Effects of tryptophan and of 5-hydroxytryptamine receptor subtype agonists on feeding

A systematic investigation of the differential roles for ventral tegmentum serotonin 1- and 2-type receptors on food intake in the rat

Central serotonin (5-HT) pathways are known to influence feeding and other ingestive behaviors. Although the ventral tegmentum is important for promoting the seeking and consumption of food and drugs of abuse, the roles of 5-HT receptor subtypes in this region on food intake have yet to be comprehensively examined. In these experiments, food restricted rats were given 2-h access to rat chow; separate groups of non-restricted animals had similar access to a sweetened fat diet. Feeding and locomotor activity were monitored following ventral tegmentum stimulation or blockade of 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2B, or 5-HT2C receptors. 5-HT1A receptor stimulation transiently inhibited rearing behavior and chow intake in food-restricted rats, and had a biphasic effect on non-restricted rats offered the palatable diet. 5-HT1B receptor agonism transiently inhibited feeding in restricted animals, but did not affect intake of non-restricted rats. In contrast, 5-HT1B receptor antagonism decreased palatable feeding. Although stimulation of ventral tegmental 5-HT2B receptors with BW723C86 did not affect hunger-driven food intake, it significantly affected palatable feeding, with a trend for an increasing intake at 2.0µg/side but not at 5.0µg/side. Antagonism of the same receptor modestly but significantly inhibited feeding of the palatable diet at 5.0µg/side ketanserin. Neither stimulation nor blockade of 5-HT2A or 5-HT2C receptors caused prolonged effects on intake or locomotion. These data suggest that serotonin's effects on feeding within the ventral tegmentum depend upon the specific receptor targeted, as well as whether intake is motivated by food restriction or the palatable nature of the offered diet.

Effects of oral and intravenous administration of buspirone on food-cocaine choice in socially housed male cynomolgus monkeys

Drugs acting at D3 dopamine receptors have been suggested as medications for cocaine dependence. These experiments examined the effects of intravenously and orally administered buspirone, a D2-like receptor antagonist with high affinity for D3 and D4 receptors, on the relative reinforcing strength of cocaine in group-housed male cynomolgus monkeys. Use of socially housed monkeys permitted the assessment of whether social status, known to influence D2-like receptor availability, modulates the behavioral effects of buspirone. Buspirone was administered acutely to monkeys self-administering cocaine under a food-drug choice procedure in which a cocaine self-administration dose-effect curve was determined daily. When administered by either route, buspirone significantly decreased cocaine choice in dominant-ranked monkeys. In subordinate monkeys, however, i.v. buspirone was ineffective on average, and oral buspirone increased choice of lower cocaine doses. The effects of buspirone only differed according to route of administration in subordinate monkeys. Moreover, it is noteworthy that the effects of buspirone were similar to those of the D3 receptor-selective antagonist PG01037 and qualitatively different than those of less selective drugs that act at D2-like or serotonin (5-HT)1A receptors, suggesting a D3 and possibly D4 receptor mechanism of action for buspirone. Taken together, the data support the utility of drugs targeting D3/D4 receptors as potential treatments for cocaine addiction, particularly in combination with enriching environmental manipulations.

Serotonin 1A, 1B, and 7 receptors of the rat medial nucleus accumbens differentially regulate feeding, water intake, and locomotor activity

Serotonin (5-HT) signaling has been widely implicated in the regulation of feeding behaviors in both humans and animal models. Recently, we reported that co-stimulation of 5-HT1&7 receptors of the anterior medial nucleus accumbens with the drug 5-CT caused a dose-dependent decrease in food intake, water intake, and locomotion in rats (Pratt et al., 2009). The current experiments sought to determine which of three serotonin receptor subtypes (5-HT1A, 5-HT1B, or 5-HT7) might be responsible for these consummatory and locomotor effects. Food-deprived rats were given 2-h access to rat chow after stimulation of nucleus accumbens 5-HT1A, 5-HT1B, or 5-HT7 receptors, or blockade of the 5-HT1A or 5-HT1B receptors. Stimulation of 5-HT1A receptors with 8-OH-DPAT (at 0.0, 2.0, 4.0, and 8.0 μg/0.5 μl/side) caused a dose-dependent decrease in food and water intake, and reduced rearing behavior but not ambulation. In contrast, rats that received the 5-HT1B agonist CP 93129 (at 0.0, 1.0, 2.0 and 4.0 μg/0.5 μl/side) showed a significant dose-dependent decrease in water intake only; stimulation of 5-HT7 receptors (AS 19; at 0.0, 1.0, and 5.0 μg/0.5 μl/side) decreased ambulatory activity but did not affect food or water consumption. Blockade of 5-HT1A or 5-HT1B receptors had no lasting effects on measures of food consumption. These data suggest that the food intake, water intake, and locomotor effects seen after medial nucleus accumbens injections of 5-CT are due to actions on separate serotonin receptor subtypes, and contribute to growing evidence for selective roles of individual serotonin receptors within the nucleus accumbens on motivated behavior.

The neurochemistry of human aggression

Various data from scientific research studies conducted over the past three decades suggest that central neurotransmitters play a key role in the modulation of aggression in all mammalian species, including humans. Specific neurotransmitter systems involved in mammalian aggression include serotonin, dopamine, norepinephrine, GABA, and neuropeptides such as vasopressin and oxytocin. Neurotransmitters not only help to execute basic behavioral components but also serve to modulate these preexisting behavioral states by amplifying or reducing their effects. This chapter reviews the currently available data to present a contemporary view of how central neurotransmitters influence the vulnerability for aggressive behavior and/or initiation of aggressive behavior in social situations. Data reviewed in this chapter include emoiric information from neurochemical, pharmaco-challenge, molecular genetic and neuroimaging studies.

Investigating the motivational mechanism of altered saline consumption following 5-HT(1A) manipulation

The precise role played by serotonin (5-HT) in taste--an issue of great interest given the involvement of serotonin in human sensory and eating disorders--is a matter of considerable debate, perhaps because of the variety of methodologies that have been brought to bear by different researchers. Here, we use multiple methods to reveal the motivational mechanism whereby 5-HT(1A) receptor activation modulates drinking behavior. Subcutaneous injections of the selective 5-HT(1A) agonist 8-hydroxy-2-di-n-propylamino-tetralin (8-OH-DPAT), a drug that reduces 5-HT release by acting on presynaptic autoreceptors, dose-dependently increased consumption of 0.45 M NaCl in a one-bottle test. In a two-bottle test, however, 8-OH-DPAT-treated animals (30 microg/kg/ml) demonstrated decreased NaCl preference--although our detection of this effect was obscured by adaptation to the drug across days. Rats' performance in a brief access test confirmed that 8-OH-DPAT decreased preference for saline by both increasing water consumption and decreasing NaCl consumption. Finally, taste reactivity tests demonstrated that the latter result does not reflect decreased NaCl palatability. Overall, the results suggest that 8-OH-DPAT-induced 5-HT hypofunction increases thirst without substantially affecting the palatability of NaCl.

Influence of serotonergic transmission and postsynaptic 5-HT2C action on the feeding behavior of Coturnix japonica (Galliformes: Aves)

We investigated the role of 5-HT2C receptors and serotonergic transmission in the feeding behavior control of quails. Administration of serotonin releaser, fenfluramine (FEN) and 5-HT2C agonists, mCPP and MK212, 1.0 and 3.3 mg/Kg induced significant inhibition of food intake in previously fasted fowls (0.71 ± 0.18 g and 0.47 ± 0.2 g; 0.49 ± 0.22 g and 0.48 ± 0.29 g; 0.82 ± 0.13 g and 0.71 ± 0.16 g, respectively). Control groups ranged from 2.89 ± 0.21 g to 2.97 ± 0.22 g, 60 min after reintroduction of food, P < 0.0001). Similar results were obtained with normally fed quails. Both serotonin releaser and 5-HT2C agonists, in a 3.3 mg/Kg dose, induced hypophagy (FEN, 0.78 ± 0.08 g; mCPP, 0.89 ± 0.07 g; MK212, 1.25 ± 0.17 g vs. controls, 2.05 ± 0.12 g, 120 min after food was presented, P < 0.0001 to P < 0.01). Previous administration of 5-HT2C antagonist, LY53857 (5.0 mg/Kg) blocked the hypophagic response induced by 5-HT2C agonists 60 min after food was reintroduced. Current data show a modulatory role of serotonin release and postsynaptic 5-HT2C receptors in the feeding behavior of quails.

Influence of 5-HT1A agonist on the feeding behavior of Coturnix japonica (Galliformes: Aves)

In this study, we investigate the effect of serotonin receptor 5-HT1A stimulation on the feeding behavior of quails (Coturnix japonica). The administration of 5-HT1A agonist, 8-OH-DPAT (0.05 to 5.0 mg/Kg) dose-dependently inhibited the food intake in normally fed quails. Greater inhibition was attained with 5.0 mg/kg (0.93 ± 0.21 g vs. 5.83 ± 0.25 g, P < 0.05, 2 h after food offer). A comparable response was obtained from previously fasted quails. At end of 2 h, a higher dose of 8-OH-DPAT induced more intense hypophagy (1.59 ± 0.41 g vs. 6.85 ± 1.04 g, P < 0.0001). Previous treatment with the antagonist 5-HT1A/beta-adrenergic, propranolol, failed to block the inhibitory action of 8-OH-DPAT, but instead, intensified it (controls, 5.22 ± 1.09 g; 8-OH-DPAT, 1.41 ± 0.19 g; propranolol + 8-OH-DPAT, 0.44 ± 0.25 g, P < 0.01, for all comparisons). The administration of an isolated higher dose of propranolol induced a hypophagic action (controls, 4.5 ± 0.8 g vs. propranolol, 2.0 ± 0.2 g, P < 0.01). Current outcomes suggest a possible role of 5-HT1A receptor on the feeding behavior of quails, as opposed to mammals. On the other hand, the intensified hypophagy induced by previous administration of propranolol raises the hypothesis of a beta-adrenergic excitatory mechanism that controls the feeding behavior of quails.

Evidence indicating participation of the serotonergic system in controlling feeding behavior in Coturnix japonica (Galliformes: Aves)

We investigated participation of the brain serotonergic system in food intake control by using oral and systemic administration of serotonin precursors in quails (Coturnix japonica). Dietary supplemental tryptophan (0.1-50.0 g/kg) provoked a dose-dependent inhibition of food intake during a 5-h observation period, which persisted up to 24 h for doses of 30.0 and 50.0 g/kg. Normally fed and fasted animals treated with hydroxytryptophan (12.5-50.0 mg/kg) by the intracoelomic route showed an acute inhibition of food intake. Hypophagia in fasted birds was only effective when the precursor was administered immediately before food presentation. A similar response was obtained by administering serotonin (0.125-2.5 mg/kg, sc), with animals showing a hypnogenic response within the first ten minutes after administration, suggesting that, in contrast to mammals, the amine crosses the blood-brain barrier in quails. Administration of hydroxytryptophan at all doses tested induced significant dipsogenic behavior despite the concomitant hypnogenic response. The results suggest the involvement of serotonergic pathways in food intake control in quails and also show, for the first time, hypnogenic action induced by serotonin and a hyperdipsic effect elicited by hydroxytryptophan.

A reassessment of the role of serotonergic system in the control of feeding behavior

The role of serotonergic system in the feeding behavior was appraised by electrolytic lesions in the dorsal raphe nucleus (DRN) and administration of para-chlorophenylalanine (PCPA, 3 mg/5 microl, icv). Chronic evaluations were accomplished through 120 and 360 days in PCPA-injected and DRN-lesioned rats, respectively. Acute food intake was evaluated in fasted rats and submitted to injection of PCPA and hydroxytryptophan (LHTP, 30 mg/kg, ip). DRN-lesioned rats exhibited 22-80% increase in food intake up to sixth month, whereas the obesity was evident and sustained by whole period. In PCPA-injected rats was observed an initial increase in the food intake followed by hypophagy from 25th to 30th day and a transitory increase of body weight from 5th to 60th day. In the acute study, the LHTP reverted partially the PCPA-induced increase in food intake of fasted rats suggesting a sustained capacity of decarboxylation of precursor by serotonergic neurons. Slow restoration of the levels of food intake in DRN-lesioned rats reveals a neuroplasticity in the systems that regulate feeding behavior. A plateau on the body weight curve in lesioned rats possibly represents the establishment of a new and higher set point of energetic balance.

Effects of serotonin 5-HT(1/2) receptor agonists in a limited-access operant food intake paradigm in the rat

Hypophagic effects of serotonergic drugs have mostly been investigated in free-feeding paradigms and are generally ascribed to drug-induced acceleration of satiety, or to behavioral disruption. The present study investigated the hypophagic effects of various 5-HT(1/2) receptor agonists in an operant paradigm. Because of its limited duration (10-min session) the procedure was considered to be relatively insensitive to satiety processes. The behavioral specificity of the hypophagic effect was assessed by additional testing of the compounds in a locomotor activity assay. Male Wistar rats, maintained at about 80% of their free-feeding weights, were trained to acquire stable operant responding in daily fixed ratio:10 food-reinforced sessions; after which they were tested once a week with a 5-HT receptor agonist. Each compound dose-dependently suppressed the number of earned pellets after i.p. administration: DOI (5-HT(2A/2C) receptor agonist; ED(50): 0.36 mg/kg), TFMPP (5-HT(1B/2C/2A); 0.37 mg/kg), m-CPP (5-HT(2C/1B/2A); 0.54 mg/kg), ORG 37684 (5-HT(2C/2A); 0.85 mg/kg), CP-94,253 (5-HT(1B); 2.09 mg/kg), BW 723C86 (5-HT(2B); 6.26 mg/kg) and ipsapirone (5-HT(1A); 10.17 mg/kg). When tested at the dose equivalent to the ED(50) value in the operant paradigm, only ORG 37684 and DOI weakly suppressed activity counts in a locomotor activity assay; suggesting that the inhibition of operant food intake obtained with the other compounds at these doses is not a direct consequence of unconditioned motor effects. It is suggested that the hypophagic effect induced by relatively low doses of CP-94,253, TFMPP and m-CPP, and by moderate doses of ipsapirone and BW 723C86, is partly due to a drug-induced suppression of appetite. Although the exact contribution of the diverse 5-HT(1/2) receptor subtypes to appetite control remains to be studied in more detail, it is hypothesized that activation of 5-HT(1B) and/or 5-HT(2C) receptors attenuates appetite.

Male and female 5-HT(1B) receptor knockout mice have higher body weights than wildtypes

5-HT(1B) receptors have a regulatory role in serotonergic activity and influence feeding behavior and body weight. Because the absence of 5-HT(1B) receptors may cause changes in this regulation, body weight was measured in male and female 5-HT(1B) receptor knockout (5-HT(1B) KO) and wildtype (WT) mice from weaning until the age of 30 weeks. In both genders, 5-HT(1B) KO mice had a higher body weight than WT mice (17% and 9%, respectively). Body weight was significantly higher for males over the entire period and for females from Week 18 onwards. Absolute food and water consumption were related to body weight. However, relative to body weight, males consumed more than females. 5-HT(1B) KO males drank strikingly more water. Housing mice singly reduced food and water intake in males, but not in females. Plasma leptin levels and most organ weights did not differ between genotypes, indicating that higher body weight in 5-HT(1B) KO mice is not related to obesity. Relative to body weight, brains and adrenals were larger in females, while heart and liver were smaller. Kidneys were smaller in females, but larger in 5-HT(1B) KO mice, while lungs showed opposite effects. Spleen and testes were smaller in 5-HT(1B) KO mice. Although 5-HT(1B) KO males are more aggressive, testosterone levels were not different from WT mice. Basal corticosterone levels were similar in all groups and increased in response to mild stress, particularly in females. Lifelong absence of 5-HT(1B) receptors in mice resulted in clear phenotypic differences in body weights and food and water intake. Lacking this receptor increases body growth, without signs of obesity. A potential genetic background effect influencing this phenotype is discussed.

Effects of serotonin 5-HT(1) and 5-HT(2) receptor agonists in a conditioned taste aversion paradigm in the rat

Although 5-HT(1/2) receptor agonists can inhibit ingestive behavior, it remains unclear whether this effect is confounded by drug-induced "malaise." The present study assessed the potential of such compounds to induce conditioned taste aversion (CTA), a possible correlate of aversive stimulus properties. Male Wistar rats were tested in a two-bottle saccharin versus water choice paradigm. DOI [5-HT(2A/2C) receptor agonist; ED(50) (95% confidence limits) in mg/kg, IP: 0.29 (0.14-0.63)], m-CPP [5-HT(2C/1B); 1.69 (0.96-2.99)], TFMPP [5-HT(1B/2C); 2.45 (1.46-4.11)], ORG 37684 [5-HT(2C); 2.96 (1. 17-7.52)], BW 723C86 [5-HT(2B); 3.49 (1.29-9.47)], CP-94,253 [5-HT(1B); 3.74 (1.54-9.08)], and ipsapirone [5-HT(1A); 20.15 (11. 25-36.09)] dose dependently suppressed saccharin preference, with potencies that correlated with their previously reported potencies to inhibit ingestive behavior in operant- and free feeding paradigms. Although these results did not necessarily imply that such hypophagic effects result from a drug-induced "malaise," it can be hypothesized that they involve, at least partly, the same physiological mechanism/substrate underlying CTA. As the hypophagic effects of serotonergic compounds have been ascribed to their effects on satiety processes and generally occur at doses that are lower than those inducing CTA, it is speculated that weak activation of this substrate results in satiety, whereas strong activation will result in aversive effects, or drug-induced "malaise."

Effects of selected serotonin 5-HT(1) and 5-HT(2) receptor agonists on feeding behavior: possible mechanisms of action

Serotonin (5-HT) receptor agonists with high affinity for the different subtypes (i.e. 5-HT(1A-1F), 5-HT(2A-2C)) of the 5-HT(1)- and 5-HT(2) receptor families have been shown to affect ingestive behavior. It has been assumed that: (1) stimulation of hypothalamic 5-HT(2C) or 5-HT(1B) receptors leads to a behaviorally specific hypophagic effect by accelerating satiety processes; (2) stimulation of 5-HT(2A) receptors leads to a disruption of the feeding cascade; and (3) stimulation of 5-HT(1A) and 5-HT(2B) receptors leads to a hyperphagic effect. The present paper reviews studies performed with the relatively selective receptor agonists ipsapirone (5-HT(1A)), CP-94,253 (5-HT(1B)), BW 723C86 (5-HT(2B)) and ORG 37684 (5-HT(2C)), as well as the nonselective receptor agonists TFMPP (5-HT(1B/2C)), m-CPP (5-HT(2C/1B)) and DOI (5-HT(2A/2C)) in a variety of feeding paradigms in rats, both after systemic and local injection. These studies support a role for other neuroanatomical regions (i.e. brain stem) and behavioral mechanisms (i.e. appetitive processes) in the hypophagic effects of these compounds, possibly as a function of the administered dose. Studies with 5-HT receptor antagonists indicate that the proposed role of particular 5-HT(1/2) receptor subtypes in the hypophagic effects of these 5-HT receptor agonists may be more complicated than originally thought. Further characterization of the role of 5-HT(1/2) receptor subtypes in the control of ingestive behavior will require extensive pharmacological and behavioral studies, using more selective receptor agonists and antagonists and different behavioral procedures, as well as verification in transgenic animals.

Behavioral satiety sequence (BSS) for the diagnosis of drug action on food intake

The Behavioral Satiety Sequence (BSS) is the name given to the orderly transitions of eating, activity grooming and resting measured during the postingestive period. Because the BSS is considered to reflect the operations of natural physiological processes underlying satiety, the sequence can be used to discriminate between different drugs (and other manipulations) that reduce food intake via these natural physiological mechanisms or those that do so by interference. The BSS is only produced by the presence of a caloric load in the gut, and the preabsorptive satiety factors (such as CCK) the caloric load triggers. The BSS is most accurately defined by continuous observation rather than time or event sampling techniques [Partial Time Sampling (PTS) or Momentary Time Sampling (MTS)]. Continuous observation also allows the true duration and true frequency of each behavior to be analyzed. Continuous observation can be used to determine if the profiles associated with the reduction in food intake is caused by nausea, sedation, hyperactivity, or altered palatability of food. At the present time is it possible to identify a number of drugs whose suppression of food intake is associated with the disruption or preservation of the BSS. Drugs that increase synaptic 5-HT activity such d-fenfluramine, fluoxetine. and sibutramine all preserve the BSS and advance the onset of resting. The 5-HT1b/2c agonists mCPP and TFMPP and the 5-HT1b agonist CP-94,253 produce similar effects. However, the 5-HT2 agonist DOI and the 5-HT1a/1b agonist RU-24969 disrupt the BSS by inducing hyperactivity as does amphetamine. The 5-HT2 agonist MK-212 disrupts the BSS by inducing sedation. Selective dopamine agonists, at low doses, such as SKF-38393 (DA1) and LY-171555 (DA2) also preserve the BSS. However, detailed behavioral analysis of the effects of many recently discovered putative satiety factors remains to be carried out.

5HT1A receptor antagonists enhance the functional activity of fluoxetine in a mouse model of feeding

Fluoxetine has been reported to suppress food intake in animal models of feeding. Fluoxetine increases extracellular serotonin in the brain. 5HT1A autoreceptors regulate synaptic levels of serotonin. A combination of a 5HT1A receptor antagonist and fluoxetine has been previously reported to enhance extracellular levels of serotonin over what is obtained with fluoxetine alone. Thus, a combination of fluoxetine and a 5HT1A antagonist could enhance the ability of fluoxetine to suppress appetite. Fluoxetine was tested in a model of feeding, in which CD-1 mice were trained to drink sweetened condensed milk. Fluoxetine was found to attenuate milk drinking, in a dose-dependent manner, at doses greater than 10 mg/kg, i.p. A 10 mg/kg dose of fluoxetine, which was ineffective by itself, was then combined either with 5-hydroxytryptophan (5HTP), a serotonin precursor, or with S(-) pindolol, a 5HT1A/beta adrenergic receptor antagonist or with LY206130, a more selective 5HT1A receptor antagonist. These treatment paradigms resulted in significant attenuation of the consumption of sweetened condensed milk. Since fluoxetine has been shown to be useful in the treatment of eating disorders and to promote weight loss in obese humans, although at doses greater than those required for the treatment of depression, a combination of fluoxetine with a 5HT1A receptor antagonist could be of clinical utility in the treatment of eating disorders and obesity.

A single pretreatment with 8-OH-DPAT reduces behavioral indices of serotonin 1A receptor activation in ovariectomized rats

The effects of prior treatment with 8-OH-DPAT on 8-OH-DPAT-induced eating behavior, hypothermia, flat body posture and forepaw treading were examined in ovariectomized rats. Twenty-four hours after a single pretreatment with 0.25 mg/kg of the drug, the eating behavior and flat body posture induced by 8-OH-DPAT were reduced relative to that seen following the first treatment with the drug. In contrast, within 24 hr of the prior drug treatment, the drug's effects on forepaw treading and rectal temperature were unchanged. However, 7 days after prior treatment with 8-OH-DPAT, eating behavior, flat body posture and hypothermia induced by the drug were suppressed. Only forepaw treading remained unchanged. These findings are discussed in reference to similar studies of 5-HT1A receptor agonist-induced modulation of behaviors in male rats.

RS-102221: a novel high affinity and selective, 5-HT2C receptor antagonist

The 5-HT2C receptor is one of three closely related receptor subtypes in the 5-HT2 receptor family. 5-HT2A and 5-HT2B selective antagonists have been described. However, no 5-HT2C selective antagonists have yet been disclosed. As part of an effort to further explore the function of 5-HT2C receptors, we have developed a selective 5-HT2C receptor antagonist, RS-102221 (a benzenesulfonamide of 8-[5-(5-amino-2,4-dimethoxyphenyl) 5-oxopentyl]-1,3,8-triazaspiro[4.5]decane-2,4-dione). This compound exhibited nanomolar affinity for human (pKi = 8.4) and rat (pKi = 8.5) 5-HT2C receptors. The compound also demonstrated nearly 100-fold selectivity for the 5-HT2C receptor as compared to the 5-HT2A and 5-HT2B receptors. RS-102221 acted as an antagonist in a cell-based microphysiometry functional assay (pA2 = 8.1) and had no detectable intrinsic efficacy. Consistent with its action as a 5-HT2C receptor antagonist, daily dosing with RS-102221 (2 mg/kg intraperitoneal) increased food-intake and weight-gain in rats. Surprisingly, RS-102221 failed to reverse the hypolocomotion induced by the 5-HT2 receptor agonist 1-(3-chlorophenyl)piperazine (m-CPP). It is concluded that RS-102221 is the first selective, high affinity 5-HT2C receptor antagonist to be described.

The 5-HT1B receptor agonist CP-94,253 reduces food intake and preserves the behavioural satiety sequence

The 5-HT1B receptor has been proposed as a link in the relationship between serotonin and satiety. However, the anorexia induced by RU-24969, a 5-HT1A/1B agonist, has been shown to delay behaviours associated with the onset of satiety, increase the frequency of intake and increase some nonfeeding activities. CP-94,253 is a highly selective agonist at the 5-HT1B receptor site with less affinity for other receptor sites, and has been used in this study to determine if agonism of 5-HT1B receptor sites adjust the behavioural satiety sequence in a way consistent with satiety. The effects of an ED50 of dose of CP-94,253 (5.0 mg/kg, IP) on eating behaviour of the fasted rat were monitored. Temporal profiles of behaviour duration and frequency were generated. Food intake was reduced 57% by CP-94,253 (p < 0.0005). Analysis confirmed that CP-94,253 reduced rearing (p < 0.05) and increased resting (p < 0.05) but preserved the overall pattern of the satiety sequence. Increased locomotion was not significant and caused no gross disruption of the behavioural repertoire. Such changes are also produced by prefeeding. In a second experiment, RU-24969 (1.0 mg/kg, IP) reduced food intake by only 30% (p < 0.05) However, RU-24969 markedly disrupted the satiety sequence profile, which did not resemble the profile produced by prefeeding. RU-24969 presumably induces this disruption via a mechanism other than 5-HT1B activation. The use of CP-95,253 demonstrates that selective activation of 5-HT1B receptor sites is sufficient to reduce food intake and enhance satiety as represented by the behavioural satiety sequence.

Changes in responsiveness to serotonin on rat ventromedial hypothalamic neurons after food deprivation

The effects of food deprivation on responsiveness of neurons in the ventromedial nucleus of the hypothalamus (VMH) to serotonin (5-HT), norepinephrine (NE), gamma-aminobutyric acid (GABA), and neuropeptide Y (NPY) were investigated using brain slices in vitro along with behavioral changes in vivo during fasting. Adult male rats were fasted for 48 h starting at the beginning of the dark phase (lights on: 0700-1900 h). The animals showed a significant loss of body weight on the second day of fasting and an increase in food consumption on the first day of refeeding. During fasting, voluntary locomotor activity was significantly increased in the light phase but not during the dark phase. Plasma catecholamine levels were not affected by fasting. In vitro electrophysiological study showed that, in normally fed rats, 5-HT and NE induced both excitatory and inhibitory responses, while GABA and NPY intensively suppressed unit activity in the VMH. Food deprivation for 48 h significantly changed the responsiveness of VMH neurons to 5-HT, for instance, the ratio of neurons whose activity was facilitated by 5-HT was significantly decreased. The responsiveness of VMH neurons to NE, GABA, and NPY was not affected by food deprivation. These results suggest that food deprivation decreases the facilitatory response of VMH neurons to 5-HT, and that this change in responsiveness to 5-HT is at least partially involved in the increase in food intake motivation and locomotor activity during fasting.