Effects of Raclopride in the Core of the Nucleus Accumbens on Ethanol Seeking and Consumption: The Use of Extinction Trials to Measure Seeking

BACKGROUND

A previous study using a sipper procedure of ethanol self-administration found that blockade of the D2 dopamine (DA) receptors in the nucleus accumbens resulted in a reduction in ethanol-seeking behavior with only slight effects on ethanol drinking. However, because of procedural matters in that study, it was unclear as to the extent of the reduction in seeking behavior that occurred. This study expanded that study to examine in more depth the role of DA transmission in the nucleus accumbens in ethanol-seeking and consummatory behaviors.

METHODS

Male Long-Evans rats were initiated to self-administer 10% ethanol with a sipper-tube procedure. Once initiated, in a once-a-day session, pressing a lever 30 times resulted in a sipper tube containing the ethanol solution being made available for 20 min. By using extinction trials in which no sipper was presented and responses were recorded for 20 min, a measure of ethanol seeking, with no effects of consumption, could be obtained. Bilateral microinjections of 1.0, 3.0, and 10.0 microg of raclopride into the nucleus accumbens were tested on both consummatory and extinction trials.

RESULTS

There were significant decreases in ethanol-seeking responses at both the 3.0- and 10.0-microg doses of raclopride, whereas no effects of those doses on consumption were observed. The effects on extinction responding were the same for the first run of responses as for total responding, without effecting rates of responding.

CONCLUSIONS

These findings replicate and expand the initial study with this model of ethanol self-administration and indicate that DA transmission at the D2 receptor in the nucleus accumbens is important for processing information related to stimulus control and goal-directed behavior. The results also suggest that DA has at most a minor role in controlling ethanol consumption once a drinking bout has begun.

Accumbal dopamine concentration during operant self-administration of a sucrose or a novel sucrose with ethanol solution

The goal of the current study was to determine the effect of operant self-administration of (1) 10% sucrose and (2) a first-time solution of 10% sucrose with 5% or 10% ethanol, on dopamine concentration in the nucleus accumbens. We used an operant procedure that distinguished lever pressing (an appetitive behavior) from drinking to better assess the effect of fluid consumption on accumbal dopamine activity. Male Long-Evans rats were trained to bar press by using 10% sucrose reinforcement, and they were required to emit an escalating number of bar presses across daily sessions. Completion of the response requirement resulted in 20 min of access to the solution. Microdialysis samples were collected before, during, and after bar pressing and drinking, and content of ethanol and dopamine was determined. Dopamine concentration in the dialysate was slightly, but significantly, increased in both groups during lever pressing. However, after consumption began, dopamine concentration increased in the sucrose, but not in the sucrose with ethanol, group, followed by a return to baseline values. Ethanol consumption was low (0.27 +/- 0.02 g/kg) and corresponded to low dialysate ethanol concentrations, which appeared within 5 min of drinking. These results demonstrate that operant self-administration of sucrose increases accumbal dopamine concentration during consummatory phases of behavior, but that a similar increase is not apparent when a novel, perhaps aversive, solution (sucrose with ethanol) is presented. This difference may be due to the sensory-related stimulus properties of each solution. In addition, oral self-administration of ethanol at 0.27 +/- 0.02 g/kg over 20 min is not sufficient for stimulation of dopamine activity in the nucleus accumbens.

Dopamine activity in the nucleus accumbens during consummatory phases of oral ethanol self-administration

BACKGROUND This present study was designed to clarify the role of dopamine in the nucleus accumbens during operant ethanol self-administration by separating bar pressing (ethanol seeking) from ethanol consumption. Furthermore, we sought to define the relationship between ethanol in the brain and the accumbal dopamine response after oral self-administration of ethanol.

METHODS

Two separate groups of male Long-Evans rats were trained to bar press with 10% ethanol or water. Rats were trained to elicit an escalating number of bar presses across daily sessions before gaining access to the drinking solution for 20 min. Microdialysis was performed before (during a waiting period), during, and after bar pressing and drinking. A handling control group was included, but did not receive training.

RESULTS

A significant increase in dopamine occurred during placement of the rats into the operant chamber in trained rats and handling controls. The lever-pressing period did not produce an increase in dialysate dopamine. Accumbal dopamine was increased in the first 5 min of ethanol, but not water, consumption. Ethanol appeared in the dialysate sample following ethanol availability, and peak concentrations were reached at 10 min. Most of the ethanol and water consumption occurred within 5 min of fluid access. The probes were distributed in the core (32%), shell (32%), and core plus shell (36%) regions of the nucleus accumbens.

CONCLUSIONS

The enhancement of dopamine during transfer into the operant chamber does not depend on anticipation or operant training with ethanol or water reinforcement. Furthermore, the difference between the time course of accumbal dopamine and ethanol in dialysates suggests that the dopamine response is not solely due to pharmacological effects of ethanol. The dopamine response may be associated with the stimulus properties of ethanol presentation, which would be strongest during consumption.

Devaluation of ethanol reinforcement

Postingestive CNS pharmacologic effects of ethanol are often assumed to provide the major stimuli for development and maintenance of ethanol self-administration in rats. However, there is little direct evidence to support this assumption. In all procedures that have been used to initiate ethanol intake in rats, some type of taste adaptation or taste conditioning could account for the increased and maintained ethanol intake. Thus, it remains critical to demonstrate that increased ethanol intake is related to postingestive CNS actions of ethanol, and not to a positive shift in the hedonic taste value of the solution. Two experiments were performed to examine this question. In both studies, rats were trained to self-administer 20% ethanol by using a sucrose-substitution initiation procedure. The rats were required to press a lever 25 or 30 times to gain access to 20% ethanol for 20 min from a sipper tube. Once initiated, extinction sessions were used to determine the strength of ethanol seeking by measuring the number of lever presses that occurred in 20 min with no presentation of the ethanol solution. After initial training, the rats were split into two groups: one that received pairings of a gavage of ethanol (1 g/kg), followed after 10 min by a lithium chloride (LiCl) injection (paired group), and one that also received ethanol gavage and LiCl injections, but separated by 24 h (unpaired group). This pairing of postingestive effects with the illness induced by LiCl injection has been shown to devalue other food and fluid reinforcers. In Experiment 1, the rats received four pairings, one after the other with no behavioral testing between. In Experiment 2, the rats received three pairings and were tested for devaluation after each pairing. Results from both experiments showed significant decreases in seeking behavior in both groups, but seeking behavior was decreased significantly greater in the paired group, even though neither group had access to ethanol during the extinction testing periods. In Experiment 1, when ethanol became available after the devaluation procedure, the pattern of intake in the paired group was unchanged early in the sipper tube availability period, supporting the suggestion that the devaluation effect was not mediated by taste stimuli. These findings support the assumption that postingestive effects contribute to the reinforcement produced by self-administered ethanol in rats.

Determination of an estradiol dose-response relationship in the modulation of ethanol intake

BACKGROUND

Estradiol (E2) potentiates the self-administration of numerous psychoactive drugs in female rodents. An analogous modulatory role of E2 on ethanol consumption remains unresolved because of examination of limited doses. The purpose of this study was to delineate a dose-response relationship for E2 on ethanol intake with an extended range and number of E2 doses.

METHODS

Female Long-Evans rats had continuous access (22 hr/day) to both 10% ethanol (10E) solution and water. After the establishment of stable 10E intake baselines, rats were assigned to one of seven dose groups balanced for 10E intake [sham-operated (Shm) or ovariectomized (Ovx) plus E2 (microgram/kg)]: Shm + 0, Ovx + 0, Ovx + 0.05, Ovx + 0.15, Ovx + 0.5, Ovx + 1.5, and Ovx + 5. Ethanol preference drinking was evaluated during 25 consecutive days of E2 replacement treatment, and trunk blood was collected for the determination of plasma E2 and progesterone concentrations.

RESULTS

Chronic E2 replacement regimens (0.05-1.5 micrograms/kg) dose-dependently augmented 10E intakes and ethanol preference ratios without concomitantly altering water consumption. Despite the maintenance of 2- to 3-fold greater plasma E2 levels, a supraphysiologic E2 dose (5 micrograms/kg) failed to precipitate ethanol intakes in excess of levels observed after treatment with a high physiologic E2 dose (1.5 micrograms/kg). Plasma progesterone concentrations were significantly increased in treatment groups (1.5 and 5 micrograms/kg E2) that exhibited corresponding significant increases in ethanol consumption.

CONCLUSIONS

A positive dose-response relationship between E2 and ethanol intake (incremental increases in E2 dose corresponded to incremental increases in intake) was apparently limited to a physiologic concentration range, because a supraphysiologic dose failed to elicit an additional stepwise increase in ethanol intake. These findings stipulate a modulatory role for E2 in the regulation of moderate ethanol intake and suggest that endogenous fluctuations of E2 may alter the propensity toward consumption in women and in female animal models of ethanol self-administration.

Failure of a schedule-induction procedure to increase ethanol intake in an established limited-access self-administration condition

Determining mechanisms that can increase ethanol consumption during a single drinking bout is central to understanding alcohol abuse. When rodents are used as models to study excessive drinking, most often limited and transient increases in bout size are found with various manipulations. In a variety of studies, investigators have reported that schedule-induced drinking can result in excessive consumption of either water or alcohol (ethanol) during a single drinking period in food-restricted rats. The question examined in this experiment was, Could a schedule-induction paradigm increase bout size in nondeprived rats already self-administering ethanol? After the rats were trained to self-administer a 10% (volume/volume) ethanol solution in a fixed daily drinking session, non-response-contingent presentation of a 10% (weight/volume) sucrose solution, on a fixed-time, 120-s schedule, was used to determine whether additional ethanol consumption could be induced. This was followed by the use of a fixed-time, 300-s schedule and then, by using the fixed-time, 120-s schedule, with the presentation of a 2% (weight/volume) sucrose solution. None of these conditions induced an increase in ethanol self-administration. The results indicate that factors that control ethanol bout size in the nondeprived rat are such that the standard schedule-induction condition seems to be ineffective if an ethanol bout has occurred in the recent past.

Dopaminergic involvement in medial prefrontal cortex and core of the nucleus accumbens in the regulation of ethanol self-administration: a dual-site microinjection study in the rat

The complex mesolimbic-mesocortical system involved with behavioral selection has been implicated in the control of ethanol self-administration. However, the nature of the interactions within this multiple-structured system in ethanol intake regulation remains unclear. Although the role of dopamine (DA) in the prefrontal cortex and the nucleus accumbens has been examined individually, the interaction of DA activity in both structures at the same time remains to be examined. Male, Long-Evans rats were initiated to self-administer ethanol in an operant situation using the sucrose-substitution procedure. Following initiation, bilateral cannula guides were located to allow microinjection in the medial prefrontal cortex (mPFC) and the core of the nucleus accumbens. The DA D2/D3 agonist quinpirole (10.0-microg dose in the prefrontal cortex; 4.0-microg dose in n. accumbens) and the D2 antagonist raclopride (0.05-microg dose in prefrontal cortex; 1.0-microg dose in the nucleus accumbens) were then tested in each site alone and in combination in both sites in each rat. Changes in total responding, ethanol intake, and the pattern of responding were analyzed. Single-site injections replicated most of our previous findings for these doses. Changes in single-site effects were found when dual-site injections were performed, with altered input from the prefrontal areas impacting the effects of accumbens injections. Based on these interactions, our hypothesis that the prefrontal area is involved with the onset and offset of drinking, while the nucleus accumbens is involved with maintaining the ongoing behavior, remains viable.

Measuring the appetitive strength of ethanol: use of an extinction trial procedure

Assessing the motivational level related to gaining access to ethanol (appetitive strength) is critical in understanding the human behavioral processes described as "craving." Use of an animal model that separates the behavior required to obtain the opportunity to consume ethanol from the actual consumption allows for independent measures of appetitive strength that are not intermixed with the effects of the consumed ethanol. In this model, two different methods have been applied to determine the appetitive strength: an across-sessions breakpoint procedure and single-session extinction trials. Although both methods provide an estimate of appetitive strength, the current studies were performed to provide some comparison between them. From the results and comparison with findings from prior studies, it was determined that the breakpoint procedure gave the most stable and potentially least influenced measure of appetitive strength. On the other hand, extinction trials in general provided a stable measure, somewhat different from breakpoint, that could be influenced by behavioral effects other than those related to ethanol consumption. Ethanol intakes were in a pharmacologically relevant range, averaging approximately 1.0 g/kg during 20 min of access to the solutions. Both the breakpoint and extinction measures provide additional evidence of a separation between the behavioral processes of ethanol seeking and ethanol drinking, as there was no relation between lever-press responding and ethanol intake. Thus, when choosing a method, it is possible to assess the appetitive strength of ethanol. However, care should be exercised to avoid misinterpretation of results.

Behavioral measures of alcohol self-administration and intake control: rodent models

This review provides a brief historical overview of the behavioral paradigms that have been used to study alcohol consumption using rats as subjects, and then critically evaluates these models' ability to address the complexities involved in ethanol-seeking and self-administration. Many of these models have been influenced by a "behavioral pharmacology" approach, and therefore have studied oral ethanol reinforcement in a manner similar to food and water reinforcement. Because of this, these models have failed to adequately assess the complex seeking responses that are an integral part of ethanol-motivated behaviors. As an alternative, an appetitive-consummatory approach that procedurally separates and measures the two phases of behavior is suggested, and recent data using this model are reviewed. It is important that animal models that are to be used to evaluate underlying physiological mechanisms of the control of ethanol self-administration accurately address all of the complex behaviors that are involved in intake control.

New neuronal networks involved in ethanol reinforcement

This article represents the proceedings of a symposium at the 2002 ISBRA/RSA meeting in San Francisco. The organizers were Kalervo Kiianmaa and Andrey E. Ryabinin. The chairs were Kalervo Kiianmaa and Jörgen A. Engel. The presentations were (1) The role of opioidergic and dopaminergic networks in ethanol-seeking behavior, by Kalervo Kiianmaa and Petri Hyytiä; (2) Interaction between the dopamine systems in the prefrontal cortex and nucleus accumbens during ethanol self-administration, by Herman H. Samson; (3) Neurochemical and behavioral studies on ethanol and nicotine interactions, by Jörgen A. Engel, Lennart Svensson, Bo Söderpalm, and Anna Larsson; (4) Involvement of the GABA receptor in alcohol reinforcement in sP rats, by Giancarlo Colombo and Giovanni Vacca; (5) Neuroactive steroids and ethanol reinforcement, by Deborah A. Finn, and (6) Potential contribution of the urocortin system to regulation of alcohol self-administration, by Andrey E. Ryabinin and Ryan K. Bachtell.(B)

Assessment of sucrose and ethanol reinforcement: the across-session breakpoint procedure

We have demonstrated previously that the use of an across-session progressive ratio procedure yields breakpoint values for 10% ethanol (10E) that are stable and comparable to those measured for other drugs of abuse [Alcohol. Clin. Exp. Res. 23 (1999) 1580]. The aims of the present experiment were twofold: (1). to determine whether this procedure is sensitive to changes in reinforcer magnitude using a reinforcer previously demonstrated to affect operant responding in a predictable fashion and (2). to determine whether ethanol reinforcement produced similar changes in behavior. Male, Long-Evans rats were trained to respond for either 3% sucrose (3S) or 10E using the sipper tube appetitive/consummatory procedure where the completion of a single response requirement results in access to a liquid solution for 20 min. Three successive breakpoints were determined for this "baseline" solution by increasing the response requirement each day until it was not completed. The concentration of the solutions was then manipulated such that breakpoints for the Sucrose Group were assessed for 1%, 3%, 5% and 10% sucrose, and breakpoints for the Ethanol Group were assessed for 2%, 5%, 10% and 20% ethanol. The concentration manipulation showed that sucrose concentration had a greater impact on seeking and consumption than did ethanol concentration. Breakpoints in the Sucrose Group were highly correlated with sucrose concentration, whereas in the Ethanol Group, breakpoint was unrelated to ethanol concentration. Ethanol intake patterns suggested that pharmacological factors might have been regulating intake, and that when physiologically detectable amounts of ethanol were consumed, there was a dissociation between seeking and intake with slightly elevated ethanol seeking. Overall, the across-session breakpoint procedure confirmed that sweet taste was highly related to seeking and consumption, whereas ethanol-motivated responding may be controlled by different regulatory mechanisms that are distinct to seeking and consumption.

Ethanol and sucrose self-administration components: effects of drinking history

Results of previous studies have shown that when rats consume higher concentrations of ethanol during initiation both the amount consumed and the pattern of consumption change with the return to a lower concentration. In this study, an across-sessions breakpoint procedure in the sipper-tube model was used to examine the effect that experience with drinking higher concentrations (a concentration manipulation) of both ethanol and sucrose had on appetitive and consummatory behaviors. A follow-up study was then conducted in the ethanol-consuming group with across-session breakpoint and intake examined before, during, and after a 3% sucrose/10% ethanol solution was presented in the sipper tube. As ethanol concentration increased, intake was not changed. Exposure to higher ethanol concentrations had no effect on the amount of 10% ethanol consumed when retested. The exposure tended to increase appetitive behavior (breakpoint), but this effect was not unique to ethanol, as rats self-administering 3% sucrose showed a similar increase. When the combined ethanol-sucrose solution was available, a significant increase in both intake and appetitive responding occurred; however, there was no change from prior intake or breakpoint when 10% ethanol was retested. That the addition of sucrose to the ethanol solution significantly increased appetitive and consummatory behaviors supports the suggestion that the composition of the alcoholic beverage can have a strong influence over the control of self-administration. Because most consumption of ethanol by human beings is in solutions that contain mixers that alter the taste of the solution, this taste factor needs to be considered in the regulation of ethanol drinking.

Effects of self-administered ethanol or water preloads on appetitive and consummatory behavior in the alcohol-preferring (P) rat

OBJECTIVE

Ethanol intake control in the selected alcohol-preferring lines of rats appeared to have shifted in some lines for both increased ethanol seeking and increased consumption once ethanol was available. It was unknown whether a small preload of ethanol would alter either the seeking or the consumption in a selected line. This study examined this issue.

METHOD

Alcohol-preferring (P) rats from Indiana University School of Medicine were initiated to drink ethanol using a sucrose-substitution procedure and a single daily limited-access trial. Following 30 responses on a lever, a sipper tube containing 10% ethanol extended into the operant chamber for 20 minutes. Self-administered ethanol and water preloads were tested prior to either a regular session or an extinction session. In extinction sessions, no access to the sipper tube occurred, and the number of responses occurring during 20 minutes was taken as a measure of ethanol seeking.

RESULTS

The ethanol and water preloads had no effect on the following ethanol consumption at any time during the experiment. However, the first two ethanol preloads significantly reduced extinction responding, which did not recover to the levels observed prior to the preload tests.

CONCLUSIONS

The data support the conclusion that ethanol seeking in the P rat can be influenced by environmental history, whereas consummatory behavior appears to be under more explicit genetic control. This gene-environment interaction suggests that, in the P rat, seeking behavior, initially set at higher levels than observed for nonselected lines, can be modified by certain environmental experiences.

Operant self-administration of ethanol in Sardinian alcohol-preferring rats

BACKGROUND

"Work" for ethanol, that is, the ability of a laboratory animal to press a lever to gain access to ethanol, has been proposed as (a) a requirement for definition of an animal model of alcoholism and (b) a measure of ethanol-reinforcing properties. The present study evaluated oral self-administration of ethanol under an operant (lever pressing) procedure in selectively bred Sardinian alcohol-preferring (sP) and alcohol-nonpreferring (sNP) rats.

METHODS

Rats from both lines were initiated to self-administer 10% ethanol, on a fixed ratio 1 schedule and in daily 30 min sessions, by using the Samson sucrose fading procedure. Subsequently, rats were exposed to increasing concentrations of ethanol up to 30% on a fixed ratio 4 schedule. Finally, the extinction responding for ethanol, defined as the maximal number of lever responses reached by each rat in the absence of ethanol reinforcement, was determined.

RESULTS

The results indicated that sP rats acquired and maintained lever pressing for ethanol, self-administering mean amounts of ethanol in the range of 0.6 to 1.1 g/kg/session, which gave rise to mean blood ethanol levels in the 30 to 45 mg% range. Extinction responding for ethanol in sP rats averaged 73. In contrast, once sucrose was faded out, sNP rats displayed minimal levels of responding for ethanol, and extinction responding averaged 6.

CONCLUSIONS

The results of the present study extend to the sP/sNP rat lines the finding that ethanol can be established as a reinforcer in selectively bred alcohol-preferring rats, whereas it has modest, if any, reinforcing properties in alcohol-nonpreferring rats.

Ethanol- and sucrose-reinforced appetitive and consummatory responding in HAD1, HAD2, and P rats

INTRODUCTION

Ethanol-preferring (P) rats and high-alcohol-drinking (HAD1 and HAD2) rats have been selectively bred to consume greater amounts of ethanol than nonselected rat strains. These three rat lines also show increased levels of responding for ethanol in operant paradigms that assess a combined appetitive/consummatory response.

METHODS

The present experiment used a model of reinforced responding that procedurally separates the appetitive, or seeking, response requirement from consummatory responding to compare seeking and intake responding in P and HAD rats. Subjects (n = 7 or 8 per group) were trained to make 25 lever-press responses, which were followed by 20 min of access to a sipper tube spout containing either 10% ethanol (10E) or (in separate groups of subjects) 3% sucrose (3S). After training, a single nonreinforced session was conducted to assess the limit to appetitive responding under extinction conditions. After this single nonreinforced session, three successive across-session breakpoint determinations were made for 10E and 3S by increasing the response requirement over days until subjects failed to complete the requirement. A final extinction session was then conducted.

RESULTS

Appetitive responding during both the nonreinforced and breakpoint sessions indicated that P rats made significantly more responses overall than HAD rats in both the ethanol and sucrose groups. P rats also consumed more sucrose than HAD rats, with no differences in ethanol consumption between the lines (1.0-1.5 g/kg/20 min). Appetitive responding in the HAD rats in the ethanol groups was comparable to that reported previously for nonselected Long-Evans rats.

CONCLUSIONS

These findings indicate that appetitive and consummatory processes are distinct and that P rats have an increased tendency to both seek and drink ethanol and sucrose solutions, making this selected line useful when modeling both "craving" and "loss of control" related behaviors involved in excessive alcohol consumption.

Repeated nicotine injections decrease operant ethanol self-administration

Nicotine and alcohol are two of the most used drugs in the United States. However, it is not clear whether the co-use of these drugs is due to pharmacological or environmental reasons, or perhaps related to both. Although results from previous studies in animal models seem to indicate that nicotine has an effect on ethanol consumption, little has been done to determine how nicotine affects appetitive and consummatory phases of ethanol self-administration. In this study, we examined the effect of repeated treatment with nicotine (0, 0.35, and 0.7 mg/kg, s.c.), given 30 min before a daily operant session, on appetitive and consummatory phases of Long-Evans rats self-administering 10% (vol./vol.) ethanol in a sipper-tube model. Ethanol intake (consummatory phase) decreased at both doses of nicotine tested, and lever pressing (appetitive phase) decreased after injection of the high dose of nicotine. These results support the suggestion that nicotine affects ethanol self-administration. However, in this model, the findings demonstrate a reduction in drinking, rather than the enhancement that has been shown in findings obtained from other studies.

Separate measures of ethanol seeking and drinking in the rat: effects of remoxipride

Remoxipride, a dopamine D(2) antagonist, decreases responding that results in the presentation of small amounts (approximately 0.1 ml) of ethanol in limited-access paradigms. This type of operant response is a combined appetitive/consummatory response that is differentially affected by changing stimulus properties of consumed ethanol (i.e., taste, pharmacology) over the course of the session. In the present experimental design, ethanol-directed appetitive and consummatory responses were procedurally separated to investigate the specific effects of remoxipride on these distinct behaviors. Male Long-Evans rats were trained to make a series of lever-press responses once each day that resulted in access to a sipper tube spout containing 10% ethanol for 20 min. Three doses of remoxipride were tested: 5.0, 10.0, and 15.0 mg/kg (-30 min, i.p.). In Experiment 1, a response requirement of 20 was used, and both reinforced and nonreinforced sessions were examined. In nonreinforced sessions, subjects were permitted to lever press for 20 min, after which the session ended without sipper tube presentation. These sessions were conducted to remove the possibility that limiting responding might obscure a drug effect on the seeking response. In Experiment 2, a low response requirement (4) was used to investigate the effects of remoxipride on ethanol intake. Average baseline ethanol intake (Experiment 1) was 0.69 g/kg, with blood ethanol concentrations at the end of the session at 64 mg%. At all doses tested, remoxipride had no effect on the measures of ethanol consumption (e.g., total intake, lick latency, lick rate) in either experiment. However, remoxipride dose dependently decreased the number of appetitive responses made, while having no effect on response latency or rate, during both reinforced and nonreinforced sessions in Experiment 1. In these experiments, the systemic antagonism of the dopamine D(2) receptor decreased ethanol seeking without causing a general impairment of motor function. The procedural separation of seeking and intake responses revealed that appetitive responding was more sensitive than consummatory responding to remoxipride treatment.

Effect of ethanol self-administration on mu- and delta-opioid receptor-mediated G-protein activity

BACKGROUND

This study examined the effects of ethanol self-administration on mu- and delta-opioid receptor-mediated G-protein activity in specific brain regions of male Long Evans rats.

METHODS

Rats were trained to self-administer ethanol by using a home-cage modification of the sucrose substitution paradigm. After 30 to 40 days of sucrose or sucrose/15% ethanol self-administration (20 min sessions, Monday-Friday), rats were killed for autoradiographic assays. Coronal sections of brains from sucrose and ethanol self-administering rats were collected and processed for basal and mu- and delta-stimulated [35S]guanosine-5'-O-(gamma-thio)-triphosphate (GTPgammaS) binding. Sections were exposed to film and then analyzed by using computer-assisted densitometry to determine levels of basal and agonist-stimulated [35S]GTPgammaS binding.

RESULTS

Mu-opioid-stimulated [35S]GTPgammaS binding was decreased in the prefrontal cortex of brains from ethanol compared with sucrose self-administering rats. Mu-opioid-stimulated [35S]GTPgammaS binding was unchanged in the cingulate cortex, caudate-putamen, nucleus accumbens, amygdala, hypothalamus, thalamus, and locus ceruleus of ethanol compared with sucrose self-administering rats. Basal and delta-opioid-stimulated [35S]GTPgammaS binding did not differ between the two groups in the prefrontal cortex or any other region analyzed.

CONCLUSIONS

These data demonstrate decreased mu-opioid-mediated G-protein activity in the prefrontal cortex of ethanol self-administering rats and suggest an interaction between ethanol and mu-opioid receptors in this region.

Microanalysis of ethanol self-administration: estrous cycle phase-related changes in consumption patterns

BACKGROUND

Female rodent and reproductive cycle-related patterns in ethanol consumption have been identified grossly, but the regulatory processes that underlie these patterns remain to be clarified. The evaluation of consummatory bout dynamics may be useful in determining the effects of a changing endogenous state (ovarian hormone fluctuations) on ethanol consumption patterns. This study assessed the microstructural components of ethanol intake patterns across the estrus cycle.

METHODS

Cumulative licking of 13 female Long-Evans rats was recorded during continuous access (22 hr/day) to 10% ethanol and tap water solutions throughout a 10 week period. Drinking bouts were defined as 20 or more successive licks separated by less than a 60 sec pause. Vaginal smears were obtained daily from each rat between 1100 and 1130 hr to assess estrus cycle phase. ANOVAs were conducted to evaluate estrus cycle effects on consumption pattern parameters.

RESULTS

Total daily ethanol intakes were unaffected by estrus cycle phase. Ethanol bout frequency was significantly greater during proestrus compared with all other phases, whereas ethanol bout sizes were significantly attenuated during proestrus compared with estrus and diestrus. Experience with ethanol was associated with a significant decline in bout frequency and a significant elevation in bout size across all cycle phases, which indicated that initiation of ethanol self-administration occurred over time. A significant main effect of hour and a cycle phase by hour interaction were found for the temporal distribution of ethanol lick responses throughout the estrus cycle, and a progressive shift in lick patterns evolved between estrus and the subsequent proestrus.

CONCLUSIONS

The estrus cycle phase-related changes in the microstructural components of ethanol intake suggest that female rats experience periods of altered sensitivity to the neurobiological and reinforcing effects of ethanol. Furthermore, these findings implicate cyclical fluctuations in ovarian hormones and other neuromodulators in the regulation of ethanol consumption patterns throughout the estrus cycle of the female rat.

Ethanol self-administration in Maudsley reactive and Maudsley nonreactive inbred rats

This study was performed to investigate ethanol self-administration in inbred Maudsley rats, which were selected for differences in stress susceptibility and which often differ in their home cage ethanol consumption. Adult, male, Maudsley reactive (MR/Har) and Maudsley nonreactive (MNRA/Har) rats were tested in a standard protocol for the sucrose-substitution procedure for the initiation of self-administration of ethanol in an operant setting. Before and after initiation for self-administration in the operant setting, rats were tested for home cage consumption of 10% (vol./vol.) ethanol in a two-bottle test for 14 consecutive days. During the sucrose-substitution procedure, MNRA/Har rats consumed more sucrose and ethanol than did MR/Har rats. In addition, MNRA/Har rats self-administered a greater amount of ethanol during a concentration manipulation with the use of a fixed ratio (FR) 4 response requirement. However, both strains self-administered low amounts of 10% ethanol (MNRA/Har, 0.15 g/kg/day; MR/Har, 0.08 g/kg/day) after concentration manipulation compared with those observed in outbred rats and alcohol-preferring rats tested under identical conditions in other studies. Both MR/Har and MNRA/Har rats markedly increased their ethanol intake in the home cage after the initiation protocol, but there was no difference between MR/Har and MNRA/Har on that measure. The failure of MR/Har rats to self-administer ethanol was inconsistent with their home cage drinking in other studies, and this is distinctly different from the self-administration pattern of high-alcohol-drinking rat lines tested in this paradigm.

Reinstatement of ethanol seeking responding after ethanol self-administration

Understanding the processes related to resumption of alcohol-seeking behavior after a small, single exposure to alcohol could be important in treating alcoholic relapse. We used a new ethanol self-administration model to determine the potential role of ethanol self-administration in reinstatement of seeking behavior. Long-Evans rats were initiated to self-administer either 10% ethanol or 3% sucrose in a sipper procedure. This procedure required that the rat make a fixed number of lever presses to gain access to a sipper tube for 20 min. Patterns of responding and tube-licking as well as volume intakes were recorded. Both within-session and across-session extinction/reinstatement procedures were tested with a brief ethanol self-administration exposure as the reinstatement event. Self-administration of small amounts of 10% ethanol (1.3 ml) in ethanol-trained rats and small amounts of 3% sucrose (1.4 ml) in sucrose-trained rats resulted in modest reinstatement lever pressing. Although some reinstatement occurred, the amounts of lever pressing were minimal. These findings support the suggestion that self-administered ethanol in this experimental paradigm does not increase ethanol-seeking behavior.

Ethanol consumption in the female Long-Evans rat: a modulatory role of estradiol

The examination of various gonadal hormone manipulations on ethanol intake in human subjects and in rodent models has resulted in disparate findings. In the present study, we examined the effects of ovariectomy and subsequent estradiol (E(2)) replacement on ethanol intake in a within-subject design, as well as assessed the relevance of reproductive status on the efficacy of an E(2) stimulus in eliciting consumption. Female Long-Evans rats (n = 24) were given access to 10% ethanol and water in a continuous-access paradigm. After establishment of baseline intake values, rats were divided into four groups: sham/placebo (Shm+P), sham/estradiol (Shm+E(2)), ovariectomized/placebo (Ovx+P), and ovariectomized/estradiol (Ovx+E(2)). Rats in the Ovx+P group were found to have a large and permanent decline in ethanol intake that persisted more than 3 months postsurgery. Administration of E(2) to Ovx+E(2) rats was associated with restoration of ethanol consumption to baseline levels. When Shm+E(2) and Ovx+E(2) groups were compared, reproductive status was found to be a determining factor in the efficacy of E(2) to elicit ethanol intake. Together, these findings provide evidence that ovarian hormones, particularly estradiol, exert activational effects on estrogen-responsive substrates to modulate ethanol consumption in the adult female rat.

Effects of self-administered alcohol or sucrose preloads on subsequent consumption in the rat

OBJECTIVE

The initial drink of alcohol is often conceptualized as "priming" the individual for the following drinking bout. For the alcoholic, this priming effect has been considered a key for the loss of control that then occurs. Although there have been a few animal studies examining the effects of an investigator-administered ethanol preload on subsequent ethanol self-administration, the effects of a small self-administered oral preload on subsequent consumption have not been examined.

METHOD

Adult, male rats, initiated to self-administer ethanol using the sucrose-substitution procedure, were given brief access periods to drink ethanol or water, 5 minutes prior to a second opportunity to press a lever for an additional 20-minute access to a 10% ethanol solution. A second group of rats were trained to press a lever to gain access to a 3% sucrose solution, and the effects of sucrose or water preloads were examined and compared with results in the ethanol group.

RESULTS

In the ethanol group, both the ethanol and water preload intakes increased as preload access time increased and were not different from each other. However, ethanol preloads at the longer access times (60 seconds and 120 seconds) decreased subsequent ethanol consumption and at the highest time also affected ethanol-seeking behavior. Equal volumes of water intake at these longer access times had no effects on subsequent ethanol consumption. In the sucrose group, sucrose preload intakes increased as access time increased, but water preload intakes did not. Neither sucrose nor water preloads had any effect on subsequent sucrose consumption.

CONCLUSIONS

The data failed to find any priming effect of ethanol preloads in terms of increased subsequent ethanol consumption. It appears that a major factor in the regulation of ethanol intake for the rat in this training procedure is the postingestional effects of ethanol, because taste stimuli did not appear to be important. However, it appears that these ethanol postingestive stimuli are not identical to those involved in the regulation of sucrose consumption.

Muscimol injected into the medial prefrontal cortex of the rat alters ethanol self-administration

The role of the rodent prefrontal cortex in the regulation of ethanol self-administration has not been widely explored. Understanding the role of GABAergic transmission in this area in relation to ethanol self-administration is important, as the GABA system may be one of several targets for alcohol's actions in the brain. Rats were initiated to drink 10% ethanol from a dipper using a sucrose-substitution procedure. When baseline behavior was stable, bilateral microinjections of muscimol (a GABA(A) agonist) into the prefrontal cortex were tested at doses of 17.5, 30, 100 and 300 ng/microl. Ethanol self-administration was decreased by approximately 40% at the 30-ng dose and 30% at the 100-ng dose. No effects were observed at either the 17.5- or 300-ng dose. The effect on the pattern of self-administration was to shorten the size of the first run of drinking without affecting the rate of drinking. The hypothesis is put forward that the injections increased glutamatergic output to the nucleus accumbens (nAcc) that in turn increased accumbens output. This increased output is proposed as similar to the effects of dopaminergic (DA) manipulations within this system.

Effects of raclopride in the nucleus accumbens on ethanol seeking and consumption

BACKGROUND

Nucleus accumbens dopamine has been shown to play a role in the processing of behaviorally relevant stimuli and to mediate ethanol-reinforced responding. Previous research that used a fixed-ratio schedule of responding maintained by the presentation of small dippers (0.1 ml) of ethanol demonstrated that the dopamine D2 antagonist, raclopride, decreased total responding for ethanol by both delaying the onset of responding and causing the early termination of lever-press behavior. Because these studies required animals to continuously respond to obtain access to small amounts of ethanol over a period of self-administration, this procedure assessed a combination of appetitive (seeking) and consummatory (drinking) behavior. The paradigm used in the present study separated the appetitive or seeking response from the consummatory response to assess the effects of raclopride on both types of ethanol-related behaviors.

METHODS

Male Long-Evans rats were trained to emit a fixed number of lever-press responses that resulted in access to a drinking tube that contained 10% ethanol for 20 min, once each day. We measured the effects of microinjections of raclopride (1.0, 3.0, and 10.0 microg/subject) into the nucleus accumbens, before the sessions, on appetitive and consummatory responding.

RESULTS

Raclopride delayed the onset of ethanol-seeking (appetitive responding) at all doses and decreased the number of responses made at the low and high doses. The rate of responding, however, was unaffected. Raclopride had no effect on the latency to begin consuming ethanol or on any of the characteristics of the initial bout of ethanol intake at all doses tested. Total ethanol intake was decreased, after an initially "normal" pattern of self-administration, following only the highest dose of raclopride. Mean ethanol intake (g/kg) was 0.54 (+/-0.03) after no injection, 0.51 (+/-0.04) after sham treatment, and 0.38 (+/-0.05) after 10 microg of raclopride.

CONCLUSIONS

The procedural separation of the seeking and intake responses used in this experiment allowed us to assess the effects of dopamine receptor antagonism in the nucleus accumbens on these two different behaviors. Overall, appetitive responding that preceded the delivery of an ethanol solution was more sensitive to raclopride treatment than was consummatory responding. These findings are consistent with a stimulus-processing function of the mesolimbic dopamine system.

Effect of naloxone on appetitive and consummatory phases of ethanol self-administration

BACKGROUND

The opioid system has been implicated in ethanol self-administration. Morphine, an opiate agonist, can sometimes increase the amount of ethanol consumed, and opiate antagonists such as naloxone and naltrexone decrease the amount of ethanol consumed in both animals and humans. The objective of this study was to examine the effect of naloxone on appetitive (or seeking) and consummatory behaviors by using an operant model developed to separate these two phases of self-administration.

METHODS

Intraperitoneal injections of naloxone (0.3-10 mg/kg) or vehicle were given before operant self-administration sessions to assess the effect on lever pressing (appetitive behavior) and subsequent consumption. Effects were measured in two groups of rats: one self-administered a 3% sucrose solution and the other a 10% ethanol solution.

RESULTS

Naloxone dose-dependently decreased ethanol and sucrose consumption by an earlier cessation of drinking in the session compared with vehicle injection days. There were some effects on appetitive responding after treatment with naloxone, but none was statistically significant.

CONCLUSIONS

Naloxone may decrease ethanol self-administration by decreasing the postingestive or pharmacological effects of alcohol. This model provides a new method for examining the effects of potential pharmacotherapeutics on alcohol self-administration behavior.

Measuring ethanol-seeking behavior: the effect of using repeated extinction trials

The development of a reliable measure of the level of ethanol-seeking behavior in an animal model is important to understanding the concept of craving. However, most existing models do not allow for the separation of the behavior associated with obtaining ethanol from that involved in consumption of ethanol. In this study, we determined the ability of repeated, single-session extinction tests in an appetitive and consummatory procedure of ethanol self-administration to assess the level of seeking behavior. The findings indicated that there were no major effects of previous extinction trials on later trials, when there were at least four reinforced sessions between tests. During reinforced sessions, the rats were consuming an average of 0.80 g of ethanol per kilogram of body weight in less than 20 min from a sipper tube. In addition, the amount of extinction responding was found to be similar to a previous measure of the appetitive strength of ethanol by using a breakpoint procedure. This method of repeated extinction tests seems to be valuable for examining the effects of pharmacological treatments that might alter ethanol seeking.

Effects of SR141716A on ethanol and sucrose self-administration

BACKGROUND

Previous studies have demonstrated that administration of central cannabinoid receptor (CB1) ligands can produce marked effects on ingestive behaviors. However, the possible relationship to ethanol self-administration has not been fully examined. The present series of experiments was designed to characterize further the role of CB1 receptors in appetitive and consummatory behaviors related to sucrose and ethanol.

METHODS

To determine the relative contribution of CB1 receptors to ethanol seeking and consumption, a series of experiments was designed using the sipper-tube model. In this paradigm, the appetitive and consummatory phases of ethanol and sucrose self-administration are separated. In the appetitive phase, animals are required to complete a response requirement (16 lever presses) within 20 min. If the requirement is successfully completed, access to a sipper tube containing either sucrose or ethanol (consummatory phase) is made available for 20 min.

RESULTS

In the ethanol condition, the CB1 receptor antagonist SR141716A (0.3-3.0 mg/kg, ip) produced dose-related decreases in the probability of response requirement completion without significantly affecting latency to first lever press or overall lever press rate. In the sucrose condition, SR141716A (0.3-3.0 mg/kg, ip) increased first lever press latency without affecting lever press rate. In the consummatory phase, SR141716A (0.3-3.0 mg/kg, ip) administration markedly decreased total intake and the total number of licks for both ethanol and sucrose.

CONCLUSIONS

These data indicate that CB1 receptors are involved in mediating both appetitive and consummatory aspects of ingestive behaviors related to sucrose and ethanol.

Effects of acamprosate on ethanol-seeking and self-administration in the rat

BACKGROUND

Acamprosate (calcium acetyl homotaurinate) has been used clinically to treat relapse in alcoholics. In rats, it has been shown to decrease ethanol, but not water, self-administration after ethanol deprivation.

METHODS

To further investigate the effect of acamprosate on reinforced behaviors in rats, the present experiment used: (1) both ethanol and sucrose reinforcer solutions to better assess the distinct effects of acamprosate on ethanol-directed behaviors, and (2) an operant model that procedurally separates the "cost" to begin drinking from consuming the reinforcer solutions to dissociate the effects of acamprosate on appetitive versus consummatory processes. In daily sessions (5 days/week), rats (n = 6/group) were trained to make 30 lever-press responses to gain access for 20 min to a sipper tube containing either ethanol (10%) or sucrose (3%). After stable responding, acamprosate treatment was given. Three doses were tested (50, 100, and 200 mg/kg/injection, intraperitoneally), one dose per week. Each week, a total of four injections were given (21 and 2 hr before the operant sessions over 2 consecutive days).

RESULTS

At these doses, acamprosate had no effect on the measures of appetitive responding for either solution. However, all doses reliably decreased ethanol consumption on the 2nd day of treatment (from an average of 0.83 to 0.63 g/kg). Analysis of the pattern of ethanol consumption showed that the effects of acamprosate occurred after the onset of a normal pattern of intake, as measured by lick rate and size of the initial bout of drinking, which suggested that acamprosate is most effective when combined with the pharmacological effects of ethanol. Sucrose intake was unaffected by all acamprosate treatments, which indicated that the treatment effects were specific to ethanol and not due to a general decrease in consummatory behavior.

CONCLUSIONS

Overall, these results suggest that acamprosate is effective at reducing total ethanol intake, but may not reliably alter subjects propensity to begin a drinking bout as measured by this model. However, whether this applies to the clinical use of acamprosate, where other types of reinforcement may also precipitate relapse drinking, is not certain.

Injected muscimol in pedunculopontine tegmental nucleus alters ethanol self-administration

The pedunculopontine tegmental nucleus (PPN) has been implicated in a variety of behavioral functions, including stimulus selection. Given the PPN interactions with the mesolimbic system, it was considered important to determine its involvement in ethanol self-administration. Long-Evans male rats were trained to self-administer 10% ethanol by using a sucrose-substitution procedure. After implantation of cannula guides, microinjections of 30, 100, and 300 ng of muscimol into the PPN before the self-administration session were performed. Ethanol self-administration was decreased at the 300-ng dose, in a manner similar to the actions of dopamine agonists microinjected in the nucleus accumbens. It is hypothesized that loss of PPN cholinergic input to the mesolimbic system affects the integrative activity of the nucleus accumbens and underlies the observed change in ethanol self-administration behavior.

The microstructure of ethanol drinking: genetic and behavioral factors in the control of drinking patterns

The concept of craving can be examined in many different ways, depending upon the individual definition of the term. Using the concepts and procedures of regulatory behavior analysis, this review explores behavioral studies in rats that have some relationships to some of the possible processes that underlie the concept of craving in humans. Data are reviewed from studies employing both limited and continuous access to ethanol, examining the role of access availability, ethanol initiation, response cost, time since last access, composition of the ethanol containing solution and genetic selection. From this review, it is clear that rat models can implicate important variables involved in the control of human alcohol consumption.

A new assessment of the ability of oral ethanol to function as a reinforcing stimulus

BACKGROUND

In animal studies, the ability of ethanol to function as a reinforcer has been described as weak to moderate. This is in contrast with the human condition, where the consumption of alcoholic beverages can result in a variety of unwanted drinking behaviors. However, when the ethanol self-administration pattern is examined, animal studies over the last several years indicate that the ability of ethanol presentation to maintain behavior may be greater than originally assumed.

METHODS

We reevaluated the ability of ethanol to function as a reinforcing stimulus in two paradigms by using an analysis of drinking bout characteristics. Data from previous studies that employed two self-administration models were analyzed. With the "dipper" model, small amounts of ethanol are presented after each completion of a response requirement; with the "sipper" model, the animal is allowed access to a drinking tube that contains ethanol for an extended period after completing a single response requirement. For both models, the consumption pattern could be characterized as occurring in a bout. Each drinking bout was divided into runs within the bout, and run rates and size were analyzed. As well, in the sipper model, the data on response requirement size were reviewed to demonstrate the ability of ethanol presentation to maintain high levels of responding in this model.

RESULTS

From this assessment, we suggest that ethanol presentation in non-food- or non-water-restricted rats is as reinforcing as many other stimuli generally considered to be strong reinforcers (i.e., food in food-restricted rats). Using run size, we demonstrated that intake control appears to be regulated by shifts in run size during the bout and not run rate.

CONCLUSIONS

Assessment of the pattern of ethanol consummatory bouts and the behaviors that precede them is critical in understanding how ethanol functions as a reinforcer. By using a drinking pattern analysis, the shifts in the momentary salience of the ethanol stimulus can be evaluated in these animal models. In addition, the separation of responding required to gain access to ethanol from consumption of ethanol demonstrated that ethanol presentation in this procedure can be a strong reinforcer for rats.

Effects of prior ethanol exposure on ethanol self-administration in a continuous access situation using retractable drinking tubes

To examine whether exposure to ethanol influences subsequent ethanol consumption using a continuous access procedure, two groups of rats were given differing initial exposure to ethanol. One group underwent a sucrose-substitution initiation procedure. The second group received abbreviated initiation consisting of one-session exposure to each ethanol/sucrose combination used in standard initiation. The animals were then provided with 23 h/day access to ethanol (10%, v/v) from a retractable drinking tube. Food pellets were available following a single-lever press, and water was available from a sipper tube. After 5 weeks, the data indicated that few significant differences existed between the groups on total ethanol (g/kg), food or water consumed. The overall intake (g/kg/day), number of ethanol bouts per day, and amount consumed per bout (g/kg/bout) were substantially lower than observed in previous research using ethanol presented in a dipper. However, differences in g/kg per ethanol bout did differ significantly between the two groups with the group receiving standard initiation showing more ethanol consumed per bout. These data agree with our previous work indicating that initiation results in larger drinking bouts.

Initiation of ethanol self-administration in the rat using sucrose substitution in a sipper-tube procedure

RATIONALE

The concepts of appetitive and consummatory behaviors provide a framework for examining ethanol-drinking behavior. However, traditional studies of ethanol self-administration using dipper procedures make separating the appetitive from the consummatory components difficult.

OBJECTIVE

This study compared the ability to initiate ethanol self-administration using a new sipper-tube self-administration procedure with the older established sucrose-substitution initiation model that employed dipper presented reinforcement. The new model was developed to allow for an assessment of the appetitive and consummatory components in ethanol self-administration.

METHODS

For the sipper-tube procedure, the rats were initiated to self-administer ethanol using a sucrose-substitution procedure that provided limited access to a sipper tube containing ethanol. This procedure required the completion of a fixed ratio requirement (FR4) in order to gain access to a sipper tube for 20 min. Initially, a 20% sucrose solution with no ethanol was provided in the sipper tube. Over sessions, the concentration of sucrose was reduced and the ethanol concentration increased, until 10% ethanol in water was the solution presented. A second group of animals was initiated to self-administer ethanol using the dipper-presentation procedure employed in our laboratory for many years. This group was used for comparison of the effectiveness of initiation in the sipper-tube procedure.

RESULTS

Following initiation, the sipper-tube rats self-administered 10% ethanol in water with intakes averaging 0.75 g/kg during the 20-min drinking period. Increasing the ethanol concentrations as high as 20%, increased intakes as high as 1.5 g/kg. The ethanol intakes observed were similar to those obtained with the dipper initiation procedure but occurred in one-third of the time.

CONCLUSIONS

The sipper-tube procedure employed here results in similar ethanol self-administration behavior as has been found with a dipper presentation procedure. More importantly, however, it allows for a separation of the appetitive and consummatory components of ethanol self-administration. This separation may prove useful for examining the strength of ethanol-seeking behaviors without the confound of increasing levels of ethanol interacting with the appetitive seeking behaviors.

Breakpoint determination and ethanol self-administration using an across-session progressive ratio procedure in the rat

BACKGROUND

Progressive ratio schedules are used to determine the "breakpoint" or limit to the amount of "work" that a subject is willing to perform to obtain a reinforcer. Reinforcing efficacy is inferred from the breakpoint values, which are typically measured in a single session by increasing the number of responses required for successive reinforcer presentations. This procedure is not feasible, however, when assessing the reinforcing efficacy of a substance that can change as a function of its physiological actions during self-administration, as in the case of ethanol.

METHODS

The present study made use of a procedure that increased the response requirement across single daily sessions rather than within a session. Completion of the response requirement in each daily session resulted in the presentation of a drinking tube that allowed for self-administration of ethanol for a 20-min period. This procedure made possible the assessment of ethanol-directed appetitive (number of lever presses) and consummatory (number of licks and intake volume) behaviors. Reliable responding for 10% ethanol was initiated using sucrose-substitution on a fixed ratio (FR) 4 schedule in male Long Evans rats. Then four successive breakpoint determinations were made which were separated by a return to the FR4 schedule to re-establish baseline responding.

RESULTS

The results indicated that there was an increase in breakpoint values from the first to the second determination, which was then stable over the following three determinations. Individual rats reached breakpoints as high as 240 lever presses to receive access to 10% ethanol and maintained ethanol intake over sessions in the 1.0 g/kg range. Ethanol intake (g/kg), however, was stable across all four determinations (mean 0.86 +/- 0.06 to 1.01 +/- 0.10). Moreover, ethanol intake was not related to the preceding appetitive responding, as no differences between intake on the session before a breakpoint (high FR) and the following baseline period (FR4) were observed.

CONCLUSIONS

This model provides an assessment of the distinct mechanisms that mediate ethanol-seeking versus ethanol consumption in subjects that drink measurable amounts of ethanol, with the appetitive behaviors not altered by the pharmacological effects of ethanol.

Chronic ethanol self-administration in a continuous-access operant situation: the use of a sucrose/ethanol solution to increase daily ethanol intake

The addition of sucrose to an ethanol solution increases both limited- and continuous-access ethanol consumption. The present study examined if the increased intakes in a continuous-access condition could produce withdrawal signs indicating physical dependence on ethanol. Rats were maintained in a continuous-access operant situation in which one lever press on one lever resulted in the presentation of a food pellet, whereas one lever press on a second lever presented 0.1 ml of fluid in a dipper. Water was available from a drinking spout. Ten rats received a 10% sucrose/20% ethanol mixture in the dipper and six rats 10% sucrose. After 30 days the animals were tested for withdrawal signs after 8 h without ethanol using an activity test and response to key shaking. They were then given an additional 30 days of access to the solutions and retested for withdrawal. This was followed by a final 30 days of access and a third withdrawal test. Over the 90 days, the sucrose/ethanol group consumed 8-10 g of ethanol per kilogram of body weight per day. Over this time both groups gained weight. At the third withdrawal test, a significant reduction in activity occurred in the ethanol-drinking group, compared with the sucrose group. No severe withdrawal effects were observed to the key shake test. The results suggest that the higher ethanol intakes previously observed using this sucrose/ethanol solution can be maintained over long periods of time. Although this intake was not sufficient to produce severe withdrawal signs, the results suggest that longer exposure might result in more severe ethanol dependence.

Blood ethanol concentrations in rats drinking sucrose/ethanol solutions

BACKGROUND

The addition of sucrose to ethanol solutions results in a substantial increase in ethanol self-administration by rats that are deprived of neither food nor water. However, if sucrose alters ethanol absorption or metabolism, resulting in blood ethanol concentrations (BECs) not different from those resulting from lower intakes of ethanol/water solutions, then the usefulness of sucrose/ethanol mixtures in increasing ethanol consumption is questionable. The present study was conducted to determine whether the addition of sucrose to ethanol solutions altered BECs in an operant self-administration paradigm.

METHODS

Tail blood (from male Long-Evans rats) was collected 30 min after the intake of four different solutions, i.e., 5% sucrose/20% ethanol, 5% sucrose/10% ethanol, 2% sucrose/10% ethanol, and 10% ethanol.

RESULTS

Ethanol intakes (mean, 1.57+/-0.21 g/kg) and BECs (mean, 78.4+/-9.3 mg/100 ml) were highest when 5% sucrose was added to the ethanol solution. Moreover, the ratios between ethanol intakes and resulting BECs were approximately the same for all solutions.

CONCLUSIONS

These findings indicate that, under the conditions of this procedure, the BEC reached is dependent on the amount of ethanol consumed and is not influenced by the addition of sucrose to the solution.