G. Shah

RAPID TOLERANCE AS AN INDEX OF CHRONIC TOLERANCE

Hypothermia and motor impairment (tilt-plane test) were used to assess the phenomenon of rapid cross-tolerance between ethanol and pentobarbital in rats. The hypothermic and motor-impairment responses were significantly reduced on day 2 in animals receiving ethanol on day 1, compared to the control group pretreated with saline. Ethanol pretreatment, however, did not result in rapid cross-tolerance to pentobarbital on either test. Pentobarbital pretreatment on day 1 resulted in rapid tolerance to pentobarbital on day 2. However, in contrast to the lack of rapid cross-tolerance to pentobarbital after pretreatment with ethanol, pentobarbital pretreatment clearly conferred rapid cross-tolerance to ethanol. Determination of ethanol and pentobarbital blood levels suggested that pharmacokinetic alterations did not contribute significantly to the observed rapid tolerance and cross-tolerance. The asymmetry of rapid cross-tolerance seen in these studies mimics the results obtained by us in chronic tolerance and cross-tolerance studies reported recently. These results suggest that rapid tolerance and cross-tolerance can be used as predictors of chronic tolerance and cross-tolerance.

Effect of NMDA receptor antagonists on rapid tolerance to ethanol

Hypothermia and motor impairment (tilt-plane test) were used to assess whether N-methyl-D-aspartate (NMDA) receptors play a role in the development of rapid tolerance to ethanol, i.e., tolerance to a second dose of ethanol given 24 h after the effect of the first dose of ethanol had disappeared. Results showed that (+)-MK-801 and ketamine blocked the development of rapid tolerance to ethanol on both tests. Since these drugs did not modify blood or brain alcohol levels in any of the groups, the blockade of ethanol rapid tolerance cannot be attributed to changes in pharmacokinetics of ethanol. These data suggest that the role of NMDA receptors in ethanol tolerance may be similar to their role in memory and learning, involving a facilitation of transmission in certain synapses.

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.

Differential inhibition by NMDA antagonists of rapid tolerance to, and cross-tolerance between, ethanol and chlordiazepoxide

We have recently found that the non-competitive N-methyl-D-aspartate (NMDA) antagonists, (+)MK-801 and ketamine, block the development of rapid tolerance to ethanol. In the present report we show that they also block rapid cross-tolerance from chlordiazepoxide to ethanol as well as ethanol to chlordiazepoxide. However, NMDA antagonists fail to block the development of rapid tolerance to chlordiazepoxide. Our results suggest that NMDA antagonists may affect not only the acquisition of rapid tolerance or cross-tolerance to sedatives but also the ability to express that tolerance or cross-tolerance, depending on the drugs used. It is also possible that the phenomena of rapid tolerance and rapid cross-tolerance have basic differences not previously reported in the literature.

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)

Comparison of sensitivity and alcohol consumption in four outbred strains of rats

Differences in alcohol consumption and in sensitivity to the effects of ethanol were investigated in four outbred rat strains: Fischer 344, Long-Evans, Sprague-Dawley and Wistar. Alcohol consumption was measured in all four strains in three separate subgroups for each strain, using three different concentrations of ethanol (5, 10 and 20% v/v). An intermittent forced alternate-day ethanol presentation procedure (ethanol as the sole fluid for one day followed by only water the next day), as well as a two-bottle choice paradigm, were employed for this purpose. Ethanol-induced hypothermia and motor impairment (tilting plane test) were used to assess sensitivity. Significant differences in alcohol consumption were found among these strains. The Long-Evans strain consumed the highest and Fischer 344 the lowest amount of ethanol. Wistar and Sprague-Dawley were intermediate. However, the strains did not differ in sensitivity to ethanol. Similarly, determination of sensitivity to ethanol on day 0 in separate groups of these four strains (same age and weight, and obtained at the same time from the same supplier) did not reveal graded differences in sensitivity (hypothermia and motor impairment) corresponding to differences in alcohol consumption. These results suggest that sensitivity does not correlate with alcohol consumption.

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.