Department of Psychology and Center of Alcohol Studies, Rutgers University, New Brunswick, NJ 08903, USA
* Author to whom correspondence should be addressed at: Center of Alcohol Studies, Rutgers University, 607 Allison Road, Piscataway, NJ 08854 8001, USA. Tel.: 732 445 3595; Fax: 732 445 3500; E-mail: tomie{at}rci.rutgers.edu
(Received 25 September 2003; in revised form 5 January 2004; accepted 4 February 2004)
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ABSTRACT |
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INTRODUCTION |
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Tomie and his associates (Tomie et al., 2002a,b
) modified the autoshaping apparatus by replacing the retractable lever with a retractable sipper tube CS. Insertion of the sipper tube CS into the chamber immediately before the response-independent delivery of the food US induced sipper CS-directed approach and contact responses, culminating in mouthing, chewing, licking and swallowing responses, resulting in drinking of the 6% ethanol (v/v) solution in the sipper CS. Several effects suggest that autoshaping contributes to ethanol drinking. For example, autoshaping of sipper CS-directed ethanol drinking increases as a function of experience with sipper CSfood US pairings (Tomie et al., 2002a
,b
, 2003a
,b
). Ethanol drinking induced by sipper CSfood US pairings is greater than in controls provided with randomly related presentations of sipper CS and food US (Tomie et al., 2002a
, 2003a
,b
). Thus, autoshaping procedures induce more ethanol drinking than is due to factors related to pseudoconditioning.
The present study explores the effects of autoshaping procedures employing social opportunity US, rather than food US, on ethanol drinking in rats. The autoshaping apparatus was modified by replacing the pellet dispenser with a guillotine door that separated the autoshaping chamber from a stainless steel wire mesh cage housing a conspecific male rat. Effects of repeated sipper tube CSsocial opportunity US pairings (Group Paired/Ethanol) on sipper tube CS-directed drinking were evaluated, relative to control groups that either received the CS and US randomly (Random/Ethanol), or received the CS but no US (Sipper Only), or water rather than ethanol in the sipper CS (Paired/Water).
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MATERIALS AND METHODS |
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Apparatus
Experimental chambers for autoshaping were four locally constructed Plexiglas cubicles (24 cm x 24 cm x 26 cm; L x W x H) with a floor consisting of stainless steel rods. The right wall of each chamber was an intelligence panel equipped with a retractable stainless steel sipper tube and a stainless steel guillotine door. The sipper tube contained a stainless steel ball-bearing with an inserted rubber stopper used for holding the fluid in a 50 ml Plexiglas graduated tube (Model 58320, Kimble-Kontes, Vineland, NJ). The graduated tube was mounted on a mechanical bottle insertion mechanism (BCS Machine, Plainfield, NJ), which inserted the stainless steel sipper tube through an aperture located 4 cm above the floor and 6 cm to the left of the front wall. The sipper insertion mechanism moved the sipper tube a total of 2.75 cm from fully retracted to fully inserted. In the fully inserted position, the tip of the sipper tube was 0.5 cm into the chamber. The guillotine door (11 cm x 13 cm; H x W) separated the autoshaping chamber from a stainless steel wire mesh cage (20 cm x 10 cm x 12 cm; L x W x H) that during some of the procedures housed a conspecific male of the same age and weight as the subject. The left edge of the guillotine door was located 0.5 cm to the right of the back wall. The guillotine door was operated by a pulley system connected to a mechanical door opening mechanism (BCS Machine, Plainfield, NJ). Each chamber was powered by a 28 V DC power supply, and session events were controlled by IBM-compatible PCs equipped with I/O relay cards (Model PCL-725, JDR Microdevices, San Jose, CA).
Drugs
Bulk ethanol (95%) was obtained from Rutgers University Chemical Stores. Ethanol was diluted in tap water to produce all ethanol concentrations (volume to volume, v/v) employed in the study.
Autoshaping procedure
Rats were run 5 days per week and received 35 autoshaping trials per session. Prior to each autoshaping session, rats were individually weighed and then immediately placed in the experimental chambers. Rats were randomly assigned to one of four groups. For rats in the Ethanol/Paired group (n = 12), the ethanol sipper CS was inserted into the autoshaping chamber for 10 s immediately prior to the social opportunity US, which consisted of the raising of the guillotine door separating the subject from the conspecific male rat in the wire mesh cage for 15 s. Control groups received water rather than ethanol in the sipper CS (Paired/Water, n = 6), or ethanol sipper CS and social opportunity US randomly with respect to one another (Random/Ethanol, n = 11), or ethanol sipper CS but no social opportunity US (Sipper Only, n = 18). For all groups, the mean intertrial interval (ITI) was 60 s (15 s), and the session duration was 50 min. Volume of fluid consumed (ml) during each autoshaping session was determined by recording the volume in the tube immediately before and after each session. Unsystematic observations revealed that the raising of the guillotine door was often followed by social interactions between the subject rat and the conspecific male rat housed in the wire mesh cage. Behaviors commonly observed in subject rats were rearing, sniffing, nose poking, and pawing, which were directed at the wire mesh surface of the holding cage that separated the test chamber from the holding cage.
During the first 22 sessions of autoshaping, for the three groups receiving ethanol in the sipper CS (Ethanol/Paired, Ethanol/Random, and Sipper Only), the ethanol sipper CS contained 3% ethanol (v/v), followed by nine sessions (2331) of autoshaping with 4% ethanol (v/v), three sessions (3234) of autoshaping with 8% ethanol (v/v), seven sessions (3541) of autoshaping with 10% ethanol (v/v), four sessions (4245) of autoshaping with 12% ethanol (v/v), four sessions (4649) of autoshaping with 14% ethanol (v/v), and four sessions (5053) of autoshaping with 16% ethanol (v/v). Training was conducted with each ethanol concentration until variability in g/kg ethanol intake and g/kg fluid intake did not vary by more by 10% between three consecutive sessions. For the Paired/Water group, for the entire duration of the study, the sipper CS contained tap water (0% ethanol).
Blood-ethanol assays
Immediately following the 53rd autoshaping session, all rats were sacrificed by rapid decapitation and trunk blood samples taken from each rat were collected in heparinized tubes. Samples were centrifuged, frozen, and then assayed for blood-ethanol levels using Alcohol Ethyl Enzymatic Alcohol Assay Kit (Product number 33C-1KT, Sigma Diagnostics, St. Louis, MO).
Statistics
For each subject in each autoshaping session, mililiters of fluid consumed and body weight (kg) were measured, then grams of ethanol consumed per kg of body weight (g/kg ethanol intake), and grams of fluid consumed per kg of body weight (g/kg fluid intake) were derived. Effects of autoshaping sessions and ethanol concentrations on mean g/kg ethanol intake and mean g/kg fluid intake were assessed by separate mixed design two-way repeated-measures multivariate analysis of variance using MANOVA (Systat Statistical Software, Richmond, CA). Statistical adjustments were made to maintain family-wise P-values at an alpha level of 0.05 by dividing 0.05 by the number of effects reported in the family-wise analysis. Fisher's Least Significant Difference (LSD) test provided pair-wise comparisons at individual points (alpha = 0.05). All analyses of the effects of ethanol concentration were conducted on the mean of the last three sessions of training with each concentration. Effects of Groups on mean blood-ethanol levels were assessed by one-way analysis of variance using ANOVA (Systat).
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RESULTS |
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DISCUSSION |
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Ethanol intake in the Paired/Ethanol and Random/Ethanol groups was not entirely due to the intermittent presentations of the ethanol sipper CS per se, as revealed by the much lower levels of ethanol intake observed in the Sipper Only group. This indicates that the intermittent presentations of the social opportunity US contributed to ethanol sipper CS-directed ethanol drinking in the Paired/Ethanol and Random/Ethanol groups. Direct comparisons of ethanol intake in the Paired/Ethanol and Sipper Only groups revealed that social opportunity US contributed to the faster rate of initiation of intake of 3% ethanol and to the more rapid escalation of ethanol intake observed when higher concentrations of ethanol were available in the sipper CS. Thus, social opportunity US serves to exaggerate g/kg ethanol intake from the sipper CS and this effect was evident across a broad range of ethanol concentrations.
The induction of g/kg ethanol intake by social opportunity may be due to several factors. For example, unsystematic observation indicated that rats in the Paired/Ethanol and Random/Ethanol groups were more active than rats in the Sipper Only group, indicating that social opportunity may have induced arousal and motor activation. Another possibility is that social opportunity is rewarding, and intermittent presentations of social opportunity reward induce adjunctive polydipsia or excessive ethanol drinking (Falk et al., 1972). Finally, it should be noted that the social opportunity US includes the opening of the door, and the opportunity to see out of the chamber may be rewarding (Reed et al., 1996
).
Gradually exposing rats to increasingly concentrated ethanol solutions in the home cage, a procedure known as acclimation, has been reported to maximize ethanol consumption (Veale and Myers, 1969; Amit et al., 1970; Wise, 1973
); thus, acclimation may have contributed to the high levels of ethanol intake observed here. Our procedures differ from those typically employed in studies reporting acclimation effects in several ways. Our study does not include a control that received exposure only to high concentrations of ethanol, and our rats were exposed to increasingly concentrated ethanol solutions in a test chamber, away from the home cage. Most significantly, acclimation procedures did not induce high levels of ethanol intake in the Sipper Only group but did so in the Paired/Ethanol and Random/Ethanol groups. Acclimation procedures have also been employed in autoshaping studies with food US inducing high ethanol intake of unsweetened 16% ethanol in food-deprived rats (Tomie et al., 2003b
) and high intake of unsweetened 28% ethanol in rats deprived of neither food nor water (Tomie et al., 2003a
).
The effect of the social opportunity US on sipper CS-directed drinking was far more evident when the sipper CS contained ethanol as compared to when the sipper CS contained water. Higher mean daily g/kg fluid intake was observed in the Paired/Ethanol group than in the Paired/Water control, and this difference was evident across a broad range of ethanol concentrations (3 to 16% v/v). Studies reporting autoshaping of ethanol drinking employing food US have also noted that daily fluid intake in the Paired/Ethanol groups was higher than in the Paired/Water groups (Tomie et al., 2002b, 2003b
); however, it should be noted that water drinking induced by food US was far higher than the water drinking induced by social opportunity US reported here. As noted by Tomie and his associates, prandial drinking effects (Kisileff, 1969
) and schedule-induced polydipsia effects (Falk, 1966
; Roehrs et al., 1976
; DeCarolis et al., 2003
) may serve to increase water drinking in situations employing food US. Intermittent presentations of food may induce more prandial drinking and schedule-induced drinking than is induced by social opportunity, and this may account for the lower levels of water drinking in autoshaping studies using social opportunity as compared to food.
The high levels of ethanol intakes observed in the Paired/Ethanol and Random/Ethanol groups were comparable and this indicates that additional ethanol drinking was not induced by temporally contiguous pairings of the CS and US. This is in contrast to the results obtained in previous food studies reporting autoshaping of ethanol drinking, which typically report 20 to 30% more ethanol drinking in the Paired condition relative to the Random condition (Tomie et al., 2002a
,b
, 2003a
,b
). This difference is consistently observed even though there are factors related to food US that might be expected to increase ethanol drinking in the Random condition, including prandial drinking (Neill et al., 1994
) and schedule-induced polydipsia (Falk et al., 1972
). On the other hand, there are several aspects of social opportunity US that may also contribute to elevations of drinking in the Random/Ethanol group. For example, sounds and odors from the rat behind the door that are observed by the subject rat during the interval of time between US presentations would serve to make the social opportunity US less punctate and discrete, increasing the likelihood of pairings of sounds and odor stimuli with the ethanol sipper CS. Perhaps social opportunity is an appetitive Pavlovian US that does not require close temporal contiguity with the sipper CS to induce Pavlovian autoshaping CR performance. There is abundant evidence in the Pavlovian conditioning literature, for example, that gastrointestinal distress US experienced hours after a novel flavor CS induces Pavlovian taste-aversion CRs (Logue, 1979
).
Low levels of ethanol intake were observed in the Sipper Only group, even though they received gradually increasing concentrations of ethanol solutions (i.e. acclimation procedures) in the sipper CS. This may be due to a number of factors. Rats in the Sipper Only group were deprived of neither food nor water, thus drinking for the caloric value of ethanol (Samson, 1986) was unlikely to contribute substantially to ethanol intake. In addition, rats in the Sipper Only group, had access to only unsweetened ethanol solutions in the sipper CS, therefore, initiation and escalation of ethanol intake were not confounded by the effects of sweeteners, which have been widely used in studies with rats, to facilitate the initiation of ethanol drinking in self-administration procedures (Samson, 1986
, 1999
; Weiss et al., 1990
), as well as in schedule-induction procedures (Samson and Falk, 1974
). Despite the fact that all rats in this study were deprived of neither food nor water and had access only to unsweetened ethanol solutions, at the higher ethanol concentrations, mean daily ethanol intake for rats in the Paired/Ethanol and Random/Ethanol groups was over 1.0 g/kg.
While the facilitation of ethanol drinking by social interaction opportunities is well established in human beings (Koposov et al., 2002; for review see Pohorecky, 1981
), a similar effect is far less evident in studies employing rats (Pohorecky, 1990
; van Erp and Miczek, 2001
). In fact, the opposite effect has often been reported. For example, animal researchers (for review see Pohorecky et al., 1995
) have noted that social interaction opportunity afforded by group housing conditions may reduce ethanol drinking relative to isolation housed controls (Parker and Radow, 1974
; Kulkosky et al., 1980
; c.f., Heminway and Furumoto, 1972
; Wolffgramm, 1990
), and bouts of aggression between conspecific males typically results in the lowering of ethanol intake (van Erp and Miczek, 2001
; van Erp et al., 2001
). Procedures employed to study effects of social interaction opportunities on ethanol intake in animals differ in many ways, including housing conditions, duration and nature of social interaction, timing of ethanol availability relative to social opportunity, ethanol intake procedures, and measures of ethanol intake (for reviews, see Pohorecky, 1990
, 1995
). While the present procedures clearly provide evidence of augmentation of ethanol intake by social opportunity, further experimental analysis of the precise features of the social opportunity US complex employed in the present procedures that contributed to the initiation and escalation of ethanol intake is required.
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ACKNOWLEDGEMENTS |
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