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

Cannabinoids, Chemical Senses, and Regulation of Feeding Behavior

The herb Cannabis sativa has been traditionally used in many cultures and all over the world for thousands of years as medicine and recreation. However, because it was brought to the Western world in the late 19th century, its use has been a source of controversy with respect to its physiological effects as well as the generation of specific behaviors. In this regard, the CB1 receptor represents the most relevant target molecule of cannabinoid components on nervous system and whole-body energy homeostasis. Thus, the promotion of CB1 signaling can increase appetite and stimulate feeding, whereas blockade of CB1 suppresses hunger and induces hypophagia. Taste and flavor are sensory experiences involving the oral perception of food-derived chemicals and drive a primal sense of acceptable or unacceptable for what is sampled. Therefore, research within the last decades focused on deciphering the effect of cannabinoids on the chemical senses involved in food perception and consequently in the pattern of feeding. In this review, we summarize the data on the effect of cannabinoids on chemical senses and their influences on food intake control and feeding behavior.

Conditioned flavor preferences in animals: Merging pharmacology, brain sites and genetic variance

The elucidation of the behavioral, neurochemical, neuroanatomical and genetic substrates mediating the development of conditioned flavor preferences (CFP) is one of the multi-faceted scientific contributions that Dr. Anthony Sclafani has made to the study of food intake. This review summarizes the results of thirty-five publications over nearly twenty years of collaborations between the Sclafani and Bodnar laboratories. This includes the different approaches employed to study the orosensory (flavor-flavor) and post-ingestive (flavor-nutrient) processes underlying CFP including its acquisition (learning) and expression. It describes how CFP is elicited by different sugars (sucrose, glucose, fructose) and fats (corn oil) in rats, and how strain-specific CFP effects can be observed through the use of inbred mouse strains to evaluate genetic variance. The roles of pharmacological substrates (dopamine, glutamate, opioids, acetylcholine, GABA, cannabinoids) mediating sugar- and fat-CFP acquisition and expression are elucidated. Finally, neuroanatomical sites of action (nucleus accumbens, amygdala, medial prefrontal and orbital frontal cortices, lateral hypothalamus) are evaluated at which dopamine signaling mediates acquisition and expression of different forms of CFP.

Neuropharmacology of learned flavor preferences

Innate and learned flavor preferences influence food and fluid choices in animals. Two primary forms of learned preferences involve flavor-flavor and flavor-nutrient associations in which a particular flavor element (e.g., odor) is paired with an innately preferred flavor element (e.g., sweet taste) or with a positive post-oral nutrient consequence. This review summarizes recent findings related to the neurochemical basis of learned flavor preferences. Systemic and central injections of dopamine receptor antagonists implicate brain dopamine signaling in both flavor-flavor and flavor-nutrient conditioning by the taste and post-oral effects of sugars. Dopamine signaling in the nucleus accumbens, amygdala and lateral hypothalamus is involved in one or both forms of conditioning and selective effects are produced by D1-like and D2-like receptor antagonism. Opioid receptor antagonism, despite its suppressive action on sugar intake and reward, has little effect on the acquisition or expression of flavor preferences conditioned by the sweet taste or post-oral actions of sugars. Other studies indicate that flavor preference conditioning by sugars is differentially influenced by glutamate receptor antagonism, cannabinoid receptor antagonism and benzodiazepine receptor activation.

Opioid receptor antagonism in the nucleus accumbens fails to block the expression of sugar-conditioned flavor preferences in rats

In our prior studies, systemic administration of the opioid receptor antagonist naltrexone (NTX) did not block flavor preference conditioning by the sweet taste or post-oral actions of sugar despite reducing intake. Because opioid signaling in the nucleus accumbens (NAc) is implicated in food reward, this study determined if NTX administered into the NAc would block the expression of sugar-conditioned preferences. In Experiment 1, food-restricted rats with bilateral NAc shell or core cannulae were trained to drink a fructose (8%)+saccharin (0.2%) solution mixed with one flavor (CS+) and a less-preferred 0.2% saccharin solution mixed with another flavor (CS-) during one-bottle sessions. Two-bottle tests with the two flavors mixed in saccharin solutions occurred 10 min following total bilateral NAc shell or core doses of 0, 1, 25 and 50 microg of NTX. The rats preferred the CS+ over CS- following vehicle (80%) and all NTX doses in the shell and core. The CS+ preference was reduced to 64% and 72% by 50 microg NTX in the shell and core, although only the core effect was significant. In Experiment 2, food-restricted rats were trained to drink one flavored saccharin solution (CS+) paired with an intragastic (IG) glucose (8%) infusion and a second flavored saccharin solution (CS-) paired with an IG water infusion. In subsequent two-bottle tests, the rats displayed significant preferences for the CS+ (81-91%) that were unaltered by any NTX dose in the shell or core. CS+ intake, however, was reduced by NTX in the shell, but not the core. These data indicate that accumbal opioid antagonism slightly attenuated, but did not block the expression of sugar-conditioned flavor preferences. Therefore, while opioid drugs can have potent effects on sugar intake they appear less effective in altering sugar-conditioned flavor preferences.

Rapid acquisition of conditioned flavor preferences in rats

Rats learn to prefer flavors paired with the post-oral effects of glucose. The present study examined how rapidly they acquire this preference. In Experiment 1, food-restricted rats were given repeated three-session training/testing cycles: one 30-min session with a CS+ flavor paired with intragastric (IG) infusion of 16% glucose, another session with a CS- flavor paired with IG water, and a third session with a choice between the flavors with their infusates. The rats preferred the CS+ (69%) in the first choice session, and preference increased across the six cycles to 86%. These data demonstrate that the post-oral reinforcing action of glucose is potent enough to support one-trial learning. In Experiment 2, two groups of rats were trained in the same way, with the CS+ flavor paired with IG infusion of 16% glucose or 7.1% corn oil emulsion, but tests were conducted under extinction conditions, with both CS+ and CS- flavors paired with IG water. Significant preference for the CS+ was acquired more rapidly with glucose (71% CS+ in test 1) than with oil (69% CS+ in test 4). Consistent with previous work, the post-oral stimulation by glucose was more potent than that of isocaloric oil emulsion in conditioning preferences. The last experiment examined the acquisition rate for a flavor-taste conditioned preference. Rats were trained with a CS+ flavor mixed into an 8% fructose + 0.2% saccharin solution and a CS- flavor in 0.2% saccharin. The same three-session training/testing cycles were used, and in the tests the flavors were presented in saccharin. A significant 74% preference for the CS+ flavor was apparent by the second test. Together these studies show that the acquisition of flavor preferences, whether based on flavor-taste or flavor-nutrient associations, can be quite rapid.