Apomorphine anorexia: a further pharmacological characterization

Effects of mu-CPP and mesulergine on dietary choices in deprived rats: possible mechanisms of their action

Although it has been well established that compounds that stimulate 5-HT(2C) and/or 5-HT(1B) receptors induce hypophagia by promoting satiety process, the relative role of these receptor subtypes in dietary choices remains to be fully determined. m-CPP is considered a useful probe of 5-HT(2C) receptor function in vivo and its administration reduces food intake and appetite in humans and rats. Conversely, the non-selective 5-HT(2C) receptor antagonist mesulergine elicits feeding in rats. Food intake and dietary choices were measured in a food-deprivation experimental protocol employing male Wistar rats. Animals were given access for a 4-h period to a pair of isocaloric diets. These two diets were enriched in protein or carbohydrate proportions, respectively, but fat content was held constant. The mixed 5-HT(2C/1B) receptor agonist, m-CPP, led to a dose-dependent hypophagia, due to substantial reduction in carbohydrate consumption while protein intake was spared (0.62, 1.25 and 2.50 mg/kg i.p., respectively). The non-selective 5-HT(2C) receptor antagonist and also D2 agonist, mesulergine, on its own produced a significant dose-dependent increase in both protein and carbohydrate diets (1.0 and 3.0 mg/kg i.p., respectively). Combined treatment with m-CPP, at its maximum effective dose, and mesulergine dose-dependently reversed m-CPP-induced hypophagia, during the 4-h test period. In order to clarify the effects of mesulergine on dietary choices since it is simultaneously a dopamine agonist besides its antiserotonergic properties, the D2 agonist apomorphine was also used. Apomorphine caused a dose-dependent increase in protein intake while carbohydrate and total food intake remained nearly unchanged (0.5 and 1.0 mg/kg i.p., respectively). It is concluded that the mesulergine-induced hyperphagic response on both diets is the expression of a dual mode of action, due to its 5-HT(2C) antagonist activity together with D2 agonist properties. The results further indicate that the activation of hypothalamic 5-HT(2C) receptors may be involved in both protein sparing and carbohydrate suppressing effects of 5-HT (m-CPP-like effect), whereas an important role in increase of protein consumption seems to have the dopaminergic system probably through D2 receptors (apomorphine-like and mesulergine-like effects, respectively).

Dopamine receptor subtype agonists and feeding behavior

Stimulation or blockade of various dopamine receptor subtypes is associated with reduced feeding. For example, D2 receptor agonists suppress feeding in food-deprived and free-feeding rats, and in rats given access to a highly palatable diet. Similarly, reduced food intake is associated with the actions of diverse D1 receptor agonists, and these compounds can interact synergistically with D2 receptor agonists to potentiate reductions in feeding. Using microstructural analysis to compare D1 and D2 agonist effects, specific differences emerge in their modes of action. D1 agonists reduce the duration of feeding, primarily by decreasing the frequency of feeding bouts, whereas D2 agonists reduce the local rate of eating. However, since D1 agonists uniquely reduce feeding in the absence of other behavioral impairments and are less disruptive of the pattern of feeding behavior, it has been suggested that D1 agonists are more likely than D2 agonists to act on central mechanisms regulating food intake. Moreover, only D1 agonists are effective in suppressing sucrose sham-feeding, suggesting that D1 receptor stimulation may promote satiety. Nevertheless, many questions remain. For example, antagonist studies have implicated 5-HT receptor stimulation in the anorectic effects of D1 agonists, suggesting that further pharmacological and behavioral analyses of receptor-subtype agonist effects are required. Above all, recent developments in the classification of dopamine receptor subtypes reveal the need for new studies examining the involvement of D3, D4 and D5 receptors in feeding.

Effects of dopamine receptor agonists on food intake and rumen motility in dwarf goats

In ruminants, the dopaminergic regulation of feeding behaviour has not been investigated. Therefore, the effects of dopamine receptor agonists and antagonists on food intake and forestomach motility were studied in dwarf goats. Goats treated i.v. with bromocriptine (1 micrograms or 2.5 micrograms/kg body wt/min during 10 min) ate less food than when treated with saline. This inhibitory effect on food intake could not be prevented by the peripheral dopamine receptor antagonist domperidone (0.5 mg/kg body wt i.v.). In contrast, dopamine (i.v. 20 micrograms/kg body wt/min during 15 min), levodopa (i.v. 40 micrograms/kg body weight during 10 min), apomorphine (i.v. 2 micrograms/kg body wt/min during 10 min) and lisuride (i.v. 0.2 microgram/kg body wt/min during 15 min and 0.5 microgram/kg body wt during 10 min) failed to modify food intake. Given in association with benserazide, a decarboxylase inhibitor (i.v. 20 micrograms/kg body wt/min during 10 min), levodopa was still inactive as an anorectic agent. Levodopa, bromocriptine and lisuride administered at similar dose rates to those which were used in the food intake experiments, induced some clinical signs including inhibition of forestomach contractions. The inhibition of rumen contractions induced by these drugs was completely antagonized by domperidone pretreatment. These results, together with earlier in vivo and in vitro observations, suggest that the inhibitory effects of dopamine receptor agonists on forestomach contractions are due to interactions with peripheral dopaminergic receptors. The change in smooth muscle tension, which leads to a change in the signals transmitted via vagal afferents to the central nervous system, probably does not modify feeding behaviour in dwarf goats.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for dopamine D-1 receptor-mediated facilitatory and inhibitory effects on feeding behaviour in rats

Early studies on dopamine agonists and feeding behaviour showed that apomorphine, administered either centrally or systemically, reduced food intake. While this anorectic effect was later analysed in some detail, it was also recognized that, under some circumstances, apomorphine could increase food consumption. Once the distinction between dopamine D-1 and D-2 receptors had been drawn, and receptor-specific agonists and antagonists had become available, the way was clear to investigate the potential roles of D-1 and D-2 receptor subtypes in relation to ingestional behaviour. This paper reviews evidence which indicates, first of all, that D-1 receptor agonists reduce food intake relatively specifically and that the effect cannot be attributed to the induction of intrusive mouth movements or to general behavioural changes (e.g. increased arousal or sedation). We show that dopamine D-1 agonists reduce sucrose sham-feeding in the gastric-fistulated rat. Secondly, however, we provide initial evidence that D-1 agonist activity can have a facilitatory effect on feeding behaviour. More specifically, the D-1 agonist, SKF 38393, increased the response to a highly palatable food in a food-preference test. This result is compared with published reports that injections of either D-amphetamine or morphine into the nucleus accumbens increases feeding behaviour. We speculate that D-1 receptors in the nucleus accumbens may be involved in the mediation of preference for more palatable foods.

Stimulation of D1 and D2 dopamine receptors produces additive anorectic effects

In food-deprived mice the D1 dopamine agonist SKF 38393 induced dose dependent anorexia (ED50 = 2.6 mg/kg). This effect was reversed by the D1 antagonist SCH 23390. In similar conditions, the D2 dopamine agonist RU 24926 also induced dose dependent anorexia (ED50 = 0.19 mg/kg). This effect was reversed by the D2 antagonist (+/-) sulpiride. The mixed D1/D2 agonist apomorphine also induced an anorectic effect (150 micrograms/kg sc) which was completely reversed by (+/-) sulpiride (25 mg/kg, ip) but unaffected by SCH 23390 (5-30 micrograms/kg). The dose response curve obtained by associating SKF 38393 (2.5 mg/kg) with increasing doses of RU 24926 was roughly parallel to that obtained with RU 24926 alone. This indicates that effects of two drugs were additive. Although both D1 and D2 receptors regulate food consumption, the anorectic effect of apomorphine appears to involve only D2 receptors.

Evaluation of dopamine receptor involvement in rat feeding behaviour

1. The anorectic effect of dopamine agonists and antagonists were studied in rats. 2. Dopamine agonists bromocriptine, quinpirole or SKF 38393 treatment induced, a dose-dependent anorexia in rats. 3. Anorectic effect of bromocriptine was decreased in animals pretreated with pimozide (D-2 antagonist), but not by sulpiride (D-2 antagonist) or SCH 23390 (D-1 antagonist) pretreatment. 4. Anorexia induced by quinpirole was decreased by sulpiride or pimozide, but not by SCH 23390 administration. 5. While sulpiride and SCH 23390 failed to antagonize the anorectic response of SKF 38393, methergoline (5-HT antagonist) decreased anorexia induced by the drug. 6. A combination of quinpirole with SKF 38393 did not elicit potentiated anorectic response. 7. Decrease in food intake induced by bromocriptine, quinpirole or SKF 38393 was potentiated in reserpinized animals, although single administration of reserpine also induced a marked decrease in feeding. 8. Single administration of sulpiride, pimozide or methergoline did not change the feeding behaviour of rats, but SCH 23390 induced anorexia. 9. It is concluded that D-2 activation may induce inhibition of feeding and anorexia induced by SKF 38393 may be mediated through serotonergic mechanism(s).

A comparison of benzodiazepine, serotonin, and dopamine agents in the taste-reactivity paradigm

Previous studies have shown that rats' positive, palatability-dependent consummatory reactions to infused tastes are selectively facilitated by a benzodiazepine agonist (chlordiazepoxide), and that this effect can be blocked by the coadministration of benzodiazepine antagonists (e.g., Ro 15-1788). The purpose of the present study was to determine whether agents acting at other receptor sites (dopaminergic, serotonergic), which have been shown to modulate food consumption, might also modify rats' palatability-dependent reactivity to infused tastes. In this experiment, the benzodiazepine agonist, diazepam, facilitated positive palatability reactions, while dopaminergic agents (haloperidol, apomorphine, amphetamine) had no significant effects on either positive or aversive reactions. The putative 5-HT1A agonists, buspirone and gepirone, had a general inhibitory action on both positive and aversive palatability reactions. These results are surprising in view of the effects of serotonergic and dopaminergic agents on food and fluid intake. Our results suggest that the benzodiazepine receptor system may play a special role in the neural control of appetite through its enhancement of the positive palatability of tastes. Dopamine systems, by contrast, appear to control food intake by modulating processes that are independent of food affect evaluation.

The anorectic effect of SK&F 38393, a selective dopamine D1 receptor agonist: a microstructural analysis of feeding and related behaviour

An observational study was undertaken to provide a microstructural analysis of the effects of the selective dopamine D1 receptor agonist, SK&F 38393, on feeding and associated behaviours in the rat. Adult, male non-deprived rats were adapted to eating a meal of sweetened mash in a 30-min period. SK&F 38393 (3.0 and 10 mg/kg, SC) significantly reduced food consumption). It also reduced the frequency of feeding bouts and the local rate of eating, while there was an increase, at 10 mg/kg, in the mean duration of individual feeding episodes. The D1 receptor agonist also reduced the total duration of locomotor activity, but did not affect the total duration of rearing, grooming, sniffing, oral behaviours (other than feeding), or resting. The frequency of bouts of locomotion, rearing, grooming, and sniffing were reduced by SK&F 38393. Consideration of the time-courses for the several responses suggested that SK&F 38393 did not materially affect the form and sequence of behavioural responses in the test, although some changes occurred.

The selective dopamine D1 receptor agonist SK&F 38393: its effects on palatability- and deprivation-induced feeding, and operant responding for food

A series of experiments investigated the involvement of the dopamine D1 receptor subtype in relation to feeding responses. The selective D1 agonists, SK&F 38393 (1.0-20 mg/kg) and SK&F 75760 (5 mg/kg), significantly reduced palatable food consumption in nondeprived rats. The anorectic effect of SK&F 38393 (10 mg/kg) was additive with that of the selective D2 receptor agonist, N-0437 (0.3 mg/kg). In nondeprived mice, SK&F 38393 had a stereoselective effect to reduce palatable food intake. At a peripherally-selective dose (3.0 mg/kg), the peripheral dopamine D1 receptor agonist, fenoldopam, had no effect on food intake. At 10 mg/kg, however, it exhibited anorectic properties, although this may have been due to some penetration of the blood-brain barrier. In rats adapted to a food-deprivation schedule, SK&F 38393 (3.0-30 mg/kg) produced significant dose-dependent reductions in consumption of powdered chow and in lever-pressing for food pellets on a FR8 schedule of reinforcement. In rats adapted to a water-deprivation schedule, SK&F 38393 (3.0-30 mg/kg) was substantially less effective in reducing water intake. The results are discussed in terms of a possible selective effect of D1 agonist activity on feeding behaviour.

Microstructural analysis of the anorectic effect of N-0437, a highly selective dopamine D2 agonist

Drug actions, mediated by dopamine D2 receptors, have been shown to reduce food consumption in rodents. The present study used a microstructural approach to feeding responses to determine the behavioural changes which underlie the anorectic effect of a selective D2 agonist, N-0437. Non-deprived male rats were trained to consume a palatable, sweetened mash in a 30 min test under familiar test conditions. N-0437 (1.0 and 3.0 mg/kg) significantly reduced food intake, but had no effect on the duration of feeding, the duration and frequency of feeding bouts, or on the time course of feeding. Its anorectic effect depended upon a selective reduction in the rate of eating. Microstructural analysis of other behavioural changes which followed treatments with N-0437 indicated that, at 0.3 mg/kg, the drug may have selective dopamine autoreceptor activity, but at 1.0 and 3.0 mg/kg it acts postsynaptically at D2 receptors. The results show that the anorectic effects of N-0437 can be clearly distinguished from the effects of psychomotor stimulants like D-amphetamine or cocaine, but they overlap in part with the effect of the mixed D1/D2 agonist, apomorphine. The results are discussed in relation to a proposed D2 receptor-mediation of one component of the behavioural changes that underlie feeding satiation.

Characteristics of the dopamine receptors involved in the anorectic effects of apomorphine in mice

In food-deprived mice apomorphine injected SC induced a brief (15-30 min) dose-dependent (30-150 micrograms/kg) reduction in food intake. This effect occurred in naive mice as well as in mice habituated to a food deprivation procedure. The anorectic effect of apomorphine (150 micrograms/kg SC) was antagonized by sulpiride (ID50 = 8.6 mg/kg) and by haloperidol (ID50 = 66 micrograms/kg) but domperidone was ineffective (250 micrograms/kg). Mice submitted to a semi-chronic (6 d) blockade of dopamine receptors by haloperidol or injected intracerebroventricularly with 125 micrograms 6-hydroxydopamine 21 d before testing failed to develop a hypersensitivity to the anorectic effect of apomorphine (60 micrograms/kg). Although a single apomorphine injection (5 mg/kg) induced tolerance to the hypothermic effect of a second apomorphine injection of 150 micrograms/kg, it did not modify the anorectic effect. Repeated apomorphine injection (5 x 5 mg/kg) resulted in a slight but significant reduction in apomorphine-induced anorexia. A similarly significant reduction was not observed in mice submitted to repeated injections of dexamphetamine (5 x 5 mg/kg).

Dopamine D-2 antagonists reverse apomorphine-induced decreased water intake in the rat: prediction of antipsychotic drugs with few extrapyramidal side-effects?

Water intake in water deprived rats was decreased by administration of a low dose of apomorphine (0.1 mg/kg s.c.). This dose is too low to induce hyperactivity and stereotypies. Four different dopamine (DA) D-2 antagonists were used to counteract this effect of apomorphine; haloperidol [an antipsychotic inducing extrapyramidal side-effects (EPS)], sulpiride (an antipsychotic inducing less EPS than haloperidol), metoclopramide (not used as an antipsychotic but inducing EPS) and domperidone (not passing through the blood brain barrier). Domperidone did not counteract the apomorphine effect, indicating a central mechanism of action for apomorphine. Metoclopramide did not counteract the apomorphine effect and, in higher doses, water intake was even further reduced. Sulpiride completely counteracted the apomorphine effect but, in higher doses, did not by itself reduce water intake. Haloperidol counteracted the apomorphine effect in a small dose-range and caused a further reduction in the water intake when given in high doses. The results can be explained by the existence of two subpopulations of D-2 receptors related to different functions. The model described may be used in screening experiments aimed at finding new antipsychotic drugs with a low incidence of EPS.

Behavioural microanalysis of the role of dopamine in amphetamine anorexia

A microstructural analysis paradigm was used to study amphetamine anorexia. Doses above 0.40 mg/kg significantly reduced food intake by reducing eating time; in contrast, eating rate was increased at these doses. Examination of the frequency distribution of interresponse times (IRTs) revealed a significant shift to shorter IRTs at doses as low as 0.125 mg/kg. Pimozide blocked amphetamine anorexia at 0.5 and 1.0 mg/kg, suggesting that at both doses amphetamine anorexia has a dopaminergic substrate. However, the atypical neuroleptic thioridazine did not antagonize amphetamine. Furthermore, effects of amphetamine were additive with those of apomorphine, administered at a dose known to suppress feeding by inhibiting mesolimbic DA neurons. These results provide evidence against an involvement of the mesolimbic DA system in amphetamine anorexia.

[Pharmacological and molecular aspects of the regulation of eating behavior. With special reference to the role of catecholamines and effects of amphetamine]

Among the numerous endogenous substances involved in the regulation of feeding behaviours, the catecholamines are in the front rank. The numerous studies devoted to this aspect of catecholamines emphasize the importance and complexity of their intervention. Depending on the cerebral structures on which they act and on whether noradrenaline or dopamine are concerned, orexigenic or anorexigenic effects have been described. Alpha-2 and beta adrenergic receptors as well as D1 and D2 dopaminergic receptors participate in these effects. Amphetamine, which is an indirect catecholaminergic agonist, mobilizes neuronal catecholamines and fosters their various effects. Moreover, it exercises direct effects by its association with sites borne by glycaemia-sensitive neurons. This target seems to be common to a wide variety of anorectic agents. They are thought to reproduce on this hypothalamic "glucostat" the effect of a high blood glucose level, thus triggering off signals of satiety. In this unifying hypothesis, the diverse pharmacological profiles these agents are known to possess would result from associated properties.

Changes in dopamine autoreceptor sensitivity in an animal model of depression

Rats exposed for 6 weeks to a variety of mild unpredictable stressors showed reduced consumption of a preferred sucrose solution. The deficit was apparent after 1 week of stress and was maintained for at least 2 weeks after termination of the stress regime. Sucrose preference was unaffected by 2 weeks of treatment with the tricyclic antidepressant DMI but returned to normal after 3 weeks of DMI treatment. Subsensitivity to the anorexic effect of a low dose of apomorphine was seen in vehicle-treated stressed animals, and in unstressed animals following withdrawal from DMI. In both cases, the changes resulted from a failure of apomorphine to reduce eating time (rather than from changes in eating rate); this effect is assumed to represent a subsensitive response to stimulation of dopamine cell body autoreceptors. As the same effect is seen in anhedonic stressed animals and in animals withdrawn from DMI, it is concluded that dopamine autoreceptor desensitization probably does not contribute to clinical improvement following chronic antidepressant treatment.

Apomorphine anorexia: the role of dopamine receptors in the ventral forebrain

The inhibition of feeding following the administration of apomorphine, systemically or directly into the nucleus accumbens/ventral striatum, was studied using a microstructural analysis paradigm. On systemic administration, apomorphine reduced food consumption, eating rate and eating time; the effects were blocked by sulpiride but not by SCH-23390. Two doses of apomorphine were administered centrally. Both doses reduced total food intake and eating rate; only the higher dose also reduced eating time; all of these effects were blocked by sulpiride pretreatment. Only the lower dose reduced locomotor activity and rearing in the open field. The results suggest that apomorphine reduces eating rate by an action on dopamine (DA) axon terminal autoreceptors. We have previously demonstrated that apomorphine reduces eating time by an action on DA cell body autoreceptors. Therefore, the two populations of DA autoreceptors appear to be differentially involved in behaviour.

Neuropharmacology of drugs affecting food intake

The importance of the central monoamines NE, DA and 5-HT in ingestive behavior has inevitably resulted in considerable effort being expended in attempting to implicate these monoamines in the mechanism of action of anorectic drugs. The statements that amphetamine-induced anorexia is unlikely to be due to central serotoninergic systems and that central noradrenergic and dopaminergic systems are not implicated in the appetite suppressant effect of fenfluramine are in all probability correct. However, to attribute the ability of drugs to decrease food intake unequivocally to a specific effect on central monoaminergic systems is almost certainly an oversimplification, due to the fact that other putative neurotransmitters, such as GABA and peptides, play a critical role in eating. This can be achieved either directly or by modulating the release of other transmitters. An added complication in attempting to correlate a specific neurochemical process to a behavioral effect, such as anorexia, is the complexity of the central actions of the drug. At best, a predominant but not an exclusive process can be identified. Perhaps the in-built constraint of attempting to correlate a specific neurochemical effect to the desired action of a drug is accountable for the absence of a second generation of centrally acting anorectic drugs. Dramatic progress has been made in elucidating the factors involved in ingestive behavior over the last 5-10 years. This information should, and must, provide the catalyst for more efficacious anorectic drugs because obesity represents one of the few major diseases for which adequate drug therapy does not exist.