Ethanol consumption in rats selectively bred for differential saccharin intake

Acquisition of i.v. cocaine self-administration in adolescent and adult male rats selectively bred for high and low saccharin intake

Adolescence and excessive intake of saccharin have each been previously associated with enhanced vulnerability to drug abuse. In the present study, we focused on the relationship between these two factors using male adolescent and adult rats selectively bred for high (HiS) and low (LoS) levels of saccharin intake. On postnatal day 25 (adolescents) or 150 (adults), rats were implanted with an intravenous catheter and trained to self-administer cocaine (0.4 mg/kg) using an autoshaping procedure that consisted of two 6-h sessions. In the first 6 h, rats were given non-contingent cocaine infusions at random intervals 10 times per hour, and during the second 6-h session, rats were allowed to self-administer cocaine under a fixed ratio 1 (FR 1) lever-response contingency. Acquisition was defined as a total of at least 250 infusions over 5 consecutive days, and rats were given 30 days to meet the acquisition criterion. Subsequently, saccharin phenotype scores were determined by comparing 24-h saccharin and water consumption in two-bottle tests to verify HiS/LoS status. Adolescent LoS rats had a faster rate of acquisition of cocaine self-administration than adult LoS rats; however, adolescent and adult HiS rats acquired at the same rate. Both HiS and LoS adolescents had significantly higher saccharin phenotype scores than HiS and LoS adults, respectively. Additionally, saccharin score was negatively correlated with the number of days to meet the acquisition criterion for cocaine self-administration, but this was mostly accounted for by the HiS adolescents. These results suggest that during adolescence, compared with adulthood, rats have both an increased avidity for sweets and vulnerability to initiate drug abuse.

Impulsivity (delay discounting) for food and cocaine in male and female rats selectively bred for high and low saccharin intake

Previous research in rats indicates that delay discounting for food, a model of impulsivity, predicted the rate of acquisition of cocaine self-administration. In other studies, rats bred for high saccharin intake (HiS) acquired cocaine self-administration at higher rates than those with low saccharin intake (LoS), and female (F) rats acquired cocaine self-administration more rapidly than males (M). The purpose of this study was to examine a possible connection between impulsivity, saccharin intake, and sex by comparing M and F rats from the HiS and LoS selectively bred lines on measures of impulsivity; i.e., their rate of delay discounting for food or i.v. cocaine infusions. The adjusting delay procedure allowed rats access to 2 response levers, and a pellet dispenser or an i.v. drug infusion pump. In 4 groups (HiS M, HiS F, LoS M, LoS F) responses under a fixed-ratio (FR) 1 schedule on one lever resulted in one 45 mg pellet immediately, and responses on the other lever resulted in 3 or 6 pellets after a delay. Four additional groups received either a small cocaine (0.2, 0.4, or 0.8 mg/kg) infusion immediately or a delayed larger infusion (3x the amount of the small infusions). The delay to the larger reinforcer began at 6 s and increased or decreased by 1 s following responses on the delay or immediate levers, respectively. A mean adjusted delay (MAD) was calculated over 30 choice trials during each daily 3-hour session, and it was used as a quantitative measure of impulsivity. In groups maintained by food, HiS rats were more impulsive (lower MADs) than LoS rats, and LoS females were more impulsive than LoS males. There were no phenotype or sex differences in delay discounting for cocaine. Understanding the relationship between impulsivity and other predictors of drug abuse (e.g., sex, saccharin intake) is important in developing prevention and treatment strategies.

Regulation of intravenous cocaine self-administration in rats selectively bred for high (HiS) and low (LoS) saccharin intake

RATIONALE

Rats selectively bred for high saccharin (HiS) intake consume more alcohol and acquire intravenous (i.v.) cocaine self-administration more rapidly than their low saccharin (LoS)-consuming counterparts. The present experiment was designed to determine whether HiS and LoS rats differed in other aspects of drug abuse.

OBJECTIVE

The purpose of the present experiment was to use a two-lever dose self-selection procedure to investigate the regulation/dysregulation of i.v. cocaine self-administration in female HiS and LoS rats.

MATERIALS AND METHODS

HiS and LoS rats were trained to self-administer eight different doses of cocaine during daily 5-h sessions, with the cocaine doses ranging from 0.2 to 1.6 mg/kg in steps of 0.2 mg/kg. The dose size increased after a response on one lever (infusion duration lengthened by 3 s) and decreased after a response on the other lever (infusion duration shortened by 3 s), with a lower limit of 0 s and with an upper limit of 24 s (a corresponding range of 0 to 1.6 mg/kg); the animals increased or decreased their self-administered dose in nine discrete steps.

RESULTS

The HiS rats showed less precise regulation of their postinfusion interval (PII) than LoS rats based on the size of the previously self-administered cocaine dose. Correlations between these variables were lower for HiS than for LoS rats during the acquisition and maintenance phases, and HiS rats had lower PIIs than LoS rats after many of the cocaine doses. The HiS rats also self-administered significantly more cocaine infusions during the maintenance phase than the LoS rats, especially at the highest dose.

CONCLUSIONS

These data indicate that HiS rats are more likely to self-administer more cocaine infusions, at higher doses, and with less precise dose-time regulation than LoS rats. Thus, rats selectively bred for HiS showed less inhibitory control over their cocaine intake than LoS rats, suggesting that a genetic predisposition for saccharin preference is related to the rewarding effects of i.v. cocaine.

Sweet and bitter taste of ethanol in C57BL/6J and DBA2/J mouse strains

Studies of inbred strains of rats and mice have suggested a positive association between strain variations in sweet taste and ethanol intake. However, strain associations by themselves are insufficient to support a functional link between taste and ethanol intake. We used conditioned taste aversion (CTA) to explore the sweet and bitter taste of ethanol and ability to detect sucrose, quinine and ethanol in C57BL/6J (B6) and DBA/2J (D2) mouse strains that are frequently used in alcohol research. The present study showed that C57BL/6J mice generalized taste aversions from sucrose and quinine solutions to 10% ethanol and, reciprocally, aversions to 10% ethanol generalized to each of these solutions presented separately. Only conditioned aversions to quinine generalized to ethanol in the DBA/2J strain but an aversion conditioned to ethanol did not generalize reciprocally to quinine. Thus, considering these two gustatory qualities, 10% ethanol tastes both sweet and bitter to B6 mice but only bitter to D2. Both strains were able to generalize taste aversions across different concentrations of the same compound. B6 were able to detect lower concentrations of quinine than D2 but both strains were able to detect sucrose and (in contrast to previous findings) ethanol at similar concentrations. The strain-dependent gustatory profiles for ethanol may make an important contribution to the understanding of the undoubtedly complex mechanisms influencing high ethanol preference of B6 and pronounced ethanol avoidance of D2 mice.

Escalation of i.v. cocaine self-administration and reinstatement of cocaine-seeking behavior in rats bred for high and low saccharin intake

RATIONALE

Rats selectively bred for high saccharin (HiS) intake consume more alcohol, acquire intravenous (i.v.) cocaine self-administration more rapidly, and show more dysregulated patterns of cocaine self-administration than their low saccharin-consuming (LoS) counterparts.

OBJECTIVES

The purpose of the present study was to determine whether HiS and LoS rats also differ in the escalation, maintenance, extinction, and reinstatement of i.v. cocaine self-administration.

MATERIALS AND METHODS

Two experiments were conducted in separate groups of rats. In the first experiment, HiS and LoS female rats were allowed to self-administer cocaine [0.4 mg/kg; fixed ratio (FR) 1] under short (ShA, 2 h per day) or long (LgA, 12 h per day) access conditions for 21 days. Session lengths were subsequently equated (2 h), and FR1-maintained cocaine self-administration was examined. In the second experiment, additional groups of HiS and LoS female rats were given access to cocaine (0.4 mg/kg; FR 1) self-administration during 2-h sessions for 10 days. Subsequently, saline was substituted for cocaine, and responding was extinguished. After a 14-day extinction period, saline- and cocaine-[5, 10, and 15 mg/kg, intraperitoneal (i.p.)] induced reinstatement of drug-seeking behavior was measured.

RESULTS

HiS LgA rats escalated their cocaine intake more rapidly than LoS rats, and during the 2 h sessions after escalation cocaine self-administration was significantly higher in HiS LgA rats, compared to LoS LgA rats. HiS rats responded on the cocaine-paired lever more than LoS rats during maintenance, extinction, and cocaine-(15 mg/kg) induced reinstatement.

CONCLUSIONS

These results suggest that HiS and LoS rats have distinct drug-seeking and drug-taking profiles. The HiS and LoS rats differ along a wide range of behavioral dimensions and represent an important model to study the interactions of excessive intake of dietary substances and vulnerability to drug abuse.

Wheel running as a predictor of cocaine self-administration and reinstatement in female rats

Avidity for behaviors mediated by nondrug rewards, such as novelty seeking or intake of sweets or fats, is predictive of enhanced vulnerability to the locomotor-activating and rewarding effects of drugs of abuse. The purpose of the present study was to determine whether avidity for wheel running was predictive of subsequent cocaine-induced locomotor activity, cocaine self-administration, and cocaine-seeking behavior in rats. Rats with high (HiR) and low (LoR) levels of wheel running were selected from an outbred sample of Wistar rats. These rats were first tested for their locomotor response to an acute injection of cocaine (10 mg/kg, i.p.). Subsequently, a multi-phase self-administration procedure was used to examine the effect of wheel running on the maintenance, extinction, and cocaine-induced reinstatement of cocaine-seeking behavior in HiR and LoR rats. The results indicate no significant differences between HiR and LoR rats in the cocaine-induced stimulation of locomotor activity. During maintenance, HiR rats self-administered more cocaine than LoR rats. While there were no group differences in saline self-administration behavior during extinction, HiR rats showed higher cocaine-induced reinstatement than LoR rats. Rats that were previously high responders to novelty (day 1 in locomotor track) also showed significantly higher reinstatement than low novelty responders. These results suggest that a propensity for wheel running is associated with increased vulnerability for cocaine self-administration and reinstatement and that HiR rats are more motivated than LoR rats to seek cocaine.

Ethanol withdrawal and proclivity are inversely related in rats selectively bred for differential saccharin intake

Withdrawal severity and voluntary alcohol consumption are inversely related in rats and mice. The present study demonstrated this empirical relation and extended it in two ways. First, the rats were selectively bred for low (LoS) and high (HiS) saccharin intake, a phenotype that correlates positively with ethanol intake and inversely with emotional reactivity. Withdrawal has not yet been studied in these rats. Second, proclivity to consume ethanol was measured as conditioned preference for an ethanol-paired flavor. After 2 weeks of forced exposure to ethanol and a period of abstinence, LoS rats showed elevated acoustic startle; HiS rats did not (Exp. 1). When ethanol- and no-ethanol solutions were available freely during conditioning, both LoS and HiS rats preferred a flavor paired with 4% ethanol, but only HiS rats preferred a flavor paired with 10% ethanol (Exp. 2A); when exposure to the two solutions was controlled, all groups except LoS males preferred flavors paired with 4% or 10% ethanol (Exp. 2B). Thus, as predicted, withdrawal was more severe in the line with less ethanol proclivity (LoS). These results implicate basic associative and affective processes in individual differences in patterns of alcohol use.

Brain stimulation reward performance and sucrose maintained behaviors in alcohol-preferring and -nonpreferring rats

BACKGROUND

The relation between ethanol (EtOH) preference and sensitivity to brain stimulation reward (BSR) was examined under multiples schedules of reinforcement in the current study. For comparison, the study also examined the relation between EtOH preference and motivation for a sweet, palatable sucrose solution under similar schedules of reinforcement.

METHODS

To investigate sensitivity to BSR performance, alcohol-preferring and -nonpreferring rats were tested using the curve-shift (rate-frequency) paradigm under several intensity levels during a 20-min session. Animals were first trained under an optimal current intensity, which produced maximal responding (i.e., 100%) across a series of descending frequencies (i.e., 300-20 Hz). BSR was then evaluated at 100%, 75%, and 50% of the optimal current. The sensitivity of the curve-shift method was further evaluated under the animal's optimal current using the FR1, FR6, and FR12 schedules. To examine responding for the sucrose solution, a separate group of alcohol-preferring and -nonpreferring rats was initially stabilized on an FR1 schedule and then subsequently on FR6 and FR12 schedules.

RESULTS

The results demonstrated that reducing the reinforcing efficacy of BSR via reduction in current intensity/reinforcement schedule produced marked orderly rightward shifts in the rate-frequency curves relating responding to stimulation frequency in both rat lines. However, no differences were found between the lines with either manipulation. Specifically, both lines demonstrated orderly reductions in response rate and increases in BSR threshold parameters (i.e., half maximal frequency/responding, minimum and maximum frequencies). In contrast to BSR, genetic selection for EtOH preference was highly associated with responding for the sweet, palatable sucrose solution. The association was even more salient as the reinforcement schedule increased (i.e., reward cost).

CONCLUSION

The results demonstrate that responding for BSR is not associated with EtOH preference, insofar as alcohol-preferring and -nonpreferring rats respond similarly under an array of reinforcement schedules and current intensities. In contrast, genetic selection for EtOH preference is highly associated with responding for a palatable sucrose reward, and the relation increases as the reward cost for the sucrose increases. These findings suggest that similar/overlapping mechanisms of action regulate the reinforcing properties of EtOH and sucrose but that overlapping yet distinct neuronal mechanism may modulate the reward characteristics of BSR and EtOH preference.

No relationship between sequence variation in protein coding regions of the Tas1r3 gene and saccharin preference in rats

Nearly all mammalian species like sweet-tasting foods and drinks, but there are differences in the degree of 'sweet tooth' both between species and among individuals of the same species. Some individual differences can be explained by genetic variability. Polymorphisms in a sweet taste receptor (Tas1r3) account for a large fraction of the differences in consumption of sweet solutions among inbred mouse strains. We wondered whether mice and rats share the same Tas1r3 alleles, and whether this gene might explain the large difference in saccharin preference among rats. We conducted three experiments to test this. We examined DNA sequence differences in the Tas1r3 gene among rats that differed in their consumption of saccharin in two-bottle choice tests. The animals tested were from an outbred strain (Sprague-Dawley; experiment 1), selectively bred to be high- or low-saccharin consumers (HiS and LoS; experiment 2), or from inbred strains with established differences in saccharin preference (FH/Wjd and ACI; experiment 3). Although there was considerable variation in saccharin preference among the rats there was no variation in the protein-coding regions of the Tas1r3 gene. DNA variants in intronic regions were detected in 1 (of 12) outbred rat with lower-than-average saccharin preference and in the ACI inbred strain, which also has a lower saccharin preference than the FH/Wjd inbred partner strain. Possible effects of these intronic nucleotide variants on Tas1r3 gene expression or the presence of T1R3 protein in taste papillae were evaluated in the ACI and FH/Wjd strains. Based upon the results of these studies, we conclude that polymorphisms in the protein-coding regions of the sweet receptor gene Tas1r3 are uncommon and do not account for individual differences in saccharin preference for these strains of rats. DNA variants in intron 4 and 5 are more common but appear to be innocuous.

Escalation of intravenous cocaine self-administration, progressive-ratio performance, and reinstatement in rats selectively bred for high (HiS) and low (LoS) saccharin intake

RATIONALE

Rats selectively bred for high saccharin (HiS) intake consume more alcohol and acquire intravenous (IV) cocaine self-administration more rapidly than their low saccharin (LoS) consuming counterparts.

OBJECTIVES

The purpose of the present study was to determine whether HiS and LoS rats also differ in the escalation, maintenance, and reinstatement of IV cocaine self-administration.

METHODS

LoS and HiS female rats were allowed to self-administer cocaine [0.4 mg/kg; fixed ratio (FR) 1] under short (ShA, 2 h per day) or long (LgA, 12 h per day) access conditions for 21 days. Session lengths were subsequently equated (2 h) and (1) FR1-maintained cocaine (0.4 mg/kg) self-administration, (2) progressive ratio (PR)-maintained cocaine (0.2-1.6 mg/kg) self-administration, and (3) saline-induced and cocaine (10 mg/kg, IP)-induced reinstatement of drug-seeking behavior were examined.

RESULTS

HiS LgA rats escalated their cocaine intake more rapidly and self-administered more cocaine (mg/kg) than LoS LgA rats; however, there was no LoS versus HiS phenotype difference in the number of infusions self-administered by Day 21. Post-escalation cocaine self-administration under an FR1 schedule did not differ as a function of phenotype (LoS versus HiS) or access condition (ShA versus LgA); however, LoS rats responded more for cocaine under the PR schedule than HiS rats, and they showed a greater reinstatement of cocaine-seeking behavior than HiS rats. In contrast, ShA versus LgA did not affect PR or reinstatement performance in the LoS and HiS groups.

CONCLUSIONS

These results suggest that LoS and HiS rats have distinct drug-seeking and drug-taking profiles that differ as a function of the experimental phase and access condition. The LoS and HiS rats differ along a wide range of behavioral dimensions and represent an important model to study the interactions of feeding, emotionality, and other factors related to vulnerability to drug abuse.

Sweet liking, novelty seeking, and gender predict alcoholic status

BACKGROUND

The relationship between a hedonic response to sweet taste and a propensity to excessive alcohol drinking is supported by both animal and human studies. There is evidence indicating that the tendency to rate more concentrated sweet solutions as the most pleasurable (i.e., sweet liking) is associated with the genetic vulnerability to alcoholism. However, sweet liking by itself is insufficient to predict the alcoholic status of the individual. Our previous study indicated that alcoholic status can be predicted by a combination of hedonic response to sweet taste and personality profile as measured by the Tridimensional Personality Questionnaire (TPQ). This study was designed to further test this hypothesis.

METHODS

Participants were 165 patients admitted to a residential treatment program for the treatment of alcoholism, drug dependence and/or interpersonal problems secondary to substance-abusing family members. In addition to a routine medical examination, on the 24th day after admission, patients completed the TPQ, the standard sweet taste test was conducted, and paternal family history of alcoholism was evaluated.

RESULTS

Sweet liking was strongly associated with a paternal history of alcoholism. The odds of receiving an alcohol dependence diagnosis were shown to increase, on the average, by 11% for every one-point increase in the TPQ novelty-seeking score in sweet-liking but not in sweet-disliking subjects. Gender contributed independently to the probability of alcohol dependence, with males exhibiting higher rates of alcoholism than females.

CONCLUSIONS

These findings support the hypothesis that a hedonic response to sweet taste is associated with a genetic risk for alcoholism. Alcoholic status may be predicted by a combination of sweet liking, the TPQ novelty-seeking score, and gender in a mixed group of alcoholic, polysubstance-dependent, and psychiatric patients.

Alcohol Activates a Sucrose-Responsive Gustatory Neural Pathway

A strong positive association exists between the ingestion of alcohol and sweet-tasting solutions. The neural mechanisms underlying this relationship are unknown, although recent data suggest that gustatory substrates are involved. Here, we examined the role of sweet taste receptors and central neural circuits for sugar taste in the gustatory processing of ethanol. Taste responses to ethanol (3, 5, 10, 15, 25, and 40% vol/vol) and stimuli of different taste qualities (e.g., sucrose, NaCl, HCl, and quinine-HCl) were recorded from neurons of the nucleus of the solitary tract in anesthetized rats prior to and after oral application of the sweet receptor blocker gurmarin. The magnitude of ethanol-evoked activity was compared between sucrose-responsive ( n = 21) and sucrose-unresponsive ( n = 20) neurons and the central neural representation of ethanol taste was explored using multivariate analysis. Ethanol produced robust concentration-dependent responses in sucrose-responsive neurons that were dramatically larger than those in sucrose-unresponsive cells. Gurmarin selectively and similarly inhibited ethanol and sucrose responses, leaving NaCl, HCl, and quinine responses unaltered. Across-neuron patterns of response to ethanol were most similar to those evoked by sucrose, becoming increasingly more so as the ethanol concentration was raised. Results implicate taste receptors for sucrose as candidate receptors for ethanol and reveal that alcohol and sugar taste are represented similarly by gustatory activity in the CNS. These findings have important implications for the sensory and reward properties of alcohol.

Association Between Sweet Preference and Paternal History of Alcoholism in Psychiatric and Substance Abuse Patients

BACKGROUND

The relationship between preference for stronger sweet solutions and propensity to excessive alcohol drinking is supported by both animal and human studies. This study was designed to test the hypothesis that sweet preference is associated with the genetic risk of alcoholism as measured by a paternal history of alcoholism.

METHODS

Participants were 180 patients admitted to a residential treatment program for the treatment of alcoholism, drug dependence, or psychiatric conditions. In addition to a routine medical examination, patients completed the standard sweet preference test twice (on the 9th and 24th days after admission), and the family history of alcoholism was evaluated.

RESULTS

Sweet preference was shown to be stable over time. It was strongly associated with a paternal history of alcoholism, with family history-positive patients approximately 5 times more likely to prefer stronger sweet solutions than family history-negative subjects. Such factors as dependence on alcohol, cocaine, opiates, cannabis, other drugs (including prescription drugs), and tobacco smoking, as well as demographics (gender and age), did not significantly interfere with association between sweet preference and paternal history of alcoholism.

CONCLUSIONS

These findings provide some support for the hypothesis that preference for stronger sweet solutions is associated with a genetic predisposition to alcoholism as measured by a paternal history of alcoholism.

Family History of Alcoholism and Response to Sweets

BACKGROUND

The relationship between a hedonic response to sweet tastes and a propensity to excessive alcohol drinking is supported by both animal and human studies. This study was designed to test the hypothesis that the genetic risk for alcoholism as measured by a paternal history of alcoholism in young social drinkers is associated with sweet-liking, defined as rating the strongest offered sucrose solution (i.e., 0.83 M) as the most palatable during the standard sweet test.

METHODS

Participants were 163 subjects (39% male) without a lifetime history of alcohol or drug abuse or dependence. Eighty-one subjects had a paternal history of alcoholism (FH+), and 82 did not (FH-). Each subject rated a series of sucrose solutions for intensity of sweetness and palatability. Subjects were categorized as sweet-likers if they rated the highest sucrose concentration as the most pleasurable.

RESULTS

The estimated odds of being a sweet-liker were 2.5 times higher for FH+ than for FH- subjects. FH+ subjects disliked the tastes of the two weakest offered sucrose concentrations (0.05 and 0.10 M), whereas FH- subjects reported these tastes to be neutral.

CONCLUSIONS

The results of this study support the hypothesis that sweet-liking is associated with a genetic vulnerability to alcoholism.

Sugars: hedonic aspects, neuroregulation, and energy balance

The prevalence of obesity has increased dramatically in recent years in the United States, with similar patterns seen in several other countries. Although there are several potential explanations for this dramatic increase in obesity, dietary influences are a contributing factor. An inverse correlation between dietary sugar intake and body mass index has been reported, suggesting beneficial effects of carbohydrate intake on body mass index. In this review we discuss how sugars interact with regulatory neurochemicals in the brain to affect both energy intake and energy expenditure. These neurochemicals appear to be involved in dietary selection, and sugars and palatable substances affect neurochemical changes in the brain. For example, rats that drink sucrose solutions for 3 wk have major changes in neuronal activity in the limbic area of the brain, a region involved in pleasure and other emotions. We also investigate the relations between sucrose (and other sweet substances), drugs of abuse, and the mesolimbic dopaminergic system. The presence of sucrose in an animal's cage can affect the animals desire to self-administer drugs of abuse. Also, an animal's level of sucrose preference can predict its desire to self-administer cocaine. Such data suggest a relation between sweet taste and drug reward, although the relevance to humans is unclear. Finally, we address the influence of sugar on body weight control. For example, sucrose feeding for 2 wk decreases the efficiency of energy utilization and increases gene expression of uncoupling protein 3 in muscle, suggesting that sucrose may influence uncoupling protein 3 activity and contribute to changes in metabolic efficiency and thus regulation of body weight.

Differential responding for brain stimulation reward and sucrose in high-alcohol-drinking (HAD) and low-alcohol-drinking (LAD) rats

BACKGROUND

This study examined the associations among selective breeding for alcohol preference, intake of sweet solutions, and responding for brain stimulation reward (BSR), a nonoral reinforcer, in alcohol-preferring high-alcohol-drinking (HAD)-1 and nonpreferring low-alcohol-drinking (LAD)-1 rats.

METHODS

Adult male HAD-1 and LAD-1 rats were trained to lever press for medial forebrain bundle stimulation. Current intensity was varied in separate sessions to generate a rate/intensity function. To further examine BSR responding, the animals responded for stimulation at 100 Hz and at a fixed current intensity on an FR1 schedule. In subsequent sessions, the schedule was increased to FR6 and then to FR12. To examine responding for the sucrose solution, we trained a separate group of HAD-1/LAD-1 rats to bar press for sucrose on an FR1 schedule. Similar to the BSR experiment, in following sessions, the schedule was increased to an FR6 and then to an FR12 schedule.

RESULTS

No significant differences were observed between the two rat lines across a range of current intensities. As the reinforcement schedule increased, HAD-1 rats exhibited a dramatic decrease in BSR responding, whereas the LAD-1 rats displayed a more protracted reduction. In contrast to BSR, marked elevations in responding were observed for sucrose as the schedule increased. However, in HAD-1 rats, response rates were similar on the FR6 and FR12 schedules, whereas LAD-1 rats showed a reduction in response rates from the FR6 to FR12 schedule. Furthermore, HAD-1 rats exhibited significantly more responses compared with LAD-1 rats across the three reinforcement schedules. An analysis of the response profile for the three reinforcement schedules suggested that few if any postreinforcement pauses were exhibited when the reinforcer was BSR compared with sucrose in both lines.

CONCLUSION

Medial forebrain bundle BSR is a powerful reinforcer in both HAD-1 and LAD-1 lines. However, BSR responding was not associated with selective breeding for alcohol preference. In contrast, selective breeding for alcohol preference was associated with sucrose consumption, especially as the amount of work increased. The lack of correspondence between BSR and sweet taste rewards in HAD-1 and LAD-1 lines may suggest important differences yet an overlapping brain reward mechanism in the control of motivated behaviors in these selected lines.

Sex differences in the effects of baclofen on the acquisition of intravenous cocaine self-administration in rats

Baclofen, a GABA(B) agonist, decreases both the maintenance and reinstatement of i.v. cocaine-reinforced responding in rats. In the present experiment the effects of baclofen were extended to a comparison of male and female rats during the acquisition of i.v. cocaine self-administration. Four groups of rats were trained to self-administer i.v. cocaine (0.2 mg/kg) under a fixed-ratio 1 (FR 1) schedule using an autoshaping procedure. The criterion for acquisition was a 5-day period during which a mean of 100 cocaine infusions were administered. Rats were given 30 days to reach this criterion. Male and female groups (n=10-13) were pretreated with i.p. injections of baclofen (2.5 mg/kg) or vehicle 30-min prior to the sessions. A subset of rats (N=5) that did not acquire cocaine self-administration continued to be exposed to the acquisition procedure after baclofen treatment ended. Pretreatment with baclofen decreased both the rate of acquisition of cocaine self-administration and the percentage of rats meeting the acquisition criterion to a greater extent in females than in males. Female rats that did not meet the acquisition criterion with baclofen treatment, acquired within a few days after treatment ended. The findings confirm previous reports of enhanced acquisition of cocaine self-administration in females versus males, and they indicate that baclofen suppressed the acquisition of cocaine self-administration significantly more in females than in males.

Responses to basic taste qualities in rats selectively bred for high versus low saccharin intake

Rats selectively bred for relatively high (HiS) and relatively low (LoS) saccharin intake were offered sweet (sucrose), bitter (quinine, sucrose octaacetate), salty (sodium chloride), starchy (Polycose((R))), and sour (citric acid) solutions at several concentrations; sucrose/quinine, and sucrose/citric acid mixtures were also tested. Compared to HiS rats, LoS rats displayed weaker preferences for and lower consumption of sweet, salty, and starchy solutions. HiS and LoS rats did not differ in responses to simple bitter or sour solutions or to adulteration of sucrose with citric acid. However, quinine adulteration reduced sucrose preference more among LoS rats. Thus, selection on a saccharin intake phenotype has yielded line differences on all hedonically positive tastants and, probably as a consequence of that difference, greater finickiness specifically towards bittersweet solutions in the low saccharin-consuming line. Additional work can clarify the psychobiological mechanisms for the phenotypic difference and, potentially, the reasons for its relationship to measures of emotionality.