Effect of raphe lesions on the development of chronic tolerance to pentobarbital and cross-tolerance to ethanol

Effects of acute ethanol exposure on anxiety measures and epigenetic modifiers in the extended amygdala of adolescent rats

Epigenetic mechanisms appear to play an important role in neurodevelopment. We investigated the effects of acute ethanol exposure on anxiety measures and function of histone deacetylases (HDAC) and DNA methyltransferases (DNMT) in the amygdala and bed nucleus of stria terminalis (BNST) of adolescent rats. One hour after ethanol exposure, rats were subjected to anxiety measures. A subset of adolescent rats was exposed to two doses (24 h apart) of ethanol (2 g/kg) to measure rapid ethanol tolerance to anxiolysis. The HDAC and DNMT activities and mRNA levels of DNMT isoforms were measured in the amygdala and BNST. The lower dose of ethanol (1 g/kg) produced neither anxiolysis, nor inhibited the HDAC and DNMT activities in the amygdala and BNST, except DNMT activity in BNST was attenuated. Anxiolysis by ethanol was observed at 2 and 2.25 g/kg, whereas higher doses (2.5 and 3 g/kg) were found to be sedative. DNMT activity in the amygdala and BNST, and nuclear HDAC activity in the amygdala, but not in the BNST were also inhibited by these doses of ethanol. A lack of tolerance was observed on ethanol-induced inhibition of DNMT activity in the amygdala and BNST, and nuclear HDAC activity in the amygdala, as well to anxiolysis produced by ethanol (2 g/kg). The DNMT1, DNMT3a, and DNMT3b mRNA expression in the amygdala was not affected by either 1or 2 doses of 2 g/kg. However, DNMT1 and DNMT3a expression in the BNST was increased, whereas DNMT3l mRNA was decreased in the amygdala, after 2 doses of 2 g/kg ethanol. These results suggest that reduced sensitivity to anxiolysis and the lack of rapid tolerance to the anxiolytic effects of ethanol and inhibition of HDAC and DNMT functions may play a role in engaging adolescents in binge drinking patterns.

Age differences in the expression of acute and chronic tolerance to ethanol in male and female rats

BACKGROUND

Ontogenetic differences in response to ethanol (EtOH) challenge have been observed under a variety of circumstances, including varying reports of developmental differences in the expression of tolerance to EtOH. The purpose of the present experiment was to further explore potential differences in acute (AT) and chronic (CT) tolerance expression between adolescent and adult, male and female Sprague-Dawley rats, using the social interaction test.

METHODS

AT and CT to the social suppressing effects of a moderate dose of EtOH was assessed in adolescent and adult rats following intraperitoneal injections of 2.0 g/kg EtOH or saline daily for 10 days. At test, adults and adolescents were challenged with 1.0 or 1.25 g/kg EtOH, respectively, with AT and CT assessed at 5 and 25 minutes postinjection using ratios of impairment to brain ethanol concentrations (BrECs) at each time period (CT) and within-session declines in impairment relative to BrECs (AT).

RESULTS

In adolescents, 10 days of EtOH pre-exposure resulted in evidence of CT at 25 minutes postinjection, perhaps associated with an enhanced expression of AT. Among adults, signs of CT were seen at 5 minutes postinjection in adults, and may reflect neuroadaptations unassociated with AT, as with evidence of tolerance emerging only in adult control animals repeatedly exposed to saline injection prior to EtOH challenge on test day. Sex differences in tolerance expression were not observed at either age.

CONCLUSIONS

Our results show ontogenetic differences between adolescents and adults in the short- and long-term neuroadaptations that they express in response to repeated perturbations with EtOH. Together these findings add age of exposure and time of testing within the intoxication period as critical variables to be considered when exploring the complex relationship between AT and CT.

The Effects of NMDA and GABAA Pharmacological Manipulations on Acute and Rapid Tolerance to Ethanol During Ontogeny

BACKGROUND

Sensitivity to several ethanol effects increases during ontogeny, perhaps in part because of a notable decline in acute tolerance. In contrast, rapid tolerance to ethanol-induced sedation emerges slowly during ontogeny. This study tested the hypothesis that ontogenetic differences in glutamate and/or gamma-aminobutyric acid systems influence tolerance expression.

METHODS

Sprague-Dawley rats at postnatal day (P)26 or P70 received (+)MK-801, muscimol, or saline before ethanol (3.5 or 4.5 g/kg) or saline on day 1 and ethanol only on day 2. Loss of and time to regain the righting reflex and blood alcohol levels at recovery were recorded. The presence of acute tolerance was indicated as a positive slope of the linear regression of blood alcohol levels at recovery versus ethanol dose. Rapid tolerance was estimated on day 2 by comparing animals given ethanol only on day 2 with those given ethanol on both days.

RESULTS

Acute tolerance on day 1 only was observed at P26; this was disrupted by (+)MK-801 but not muscimol. Evidence for acute tolerance also emerged in adults on day 2. Whereas both drugs increased ethanol sedation at both ages, they did not facilitate ontogenetic expression of rapid tolerance: rapid tolerance was not evident at P26 regardless of pretreatment when indexed in terms of recovery time.

CONCLUSIONS

These data provide further evidence for an ontogenetic dissociation in the expression of acute and rapid tolerance to ethanol-induced sedation. Pharmacological attenuation of the expression of acute tolerance was sufficient but not necessary to delay recovery of righting after ethanol. The greater propensity of young animals to develop acute tolerance, seemingly modulated in part by NMDA receptors, may contribute to their relative resistance to ethanol, although other factors, including pharmacokinetic factors, also contribute to their more rapid recovery from ethanol sedation.

Rapid and Chronic: Two Distinct Forms of Ethanol Tolerance in Drosophila

BACKGROUND

Ethanol tolerance, defined as a reduction in the intensity of the effects of ethanol upon continuous or repeated exposure, is a hallmark of alcoholism. Tolerance may develop at the cellular or neural systems levels. The molecular changes underlying ethanol tolerance are not well understood. We therefore explored the utility of Drosophila, with its accessibility to genetic, molecular, and behavioral analyses, as a model organism to study tolerance development in response to different ethanol-exposure regimens.

METHODS

We describe a new assay that quantifies recovery from ethanol intoxication in Drosophila. Using this recovery assay, we define ethanol pre-exposure paradigms that lead to the development of tolerance. We also use the inebriometer, an assay that measures the onset of intoxication, to study the effects of pharmacological and genetic manipulations on tolerance development.

RESULTS

We show that flies develop different forms of ethanol tolerance: rapid tolerance, induced by a single short exposure to a high concentration of ethanol, and chronic tolerance, elicited by prolonged exposure to a low concentration of the drug. Neither rapid nor chronic tolerance involves changes in ethanol pharmacokinetics, implying that they represent functional rather than dispositional tolerance. Chronic and rapid tolerance can be distinguished mechanistically: chronic tolerance is disrupted by treatment with the protein synthesis inhibitor cycloheximide, whereas rapid tolerance is resistant to this treatment. Furthermore, rapid and chronic tolerance rely on distinct genetic pathways: a mutant defective for octopamine biosynthesis shows reduced rapid tolerance but normal chronic tolerance.

CONCLUSIONS

Flies, like mammals, develop tolerance in response to different ethanol-exposure regimens, and this tolerance affects both the onset of and the recovery from acute intoxication. Two forms of tolerance, rapid and chronic, are mechanistically distinct, because they can be dissociated genetically and pharmacologically.

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.

Interaction between N-methyl-D-aspartate (NMDA) and serotonin (5-HT) on ethanol tolerance

Earlier work from this laboratory had shown that 5-HT is involved in the development of tolerance to ethanol, and that enhancement of 5-HT levels by L-tryptophan accelerated tolerance development. To explore the possibility that NMDA receptors are involved in the 5-HT effect on tolerance, we examined the effect of a noncompetitive N-methyl-D-aspartate (NMDA) antagonist [(+)MK-801] on the ability of L-tryptophan to enhance tolerance to the effect of ethanol on tilt-plane test performance by the rat. L-Tryptophan treatment resulted in the development of rapid tolerance to a dose of ethanol that failed to produce such tolerance by itself. However, prior administration of (+)MK-801 blocked the L-tryptophan effect on rapid tolerance development, in a dose-dependent manner. These results suggest that NMDA receptors are involved in the 5-HT enhancement of ethanol tolerance.

Different patterns of behavior produced by haloperidol, pentobarbital, and dantrolene in tests of unconditioned locomotion and operant responding

Three motor-impairing drugs with different putative mechanisms of action (haloperidol 0.00, 0.075, 0.15, 0.30 mg/kg IP; pentobarbital 0.00, 4.5, 9, 12 mg/kg IP; and dantrolene 0.00, 5, 7.5, 10 mg/kg IP) produced strikingly similar patterns of dose-dependent attenuation in unconditioned locomotor behavior. However, the same drugs and doses produced highly divergent patterns of disruption when tested using different groups of rats in a food-rewarded operant task, which included both response initiation and maintenance components (FR1-FR1 two lever chain). Haloperidol animals began the session as fast as vehicle animals and slowed dose-dependently across trials; pentobarbital animals started off significantly slower than controls but soon achieved comparable speeds; and dantrolene animals were slower throughout the session. These results suggest that the observed neuroleptic-induced deterioration in responding over trials, especially in response initiation, was not simply a result of motoric disruption. Rather, the profile of this deterioration is consistent with the anhedonia hypothesis of neuroleptic action and supports the view that dopamine neurons are involved in the biological basis of food reward.

Effect of raphe lesions on the development of acute tolerance to ethanol and pentobarbital

The effect of electrolytic lesions in the median and dorsal raphe nuclei was tested on acute tolerance development to ethanol and pentobarbital in the rat, as measured by motor impairment on the moving belt test. Acute tolerance to ethanol (1.7 g/kg, IP) or pentobarbital (17.5 mg/kg, IP) was monitored at 12.5, 25, or 50 min in separate subgroups tested only once each. One week of recovery was allowed between ethanol and pentobarbital tests. Median raphe lesions delayed the development of acute tolerance, whereas dorsal raphe lesions produced a negligible effect. These results were seen with both ethanol and pentobarbital. The mesolimbic 5-HT pathway from the median raphe nucleus is important in the development of acute tolerance to ethanol and pentobarbital, as was shown to be the case previously for chronic tolerance.