Pharmacology of flavor preference conditioning in sham-feeding rats: effects of dopamine receptor antagonists

A 40-year analysis of central neuroanatomical and neurochemical circuits mediating homeostatic intake and hedonic intake and preferences in rodents

This perspective review was written in response to the celebration of the 60th anniversary of the journal, Brain Research, and covers the evolving focus of my laboratory's work over 40 years in the neurobiological substrates of ingestive behavior in rodents. Following our initial work examining the effects of systemic and ventricular administration of general and selective opioid receptor agonists and antagonists on food intake under spontaneous, deprivation, glucoprivic and hedonic conditions, my laboratory in close collaboration with Drs. Gavril Pasternak and Ying-Xian Pan utilized an antisense oligodoxynucleotide knock-down technique affecting MOR-1, DOR-1, KOR-1 and ORL-1 genes as well as against G-protein subunits to study receptor mediation of opioid receptor agonist-induced feeding as well as feeding following regulatory challenges. Our laboratory employed intracerebral microinjection techniques to map limbic nucleus accumbens and ventral tegmental area central brain circuits mediating homeostatic and hedonic feeding responses through the use of selective mu, delta1, delta2 and kappa opioid receptor subtype agonists in combination with general and selective opioid, dopamineric, glutamatergic and GABAergic antagonists administered into the same site or the reciprocal site, allowing for the identification of a distributed brain network mediating these ingestive effects. Our laboratory in close collaboration with Dr. Anthony Sclafani then focused on the pharmacological, neuroanatomical and learning mechanisms related to the development of sugar- (sucrose, glucose and fructose) and fat- (corn oil) conditioned flavor preferences (CFP) in rats, and on murine genetic variance in food intake, preferences and the process of appetition.

Acquisition and expression of sucrose conditioned flavor preferences following dopamine D1, opioid and NMDA receptor antagonism in C57BL/6 mice

The study of inbred mouse strains is a useful animal model to assess differences in ingestive behavior responses, including conditioned flavor preferences (CFP). C57BL/6, BALB/c and SWR inbred mice display differential sensitivity to dopamine (DA) D1, opioid and muscarinic cholinergic receptor antagonism of sucrose or saccharin intake as well as to muscarinic cholinergic antagonism of acquisition (learning) of sucrose-CFP. Given that DA D1, opioid and N-methyl-D-aspartate (NMDA) receptor antagonists differentially alter sucrose-CFP in BALB/c and SWR inbred mice, the present study examined whether systemic administration of naltrexone, SCH23390 or MK-801 altered acquisition and expression of sucrose-CFP in C57BL/6 mice. In acquisition experiments, male food-restricted C57BL/6 mice were treated with vehicle, naltrexone (1, 5 mg/kg), SCH23390 (50, 200 nmol/kg) or MK-801 (100, 200 µg/kg) 30 min prior to each of ten daily sessions in which they alternately consumed a flavored (CS+, e.g. cherry) 16% sucrose solution and a differently-flavored (CS-, e.g. grape) 0.05% saccharin solution followed by six two-bottle CS choice tests mixed in 0.2% saccharin without injections. SCH23390 and MK-801, but not naltrexone eliminated sucrose-CFP acquisition in food-restricted C57BL/6 mice. In expression experiments, food-restricted C57BL/6 mice underwent the ten training sessions without injections followed by two-bottle CS choice tests 30 min following vehicle, naltrexone (1, 5 mg/kg), SCH23390 (200, 800 nmol/kg) or MK-801 (100, 200 µg/kg). SCH23390 more effectively reduced the magnitude of sucrose-CFP expression than naltrexone or MK-801 in food-restricted C57BL/6 mice. Thus, dopamine D1 and NMDA receptor signaling is essential for learning of sucrose-CFP in C57BL/6 mice.

Murine genetic variance in muscarinic cholinergic receptor antagonism of acquisition and expression of sucrose-conditioned flavor preferences in three inbred mouse strains

Conditioned flavor preferences (CFP) are elicited by sucrose relative to saccharin in inbred mice with both the robustness of the preferences and sensitivity to pharmacological receptor antagonists sensitive to genetic variance. Dopamine, opioid and N-methyl-d-aspartate receptor antagonists differentially interfere with the acquisition (learning) and expression (maintenance) of sucrose-CFP in BALB/c and SWR inbred mice. Further, the muscarinic cholinergic receptor antagonist, scopolamine (SCOP) more potently reduces both sucrose and saccharin intake in BALB/c and C57BL/6 relative to SWR inbred mice. The present study examined whether SCOP altered the expression and acquisition of sucrose-CFP in BALB/c, C57BL/6 and SWR mice. In expression experiments, food-restricted mice alternately consumed a flavored (CS+, e.g., cherry, 5 sessions) 16% sucrose solution and a differently-flavored (CS-, e.g., grape, 5 sessions) 0.05% saccharin solution. Two-bottle CS choice tests with the two flavors mixed in 0.2% saccharin solutions occurred following vehicle or SCOP at doses of 1 or 5 mg/kg. SCOP significantly reduced the magnitude of the expression of sucrose-CFP in BALB/c, but not either C57BL/6 or SWR mice. In acquisition experiments, separate groups of BALB/c, C57BL/6 and SWR mice were treated prior to acquisition training sessions with vehicle or 2.5 or 5 mg/kg SCOP doses that was followed by six two-bottle CS choice tests without injections. SCOP dose-dependently reduced (1 mg/kg) and eliminated (2.5 mg/kg) the acquisition of sucrose-CFP in BALB/c mice, and reduced the magnitude of acquisition of sucrose-CFP in SWR mice. In contrast, neither SCOP dose affected the acquisition of sucrose-CFP in C57BL/6 mice. Thus, muscarinic cholinergic receptor signaling is essential for the learning of sucrose-CFP in BALB/c mice, to a lesser degree in SWR mice, but not in C57BL/6 mice. Murine genetic variance differentially modulates muscarinic cholinergic receptor control of sweet intake per se relative to learned conditioned flavor preferences of sweets.

Acquisition and expression of fat-conditioned flavor preferences are differentially affected by NMDA receptor antagonism in BALB/c and SWR mice

Conditioned flavor preferences are elicited by fat (Intralipid) in inbred mouse strains with BALB/c and SWR mice displaying among the most robust preferences. Dopamine D₁ and opioid receptor antagonism differentially reduces the acquisition (learning) and expression (maintenance) of fat-conditioned flavor preferences in these two strains. Because noncompetitive NMDA receptor antagonism with MK-801 differentially altered sugar-conditioned flavor preferences in these strains, and because NMDA receptors are involved in fat intake, the present study examined whether MK-801 differentially altered expression and acquisition of fat (Intralipid)-conditioned flavor preferences in BALB/c and SWR mice. In expression studies, food-restricted male mice alternately consumed a flavored (CS+, e.g., cherry, 5 sessions) 5% Intralipid solution and a differently-flavored (CS-, e.g., grape, 5 sessions) 0.5% Intralipid solution. Two-bottle CS choice tests occurred following vehicle or MK-801 (100, 200µg/kg). MK-801 blocked expression of Intralipid-CFP at both doses in BALB/c mice, but only at the 100µg/kg dose in SWR mice. In acquisition studies, groups of BALB/c (0, 100µg/kg) and SWR (0, 100µg/kg) male mice were treated prior to the ten acquisition training sessions followed by six 2-bottle CS choice tests without injections. MK-801 eliminated acquisition of Intralipid-conditioned flavor preferences in BALB/c mice, and actually changed the preference to an avoidance response in SWR mice. Thus, NMDA receptor signaling appears essential especially for the learning of fat-conditioned flavor preferences in both mouse strains.

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.

Simultaneous Detection of c-Fos Activation from Mesolimbic and Mesocortical Dopamine Reward Sites Following Naive Sugar and Fat Ingestion in Rats

This study uses cellular c-fos activation to assess effects of novel ingestion of fat and sugar on brain dopamine (DA) pathways in rats. Intakes of sugars and fats are mediated by their innate attractions as well as learned preferences. Brain dopamine, especially meso-limbic and meso-cortical projections from the ventral tegmental area (VTA), has been implicated in both of these unlearned and learned responses. The concept of distributed brain networks, wherein several sites and transmitter/peptide systems interact, has been proposed to mediate palatable food intake, but there is limited evidence empirically demonstrating such actions. Thus, sugar intake elicits DA release and increases c-fos-like immunoreactivity (FLI) from individual VTA DA projection zones including the nucleus accumbens (NAC), amygdala (AMY) and medial prefrontal cortex (mPFC) as well as the dorsal striatum. Further, central administration of selective DA receptor antagonists into these sites differentially reduce acquisition and expression of conditioned flavor preferences elicited by sugars or fats. One approach by which to determine whether these sites interacted as a distributed brain network in response to sugar or fat intake would be to simultaneous evaluate whether the VTA and its major mesotelencephalic DA projection zones (prelimbic and infralimbic mPFC, core and shell of the NAc, basolateral and central-cortico-medial AMY) as well as the dorsal striatum would display coordinated and simultaneous FLI activation after oral, unconditioned intake of corn oil (3.5%), glucose (8%), fructose (8%) and saccharin (0.2%) solutions. This approach is a successful first step in identifying the feasibility of using cellular c-fos activation simultaneously across relevant brain sites to study reward-related learning in ingestion of palatable food in rodents.

Dopamine D1 and opioid receptor antagonists differentially reduce the acquisition and expression of fructose-conditioned flavor preferences in BALB/c and SWR mice

Sugar appetite is influenced by unlearned and learned preferences in rodents. The present study examined whether dopamine (DA) D1 (SCH23390: SCH) and opioid (naltrexone: NTX) receptor antagonists differentially altered the expression and acquisition of fructose-conditioned flavor preferences (CFPs) in BALB/c and SWR mice. In expression experiments, food-restricted mice alternately (10 sessions, 1h) consumed a flavored (e.g., cherry) 8% fructose+0.2% saccharin solution (CS+) and a differently-flavored (e.g., grape) 0.2% saccharin solution (CS-). Two-bottle CS choice tests (1h) occurred 0.5h following vehicle: SCH (200 or 800 nmol/kg) or NTX (1 or 5mg/kg). SCH, but not NTX significantly reduced CS+ preference in both strains. In acquisition experiments, 0.5h prior to 10 acquisition training sessions, vehicle, SCH (50 nmol/kg), NTX (1 mg/kg) or Limited Control vehicle treatments were administered, followed by two-bottle CS choice tests without injections. SCH and NTX reduced training intakes in both strains. BALB/c mice displayed hastened extinction of the fructose-CFP following training with SCH, but not NTX. SCH eliminated fructose-CFP acquisition in SWR mice, whereas NTX hastened extinction of the CFP. These results are compared to previous drug findings obtained with sucrose-CFPs in SWR and BALB/c mice, and are discussed in terms of differential effects of these sugars on oral and post-oral conditioning.

Dopamine receptor signaling in the medial orbital frontal cortex and the acquisition and expression of fructose-conditioned flavor preferences in rats

Systemic dopamine (DA) D1 (SCH23390: SCH) and D2 (raclopride: RAC) antagonists blocked fructose-conditioned flavor preference (CFP) acquisition and expression. Fructose-CFP acquisition was eliminated by medial prefrontal cortex (mPFC) SCH and mPFC or amygdala (AMY) RAC. Fructose-CFP expression was reduced following SCH or RAC in AMY or nucleus accumbens (NAc). The present study examined fructose-CFP acquisition and expression following SCH and RAC in the medial orbital frontal cortex (MOFC), another ventral tegmental area DA target. For fructose-CFP acquisition, five groups of rats received vehicle, SCH (24 or 48 nmol) or RAC (24 or 48 nmol) in the MOFC 0.5h prior to 8 training sessions with one flavor (CS+/Fs) mixed in 8% fructose and 0.2% saccharin, and another flavor (CS-/s) mixed in 0.2% saccharin. In six 2-bottle choice tests in 0.2% saccharin, similar fructose-CFP preferences occurred in groups trained with vehicle (76-77%), SCH24 (69-78%), SCH48 (70-74%) and RAC48 (85-92%). RAC24-trained rats displayed significant CS+ preferences during the first (79%) and third (71%), but not second (58%) test pair. For fructose-CFP expression, rats similarly trained with CS+/Fs and CS- solutions received 2-bottle choice tests following MOFC injections of SCH or RAC (12-48 nmol). CS+ preference expression was significantly reduced by RAC (48 nmol: 58%), but not SCH relative to vehicle (78%). A control group receiving RAC in the dorsolateral prefrontal cortex displayed fructose-CFP expression similar to vehicle. These data demonstrate differential frontal cortical DA mediation of fructose-CFP with mPFC D1 and D2 signaling exclusively mediating acquisition, and MOFC D2 signaling primarily mediating expression.

Roles of NMDA and dopamine D1 and D2 receptors in the acquisition and expression of flavor preferences conditioned by oral glucose in rats

Animals learn to prefer flavors associated with the intake of sugar (sucrose, fructose, glucose) and fat (corn oil: CO) solutions. Conditioned flavor preferences (CFP) have been elicited for sugars based on orosensory (flavor-flavor: e.g., fructose-CFP) and post-ingestive (flavor-nutrient: e.g., intragastric (IG) glucose-CFP) processes. Dopamine (DA) D1, DA D2 and NMDA receptor antagonism differentially eliminate the acquisition and expression of fructose-CFP and IG glucose-CFP. However, pharmacological analysis of fat (CO)-CFP, mediated by both flavor-flavor and flavor-nutrient processes, indicated that acquisition and expression of fat-CFP were minimally affected by systemic DA D1 and D2 antagonists, and were reduced by NMDA antagonism. Therefore, the present study examined whether systemic DA D1 (SCH23390), DA D2 (raclopride) or NMDA (MK-801) receptor antagonists altered acquisition and/or expression of CFP induced by oral glucose that should be mediated by both flavor-flavor and flavor-nutrient processes. Oral glucose-CFP was elicited following by training rats to drink one novel flavor (CS+, e.g., cherry) mixed in 8% glucose and another flavor (CS-, e.g., grape) mixed in 2% glucose. In expression studies, food-restricted rats drank these solutions in one-bottle sessions (2 h) over 10 days. Subsequent two-bottle tests with the CS+ and CS- flavors mixed in 2% glucose occurred 0.5 h after systemic administration of vehicle (VEH), SCH23390 (50-800 nmol/kg), raclopride (50-800 nmol/kg) or MK-801 (50-200 μg/kg). Rats displayed a robust CS+ preference following VEH treatment (94-95%) which was significantly though marginally attenuated by SCH23390 (67-70%), raclopride (77%) or MK-801 (70%) at doses that also markedly reduced overall CS intake. In separate acquisition studies, rats received VEH, SCH23390 (50-400 nmol/kg), raclopride (50-400 nmol/kg) or MK-801 (100 μg/kg) 0.5 h prior to ten 1-bottle training trials with CS+/8%G and CS-/2%G training solutions that was followed by six 2-bottle CS+ vs. CS- tests in 2% glucose conducted without injections. The significant and persistent CS+ preferences observed in the VEH (94-98%) group was significantly reduced by rats receiving SCH23390 at 400 nmol/kg (65-73%), raclopride at 200 or 400 nmol/kg (76-82%) or MK-801 at 100 μg/kg (68-69%). Thus, systemic DA D1 and DA D2 receptor antagonism produced smaller reductions in the expression of oral glucose-CFP relative to fructose-CFP or IG-glucose-CFP. Correspondingly, systemic DA D1, DA D2 and NMDA receptor antagonism also produced smaller reductions in the acquisition of oral glucose-CFP relative to fructose-CFP or IG-glucose-CFP. These data suggest, but do not prove, that the magnitude and persistence of these receptor antagonist effects upon sugar-CFP might depend upon the individual or combined engagement of flavor-flavor and flavor-nutrient processes.

Neuroendocrine circuits governing energy balance and stress regulation: functional overlap and therapeutic implications

Significant comorbidities between obesity-related metabolic disease and stress-related psychological disorders suggest important functional interactions between energy balance and brain stress integration. Largely overlapping neural circuits control these systems, and this anatomical arrangement optimizes opportunities for mutual influence. Here we first review the current literature identifying effects of metabolic neuroendocrine signals on stress regulation, and vice versa. Next, the contributions of reward-driven food intake to these metabolic and stress interactions are discussed. Lastly, we consider the interrelationships between metabolism, stress, and reward in light of their important implications in the development of therapies for metabolism- or stress-related disease.

Effect of dopamine D1 and D2 receptor antagonism in the lateral hypothalamus on the expression and acquisition of fructose-conditioned flavor preference in rats

The attraction to sugar-rich foods is influenced by conditioned flavor preferences (CFP) produced by the sweet taste of sugar (flavor-flavor learning) and the sugar's post-oral actions (flavor-nutrient) learning. Brain dopamine (DA) circuits are involved in both types of flavor learning, but to different degrees. This study investigated the role of DA receptors in the lateral hypothalamus (LH) on the flavor-flavor learning produced the sweet taste of fructose. In an acquisition study, food-restricted rats received bilateral LH injections of a DA D1 receptor antagonist (SCH23390), a D2 antagonist (RAC, raclopride) or vehicle prior to 1-bottle training sessions with a flavored 8% fructose+0.2% saccharin solution (CS+/F) and a less-preferred flavored 0.2% saccharin solution (CS-). Drug-free 2-bottle tests were then conducted with the CS+ and CS- flavors presented in saccharin. The fructose-CFP did not differ among groups given vehicle (76%), 12 nmol SCH (78%), 24 nmol (82%) or 24 nmol RAC (90%) during training. In an expression study with rats trained drug-free, LH injections of 12 or 24 nmol SCH or 12-48 nmol RAC prior to 2-bottle tests did not alter CS+ preferences (77-90%) relative to vehicle injection (86%). Only a 48 nmol SCH dose suppressed the CS+ preference (61%). The minimal effect of LH DA receptor antagonism upon fructose flavor-flavor conditioning differs with the ability of LH SCH injections to block the acquisition of glucose flavor-nutrient learning.

Critical role of NMDA but not opioid receptors in the acquisition of fat-conditioned flavor preferences in rats

Animals learn to prefer flavors associated with the intake of dietary fats such as corn oil (CO) solutions. We previously reported that fat-conditioned flavor preferences in rats were relatively unaffected by systemic treatment with dopamine D1 and D2 antagonsits. The present study examined whether systemic opioid (naltrexone, NTX) or NMDA (MK-801) receptor antagonists altered the acquisition and/or expression of CO-CFP. The CFP was produced by training rats to drink one novel flavor (CS+, e.g., cherry) mixed in a 3.5% CO solution and another flavor (CS-, e.g., grape) in a 0.9% CO solution. In expression studies, food-restricted rats drank these solutions in one-bottle sessions (2 h) over 10 d. Subsequent two-bottle tests with the CS+ and CS- flavors mixed in 0.9% CO solutions occurred 0.5h after systemic administration of vehicle (VEH), NTX (0.1-5 mg/kg) or MK-801 (50-200 μg/kg). Rats displayed a robust CS+ preference following VEH treatment (85-88%) which was significantly though moderately attenuated by NTX (69-70%). The lower doses of MK-801 slightly reduced the CS+ preference; the high dose blocked the CS+ preference (49%) but also markedly reduced overall CS intake. In separate acquisition studies, rats received VEH or NTX (0.1, 0.5, 1mg/kg) or MK-801 (100 μg/kg) 0.5h prior to 1-bottle training trials with CS+/3.5% CO and CS-/0.9% CO training solutions. Additional Limited VEH groups were trained with intakes limited to that of the NTX and MK-801 groups. Subsequent two-bottle CS+ vs. CS- tests were conducted without injections. Significant and persistent CS+ preferences were observed in VEH (77-84%) and Limited VEH (88%) groups. NTX treatment during training failed to block the acquisition of CO-CFP although the magnitude of the CS+ preference was reduced by 0.5 (70%) and 1.0 (72%) mg/kg doses relative to the Limited VEH treatment (88%). In contrast, MK-801 (100 μg/kg) treatment during training blocked the acquisition of the CO-CFP. These data suggest a critical role for NMDA, but not opioid receptor signaling in the acquisition of a fat conditioned flavor preferences, and at best limited involvement of NMDA and opioid receptors in the expression of a previously learned preference.

Double-dissociation of D1 and opioid receptor antagonism effects on the acquisition of sucrose-conditioned flavor preferences in BALB/c and SWR mice

Sugar appetite is influenced by unlearned attractions to sweet taste and learned responses to sugars' taste and post-ingestive actions. In rats, sugar-conditioned flavor preferences (CFP) are attenuated by dopamine D1 (SCH23390: SCH), but not by opioid (naltrexone: NTX), receptor antagonism. Sucrose-CFP occurs in BALB/c and SWR inbred mice that differ in their suppressive effects of SCH and NTX on sucrose intake. The present study examined whether SCH and NTX altered expression of previously learned sucrose-CFP and acquisition (learning) of sucrose-CFP in these strains. In Experiment 1, food-restricted mice were trained (10 one-bottle sessions) to drink a more-preferred flavored (e.g., cherry) 16% sucrose solution (CS+/Sucrose) on odd-numbered days, and a less-preferred flavored (e.g., grape) 0.05% saccharin solution (CS-/Saccharin) on even-numbered days. Two-bottle tests with the flavors mixed in 0.2% saccharin occurred 30 min following vehicle (Veh), SCH (50-800 nmol/kg) or NTX (1-5mg/kg) assessing preference expression. CS+ preference expression in BALB/c and SWR mice following Veh were significantly reduced by SCH and NTX. In Experiment 2, separate groups of BALB/c and SWR mice received Veh, SCH (50 nmol/kg) or NTX (1mg/kg) injections 30 min prior to daily one-bottle training sessions with the CS+/Sucrose and CS-/Saccharin solutions assessing preference acquisition. Subsequent two-bottle tests with the CS+ vs. CS- solutions were conducted without injections. CS+/Sucrose training intakes were reduced by SCH in both strains and by NTX in BALB/c mice. In the initial two-bottle test, sucrose-CFP acquisition was significantly reduced in BALB NTX (54%), but not in BALB SCH (77%) groups relative to the BALB Veh group (85%). In contrast, sucrose-CFP acquisition was significantly reduced in SWR SCH (61%), but not in SWR NTX (83%) groups relative to the SWR Veh group (86%). DA D1 and opioid receptor signaling modulate acquisition and/or expression of sucrose-CFP in mice with significant strain differences observed.

Dopamine signaling in the medial prefrontal cortex and amygdala is required for the acquisition of fructose-conditioned flavor preferences in rats

Systemic administration of dopamine (DA) D1 (SCH23390: SCH) and D2 (raclopride: RAC) antagonists blocked both acquisition and expression of fructose-conditioned flavor preferences (CFP). It is unclear what brain circuits are involved in mediating these effects. The present study investigated DA signaling within the nucleus accumbens shell (NAcS), amygdala (AMY) and medial prefrontal cortex (mPFC) in the acquisition and expression of fructose-CFP. In Experiment 1, separate groups of rats were injected daily in the NAcS or AMY with saline, SCH (24 nmol) or RAC (24 nmol) prior to training sessions with a flavor (CS+) mixed with 8% fructose and 0.2% saccharin (CS+/F) and a different flavor (CS-) mixed with only 0.2% saccharin. In the two-bottle choice tests with 0.2% saccharin, only rats injected with RAC in the AMY failed to acquire a CS+ preference (45-54%). In Experiment 2, new rats were identically trained, but saline, SCH and RAC were injected in the mPFC. In subsequent two-bottle choice tests, SCH- and RAC-treated rats failed to exhibit a CS+ preference (50-56%). In Experiment 3, new rats were trained with CS+/F and CS- without injections. Subsequent two-bottle choice tests were then conducted following bilateral injections of SCH or RAC in the mPFC at total doses of 0, 12, 24 and 48 nmol. Expression of the CS+ preference failed to be affected by either antagonist, indicating that the mPFC is not involved in the maintenance of this preference. These data indicate that the acquisition of fructose-CFP is dependent on DA signaling in the mPFC and AMY.

Roles of dopamine D1 and D2 receptors in the acquisition and expression of fat-conditioned flavor preferences in rats

Sugars and fats elicit innate and learned flavor preferences with the latter mediated by flavor-flavor (orosensory) and flavor-nutrient (post-ingestive) processes. Systemic dopamine (DA) D1 (SCH23390: SCH) and D2 (raclopride: RAC), but not opioid antagonists blocked the acquisition and expression of flavor-flavor preferences conditioned by sugars. In addition, systemic D1, but not D2 or opioid antagonists blocked the acquisition of flavor-nutrient preferences conditioned by intragastric (IG) sugar infusions. Given that DA antagonists reduce fat intake, the present study examined whether systemic D1 or D2 antagonists altered the acquisition and/or expression of conditioned flavor preferences (CFP) produced by pairing one novel flavor (CS+, e.g., cherry) with a 3.5% corn oil (CO: fat) solution relative to another flavor (CS-, e.g., grape) paired with a 0.9% CO solution. In an expression study, food-restricted rats were trained to drink either flavored 3.5% or 0.9% CO solutions on alternate days. Subsequent two-bottle tests with the CS+ and CS- flavors mixed in 0.9% CO solutions occurred 0.5h after systemic administration of vehicle (VEH), SCH (50-800 nmol/kg) or RAC (50-800 nmol/kg). The rats displayed a robust CS+ preference following VEH treatment (87-88%) the expression of which was attenuated by treatment with moderate doses of RAC, and to a lesser degree, SCH. In an acquisition study, six groups of rats received VEH, SCH (25, 50, 200 nmol/kg) or RAC (50, 200 nmol/kg) 0.5 h prior to 1-bottle training trials with CS+ flavored 3.5% and CS- flavored 0.9% (CS-) CO solutions. A seventh Limited VEH group was trained with its training intakes limited to that of the SCH and RAC groups. Subsequent two-bottle tests were conducted with the CS+ and CS- flavors presented in 0.9% CO without injections. Significant and persistent CS+ preferences were observed in VEH (75-82%), Limited VEH (70-88%), SCH25 (75-84%), SCH50 (64-87%), SCH200 (78-91%) and RAC200 (74-91%) groups. In contrast, the group trained with RAC50 displayed a significant initial CS+ preference (76%) which declined over testing to 61%. These data indicate limited DA D1 and D2 receptor signaling involvement in the expression and acquisition of a fat-CFP relative to previous robust effects for sugar-CFP.

Differential effects of dopamine receptor D1-type and D2-type antagonists and phase of the estrous cycle on social learning of food preferences, feeding, and social interactions in mice

The neurobiological bases of social learning, by which an animal can 'exploit the expertise of others' and avoid the disadvantages of individual learning, are only partially understood. We examined the involvement of the dopaminergic system in social learning by administering a dopamine D1-type receptor antagonist, SCH23390 (0.01, 0.05, and 0.1 mg/kg), or a D2-type receptor antagonist, raclopride (0.1, 0.3, and 0.6 mg/kg), to adult female mice prior to socially learning a food preference. We found that while SCH23390 dose-dependently inhibited social learning without affecting feeding behavior or the ability of mice to discriminate between differently flavored diets, raclopride had the opposite effects, inhibiting feeding but leaving social learning unaffected. We showed that food odor, alone or in a social context, was insufficient to induce a food preference, proving the specifically social nature of this paradigm. The estrous cycle also affected social learning, with mice in proestrus expressing the socially acquired food preference longer than estrous and diestrous mice. This suggests gonadal hormone involvement, which is consistent with known estrogenic regulation of female social behavior and estrogen receptor involvement in social learning. Furthermore, a detailed ethological analysis of the social interactions during which social learning occurs showed raclopride- and estrous phase-induced changes in agonistic behavior, which were not directly related to effects on social learning. Overall, these results suggest a differential involvement of the D1-type and D2-type receptors in the regulation of social learning, feeding, and agonistic behaviors that are likely mediated by different underlying states.

Neuropsychiatric adverse effects of centrally acting antiobesity drugs

INTRODUCTION

Central neurochemical systems including the monoamine, opioid, and cannabinoid systems have been promising targets for antiobesity drugs that modify behavioral components of obesity. In addition to modulating eating behavior, centrally acting antiobesity drugs are also likely to alter emotional behavior and cognitive function due to the high expression of receptors for the neurochemical systems targeted by these drugs within the fronto-striatal and limbic circuitry.

METHODS

This paper reviewed the neuropsychiatric adverse effects of past and current antiobesity drugs, with a central mechanism of action, linking the adverse effects to their underlying neural substrates and neurochemistry.

RESULTS

Antiobesity drugs were found to have varying neuropsychiatric adverse event profiles. Insomnia was the most common adverse effect with drugs targeting monoamine systems (sibutramine, bupropion and tesofensine). These drugs had some positive effects on mood and anxiety and may have added therapeutic benefits in obese patients with comorbid depression and anxiety symptoms. Sedation and tiredness were the most common adverse effects reported with drugs targeting the m-opioid receptors (i.e., naltrexone) and combination therapies targeting the opioid and monoamine systems (i.e., Contrave™). Cognitive impairments were most frequently associated with the antiepileptic drugs, topiramate and zonisamide, consistent with their sedative properties. Drugs targeting the cannabinoid system (rimonabant and taranabant) were consistently associated with symptoms of anxiety and depression, including reports of suicidal ideation. Similar adverse events have also been noted for the D₁/D₅ antagonist ecopipam.

CONCLUSION

These findings highlight the need to assess neuropsychiatric adverse events comprehensively using sensitive and validated methods early in the clinical development of candidate antiobesity drugs with a central mechanism of action.

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.

Role of amygdala dopamine D1 and D2 receptors in the acquisition and expression of fructose-conditioned flavor preferences in rats

Systemic administration of dopamine D1-like (SCH23390) and, to a lesser degree D2-like (raclopride), receptor antagonists significantly reduce the acquisition and expression of fructose-conditioned flavor preferences (CFP) in rats. Given the role of dopamine in the amygdala (AMY) in the processing and learning of food reward, the present study examined whether dopamine D1-like or D2-like antagonists in this site altered acquisition and/or expression of a fructose-CFP. In Experiment 1, food-restricted rats with bilateral AMY cannulae were trained to drink a fructose (8%)+saccharin (0.2%) solution mixed with one flavor (e.g., grape, CS+/Fs) and a less-preferred 0.2% saccharin solution mixed with another flavor (e.g., cherry, CS-/s) during one-bottle (16 ml) sessions. Two-bottle tests with the two flavors mixed in saccharin solutions (CS+/s, CS-/s) occurred 10 min following total bilateral AMY doses of 0, 12, 24 and 48 nmol of SCH23390 or raclopride. Preference for CS+/s over CS-/s was significantly reduced relative to vehicle baseline by the 48 nmol doses of SCH23390 and raclopride (from 77% to 66% and 68%), but not lower doses. In Experiment 2, rats received bilateral AMY injections (12 nmol) of SCH23390 (D1 group) or raclopride (D2 group) 10 min prior to one-bottle training sessions with CS+/Fs and CS-/s. Yoked Control rats received vehicle and were limited to the CS intakes of the D1 and D2 groups; untreated controls were not injected or limited to drug group intakes during training. Subsequent 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 6 tests sessions in the D1 group, but not in the D2 group. These data indicate that dopamine D1-like and D2-like antagonists significantly attenuated the expression of the previously acquired fructose-CFP, and did not block acquisition of the fructose-CFP. D1-like antagonism during training hastened extinction of the fructose-CFP. The results are similar to those produced by dopamine D1-like and D2-like antagonist injections into the nucleus accumbens shell which suggests that flavor conditioning involves a regionally distributed brain network.

Natural addiction: a behavioral and circuit model based on sugar addiction in rats

The distinction between natural addiction and drug addiction is interesting from many points of view, including scientific and medical perspectives. "Natural addictions" are those based on activation of a physiobehavioral system, such as the one that controls metabolism, foraging, and eating to achieve energy balance. "Drug addictions" activate many systems based on their pharmacology. This review discusses the following questions: (1) When does food produce a natural addiction? Sugar causes signs of addiction if the scheduling conditions are appropriate to cause binge eating. (2) Why does addictive-like behavior result? Bingeing on a 10% sucrose solution repeatedly releases dopamine in the nucleus accumbens, and it delays the release of acetylcholine, thereby postponing satiety. Opioid involvement is shown by withdrawal caused by naloxone or food deprivation. Bingeing, withdrawal, and abstinence-induced motivation are described as the basis for a vicious cycle leading to excessive eating. (3) Which foods can lead to natural addiction? A variety of sugars, saccharin, and sham feeding are compared with bingeing on high-fat diets, which seem to lack sugar's opioid-withdrawal characteristic. (4) How does natural food addiction relate to obesity? Low basal dopamine may be a common factor, leading to "eating for dopamine." (5) In a neural model, the accumbens is depicted as having separate GABA output pathways for approach and avoidance, both controlled by dopamine and acetylcholine. These outputs, in turn, control lateral hypothalamic glutamate release, which starts a meal, and GABA release, which stops it.

Genetic variance contributes to dopamine receptor antagonist-induced inhibition of sucrose intake in inbred and outbred mouse strains

Preference and intake of sucrose varies across inbred and outbred strains of mice. Pharmacological analyses revealed that the greatest sensitivity to naltrexone-induced inhibition of sucrose (10%) intake was observed in C57BL10/J and C57BL/6J strains, whereas 129P3/J, SWR/J and SJL/J strains displayed far less sensitivity to naltrexone-induced inhibition of sucrose intake. Given that dopamine D1 (SCH23390) and D2 (raclopride) receptor antagonism potently reduce sucrose intake in outbred rat and mouse strains, the present study examined the possibility of genetic variance in the dose-dependent (50-1600 nmol/kg) and time-dependent (5-120 min) effects of these antagonists upon sucrose (10%) intake in the eight inbred (BALB/cJ, C3H/HeJ, C57BL/6J, C57BL/10J, DBA/2J, SJL/J, SWR/J and 129P3/J) and one outbred (CD-1) mouse strains previously tested with naltrexone. SCH23390 significantly reduced sucrose intake across all five doses in 129P3/J and SJL/J mice, across four doses in C57BL/6J and BALB/cJ mice, across three doses in DBA/2J, SWR/J, C3H/HeJ and C57BL/10J mice, but only at the two highest doses in CD-1 mice. SCH23390 was 2-3-fold more potent in inhibiting sucrose intake in 129P3/J and SJL/J mice relative to CD-1 mice. In contrast, only the highest equimolar 1600 nmol/kg dose of raclopride significantly reduced sucrose intake in the BALB/cJ, C3H/HeJ, C57BL/6J, C57BL/10J, DBA/2J, SJL/J and 129P3/J, but not the SWR/J and CD-1 strains. The present and previous data demonstrate specific and differential patterns of genetic variability in inhibition of sucrose intake by dopamine and opioid antagonists, suggesting that distinct neurochemical mechanisms control sucrose intake across different mouse strains.

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.

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.

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.

Dopamine D2 receptors contribute to increased avidity for sucrose in obese rats lacking CCK-1 receptors

Accumulating evidence has indicated a link between dopamine signaling and obesity in both animals and humans. We have recently demonstrated heightened avidity to sapid sweet solutions in the obese cholecystokinin (CCK)-1 receptor deficient Otsuka Long Evans Tokushima fatty (OLETF) rat. To investigate the dopamine dependence and the respective contribution of D1 and D2 receptor subtypes in this phenomenon, real and sham intake of 0.3 M sucrose solution was compared between prediabetic, obese OLETF and age-matched lean Long-Evans Tokushima Otsuka (LETO) cohorts following peripheral (i.p.) administration of equimolar doses (50-800 nmol/kg) of the D1 (R-(+) 7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine, SCH23390) and D2 (raclopride) selective receptor antagonists. Both antagonists were potent in reducing sucrose intake in both strains with both drugs suppressing sham intake starting at lower doses than real intake (200 nmol/kg vs. 400 nmol/kg for SCH23390, and 400 nmol/kg vs. 600 nmol/kg for raclopride, respectively). Furthermore, when percent suppression of intake, a measure that controlled for the higher baseline sucrose intake by obese rats was analyzed, OLETF rats expressed an increased sensitivity to raclopride in reducing ingestion of sucrose with a 1.7- and 2.9-fold lower inhibitory dose threshold (ID50) for real and sham intake conditions, respectively, compared with LETO controls. In contrast, SCH23390 caused no differential strain effect with respect to dosage whether sucrose was real or sham fed. These findings demonstrate that D2 receptors are involved in heightened increased consumption of sucrose observed in the OLETF obese rat.

Randomized controlled trials of the D1/D5 antagonist ecopipam for weight loss in obese subjects

OBJECTIVE

To evaluate the efficacy and safety of the selective dopamine D1/D5 antagonist ecopipam for the treatment of obesity.

RESEARCH METHODS AND PROCEDURES

Four randomized, double-blind, multicenter trials compared ecopipam (n=1667) and placebo (n=1118) in obese subjects including type 2 diabetic subjects. Subjects received oral ecopipam 10, 30, or 100 mg daily for 12 weeks (Phase 2) or 50 or 100 mg daily for 52 weeks (Phase 3) combined with a weight loss program. Primary efficacy variables were the proportion of subjects with>or=5% weight loss from baseline at 12 weeks (Phase 2) or the distribution of percentage weight loss from baseline at 52 weeks (Phase 3).

RESULTS

In the Phase 2 study, 26% of subjects administered ecopipam 100 mg vs. 6% of placebo subjects achieved>or=5% weight loss after 12 weeks (p<0.01). In the Phase 3 studies, ecopipam 100 mg produced a 3.1% to 4.3% greater weight loss than placebo at 52 weeks. More subjects administered ecopipam vs. placebo achieved a 5% to 10% or >10% weight loss in two non-diabetic phase 3 trials. Ecopipam-treated subjects also maintained more weight loss compared with placebo subjects at 52 weeks. Phase 3 studies were discontinued because of unexpected psychiatric adverse events (ecopipam 31% vs. placebo 15%), including depression, anxiety, and suicidal ideation.

DISCUSSION

Ecopipam was effective for achieving and maintaining weight loss in obese subjects, including type 2 diabetic subjects; however, the adverse effects on mood observed in the Phase 3 studies exclude its projected use in weight management.

Effort-related functions of nucleus accumbens dopamine and associated forebrain circuits

BACKGROUND

Over the last several years, it has become apparent that there are critical problems with the hypothesis that brain dopamine (DA) systems, particularly in the nucleus accumbens, directly mediate the rewarding or primary motivational characteristics of natural stimuli such as food. Hypotheses related to DA function are undergoing a substantial restructuring, such that the classic emphasis on hedonia and primary reward is giving way to diverse lines of research that focus on aspects of instrumental learning, reward prediction, incentive motivation, and behavioral activation.

OBJECTIVE

The present review discusses dopaminergic involvement in behavioral activation and, in particular, emphasizes the effort-related functions of nucleus accumbens DA and associated forebrain circuitry.

RESULTS

The effects of accumbens DA depletions on food-seeking behavior are critically dependent upon the work requirements of the task. Lever pressing schedules that have minimal work requirements are largely unaffected by accumbens DA depletions, whereas reinforcement schedules that have high work (e.g., ratio) requirements are substantially impaired by accumbens DA depletions. Moreover, interference with accumbens DA transmission exerts a powerful influence over effort-related decision making. Rats with accumbens DA depletions reallocate their instrumental behavior away from food-reinforced tasks that have high response requirements, and instead, these rats select a less-effortful type of food-seeking behavior.

CONCLUSIONS

Along with prefrontal cortex and the amygdala, nucleus accumbens is a component of the brain circuitry regulating effort-related functions. Studies of the brain systems regulating effort-based processes may have implications for understanding drug abuse, as well as energy-related disorders such as psychomotor slowing, fatigue, or anergia in depression.

NMDA receptor in conditioned flavor-taste preference learning: blockade by MK-801 and enhancement by D-cycloserine

Conditioned flavor-taste preference (CFTP) is a robust form of learning in which animals acquire a preference for a flavor (e.g. Kool-Aid) previously mixed with a highly preferred tastant (e.g. fructose) over a flavor previously mixed with a less-preferred tastant (e.g. saccharin). Here, the role of the N-methyl-D-aspartate (NMDA) glutamate-glycine receptor (NR) was probed using systemic MK-801, a non-competitive antagonist, and D-cycloserine (DCS), a glycine agonist. Rats were injected with MK-801 (100 microg/kg) or vehicle 30 min prior to a daily 2-h conditioning session with 1-bottle access to a Kool-Aid flavor (grape or cherry) mixed with either 8% fructose (CS+/F) or 0.2% saccharin (CS-/S). CFTP expression was measured in 2-bottle preference tests between the Kool-Aid flavors mixed with 0.2% saccharin (CS+/S vs. CS-/S). While vehicle-treated rats acquired a preference for CS+/S over CS-/S, MK-801 prior to conditioning completely blocked CFTP learning. The effect of MK-801 was specific to CFTP acquisition, because follow-up experiments demonstrated that MK-801 did not induce a conditioned taste aversion, cause state-dependent learning, or affect CFTP expression. In a second approach, rats were injected with DCS (15 mg/kg) 60 min prior to daily conditioning. In contrast to MK-801, administration of DCS prior to conditioning enhanced CFTP learning (but not reversal conditioning). These results demonstrate that NR neurotransmission is critical for CFTP learning. Furthermore, enhancement of CFTP learning by DCS suggests that endogenous levels of glycine or D-serine may be a limiting factor in CFTP learning.

Unconditioned stimulus devaluation effects in nutrient-conditioned flavor preferences

Experiments with different temporal relations between the conditioned stimulus (CS) and the unconditioned stimulus (US) in conditioning assessed whether US devaluation effects can be obtained after nutrient-conditioned flavor preference learning. One flavor (CScarb) was paired with a carbohydrate, Polycose; a 2nd flavor (CSprot) was paired with a protein, casein; and a 3rd flavor (CS-) was presented by itself. Following conditioning, one of the nutrients was devalued through pairings with lithium chloride in the absence of the CS flavors. In a subsequent 2-bottle test, rats preferred CScarb over CSprot; however, this preference was smaller when the carbohydrate was devalued than when the protein was devalued. Results suggest that CS flavors are able to form associations with the sensory features of nutrient USs under a wide variety of circumstances.

Altered taste sensitivity in obese, prediabetic OLETF rats lacking CCK-1 receptors

Otsuka Long-Evans Tokushima fatty (OLETF) rats lack the CCK-1 receptor, are hyperphagic, progressively become obese, and develop type-2 diabetes. We recently demonstrated an increased preference for both real and sham feeding of sucrose in this strain, suggesting altered orosensory sensitivity. To investigate taste functions, we used an automated gustometer with 10-s access to different concentrations of various sapid stimuli. Tests were repeated at 10 and 18 wk of age to assess the early and advanced stages of prediabetes, respectively. Compared with age-matched, nonmutant controls, the OLETF rats showed higher avidity for sucrose at both ages. This difference increased as a function of age and tastant concentration. An exaggerated response also occurred for saccharin, alanine, and fructose, but not for Polycose. Similarly, OLETF rats consumed monosodium-glutamate more at the lower concentrations compared with controls, an effect that age also accentuated. In contrast, there was no statistical strain or age differences in responses to NaCl, MgCl2, citric acid, quinine-HCl, and the trigeminal stimulus capsaicin. These findings demonstrate that compared with controls, OLETF rats differ in their gustatory functions with an overall augmented sensitivity for sweet that progresses during prediabetes. This effect explains their overconsumption of sweet solutions and may contribute to the overall hyperphagia and obesity in this strain.

Increased oral and decreased intestinal sensitivity to sucrose in obese, prediabetic CCK-A receptor-deficient OLETF rats

CCK-A receptor-deficient Otsuka Long-Evans Tokushima fatty (OLETF) rats are hyperphagic and develop obesity and Type 2 diabetes. In this strain, taste preference functions have not been investigated. Therefore, a series of short-access, two-bottle tests were performed in age-matched prediabetic OLETF and nonmutant Long-Evans Tokushima Otsuka (LETO) rats to investigate preference for sucrose (0.03, 0.1, 0.3, or 1.0 M) presented with a choice of water. To discern orosensory from postgastric factors that may contribute to this preference, in a separate experiment, rats were allowed to sham feed sucrose in the absence or presence of duodenal sucrose infusion (0.3, 0.6, or 1.0 M). In the two-bottle real-feeding tests, OLETF rats exhibited a greater preference for 0.3 M sucrose (91.2 +/- 1.7 and 78.5 +/- 3.4% for OLETF and LETO, respectively; P < 0.01) and 1.0 M sucrose (65.3 +/- 1.2 and 57.5 +/- 2.7% for OLETF and LETO, respectively; P < 0.05) than LETO rats. OLETF rats also sham fed less of the lowest (0.03 M; 33.8 +/- 4.8 and 58.3 +/- 7.3 ml for OLETF and LETO, respectively; P < 0.05) and more of the highest (1.0 M; 109.9 +/- 6.5 and 81.0 +/- 3.9 ml for OLETF and LETO, respectively; P < 0.01) concentration of sucrose relative to LETO rats. Finally, intraduodenal sucrose infusions (0.6 and 1.0 M) produced a smaller reduction of 0.3 M sham sucrose intake [14.1 +/- 8.1 vs. 52.5 +/- 3.3 ml and 49.4 +/- 8.0 vs. 82.4 +/- 3.2 ml for 0.6 M (P < 0.01) and 1.0 M (P < 0.05) infusions in OLETF and LETO, respectively]. These findings demonstrate that OLETF rats display an increased preference for sucrose, an effect that is at least partially influenced by the orosensory stimulating effect of sucrose. This enhanced responsiveness to oral stimulation, coupled with the deficit in responding to the postingestive feedback of intestinal sucrose, may contribute additively to the development of hyperphagia and weight gain in OLETF rats.

Naltrexone does not prevent acquisition or expression of flavor preferences conditioned by fructose in rats

The effects of the general opioid antagonist, naltrexone, on the acquisition and expression of flavor preferences conditioned by the sweet taste of fructose were examined. Food-restricted rats were trained over eight daily alternating one-bottle sessions (2 h) to drink an 8% fructose solution containing one novel flavor (CS+/F) and a less preferred 0.2% saccharin solution containing a different flavor (CS-/S). Four groups of rats were treated daily with either saline (control group) or naltrexone doses of 0.1, 1.0, or 5.0 mg/kg during training. Preferences were assessed in two-bottle tests with the CS+/S and CS-/S flavors presented in 0.2% saccharin solutions following saline injections. Naltrexone dose-dependently reduced fructose and saccharin intakes during training, confirming the drug's well-known suppressive effect on the intake of sweet solutions. Despite their reduced training intakes, the naltrexone groups displayed preferences for the CS+/S over the CS-/S (72-86%) that were similar to that of the control group (78%). The effect of naltrexone on the expression of the CS+/S flavor preference was evaluated by treating control rats with naltrexone (0.1-5 mg/kg) prior to CS+/S vs. CS-/S choice tests. The drug doses produced a dose-dependent reduction in CS+/S intake but did not significantly attenuate the CS+/S preference. These data are consistent with the relative inability of naltrexone to reduce flavor-flavor conditioning by sucrose in sham-feeding rats and flavor-nutrient conditioning in rats receiving intragastric sucrose infusions. In contrast, dopamine antagonists reduce both sucrose- and fructose-conditioned flavor preferences, which indicates the sensitivity of these conditioning paradigms to neuropharmacological manipulations. These data indicate that the endogenous opioid system, unlike the dopamine system, does not play a major role in either the acquisition or expression of flavor preference learning as measured in two-bottle choice tests.

Dopamine D1 and D2 antagonists reduce the acquisition and expression of flavor-preferences conditioned by fructose in rats

The effects of dopamine (DA) D(1) and D(2) receptor antagonists on the acquisition and expression of flavor-preferences conditioned by the sweet taste of fructose were examined. Food-restricted rats were trained over eight alternating one-bottle sessions to drink an 8% fructose solution containing one novel flavor (CS+) and a less preferred 0.2% saccharin solution containing a different flavor (CS-). Three groups of rats were treated daily with either vehicle (control group), SCH23390 (200 nmol/kg; D(1) group), or raclopride (200 nmol/kg; D(2) group) during training. Additional groups of vehicle-treated rats had their daily training intakes matched to that of the D(1) and D(2) groups. Preferences were assessed in two-bottle tests with the CS+ and CS- flavors presented in 0.2% saccharin solutions following doses of 0, 50, 200, 400, or 800 nmol/kg of either D(1) or D(2) antagonists. The D(1) and D(2) groups, unlike the control and yoked-control groups, failed to display a significant CS+ preference in the two-bottle tests following vehicle treatment. In addition, treatment with SCH23390 prior to the two-bottle tests blocked the expression of the CS+ preference in the control groups. Pretest raclopride treatment attenuated the CS+ preference at some dose levels. Raclopride also attenuated the preference for fructose in rats given two-bottle training with the CS+/fructose (CS+/F) and CS-/saccharin (CS-/S) solutions. These findings indicate that D(1) and D(2) antagonists block flavor-preference conditioning by sweet taste and that D(1), and to a lesser extent D(2), receptor antagonists attenuate the expression of a previously acquired preference.

Feeding induced by food deprivation is differentially reduced by G-protein alpha-subunit antisense probes in rats

Antisense oligodeoxynucleotide (AS ODN) probes directed against the alpha-subunit of different G-proteins have been used to differentiate feeding responses in rats elicited by different opioid agonists, including morphine, beta-endorphin and dynorphin. Furthermore, antisense probes directed against G(o)alpha, but not G(s)alpha, G(q)alpha or G(i)alpha, significantly reduced nocturnal feeding in rats. The present study examined whether food intake and weight changes elicited by 24 h of food deprivation were significantly altered by ventricular administration of antisense probes directed against either G(i)alpha(1), G(i)alpha(2), G(i)alpha(3), G(s)alpha, G(o)alpha, G(q)alpha or G(x/z)alpha as well as a control nonsense probe in rats. Deprivation-induced weight loss was significantly enhanced by antisense probes directed against G(s)alpha and G(x/z)alpha, whereas weight recovery 24 h following reintroduction of food was significantly reduced by antisense probes directed against G(i)alpha(2), G(q)alpha and G(o)alpha. Selective antisense probe effects were noted for deprivation-induced intake with G(s)alpha and G(q)alpha probes exerting the greatest reductions, G(x/z)alpha, G(i)alpha(2), and G(i)alpha(3) probes exerting lesser effects, and G(i)alpha(1) and G(o)alpha probes failing to affect deprivation-induced intake. Importantly, the nonsense control probe failed to alter deprivation-induced intake or weight. The reductions in deprivation-induced intake by AS ODN probes directed against G(s)alpha or G(q)alpha were not accompanied by any evidence of a conditioned taste aversion. These data indicate important distinctions between G-protein mediation of different effector signaling pathways mediating feeding responses elicited under natural (e.g. nocturnal feeding) and regulatory challenge (e.g. food deprivation) conditions.

Alterations in food intake elicited by GABA and opioid agonists and antagonists administered into the ventral tegmental area region of rats

Food intake is significantly increased following administration of mu-selective opioid agonists into the ventral tegmental area (VTA) region acting through multiple local opioid receptor subtypes. Since GABA receptor agonists in the VTA region are capable of eliciting feeding, the present study investigated whether feeding elicited by the mu-selective opioid agonist [D-Ala(2), NMe(4), Gly-ol(5)]-enkephalin (DAMGO) in the VTA region was altered by pretreatment into the same site with equimolar doses of either GABA(A) (bicuculline) or GABA(B) (saclofen) antagonists, and further, whether pretreatment with either general opioid or selective GABA receptor antagonists decreased feeding elicited by GABA(A) (muscimol) or GABA(B) (baclofen) agonists in the VTA region. DAMGO-induced feeding in the VTA region was dose-dependently decreased following pretreatment with either GABA(A) or GABA(B) antagonists in the absence of significant alterations in food intake by the antagonists per se. However, the presence of short-lived seizures following bicuculline in the VTA region suggests that this ingestive effect was caused by nonspecific actions. In contrast, GABA(B) receptors are involved in the full expression of mu-opioid agonist-induced feeding in this region since saclofen failed to elicit either seizure activity or a conditioned taste aversion. Pretreatment with naltrexone in the VTA region reduced intake elicited by baclofen, but not muscimol. Finally, baclofen-induced feeding was significantly reduced by saclofen, but not bicuculline, pretreatment in the VTA region. Therefore, possible coregulation between GABA(B) and opioid receptors in the VTA region, as suggested by immunocytochemical evidence, is supported by these behavioral effects upon ingestion.

Differential actions of dopamine receptor antagonism in rats upon food intake elicited by either mercaptoacetate or exposure to a palatable high-fat diet

Selective dopamine receptor antagonists have been shown to reduce food intake of rats under such regulatory challenge conditions as food deprivation and 2-deoxy-D-glucose-induced glucoprivation, and under such palatable conditions as acute exposure to sucrose solutions. Food intake is increased following either pretreatment with the free fatty acid oxidation inhibitor, mercaptoacetate (MA), or acute exposure to a palatable high-fat source. The present study examined whether equimolar doses (50-800 nmol/kg, s.c.) of either the selective D(1) receptor antagonist, SCH23390, or the selective D(2) receptor antagonist, raclopride, would alter food intake elicited by either MA (70 mg/kg, i.p.) or acute exposure to a high-fat diet (67% ground rat chow, 33% vegetable shortening). SCH23390 significantly and dose-dependently reduced MA-induced feeding with the two higher (400 and 800 nmol/kg) doses eliminating this response after the first 2 h and the two lower (50 and 200 nmol/kg) doses preventing the occurrence of significant MA-induced feeding. Raclopride eliminated MA-induced feeding at the highest dose, and produced dose-dependent reductions at lower doses. A different pattern of dopamine antagonist effects emerged for high-fat intake. The identical dose range of SCH23390 failed to alter high-fat intake. In contrast, whereas the highest (800 nmol/kg) dose of raclopride significantly reduced high-fat intake after 1 h, the middle (200 and 400 nmol/kg) doses of raclopride significantly increased high-fat intake after 2 h. These data are discussed in terms of the modulatory actions of dopamine upon food intake, of the differential actions of dopamine receptor subtypes upon intake under challenge and palatable conditions, and of the potential participation of presynaptic and postsynaptic receptor populations in these responses.

D1 but not D2 dopamine receptor antagonism blocks the acquisition of a flavor preference conditioned by intragastric carbohydrate infusions

The effects of dopamine D1 (SCH23390) and D2 (raclopride) receptor antagonists on the acquisition and expressions of flavor preferences conditioned by the postingestive actions of sucrose were investigated. Food-restricted rats were trained in one-bottle sessions to associate one flavored saccharin solution (CS+) with intragastric (i.g.) infusions of 16% sucrose, and another flavored saccharin solution (CS-) with water infusions. Flavor preferences were then measured in two-bottle tests. In Experiment 1A, rats that received the D2 antagonist (raclopride, 200 nmol/kg; RAC group) throughout training consumed less CS+ and CS- than did saline-treated Control rats; a saline-treated Yoked group had its intake limited to that of the RAC group. All three groups displayed CS+ preferences during two-bottle tests when treated with saline or raclopride, except at doses that greatly suppressed intake. Experiment 1B obtained similar results with rats treated with 400 nmol/kg raclopride throughout training. In Experiment 2, rats that received the D1 antagonist (SCH23390, 200 nmol/kg; SCH group) throughout training consumed less CS+ and CS- than did saline-treated Control rats; a saline-treated Yoked group had its intake limited to that of the SCH group. Unlike the Control and Yoked groups, the SCH group failed to prefer the CS+ to the CS- in two bottle tests. SCH23390 treatment during two-bottle testing did not block CS+ preference in the Control or Yoked groups, except at doses that greatly suppressed intake. We conclude that D1, but not D2, dopamine receptors are critically involved in the acquisition of a sucrose-conditioned flavor preference, and both receptor subtypes have a more limited role in the expression of this preference.

Role of D(1) and D(2) dopamine receptors in the acquisition and expression of flavor-preference conditioning in sham-feeding rats

The present study examined the effects of D(1) and D(2) antagonists on flavor-preference conditioning by the sweet taste of sucrose. All sessions were conducted under sham-feeding conditions to minimize post-ingestive influences. The rats were trained in alternating, one-bottle sessions to sham-feed a 16% sucrose solution containing one novel flavor (CS+) and a less-preferred 0.2% saccharin solution containing a different flavor (CS-). Three groups of food-restricted rats were treated with either vehicle (control group), the D(1) antagonist, SCH23390 (200 nmol/kg), or the D(2) antagonist, raclopride (200 nmol/kg) during one-bottle training. A fourth group (yoked group) was vehicle-treated and its training intakes were matched to that of the D(1) and D(2) drug groups. Preferences were assessed in two-bottle tests with the CS+ and CS- flavors presented in mixed 8% sucrose+0.1% saccharin solutions following systemic doses of 0, 200, or 800 nmol/kg of either the D(1) or D(2) antagonists. All groups significantly preferred the CS+ flavor in vehicle tests, although the preferences were weaker in the D(1), D(2), and yoked groups compared to the control group. All groups selectively reduced their CS+ intakes when treated with either D(1) or D(2) antagonists during two-bottle testing, and the CS+ preference was blocked at the higher doses. These data show that D(1) and D(2) receptor antagonists block the expression of a sucrose-conditioned preference, but produces substantially lesser effects upon the acquisition of this form of flavor conditioning.