The contribution of environmental cues to cross-tolerance between ethanol and pentobarbital

Behavioral sensitization to ethanol is modulated by environmental conditions, but is not associated with cross-sensitization to allopregnanolone or pentobarbital in DBA/2J mice

RATIONALE

The ability of ethanol to facilitate GABA(A) receptor-mediated transmission may result in GABA(A) receptor alterations during repeated ethanol administration, and lead to dynamic behavioral changes, including sensitization to the locomotor stimulant effect of ethanol. Since alterations in GABA(A) receptors are likely to alter sensitivity to GABAergic drugs such as 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) and pentobarbital, we determined whether enhanced sensitivity to ethanol was associated with enhanced sensitivity (cross-sensitization) to these drugs. Two procedures that produced differences in the magnitude of expression of ethanol-induced locomotor sensitization were used.

METHODS

After habituation to testing procedures for 2 days, female DBA/2J mice were injected with ethanol or saline for 12 days. On the following day, locomotion was recorded after a challenge injection of ethanol (2 g/kg), allopregnanolone (10 or 17 mg/kg), or pentobarbital (10 or 20 mg/kg). Due to evidence that exposure to the test chambers influenced sensitization, in some experiments, mice were exposed to the test apparatus on the day prior to challenge.

RESULTS

Exposure to the test apparatus prior to drug challenge attenuated the expression of ethanol sensitization, compared with mice without this pre-exposure. Cross-sensitization was not observed to either allopregnanolone or pentobarbital under any condition; however, some groups of repeated ethanol-treated mice displayed tolerance to the initial stimulant effects of allopregnanolone and pentobarbital.

CONCLUSIONS

These studies indicate that behavioral sensitization to ethanol is not associated with cross-sensitization to pentobarbital or allopregnanolone, and that the expression of ethanol sensitization is influenced by the relative novelty of the test chamber. In addition, these results do not support a mechanism in which alterations in the neurosteroid or barbiturate modulatory sites of the GABA(A) receptor are responsible for the expression of sensitization to the locomotor stimulant effects of ethanol.

Research on tolerance: what can we learn from history?

The concept of tolerance to ethanol has evolved gradually over the past two centuries, and all of the basic clinical features, as they are now understood, have been clearly recognized for nearly 100 years. The basic mechanisms involved in central nervous system tolerance, however, have been elucidated only in the past 20 to 30 years. Little progress was made as long as tolerance was viewed as a purely cellular or physiological adaptation to alcohol, and researchers used overly simple paradigms based on mere exposure to the drug. With the recognition that learning, both operant and associative, can play a major role in the development of tolerance to alcohol and cross-tolerance to other drugs, a radical change in research approaches became possible. Most of the neural mechanisms related to learning and memory are now known to be involved in the development and retention of tolerance, and the simplistic models used in earlier research must now be abandoned. Nevertheless, a review of the history of past research points to a number of important lessons for future work, including the following: (1) many of the present concepts were enunciated by astute observers many decades ago, and research was hindered because this older literature was forgotten; (2) for many decades progress was slow because of a narrow focus on specific techniques, questions, and hypotheses that overlooked important research in related disciplines; (3) the course of research is often irregular, and past questions may have to be revisited with new approaches--but these are more likely to be fruitful if based on knowledge of past history; and (4) excellent researchers often obtain apparently contradictory findings, but the disagreements may hold the key to deeper understanding of the phenomena, and should not be brushed over by ignoring the minority findings and interpretations. As in all scientific research, the most important requirement for major progress is the formulation of good questions or hypotheses: the results yielded by the best available techniques can be only as good as the questions they are meant to answer.

Effect of lipid solubility on the development of chronic cross-tolerance between ethanol and different alcohols and barbiturates

Tolerance to ethanol and cross-tolerance to other alcohols (n-propanol, n-butanol, t-butanol, isobutanol, t-amyl alcohol, n-amyl alcohol, and benzyl alcohol) and barbiturates (pentobarbital, secobarbital, amobarbital, thiopental, barbital and phenobarbital) that differ in lipid:water partition coefficient was examined in rats after chronic pretreatment with ethanol. Tolerance and cross-tolerance were studied with three different measures (hypothermia, tilt-plane, and rotarod). Tolerance to ethanol resulted in significant cross-tolerance to alcohols with low lipid solubility (n-propanol and t-butanol), whereas no cross-tolerance was seen with alcohols of high lipid solubility (isobutanol, n-amyl alcohol, t-amyl alcohol and benzyl alcohol). Cross-tolerance to n-butanol (which has intermediate lipid solubility) appeared to be metabolic rather than functional. Tolerance to ethanol also resulted in significant cross-tolerance to barbital and phenobarbital, but not to pentobarbital, secobarbital, amobarbital or thiopental. These studies suggest that lipid solubility is an important factor in relation to specificity of cross-tolerance to alcohols and barbiturates.

Circadian variation in tolerance to the hypothermic action of CNS drugs

Male SAF mice (30-35 g) or male Sprague-Dawley rats (180-250 g) were used to study the circadian variation in tolerance to the hypothermic action of ethanol, apomorphine, and nicotine. Animals were treated for 2 or 3 consecutive days during the light phase (1000, 1400, or 1800 h) or the dark phase (2200, 0200, or 0600 h) and hypothermia produced measured. In one experiment, repeated injections of 20% ethanol (3 g/kg, IP) to mice resulted in varying degrees of hypothermia depending upon the time of injection. Tolerance to hypothermic action was observed only in animals treated during the light phase. On the contrary, the hypothermic response in animals treated during the dark phase increased. In another experiment, apomorphine (15 mg/kg, IP) was used and tolerance to apomorphine-induced hypothermia observed following repeated injections during the light phase with maximum tolerance noticed at 1400 h. In the third experiment, nicotine (2 mg/kg, IP) was repeatedly administered and resulted in tolerance development when given during the light phase. These results indicate that the rapid development of tolerance to CNS drugs studied is a diurnally controlled phenomenon.

Development of rapid tolerance to pentobarbital and cross-tolerance to ethanol on a motor performance test with intoxicated practice

Male Wistar rats given a single moderate dose (1.7 mg/kg, IP) of pentobarbital (PB), followed by six trials on the moving belt apparatus during the next hour, showed tolerance to the motor-impairing effects of a second dose of 17 mg/kg given 24 h later. A control group that received saline before the first test showed the usual initial sensitivity when tested with PB 24 h later. Three weeks later, the first group showed cross-tolerance to the effects of ethanol (1.7 g/kg, IP) on the same test, while the second group did not. These findings support the suggestion that rapid tolerance is closely similar to chronic tolerance and that the contribution of intoxicated practice results in a long-lasting component that applies to cross-tolerance to ethanol on the same test.

Arginine-8-vasopressin potentiates acute ethanol intoxication

AVP maintains ethanol (EtOH) tolerance after cessation of chronic EtOH treatment. However, the acute interaction of AVP and EtOH has not been well characterized. Rats were trained on a moving belt and the EtOH dose-response relationship (range 1.0-2.0 g/kg) was determined after pretreatment with saline, AVP (2.5-40 micrograms SC or 10 ng ICV), the AVP-V1 receptor antagonist [Des-Gly9,d(CH2)5(1),O-Et-Tyr2, Val4,Arg8]-vasopressin (10 ng ICV), or AVP in combination with the V1 antagonist. AVP produced a 16% decrease in the EtOH ED50 when given either SC or ICV; this decrease, which appears to represent true potentiation rather than additivity, was prevented by the preadministration of the V1 antagonist. Other rats were made EtOH-tolerant by 7 daily injections of either EtOH alone (1.8 g/kg IP) or EtOH (1.5 g/kg IP) + AVP (10 micrograms SC), followed by a practice session on the moving belt. In both sets of tolerant animals, AVP potentiation of acute EtOH effects was still seen on day 6. The mechanism of AVP potentiation of EtOH-induced impairment is unknown, but the failure of the V1 antagonist alone to alter the effect of EtOH suggests that endogenous AVP is not involved directly in modulating EtOH intoxication.