Environmental lighting has a selective influence on ethanol intake in rats.

The effect of lighting condition on levels of absolute ethanol intake were systematically examined in the present study. Wistar rats were exposed to one of three lighting conditions: constant light, constant dark, and a standard 12/12 light/dark cycle. The animals were acclimatized to lighting conditions for 2 weeks prior to ethanol (EtOH) acquisition with water and food available ad lib. EtOH was then presented in increasing concentrations from 2% (v/v; 95% with tap water) to 10% on alternate days in free choice with water. Immediately following the acquisition phase, a maintenance period was initiated that began with everyday presentations of 10% EtOH solution in free choice with water. After 10 days, lighting conditions for the constant light and dark groups were switched to normal lighting (12/12 light/ dark). EtOH and water intake were recorded for an additional 10 days. Rats exposed to constant light during EtOH acquisition and maintenance consumed less EtOH during the maintenance period than rats exposed to normal lighting conditions. When lighting conditions were switched to a normal cycle, water consumption increased significantly but EtOH intake did not change. Rats living in constant dark during EtOH acquisition and maintenance consumed less EtOH during the acquisition period when compared with rats living in normal lighting conditions. Unlike animals trained under constant lighting, switching to normal lighting conditions had no effect on EtOH or water intake. There were no differences in water consumption levels among the groups during acquisition and maintenance, suggesting a specificity of the effects of lighting condition on EtOH intake. The present study, therefore, has attempted to show that an environmental variable such as lighting may exert a selective influence on EtOH self-selection in rats.

Differences in the consumption of ethanol and flavored solutions in three strains of rats.

Several studies have shown a correlation between ethanol consumption and the intake of flavored solutions in rats, particularly sweet solutions. This observation, however, has not been shown in all strains of rats. The present study examined whether the intake of ethanol and that of flavored solutions would be related in Lewis (LEW), Wistar (WIS), and Wistar Kyoto (WKY) rats. During phase I, all rats were presented with water and a flavored solution following a continuous access paradigm as developed by Overstreet et al.: quinine (0.25% wt/vol), saccharin (0.1% wt/vol), ethanol (ETOH) (10% vol/vol), and saccharin-quinine (SQ) solutions (0.4% wt/vol-0.04% wt/vol). During phase II, fluid presentations were reduced to a 10-min limited access schedule and were presented in the same order. Results showed strain differences in intake and preference for ETOH and SQ during both phases, but not in quinine or saccharin intake. ETOH and saccharin intake were only correlated in the LEW strain during limited access drinking, while ETOH and SQ intake were correlated in the LEW strain as well as when all strains were collapsed during continuous drinking. These findings suggested that any association between ETOH and sweet intake may not be generalizable to all rat strains. The animals used in this study may have differed in taste sensitivity, as low ETOH-consuming LEW rats were sensitive to the bitter taste of quinine alone, as well as when mixed with saccharin. Sensitivity to bitter tastes may be an important predictor of low ETOH consumption and/or preference. These data provide further evidence for the role of taste factors in the mediation of voluntary ETOH consumption in rats.

Lack of effect of dopamine D2 blockade on ethanol intake in selected and unselected strains of rats.

Previous research has suggested that brain catecholamines may be involved in regulating ethanol intake. This study was designed to look more specifically at dopamine (DA) and whether DA D2 receptor blockade with the antagonist pimozide would alter ethanol consumption in rats. Subjects were male Maudsley Reactive and Wistar rats, the former previously shown to consume larger amounts of ethanol than the latter. Both strains were screened for ethanol intake by presentation of ethanol solutions (free choice with water) in increasing steps from 2% to 10% (v/v) on an alternate-day schedule. Following the screening period, animals were switched to a schedule of everyday presentation of the 10% (v/v) ethanol solution (free choice with water) for 10 baseline days. Animals were then divided into high and low drinking levels according to whether their mean baseline ethanol intake (g/kg) fell within +/- 0.5 SD of the mean intake of their group (Maudsley Reactives: mean = 2.55 g/kg, low drinkers < 1.63, high drinkers > 3.47; Wistars: mean = 2.17 g/kg, low drinkers < 1.53, high drinkers > 2.82). The animals were assigned to one of five treatment groups for 5 subsequent days where they received IP injections of pimozide (0.08, 0.24, or 0.48 mg/kg), tartaric acid, or saline. Following the treatment period, ethanol consumption was recorded for 5 posttreatment days. No significant differences due to treatment were observed for either intake or preference of ethanol across treatments, drinking groups, or strains. The results obtained in the present study suggested that interference in DA neurotransmission through administration of the D2 antagonist pimozide does not significantly alter ethanol consumption in either MR or Wistar animals.

Effects of naltrexone on the intake of ethanol and flavored solutions in rats.

It has been suggested that the endogenous opioid system may mediate the intake of preferred fluids, perhaps through an attenuation of reinforcement properties causing a subsequent shift in palatability. The purpose of the present study was to investigate the effects of the nonspecific opiate antagonist naltrexone on the intake of 10% ethanol, 0.1% saccharin, 0.0006% quinine, 0.4% saccharin + 10% ethanol, and 0.4% saccharin + 0.04% quinine solutions. Fluid intake was measured in male Long-Evans and Wistar rats under 24-h continuous and 30-min limited-fluid-access drinking paradigms. All rats received injections of naltrexone hydrochloride (10 mg/kg, i.p.) for 5 days after baseline intake measures and were monitored for a further 5 days (after-treatment phase). Results indicated that naltrexone did not affect intake of any solution when fluids were available over 24 h. However, under limited-access conditions, naltrexone caused a decrease in the intake of all fluids except quinine in both rat strains. On the basis of these findings, it is possible that the effects of this dose of naltrexone were not due to any true conditioning effect on the reinforcement properties of ethanol, but perhaps to some nonspecific effect of the drug, such as an alteration in palatability or an attenuation of locomotor activity. As well, due to the inconsistent results in fluid intake across drinking paradigms, the present findings do not provide evidence for an effective role for opiate mediation in ethanol intake as well as any ethanol-sweet fluid intake interactions.

Relative taste thresholds for ethanol, saccharin, and quinine solutions in three strains of rats nonselected for ethanol: a comparative study.

C. P. Richter and K. H. Campbell (1940b) originally defined taste threshold as "the point at which the rats first indicated that they recognized a difference between the distilled water and the solutions" (p. 34). The present study sought to apply this simple behavioral measure to the investigation of strain differences in taste sensitivities, particularly with respect to predictive relationships in ethanol, saccharin, and quinine preference. Fawn-Hooded, Lewis, and Wistar rats were presented with gradual increments in concentration of ethanol (0.01-15%; C. P. Richter & K. H. Campbell, 1940a), saccharin (0.002-3%) or quinine (0.0001-0.0055). Results showed that although intake for saccharin was similar in all strains, consumption of ethanol and quinine differed among the groups. Although previous research has proposed that sweet preference is a promising behavioral marker for ethanol preference, these results suggested that bitter preference may be a more reliable predictor of ethanol preference in rats.

Do taste factors contribute to the mediation of ethanol intake? Ethanol and saccharin-quinine intake in three rat strains.

Several recent studies have suggested that ethanol-preferring rodents may also have an affinity for sweet solutions (saccharin, sucrose) and, conversely, that saccharin preference may predict ethanol preference. The purpose of the present investigation was to determine whether intake of ethanol and saccharin-quinine (SQ) solutions would be related in three nonselected strains of rats who differ in their ethanol preference: Lewis, Wistar Kyoto, and Wistar. In the first phase of the experiment, all animals were presented with an ascending series of ethanol solutions (2 to 10%) in free choice with water, followed by a 10-day maintenance period of 10% ethanol with water. In the second phase, the same animals were presented with an ascending series of SQ solutions (saccharin: 0.4%, quinine: 0.001 to 0.04%) in free choice with water, followed by a 10-day maintenance period of 0.4% saccharin with 0.04% quinine and water. The results revealed an absence of a direct relationship between ethanol and SQ consumption. The ethanol-nonpreferring Lewis rats showed a greater preference for the SQ solutions than Wistar Kyoto rats, whereas the ethanol-preferring Wistar Kyoto strain consistently consumed significantly less SQ. Wistar rats showed relatively stable consumption levels for both solutions that fell between those of the other two strains. These results suggested that the relationship between ethanol and SQ preference in rats was not a direct one and did not support the findings in the literature of a simple overall positive relationship between sweet and ethanol preference. These data do, however, provide further evidence for taste factors in the mediation of self-selection of ethanol in rats.