A. E. LeBlanc

Acute tolerance to ethanol in the rat

Male Wistar rats were examined on the moving belt test at approximately 10, 30 and 60 min after administration of ethanol in doses ranging from 1.0–2.8 g/kg. Immediately after the test, each animal was sacrificed and ethanol concentrations were measured. The regression line of impairment as a function of brain alcohol concentration showed a progressive shift toward higher brain levels with increasing time after alcohol administration. The results confirm the existence of acute tolerance to ethanol, as defined by reduced impairment of function for a given blood level on the falling versus the rising arm of the blood alcohol curve. Confusion of interpretation due to arterio-venous differences in alcohol concentration was ruled out by simultaneous measurements in arterial blood and brain. Practice effects were ruled out by testing each animal only once.

Behavioral augmentation of tolerance to ethanol in the rat

Three groups of rats were trained on a circular maze task, then were tested under the influence of ethanol. Thereafter, all three groups received ethanol daily, on different schedules. One group (“psychological”) received ethanol (1.2 g/kg i.p.) just before each treatment session; another group (“physiological”) received the same dose immediately after the session; and the control group received only saline. All three groups were tested under ethanol every fourth day. The psychological group showed significant tolerance by the second test day, and maximal tolerance by the fourth. The physiological group reached the same maximum tolerance by the sixth test day, while the controls showed no increase in tolerance. Addition of daily gavage with ethanol (6 g/kg) did not modify the level of tolerance in the psychological or physiological groups, but raised the controls to the same level as the others. None of the changes in tolerance were attributable to increased rate of ethanol elimination. It is concluded that production of tolerance by these various techniques is distinguishable only with respect to rate.

Acquisition and loss of behaviorally augmented tolerance to ethanol in the rat

The phenomenon of behavioral augmentation of tolerance (BAT) to ethanol (EtOH) in the rat was replicated in studies using the moving belt test of intoxication. Rats performing the test daily under the influence of EtOH (2.2 or 2.5 g/kg i.p.) developed tolerance more rapidly than those receiving the same dose after each daily session on the belt. However, both groups reached the same maximum level of tolerance. Acceleration of tolerance by BAT was proportional to the frequency of performance under the influence of EtOH when total exposure to EtOH was held constant. The degree of tolerance produced by BAT could not be increased by daily gavage with a large dose (6 g/kg) of EtOH. After termination of EtOH administration, tolerance produced by BAT was lost at the same rate, whether or not daily alcohol-free sessions on the belt test were given. These findings are consistent with the hypothesis that BAT and conventionally produced tolerance differ only in rate.

Generalization of behaviorally augmented tolerance to ethanol, and its relation to physical dependence

Adult rats, required to perform a motor coordination task while under the influence of ethanol (2.2 g/kg) daily for 12 days, developed significant tolerance to the effects of ethanol, not only on this test but also on a food-motivated maze task. At the same time, they showed reduced shock threshold for production of a startle response, and reduced open field test scores 23 hrs after the last dose of ethanol. Other animals receiving the same dose of ethanol immediately after each training session on the coordination task, and others performing the task and receiving no ethanol, showed no tolerance on either test and no change in shock threshold or open field behavior. The results suggest that behaviorally augmented tolerance depends on basic neuronal adaptive changes indistinguishable from those accompanying physiological tolerance and dependence.

The effects of chronic administration of ethanol on startle thresholds in rats

The thresholds for startle responses to electric shock were measured in adult male Wistar strain rats given ethanol daily in doses rising from 3 to 7 g/kg over a 30-day period, and in controls receiving equicaloric doses of sucrose. Tests made 23, 36, or 47 h after ethanol (i.e., during partial or complete ethanol withdrawal) gave threshold values significantly lower than those obtained with sucrose-treated controls. The difference became greater after longer ethanol treatment and larger doses. However, when threshold measurements were made under the acute influence of ethanol in the experimental group, the mean values were virtually equal to those of the sucrose controls. This normalization, by ethanol, of a disturbance produced by absence of ethanol in a chronically treated animal is indicative of physical dependence. Following termination of ethanol treatment there was a gradual return of startle thresholds almost to control values over a relatively short period, indicating that the changes underlying the hyperexcitability are readily reversible.

Effect of acute and chronic ethanol administration on serotonin turnover in rat brain.

In rats treated acutely and chronically with ethanol, brain serotonin (5-HT) turnover was determined by measuring in the same animals the rate of accumulation of 5-HT and the rate of decline of 5-hydroxyindoleacetic acid (5-HIAA) in the brain after administration of the MAO inhibitor pargyline-HCl. In the acute study, 5-HT turnover was not altered after a single 4 g/kg oral dose of ethanol. In two separate studies, it was shown that chronic administration of ethanol in the form of a liquid diet for 32 days also did not alter brain 5-HT turnover, even when the rats had developed a significant degree of tolerance to the motor-impairing effects of ethanol. These results suggest that tolerance to ethanol is unrelated to any change in brain 5-HT turnover.

Effect of modification of brain serotonin (5-HT), norepinephrine (NE) and dopamine (DA) on ethanol tolerance.

Rats were permanently depleted of brain dopamine (DA), serotonin (5-HT), 5-HT+norepinephrine (NE), or NE +DA by intraventricular injection of either 5,7-dihydroxytryptamine (5,7-DHT) or 6-hydroxydopamine (6-OHDA) with or without pretreatment with desmethylimipramine (DMI). Following 1 week of recovery from surgery, daily treatment with ethanol (5 g/kg, PO) or isocaloric sucrose was carried out for a period of 20–25 days. Testing at 5-day intervals showed that chronic ethanol treatment produced tolerance to the hypothermic and motor impairing effects of ethanol. Depletion of 5-HT alone retarded tolerance, while depletion of NE or DA alone produced no effect. Combined depletion of both NE and 5-HT, however, completely inhibited tolerance development. The inhibition of tolerance development by combined depletion of both NE and 5-HT is dicussed in terms of a reciprocal relationship between these two systems.

Initial sensitivity versus acquired tolerance to ethanol in rats selectively bred for ethanol sensitivity

Chronic tolerance to ethanol-induced sleep, motor impairment (moving belt test), and hypothermia were examined in two lines of rats that had been selectively bred for their different initial sensitivities to ethanol. In agreement with previous work (Mayer et al. 1982, 1983), the least-affected (LA) rats were found to be less sensitive than their most-affected (MA) counterparts in all three tests. Chronic treatment with ethanol resulted in a more rapid and more marked tolerance development in MA animals than in LA ones. The two lines did not differ in final level of tolerance achieved for either sleep time or hypothermia. However, significant differences were observed with respect to the moving belt test, in that at the end of chronic ethanol treatment the MA animals were more resistant to ethanol than the LA ones. These studies support the existence of a relationship, but not necessarily a direct genetic linkage, between initial sensitivity and acquired tolerance.

Role of serotonergic and adrenergic systems in alcohol tolerance

Abstract 1. 1. This paper attempts to provide an overview of our research which has led us to the concept of tolerance as adaptation to drug effects rather than adaptation to the drug itself. 2. 2. This notion can be expanded to include drug tolerance as an example of a range of adaptive manifestations of neuroplasticity. 3. 3. The suggestion that serotonin (5-HT) is involved in general adaptive processes gave impetus to exploration of its role in tolerance. 4. 4. The paper focusses, therefore, primarily on the line of research that started with whole brain manipulation of brain 5-HT and evolved into a study of the effects of discrete and localized depletion of 5-HT. 5. 5. The evidence for the role of median raphe nucleus, the limbic 5-HT, central norepinephrine (NE), and Des-Gly 9 (Arg 8 ) vasopressin (DGAVP) in ethanol tolerance is also provided, with the relationship between the median raphe nuclei and the limbic system most clearly delineated.

Role of serotonin in tolerance to ethanol and barbiturates: evidence for a specific vs. non-specific concept of tolerance.

The results of our recent investigations have suggested that tolerance and cross-tolerance development to motor-impairing and hypothermic effects of ethanol was slowed when brain serotonin (5-HT) was extensively depleted by treatment with p-chlorophenylalanine (p-CPA). These findings have been extended by the observation that p-PCA also slowed the development of tolerance to motor-impairing effects of barbital whether tolerance was tested repeatedly in the same animal or in separate subgroups being tested only once. Additional support was provided by the demonstration that intracerebral injection of 5,7-dihydroxytryptamine (-DHT), which is known to deplete 5-HT markedly, also slowed the development of tolerance to motor-impairing and hypothermic effects of ethanol. In addition, when brain 5-HT level was elevated by administration of L-tryptophan, the rate of tolerance development to ethanol, as measured by motor impairment and hypothermia, was accelerated. In contrast to 5,7-DHT, intracerebral injection of 5,6-DHT was surprisingly found to accelerate the development of tolerance to ethanol. Upon further investigation, however, it was determined that the 5,6-DHT treatment depleted brain 5-HT levels by only 20% and, in addition, resulted in the development of supersensitivity. These results further confirm and extend the generality of our observations that 5-HT may be involved in the development of tolerance and cross-tolerance to sedatives. The possibility of a non-specific vs. specific effect of the serotoninergic system (as well as other aminergic systems) in tolerance and neuroplasticity deserves further investigation. The possible significance of these findings and the role of 5-HT (and noradrenaline) in the mechanism of tolerance are discussed in terms of analogy to enzyme or receptor mechanism.