Effects of the 5-HT uptake inhibitors, femoxetine and paroxetine, and a 5-HT1A/B agonist, eltoprazine, on the behavioural satiety sequence

Night and day: diurnal differences in the behavioural satiety sequence in male rats

The behavioural satiety sequence (BSS) is an important method for assessing the selectivity of treatment (physiological, pharmacological and/or genetic) effects on food intake in rodents. The concept describes the natural progression from feeding to resting, with the transition between the two a useful biomarker of behavioural satiety. Although treatments can accelerate (anorexigenics) or delay (orexigenics) this transition without disrupting behavioural structure, the detection of such changes depends upon the timing of the transition under control conditions. Fasting and presatiation are known to affect this timing. However, recent observations in our laboratory have suggested that phase of testing (light or dark) might also be an important consideration. The present study therefore directly compared food intake and the BSS in thoroughly habituated male rats maintained either on a normal light cycle and tested during the light phase or on a reversed light cycle and tested during the dark phase. The results show that phase of testing had relatively little impact on food intake or diverse measures of ingestive and non-ingestive behaviour. Although modest differences were detected in locomotion, grooming and scratching (higher in dark phase), by far the largest difference concerned resting behaviour which had both a later onset and a much lower peak level during dark phase testing. Importantly, these behavioural differences delayed the transition between eating and resting. The potential contribution of diurnal differences in rate of eating is discussed as are the implications of these findings for future studies on the neurobiology of feeding behaviour.

Mice overexpressing the 5-hydroxytryptamine transporter show no alterations in feeding behaviour and increased non-feeding responses to fenfluramine

RATIONALE

The 5-HT transporter (5-HTT) is implicated in the regulation of appetite. Expression of the 5-HTT varies in the human population, and this variation may determine both individual differences in feeding and abnormal feeding behaviours such as eating disorders.

OBJECTIVES

The effects of 5-HTT expression on feeding and satiety were examined in a transgenic mouse model of 5-HTT overexpression.

MATERIALS AND METHODS

We measured free-feeding food intake and observed the behavioural satiety sequence (BSS) after food deprivation in mice at baseline and after administration of the anorectic drug fenfluramine.

RESULTS

5-HTT overexpressing mice were both lighter and shorter than their wildtype littermates. Despite this size difference, food intake by transgenic and wildtype mice did not differ. There was no effect of genotype on the BSS or on food intake during the test at baseline. Increasing doses of fenfluramine reduced food intake in a similar manner in both transgenic and wildtype mice. After 0.3 and 1 mg/kg fenfluramine, the temporal pattern of the BSS was the same for both groups, whereas 3 and 10 mg/kg fenfluramine disrupted the BSS. In transgenic mice, this disruption was evident at the 3 mg/kg dose, while in wildtypes, it emerged only at the 10-mg/kg dose.

CONCLUSION

These data suggest that overexpression of the 5-HTT does not lead to alterations in feeding or satiety in food-deprived mice but does increase the occurrence of other non-feeding behaviours in response to the 5-HT releasing agent fenfluramine.

Serotonergic drugs : effects on appetite expression and use for the treatment of obesity

Over 35 years of research suggests that endogenous hypothalamic serotonin (5-hydroxytryptamine) plays an important part in within-meal satiation and post-meal satiety processes. Thus, the serotonin system has provided a viable target for weight control, critical to the action of at least two effective anti-obesity treatments, both producing clinically significant weight loss over a year or more. Numerous serotonin receptor subtypes have been identified; of these, serotonin 5-HT1B and 5-HT2C receptors have been specifically recognised as mediators of serotonin-induced satiety.A number of serotonergic drugs, including selective serotonin reuptake inhibitors (SSRIs), dexfenfluramine and 5-HT2C receptor agonists, have been shown to significantly attenuate rodent bodyweight gain. This effect is strongly associated with marked hypophagia and is probably mediated by the hypothalamic melanocortin system. Additionally, sibutramine, dexfenfluramine, fluoxetine and the 5-HT2C receptor agonist chlorophenylpiperazine (mCPP) have all been shown to modify appetite in both lean and obese humans, resulting in reduced caloric intake. Clinical studies demonstrate serotonergic drugs specifically reduce appetite prior to and following the consumption of fixed caloric loads, and cause a reduction in pre-meal appetite and caloric intake at ad libitum meals. Weight loss in the obese has also been produced by treatment with both the serotonin precursor 5-hydroxytryptophan and the preferential 5-HT2C receptor agonist mCPP.A new generation of 5-HT2C receptor selective agonists have been developed and at least one, lorcaserin (APD356), is currently undergoing clinical trials. In addition, 5-HT6 receptor antagonists such as PRX-07034 and BVT74316 have been shown to potently reduce food intake and bodyweight gain in rodent models and have recently entered clinical trials. However, the role of the 5-HT6 receptor in the expression of appetite remains to be determined. The hope is that these drugs will not only be free of their predecessors' adverse effect profiles, but will also be equally or more effective at regulating appetite and controlling bodyweight.

The 5-HT2 receptor agonist MK-212 reduces food intake and increases resting but prevents the behavioural satiety sequence

The 5-HT2c receptor is implicated in the relationship between serotonin and satiety. However, anorexia induced by the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) has been shown to delay, not advance behaviours associated with the onset of satiety, fragmenting eating behaviour, 6-chloro-2-(1-piperazinyl)pryazine (MK-212) is also a selective agonist at the 5-HT2 receptor sites. MK-212 has greater affinity for 5-HT2c receptor sites than DOI. The effects of an ED50 dose of MK-212 (5.0 mg/kg i.p.) on the eating and other behaviours of the fasted rat were continuously monitored following the presentation of food. Continuous monitoring provides the most powerful and valid form of behavioural analysis. Temporal profiles of behaviour duration (dur) and frequency (frq) were generated. Food intake was reduced 54% by MK-212 (p < .001). The frequency of grooming was reduced (p < .01). Locomotion (dur p < .001, frq p < .001), rearing (dur p < .0005, frq p < .005) and sniffing (dur p < .05, frq p < .0001) were all reduced. The duration of resting increased (p < 0.01). This is consistent with enhanced satiety. However, the Behavioural Satiety Sequence was not present after the administration of MK-212 (5.0 mg/kg). The temporal structure of behaviour produced by MK-212 was quite different from that produced by pre-feeding. Initially resting dominated the behavioural profile. Eating increased over time from a suppressed state in the initial stages of the observation period. This lack of appearance of the Behavioural Satiety Sequence is more similar to a state of hyper-sedation than to DOI induced hyper-activity. The time course of this sedation would not have been picked up by a simple categorical analysis of behaviour. Hence, temporal analysis is an essential tool in understanding of drug induced anorexia.

Serotonergic control of the organization of feeding and satiety

Studies in which serotonergic drugs were administered either systemically or directly into central sites have implicated 5-HT in the inhibitory control of feeding in mammals. In animal models and in humans, 5-HT agonists such as fenfluramine, fluoxetine and sertraline reduced the rate of eating and the size of meals in a manner suggesting that increasing serotonergic neurotransmission specifically enhanced satiation. In rodents, directly acting agonists at 5-HT1B, 5-HT2C or 5-HT2A receptors decreased food intake but by different behavioral mechanisms. Stimulation of the 1B and 2C subtypes may probe physiological roles in feeding and satiety. The former receptors may be involved primarily in regulating meal size and the latter more in controlling eating rate. Activation of both may be required for complete expression of behavioral satiety. By contrast, stimulating 2A sites may simply disrupt the continuity of feeding. Drugs that stimulate 5-HT1A autoreceptors increase food intake, presumably by acutely reducing the firing of serotonergic neurons in the brain. The hypothalamic paraventricular nucleus (PVN) has been proposed as an important terminal field in the forebrain that is involved in 5-HT's satiety role although recent studies implicate extra-PVN regions in this function. Peripherally administered 5-HT also decreases food intake in rats in a behaviorally specific manner. Studies with antagonists and with structural analogs of 5-HT revealed that 5-HT's peripheral satiety action involves 5-HT1-like and 5-HT2-like mechanisms. Thus, within and outside the brain, multiple pharmacological and behavioral mechanisms contribute to serotonergic functions in ingestion. The rich body of data from preclinical investigation in animals provides the foundation for therapeutic development in humans.

Effects of serotonin and the serotonin blocker metergoline on meal patterns and macronutrient selection

Serotonin [5-hydroxytryptamine(5-HT)] in the paraventricular nucleus (PVN) of rats has a suppressive effect on feeding behavior and causes a selective decrease in carbohydrate ingestion, specifically at the onset of the natural (dark) feeding period. Studies conducted here provide further evidence for this phenomena, showing a similar dose-related decrease in carbohydrate ingestion at dark onset after PVN injection of 5-HT or of the agonists, d-norfenfluramine or fluoxetine, which act through endogenous 5-HT. To further characterize the effects of this indoleamine on the macrostructure of feeding, a computer-automated data acquisition system was used to analyze macronutrient feeding patterns in freely feeding animals maintained on the pure diets of protein, carbohydrate, and fat. Results indicate that PVN administration of 5-HT at dark onset decreases intake of the carbohydrate nutrient by decreasing meal size, feeding time, and feeding rate for this nutrient and increasing the satiating effect of carbohydrate. These effects, which occur specifically during the first meal after injection, are opposite those seen after peripheral administration of the 5-HT receptor antagonist, metergoline. This drug stimulates feeding through a selective increase in carbohydrate intake, characterized by an increase in meal size, percent composition, and feeding time for this nutrient and a decrease in the satiety ratio for carbohydrate. These results implicate the serotonergic system in the termination of carbohydrate-rich meals that are prevalent during the early hours of the natural feeding cycle.

Effects of chronically administered fluoxetine and fenfluramine on food intake, body weight and the behavioural satiety sequence

Rats were administered fenfluramine (FF: 3 mg/kg) or fluoxetine (FX: 6 mg/kg) daily for 3 weeks. On acute administration, FF suppressed consumption of 35% sucrose (in a 40 min test) and overnight chow intake. Repeated administration saw the rapid development of extensive tolerance to these effects. FF had no effects on body weight, and no withdrawal effects were apparent. FX reduced chow intake and body weight throughout the treatment period, but there was evidence of some tolerance to the suppression of chow intake and sucrose drinking. Following FX withdrawal, normal body weight was restored in 4 days; food intake was normal during this period. A delayed rebound hyperphagia commenced on day 5 of withdrawal, and persisted for at least 6 days. The behavioural satiety sequence (drinking--activity--grooming--resting) was disrupted by acute FF; on chronic treatment, FF advanced the onset of postprandial resting, but also increased drinking time. FX advanced the behavioural satiety sequence on acute administration, but not after chronic treatment. We consider the implications of these results for the use of resting behaviour as an indicator of postprandial satiety.