Learned tolerance to ethanol in the spinal cord

Increased alcohol consumption in urocortin 3 knockout mice is unaffected by chronic inflammatory pain

AIMS

Stress neurocircuitry may modulate the relationship between alcohol drinking and chronic pain. The corticotropin-releasing factor (CRF) system is crucial for regulation of stress responses. The current study aimed to elucidate the role of the endogenous CRF ligand Urocortin 3 (Ucn3) in the relationship between alcohol drinking behavior and chronic pain using a genetic approach.

METHODS

Ucn3 (KO) and wildtype (WT) littermates were subjected to a 24-h access drinking procedure prior to and following induction of chronic inflammatory pain.

RESULTS

Ucn3 KO mice displayed significantly increased ethanol intake and preference compared with WT across the time course. There were no long-term effects of chronic pain on alcohol drinking behavior, regardless of genotype, nor any evidence for alcohol-induced analgesia.

CONCLUSION

The increased drinking in Ucn3 KO supports a role for this peptide in alcohol-related behavior. These data suggest the necessity for more research exploring the relationship between alcohol drinking, chronic pain and the CRF system in rodent models.

Alcohol dependence as a chronic pain disorder

Dysregulation of pain neurocircuitry and neurochemistry has been increasingly recognized as playing a critical role in a diverse spectrum of diseases including migraine, fibromyalgia, depression, and PTSD. Evidence presented here supports the hypothesis that alcohol dependence is among the pathologies arising from aberrant neurobiological substrates of pain. In this review, we explore the possible influence of alcohol analgesia and hyperalgesia in promoting alcohol misuse and dependence. We examine evidence that neuroanatomical sites involved in the negative emotional states of alcohol dependence also play an important role in pain transmission and may be functionally altered under chronic pain conditions. We also consider possible genetic links between pain transmission and alcohol dependence. We propose an allostatic load model in which episodes of alcohol intoxication and withdrawal, traumatic stressors, and injury are each capable of dysregulating an overlapping set of neural substrates to engender sensory and affective pain states that are integral to alcohol dependence and comorbid conditions such as anxiety, depression, and chronic pain.

Effects of ethanol on deep pain evoked by formalin injected in TMJ of rat

It has been reported that ethanol can alter nociceptive sensitivity from superficial tissues, such as skin and subcutaneous region. However, the influence of ethanol on deep pain conditions is not understood. The aim of this study was to demonstrate the acute, chronic and ethanol withdrawal effects on nociceptive behavioral responses induced by the injection of formalin into the temporomandibular joint (TMJ) region of rats. In experiment 1, rats were injected with ethanol (2,5 g/Kg, i.p.) or an equal volume of saline 15 min before the administration of formalin (1.5%) into the TMJ. Rats pretreated with ethanol showed a decrease in nociceptive behavioral responses. In experiment 2, rats were given an ethanol solution (6.5%) or tap water to drink for 4 and 10 days. On day 4, the animals (ethanol group) showed amounts of analgesia when submitted to the TMJ formalin test. Tolerance to the antinociceptive effects was observed on day 10. Behavioral hyperalgesia was verified 12 hr after withdrawal in another group that drank ethanol for 10 days. These results show that ethanol can affect the nociceptive responses related to deep pain evoked by the TMJ formalin test.

Dissipation of contingent tolerance to the anticonvulsant effect of diazepam: effect of the criterion response

The effect of convulsive stimulations on the dissipation of tolerance to the anticonvulsant effect of diazepam was investigated using the kindled-convulsion model. Amygdala-kindled rats were rendered tolerant to diazepam's anticonvulsant effect by 25 "bidaily" (one/48 h) diazepam injections (2.5 mg/kg), each followed 1 h later by a convulsive stimulation. They were then divided into nine groups for the tolerance-dissipation phase of the experiment. Of the nine groups, three received bidaily control handling for one trial, three trials, or seven trials; three received bidaily saline injections, each 1 h before a convulsive stimulation, for one, three, or seven trials; and three received bidaily diazepam injections, each 1 h after a convulsive stimulation, for one, three, or seven trials. Finally, each rat received a tolerance-retention test (i.e., a diazepam injection followed 1 h later by a convulsive stimulation) 48 h after its last tolerance-dissipation trial. The tolerance dissipated gradually but completely over the 4-, 8-, and 16-day test intervals in the rats that received a convulsive stimulation before each injection during the tolerance-dissipation phase, whether they were injected with saline or diazepam; in contrast, tolerance did not dissipate in the rats that received saline injections but no stimulations. Remarkably, the discontinuance of the bidaily diazepam injections, even for 16 days, was not sufficient to dissipate the tolerance that had developed to diazepam's anticonvulsant effect; nor was the continuation of the bidaily diazepam injections sufficient to keep tolerance from dissipating.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of intoxicated practice on the development of acute tolerance to the motor impairment effect of ethanol

The influence of practice while under intoxication was tested on the development of acute tolerance to the motor impairment effect of ethanol. In experiment 1, the motor impairment effect induced by an IP injection of 1.8 g/kg ethanol was quantified after various intervals in separate groups of animals. Lower impairment scores were observed in rats tested at 30 and 45 min after ethanol administration than in those tested at 15 min. In group that was tested repeatedly after ethanol administration, intoxication decreased more rapidly and to a greater extent. The same phenomenon was observed in experiment 2 when a higher dose of ethanol (2.2 g/kg) and later testing (60-180 min after ethanol administration) were employed. To maintain constant blood ethanol levels, those tested at later times received a supplementary dose of ethanol. Impairment scores were lower in rats tested at later times than in those tested earlier. Again, the impairment scores for the practice group decreased more rapidly and to a greater extent. Blood ethanol levels among various groups were essentially the same. Acute tolerance to ethanol can develop without opportunity for practice while under intoxication. Intoxicated practice, however, can facilitate acute tolerance development.

Learning factor in rapid tolerance to ethanol-induced motor impairment

Exposure of male Wistar rats to a single moderate dose (1.7 g/kg, IP) of ethanol (EtOH), followed by intensive intoxicated practice on the moving belt apparatus (a total of 12 min during the first hour after EtOH injection), results in functional tolerance to the motor-impairing effects of a second dose given either 8 or 24 h later. In the absence of intoxicated practice, or after a considerably reduced opportunity for it (a total of 4 min during the first hour after EtOH injection), the same dose of EtOH fails to produce tolerance. Thus, not only the opportunity to practice, but also its extent and possibly its quality are important determinants in the rapid development of intersessional tolerance. In contrast to its rapidity of development, no significant loss of this tolerance is evident three weeks after the tolerance acquisition sessions.

Contingent suppression of tolerance to haloperidol: a dose-response analysis

Rats were given injections of haloperidol (HAL; 0.625 or 2.5 mg/kg) either before (Before groups) or after (After groups) access to sweetened milk on alternate days. Controls (Saline groups) were given injections of saline. At biweekly intervals ("test days"), all groups were given pretest injections of the drug in order to monitor the development of tolerance in the After and Saline groups. Rats in the Before groups showed no tolerance to the initial suppression of milk intake. In contrast, rats in the After groups had greater intakes, although the level of intake declined on subsequent test days in the group given the lower dose. Rats in the Saline groups drank less on the test days than any of the other groups, suggesting that sensitization occurred. These results are consistent with previous findings (29) that tolerance to HAL is suppressed following pretest injections of the drug. The degree of suppression appears to be inversely related to the frequency of such injections.

Factors regulating ethanol tolerance

Ethanol tolerance is a complex phenomenon. Its development is governed by pharmacological and behavioural, as well as genetic factors. The doses of ethanol employed and the duration of ethanol treatment are important pharmacological variables. Behavioural factors such as experience of intoxication and Pavlovian conditioning may also affect the development or manifestation of ethanol tolerance. Genetic factors can influence the development of tolerance directly or/and indirectly through its influence on the initial sensitivity to ethanol. The relevance and implication of tolerance, particularly conditioned tolerance in alcohol abuse and alcoholism, are discussed.

Roles of intoxicated practice in the development of ethanol tolerance

The development of tolerance to the motor impairment effect of ethanol was examined in separate groups of rats receiving and not receiving intoxicated practice. Tolerance to the motor impairment effect of ethanol developed whether or not rats received intoxicated practice during chronic ethanol treatment. Depending on the treatment dosage and test dose, intoxicated practice might enhance the level of tolerance attained. Tolerance to other effects of ethanol (hypothermia and narcosis) developed as a function of the treatment dosage. Intoxicated practice on the moving belt did not modify the development of tolerance to these effects of ethanol. Tolerance to the motor impairment effect of ethanol, however, was retained much longer in the intoxicated practice group following the termination of ethanol treatment.

Response latencies in the tail-flick test depend on tail skin temperature

Tail skin temperatures and tail-flick latencies were simultaneously recorded in male Sprague-Dawley rats exposed to various ambient temperatures (22-30 degrees C). There was a positive correlation between tail skin temperature and ambient temperature and a negative correlation between tail-flick latency and ambient temperature. Importantly, a highly significant negative correlation was present between tail-flick latency and skin temperature, even at constant ambient temperature (22.1 or 23.3 degrees C). Thus, the results of tail-flick testing are highly affected by skin temperature and factors altering the skin temperature must be considered when tail-flick latencies are interpreted in terms of nociception.

[Occurrence of the alcoholic tolerance phenomenon and its relation to the daily frequency of ethanol doses and with the presence of a withdrawal interval, in an experimental model of alcoholism in rats]

In this paper we present the effects of the number of daily toxic doses of alcohol and of the duration of withdrawal period on alcohol tolerance in rats submitted to cycles of 21 doses each. It was observed that alcohol tolerance was increased in animals which received less number of alcohol doses by day and that some degree of tolerance can be detected even after a withdrawal interval of 15 days.

Analysis of the role of behavioural factors in the development of tolerance to the benzodiazepine midazolam

Two experiments were conducted on the acute and chronic effects of the short-acting benzodiazepine midazolam on fixed ratio schedule-maintained operant responding in rats. Acute administration of midazolam produced suppression of responding in large doses but elevation of responding in small doses. Following intermittent (every third day) chronic treatment tolerance developed rapidly to the rate-suppressant effects of large doses of midazolam but did not develop to the rate-elevating effects of small doses, even after chronic treatment with large doses of the drug. Thus, when tolerance was measured in terms of shifts in dose-effect curves, it was manifested as a non-parallel shift in the curve after chronic treatment. Since tolerance developed to the effects of large but not small doses, the observed tolerance could not be attributed to changes in disposition of the drug. The development of tolerance did not depend on whether the drug was given before or after behavioural testing. These findings contrast with data reported in behavioural studies with other sedative-hypnotics (ethanol, barbiturates). Animals tolerant to midazolam showed no cross-tolerance to ethanol, a drug for which there is reliable evidence indicating that behavioural factors play a role in acquisition of tolerance. Tolerance to midazolam cannot therefore be explained in terms of learned strategies acquired as a result of drug-induced loss of rewarding stimuli. This conclusion contrasts with recent suggestions (File, 1985; File and Pellow, 1985) that tolerance to benzodiazepines may be mediated by instrumental conditioning processes.

The contribution of classical conditioning to tolerance to the antinociceptive effects of ethanol

In rats, ethanol increases the latency of the tail-flick reflex to radiant heat. Three experiments examined the contribution of classical conditioning to the acquisition of tolerance to this antinociception. Experiment 1 showed that the antinociception produced by ethanol was dose dependent. The results of Experiment 2 demonstrated that rats exposed to a series of ethanol injections paired with a distinctive environment developed tolerance to this antinociception. In Experiment 3, tolerance was more pronounced in animals that had been exposed to ethanol and tested in the distinctive environment than in animals that had received ethanol in a nondistinctive environment. In contrast to previous reports in the literature, these results show that animals need not practice the tail-flick reflex while intoxicated in order to develop tolerance. Additionally, the data suggest that classical conditioning may contribute to tolerance to the antinociceptive effects of ethanol.

Learned and pharmacologically-induced tolerance to ethanol and cross-tolerance to morphine and clonidine

The development of tolerance to the inhibitory effect of ethanol on the tail-flick reflex was studied in the spinal rat. This preparation was used in order to avoid uncontrolled learning effects. Tolerance due to intoxicated practice (learned tolerance) and tolerance due to mere ethanol exposure (pharmacologically-induced tolerance) were studied in separate experiments. It was found that that learned tolerance to ethanol also caused tolerance to morphine and clonidine, whereas pharmacologically-induced tolerance did not have the same effect. The results challenge the concept of "behaviorally augmented tolerance" and suggest that learned and pharmacologically-induced tolerance involve different basal mechanisms in the CNS.

Learned tolerance to ethanol in a spinal reflex separated from supraspinal control

We have recently reported that ethanol-induced inhibition of the tail flick reflex in intact and spinal rats is diminished during an eight day period if the animals are tested daily under the influence of ethanol. Ethanol only, or testing before ethanol administration, is not followed by tolerance. In the present study we used the tail flick testing of spinal rats to investigate the effect on tolerance development of repetitively triggering the tail flick reflex during intoxication, and of just placing the intoxicated animals in the test apparatus. We also investigated if damage to the tail tissue, due to repetitive prolonged test exposure, would facilitate the reflex and thereby reduce the inhibitory effect of ethanol. The results indicated that triggering of the reflex in the presence of ethanol was necessary for the tolerance to develop. Facilitation of the tail flick reflex, due to damage of the tail tissue, was not revealed. Thus the tolerance observed seems to be caused by an adaption to ethanol learned by structures involved in the tail flick reflex.