Sign Tracking in an Enriched Environment: A Potential Ecologically Relevant Animal Model of Adaptive Behavior Change.

When an object conditioned stimulus (CS) is paired with a food unconditioned stimulus (US), anticipatory goal-directed action directed at the US location (goal tracking) is accompanied by behavior directed at the object CS (sign tracking). Sign-tracking behavior appears to be compulsive and habit-like and predicts increased vulnerability to the addictive potential of drugs in animal models. A large body of the literature also suggests that environmental enrichment protects against the development of addiction-prone phenotypes. Thus, we investigated whether compulsive-like sign tracking develops in environmentally enriched rats trained directly in their enriched home environment. We demonstrate that adolescent enriched-housed male Sprague-Dawley rats readily sign track a 5% ethanol bottle CS in their home environment and at a rate higher than adolescent standard-housed rats. We also show that enriched adolescent rats sign track less than enriched adult-trained rats and that acute isolation stress affects sign- and goal-tracking performance of adolescents and adults differently. Sign tracking increased more in the adult than the adolescent rats. Whereas the younger rats showed a decrease in goal tacking after the final stressor manipulation, the adults showed increased goal tracking. Our results are consistent with recent studies, which suggest that although sign tracking performance is compulsive-like, it is not as inflexible and habit-like as previously assumed. Testing in an enriched home environment with object CSs having greater affordance than "neutral" lever CSs may provide greater ecological relevance for investigating the development and expression of adaptive and compulsive-like behaviors in translational research.

An animal rights attitude survey of undergraduate psychology students.

An animal rights attitude survey of 46 statements on various issues related to animal rights was given to 112 freshmen who were near the end of their first college course in introductory psychology and to 63 junior and senior psychology majors. A factor analysis yielded a multidimensional structure with attitudes toward animal research, nonresearch, environment, and evolution as factors. Beginning psychology students had a more negative attitude toward animal research than did psychology majors; however, psychology majors displayed a more positive attitude toward the environment and toward animal rights issues not involving animal research.

Differential sensitivity of operant behaviors to changes in the concentration of a sucrose reinforcer: effects of pimozide.

The sensitivity of operant response rates to changes in a sucrose reinforcer was examined in well-trained animals maintained on a variable ratio (VR) or variable interval (VI) schedule (experiment 1). Although VR performance showed greater resistance to small reductions in the concentration of the sucrose reinforcer than VI performance, VR performance was more sensitive to large reductions in the sucrose concentration. Despite this differential sensitivity only the smallest dose of pimozide (0.125 mg/kg) differentially affected these behaviors by reducing VI rates without affecting VR rates. These and other results support the view that low doses of pimozide reduce the hedonic impact of the reinforcer. The results also indicate that the attenuation of operant responding by higher doses (0.25, 0.5, and 1.0 mg/kg) cannot be solely a result of the blunting of reward. Experiment 2 demonstrated that when rats drink in daily, brief one-bottle tests they show greater resistance to reductions in the sucrose concentration than when they lever-press for sucrose, and require a higher dose of pimozide (2.0 mg/kg) to attenuate consumption. Together the results of both experiments suggest that the greater the resistance to reductions in the reinforcement value, the greater the dose of pimozide necessary to attenuate performance. We discuss the importance of attaining a more complete understanding of the factors in control of operant performance in order to better assess the effects of neuroleptics on reward.

Midazolam withdrawal and discriminative motor control: effects of FG 7142 and Ro 15-1788.

Rats chronically drank either water or midazolam solution (0.1 mg/ml) in daily, 3-h schedule-induced polydipsia sessions and were evaluated in daily motor control sessions after polydipsia when midazolam metabolite levels had fallen to zero (withdrawal). Under midazolam polydipsia, animals orally self-administered between 21 and 38 mg/kg daily. The effect of acute drug administration [midazolam (0.75-3 mg/kg, SC), FG 7142 (1-8 mg/kg, IP), Ro 15-1788 (10-20 mg/kg, IP)] on motor control performance was similar after either chronic water or midazolam polydipsia. Thus chronic, oral midazolam self-administration did not lead to tolerance to the motor impairment produced by SC midazolam, nor did the daily discontinuation lead to impaired motor performance, nor had these performances, which occurred after daily elevated midazolam metabolite levels had reached zero (withdrawal), become sensitized to the effects of either the benzodiazepine inverse agonist FG 7142 or the agonist Ro 15-1788.

Fat appetite in rats: the response of infant and adult rats to nutritive and non-nutritive oil emulsions.

Fat appetite was studied in rats using corn oil and mineral oil emulsions. In Experiment 1 ingestive responses to intraoral infusions were measured in rat pups 6-15 days of age. By 12-15 days of age pups responded more to oil emulsions (10% and 30%) than they did to water or emulsifier solution. The corn and mineral oil emulsions were almost as effective as milk but less effective than sucrose (0.3M) in stimulating ingestion. Experiments 2 and 3 examined the acceptance and preference for oil emulsions in adult rats. The corn oil and mineral oil emulsions were equally acceptable to non-deprived rats, as measured by 3-min and 30-min one-bottle tests. Food deprivation increased the one-bottle intake of both emulsions. In two-bottle tests, rats displayed a slight corn oil preference when non-deprived, but developed a strong preference when food deprived. Taken together, the results suggest that rats have an unlearned attraction to the orosensory qualities of emulsified oils and they learn to prefer corn oil based on its postingestive nutritive effects.

Schedule-induced cocaine drinking: choice between cocaine and vehicle.

Rats were exposed to daily 3-hr schedule-induced polydipsia sessions (fixed-time 1-min food-pellet delivery) with two drinking fluids available: cocaine solution and water. Fluid position was alternated daily. Polydipsia occurred mostly from a preferred-side spout (position preference) until cocaine solution concentration was increased to between 0.52 and 1.04 mg/ml and animals drank mostly water. Within a lower concentration range (0.28-0.6 mg/ml) maximum session cocaine intakes ranged from 54.3 to 120.1 mg/kg. Postsession serum cocaine levels were about 200 ng/ml. At individually chosen cocaine solution concentrations, the addition of saccharin to the solution did not increase cocaine intake, but a compound solution (saccharin plus glucose) did. With progressive dilution of the compound vehicle, an almost complete preference for cocaine solution was maintained. But with a return to water as the vehicle, animals reverted to a position preference after a few sessions, although one maintained a clear cocaine preference. Schedule-induced polydipsia produced chronic, oral self-administration of cocaine resulting in pharmacologically significant intakes and serum levels.

Ontogeny of polycose and sucrose appetite in neonatal rats.

Rat pups 3 to 15 days of age were infused with sucrose or Polycose solutions (.03 or .3 M) through oral cannulas and their solution intake, mouthing behavior, and general locomotor activity were recorded. Overall, the pups displayed similar ingestive responses to Polycose and sucrose. They discriminated between .3 M sucrose and water as early as 6-days of age, and between .3 M Polycose and water as early as 9-days of age. However, at the .03 M concentration the pups responded to Polycose before they responded to sucrose. The results confirm previous reports that rat pups have an unlearned preference for sucrose and demonstrate that a similar preference exists for starch-derived polysaccharides. The Polycose appetite displayed by the neonatal rats is consistent with results obtained with adult animals which suggest that rats have taste receptors for starch-derived polysaccharides as well as for sugars.

Starch-induced overeating and overweight in rats: influence of starch type and form.

In Experiment 1, adult female rats were fed, in addition to chow and water, a carbohydrate source that consisted of pure amylopectin corn starch or hydrolyzed corn starch (Polycose) in either a dry powder form or a hydrated gel form. Over the 30-day test periods, carbohydrate intake, total food intake, and body weight gain were greater with the Polycose than with the amylopectin, and greater with the gel form than with the powder form of the carbohydrates. The amylopectin gel produced overeating and overweight relative to a chow-fed control group, although the effects were less than that obtained with the Polycose gel. In a second experiment, test meals of the carbohydrate gels produced larger postmeal increases in plasma glucose than did the carbohydrate powders. There was no effect of carbohydrate type (amylopectin vs. Polycose) on the plasma glucose response. In Experiment 3, the addition of amylopectin to a Polycose gel reduced carbohydrate and total caloric intake. Both orosensory and postingestive factors may contribute to the differential food intake and body weight gains produced by the different types (Polycose vs. amylopectin) and forms (gel vs. powder) of carbohydrates.

Effect of naloxone on conditioned suppression in rats.

Painful stimuli are known to engage an endorphin analgesic system that can be reversed by the opiate antagonist, naloxone. Naloxone, then, should increase the effectiveness of aversive unconditioned stimuli (USs) in Pavlovian fear conditioning. Consistent with this hypothesis, naloxone administered during the acquisition of conditioned suppression in rats enhanced posttrial suppression and preconditioned stimulus (pre-CS; context-controlled) suppression. Furthermore, it enhanced CS-elicited suppression during extinction when administered during acquisition but not when administered only during extinction. Thus naloxone does not enhance an already existing fear nor enhance the memory of previous conditioning; instead, it enhances the conditioning of fear presumably by making the aversive US more painful.

Effects of SOA and saccharin adulteration on Polycose preference in rats.

Female rats were tested for their preference for a 32% Polycose solution, and for 32% Polycose solutions made bitter or sweet by adulteration with either 0.05% sucrose octa acetate (SOA) or 0.2% saccharin during 24 hr/day two-bottle tests. The rats preferred the saccharin-Polycose to the Polycose, the Polycose to the SOA-Polycose, and strongly preferred the saccharin-Polycose to the SOA-Polycose solution. Additional rats that were given only a Polycose or an SOA-polycose solution, in addition to chow and water, consumed similar amounts of solution, and consumed as much as did the rats given the saccharin-Polycose solution. Thus, while SOA adulteration reduces, and saccharin adulteration increases, the relative palatability of Polycose, they do not alter the animal's intake when only one solution is available. In a second experiment rats were given the choice between a 32% Polycose solution and pure Polycose powder, between an SOA-Polycose solution and Polycose powder, or between an SOA-Polycose solution and sucrose powder during both 30-min/day and 24-hr/day preference tests. During the initial short-term test the rats preferred the Polycose solution and, to a lesser degree, the SOA-Polycose solution to the Polycose powder, but they strongly preferred the sucrose powder to the SOA-Polycose solution. During the subsequent long-term test, however, the rats developed a preference for the SOA-Polycose solution over the sucrose powder. The mechanism responsible for this preference switch is not certain, but it may be related to the postingestive effects of the solution, and may also be responsible for the rat's hyperphagic response to carbohydrates in solution form but not in powder form.

Influence of saccharide length on polysaccharide appetite in the rat.

The appetite of female rats for saccharides of different chain lengths was assessed in brief (5-min) one-bottle acceptance and two-bottle preference tests. The saccharides (and their lengths in glucose units) included glucose (1), maltose (2), maltotriose (3), maltooligosaccharide (4-8), maltopolysaccharide (average length 43), and Polycose (1 to 30+); 0.0125 and 0.025 molar solutions of these saccharides were tested. The results revealed that the rats' order of preference for these saccharides was as follows: maltooligosaccharide greater than maltotriose = maltose greater than glucose; and maltooligosaccharide greater than Polycose = maltopolysaccharide greater than maltose. Thus, as saccharide chain length increased from 1 to 4-8 glucose units palatability increased, but with further increases in chain length palatability declined somewhat. Previous findings have suggested that rats have a "polysaccharide" taste receptor and the present results indicate that the receptor is maximally (or near-maximally) stimulated by saccharides of 4-8 glucose units in length.

Effects of gustatory deafferentation on Polycose and sucrose appetite in the rat.

Recent studies have revealed that rats are strongly attracted to the taste of starch-derived polysaccharides, and suggest that the taste receptors involved differ from those that respond to sucrose. The present study examined the possibility that different gustatory nerves mediate the rat's taste and appetite for polysaccharides and sucrose. This was accomplished by measuring the effects of selective gustatory nerve transection on the intake of Polycose and sucrose solutions in nondeprived female rats. Bilateral transection of the chorda tympani nerve produced comparable reductions in Polycose and sucrose intake, but bilateral transection of the glossopharyngeal nerve selectively reduced the intake of Polycose. Bilateral transection of the greater superficial petrosal nerve, and to a lesser degree, the pharyngeal branch of the vagus nerve, increased sucrose intake without affecting Polycose intake. These results indicate that while no single gustatory nerve mediates sucrose or polysaccharide taste, there is some specialization of function within the peripheral gustatory system. Combined bilateral transections of all four gustatory nerves produced the greatest reduction in solution intake, and reduced Polycose and sucrose consumption to the same degree. The suppressive effect was only partial, however, which indicates that relatively few intact taste receptors are required to maintain the rat's appetite for sugar and polysaccharide solutions.

Effects of anosmia on Polycose appetite in the rat.

Female rats made temporarily anosmic by intranasal treatment with zinc sulphate showed little or no reduction in their intake of a polysaccharide (Polycose) solution. This finding, along with the previous finding that experimentally-induced hypoguesia suppresses Polycose intake, indicates that olfaction is much less important than gustation in the mediation of polysaccharide appetite.

Starch preference in rats.

Several experiments examined the preference of adult female rats for starch and starch-derived polysaccnarides using short- and long-term two-choice tests. In Experiment 1, food-deprived rats displayed an immediate preference for powdered corn starch over powdered cellulose in 30-min/day tests. This supports the hypothesis that rats have starch (polysaccharide) taste receptors. The rats also preferred the starch powder to Polycose powder. This preference was unexpected since Polycose, being partially hydrolyzed corn starch, should be a more effective taste stimulus than corn starch. Experiment 2 revealed that nondeprived rats also preferred starch to Polycose in 30-min/day tests, but reversed their preference in 24-hr/day tests; the preference reversal was attributed to postingestive factors. Additional experiments demonstrated that rats preferred branched-chain starch (amylopectin) to unbranched starch (amylose) in both short- and long-term tests. Rats also preferred amylopectin to Polycose in short-term tests when the saccharides were in powder form, but preferred Polycose to amylopectin when the saccharides were in liquid or gel form. The preference for amylopectin powder was not due to the noncarbohydrate constituents (fatty acids, proteins, others) in the starch since rats also preferred chemically purified amylopectin to Polycose powder. Additional tests suggested that texture differences do not explain the rats' preference for starch powder over Polycose powder. Taken together, the results demonstrate that rats have a robust preference for starch but the orosensory determinants of this preference are not completely understood.

Sex differences in polysaccharide and sugar preferences in rats.

The preferences of adult male and female rats for sucrose and polysaccharide (Polycose) solutions were measured in three experiments. In Experiment 1, male and female rats did not differ in their preference for molar solutions of sucrose or Polycose as measured by 24-hr solution vs. water tests. In Experiment 2, however, male rats displayed a greater preference for Polycose, and females a greater preference for sucrose as measured in 3-min and 30-min/day sucrose vs. Polycose tests. Both sexes preferred Polycose to sucrose at low concentrations, but switched their preference at higher concentrations. Experiment 3 revealed that both male and female rats strongly preferred 32% sucrose to 32% Polycose in 30-min/day two-solution tests, but this preference did not persist when the rats were given 24-hr/day access to the two solutions. Male rats lost their sucrose preference, while female rats developed a preference for Polycose over sucrose. The sexes also differed in that the female rats increased their total caloric intake more and consumed proportionately more carbohydrate than did the male rats during the 24-hr/day test. The findings demonstrate that while both male and female rats have an avid appetite for polysaccharide and sucrose solutions, they differ in their short-term and long-term relative preferences for the two carbohydrates, as well as in their caloric and diet selection responses to the carbohydrate solutions. The possible functional significance of these sex differences are discussed.

Conditioned suppression tests of the context-blocking hypothesis: testing in the absence of the preconditioned context.

Two experiments are reported that use rats in a conditioned suppression situation. The experiments, designed to remove confounds that have complicated interpretations of prior research, tested the context-blocking hypothesis, the proposition that static apparatus cues or conditioning contexts can block conditioning to discrete conditioned stimuli (CSs). Experiment 1, like previous work, tested for conditioning to the target CS in the same context that had been preconditioned and in which target conditioning had occurred; the experiment demonstrated a context-blocking like effect. Experiment 2 tested for conditioning not only in the preconditioned context but also in a nonpreconditioned context. Evidence for context blocking appeared similar in the two test situations. This suggests that conditioned contexts block the acquisition of associative strength by discrete CSs at the time of target conditioning (e.g., Rescorla & Wagner, 1972) and not through performance factors at the time of testing (e.g., Gibbon & Balsam, 1981).

Signaled tailshock is perceived as similar to a stronger unsignaled tailshock: implications for a functional analysis of classical conditioning.

Water-deprived rats given fixed-electrode, variable-intensity tailshock at random times rated each trial by pressing either a "high-aversiveness" or "low-aversiveness" lever in order to obtain water. Trials on which a warning signal preceded tailshock resulted in more "high-aversiveness" leverpressing than did otherwise equivalent unsignaled trials. The magnitude of this effect increased and decreased as a function of several parameters including signal-shock interval, signal duration, and range and absolute value of shock intensities but was never reversed despite efforts to achieve such a reversal. Variation in the size of the effect as a function of signal parameters as well as lick suppression scores indicated that the signal had acquired aversive characteristics, which suggests that the effect of the signal on lever choice was due largely to the aversiveness of the signal summating with the aversiveness of the tailshock. Several hypotheses concerning factors that might have either masked or prevented classically conditioned preparatory responses elicited by the signal from reducing tailshock aversiveness were tested and rejected. Despite the greater aversiveness of the signaled condition, when given the choice of receiving or not receiving the signal, the animals displayed a preference for signaled tailshock. Implications for the role of preparatory responding both in the preference-for-signaled-shock phenomenon and in classical conditioning are discussed.