Problems of concept and vocabulary in the anhedonia hypothesis

Complex motor and sensorimotor functions of striatal and accumbens dopamine: involvement in instrumental behavior processes

The suggestions that dopamine (DA) systems are involved in "motor control" and "reward" represent the classic working hypotheses on the behavioral functions of these systems. The research generated by these hypotheses has yielded results that are far more complicated than the simplest form of either hypothesis would indicate. Pharmacological or lesion-induced interference with DA function does not suppress all aspects of movement control, nor all aspects of reward, nor all aspects of motivation. The deficits produced by interference with DA systems are selective and dissociative in nature, affecting some aspects of motor or motivational function, but leaving others basically intact. In some sense the hypotheses that DA is involved in "motor" or "reward" or "motivational" processes are partly correct, but the processes to which these terms refer are too broad to offer an accurate and detailed description of the behavioral functions of brain DA. A review of the literature on the behavioral pharmacology of DA suggests that the behaviors most easily disrupted by DA antagonists are highly activated and complex learned instrumental responses that are elicited or supported by mild conditioned stimuli, and maintained for considerable periods of time. It is proposed that DA in accumbens and striatum modulates the ability of neocortical and limbic areas involved in sensory, associative, and affective processes to influence complex aspects of motor function, and also modulates the execution of complex motor acts organized by the neocortex. Thus, interference with DA systems produces a "subcortical apraxia", which dissociates complex stimulus processes from complex motor processes, but leaves aspects of those processes intact.

Effects of dopamine D-1 and D-2 antagonists on decision making by rats: no reversal of neuroleptic-induced attenuation by scopolamine

The effects of the dopamine (DA) D-1 antagonist SCH 23390 and the D-2 antagonists haloperidol and sulpiride on decision making in thirsty rats were investigated. The problem given to the rats was to choose between two behaviours, locomoting or lever-pressing, in order to obtain rewards (water). SCH23390 and haloperidol dose-dependently reduced the number of rewards obtained. However, only minor effects were seen on the decision making parameters. DA is thus important for initiation and/or performance of learned behavioural acts, not for information sampling and processing leading to the decision of which behaviour to perform. There were no differences between D-1 and D-2 antagonism in this respect. After sulpiride, no significant effects were observed. A specific behavioural prolife obtained by extinction was not mimicked by any of the doses of neuroleptics tested. It has previously been shown that performance per se of the two behavioural options used in the task can be reinstated by scopolamine after haloperidol attenuation. However, when these options had to be organized into functional sequences to reach a goal, haloperidol-induced attenuation could not be counteracted by scopolamine.

Blockade by neuroleptics of water intake and operant responding for water in the rat: anhedonia, motor deficit, or both?

Four different neuroleptic drugs, haloperidol, metoclopramide, sulpiride and cis-flupenthixol, were tested for their ability to attenuate an operant lever-pressing response with water as reward and the corresponding consummatory act, i.e., the non-conditioned water intake. All four neuroleptic drugs tested more potently attenuated the operant lever-pressing response than the consummatory water intake, just as the conditioned avoidance response previously has been found to be more potently attenuated than the non-conditioned escape reaction. The results suggest that a certain class of learned behaviors, labelled operant or instrumental behaviors, are more susceptible to the attenuating effects of neuroleptic drugs than the class of behaviors labelled non-conditioned consummatory acts. It was further concluded that the attenuation of the lever-pressing response could be explained by a decreased ability of the animals to initiate or perform the required operant response (i.e., a motor deficit) while the attenuated water intake caused by higher doses of the neuroleptics could be interpreted as a motivational effect (e.g., "anhedonia"). When studying the effects of the neuroleptic drugs it is therefore of great importance to know whether the behavior measured in the particular experimental design used is operant or consummatory. The implications of the findings are discussed.

Haloperidol and nonreinforcement produce different patterns of response slowing in a food reinforced runway task

Rats were trained to traverse a runway for food reward and speeds were measured in the start, run and goal segments of the alley. After eight days of acquisition training, subjects were tested for four days under conditions of either extinction or haloperidol pretreatment. Although both haloperidol and extinction produced a suppression of the running response, the pattern of this suppression in the three alley segments was quite different for the two conditions. Haloperidol tended to be more effective than extinction in slowing start speeds but less effective than extinction in reducing run and goal speeds. This differential effect of haloperidol and extinction on speeds in different alley segments provides further evidence that haloperidol-induced impairments in performance cannot result entirely from a blunting of primary reinforcement.

Dopamine and depression: a review of recent evidence. II. Theoretical approaches

This paper discusses 3 behavioural approaches to depression, 'learned helplessness', reward system dysfunction and reduced responsiveness to the environment, and the role of dopamine (DA) in their related animal models. The meso-limbic and nigrostriatal DA system appears to be related primarily to responsiveness to the environment. This conclusion is discussed in relation to the clinical symptomatology of depression.

Discriminating between reward and performance: a critical review of intracranial self-stimulation methodology

Despite numerous pharmacological investigations of intracranial self-stimulation (ICSS), the substrates of this behavior have yet to be completely understood. In view of the likelihood that inadequate methodology has hindered the quest for these substrates, the present review was undertaken. Criteria for ICSS methodology should include not only the ability to discriminate reward from gross performance deficit, but also adequate capacity (ability to generate experimental data at a reasonable rate). For numerous reasons, bar-pressing on a continuous reinforcement schedule fails the first criterion despite its ease and rapidity. The use of partial reinforcement schedules may alleviate some of these shortcomings. Analysis of drug-induced response decrement patterns can discriminate gross motoric incapacity from other variables, although the question of subtle response maintenance deficits remains to be answered. Measurements of response rates using alternative operants do not differentiate reward and performance adequately. More promising, "rate-free" measures using locomotion as an operant include the two-platform method of Valenstein and the "locus of rise" method. Comparison of drug effects on ICSS with those on alternate tasks are fraught with pitfalls including the problems of assuring equivalent rates and patterns of responding. The use of differential electrode placements is ideally suited for neurochemically well-characterized drugs, particularly if "double dissociations" can be established during studies of multiple placements. Presentation of different current intensities or frequencies permits the compilation of rate-intensity functions, and drug-induced shifts in these functions have considerable analytical power. Self-regulation of current intensity constitutes a powerful tool that has yet to realize its full potential in the pharmacological study of ICSS. Extensive studies involving self-regulation of stimulation duration ("shuttlebox" studies) suggest that this method may be highly versatile despite several practical difficulties. It is concluded that at least six of these methods appear to do a reasonable job of excluding gross performance deficit. However, the possible influences of other factors, such as subtle response maintenance deficit, incentive or arousal, remain to be resolved in view of the multifactorial nature of ICSS. Multiple tests for ICSS drug or lesion studies are advocated whenever feasible, as no single test appears capable of resolving all theoretical complexities.

Self-stimulation response decrement patterns differentiate clonidine, baclofen and dopamine antagonists from drugs causing performance deficit

Fouriezos and co-workers have reported that rats treated with the neuroleptics, pimozide or d-butaclamol, barpress at baseline rates at the start of an intracranial self-stimulation (ICSS) session, but cease responding within a few minutes. They suggested that this response decrement pattern (RDP) resembles natural extinction, indicating attenuation of reward by neuroleptic treatment. In the present experiments, the RDPs produced by several different drug classes were systematically compared. Two dopamine receptor antagonists, haloperidol and metoclopramide, produced an extinctionlike RDP. In contrast, the alpha-1 adrenoceptor blocker, prazosin, and the muscle relaxant, methocarbamol, caused uniformly low response rates that did not decrease further as the session progressed. Clonidine, an alpha-2 adrenoceptor agonist, and baclofen, a novel GABAB receptor agonist, were associated with RDPs that resembled those of the dopamine antagonists tested. Analysis of drug-induced RDPs is characterized as a valuable tool for exploring the nature of drug effects on ICSS responding.