Effect of chronic pentobarbital treatment on the development of cross-tolerance to ethanol and barbital

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.

Cross-tolerance between ethanol and neurotensin in mice selectively bred for ethanol sensitivity

Neurotensin (NT), a tridecapeptide that satisfies criteria as a neurotransmitter, mimics many actions of ethanol, and evidence indicates that some of the acute effects of ethanol are mediated in part by NT. Recent studies have shown that chronic ethanol treatment produced a downregulation of NT receptors in mesolimbic brain regions of long sleep (LS) mice and that reduced NT binding capacity was associated with acquisition and decay of tolerance to ethanol-induced locomotor inhibition and hypothermia in these mice. The present study was undertaken to determine whether cross-tolerance develops between NT and ethanol and whether chronic NT infusion produces NT receptor downregulation. Animals chronically treated with ethanol were tolerant to NT-mediated locomotor inhibition at a dose of 1.8 pmol NT, ICV, and were tolerant to NT-induced hypothermia at 1.8 and 6.0 pmol NT. Following repeated injections or continuous infusion of NT ICV, LS mice showed tolerance to both NT- and ethanol-induced hypothermia and locomotor inhibition. Indeed, ethanol doses that are hypnotic in control mice (2.8 g/kg) were not effective in abolishing locomotor activity following chronic NT administration. Results with chronic saline infusion ICV indicate that stress alters sensitivity to ethanol-induced hypothermia. Chronic infusion of NT ICV produced a region-specific downregulation of high-affinity NT receptors in the striatum. The results demonstrate that cross-tolerance develops between NT and ethanol, and further support a role for neurotensinergic systems in the actions of ethanol.

Hypothermic effect of ethanol in mice selected for differential sleep-time response to pentobarbital

The hypothermic action of ethanol was investigated in genetically distinct lines of mice selected for sleep-time response to pentobarbital for six generations. Ethanol (3 g/kg, intraperitoneally) was administered to alcohol-naive males and females from each of the unselected control, long-, and short-sleep mouse lines. Rectal temperatures were measured immediately before, and at 15, 30, 60, 90, 120, and 240 min after ethanol injection. Eight female and eight male mice from each line were sacrificed at each time point, and trunk blood was collected for plasma ethanol analysis. The results show that short-sleep mice were less hypothermic (p < 0.05) compared to long-sleep mice at 15 and 30 min after ethanol administration. However, plasma ethanol concentrations were not significantly different between the mouse lines at any time point. Therefore, the line-dependent differential ethanol-induced hypothermia observed may be a result of differences in "brain sensitivity" rather than in the rates of ethanol metabolism among the mouse lines.

Acute and subchronic benzodiazepine-barbiturate-interactions on behaviour and physiological responses of the mouse

Female NMRI mice were pretreated for 2 weeks with diazepam (D: 20 mg/kg/day), secobarbital (S: 23 mg/kg/day), or combination (D+S: 19 mg/kg/day, each) by means of the drinking fluid. A fourth group remained untreated. One day after this period the mice received an i.p. injection of one out of 16 drug combinations (crossover design: 0, 2, 4, 6 mg/kg D combined with 0, 6, 12, 18 mg/kg S). Open field behaviour, motor performance, and rectal body temperature were measured. In non-pretreated animals, D and S induced immobility, impairment of coordination and hypothermia in a dose-dependent manner. Excitation appeared after low doses of D (2 mg/kg) and high doses of S (12-18 mg/kg). Acute interactions between D and S were studied by means of isobolographic analysis. Dose-additivity indicating a common mechanism of action was confirmed for immobility, impairment of coordination, and hypothermia whereas excitation revealed a non-additive interaction and was reduced after combined administrations. After chronic pretreatment, the mode of acute drug interaction (dose-additive and non-additive, resp.) remained unchanged. Shifts of the isoboles indicated tolerance, cross-tolerance or sensitization. There was an asymmetry concerning the pretreatment with D and S. Chronic administration of D induced a tolerance to D in regard to all responses and a sensitization to S-effected motor incordination. Chronic administration of S sensitized the sedative and hypothermic responses to acute D. Metabolic tolerance could not account for the subchronic effects since distinct functional responses were concerned in different ways.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Ethanol tolerance in a genetically insensitive selected mouse line

Mouse lines genetically susceptible (COLD) or resistant (HOT) to the acute hypothermic effects of ethanol were previously shown to differ in tolerance development: HOT mice did not develop tolerance, while COLD mice did. The present experiment increased the thermal load on HOT and COLD mice by administering ethanol chronically at an ambient temperature of 4 degrees C. Under these conditions, initial hypothermic responses were as large as 10 degrees C. Both HOT and COLD mouse lines developed significant tolerance by the third daily injection. Tolerance was dose-dependent: significant tolerance was seen only at the higher doses. HOT and COLD mouse lines developed approximately equal degrees of tolerance. Results support the hypothesis of a role for functional demand in the development of tolerance.

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.

Rapid tolerance and cross-tolerance as predictors of chronic tolerance and cross-tolerance

Hypothermia and motor impairment (tilt-plane) tests were used to assess the phenomenon of rapid tolerance to ethanol and cross-tolerance to various alcohols, benzodiazepines, and barbiturates that differ in lipid:water partition coefficients. The hypothermic and motor impairment responses to ethanol were significantly reduced on day 2 in rats receiving ethanol (2 doses of 2 g/kg each for the hypothermia test and 2.3 and 1.7 g/kg for the tilt-plane test) 24 and 22 h earlier compared to the control group pretreated with saline. Ethanol pretreatment resulted in rapid cross-tolerance, on both tests, to the various alcohols (n-propanol, n-butanol, and t-butanol) and the benzodiazepines (chlordiazepoxide, diazepam, oxazepam, and flurazepam) tested. Ethanol pretreatment also conferred clear rapid cross-tolerance to barbital and phenobarbital, but did not result in rapid cross-tolerance to pentobarbital, secobarbital, amobarbital, or thiopental. The results on rapid cross-tolerance on both tests seen in these studies parallel the results obtained 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.

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.

Functional tolerance to the hypothermic effect of phenobarbital in rats

Phenobarbital elicits a hypothermic effect in rats. To determine if functional tolerance develops to this effect on temperature, rats were treated twice daily with IP injections of sodium phenobarbital (PheB) or saline. The PheB doses were increased over 21 days and then were held constant for another 23 days. On the next day (day 45) animals from both groups were given 80, 110 or 160 mg/kg PheB IP and the decrease in rectal temperature after 2 h was determined. Animals were decapitated after the temperature measurement and brain PheB levels were determined. A 1.51-fold shift in the relationship between brain level and response was found for the group given chronic PheB. These results show functional tolerance occurs to the hypothermic effect of PheB. This experiment was done in animals that were on a restricted food regimen. Rats given chronic PheB lost more weight than the group given chronic saline unless extra food was provided. We found that this occurred because the rats given chronic PheB lost more food through the wire cage floor than rats given chronic saline.

Barbiturates and the GABAA receptor complex

The GABA synapse plays an important role in the pharmacologic effects of barbiturates and the mechanisms involved in barbiturate tolerance and dependence. A synopsis of the effects which have been reported to date is found in Tables 1 and 2. Although the acute changes in neurotransmitter uptake and release are nonselective, a lag in the ability of the GABA synapse to compensate for discontinuation of barbiturate exposure may be important in the symptoms of withdrawal. Barbiturates cause changes in the properties of many receptors, but manipulations of the GABAA receptor in vivo correlate with changes in the therapeutic and toxicologic responses to barbiturates, indicating that the GABAA receptor complex plays a pivotal role in the effects of barbiturates. Experiments done in several laboratories show that barbiturate tolerance and dependence cause subtle changes in the properties of the GABAA receptor complex. These observations suggest that decreased GABA-stimulated chloride channel activity and reduced ability to modulate it may be important in causing barbiturate tolerance and the symptoms observed in withdrawal. Selection of drug-resistant rodent strains suggests that there may be genetic factors involved in drug tolerance and dependence. The complexity of the responses of the GABA synapse to both acute and prolonged exposure to barbiturates indicates that it is a valuable model for understanding how the central nervous system responds to drugs and the mechanisms involved in drug addiction.