Food cravings, appetite, and snack-food consumption in response to a psychomotor stimulant drug: the moderating effect of "food-addiction"

There is mounting evidence that many highly processed foods have addictive properties, and that some cases of compulsive overeating resemble an addiction disorder. While support for the Yale Food Addiction Scale (YFAS) as a valid diagnostic tool has been impressive and continues to increase, to date, no research has examined the food-addiction construct in response to an actual food stimulus, and in relation to direct measures of appetite and food consumption. As part of a larger community-based study of overeating in healthy adults who were predominately overweight and obese (aged 25-50 years), 136 participants completed the YFAS, of whom 23 met the diagnostic criteria for food-addiction. They took part in a 2-day, double-blind, cross-over, single-dose drug challenge using a psychomotor stimulant (methylphenidate) and placebo. Participants were first assessed on ratings of appetite and food cravings after holding and tasting their favorite snack food, after which they were able to eat all or part of the snack, as they wished. Three separate repeated-measures analysis-of-variance procedures were carried out, each with two between-subjects factors (Diagnosis: food-addiction vs. non-food addiction) and (Sex: male vs. female) and 1 within-subjects factor (Days: drug vs. placebo). As anticipated, for all three dependent variables, there was a significant main effect for Days with a response decrease from placebo to the drug condition. With respect to food cravings and appetite ratings, results indicated that the food addiction group had significantly higher scores on both variables. For food consumption, there was a significant Days × Diagnosis interaction whereby the food-addiction group showed no food-intake suppression across days compared to the non-food-addiction group who demonstrated a significant decrease in snack-food consumption with methylphenidate. The finding that the food-addiction group was resistant to the food-intake suppression typically induced by a dopamine agonist supports evidence of dopamine signaling-strength differences in individuals with compulsive overeating compared to those without this disorder. This represents the first demonstration that individuals defined by their food-addiction status have a unique pattern of food-intake following a pharmacologic challenge with such agents.

Habituation of reinforcer effectiveness

In this paper we propose an integrative model of habituation of reinforcer effectiveness (HRE) that links behavioral- and neural-based explanations of reinforcement. We argue that HRE is a fundamental property of reinforcing stimuli. Most reinforcement models implicitly suggest that the effectiveness of a reinforcer is stable across repeated presentations. In contrast, an HRE approach predicts decreased effectiveness due to repeated presentation. We argue that repeated presentation of reinforcing stimuli decreases their effectiveness and that these decreases are described by the behavioral characteristics of habituation (McSweeney and Murphy, 2009; Rankin etal., 2009). We describe a neural model that postulates a positive association between dopamine neurotransmission and HRE. We present evidence that stimulant drugs, which artificially increase dopamine neurotransmission, disrupt (slow) normally occurring HRE and also provide evidence that stimulant drugs have differential effects on operant responding maintained by reinforcers with rapid vs. slow HRE rates. We hypothesize that abnormal HRE due to genetic and/or environmental factors may underlie some behavioral disorders. For example, recent research indicates that slow-HRE is predictive of obesity. In contrast ADHD may reflect “accelerated-HRE.” Consideration of HRE is important for the development of effective reinforcement-based treatments. Finally, we point out that most of the reinforcing stimuli that regulate daily behavior are non-consumable environmental/social reinforcers which have rapid-HRE. The almost exclusive use of consumable reinforcers with slow-HRE in pre-clinical studies with animals may have caused the importance of HRE to be overlooked. Further study of reinforcing stimuli with rapid-HRE is needed in order to understand how habituation and reinforcement interact and regulate behavior.

Effects of novelty and methamphetamine on conditioned and sensory reinforcement

BACKGROUND

Light onset can be both a sensory reinforcer (SR) with intrinsic reinforcing properties, and a conditioned reinforcer (CR) which predicts a biologically important reinforcer. Stimulant drugs, such as methamphetamine (METH), may increase the reinforcing effectiveness of CRs by enhancing the predictive properties of the CR. In contrast, METH-induced increases in the reinforcing effectiveness of SRs, are mediated by the immediate sensory consequences of the light.

METHODS

The effects of novelty (on SRs) and METH (on both CRs and SRs) were tested. Experiment 1: rats were pre-exposed to 5 s light and water pairings presented according to a variable-time (VT) 2 min schedule or unpaired water and light presented according to independent, concurrent VT 2 min schedules. Experiment 2: rats were pre-exposed to 5 s light presented according to a VT 2 min schedule, or no stimuli. In both experiments, the pre-exposure phase was followed by a test phase in which 5 s light onset was made response-contingent on a variable-interval (VI) 2 min schedule and the effects of METH (0.5 mg/kg) were determined.

RESULTS

Novel light onset was a more effective reinforcer than familiar light onset. METH increased the absolute rate of responding without increasing the relative frequency of responding for both CRs and SRs.

CONCLUSION

Novelty plays a role in determining the reinforcing effectiveness of SRs. The results are consistent with the interpretation that METH-induced increases in reinforcer effectiveness of CRs and SRs may be mediated by immediate sensory consequences, rather than prediction.

Caffeine choice prospectively predicts positive subjective effects of caffeine and d-amphetamine

BACKGROUND

Individuals vary in their subjective and behavioral response to psychomotor stimulants and these differences may be associated with the likelihood of developing problematic use of these drugs. The present study sought to determine whether individual differences in caffeine choice prospectively predict subjective response to acute doses of caffeine and d-amphetamine.

METHODS

In Phase 1, Choosers and Nonchoosers of caffeine were identified using 10 independent choice trials in which subjects repeatedly chose between caffeine (200mg/70 kg) and placebo. Choosers were defined as those who chose caffeine over placebo on ≥7 of the 10 trials; Nonchoosers were those who chose placebo on ≥7 trials. In Phase 2, Choosers and Nonchoosers were compared in their subjective response to caffeine (100, 200, 400mg/70 kg) and d-amphetamine (5, 10, 20 mg/70 kg).

RESULTS

Of the 22 participants completing the study, 11 met criteria for being a caffeine Chooser and 8 were Nonchoosers. In Phase 1, Choosers reported higher ratings of positive (i.e., pleasant) and lower ratings of negative (i.e., unpleasant) effects of caffeine during the sampling sessions. In Phase 2, caffeine Choosers reported more positive subjective effects and fewer negative effects of caffeine and d-amphetamine, particularly at the highest doses examined.

CONCLUSIONS

Individual differences in caffeine reinforcement predicted subsequent subjective response to both d-amphetamine and caffeine. This observation may have clinical utility for identifying individuals who are vulnerable to the reinforcing effects of abused psychomotor stimulants.

Faster onset and dopamine transporter selectivity predict stimulant and reinforcing effects of cocaine analogs in squirrel monkeys

Although the behavioral-stimulant and reinforcing effects of cocaine and related psychomotor stimulants have been attributed to their actions at the dopamine transporter (DAT), the reinforcing effectiveness of these compounds varies. The properties that confer these differences are important considerations when developing agonist pharmacotherapies for the treatment of stimulant abuse. The present studies focused on the time course of action and pharmacological specificity of six 3-phenyltropane analogs of cocaine (RTI-112, RTI-126, RTI-150, RTI-171, RTI-177, and RTI-336) by observing their behavioral-stimulant, neurochemical, and reinforcing effects in squirrel monkeys. The faster-onset analogs (RTI-126, RTI-150, and RTI-336), and one of the slower-onset DAT selective analogs (RTI-177 and RTI-171) produced behavioral-stimulant effects, while the slower-onset nonselective analog RTI-112 did not. The time to the peak behavioral-stimulant effect and the peak caudate dopamine levels was strongly correlated, but the area under the curve of the time course of behavioral-stimulant effect and dopamine levels was not correlated. These results suggest that the rate of onset plays a more important role than duration of action in the stimulant effect of these analogs. In addition, the slower-onset nonselective analog (RTI-112) clearly did not exhibit any reinforcing effects while the faster-onset nonselective (RTI-126) and all the DAT-selective analogs showed robust reinforcing effects (RTI-150, and RTI-177) or showed trends towards reinforcing effects (RTI-336 and RTI-171). Hence, there was a general trend for compounds that had a faster onset and/or DAT selectivity to produce significant behavioral-stimulant and reinforcing effects.

Persistent cue-evoked activity of accumbens neurons after prolonged abstinence from self-administered cocaine

Persistent neural processing of information regarding drug-predictive environmental stimuli may be involved in motivating drug abusers to engage in drug seeking after abstinence. The addictive effects of various drugs depend on the mesocorticolimbic dopamine system innervating the nucleus accumbens. We used single-unit recording in rats to test whether accumbens neurons exhibit responses to a discriminative stimulus (SD) tone previously paired with cocaine availability during cocaine self-administration. Presentation of the tone after 3-4 weeks of abstinence resulted in a cue-induced relapse of drug seeking under extinction conditions. Accumbens neurons did not exhibit tone-evoked activity before cocaine self-administration training but exhibited significant SD tone-evoked activity during extinction. Under extinction conditions, shell neurons exhibited significantly greater activity evoked by the SD tone than that evoked by a neutral tone (i.e., never paired with reinforcement). In contrast, core neurons responded indiscriminately to presentations of the SD tone or the neutral tone. Accumbens shell neurons exhibited significantly greater SD tone-evoked activity than did accumbens core neurons. Although the onset of SD tone-evoked activity occurred well before the earliest movements commenced (150 msec), this activity often persisted beyond the onset of tone-evoked movements. These results indicate that accumbens shell neurons exhibit persistent processing of information regarding reward-related stimuli after prolonged drug abstinence. Moreover, the accumbens shell appears to be involved in discriminating the motivational value of reward-related associative stimuli, whereas the accumbens core does not.

Dissociation in effects of lesions of the nucleus accumbens core and shell on appetitive pavlovian approach behavior and the potentiation of conditioned reinforcement and locomotor activity by D-amphetamine

Dopamine release within the nucleus accumbens (NAcc) has been associated with both the rewarding and locomotor-stimulant effects of abused drugs. The functions of the NAcc core and shell were investigated in mediating amphetamine-potentiated conditioned reinforcement and locomotion. Rats were initially trained to associate a neutral stimulus (Pavlovian CS) with food reinforcement (US). After excitotoxic lesions that selectively destroyed either the NAcc core or shell, animals underwent additional CS-US training sessions and then were tested for the acquisition of a new instrumental response that produced the CS acting as a conditioned reinforcer (CR). Animals were infused intra-NAcc with D-amphetamine (0, 1, 3, 10, or 20 microg) before each session. Shell lesions affected neither Pavlovian nor instrumental conditioning but completely abolished the potentiative effect of intra-NAcc amphetamine on responding with CR. Core-lesioned animals were impaired during the Pavlovian retraining sessions but showed no deficit in the acquisition of responding with CR. However, the selectivity in stimulant-induced potentiation of the CR lever was reduced, as intra-NAcc amphetamine infusions dose-dependently increased responding on both the CR lever and a nonreinforced (control) lever. Shell lesions produced hypoactivity and attenuated amphetamine-induced activity. In contrast, core lesions resulted in hyperactivity and enhanced the locomotor-stimulating effect of amphetamine. These results indicate a functional dissociation of subregions of the NAcc; the shell is a critical site for stimulant effects underlying the enhancement of responding with CR and locomotion after intra-NAcc injections of amphetamine, whereas the core is implicated in mechanisms underlying the expression of CS-US associations.

Phasic firing time locked to cocaine self-infusion and locomotion: dissociable firing patterns of single nucleus accumbens neurons in the rat

The activity of single nucleus accumbens (NAcc) neurons of rats was extracellularly recorded during intravenous cocaine self-administration sessions (0.7 mg/kg per infusion, fixed ratio 1). We reported previously that NAcc neurons showed a change, usually a decrease, in firing rate during the first 1 min after the cocaine-reinforced lever press. This postpress change was followed by a progressive reversal of that change, which began within the first 2 min after the press and was not complete until the last 1 min before the next lever press (termed the change + progressive reversal firing pattern). In the present study we documented a regular pattern of locomotion that occurred in parallel with the change + progressive reversal firing pattern. This observation suggested that discharges time locked to locomotion may determine the change + progressive reversal firing pattern. However, 55% of the neurons failed to show firing time locked to locomotion that could have contributed to the change + progressive reversal firing pattern. Moreover, for all neurons, the change + progressive reversal firing pattern was apparent even if the calculation of firing rate excluded all periods of locomotion. The present data showed that the change + progressive reversal firing pattern is not solely attributable to phasic changes in firing time locked to the execution of locomotion. The change + progressive reversal firing pattern closely mirrors changes in drug level and dopamine overflow observed by previous researchers and may thus be a component of the neurophysiological mechanism by which drug level regulates drug-taking behavior during an ongoing self-administration session.

Phasic firing of single neurons in the rat nucleus accumbens correlated with the timing of intravenous cocaine self-administration

To examine potential neural mechanisms involved in cocaine self-administration, the activity of single neurons in the nucleus accumbens of rats was recorded during intravenous cocaine self-administration. Lever pressing was reinforced according to a fixed-ratio 1 schedule. On a time base comparable to the interinfusion interval, half the neurons exhibited phasic firing patterns time locked to the cocaine reinforced level press. For almost all neurons, this pattern consisted of a change in firing rate postpress, typically a decrease, followed by a reversal of that change. The postpress change was closely related to the lever press. Typically, it began within the first 0.2 min postpress and culminated within the first 1.0 min postpress. For a small portion of responsive neurons, the reversal of the postpress change was punctate and occurred within 1-3 min of either the last lever press or the next lever press so that firing was stable during much of the interinfusion interval. For the majority of neurons, the reversal was progressive; it began within 2 min after the previous level press, and it was not complete until the last 0.1-2.0 min before the next lever press. The duration of this progressive reversal, but not of the postpress change, was positively correlated with the interinfusion interval. Thus, in addition to exhibiting changes in firing related to the occurrence of self-infusion, the majority of neurons also exhibited progressive changes in firing related to the spacing of infusions. In a structure that has been shown to be necessary for cocaine self-administration, such a firing pattern is a likely neurophysiological component of the mechanism that transduces declining drug levels into increased drug-related appetitive behavior. It is, thus, a neural mechanism that may contribute to compulsive drug-maintained drug taking.

Cocaine: on-line analysis of an accumbens amine neural basis for psychomotor behavior

Dose-response studies on subcutaneous cocaine were done to ascertain its effects in nucleus accumbens in dopaminergic and serotonergic neuronal circuitry in the behaving rat with in vivo voltammetry. Simultaneously, and at each dose of cocaine, unconditioned psychomotor stimulant behavior induced by cocaine was studied in terms of multiple concurrent measures of spontaneous behavior and by activity pattern analysis, a study of spatial patterns of locomotion. Time course studies showed that the neurochemical effects of cocaine (10, 20, and 40 mg/kg SC) significantly (p less than 0.0001) increased accumbens synaptic concentrations of dopamine (DA) and concurrently and significantly (p less than 0.0001) decreased accumbens synaptic concentrations of serotonin (5-HT) in a dose response manner. Simultaneous behavioral time course studies showed that cocaine (10, 20, and 40 mg/kg SC) significantly (p less than 0.0001) increased ambulations (locomotor activity), fine movements (stereotypic movements of sniffing and grooming) and rearing behavior, while significantly decreasing agoraphobic behavior, as measured by a statistically significant increase in central ambulations (p less than 0.0001). The high dose of cocaine (40 mg/kg SC) significantly increased fine movements over those produced by the lower doses of cocaine (p less than 0.0002). One import of the findings is that the DA and 5-HT biogenic amine response occurs in a behavioral paradigm of psychomotor stimulation, which is a known measure of reinforcement. Another is that the biogenic amines DA and 5-HT are affected by cocaine in this reinforcement paradigm with exactly opposite directionality. Finally, acute cocaine administration is shown to produce a dose response inhibition of agoraphobia (fear), which is highly correlated (p = .983, p less than 0.01) with the opposing effects of cocaine on the accumbens biogenic amines, DA and 5-HT.