Individual differences in the feeding effects of amphetamine: role of nucleus accumbens dopamine and circadian factors

Consumption of palatable food decreases the anorectic effects of serotonergic, but not dopaminergic drugs in baboons

We examined the effects of periodic access to a palatable, high sugar content food (candy) in 8 male baboons on the anorectic response to d-amphetamine, which increases dopamine, and dexfenfluramine, which increases serotonin. During candy access, up to 200 candies containing 75% of energy as sugar were available during the morning on Mondays, Wednesdays and Fridays; food pellets (19% of energy as sugar) were available in the afternoon and throughout the remaining days of the week. During candy access, baboons consumed a mean of 177 pieces of candy containing 696 kcal (2.91 MJ) in the morning compared to 44 food pellets and 150 kcal (0.63 MJ) in the morning on non-candy days. Food pellet intake was lower during candy access. Complete dose-response functions for the effects of the drugs on food pellet intake on days that candy was not available were determined before, during, and after the period of access to candy. Dexfenfluramine and amphetamine produced dose-dependent decreases in food pellet intake and increases in latency to eat food pellets before, during, and after candy access. During access to candy, the dose-response function for dexfenfluramine was shifted to the right indicating the development of tolerance, while that for amphetamine was shifted to the left indicating sensitization. Only the dose-response function for dexfenfluramine returned to baseline after candy access suggesting that the difference was specific to concurrent palatable food consumption. We hypothesize that tolerance to the effects of dexfenfluramine reflects a decrease in the satiating effect of serotonin release due to repeatedly eating large amounts of palatable food.

Effects of an adenosine A2A receptor blockade in the nucleus accumbens on locomotion, feeding, and prepulse inhibition in rats

The nucleus accumbens (NAc) subserves behaviors governed by natural rewards, i.e., feeding or exploration, and has been implicated in control of prepulse inhibition (PPI), a measure of sensorimotor gating. The present study sought to determine whether a tonic stimulation of adenosine A(2A) receptors in the rat NAc is involved in control of spontaneous locomotor activity, feeding behavior, and PPI. To this end, bilateral microinfusions of a prodrug (MSX-3) (3 microg and 5 microg in 1 microl per side) of the selective A(2A) receptor antagonist MSX-2 or vehicle (1 microl per side) were administered into the NAc. Results show that blockade of intra-NAc adenosine A(2A) receptors by a high (5 microg), but not by a low (3 microg), dose of MSX-3 increased locomotor activity in an open field, reduced food intake, and delayed intake onset in food-deprived rats examined in a test cage with standard laboratory chow. Furthermore, PPI was significantly disrupted after intra-NAc infusion of 5 microg, but not 3 microg, MSX-3. These findings suggest that locomotor activity as well as intact PPI and feeding behavior rely on tonic activation of intra-NAc A(2A) receptors. The data add further support to the view that adenosine is a tonically active modulator of striatal function through actions on A(2A) receptors.

Lead exposure and dorsomedial striatum mediation of fixed interval schedule-controlled behavior

Prior studies demonstrate a critical role for mesolimbic dopamine systems, particularly nucleus accumbens, in the mediation of fixed interval (FI) schedule-controlled behavior and an enhancement of nucleus accumbens dopamine activity as a mechanism of chronic postweaning lead (Pb)-induced increases in Fl response rates. Since dorsomedial striatum, like nucleus accumbens, receives limbic input, it could also conceivably contribute to Pb-related effects on FI performance. Therefore, changes in FI schedule-controlled behavior were examined following administration of dopamine or the non-specific irreversible dopamine antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) into dorsomedial striatum of rats exposed from weaning to 0, 50 or 500 ppm Pb acetate drinking solutions. The 500 ppm exposure increased baseline FI response rates relative to both 0 and 50 ppm. Intra-dorsomedial striatum EEDQ and dopamine had no effects when examined across all animals. However, both compounds produced rate-dependent effects, i.e. increases or decreases in rate in different subjects, depending upon baseline Fl overall rates. The rate-increasing effects of intra-dorsomedial striatum dopamine actually mimicked Pb effects, increasing Fl overall and run rates and shortening postreinforcement pause times. Further, Pb exposure modulated effects of dopamine and EEDQ in dorsomedial striatum. While these collective findings conceivably suggest dorsomedial striatum as another potential site through which postweaning Pb exposure influences FI performance, this possibility is not supported by other studies that show that chronic postweaning Pb alters dopamine binding sites and evoked dopamine release in nucleus accumbens but not in dorsomedial striatum even over a year exposure period. Thus, while both regions may play a role in mediating Fl performance under normal conditions, it appears that alterations in nucleus accumbens dopamine activity may be sufficient to induce chronic postweaning Pb-induced increases in FI response rates.

Analysis of dopamine receptor antagonism upon feeding elicited by mu and delta opioid agonists in the shell region of the nucleus accumbens

The nucleus accumbens (NAcc) has been implicated as an important reward site for the mediation of unconditioned reinforcers such as food. Although both mu-selective and delta-selective opioid agonists in the NAcc induce spontaneous and palatable feeding, these effects are mediated by multiple opioid receptor subtypes within the nucleus. A role for dopaminergic mediation of feeding in the NAcc is based upon selective antagonist-induced suppression of feeding induced by systemic amphetamine. The present study investigated whether feeding elicited by infusion of either mu ([D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin) or delta(2) ([D-Ala(2), Glu(4)]-deltorphin) opioid receptor subtype agonists in the shell region of the NAcc would be modified by intra-accumbens pretreatment with equimolar (12-100 nmol) doses of either D(1)-selective (SCH23390) or D(2)-selective (raclopride) antagonists. Both opioid agonists displayed comparable magnitudes and durations of feeding responses in the NAcc. SCH23390 significantly and dose-dependently reduced mu agonist-induced feeding in the NAcc with significant reductions noted following the two higher, but not two lower doses. In contrast, raclopride pretreatment produced inconsistent effects upon mu agonist-induced feeding with limited actions across doses and test times. Further, neither SCH23390 nor raclopride pretreatment in the NAcc affected feeding elicited by the delta(2) opioid agonist. These data indicate that the role of dopamine receptors in mediating opioid-induced feeding within the shell region of the NAcc is both dependent upon the dopamine receptor subtype that was blocked (D(1) vs. D(2)) as well as the opioid receptor subtype which was being stimulated mu vs. delta(2)).

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.

Specific changes in food intake elicited by blockade or activation of glutamate receptors in the nucleus accumbens shell

Blockade of non-N-methyl-D-aspartic acid (NMDA) ionotropic glutamate receptors in the nucleus accumbens shell (AcbSh) with 6,7-dinitroquinoxaline-2,3-dione (DNQX) elicits intense feeding in satiated rats. In order to determine whether or not this feeding is part of a general behavioral activation, we observed the effect of intra-AcbSh DNQX injections on intake of solid food, liquid food, and water, and on gnawing behavior. In addition, we investigated the possibility that activation of a subset of these receptors with (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) could suppress feeding. DNQX significantly increased intake of solid and liquid food, but did not significantly affect water intake or gnawing behavior. Furthermore, injections of AMPA into the AcbSh suppressed deprivation-induced feeding and intake of a palatable 5% sucrose solution without affecting water intake in water-deprived rats. Taken together, these data suggest that DNQX is acting on a system specifically involved with the regulation of food intake.

Mesolimbic dopaminergic mechanisms underlying individual differences in sugar consumption and amphetamine hyperlocomotion in Wistar rats

Individual differences within strains of rats have been demonstrated for dopamine-mediated behaviours and responses to dopaminergic drugs. Differences in mesolimbic dopamine function may underlie individual differences in some of these behaviours, including sugar consumption and amphetamine hyperlocomotion. The present study addressed two potential mechanisms for these differences in dopamine-mediated behaviours. The possibility of functional differences in dopamine receptor subtypes was tested in LOW and HIGH sugar feeders. LOW and HIGH feeders did not differ in their response to the partial D1 agonist SKF-38393. The highest dose (2.5 mg/kg) of the D2 agonist quinpirole stimulated locomotor activity to a greater degree in a subset of HIGH sugar feeders as compared with LOW feeders. All doses of amphetamine induced a greater locomotor response in HIGH feeders as compared with LOW feeders, and HIGH feeders exhibited higher levels of extracellular dopamine in the nucleus accumbens than LOW feeders following exposure to sugar and treatment with amphetamine. These results support the interpretation that LOW and HIGH feeders exhibit differences in presynaptic nucleus accumbens dopamine function that account for the expression of individual differences in sugar consumption and response to amphetamine treatments. A subset of HIGH feeders may also exhibit greater D2 receptor function.

Individual differences in the feeding and locomotor stimulatory effects of acute and repeated morphine treatments

Rats with high propensity to ingest sugar (HIGH) show increased responsiveness to amphetamine treatments than rats with low propensity to ingest sugar (LOW). Intrinsic variation in the functioning of the mesolimbic dopamine system has been suggested to account for these individual differences. Morphine has stimulatory effects on feeding and locomotion that are in part mediated by the mesolimbic dopamine system. The present study therefore examined whether LOW and HIGH rats would exhibit differences in the feeding and locomotor stimulating effects of morphine. Morphine (1, 2, and 4 mg/kg) significantly stimulated the intake of chow and sugar in LOW rats without affecting food consumption in HIGH rats. Further, it was found that both groups of rats did most of their feeding in the first 20 min following injection, and that the stimulatory effect of morphine in LOW rats was restricted to the first hour of the 3-h test session. Repeated morphine (2 mg/kg) stimulated sugar intake in LOW but not HIGH rats, and there was no evidence of increased intake across injections. Acute administration of 5.0 mg/kg, but not 2.0 mg/kg, of morphine produced higher levels of locomotor activity in LOW rats compared to HIGH rats; repeated treatment with 5.0 mg/kg morphine produced a sensitized locomotor response in both LOW and HIGH rats. These results indicate that LOW rats exhibit increased responsiveness to the locomotor and feeding stimulatory effects of morphine compared to HIGH rats. One implication arising from these findings is that LOW and HIGH rats may be distinguished by differences in opiatergic function, as well as by differences in dopaminergic function.

Neuropeptide Y-induced operant responding for sucrose is not mediated by dopamine

Neuropeptide Y (NPY) induces feeding in previously satiated animals after injection into the hypothalamus, especially the perifornical region (PFH). NPY also appears to have rewarding properties as evidenced by its ability to produce a conditioned place preference following injection into the nucleus accumbens (N.Acc), an effect ostensibly mediated by mesolimbic dopamine (DA). Since the progressive-ratio (PR) operant schedule has also been used to assess an organism's motivation to respond for rewarding stimuli, we tested the possibility that NPY increases PR responding for sucrose pellets. Adult male rats were injected with NPY (0-235 pmol) bilaterally through cannulae aimed to terminate in the PFH. This produced a dose-dependent increase in the total number of responses made and the number of reinforcers earned. The DA receptor blocker, alpha-flupenthixol (FLU)(0-0.2 mg/kg intraperitoneally), attenuated both NPY(156 pmol)-induced and drug-free PR responding while having no effect on NPY(156 pmol)-induced free-feeding. FLU injected directly into the N.Acc (0-5 microg) also failed to reduce sucrose free-feeding. These results suggest that distinct reward mechanisms are activated during PFH NPY-induced feeding vs. PR responding, since FLU disrupted the latter but not the former.

Dopaminergic and opiate agonists and antagonists differentially decrease multiple schedule responding maintained by sucrose/ethanol and sucrose

Similar neurobiological mechanisms are hypothesized to influence ethanol- and food-related reinforcement processes. This study examined the ability of compounds with dopaminergic or opiate activity to selectively alter responding maintained by a sucrose/ethanol solution in comparison to a sucrose solution. Long-Evans rats were trained to press a lever using 5% sucrose/10% ethanol and 5% sucrose as the reinforcers on a multiple Fixed Ratio 4 Fixed Ratio 4 schedule of reinforcement. When stable responding was established, the effects of intraperitoneally administered amphetamine (0.0-3.0 mg/kg), haloperidol (0.0-1.0 mg/kg), morphine (0.0-10.0 mg/kg), and naloxone (0.0-10.0 mg/kg) were examined on total session reinforcer presentation and presentation of each reinforcer within individual multiple schedule components. Prior to drug treatment, the total number of reinforcer presentations of the sucrose/ethanol solution was significantly greater than sucrose reinforcer presentations, suggesting the sucrose/ethanol solution was a more efficacious reinforcer. All agents administered decreased responding maintained by sucrose/ethanol and sucrose. The dose-effect curves for sucrose/ethanol were shifted to the left compared to sucrose, suggesting that although the compounds did not selectively impact sucrose/ethanol-maintained responding, sucrose/ethanol-maintained responding was more sensitive to the effects of these compounds.

Individual differences in sugar consumption following systemic or intraaccumbens administration of low doses of amphetamine in nondeprived rats

Rats exhibit individual differences in their propensity to ingest sucrose and in their feeding response to low doses of amphetamine (AMP). Rats with high baseline sugar intake (HIGH) show a decrease in sugar consumption in response to AMP, while rats with low baseline sugar intake (LOW) show an increase in consumption (33,34). At present, it is not known whether LOW and HIGH rats would be differentially responsive to higher, anorexigenic doses of AMP. Thus, the present study was developed to further determine the dose-response curve for AMP's effects on sugar consumption in LOW and HIGH rats. One group of rats received IP injections of 0.05, 0.1, and 0.2 mg/kg AMP, while a second group was administered 0.25, 0.5, and 1.0 mg/kg AMP. A third group of rats received intranucleus accumbens (Acc) microinjections of low to moderate (2.0-8.0 micrograms) doses of AMP, because evidence indicates that this may be an important site of action for AMP's effects on feeding in LOW and HIGH rats. Results showed that at low doses (< or = 0.25 mg/kg), AMP stimulated sugar consumption in LOW rats and either had no effect or inhibited consumption in HIGH rats. At doses greater than 0.25 mg/kg, AMP inhibited sugar consumption in both LOW and HIGH rats. Furthermore, intra-Acc administration of AMP stimulated sugar intake in LOW rats and produced a slight, but nonsignificant, decrease in consumption in HIGH rats. Taken together, these results show that LOW and HIGH rats exhibit individual differences in their feeding response to low but not moderate to high doses of AMP. Furthermore, the evidence indicates that the Ace is an important site for AMP's facilitatory effect on sugar consumption in LOW rats.

Individual differences in the feeding response to CCKB antagonists: role of the nucleus accumbens

Cholecystokinin (CCK) decreases food intake in a variety of species when administered systemically or centrally. Moreover, both CCKA and CCKB receptor mechanisms have been implicated in CCK's effects on feeding. Previous work done in our laboratory has shown that rats exhibit significant individual differences in the consumption of sugar. Moreover, intra-nucleus accumbens (Acc) administration of CCK reduced sugar consumption in rats with high baseline sugar intake (High) but did not affect sugar consumption in rats with low baseline sugar intake (Low). Thus, CCK mechanisms may contribute to individual differences in sugar intake observed in rats. The present study examined the involvement of endogenous CCK mechanisms in the regulation of sugar intake in Low and High rats. In Experiment 1, male Wistar rats were administered either the CCKA antagonist devazepide (0.001, 0.01, 0.1 mg/kg) or the CCKB antagonist L,365-260 (0.01, 0.1, 0.5 mg/kg) IP, and their intake of sugar and powdered lab chow recorded for 1 h. Experiment 2 was identical to Experiment 1 with the exception that rats received intra-Acc administrations of the selective CCKB antagonist PD-135158 (3, 10, 30 micrograms). Results showed that blockade of CCKB, but not CCKA receptors produced an increase in sugar consumption in Low rats and a decrease in sugar consumption in High rats. These effects were obtained with both systemic and intra-Acc administrations of a selective CCKB antagonist. These results suggest that endogenous CCK contributes to the mechanism regulating sugar consumption in Low and High rats through its actions on CCKB receptors in the Acc.

Increased potency and binding of mazindol to putative brain anorectic receptors in obesity-prone rats

A class of sodium-sensitive, low affinity binding sites in the brain recognizes [3H]mazindol (MAZ). Competition for [3H]MAZ binding at these sites correlates with the anorectic potency of various phenethylamine drugs suggesting that these might be anorectic binding sites. Here [3H]MAZ binding, in the absence of sodium, was assessed by quantitative receptor autoradiography in rat brain. Binding was saturable, widespread and heterogenous with Kd = 3-229 microM and Bmax = 0.64-21.9 nmol/mg protein in various brain areas. By saturation studies, highest binding was in the somatosensory cortex, central amygdalar nucleus and bed nucleus of the stria terminalis. Hypothalamic subnuclei had intermediate and the piriform cortex had low binding. Rats were identified as prone to develop (DIO-prone) or resist (DR-prone) diet-induced obesity by their low vs. high 24 h urine norepinephrine excretion, respectively. While similar in body weight and basal 30 min intake of 4% sucrose, DIO-prone rats had 28% greater inhibition of sucrose intake by 3 mg/kg MAZ, i.p. (86 +/- 5%) than DR-prone rats (67 +/- 6%; P = 0.05). DIO-prone rats also had 23-55% higher levels of 10 nM [3H]MAZ binding in various hypothalamic and amygdalar nuclei, the somatosensory, piriform and gustatory cortices and thalamus. Given their greater sensitivity the highest dose of MAZ used and their higher binding of MAZ to putative brain anorectic receptors, DIO-prone rats might have a deficiency of an endogenous satiety factor which could predispose them to develop obesity when challenged with high energy, high sucrose diets.

Individual differences in sugar intake predict the locomotor response to acute and repeated amphetamine administration

Rats exhibit profound individual differences in their propensity to ingest sugar and in their locomotor response to AMP. Intrinsic variation in the responsiveness of mesolimbic dopamine mechanisms has been suggested to account for these individual differences. In light of this overlap, it might be expected that individual differences in one behavior would predict individual differences in the other. The present study determined whether individual differences in sugar intake would predict individual differences in the locomotor response to AMP. Male Wistar rats were divided into low and high feeders based on a median split of their sugar intake in response to saline administration and were subsequently tested for their locomotor response to either 1.0 or 1.75 mg/kg AMP in experiment 1. High sugar feeders exhibited significantly more locomotion than low sugar feeders in response to 1.75 mg/kg AMP. This difference was observed immediately after injection and continued for approximately 90 min. There was no difference between the two groups in their locomotor response to 1.0 mg/kg AMP. In experiment 2, rats receiving 1.0 mg/kg AMP in experiment 1 were tested for the development of behavioral sensitization with repeated AMP administrations. Rats were administered 1.0 mg/kg AMP across 5 test days, interspersed with days in which they received AMP treatment in their home cages to minimize conditioning effects. High sugar feeders exhibited greater behavioral sensitization than low sugar feeders with repeated AMP administration. Starting on test day 3, high sugar feeders exhibited significantly greater AMP-induced locomotor activity than low sugar feeders.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the contribution of CCKB receptor mechanisms to individual differences in amphetamine-induced locomotion

Recent evidence shows that rats exhibit individual differences in their locomotor response to amphetamine (AMP). Moreover, evidence has accumulated showing that high-AMP responders exhibit more mesolimbic dopaminergic (DAergic) activation in response to AMP treatment than low-AMP responders. Cholecystokinin (CCK) is a peptide that is colocalised with mesolimbic DA and exerts complex modulatory actions on DA function. Two CCK receptor subtypes have been identified and selective antagonists have been developed. To examine the possible contribution of endogenous CCK mechanisms to individual differences in responsivity to AMP treatment, male Wistar rats were divided into low- and high-AMP responders based on a median split of their locomotor response to AMP and the effects of the selective CCK antagonists L365-260 (CCKB; 0.01, 0.1, 0.5 mg/kg; n = 16) and devazepide (CCKA; 0.001, 0.01, 0.1 mg/kg; n = 23) were determined. Results showed that L365-260 (0.1 mg/kg) potentiated AMP-induced hyperactivity in low-AMP responders but did not affect AMP-induced hyperactivity in high-AMP responders. Devazepide was without effect in both groups of animals. This pattern of results suggests that CCKB, but not CCKA, receptor mechanisms contribute to interindividual variation in responsivity to AMP.

Nucleus accumbens dopamine-CCK interactions in psychostimulant reward and related behaviors

The present paper provides an overview of dopamine (DA) and cholecystokinin (CCK) mechanisms that contribute to psychostimulant reward-related behaviors. Three different behavioral paradigms are focused on: intravenous psychostimulant reward, psychostimulant-induced locomotor activity, and psychostimulant-induced feeding. Based on evidence derived from these different behavioral paradigms, this paper reviews data indicating that nucleus accumbens CCK and DA play an important role in mediating reward. Neurobiological mechanisms are proposed to explain the functional relationship between CCK and DA in the nucleus accumbens.