The importance of experience in the development of tolerance to ethanol hypothermia

Pregnenolone sulfate, dehydroepiandrosterone sulfate and allotetrahydrodeoxycorticosterone affect rapid tolerance to the hypothermic effect of ethanol

Our previous study showed that the neurosteroids pregnenolone sulfate (PS) and epipregnanolone stimulated and blocked, respectively, the demonstration of chronic tolerance to the incoordinating effect of ethanol. The aim of the present study was to investigate the influence of three neurosteroids on the demonstration of tolerance to ethanol-induced hypothermia in mice using the rapid tolerance paradigm. The first experiment defined the doses of ethanol that did or did not induce rapid tolerance to ethanol-induced hypothermia. In the second, the influence of pretreatment of mice with PS (0.08 or 0.15 mg/kg, i.p.) or dehydroepiandrosterone sulfate (DHEAS; 0.15 or 0.20 mg/kg, i.p.) before ethanol (4.0 g/kg, i.p.) on rapid tolerance was studied. The third experiment examined the effect of allotetrahydrodeoxicorticosterone (ALLOT; 0.10 or 0.20 mg/kg, i.p.) before ethanol (4.0 g/kg, i.p.) on rapid tolerance. Results showed that pretreatment with PS or with DHEAS significantly facilitated the demonstration of rapid tolerance, whereas pretreatment with ALLOT interfered with the demonstration of tolerance to the hypothermic effect.

Genetic differences in ethanol-induced conditioned taste aversion after ethanol preexposure

The present studies examined the development of ethanol-induced conditioned taste aversion in C57BL/6J (B6) and DBA/2J (D2) mice with a history of ethanol preexposure. In Experiment 1, adult male B6 and D2 mice received four preexposure injections of either saline or 4 g/kg ethanol over an 8-day period. After preexposure, all mice were given five conditioning trials consisting of 1-h access to 0.15% w/v saccharin solution followed immediately by ethanol injections (4 g/kg, IP) on all but the last trial. Drug-naive D2 mice showed greater reductions in saccharin intake. Ethanol preexposure reduced the development of ethanol-induced taste aversion in each strain. However, B6 mice showed little taste aversion overall, hindering the characterization of genetic differences in ethanol's preexposure effect. To address this problem, the parameters for taste conditioning were changed in Experiment 2 to more closely match degree of taste aversion in drug-naive mice across both strains. B6 and D2 mice received four preexposure injections of either saline, 2 g/kg ethanol, or 4 g/kg ethanol, or 4 g/kg ethanol. Subsequently, mice received five conditioning trials consisting of 1-h access to 0.2 M NaCl flavor followed by 4 g/kg ethanol (B6 mice) or 2 g/kg ethanol (D2 mice) on trials 1-4. Ethanol-naive mice of each strain developed similar levels of conditioned taste aversion. Ethanol preexposure produced greater retardation of conditioned aversion in B6 mice than in D2 mice. These results demonstrate genetic differences in the ability of ethanol preexposure to reduce the development of ethanol-induced conditioned taste aversion.

Chronic intragastric infusion produces tolerance to ethanol in LS and SS mice

Tolerance to ethanol's effects is seen after chronic injections and after chronic treatment with ethanol-containing liquid diets. However, tolerance associated with injections is often associated with environmental cuing (learned tolerance) and liquid diets do not allow for careful control of dose. The studies reported here demonstrate that chronic (3 g/kg every 6 h for 7 days) intragastric infusion of ethanol produces tolerance to ethanol in both LS and SS mice, as measured by sleep time and effects on open field activity and body temperature. LS mice developed more tolerance for all measures than the SS. The rate of ethanol elimination was increased slightly in both the LS and SS mouse lines following chronic ethanol infusion, which suggests that both lines developed a modest metabolic tolerance to ethanol. In contrast, the waking blood ethanol levels were altered only in the LS mice, which argues that some of the tolerance to ethanol seen in the LS mice is pharmacodynamic tolerance. Thus, chronic intragastric infusion of ethanol is a reliable method for treating animals chronically with ethanol that allows for the precise control of dose and dose interval, oral administration of ethanol, and minimal animal handling that can be associated with drug delivery, thereby reducing the risk of developing learned tolerance to ethanol.

Ambient temperature effects on taste aversion conditioned by ethanol: contribution of ethanol-induced hypothermia

Six experiments examined the effects of low (5-10 degrees C), normal (21 degrees C), or high (32 degrees) ambient temperature on conditioned taste aversion and body temperature changes produced by ethanol, lithium chloride, or morphine sulfate. Fluid-deprived rats received five to seven taste conditioning trials at 48-hr intervals. On each trial, access to saccharin at normal ambient temperature was followed by injection of drug or saline and placement for 6 hr into a temperature-controlled enclosure. Exposure to low ambient temperature facilitated, whereas exposure to high ambient temperature retarded acquisition of ethanol-induced conditioned taste aversion. The ability of an alteration in ambient temperature to influence conditioned taste aversion varied as a function of ethanol dose and was related to ambient temperature's effect on ethanol-induced hypothermia. More specifically, strength of conditioned taste aversion was negatively correlated with core body temperature after ethanol injection. Alterations in ambient temperature alone did not affect ingestion of a paired flavor solution in the absence of drug. Moreover, alterations in ambient temperature did not appear to influence conditioned taste aversion by changing ethanol pharmacokinetics. Finally, high and low ambient temperature did not affect development of taste aversion conditioned by lithium chloride or morphine sulfate. The overall pattern of data presented by these experiments supports the hypothesis that ambient-temperature influences strength of ethanol-induced conditioned taste aversion by altering the hypothermic response to ethanol. More generally, these data support the suggestion that body temperature change induced by ethanol is related to ethanol's aversive motivational effects and may be involved in modulating ethanol intake.

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.

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.

Tolerance to nicotine following chronic treatment by injections: a potential role for corticosterone

C57BL/6 male mice were injected intraperitoneally with nicotine (2.0 mg/kg) or saline three times each day (0800 h, 1300 h and 1800 h) for a period of 12 days and then tested for nicotine tolerance using a series of behavioral and physiological tests. For each of these tests, animals that received chronic nicotine treatment were significantly less sensitive to nicotine challenge than were animals that received chronic saline treatment, as indicated by shifts to the right of dose-response curves. Animals were retested for nicotine sensitivity 2 weeks following cessation of chronic nicotine injections. Tolerance to acute nicotine challenge persisted in nicotine-treated animals. Chronic nicotine treatment by injections did not alter the binding of L-[3H]-nicotine or alpha-[125I]-bungarotoxin in any of eight brain regions. Plasma corticosterone (CCS) levels were determined in animals prior to the initiation of the injection series (day 0), and on days 4, 8 and 12 of chronic treatment, immediately before the first injection of the day. CCS levels in nicotine-treated animals were elevated as compared to saline-injected controls by day 12 of treatment. Nicotine-treated animals also had elevated CCS levels 2 weeks after the last chronic injection. Nicotine-treated animals were, however, tolerant to nicotine-induced CCS release. Since previous studies from our laboratory have demonstrated that plasma CCS levels are inversely correlated with sensitivity to nicotine, it is possible that the tolerance to nicotine measured following chronic treatment by injections is due, at least in part, to the elevation in plasma CCS levels.

The relationship between ethanol-induced hyperglycemia and hypothermia: evidence of genetic correlation

The hyperglycemic and hypothermic responses to acute ethanol exposure (0, 2, 4, 6 g/kg, intraperitoneally) were examined in non-fasted mice selectively bred for sensitivity (COLD line) or insensitivity (HOT line) to ethanol-induced hypothermia. Blood samples and rectal temperatures were obtained immediately before injection and hourly for 4 hr after injection. As expected, COLD mice demonstrated greater and more prolonged reductions in body temperature than HOT mice, especially at the 4 g/kg dose (HOT: -2.58 degrees C, COLD: -5.08 degrees C). Ethanol produced significant dose-dependent elevations in blood glucose levels over the 4-hr sampling period in both lines. The greatest elevations in blood glucose levels were seen at 4 g/kg, with COLD mice (mean = 225.1 mg/dl) showing significantly greater elevations in blood glucose levels compared to HOT mice (mean = 177.0 mg/dl). These results support the hypothesis that the thermic and glycemic effects produced by ethanol are due to related neural processes that share a common genetic component.

Cellular and learned tolerances for pentobarbital hypothermia

The development of behavioral tolerance to pentobarbital-induced hypothermia, as separable from cellular and metabolic tolerance, was established. Pentobarbital (PB) was administered to 4 groups of rats, 2 groups of which received intermittent (INT) IP PB treatment. One of these groups, INT/EXP, experienced the hypothermic (measured as rectal body temperature) drug effect after PB injection. The other group, INT/NONEXP, was monitored for body temperature functions (room temperature) before receiving PB (vehicle administration) and then prevented from experiencing PB-induced hypothermia by maintenance of body temperature with a towel wrap restraint and a heating lamp. The INT/EXP group also received equivalent exposure to this towel wrap after vehicle administration. Two other groups received chronic PB treatment (IP and in ground chow), one with experience for hypothermia after injections (CHR/EXP) and one prevented from experiencing the hypothermia (CHR/NONEXP). These groups also received equivalent exposure to the body temperature (at room temperature) testing and towel wrap restraint, EXP rats after vehicle injections and NONEXP after drug injections. A postchronic test of all groups compared the extent of PB hypothermia to prechronic test effects to assess the degree of tolerance. The INT/EXP group demonstrated behavioral tolerance for PB-induced hypothermia, as contrasted with the INT/NONEXP group which demonstrated little or no tolerance. Prominent tolerance was noted in both chronic groups for PB hypothermia, without a significant difference between them. After the postchronic test, chronic treatment was discontinued for 9 days (withdrawal) followed by 9 days of extinction training (vehicle behavioral testing). The two intermittent groups demonstrated no change in the hypothermic drug response during the postwithdrawal and postextinction drug tests.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular and learned tolerances for ethanol hypothermia

Four groups of rats received ethanol: 1) intermittently while experiencing hypothermia, 2) chronically while experiencing hypothermia, 3) intermittently while protected from hypothermia, and 4) chronically while being protected from hypothermia. On postchronic testing, Group 1 showed tolerance to 2.0 and 2.3 but not 2.7 g/kg ethanol, Group 2 was tolerant to all 3 doses, Group 3 was tolerant to none, and Group 4 was tolerant only to 2.7 g/kg. On withdrawal of chronic ethanol or vehicle, Groups 1 and 2 showed trends to lose tolerance which became significant after subsequent extinction training. The treatments were repeated in other rats up to the postchronic test for tolerance, after which they were killed at 15-120 min after ethanol to assay serum and brain concentrations. Serum and brain levels of ethanol were higher in Groups 2 and 4 despite less intense hypothermia (i.e., no metabolic tolerance). Analysis of covariance indicated less tolerance in Group 1 vs. Group 2 and Group 3 vs. Group 4 for the same brain levels of ethanol (i.e., cellular tolerance in Groups 2 and 4). Therefore, both learned and cellular tolerances were observed in these subjects and appeared to be separable phenomena according to the various treatments imposed.

Enhancing effects of sound on methamphetamine-induced behavioral aberrations in the rat: a model of relapse of schizophrenia-like symptoms

Conditioning of methamphetamine (MAP)-induced behavioral aberrations, hyperactivity (HA), stereotyped (SB) and bizarre behavior (BB) in rats were examined using Pavlovian conditioning methods. A speaker-sound (SS) was used as the conditioning stimulus (CS) and MAP-induced behavior as the unconditioned stimulus (UCS). The rats belonging to the Sound Group were repeatedly administered with MAP or saline paired with SS in an observation room and those in the Soundless Group with MAP only in the housing room. After chronic administration with MAP, conditioned behavioral responses (CBR) such as HA, SB, and BB were observed more intensely in the Sound Group than in the Soundless Group while being subjected to SS. CBR were greatest on the 30th day after the last MAP administration and they did not disappear for 120 days. CBR were considered as analogous to the spontaneous relapse of schizophrenia-like symptoms. Furthermore, as a certain behavior which had been conditioned could easily be observed as CBR with both specific (SS) and nonspecific environmental cues (handling), it is difficult to distinguish the withdrawal behavioral effects of MAP that lacks those of CBR.

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.

Effect of ingestion-contingent hypothermia on ethanol self-administration

This study tested the hypothesis that ethanol intake and preference for ethanol-paired flavors are inversely related to the magnitude of hypothermia induced by ethanol. Fluid-deprived rats were given 15-min daily access to 7% ethanol in one of two flavored saccharin solutions. Consumption of one flavored ethanol solution (counterbalanced) was consistently followed by 6-hr placement in a room maintained at 32 degrees C, whereas, consumption of the other flavored ethanol solution was followed by maintenance at room temperature (21 degrees C). Animals experienced less hypothermia when ethanol was followed by exposure to 32 degrees C and eventually drank more of the flavored ethanol that preceded this exposure. Moreover, intermittent two-bottle choice tests revealed development of a preference for the flavor that preceded exposure to 32 degrees C. An "extinction" phase indicated that this preference was not due to association with the thermal environments, but depended on differences in ethanol-induced hypothermia. These results support the conclusion that oral intake of ethanol is modulated by ethanol-induced hypothermia, most likely through a conditioned taste aversion mechanism.

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.

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

The contribution of Pavlovian conditioning of environmental cues has been studied in relation to tolerance to ethanol-induced hypothermia and cross-tolerance to pentobarbital. Two groups of 12 male Sprague-Dawley rats were exposed every other day to a distinctive set of environmental cues paired with an IP injection of either ethanol 2.5 g/kg or an equivalent volume of isotonic saline. On alternating non-drug days, both groups received saline in the animal room. When they were tested for tolerance to the hypothermic effect of ethanol 2.5 g/kg and cross-tolerance to pentobarbital 25 mg/kg in each environment, tolerance and cross-tolerance in the ethanol-treated group were significantly more pronounced in the ethanol-paired environment than in the saline-paired environment. This indicates the importance of a conditional factor in tolerance and cross-tolerance in this paradigm. Determination of blood levels of ethanol and pentobarbital at various times after injection indicated that conditioned tolerance and cross-tolerance can be explained in part by dispositional factors.

Microwave attenuation of ethanol-induced hypothermia: ethanol tolerance, time course, exposure duration, and dose response studies

Four experiments were conducted to quantify the reported attenuation by microwave (MW) irradiation of ethanol-induced hypothermia. In one experiment rats were irradiated (continuous wave 2.45 GHz, specific absorption rate = 0.3 W/kg) or sham irradiated for 45 min, injected with 3.6 g/kg, 20% (v/v) ethanol (EtOH) or saline (NaCl) i.p.. Colonic temperature was monitored at 20-min intervals for 2 h. This procedure was repeated for 8 days to determine the rate of tolerance development to the hypothermic effect of ethanol. While MW irradiation did significantly attenuate EtOH-induced hypothermia, it did not enhance or retard the rate of tolerance development. To determine the duration of irradiation necessary to attenuate EtOH-induced hypothermia, groups of rats were irradiated or sham irradiated for 5, 15, 30, or 60 min prior to EtOH injection and subsequent temperature measurements. The attenuation was apparent only after 60 min of irradiation. To determine the duration of the attenuation effect after irradiation, rats were injected with EtOH or NaCl at 0, 30, 60, 120, or 480 min after 45 min of irradiation or sham irradiation. The attenuation effect was apparent among rats injected 0 to 30 min after irradiation and for the first 40 min for groups injected at 120 min. Additional rats were injected with NaCl or 0.9, 1.8, or 2.7 g/kg of EtOH i.p. following 45 min of irradiation or sham irradiation to determine if the attenuation effect depends on the dose of EtOH administered. Attenuation of EtOH-induced hypothermia was more apparent at lower doses of EtOH than at higher doses. These results indicate that the effect is an acute response to irradiation, and rule out several other potential explanations.

Chronic functional ethanol tolerance in mice influenced by body temperature during acquisition

Previous studies have found that body temperature during intoxication influences brain sensitivity to ethanol with the sensitivity being less at cool than at warm body temperatures. If this effect of temperature reflects alterations in the acute membrane perturbing action of ethanol, as suggested by in vitro studies, then body temperature reduction (hypothermia) during tolerance acquisition should reduce the effectiveness of a given ethanol concentration and, in turn, should reduce the development of chronic functional ethanol tolerance. To test this hypothesis, adult drug-naive C57BL/6J mice were injected i.p. once daily for five days with 3.6 g/kg ethanol (20% w/v) and were exposed to 34 degrees C or 25 degrees C for five hours following injection. On day 6, both ethanol acquisition groups and naive mice were injected i.p. with 4.0 g/kg ethanol and exposed to 25 degrees C. During acquisition, the group exposed to 34 degrees C had significantly higher body temperatures than the mice exposed to 25 degrees C, and there were no statistically significant differences in blood ethanol concentrations between treatment conditions. The extent of tolerance on day 6, measured by sleep-times and wake-up blood and brain ethanol concentrations versus naive mice, was significantly greater in the 34 degrees C acquisition group than in the 25 degrees C acquisition group. The results demonstrate that body temperature influences tolerance development in the manner predicted by membrane perturbation theories of anesthesia and adaptation based tolerance theories.

[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.

Influence of ambient temperature on the development and maintenance of tolerance to ethanol-induced hypothermia

The development of tolerance to the hypothermic effect of ethanol was examined during chronic ethanol treatment (5 g/kg PO daily) at various ambient temperatures (Ta). Tolerance to the hypothermic effect of ethanol, monitored at five-day intervals for 25 days, developed rapidly when ethanol treatment was carried out at 4 degrees C. On the other hand, rats receiving ethanol treatment at a Ta of 36 degrees C, at which they did not experience hypothermia, acquired tolerance more slowly, but achieved the same level of tolerance as other groups after 25 days of treatment. This cannot be accounted for by the repeated testing at 21 degrees C at five-day intervals, since it was also observed under a non-repeated testing condition. Once tolerance to the hypothermic effect of ethanol was acquired, termination of ethanol treatment resulted in the loss of tolerance, but mere prevention of the hypothermic effect of ethanol did not. These results suggest that tolerance still developed even though the organisms did not experience hypothermia during ethanol treatment. Therefore there appears to be a component of tolerance, that depends upon a direct cellular action of the drug, as distinct from the physiological consequences of that action. However, variation in the degree of physiological disturbance (hypothermia) during drug exposure can modulate the rate of development of this tolerance.

Drug interactions with ethanol. Effects on body temperature and motor impairment

Single doses of d-amphetamine, chlorpheniramine or diazepam were combined with ethanol under two conditions: (i) in drug-naive mice and (ii) in mice which had been given a single dose of ethanol 72 hr previously. Ethanol was administered orally at doses of 6.0, 3.0 or 1.5 g/kg; doses of d-amphetamine, chlorpheniramine or diazepam were given intraperitoneally. Three parameters were measured; changes in rectal temperature, forced motor coordination, as evaluated by rotarod performance and concentrations of ethanol in blood. d-Amphetamine and chlorpheniramine attenuated the hypothermia induced by ethanol but had no effect on the motor-impairing effect of ethanol. Hypothermia induced by diazepam was unaffected by ethanol, but the combination appeared to impair maximally rotarod performance. Concentrations of ethanol in blood did not differ between ethanol-naive mice and mice which had received the same dose of ethanol 72 hr previously. Changes in body temperature and intoxication have been attributed to central actions of ethanol; however, the differential results obtained from the interactions between these drugs suggest differing sensitivities of the various systems which are affected by ethanol.

Ethanol withdrawal in mice precipitated and exacerbated by hyperbaric exposure

Mice were fed an ethanol-containing liquid diet for 9 days. On removal of the diet, exposure to 12 atmospheres absolute of a mixture of helium and oxygen precipitated earlier withdrawal, increased withdrawal scores for the first 6 hours, and increased the peak withdrawal intensity compared to dependent animals exposed to control conditions. The enhanced withdrawal did not appear to reflect alterations in ethanol elimination, oxygen or helium partial pressures, body temperature, or general excitability. These results extend to chronically treated animals the evidence that hyperbaric exposure antagonizes the membrane actions of ethanol.

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.

Learned tolerance to ethanol in the spinal cord

Learning has been claimed to be of major importance in the development of tolerance to ethanol. In the present study we investigated the influence of learning on tolerance to ethanol-induced inhibition of a spinal reflex (tail-flick response) in intact and spinal rats. On day 1 and 9, groups of rats were injected with either ethanol 2.5 g/kg IP or saline 30 min prior to tail-flick testing. On days 2-8 the groups were treated differently in order to reveal the importance of the drug alone, the test alone and the combination of the two on development of tolerance. On day 10, the rats rendered tolerant in the home room were transferred to a new test room to be tested. Both in intact and spinal rats development of tolerance was observed only if the animals were repetitively tested while intoxicated. Tolerance acquired in the home room was not attenuated by transfer to a new environment. Results in the spinal rats suggested that adaptive mechanisms leading to tolerance may also be located in the spinal cord. The tolerance observed may be regarded as learned from practice while intoxicated.