The dopamine anhedonia hypothesis: A pharmacological phrenology

Deep and beautiful. The reward prediction error hypothesis of dopamine

According to the reward-prediction error hypothesis (RPEH) of dopamine, the phasic activity of dopaminergic neurons in the midbrain signals a discrepancy between the predicted and currently experienced reward of a particular event. It can be claimed that this hypothesis is deep, elegant and beautiful, representing one of the largest successes of computational neuroscience. This paper examines this claim, making two contributions to existing literature. First, it draws a comprehensive historical account of the main steps that led to the formulation and subsequent success of the RPEH. Second, in light of this historical account, it explains in which sense the RPEH is explanatory and under which conditions it can be justifiably deemed deeper than the incentive salience hypothesis of dopamine, which is arguably the most prominent contemporary alternative to the RPEH.

Affective status in relation to impulsive, motor and motivational symptoms: personality, development and physical exercise

The contributions of impulsive and risk-taking behaviour in depressive and bipolar disorders, motivational and motor behaviours in anhedonic and substance addictive states, and the factors, particularly distress and trauma, underlying the development of neuropathology in affective status are described from clinical, epidemiological and laboratory perspectives. In order to distinguish one case factor for biopsychological substrates of health, an array of self-reported characteristics, e.g., positive or negative affect, stress or energy, optimism, etc., that may be predictive or counterpredictive for the propensity for physical exercise and activity were analysed using a linear regression in twelve different studies. Several individual characteristics were found to be markedly and significantly predictive of the exercise propensity, i.e., positive affect, energy, health-seeking behaviour and character, while optimism was of lesser, though significant, importance. Several individual characteristics were found to be significantly counterpredictive: expression of BDI- and HAD-depression, major sleep problems and lack/negligence of health-seeking behaviour. The consequences of physical activity and exercise for both affective well-being, cognitive mobility and neurogenesis is noted, particularly with regard to developmental assets for younger individuals. Affective disorder states may be studied through analyses of personal characteristics that unfold predispositions for symptoms-profiles and biomarkers derived from properties of dysfunction, such as impulsiveness, temperament dimensions, anhedonia and 'over-sensitivity', whether interpersonal or to reward.

Dopamine and reward: the anhedonia hypothesis 30 years on

The anhedonia hypothesis--that brain dopamine plays a critical role in the subjective pleasure associated with positive rewards--was intended to draw the attention of psychiatrists to the growing evidence that dopamine plays a critical role in the objective reinforcement and incentive motivation associated with food and water, brain stimulation reward, and psychomotor stimulant and opiate reward. The hypothesis called to attention the apparent paradox that neuroleptics, drugs used to treat a condition involving anhedonia (schizophrenia), attenuated in laboratory animals the positive reinforcement that we normally associate with pleasure. The hypothesis held only brief interest for psychiatrists, who pointed out that the animal studies reflected acute actions of neuroleptics whereas the treatment of schizophrenia appears to result from neuroadaptations to chronic neuroleptic administration, and that it is the positive symptoms of schizophrenia that neuroleptics alleviate, rather than the negative symptoms that include anhedonia. Perhaps for these reasons, the hypothesis has had minimal impact in the psychiatric literature. Despite its limited heuristic value for the understanding of schizophrenia, however, the anhedonia hypothesis has had major impact on biological theories of reinforcement, motivation, and addiction. Brain dopamine plays a very important role in reinforcement of response habits, conditioned preferences, and synaptic plasticity in cellular models of learning and memory. The notion that dopamine plays a dominant role in reinforcement is fundamental to the psychomotor stimulant theory of addiction, to most neuroadaptation theories of addiction, and to current theories of conditioned reinforcement and reward prediction. Properly understood, it is also fundamental to recent theories of incentive motivation.

Role of brain dopamine in food reward and reinforcement

The ability of food to establish and maintain response habits and conditioned preferences depends largely on the function of brain dopamine systems. While dopaminergic transmission in the nucleus accumbens appears sufficient for some forms of reward, the role of dopamine in food reward does not appear to be restricted to this region. Dopamine plays an important role in both the ability to energize feeding and to reinforce food-seeking behaviour; the role in energizing feeding is secondary to the prerequisite role in reinforcement. Dopaminergic activation is triggered by the auditory and visual as well as the tactile, olfactory, and gustatory stimuli of foods. While dopamine plays a central role in the feeding and food-seeking of normal animals, some food rewarded learning can be seen in genetically engineered dopamine-deficient mice.

Psychiatric treatments: an exciting new century

At the beginning of the 1900s, effective psychiatric treatments were almost non-existent (Freud's discovery of the unconscious and his psychoanalysis based on this perhaps being the noteworthy exception). In many parts of the world a person with unusual behaviour was thought to be possessed by evil spirits. These demons were exorcised by such techniques as prayer, incantation, magic and the use of purgatives concocted from herbs. If these interventions were unsuccessful, more extreme measures were taken to ensure that the body would be an unpleasant dwelling place for the evil spirit. Flogging, starving, burning, and causing the person to bleed profusely were frequent forms of intervention (Atkinson et al, 1998).

Dopamine functions in appetitive and defensive behaviours

The data reviewed here are compatible with the hypothesis that telencephalic dopamine activity is elicited by motivationally significant stimuli which in turn creates a neural state in which animals are more prepared to respond to significant stimuli in the environment. This analysis may be viewed as extensions of both the sensorimotor hypothesis, which depicts dopamine as potentiating the ability of stimuli to elicit responses (Clody and Carlton, 1980; Marshall et al., 1974; White, 1986) and of the incentive motivational hypothesis, which emphasizes the importance of dopamine in responding to stimuli that serve as signals of biologically significant events (Blackburn et al., 1989a; Crow, 1973; Mogenson and Phillips, 1976). In addition, we have sought to emphasize that not all responses are equally dependent upon the integrity of forebrain dopamine activity. Some responses, such as ingestion of standard foods by hungry animals, copulation, and escape, are relatively impervious to dopamine disruption. Further, once other behaviours, such as avoidance or appetitive operant responses, have been acquired, they can be maintained at an initially high rate despite perturbation of dopamine systems, although performance deteriorates with repeated testing. This analysis has emerged from the joint consideration of how both appetitive and defensive behaviours are influenced by dopamine antagonists, along with an examination of dopamine release during sequences of behaviour. The data reviewed suggest that dopamine is involved in fundamental psychological processes through which environmental stimuli come to exert control over certain aspects of behaviour. In the future, as knowledge in this field advances, there will have to be an integration of the literature on dopamine and motivation with the literature on dopamine and motor systems. We expect that dopamine release will be seen as a mechanism by which important environmental cues, of innate or learned significance, lead to a general enhancement of motor skeletal responses directed towards distal cues. We conclude with a caveat: Caution must be exercised when attempting to infer a general role of any neurotransmitter in motivated behaviour based on the study of a limited number of motivational systems. Although neurotransmitter pathways may figure prominently in the control of certain behaviours, it is incorrect to think of neurotransmitters as having a single role in behaviour. However, when comparative analyses reveal a common thread among different motivational systems, as is becoming apparent for the general role of mesotelencephalic dopamine pathways in behaviour, then the goal of generating coherent and comprehensive theory concerning a neurotransmitter's function in behaviour will begin to be realised.(ABSTRACT TRUNCATED AT 400 WORDS)

Attenuation of drinking sweetened water following calcium channel blockade

Recent reports cite results that both cocaine-induced conditioned place preference and activity stimulation are attenuated by pretreatment with the calcium channel blocker isradipine (ISR) in rats. By blocking voltage-dependent L-type calcium channels, ISR may regulate neural dopamine release that, in turn, decreases the putative rewarding effects mediated by dopaminergic mechanisms. It is known that nonfluid deprived rats avidly consume sweetened fluids; this suggests that the sweet taste is rewarding. Three experiments were conducted to determine the effects of ISR on drinking sweetened and nonflavored water. Experiment 1 was designed to test whether ISR would attenuate the intake of a palatable solution in a dose-dependent manner. To this end, ISR was administered both peripherally (3.0-30 mg/kg) and centrally (0.3-30 micrograms/rat) prior to a solution of saccharin and d-glucose (S + G) being made available to rats (15 min/day) and intake was recorded. ISR produced dose-dependent decreases (38%-81%) in S + G intake dependent on the route of administration. In Experiment 2, water intake was measured in 18 h water-deprived rats following ISR (10 mg/kg) administration as well as comparing S + G drinking. The effect of two ISR vehicles, dimethyl sulfoxide and Tween 80, upon fluid intake was also determined. ISR injection did not attenuate water intake in 18 h water-deprived rats and the choice of vehicle did not affect the ISR-induced attenuation of S + G drinking. In Experiment 3, a single dose (30 micrograms) of ICV administered ISR, that attenuated S + G intake by approximately 44%, did not attenuate water intake in 18 h water-deprived rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of nucleus accumbens amphetamine on lateral hypothalamic brain stimulation reward

The effects of amphetamine were examined in a brain stimulation reward paradigm in which response rate was measured across a range of stimulation frequencies. Both low (0.0625 and 0.125 mg/kg) and high (1.0, 2.0 and 4.0 mg/kg) doses of systemic amphetamine decreased the stimulation frequency needed to sustain low rates of responding; high doses decreased the maximal response rates that were sustained by the highest stimulation frequencies. Ipsilateral microinjections (2.5, 5.0 and 10.0 micrograms/0.5 microliter) of D-amphetamine sulfate into the nucleus accumbens also caused shifts to the left of the rate-frequency function; no central dose caused a change in the asymptotic response rate associated with high stimulation frequencies. Contralateral injections of D-amphetamine (10.0 micrograms) also shifted the rate-frequency functions to the left, but were much less potent. Ipsilateral injections of D-amphetamine into the caudate were also less potent, suggesting nucleus accumbens rather than more dorsal tissue as the site of this behavioral action. Also less potent were the effects of L-amphetamine, ruling out non-specific effects of pH, osmolarity and the like and also ruling out noradrenergic actions as explanations of the behavioral effects of the injections. These data suggest that nucleus accumbens is a site of amphetamine's reward-enhancing and threshold-lowering effects on brain stimulation reward.

Intracranial self-administration methodologies

Intracranial drug self-administration (ICSA) offers a relatively new approach for investigating the neurobiological mechanisms involved in brain reinforcement processes. Discrete brain regions responsible for the initiation of neuronal activity associated with the response-contingent delivery of a drug reinforcer can be identified using these procedures since the drug is infused directly into a specific brain locus. In the last decade, several papers have appeared in the literature reporting the self-administration of various substances into a number of brain regions. However, different laboratories often employ diverse methodological procedures to demonstrate ICSA, and this can lead to erroneous conclusions when comparing data from different investigations. This review presents a critical evaluation of the current status of research in this area and suggests behavioral as well as methodological guidelines for future investigations to follow.

Opposite effects of unilateral forebrain ablations on ipsilateral and contralateral hypothalamic self-stimulation

Unilateral ablations of frontal cortex, rostral striatum, nucleus accumbens, septal area and olfactory tubercle decreased ipsilateral hypothalamic self-stimulation; the same ablations had the opposite effect on contralateral self-stimulation. The ablations shifted the function relating response rate to stimulation frequency (rate-frequency function) to the right for ipsilateral self-stimulation and to the left for contralateral self-stimulation, suggesting a reduction and an augmentation, respectively, of the rewarding impact of the stimulation. The inhibition of ipsilateral self-stimulation was neither total nor permanent; 20-30% shifts in threshold were seen at first, but behavior returned to near-normal levels over a period of several weeks. In contrast, the augmentation of contralateral self-stimulation showed no significant change over the same period; in this case the 20-30% shifts in threshold were immediate and permanent. The degree of change in ipsilateral threshold was positively correlated with lesion size; the degree of change in the contralateral threshold was not. Ablations restricted to cortical tissue caused a lesser degree of augmentation of contralateral self-stimulation and had no effect on ipsilateral self-stimulation. The small effects of large ablations on ipsilateral self-stimulation confirm similar observations of Huston and Stellar and their co-workers and raise questions for current theories regarding the role of dopamine in brain stimulation reward. The facilitation of contralateral self-stimulation indicates that brain stimulation reward does not involve a completely lateralized mechanism.

The role of reward pathways in the development of drug dependence

In commenting on the discovery of "opiate" receptors, Goldstein (1976) said: "It seemed unlikely, a priori, that such highly stereospecific receptors should have been developed by nature to interact with alkaloids from the opium poppy" (p. 1081). Endogenous opioid peptides and opioid receptor systems have now been identified in invertebrates that are unlikely to have had ancestors exposed to opium poppies (Kavaliers et al., 1983; Kream et al., 1980; Leung and Stefano, 1984; Stefano et al., 1980). Moreover, endogenous opioids play a role in stress-induced feeding in the slug (Kavaliers and Hirst, 1986) just as they play a role in stress-induced feeding in rodents (Lowy et al., 1980; Morley and Levine, 1980). If we are to understand the actions of opiates and other drugs of abuse we must understand them in terms of their abilities to interact with neural systems that evolved in the service of primitive biological functions, long before any serious incidence of addiction itself. The most primitive axes of the biological substrates of behavior are the axes of approach and withdrawal. Addictive drugs appear to be able to activate the mechanisms of approach, which is termed "positive reinforcement" and to inhibit the mechanisms of withdrawal, which is termed "negative reinforcement." Anatomically distinct sets of pathways have evolved to serve these two forms of reward. Activation of the medial forebrain bundle and associated structures serves positive reinforcement and induces forward locomotion. Approach and forward locomotion are the unconditioned responses to positive reinforcing stimuli such as food and sex partners, and approach to environmental objects and positive reinforcement is induced by electrical stimulation of this structure. The locomotor stimulating effects and the positive reinforcing effects of opiates and psychomotor stimulants result from their activation of this mechanism; stimulants activate the mechanism at the level of dopaminergic synapses of the nucleus accumbens, frontal cortex, and perhaps other forebrain structures, while opiates activate the system at two points: at the level of the dopaminergic synapse and at the level of the afferents to the dopaminergic cell bodies. Ethanol, nicotine, caffeine and phencyclidine stimulate both locomotor activity and dopamine turnover, but their sites of interaction with reward pathways have not yet been identified. Benzodiazepines and barbiturates stimulate locomotor activity without stimulating dopamine turnover; they may interact with reward pathways at a synapse efferent to the dopaminergic link in the pathways.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of pimozide and naloxone on latency for hypothalamically induced eating

Latency to feed in response to lateral hypothalamic electrical stimulation was assessed at a variety of stimulation frequencies using moderate stimulation intensity; shorter latencies accompanied higher stimulation frequencies. Pimozide (0.0625, 0.125, 0.25 and 0.5 mg/kg, i.p.) and naloxone (0.5, 1.0, 2.0 and 4.0 mg/kg, i.p.) increased latencies in a dose-dependent manner. At 0.5 mg/kg (4 X the lowest effective dose), pimozide blocked eating completely for most trials and in all animals. Naloxone attenuated eating at 1.0 mg/kg, but 4.0 mg/kg was not more effective than 2.0 mg/kg and did not block the behavior completely in any animal. With each drug, latencies were normal or near-normal at the beginning of each session and anorexic effects developed with repeated testing. Thus neither drug caused a simple overall impairment in performance capability. In both cases it is inferred that the proximal food cues lost the ability to maintain eating well before the distal food cues lost the ability to elicit eating. The similarity between the effects of pimozide and naloxone raises the possibility that opioid-containing and dopamine-containing neurons interact to influence feeding through common brain circuitry.

Circling from unilateral VTA morphine: direction is controlled by environmental stimuli

Crystalline morphine was applied unilaterally to the ventral tegmental area of rats. Contralateral circling was observed in traditional test chambers and inside the walls of an elevated square enclosure. When tested outside the walls of the same enclosure, the animals turned in the opposite direction, maintaining the same flank in the position nearest the wall. This dopamine-dependent circling thus reflects a more complex interaction between the side of activation and the direction of circling than is implied in current views of dopamine function.

Relative effectiveness of pimozide, haloperidol and trifluoperazine on self-stimulation rate-intensity functions

Rats were trained to lever press for 60 Hz sine wave stimulation of the medial forebrain bundle (MFB) at the level of the lateral hypothalamus. Rate intensity functions were determined by stepping down the current intensity by 0.05 log10 units every five min. Haloperidol shifted the function approximately one step to the right at 0.08 mg/kg, with lower doses producing no effect. Pimozide shifted the curve in a dose-dependent manner over the range of 0.06 to 0.24 mg/kg; 0.12 mg/kg produced an approximate one-step shift. Trifluoperazine also produced dose related shifts to the right over the range of 0.08 to 0.32 mg/kg; this agent, unlike the others, appeared to shift the curve down as well as to the right. Less than a one-step shift was produced by 0.08 mg/kg; more than a one-step shift was produced by 0.16 mg/kg. On a molar basis, the order of potency for displacing the curves to the right was haloperidol, pimozide and trifluoperazine. While this order of potency is not predicted from simple in vitro binding affinities for the D2 receptor, this may be due to differences in the penetration of these agents to central D2 receptors in vivo.

Self-administration of methionine enkephalin into the nucleus accumbens

Microinfusions of the endogenous opiate neurohumor, methionine enkephalin, into the nucleus accumbens initiated a reinforcing stimulus in a dose-related manner. The reinforcing nature of this intracranial self-administration was evaluated with intermittent schedules of reinforcement and a two-lever discrimination procedure. Opiate receptors are likely responsible for the initiation of this reinforcing stimulus since naloxone effectively blocked self-administration. These data suggest the mediation of opiate reinforcement through interactions with opiate receptors in brain regions outside the ventral tegmental area, questioning the current dopamine hypothesis for the initiation of these reinforcement processes.

Pimozide attenuates free feeding: best scores analysis reveals a motivational deficit

Pimozide treatment (1.0 and 2.0 mg/kg) decreased free feeding in rats. The animals were presented daily with 35 meal segments, each consisting of five 45-mg pellets; pimozide resulted in longer mean latencies to initiate eating, longer mean eating times per segment (duration scores) and more pellets left uneaten. The increase in durations was progressive both within and across test sessions; toward the end of the final session many pellets were left uneaten. Failure to initiate eating of the first pellet of each segment was rare, and was always preceded by failure to eat the fifth pellet of the preceding meal segment. To assess whether either the increase in latencies or the increase in durations reflected an impairment of absolute response capability, 'best scores' in the pimozide and control conditions were compared; the shortest latencies and durations in the pimozide condition were as 'good' as those of the control condition. However, the animals generally produced 'best' scores on fewer trials in the pimozide condition. An exception was on day 1 of testing, when the frequency of 'best' latencies was higher in the pimozide condition. The fact that the 'best' scores under pimozide equalled the 'best' scores under vehicle suggests that the pimozide-treated animals had the motoric capacity to respond normally. The facts that the pimozide-treated animals did not perform to the demonstrated limits of that capacity in a normal percentage of trials and that performance on days 2 and 3 of testing were 'worse' than performance on day 1 of testing suggest that pimozide causes a motivational deficit that has not been widely recognized.