Two-bottle Tests Research Articles

The effects of midazolam on the acquisition and expression of fructose- and maltodextrin-based flavour preferences

The effects of the benzodiazepine agonist midazolam on the acquisition and expression of flavour preferences were investigated. Rats (Experiment 1) were given one-bottle training with one flavoured solution (CS+) mixed with either fructose or maltodextrin and another solution (CS−) presented alone. Animals receiving 1 mg/kg midazolam during training consumed more CS− than did animals receiving vehicle injections although there was no drug effect on CS+ consumption. In two-bottle tests the CS+ was preferred to the CS− with the preference being larger in fructose trained animals. Midazolam (0.3–3 mg/kg) increased total intake but not CS+ preference. Training under midazolam reduced the CS+ preference when fructose, but not maltodextrin, was the reinforcer. In Experiment 2 training consumption was restricted to 10 ml/session. This removed the difference in CS+ preference between reinforcer types but otherwise the results were as in Experiment 1. The midazolam induced attenuation of fructose-based preferences might reflect an increase in CS− palatability during training which would reduce the difference between the reinforced and non-reinforced solutions. As maltodextrin supports preferences due to post-ingestive effects manipulation of palatability should be ineffective. Midazolam does not influence the expression of conditioned flavour preferences despite prior evidence that benzodiazepine agonists enhance palatability.

Conditioned flavor avoidance as a measure of withdrawal in rats chronically exposed to a caffeine solution

Rats were given 21 days of chronic oral caffeine. A novel flavor (Maintenance CS) was then paired with the continuation of caffeine, and a second flavor (Withdrawal CS) was paired with caffeine removal. Rats avoided the Withdrawal CS, and drank more of the Maintenance CS in a two-bottle test, suggesting that removing caffeine had induced withdrawal. The value of the Maintenance CS was investigated by comparing it to a novel flavor paired with water (Neutral CS). In a series of two-bottle tests, the Maintenance and Neutral CSs were equivalent when pitted against each other, and both were preferred to the Withdrawal CS. These results demonstrate that conditioned flavor avoidance is a useful procedure in assessing caffeine withdrawal, and by inference dependence, produced by chronic oral consumption.

Sucrose taste but not Polycose taste conditions flavor preferences in rats

Rats have an inborn preference for sweet taste and learn to prefer flavors associated with sweetness. They are also strongly attracted to the taste of glucose polymers (e.g., Polycose). This “poly” taste differs in quality from the sweet taste of sugar. To determine if poly taste, like sweet taste, conditions flavor preferences rats were trained with a distinctive flavor (CS+) added to 2% Polycose solution and a different flavor (CS−) added to plain water. In a subsequent two-bottle test the rats did not prefer the CS+ to CS− when both flavors were presented in water. In contrast, other rats significantly preferred a CS+ flavor that had been paired with 2% sucrose. Adding saccharin to a flavored Polycose solution did not improve CS+ flavor learning; rather, Polycose appeared to overshadow saccharin-induced conditioning. Flavor conditioning by a 16% Polycose solution was assessed using a sham-feeding procedure to prevent post-oral reinforcement. Although the rats sham-fed substantial amounts of the CS+ flavored Polycose solution, they failed to prefer the CS+ to the CS− flavor. This contrasts with the preference other rats displayed for a CS+ paired with sham-fed sucrose. Why attractive sweet and poly tastes differ in their ability to condition flavor preferences is not certain, although some findings suggest that they differentially activate dopamine and/or serotonin circuits involved in flavor learning.

Opioid activation in the lateral parabrachial nucleus induces hypertonic sodium intake

Opioid mechanisms are involved in the control of water and NaCl intake and opioid receptors are present in the lateral parabrachial nucleus (LPBN), a site of important inhibitory mechanisms related to the control of sodium appetite. Therefore, in the present study we investigated the effects of opioid receptor activation in the LPBN on 0.3 M NaCl and water intake in rats. Male Holtzman rats with stainless steel cannulas implanted bilaterally in the LPBN were used. In normohydrated and satiated rats, bilateral injections of the opioid receptor agonist β-endorphin (2 nmol/0.2 μl) into the LPBN induced 0.3 M NaCl (17.8±5.9 vs. saline: 0.9±0.5 ml/240 min) and water intake (11.4±3.0 vs. saline: 1.0±0.4 ml/240 min) in a two-bottle test. Bilateral injections of the opioid antagonist naloxone (100 nmol/0.2 μl) into the LPBN abolished sodium and water intake induced by β-endorphin into the LPBN and also reduced 0.3 M NaCl intake (12.8±1.5 vs. vehicle: 22.4±3.1 ml/180 min) induced by 24 h of sodium depletion (produced by the treatment with the diuretic furosemide s.c.+sodium deficient food for 24 h). Bilateral injections of β-endorphin into the LPBN in satiated rats produced no effect on water or 2% sucrose intake when water alone or simultaneously with 2% sucrose was offered to the animals. The results show that opioid receptor activation in the LPBN induces hypertonic sodium intake in satiated and normohydrated rats, an effect not due to general ingestive behavior facilitation. In addition, sodium depletion induced 0.3 M NaCl intake also partially depends on opioid receptor activation in the LPBN. The results suggest that deactivation of inhibitory mechanisms by opioid receptor activation in the LPBN releases sodium intake if excitatory signals were activated (sodium depletion) or not.

Role of systemic endocannabinoid CB-1 receptor antagonism in the acquisition and expression of fructose-conditioned flavor–flavor preferences in rats

Rats learn to prefer a flavor mixed into a fructose–saccharin solution over a different flavor mixed into a saccharin-only solution which is considered to be a form of flavor–flavor conditioning. Fructose-conditioned flavor preferences are impaired by systemic dopamine D1 and to a lesser degree, D2 receptor antagonism as well as by NMDA, but not opioid, receptor antagonism. Given the emerging role of the endocannabinoid system in mediating hedonically-driven food intake, the present study examined whether systemic administration of the inverse CB-1 receptor agonist, AM-251 would alter fructose-conditioned flavor preferences. In Experiment 1, food-restricted rats were trained over 10 sessions (30 min/day) to drink a fructose–saccharin solution mixed with one flavor (CS+/Fs) and a less-preferred saccharin-only solution mixed with another flavor (CS−/s). Subsequent two-bottle tests with the two flavors in saccharin (CS+/s, CS−/s) occurred 15 min following counterbalanced pairs of AM-251 doses of 0, 0.1, 1 or 3 mg/kg. Preference for CS+/s over CS−/s following vehicle treatment (74%) was significantly reduced by the 0.1 (67%) and 1 (65%) AM-251 doses, whereas CS+/s, but not CS−/s intake was significantly reduced by the 1 and 3 mg/kg AM-251 doses. In Experiment 2, rats received systemic injections of AM-251 (1 mg/kg) or vehicle prior to the 10 CS+/Fs and CS−/s training sessions. In subsequent two-bottle tests (drug-free) the AM-251 and control groups displayed similar preferences for the CS+ flavor (66% vs. 69%). Experiment 3 demonstrated that AM-251 significantly decreased chow intake (24 h), and 1-h intakes of fructose–saccharin and saccharin-only solutions in ad libitum-fed rats. These data indicate that functional CB-1 receptor antagonism significantly reduces the expression, but not the acquisition of fructose-conditioned flavor–flavor preferences. The endogenous endocannabinoid system is therefore implicated in the maintenance of this form of learned flavor preferences.

Activation of dopamine D1‐like receptors in nucleus accumbens is critical for the acquisition, but not the expression, of nutrient‐conditioned flavor preferences in rats

The present study investigated the role of dopamine neurotransmission within the nucleus accumbens (NAc) in flavor preference learning induced by the postoral consequences of carbohydrates. In Experiment 1, rats fitted with a gastric catheter were trained with a flavor (CS+) paired with intragastric (IG) infusions of 8% glucose and a different flavor (CS-) paired with IG water infusions. The CS+ preference was then evaluated in two-bottle preference tests following bilateral injection of the dopamine D1-like receptor antagonist SCH23390 into the NAc shell at total doses of 0, 12, 24 and 48 nmol. SCH23390 produced dose-related reductions in CS+ intake but did not block the CS+ preference except at the highest dose, which also greatly suppressed the CS intakes. In Experiment 2, new rats were injected daily in the NAc shell with either saline or SCH23390 (12 nmol), 10 min prior to training sessions with CS+ with IG glucose and CS- with IG water. In the two-bottle preference tests, the drug-treated rats, unlike the control rats, did not significantly prefer the CS+ (61 vs. 83% preference). In Experiment 3, new rats were trained with the same procedures as Experiment 2, except that brain injections were in the NAc core. In contrast to control rats, SCH-treated rats failed to prefer the CS+ to the CS- in two-bottle tests (55% vs. 89% preference). These results demonstrate that D1-like receptors in the NAc shell and core are greatly involved in the acquisition, but less so in the expression, of a flavor preference conditioned by postingestive effects of glucose.

Role of dopamine D1 and D2 receptors in the nucleus accumbens shell on the acquisition and expression of fructose-conditioned flavor–flavor preferences in rats

Systemic administration of dopamine D1 (SCH23390) and less so D2 (raclopride) receptor antagonists significantly reduce acquisition and expression of fructose-conditioned flavor preferences (CFP). Because dopamine in the nucleus accumbens shell (NAcS) is implicated in food reward, the present study examined whether NAcS D1 or D2 antagonists altered acquisition and/or expression of fructose-CFP. In Experiment 1, food-restricted rats with bilateral NAcS cannulae were trained to drink a fructose (8%) + saccharin (0.2%) solution mixed with one flavor (CS+/Fs) and a less-preferred 0.2% saccharin solution with mixed another flavor (CS−/s). Unlimited two-bottle tests with the two flavors in saccharin (0.2%: CS+/s, CS−/s) occurred 10 min following total bilateral NAcS doses of 0, 12, 24 or 48 nmol of SCH23390 or raclopride. Preference for CS+/s over CS−/s following vehicle treatment (76%) was significantly reduced by SCH23390 (48 nmol, 62%) and raclopride (24 nmol, 63%). In Experiment 2, rats received bilateral NAcS injections (12 nmol) of SCH23390 or raclopride on one-bottle training (16 ml) days. Yoked control rats received vehicle and were limited to the CS intakes of the D1 and D2 groups, whereas untreated controls without injections received their CS ration during training. Subsequent unlimited two-bottle tests revealed initial preferences of CS+/s over CS−/s in all groups that remained stable in untreated and yoked controls, but were lost over the six tests sessions in D1 and D2 groups. These data indicate that NAcS D1 and D2 antagonists significantly attenuated the expression of the fructose-CFP and did not block acquisition, but hastened extinction of fructose-CFP.

Investigating the motivational mechanism of altered saline consumption following 5-HT1a manipulation.

The precise role played by serotonin (5-HT) in taste--an issue of great interest given the involvement of serotonin in human sensory and eating disorders--is a matter of considerable debate, perhaps because of the variety of methodologies that have been brought to bear by different researchers. Here, we use multiple methods to reveal the motivational mechanism whereby 5-HT(1A) receptor activation modulates drinking behavior. Subcutaneous injections of the selective 5-HT(1A) agonist 8-hydroxy-2-di-n-propylamino-tetralin (8-OH-DPAT), a drug that reduces 5-HT release by acting on presynaptic autoreceptors, dose-dependently increased consumption of 0.45 M NaCl in a one-bottle test. In a two-bottle test, however, 8-OH-DPAT-treated animals (30 microg/kg/ml) demonstrated decreased NaCl preference--although our detection of this effect was obscured by adaptation to the drug across days. Rats' performance in a brief access test confirmed that 8-OH-DPAT decreased preference for saline by both increasing water consumption and decreasing NaCl consumption. Finally, taste reactivity tests demonstrated that the latter result does not reflect decreased NaCl palatability. Overall, the results suggest that 8-OH-DPAT-induced 5-HT hypofunction increases thirst without substantially affecting the palatability of NaCl.

CD36 gene deletion reduces fat preference and intake but not post-oral fat conditioning in mice

Several findings suggest the existence of a "fatty" taste, and the CD36 fatty acid translocase is a candidate taste receptor. The present study compared fat preference and acceptance in CD36 knockout (KO) and wild-type (WT) mice using nutritive (triglyceride and fatty acid) and nonnutritive (Sefa Soyate oil) emulsions. In two-bottle tests (24 h/day) naive KO mice, unlike WT mice, displayed little or no preference for dilute soybean oil, linoleic acid, or Sefa Soyate emulsions. At high concentrations (2.5-20%), KO mice developed significant soybean oil preferences, although they consumed less oil than WT mice. The postoral actions of fat likely conditioned these preferences. KO mice, like WT mice, learned to prefer a flavored solution paired with intragastric soybean oil infusions. These findings support CD36 mediation of a gustatory component to fat preference but demonstrate that it is not essential for fat-conditioned flavor preferences. The finding that oil-naive KO mice failed to prefer a nonnutritive oil, assumed to provide texture rather than taste cues, requires explanation. Finally, CD36 deletion decreased fat consumption and enhanced the ability of the mice to compensate for the calories provided by their optional fat intake.

Fat and carbohydrate preferences in mice: the contribution of alpha-gustducin and Trpm5 taste-signaling proteins.

Trpm5 and alpha-gustducin are key to the transduction of tastes of sugars, amino acids, and bitter compounds. This study investigated the role of these signaling proteins in the preference for fat, starch, and starch-derived polysaccharides (Polycose), using Trpm5 knockout (Trpm5 KO) and alpha-gustducin knockout (Gust KO) mice. In initial two-bottle tests (24 h/day), Trpm5 KO mice showed no preference for soybean oil emulsions (0.313-2.5%), Polycose solutions (0.5-4%), or starch suspensions (0.5-4%). Gust KO mice displayed an attenuated preference for Polycose, but their preferences for soybean oil and starch were comparable to those of C57BL/6J wild-type (WT) mice. Gust KO mice preferred starch to Polycose, whereas WT mice had the opposite preference. After extensive experience with soybean oil emulsions (Intralipid) and Polycose solutions, the Trpm5 KO mice developed preferences comparable to the WT mice, although their absolute intakes remained suppressed. Similarly, Gust KO mice developed a strong Polycose preference with experience, but they continued to consume less than the WT mice. These results implicate alpha-gustducin and Trpm5 as mediators of polysaccharide taste and Trpm5 in fat taste. The disruption in Polycose, but not starch, preference in Gust KO mice indicates that distinct sensory signaling pathways mediate the response to these carbohydrates. The experience-induced rescue of fat and Polycose preferences in the KO mice likely reflects the action of a postoral-conditioning mechanism, which functions in the absence of alpha-gustducin and Trpm5.

Oxytocin knockout (OT KO) mice overconsume palatable carbohydrate solutions, but not palatable lipid solutions.

OT KO mice display enhanced intake of nutritive and non-nutritive sweet solutions (i.e., sucrose and saccharin) compared to wild-type (WT) mice of the same C57BL/6 background strain. The present study further investigated the differential behavioral response of OT KO and WT mice to sucrose solutions, and also examined intake preferences of OT KO and WT mice for palatable but non-sweet isocaloric solutions of carbohydrate and fat. A progressive ratio operant licking procedure demonstrated that OT KO and WT mice display similar motivational drive to consume 10% sucrose. A series of two-bottle intake tests revealed that OT KO mice consume significantly larger amounts of sweet and non-sweet carbohydrate solutions (i.e., sucrose, polycose, and cornstarch) compared to WT cohorts. Intake pattern analyses revealed that OT KO mice overconsume carbohydrate solutions by initiating more drinking bouts compared to WT mice; bout sizes did not differ between the genotypes. In contrast, OT KO and WT mice did not differ in their daily or hourly evening intake of Intralipid, a palatable soybean oil emulsion, except on the first day of exposure. These findings indicate that the absence of OT in mice does not affect their appetitive drive to consume palatable sucrose solutions. Instead, the absence of OT may increase daily intake of palatable sweet and non-sweet solutions of carbohydrate (but not fat) by selectively blunting or masking processes that contribute to post-ingestive satiety. Research supported by National Institutes of Health Grants HD 44898 (JAA) and DK 31135 (AS).

Dietary fat content and flavor reinforcement by intragastric sucrose and corn oil in rats.

The influence of dietary fat level on the relative reinforcing potency of intragastric (IG) corn oil and sucrose was tested with flavor preference conditioning. Rats were given ad libitum access to a low-fat chow (LF; 12% fat kcal) or a high-fat chow (HF; 48% fat kcal) maintenance diet. They were given four 22-h/day one-bottle training sessions each with CS+F, CS+C, and CS− flavors paired with IG 7.1% oil emulsion, 16% sucrose solution and water, respectively. During training the LF and HF rats consumed equal amounts of CS+F and CS− and less of the CS+C. Overall, the training nutrient intakes of the LF group averaged 18% protein/55% carbohydrate/27% fat, whereas the HF group averaged 12/37/51%. In two-bottle choice tests the groups showed equally strong preference for the CS+F over the CS− (82–86%). They also preferred the CS+C over the CS−, but the LF rats had a stronger preference (93%) than did the HF rats (77%). The CS+C was preferred to the CS+F; the LF group's 72% preference did not differ significantly from HF group's 60% CS+C preference. The results extend prior work showing IG fat is less reinforcing than IG carbohydrate. Relative to the LF diet, the primary effect of HF diet was attenuation of carbohydrate-based flavor preference, perhaps due to greater satiation effects by carbohydrates when they comprise less of the overall diet. Supported by DK031135.

Food, water and NaCl consumption by 14 strains of rats.

We measured the voluntary intake of food, water, and NaCl by 8 male and 8 female rats from each of 3 outbred strains (Crl:CD(SD) IGS, LE, and Wistar) and 11 inbred strains (BN/SsNHsdMcwiCrl, BUF/CrCrl, COP/CrCrl, DA/OlaHsd, DahlSS/JrHsdMcwi, F344/NTac, FHH/EurMcwi, LEW/NHsd, Noble/CrCrl, PVG/OlaHsd, and SHR/NCrl). At 8-week old when tests began, there was a 182-g range in body weight (PVG-female=121±2 g, LE-male=303±4 g), a 17-g range in food intake (BN-female=11±1 g/d, SHR-male=28±2 g/d of AIN-76A diet), and a 34-mL range in water intake (BN-female=15±1 mL/d, FHH-male=49±4 mL/d of deionized water). In a series of 48-h two-bottle tests with a choice between water and progressively increasing concentrations of NaCl, all rat strains strongly preferred 3.16, 10, 31.6, and 100 mM NaCl relative to water. All except BUF, Noble and PVG preferred 178 mM NaCl relative to water. All except LEW, SHR, and DA avoided 316 mM NaCl, and all avoided 562 mM NaCl. There were few differences in NaCl preference between the sexes at any concentration. These results illustrate the substantial diversity of voluntary ingestive behavior in rats and set the stage for future genetic analyses.

Forty mouse strain survey of voluntary calcium intake, blood calcium, and bone mineral content

We measured voluntary calcium intake, blood calcium, and bone mineral content of male and female mice from 40 inbred strains. Calcium intakes were assessed using 48-h two-bottle tests with a choice between water and one of the following: water, 7.5, 25, and 75 mM CaCl 2, then 7.5, 25, and 75 mM calcium lactate (CaLa). Intakes were affected by strain, sex, anion, and concentration. In 11 strains females consumed more calcium than did males and in the remaining 29 strains there were no sex differences. Nine strains drank more CaLa than CaCl 2 whereas only one strain (JF1/Ms) drank more CaCl 2 than CaLa. Some strains had consistently high calcium intakes and preferred all calcium solutions relative to water (e.g., PWK/PhJ, BTBR T + tf/J, JF1/Ms). Others had consistently low calcium intakes and avoided all calcium solutions relative to water (e.g., KK/H1J, C57BL/10J, CE/J, C58/J). After behavioral tests, blood was sampled and assayed for pH, ionized calcium concentration, and plasma total calcium concentration. Bone mineral density and content were assessed by DEXA. There were no significant correlations between any of these physiological measures and calcium intake. However, strains of mice that had the highest calcium intakes generally fell at the extremes of the physiological distributions. We conclude that the avidity for calcium is determined by different genetic architecture and thus different physiological mechanisms in different strains.

The palatability of corn oil and linoleic acid to mice as measured by short-term two-bottle choice and licking tests

Free fatty acids (FFAs) were reported to be recognized in the oral cavity and possibly involved in fatty foods recognition. To understand the importance of oil recognition in the oral cavity, we investigated the effect of various concentrations of a fatty acid or corn oil on fluid intake as well as mice's preferences in a two-bottle choice test and a licking test. Linoleic acid (LA), which is a main component of corn oil, was used as a representative FFA. In the two-bottle choice test between a pair of different concentrations of corn oil, the mice consistently adopted the higher concentration of corn oil. In the licking test for corn oil, the licking rates for the serial concentration of corn oils (0, 1, 5, 10 and 100%) were increased in a concentration-dependent manner. On the other hand, in the two-bottle test for a pair of different concentrations of LA (0, 0.125, 0.25 and 1%), 0.25% and 1% LA were preferred to mineral oil, but 0.25% and 1% LA were preferred equally in mice. In the licking test for LA, the mice showed the largest number of initial lickings for the 1% LA, while the licking rates for the high concentration of LA decreased. These results suggest that mice could discriminate the concentration of corn oil and LA in the oral cavity. We also suggest that pure corn oil is a highly preferable solution, while an optimal concentration of LA according to the preferences of mice is a low-range concentration (0.25–1%).

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.

Individual differences in sucrose intake and preference in the rat: Circadian variation and association with dopamine D 2 receptor function in striatum and nucleus accumbens

Measurement of sucrose intake or preference is currently in widespread use in preclinical psychopharmacology, and used for predicting sensitivity to rewards, but limited information is available about the consistency of individual sucrose intake or preference. In the present study, individual differences in sucrose intake and preference in free-feeding rats were studied during the dark and light phases, and associations of these measures with the function of D 2 receptors in the striatum and nucleus accumbens were characterized. Altogether eight two-bottle tests were carried out intermittently during light and dark phase. Ten days after the last test, animals were sacrificed. Intake, and to a lesser degree preference of sucrose during the dark phase were higher as compared to the light phase, and sucrose intake, but not preference was individually very consistent across different tests, especially during the dark phase. The average dark phase sucrose intake and preference correlated positively with dopamine-dependent [ 35S]GTPγS binding in nucleus accumbens. Dopamine-dependent [ 35S]GTPγS binding in striatum correlated negatively with sucrose preference in the first test. This study has demonstrated that sucrose intake is an individually stable trait, especially when measured during the dark phase, and persistent individual differences in sucrose consumption and possibly reward sensitivity in general are related to dopamine D 2 receptor function in the nucleus accumbens. Individual differences in D 2 receptor function in the striatum may influence behaviour of rats in novel situations.

Nutrient-conditioned flavor preference and incentive value measured by progressive ratio licking in rats

Rats develop strong preferences for flavors associated with the postingestive actions of glucose and other nutrients. This preference may involve changes in both the hedonic appeal and incentive salience of the nutrient-paired flavor. The present study used a progressive ratio (PR) operant licking task to evaluate the degree to which nutrient conditioning changes the incentive value of flavors. Food restricted rats were trained to associate one flavored saccharin solution (CS+) with intragastric glucose infusions and another flavored solution (CS−) with water infusions. The rats subsequently showed a strong preference for the CS+ in two-bottle tests and also licked more for the CS+ than CS− in PR tests. PR licking for the CS+ was similar to that for an 8% fructose solution. Together with earlier data indicating that the CS− is isopreferred to a 3% fructose solution, these findings indicate that IG glucose conditioning enhances the hedonic and incentive value of the CS+ solution so that the animal responds as if the solution had a sweeter taste.

Strain differences in the acquisition of nicotine-induced conditioned taste aversion

Lewis (LEW) and Fischer (F344) rat strains differ on a variety of physiological and behavioral endpoints, including reactivity to drugs of abuse. Although they differ in drug reactivity, such assessments are generally limited to morphine and cocaine. To determine if these differences generalize to other drugs, the present study examined these strains for their reactivity to the affective properties of nicotine, specifically their sensitivity to nicotine in the conditioned taste aversion preparation. For four or five conditioning cycles given every other day, rats from both strains were allowed access to saccharin and injected with nicotine (0.1, 0.4, 0.8 mg/kg) or vehicle. On intervening days, all rats were given access to water and injected with vehicle. Under this one-bottle training and testing procedure, neither strain displayed aversions at the lowest dose of nicotine (0.1 mg/kg). Aversions were evident for both strains at 0.4 and 0.8 mg/kg, although the F344 rats acquired the aversions at 0.4 mg/kg faster and displayed a significantly greater aversion at 0.8 mg/kg than subjects from the LEW strain. For both strains, aversions were evident at all doses (and in a dose-dependent manner) when subjects were given access to saccharin and water in a two-bottle test. There were, however, no strain differences on this test. Differences between the two strains in their acquisition of nicotine-induced taste aversions were discussed in the context of aversion assessments with other compounds as well as in relation to differences in the self-administration of nicotine in the two strains.

Flavor preference conditioning as a function of fat source

Rats learn to prefer foods based, in part, on postingestive nutrient actions. This study compared the effectiveness of intragastric (IG) infusions of fat emulsions which varied in their fatty acid composition (chain length and saturation) to condition preferences for flavored saccharin solutions. In Experiment 1, food-restricted rats were trained (30 min/day) with one flavor (CS+CO) paired with IG corn oil (CO) and a second flavor (CS+MCT) paired with IG medium chain triglyceride (MCT); the fats were prepared as isocaloric emulsions. A third flavor (CS−) was paired with IG water. The rats subsequently showed a strong preference for the CS+CO (84%) and a weaker preference for the CS+MCT (65%) relative to the CS−. In a direct choice test, the CS+CO was preferred to the CS+MCT by 75%. In Experiment 2, new rats trained with flavors paired with IG corn oil and beef tallow (BT) infusions learned to prefer both the CS+CO (89%) and the CS+BT (82%) relative to the CS−, and preferred the CS+CO to the CS+BT by 67%. The same rats were trained with three new flavors paired with IG infusions of corn oil, vegetable shortening (VS), and water. The rats strongly preferred both the CS+CO (91%) and CS+VS (86%) over the CS−, and they preferred the CS+CO to the CS+VS by 64%. In Experiment 3, new rats trained with corn oil and safflower oil (SO) paired flavors preferred both the CS+CO and CS+SO to the CS−, and equally preferred the CS+CO and CS+SO in two-bottle tests. The rats were also given one-and two-bottle tests with the various fat emulsions and their preference profile was consistent with their conditioned preferences for the flavored saccharin solutions. These findings demonstrate that many different fat sources can condition flavor preferences. Fats with high polyunsaturated content and/or lower saturated fat content are the most reinforcing.