Genetic tools in the study of drug self-administration

Making animals alcoholic: shifting laboratory models of addiction

The use of animals as experimental organisms has been critical to the development of addiction research from the nineteenth century. They have been used as a means of generating reliable data regarding the processes of addiction that was not available from the study of human subjects. Their use, however, has been far from straightforward. Through focusing on the study of alcoholism, where the nonhuman animal proved a most reluctant collaborator, this paper will analyze the ways in which scientists attempted to deal with its determined sobriety and account for their consistent failure to replicate the volitional consumption of ethanol to the point of physical dependency. In doing so, we will see how the animal model not only served as a means of interrogating a complex pathology, but also came to embody competing definitions of alcoholism as a disease process, and alternative visions for the very structure and purpose of a research field.

Differences in the urocortin 1 system between long-sleep and short-sleep mice

There is evidence that the peptide urocortin 1 (Ucn1) may be involved in mediating some of the effects of ethanol. The purpose of the present study was to characterize Ucn1 immunoreactivity in mice selectively bred for either high or low sensitivity to ethanol-induced sedation, with additional differences in their response to ethanol-induced hypothermia. The brains of naïve male mice of the inbred long sleep/short sleep (ILS/ISS) selected lines were analyzed by immunohistochemistry. Significant differences were found between lines in the number of Ucn1-containing cells in the non-preganglionic Edinger-Westphal nucleus (npEW, the main source of Ucn1 in the brain); with the ISS mice having more cells. However, significant differences in the optical density of Ucn1 immunoreactivity in individual npEW cells and differences in cell area were also found between lines, with ILS mice having a greater density of Ucn1 per cell and having larger cells in the npEW. Importantly, the ILS mice also had a significantly greater number of Ucn1-positive terminal fibers than ISS mice in the lateral septum and the dorsal raphe nucleus, projection areas of Ucn1-containing neurons. These results suggest that the greater sensitivity of ILS than ISS mice to the hypothermic effects of ethanol could be mediated by stronger innervation of the dorsal raphe by Ucn1-containing fibers. In addition, these results lend further support to previous findings implicating Ucn1-containing projections from npEW to the dorsal raphe in ethanol-induced hypothermia.

Blood alcohol concentrations after scheduled access in high-alcohol-preferring mice

Development of procedures yielding substantial blood alcohol concentrations during voluntary access to an alcohol solution in mice is necessary to further characterize genetic and neurobiologic mechanisms underlying alcohol self-administration. Although, in experimental situations, some populations of mice readily drink an alcohol solution, results from previous studies have not typically revealed high blood alcohol concentrations after voluntary access, probably because of the high alcohol metabolism rate in mice. Toward development of a murine drinking model, 36 selectively bred high-alcohol-preferring mice of both sexes were subjected to a 30-min scheduled-access procedure by using saccharin fading to gradually introduce an alcohol solution. Mice had ad libitum access to food and water 24 h a day. The alcohol solution was available 1 h after the start of the dark part of the cycle for 30 min per day, 5 days per week. After complete removal of saccharin from the drinking tubes, mice consistently drank 1.4 g/kg of a 10% [volume/volume (vol./vol.)] alcohol solution in 30 min. Analysis of tail blood samples, taken immediately after the end of the 30-min access period, indicated blood alcohol concentrations were tightly correlated with alcohol intakes (range, 6-130 mg/dl; average, nearly 60 mg/dl). A concentration-response function of 10%, 12%, 15%, 18%, and 21% (vol./vol.) alcohol solutions indicated an inverted U-shaped relation between alcohol intake and alcohol concentration, with peak intake of greater than 1.75 g/kg per 30 min when a 15% alcohol solution was available. No sex differences were seen. These findings indicate the utility of this procedure in obtaining pharmacologically relevant blood alcohol concentrations after voluntary oral self-administration of an alcohol solution in mice.

Role of Fyn tyrosine kinase in ethanol consumption by mice

BACKGROUND

Mice deficient for the intracellular protein Fyn tyrosine kinase (fynZ/fynZ mice) have been reported to show increased alcohol sensitivity and lack of tolerance to the effects of ethanol. To further study the involvement of Fyn in neurobehavioral effects of alcohol, we examined ethanol consumption and relapse drinking behavior in fynZ/fynZ mice.

METHODS

FynZ/fynZ and wild-type mice were given a free choice between water and increasing concentrations of ethanol (2-16%). Once a stable baseline of 16% ethanol consumption was established, access to ethanol was withdrawn for 2 weeks and then reinstated, to measure the alcohol deprivation effect (ADE). Forced swim stress was performed thereafter on 2 consecutive days. In a final experiment we studied alcohol sensitivity by measuring ethanol-induced loss of righting reflex (LORR).

RESULTS

The concentration of available ethanol had a significant effect on ethanol consumption and preference; however, there was no significant effect of genotype on these measures. Deprivation from ethanol led to a significant increase in ethanol consumption by all mice with no significant impact of genotype on ethanol consumption or water consumption during the ADE. Two consecutive days of forced swim stress led to a significant increase in ethanol consumption; again however, genotype had no effect on stress-associated ethanol consumption. Surprisingly, however, FynZ/fynZ mice showed no differences in alcohol sensitivity when compared to wild-type animals, in contrast to previously reported results ( Miyakawa et al., 1997).

CONCLUSIONS

Deletion of the Fyn tyrosine kinase gene may be involved in ethanol sensitivity but this effect may depend on a gene-environment interaction. Fyn does not influence ethanol consumption, neither under basal conditions nor following a deprivation period or stress. This finding indicates that phosphorylation and activation of N-methyl-D-aspartate (NMDA) receptors through Fyn is not a critical mechanism in alcohol drinking or relapse behavior.

Inbred mouse strains vary in oral self-selection of nicotine

Inbred mouse strains differ in sensitivity to a first dose of nicotine and in the development of tolerance to nicotine. The experiments reported here used six inbred mouse strains (A, BUB, C3H, C57BL/6, DBA/2, ST/b) that differ in sensitivity to an acute challenge dose of nicotine to determine whether differences in oral self-selection of nicotine exist. Animals were presented with solutions containing nicotine or vehicle (water or 0.2% saccharin) and their daily intake of the two fluids was measured for 4 days starting with a 10 micrograms/ml nicotine solution. This was followed by sequential 4-day testing with 20, 35, 50, 65, 80, 100, 125, 160 and 200 micrograms/ml nicotine solutions. The strains differed dramatically in their self-selection of nicotine and in maximal daily dose (mg/kg); the rank order of the strains was C57BL/6 > DBA > BUB > A > or = C3H > or = ST/b for both the tap water and 0.2% saccharin choice experiments. Correlations between nicotine consumption and sensitivity to nicotine, as measured by a battery of behavioral and physiological responses, were also calculated. Strain differences in nicotine intake were highly correlated with sensitivity to nicotine-induced seizures. As sensitivity to nicotine-induced seizures increases, oral self-selection of nicotine decreases. This finding may suggest that this toxic action of nicotine serves to limit intake.

Enhancement of morphine self-administration in drug naive, inbred strains of mice by acute emotional stress

The primary reinforcing effect of morphine was compared in two genetically inbred strains of mice (C57BL/6 and DBA/2) using the intravenous self-administration procedure in drug naive animals. The morphine self-administration differed between the mouse strains. DBA/2 but not C57BL/6 acquired self-administration of morphine with a bell-shaped unit dose-response curve. Acute physical stress induced by electrical footshocks did not significantly affect the self-administration in both strains. Acute emotional stress induced by forcing mice to witness another mouse being subjected to acute physical stress caused a shift of the bell-shaped unit dose-response curve of morphine self-administration to the left in the DBA/2 mice. The C57BL/6 mice, which initially failed to demonstrate stable self-administration, started to self-administer morphine after emotional but not physical stress. Emotional distress may increase the individual sensitivity to the rewarding effects of morphine and may render an individual more susceptible to development of drug dependence.

Genetic control of alcohol deprivation effect in congenic mice

Male mice of A(4R) and AKM congenic resistant strains, differing in E alpha and D loci of H-2 major histocompatibility complex (MHC) only, were tested for cross-maze exploration and for behavior in unavoidable slip funnel situation. On the following 15 weeks the mice had free access to food, water, and 30% alcohol. On weeks 16-17 the individual alcohol and water intakes were measured before and after 3 days of alcohol deprivation to evaluate the alcohol deprivation effect (ADE), which is known to depend upon both genetic and environmental factors. Unlike AKMs, the A(4R)s displayed shorter latency to start cross-maze exploration, greater time of immobility, less time in avoidance attempts in the slip funnel, and demonstrated negative ADE: the short-term, postdeprivation decrease in alcohol intake. It is concluded that the ADE and some of its predictors depend upon E alpha, and/or D loci of the MHC. In the A(4R) strain, the individual correlation between the ADE and the number of immediate reentries, right, and left turns, differed significantly from zero and from the corresponding values in the AKM strain, suggesting a gene-environment interaction.

Localization of genes influencing ethanol-induced conditioned place preference and locomotor activity in BXD recombinant inbred mice

Genetic differences in ethanol's ability to induce conditioned place preference were studied in 20 BXD Recombinant Inbred (RI) mouse strains and in the C57BL/6J and DBA/2J progenitor strains. Male mice from each strain were exposed to a Pavlovian conditioning procedure in which a distinctive floor stimulus (CS+) was paired four times with ethanol (2 g/kg). A different floor stimulus (CS-) was paired with saline. Control mice were injected only with saline. Floor preference testing without ethanol revealed significant genetic differences in conditioned place preference, with some strains spending nearly 80% time on the ethanol-paired floor while others spent only 50% (i.e., no preference). Control mice showed genetic differences in unconditioned preference for the floor cues, but unconditioned preference was not genetically correlated with conditioned preference. There were also substantial genetic differences in ethanol-stimulated activity, but contrary to psychomotor stimulant theory, ethanol-induced activity on conditioning trials was not positively correlated with strength of conditioned place preference. However, there was a significant negative genetic correlation (r = -0.42) between test session activity and preference. Quantitative trait loci (QTL) analyses showed strong associations (P < 0.01) between conditioned place preference and marker loci on chromosomes 4, 8, 9, 18 and 19. Weaker associations (0.01 < P < 0.05) were identified on several other chromosomes. Analysis also yielded several significant QTL for unconditioned preference, ethanol-stimulated activity, and sensitization. Overall, these data support the conclusion that genotype influences ethanol-induced conditioned place preference, presumably via genetic differences in sensitivity to ethanol's rewarding effects. Moreover, several chromosomal regions containing candidate genes of potential relevance to ethanol-induced conditioned place preference have been identified.

Significant differences in the activity of midbrain dopamine neurons between male Fischer 344 (F344) and Lewis rats: an in vivo electrophysiological study

There is a significant difference between the Lewis and Fischer 344 rats regarding their propensity to self-administer drugs of abuse. This study compares the number and the firing pattern of spontaneously active dopamine (DA) neurons in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in these two strains of rats. This was accomplished using the technique of in vivo extracellular recording. The number of spontaneously active DA neurons in the SNC and VTA of the Lewis rat were significantly lower compared to that of the F344 rats. Furthermore, a greater percentage of SNC and VTA DA neurons in the Lewis rats exhibited a burst firing pattern compared to F344 rats. These findings indicate that F344 and Lewis rats possess differences in mesolimbic DA neuronal activity and this may be a factor in explaining the different propensities between the two strains to self-administer drugs.

Strain differences in the analgesic and reinforcing action of morphine in mice

The analgesic and reinforcing effects of morphine were compared in four strains of mice (C57BL/6, BALB/c, DBA, CBA). The analgesic action of morphine was measured in tail immersion (49 degrees C), hot plate (60 degrees C), and tail clip (four-point scale of nociceptive response) tests. The reinforcing action of morphine was studied in i.v. self-administration and conditioned place preference techniques. The results demonstrate strain differences in the analgesic and reinforcing action of morphine in mice. The relative rank order of the strains varied for the several tests as well as for the morphine effects. The lack of correlation between analgesic and reinforcing action of morphine in inbred strains supports the conclusion that analgesia and reinforcement are separate processes with different genetic control.

Operant ethanol-reinforced behavior in P, NP, HAD, and LAD rats bred for high versus low ethanol preference

These studies examined the reinforcing effects of ethanol in rats selectively bred for high versus low ethanol drinking in a two-bottle choice preference task, namely the Preferring (P), Non-Preferring (NP), High Alcohol Drinking (HAD), and Low Alcohol Drinking (LAD) rats. The results substantiate findings suggesting that genetic factors are significant in determining whether ethanol will come to serve as a reinforcer. P rats exhibited high levels of responding for ethanol compared with the water vehicle, NP and HAD rats exhibited more moderate levels of responding for ethanol, and the behavior of LAD rats suggested that ethanol served only inconsistently as a reinforcer for these rats. Overall, the results suggest the existence of distinct, biologically influenced components of ethanol drinking behavior. Preference appears to measure an inherent facilitative factor allowing animals to initiate ethanol drinking. The operant chamber paradigm appears to measure factors related to whether and to what extent ethanol will serve as a positive reinforcer following conditioned exposure to the drug. Although preferring animals generally find ethanol reinforcing there seems to be little quantitative relationship between degree of preference and whether ethanol will serve as a reinforcer. Lack of preference does not seem to be predictive of lack of reinforcement. Thus, it appears that preference for ethanol and reinforcement from ethanol are somewhat overlapping, but distinct factors that contribute to ethanol drinking. These results suggest the existence of multiple components of behavior mediated by multiple mechanisms that contribute to ethanol drinking.

Alcohol acceptance, preference, and sensitivity in mice. I. Quantitative genetic analysis using BXD recombinant inbred strains

Although the recombinant inbred strain method was designed for molecular genetic analysis of linkage, it also provides powerful quantitative genetic analyses of heritability and genetic correlations. Measures of alcohol acceptance, alcohol preference, and hypnotic dose sensitivity (HDS) were assessed in 21 strains of mice from the BXD RI series. Sex differences were found to be significant at a phenotypic level. However, heritability estimates for acceptance, preference, and HDS are similar in males and females. Heritability estimates for the three measures are approximately 0.20 for acceptance and preference, and 0.10 for HDS. Analyses of genetic correlations reveal that acceptance and preference share some degree of genetic influence, although they mostly operate under different genetically mediated mechanisms. HDS did not show a significant genetic relationship to either acceptance or preference. Strong correlations were obtained when acceptance, preference, and HDS strain means were correlated across male and female recombinant inbreds, suggesting substantial genetic similarity across sexes.

EtOH self-administration in anticipation of noise stress in C57BL/6J mice

C57BL/6J mice were studied for self-administration of ethanol (EtOH) during a signal period that preceded delivery of an environmental stressor (noise) in the home cage. Animals were given 5 weeks of conditioning in which a 5-min period of 75-dB pulsed noise (SIGNAL) preceded a 20-min period of more intense, 90-dB pulsed noise (NOISE) five times daily. EtOH (10% w/v) was then provided in a choice procedure, and drink tube contacts were monitored by computer. Mice that had received the 5 weeks of SIGNAL and NOISE pairings showed an increase in EtOH-seeking behavior, as reflected in EtOH tube contacts during the SIGNAL period. The increase was significant as compared to contacts during baseline or QUIET periods and also as compared to contacts during the same period for control (Ctrl) mice that had received only the 75-dB SIGNAL during conditioning. A subsequent test for passive avoidance confirmed that the 75-dB SIGNAL was aversive for mice that had received noise conditioning but not for Ctrl mice. In sum, the results were in accord with a priori predictions that mice would not show increased EtOH tube contacts during occurrence of intense noise itself but would show increased contacts during the signal that preceded noise. These results were interpreted as preliminary evidence that C57BL/6J mice show self-administration of EtOH in anticipation of an environmental stressor.

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.

Effects of ethanol and toluene on fixed-ratio performance in short sleep and long sleep mice

This study examined the effects of ethanol (EtOH) and toluene on fixed-ratio (FR) responding in mice selectively bred for sensitivity to the effects of EtOH on sleep time. Although the more sensitive long sleep (LS) mice showed greater EtOH-induced impairment in a motor performance task than did the less sensitive short sleep (SS) mice, changes in FR performance in the two lines did not differ in response to EtOH, regardless of route (oral or intraperitoneal) or time (40 vs. 60 min pre-session) of administration. These results emphasize the importance of considering task variables in determination of the behavior of different genotypes. In contrast to results with EtOH, the volatile inhalant toluene produced different effects on FR responding in the selected lines, with SS mice being more sensitive than LS mice.

Molecular Mechanisms Associated with Cocaine Effects

Cocaine has been shown to be a highly addictive and toxic drug. It produces these effects and a variety of other physiological and behavioral effects through its interactions with several distinct central nervous system receptor sites. We present the results of a series of studies that utilized multiple site analyses to elucidate which cocaine binding sites influence the reinforcing and toxic effects of cocaine and with what proportion of influence. The nature of cocaine interactions with monoamine transporters is also discussed, especially with the dopamine transporter, which has been shown to be the cocaine binding site that is primarily associated with the reinforcing effects of cocaine. We also provide evidence that vulnerability to both the toxic and addictive effects of cocaine may be significantly influenced by genetic differences in both humans and animals. In view of the fact that cocaine is commonly abused in a polydrug situation, we present the results of both behavioral and biochemical experiments which suggest that common biochemical pathways may mediate the reinforcing or addictive properties of drugs of abuse. Finally, we discuss research on the biochemical mechanisms associated with effects of ethanol, particularly those which may also influence cocaine self-administration and speculate on pharmacotherapeutic strategies for concurrent abuse of cocaine and ethanol.

Orally delivered cocaine functions as a positive reinforcer in C57BL/6J mice

Cocaine serves as a reinforcer across several routes of administration and species. However, whether orally delivered cocaine serves as a positive reinforcer has not been systematically established. We determined the extent to which contingent access to orally delivered cocaine would maintain lever pressing behavior in C57BL/6J mice who had a prior history of operant ethanol-reinforced behavior. The findings presented in this report demonstrate that orally delivered cocaine can serve as a reinforcer of operant behavior. A drug substitution procedure where cocaine was substituted for gradually decreasing ethanol concentrations was successful in inducing pharmacologically significant intakes of cocaine under a fixed ratio (FR) schedule of drug access. When ethanol was removed, responding for cocaine continued. As FR size was increased, proportionate increases in responding occurred except at the highest FR value. Responding maintained by cocaine significantly exceeded responding maintained by vehicle, with the mice typically consuming 6-10 mg/kg cocaine per 30-min session. The utilization of inbred strains and the procedures followed in the present studies should prove useful in determining the extent of both genetic and environmental influences on various behavioral effects of cocaine and their mechanisms of action.

Selected mouse lines, alcohol and behavior

The technique of selective breeding has been employed to develop a number of mouse lines differing in genetic sensitivity to specific effects of ethanol. Genetic animal models for sensitivity to the hypnotic, thermoregulatory, excitatory, and dependence-producing effects of alcohol have been developed. These genetic animal models have been utilized in numerous studies to assess the bases for those genetic differences, and to determine the specific neurochemical and neurophysiological bases for ethanol's actions. Work with these lines has challenged some long-held beliefs about ethanol's mechanisms of action. For example, lines genetically sensitive to one effect of ethanol are not necessarily sensitive to others, which demonstrates that no single set of genes modulates all ethanol effects. LS mice, selected for sensitivity to ethanol anesthesia, are not similarly sensitive to all anesthetic drugs, which demonstrates that all such drugs cannot have a common mechanism of action. On the other hand, WSP mice, genetically susceptible to the development of severe ethanol withdrawal, show a similar predisposition to diazepam and phenobarbital withdrawal, which suggests that there may be a common set of genes underlying drug dependencies. Studies with these models have also revealed important new directions for future mechanism-oriented research. Several studies implicate brain gamma-aminobutyric acid and dopamine systems as potentially important mediators of susceptibility to alcohol intoxication. The stability of the genetic animal models across laboratories and generations will continue to increase their power as analytic tools.