapid tolerance to ethanol. The findings suggest that NMDA antagonists block rapid tolerance by preventing some adaptation that occurs during intoxicated practice.

Characterization of the phenomenon of rapid tolerance to ethanol

Motor impairment (tilt-plane) and hypothermia tests were used to further characterize the phenomenon of rapid tolerance to ethanol. Five experiments were carried out to clarify the relationship between rapid and chronic tolerance. The first experiment demonstrated that the extent of tolerance on day 2 produced by the single dose of 4 g/kg alcohol on day 1 was similar to that resulting from two divided doses, administered 2 h apart. In the second experiment, a linear relationship between treatment dose and rapid tolerance development was demonstrated in that higher day 1 treatment doses resulted in greater rapid tolerance development. In the third, a parallel dose-response relationship, similar to that known for chronic tolerance, was observed for rapid tolerance. In the fourth experiment, we compared the development of rapid tolerance under three different conditions: (a) in groups of rats that were not subjected to testing at all (no testing); on day 1, (b) in groups of rats that were not tested on the apparatus but handled at all test times on day 1 (dummy testing); and (c) in groups of rats that were tested at all test times on day 1 (testing or intoxicated group). No testing on day 1 failed to produce rapid tolerance to ethanol whereas testing and dummy testing of animals on day 1 after pretreatment with ethanol-produced rapid tolerance to ethanol on day 2. In the last experiment, immediate posttrial administration of ketamine was found not to block rapid tolerance development. These findings provide additional support for similarities between the mechanisms of rapid and chronic tolerance.

Effect of NMDA antagonists on rapid and chronic tolerance to ethanol: Importance of intoxicated practice

Recent studies from our laboratory have shown that NMDA antagonists ((+)MK-801 and ketamine) inhibit the development of both rapid and chronic tolerance to the motor-impairing (moving belt test) and hypothermic effects of ethanol. The present experiments were designed to determine a) the generality of this inhibition, by using a different test of motor function, the tilt-plane test, and b) the possible importance of the experimental paradigm (i.e., with and without intoxicated practice), for the effect of the NMDA antagonist on ethanol tolerance. Daily administration of ethanol 3.3 g/kg for 5 days produced the same degree of tolerance on this test, whether it was given as a single dose of 3.3 g/kg before the daily training session or as divided doses of 2.3 g/kg before and 1 g/kg immediately after the session. The inhibitory effect of a single dose of (+)MK-801 (0.25 mg/kg IP) on rapid tolerance did not last longer than 1 day. Therefore, daily administration of the NMDA antagonists was necessary to block development of chronic tolerance. Daily injection of (+)MK-801 (0.25 mg/kg IP) failed to block chronic tolerance, but inclusion of a second dose of (+)MK-801 daily, and progressive increase of this second dose during the chronic treatment period did block chronic tolerance. Unlike (+)MK-801, ketamine does not have motor-impairing effects of its own, and does not potentiate those of ethanol; it was, therefore, used in the remaining experiments. Groups of rats received ethanol (3.3 g/kg) or saline, either before a daily practice session on the tilt-plane or after it.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of D-cycloserine on rapid tolerance to ethanol

We recently reported that the noncompetitive NMDA antagonists, (+)MK-801 and ketamine, block the development of rapid tolerance to ethanol. In the present article, we show that D-cycloserine (CS), an agonist at the glycine site of the NMDA receptor that enhances learning and memory, also enhances the development of rapid tolerance to ethanol. Rats were pretreated on day 1 with saline or CS, followed 30 min later by ethanol (2.3 g/kg, IP) or saline. At the end of motor impairment testing on the tilt-plane apparatus, a second injection of CS (3 mg/kg, IP, each time) or saline was given, followed 30 min later by ethanol or saline. Ethanol pretreatment alone (at this dose) did not result in rapid tolerance to ethanol on day 2. However, the group pretreated with CS and ethanol on day 1 showed significant tolerance on day 2 compared to other groups. Pretreatment with CS on day 1 did not affect the motor impairment response to the first exposure to ethanol whether this was on day 1 or day 2. In another experiment, administration of (+)MK-801 (0.25 mg/kg, IP) prior to CS abolished the rapid tolerance enhancement by CS. These findings are further evidence that the NMDA system, which requires activation by the glycine receptor, plays a major role in the development of at least some forms of ethanol tolerance.

Rapid tolerance and crosstolerance to motor impairment effects of benzodiazepines, barbiturates, and ethanol

Motor impairment (tilt-plane test) test was used to assess the phenomenon of rapid tolerance and crosstolerance to benzodiazepines, barbiturates, and ethanol. The motor impairment responses to benzodiazepines (chlordiazepoxide and diazepam) and to various barbiturates (pentobarbital, phenobarbital, and barbital) were significantly reduced on day 2 in rats that had been treated on day 1 with benzodiazepines and barbiturates, respectively, compared to the control group treated with saline on day 1. Benzodiazepine treatment on day 1 resulted in rapid crosstolerance to the motor impairment effects of ethanol on day 2. Benzodiazepine treatment, however, did not result in rapid crosstolerance to the three barbiturates (pentobarbital, barbital, and phenobarbital) tested. In contrast to the lack of rapid crosstolerance to barbiturates after treatment with benzodiazepines, barbiturate treatment clearly conferred rapid crosstolerance to benzodiazepines and to ethanol. This asymmetry of rapid crosstolerance raises the possibility that benzodiazepines and barbiturates invoke tolerance by mechanisms that are not wholly identical. Therefore, tolerance to the broad range of actions of barbiturates would include crosstolerance to the effects of benzodiazepines, whereas tolerance to benzodiazepines would include only a weak or partial crosstolerance to some of the effects of barbiturates.

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.

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.

Effect of NMDA antagonists on rapid tolerance to ethanol under two different testing paradigms

We have recently reported that pretreatment with NMDA receptor antagonists [(+)MK-801 and ketamine] inhibited the development of rapid tolerance to ethanol hypothermia and motor-impairment on day 2 in animals receiving ethanol on day 1, compared to the control group pretreated with saline. In these studies rats were tested at 30, 60, 90 and 120 min after ethanol on both day 1 and 2. In the present report we compared the development of rapid tolerance under 2 different conditions: (1) in groups of rats that were tested on the tilt-plane at all test times (Testing or Intoxicated Practice group), (2) in groups of rats that were not tested on the tilt-plane but were handled at all test times on day 1 (dummy testing). Rats were pretreated with ethanol or saline on day 1 and tested with ethanol on day 2 in all the above studies. Both testing (intoxicated practice) and dummy testing of animals on day 1 after pretreatment with ethanol produced rapid tolerance to ethanol on day 2. However, (+)MK-801 or ketamine pretreatment, which blocked rapid tolerance in the intoxicated practice testing paradigm, failed to block rapid tolerance in the dummy testing paradigm. Similar results were obtained for rapid tolerance and for the effect of ketamine in the hypothermia experiment. These findings suggest that NMDA antagonists block rapid tolerance in the intoxicated testing paradigm but not in the dummy testing paradigm. However, whether the two types of rapid tolerance tested in the present experiments are indeed different or interrelated remains to be further investigated.