Infusion of the serotonin1B (5-HT1B) agonist CP-93,129 into the parabrachial nucleus potently and selectively reduces food intake in rats

Unilateral infusion of the selective 5-HT_1B agonist, CP-93,129 (3-(1,2,5,6-tetrahydropyrid-4-yl) pyrrolo[3,2-b]pyrid-5-one) into the parabrachial nucleus (PBN) of the pons reduced food consumption by rats. The hypophagia was dose-related (ED₅₀ ≈ 1 nmol) and associated with fewer observations of feeding and more periods of inactivity. Water intake, grooming and exploratory activity were unaffected. CP-93,129 also decreased food intake when injected into the hypothalamic paraventricular nucleus, but this action was 50-fold less potent than administration into the PBN. Autoradiography demonstrated 5-HT_1B sites in the PBN; this binding was displaced by CP-93,129. The results implicate parabrachial 5-HT_1B receptors in mediating serotonergic enhancement of satiation. Received: 11 November 1997/Final version: 20 November 1997     

Biochemical and behavioral studies of the 5-HT1B receptor agonist,CP-94,253

CP-94,253, 3-(1,2,5,6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3,2-b]pyridine, a new serotonergic ligand, was found to exhibit significantly greater binding affinity at 5-HT_1B receptors than at 5-HT_1A or 5-HT_1C receptors. Saturation studies showed CP-94,253 to be a competitive inhibitor of [¹²⁵l]iodocyanopindolol binding to 5-HT_1B sites. Its competition curve with this radioligand was shifted to the right (decreased affinity) in the presence of Gpp(NH)p, indicating an agonist function for CP-94,253. Oral administration of CP-94,253 to rats caused inhibition of food intake, decrease in body weight gain, and hyperlocomotion, effects apparently elicited via activation of 5-HT_1B receptors. © 1992 Wiley-Liss, Inc.     

5-HT1B receptors, ventral orbitofrontal cortex, and aggressive behavior in mice

Rationale Systemic injections of 5-HT_1B receptor agonists have been shown to have specific anti-aggressive effects in aggressive individuals. One site of action for these drugs is the 5-HT_1B receptors in the ventral orbitofrontal cortex (VO PFC), an area that has been implicated in the inhibitory control of behavior and is a terminal region for 5-HT projections.Objective To assess the anti-aggressive effects of the 5-HT_1B receptor agonist CP-94,253 when microinjected into the VO PFC (0.1, 0.56, and 1.0 μg/0.2 μl) or into the infralimbic prefrontal cortex (IL PFC; 1.0 μg/0.2 μl) in separate groups of aggressive resident male mice. To confirm the 5-HT_1B receptor as the critical site of action for the anti-aggressive effects, the 5-HT_1B/D antagonist GR-127,935 was microinjected at 10.0 μg/0.2 μl into the VO PFC. After recovery from surgery, the anti-aggressive effects of microinjected CP-94,253 were studied during 5-min resident–intruder confrontations that were recorded and analyzed.Results Microinjections of CP-94,253 (0.56 and 1.0 μg/0.2 μl) dose-dependently reduced the frequency of attack bites and sideways threats. This effect was behaviorally specific because non-aggressive motor activities were not significantly altered by the drug. In the IL vmPFC or in an area lateral to the VO PFC, CP-94,253 (1.0 μg/0.2 μl) did not have significant behavioral effects.Conclusions The results highlight the 5-HT_1B receptors in the VO PFC as a particularly important site for the inhibition of species-typical aggressive behavior in male mice.     

5-HT1B receptors modulate components of satiety in the rat: behavioural and pharmacological analyses of the selective serotonin1B agonist CP-94,253

Abstract Rationale. 5-HT_1B receptors are thought to be one of the receptor subtypes that mediate the inhibitory control of serotonin on food intake and satiety. Objective. To use the selective 5-HT_1B receptor agonist, CP-94,253 as a probe of 5-HT_1B receptor function in feeding behaviour, and to confirm the pharmacological selectivity of CP-94,253-induced hypophagia with a range of antagonists. Methods. Dose-response functions for CP-94,253 (0, 1.25, 2.5, 5.0 mg/kg; IP) were determined in animals consuming wet mash in a 40-min test session during which time-sampled behavioural observations were collected to evaluate satiety sequences. A meal patterning study was carried out in a separate group of rats. The 5-HT_1A antagonist WAY 100,635 (0, 1.0, 3.0 mg/kg; SC), the 5-HT_1B/1D antagonist GR 127,935 (0, 3 mg/kg; IP), and the 5-HT_1B antagonist SB 224289 (0, 2.5, 5.0 mg/kg; IP) were used to confirm that 5-HT_1B receptor subtypes were responsible for the action of CP-94,253 on feeding behaviour. Results. CP-94,253 (2.5 mg/kg) reduced food intake and preserved the satiety sequence in animals consuming a diet of mash. GR 127,935 (3.0 mg/kg) and SB 224289 (2.5 mg/kg), but not WAY 100,635, attenuated the hypophagic effect of the 5-HT_1B agonist, and returned the changes in satiety sequence to control patterns. Meal patterning analyses indicated that CP-94,253 (2.5 mg/kg) reduced food intake through a decrease in meal size and duration in the absence of any alteration in the rate of eating. A hypodipsic action of CP-94,253 was also observed (2.5 and 5.0 mg/kg). Conclusion. These findings imply that 5-HT_1B receptors regulate discrete elements of satiety. We discuss the potential role of 5-HT_1B agonists for the treatment of obesity. Electronic Publication     

Anti-aggressive effects of agonists at 5-HT1B receptors in the dorsal raphe nucleus of mice

Rationale In rodents, serotonin 1B (5-HT_1B) agonists specifically reduce aggressive behaviors, including several forms of escalated aggression. One form of escalated aggression is seen in mice that seek the opportunity to attack another mouse by accelerating their responding during a fixed interval (FI) schedule. Responses preceding the opportunity to attack may reflect aggressive motivation. Objective This study investigated the effects of two 5-HT_1B receptor agonists on the motivation to fight and the performance of heightened aggression. Materials and methods Male mice were housed as “residents” and performed nose-poke responses on an FI 10-min schedule with the opportunity to briefly attack an “intruder” serving as the reinforcer. In the first experiment, the 5-HT_1B receptor agonist, CP-94,253 (0–10 mg/kg, IP), was given 30 min before the FI 10 schedule. To confirm that CP-94,253 achieved its effects via 5-HT_1B receptors, the 5HT_1B/1D receptor antagonist, GR 127935 (10 mg/kg, IP) was administrated before the agonist injection. In the second experiment, the 5-HT_1B agonist CP-93,129 (0–1.0 μg) was microinjected into the dorsal raphe 10 min before the FI 10 schedule. Results The agonists had similar effects on all behaviors. CP-94,253 and CP-93,129 significantly reduced the escalated aggression towards the intruder at doses lower than those required to affect operant responding. The highest doses of CP-94,253 (10 mg/kg) and CP-93,129 (1.0 μg) decreased the rate and accelerating pattern of responding during the FI 10 schedule; lower doses were less effective. GR 127935 antagonized CP-94,253’s effects on all other behaviors, except response rate. Conclusions These data extend the anti-aggressive effects of 5-HT_1B agonists to a type of escalated aggression that is rewarding and further suggest that these effects are associated with actions at 5-HT_1B receptors in the dorsal raphe.     

Serotonin 1B and 2C receptor interactions in the modulation of feeding behaviour in the mouse

Rationale To examine the functional relationship between 5-HT_1B receptors (5-HT_1B-R) and 5-HT_2C receptors (5-HT_2C-R) in the control of food intake. Objectives To compare the hypophagic effect of the 5-HT_2C/1B-R agonist m-chlorophenylpiperazine (mCPP), with that of the selective 5-HT_1B-R agonist CP-94,253 in both wildtype (WT) and 5-HT_2C knockout (KO) mice. Methods The hypophagic effects of mCPP (1, 3 and 5.6 mg/kg) and CP-94,253 (5, 10 and 20 mg/kg) were assessed in WT and 5-HT_2C KO mice using the behavioural satiety sequence paradigm. The effects of pre-treatment with the selective 5-HT_2C-R antagonist SB 242,084 (0.5 and 1.5 mg/kg) were assessed in two groups of WT mice, with each group given only mCPP or CP-94,253. Results The 5-HT_2C/1B receptor agonist mCPP and the selective 5-HT_1B receptor agonist CP-94,253 both suppressed food intake in WT mice. 5-HT_2C KO mice were insensitive to the hypophagic effects of mCPP but were more sensitive to CP-94,253-induced hypophagia than WT controls. mCPP induced a significant increase in post-prandial activity in 5-HT_2C KO mice, but this effect was absent in 5-HT_2C KO mice who were given CP-94,253. Data from WT mice, who were pre-treated with the 5-HT_2C receptor antagonist SB 242,084 and then challenged with either mCPP or CP-94,253, were similar to those obtained from 5-HT_2C KO mice. Conclusions 5-HT_2C-R and 5-HT_1B-R activation are each sufficient to induce a hypophagic response. However, concurrent 5-HT_2C-R inactivation can potentiate the hypophagic response to 5-HT_1B-R activation, consistent with an inhibitory role for the 5-HT_2C-R in behaviour mediated by the activation of other 5-HT receptors. These results also confirm that 5-HT_1B-R activation alone cannot account for the hyperactive response of 5-HT_2C KO mice to mCPP.     

Aggression heightened by alcohol or social instigation in mice: reduction by the 5-HT1B receptor agonist CP-94,253

Rationale: Models of heightened aggression may be particularly relevant in exploring pharmacological options for the clinical treatment of aggressive and impulsive disorders. Objectives: To investigate and compare the effects of a 5-HT_1B selective agonist, CP-94,253, on aggression that was heightened as a result of 1) social instigation or 2) alcohol treatment. Methods: Male CFW mice were administered 1.0 g/kg EtOH and were subsequently confronted by an intruder in their home cage. In a separate experimental procedure, resident male mice were instigated to aggressive behavior by brief exposure to a provocative stimulus male. To test the hypothesis that activation of the 5-HT_1B receptor subtype would preferentially attenuate heightened aggression, in comparison to the moderate levels of species-typical aggressive behaviors, the selective agonist, CP-94,253 (1.0–30 mg/kg, IP), and antagonists to the 5-HT_1B (GR 127935; 10 mg/kg, IP) and the 5-HT_1A receptor (WAY 100,635; 0.1 mg/kg IP) were used. Results: CP-94,253 suppressed non-heightened aggressive behavior (ED₅₀=7.2 mg/kg ). GR 127935, but not WAY 100,635 shifted the ED₅₀ for CP-94,253 to 14.5 mg/kg. Importantly, the anti-aggressive effects of CP-94,253 were not accompanied by locomotor sedation. Alcohol-heightened and instigation-heightened aggression were suppressed at lower doses than those necessary to suppress non-heightened aggression (ED₅₀=3.8 and 2.7 mg/kg, respectively). Conclusions: The current results support the hypothesis that activation of 5-HT_1B receptors modulates very high levels of aggressive behavior in a pharmacologically and behaviorally specific manner. Received: 5 January 1999 / Final version: 16 April 1999     

CP-135,807, a selective 5-HT1D agonist: effects in drug discrimination and punishment procedures in the pigeon

 CP-135,807 [3-(N-methylpyrrolidin-2R-ylmeth- yl)-5-(3-nitropyrid-2-yl)amino-1H-indole] binds with high affinity to central 5-HT_1D receptors, and in functional studies produces dose-dependent decreases in extracellular serotonin. These and other findings have suggested that CP-135,807 may act as a terminal 5-HT autoreceptor agonist. In an attempt to characterize the behavioral activity of selective 5-HT_1D ligands, adult male Carneau pigeons were trained to discriminate IM injections of 0.1 mg/kg CP-135,807 from saline under a two-key, fixed ratio schedule of food-reinforced key pecking. CP-135,807 and the structurally unrelated 5-HT_1D agonist CP-286,601 fully and dose-dependently substituted for the training dose. In contrast, little substitution was observed following administration of 8-OH-DPAT, a potent 5-HT_1A agonist, the 5-HT_1B agonist CP-94,253, or the serotonin reuptake inhibitor sertraline. In addition, the discriminative stimulus produced by CP-135,807 was not blocked by WAY 100,635, a selective 5-HT_1A antagonist, but was completely and dose-dependently antagonized by the selective 5-HT_1D antagonist, GR127935. In subjects trained under a multiple schedule of punished and unpunished responding, 8-OH-DPAT produced large increases in punished responding while having little effect on unpunished responding. In contrast, CP-135,807 and CP-94,253 produced no antipunishment effects, while GR127935 produced modest increases in punished responding. Collectively, these results suggest that CP-135,807 produces centrally mediated psychoactive effects that differ distinctly from those of 5-HT_1A agonists. Received: 23 April 1996 / Final version: 9 July 1996     

Binding and uptake studies with [3H]CP-93, 129, a radiolabeled selective 5-HT1B receptor ligand

3-(1,2,5,6-Tetrahydro-4-pyridyl)-5-pyrrolo[3,2-b]pyridone, CP-93, 129, is a selective agonist ligand for 5-HT_1B receptors. High affinity binding sites of [³H]CP-93, 129 were found in rat whole brain membranes, which showed K_D and B_max values similar to those for 5-HT_1B sites labeled by [³H]5-HT. Uptake of [³H]CP-93, 129 in crude rat synaptosomes was also observed, which was potently inhibited by 5-HT uptake blockers and 5-HT but not by desipramine (NE uptake blocker) or tametraline (NE and DA uptake blocker). Because of this sensitivity to 5-HT uptake inhibitors and the structural similarity of CP-93, 129 to serotonin, [³H]CP-93, 129 uptake probably occurred in 5-HT neurons.     

5-HT<Subscript>1B</Subscript> receptor inhibition of alcohol-heightened aggression in mice: comparison to drinking and running

Rationale Serotonin 5-HT_1B receptors are promising targets for the management of several mood and impulse disorders. Objective These experiments examine a 5-HT_1B agonist, CP-94,253, and attempt to distinguish between its effects on seeking to perform three rewarding behaviors: aggression, drinking, and wheel running. Materials and methods Male CFW mice perform nose-poke responses that are maintained by a fixed interval schedules of 10-min (FI10) schedule to gain access to one of three rewarding activities. The first experiment studies mice reinforced by the opportunity to confront an intruder mouse after drinking water or alcohol; the second studies mice reinforced by the presentation of alcoholic or non-alcoholic solutions (i.e., 6% ethanol, 0.05% saccharin vs 0.05% saccharin); the third studies mice reinforced by access to a running wheel. Results CP-94,253 (1.0–10 mg/kg i.p.) dose-dependently reduces aggression, drinking, and wheel running. Of these behaviors, alcohol-heightened aggression is the most sensitive to the 5-HT_1B receptor agonist (ED50 = 4.8 mg/kg). Responding for the opportunity to drink or engage in alcohol-heightened aggression is suppressed by the highest dose of CP-94,253, whereas CP-94,253 does not affect responding that is reinforced by wheel running or species-typical aggression. Conclusions These results confirm the inhibitory effects of 5-HT_1B receptor stimulation on aggressive performance and drinking. They also reveal an inhibition of voluntary wheel running, contrary to the stimulation of running in a novel, open arena. 5-HT_1B receptor agonists may be particularly useful for the treatment of aggressive behavioral disorders, but their efficacy and potency appear to be sensitive to the intensity and context of the behavior.     

Serotonin1B receptor stimulation enhances dopamine-mediated reinforcement

The effect of 5-HT_1B receptor stimulation on dopamine-mediated reinforcement in rats was investigated using intravenous self-administration of the selective dopamine uptake inhibitor GBR-12909 on an FR5 schedule of reinforcement. Pretreatment with the 5-HT_1A/1B receptor agonist CGS-12066B (1–10 mg/kg, IP) dose-dependently reduced the self-administration of GBR-12909 (83 μg/injection) by increasing the interval between drug injections, consistent with a enhancement of the reinforcing effects of GBR-12909. Additionally, CGS-12066B pretreatment (3 mg/kg, IP) shifted the dose-effect function for GBR-12909 self-administration to the left. Pretreatment with the selective 5-HT_1A receptor agonist 8-OH-DPAT (0.03– 1.0 mg/kg, SC) had no significant effect on GBR-12909 self-administration (83 μg/injection), indicating that the effect of CGS-12066B is not mediated by the 5-HT_1A receptor. Finally, CGS-12066B pretreatment (1–10 mg/kg, IP) did not alter the self-administration of cocaine (0.03–0.5 mg/injection), suggesting that the simultaneous stimulation of multiple 5-HT receptor subtypes by the indirect 5-HT agonist properties of cocaine may mask the effect of 5-HT_1B receptor stimulation on DA-mediated reinforcement. Received: 5 February 1996/Final version: 20 June 1996     

Distress vocalizations in maternally separated mouse pups: modulation via 5-HT1A, 5-HT1B and GABAA receptors

Rationale: Young rodents emit ultrasonic vocalizations (USVs) when separated from their dams and littermates. Pharmacological agents that act on GABA_A and/or 5-HT receptors and that alleviate anxiety in humans reduce the emission of these calls. Objectives: 1) to investigate specific 5-HT₁ receptor subtypes that modulate maternal separation-induced USVs in mice; 2) to assess the behavioral specificity of these effects; and 3) to compare 5-HT₁ agonists with a positive neurosteroid modulator of the GABA_A receptor complex. Methods: Seven-day old CFW mouse pups were isolated from their littermates and placed onto a 20°C surface for 4 min. USVs between 30 and 80 kHz, grid crossing, and rectal temperature were measured in separate groups of mouse pups following subcutaneous administration of 5-HT_1A and 5-HT_1B receptor agonists and antagonists, the neurosteroid allopregnanolone, or the benzodiazepine midazolam. Results: The 5-HT_1A agonists (+)8-OH-DPAT (0.01–0.1 mg/kg) and flesinoxan (0.3–1.0 mg/kg), the selective 5-HT_1B agonist CP-94,253 (0.03–30.0 mg/kg), and the mixed 5-HT_1B/2C receptor agonist TFMPP (0.1–10.0 mg/kg) dose-dependently reduced USVs. These effects were reversed by the 5-HT_1A receptor antagonist WAY 100,635 (0.1 mg/kg) or the 5-HT_1B/D receptor antagonist GR 127935 (0.1 mg/kg). The effects of TFMPP were biphasic; low doses (i.e. 0.01 and 0.03 mg/kg) increased the rate of vocalization. Midazolam and allopregnanolone also reduced USVs. The highest doses of flesinoxan, (+)8-OH-DPAT, and allopregnanolone suppressed locomotion, whereas CP-94,253, TFMPP, and midazolam stimulated motor activity. Conclusions: These experiments confirm that agonists at the 5-HT₁ receptors and a positive allosteric modulator of the GABA_A receptor complex decrease maternal separation-induced USVs in mice, with 5-HT_1B manipulations dissociating the effects on vocalizations from sedative effects. Received: 22 September 1999 / Final version: 14 December 1999     

Social instigation and aggressive behavior in mice: role of 5-HT1A and 5-HT1B receptors in the prefrontal cortex

Rationale  Social instigation is used in rodents to induce high levels of aggression, a pattern of behavior with certain parallels to that of violent individuals. This procedure consists of a brief exposure to a provocative stimulus male, before direct confrontation with an intruder. Studies using 5-HT_1A and 5-HT_1B receptor agonists show an effective reduction in aggressive behavior. An important site of action for these drugs is the ventral orbitofrontal cortex (VO PFC), an area of the brain which is particularly relevant in the inhibitory control of aggressive and impulsive behavior. Objectives  The objectives of the study are to assess the anti-aggressive effects of 5-HT_1A and 5-HT_1B agonist receptors [8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT) and CP-93,129] in the VO PFC of socially provoked male mice. To confirm the specificity of the receptor, 5-HT_1A and 5-HT_1B antagonist receptors (WAY-100,635 and SB-224,289) were microinjected into the same area, in order to reverse the agonist effects. Results  8-OH-DPAT (0.56 and 1.0 μg) reduced the frequency of attack bites. The lowest dose of CP-93,129 (0.1 μg) also decreased the number of attack bites and lateral threats. 5-HT1A and 5-HT1B receptor agonists differed in their effects on non-aggressive activities, the former decreasing rearing and grooming, and the latter, increasing these acts. Specific participation of the 1A and 1B receptors was verified by reversal of anti-aggressive effects using selective antagonists WAY-100,635 (10.0 μg) and SB-224,289 (1.0 μg). Conclusions  The decrease in aggressiveness observed with microinjections of 5-HT_1A and 5-HT_1B receptor agonists into the VO PFC of socially provoked mice, supports the hypothesis that activation of these receptors modulates high levels of aggression in a behaviorally specific manner.     

3-(1,2,5,6-Tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one: a potent and selective serotonin (5-HT1B) agonist and rotationally restricted phenolic analogue of 5-methoxy-3-(1,2,5,6-tetrahydropyrid-4-yl)indole

The synthesis and in vitro and in vivo characteristics of 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (1, CP-93,129) are described. This rotationally restricted phenolic analogue of RU-24,969 is a potent (15 nM) and selective (200x vs the 5-HT1A receptor, 150x vs the 5HT1D receptor) functional agonist for the 5-HT1B receptor. Direct infusion of 1 into the paraventricular nucleus of the hypothalamus of rats significantly inhibits food intake, implicating the role of 5-HT1B receptors in regulating feeding behavior in rodents. 3-(1,2,5,6-Tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (1) has also been shown to be biochemically discriminatory in its ability to selectively inhibit forskolin-stimulated adenylate cyclase activity only at the 5-HT1B receptor. The source of the selectivity of 1 appears to lie in the ability of a pyrrolo[3,2-b]pyrid-5-one to act as a rotationally restricted bioisosteric replacement for 5-hydroxyindole.     

5-HT1B receptors modulate release of [3H]dopamine from rat striatal synaptosomes

The effect of the selective r5-HT_1B agonist 3-(1,2,5,6-tetrahydro)-4-pyridil-5-pyrrolo [3,2-b] pyril-5-one (CP93,129) on the K⁺-evoked overflow of [³H]dopamine was studied in rat striatal synaptosomes loaded with [³H]dopamine. The aim of the study was to investigate the participation of 5-HT_1B receptors in the serotonergic modulation of striatal dopaminergic transmission. The Ca²⁺-dependent, tetrodotoxin-resistant K⁺-evoked overflow of [³H]dopamine was inhibited by CP93,129 (0.01–100 μM) in a concentration-dependent manner (IC₅₀=1.8 μM; maximal inhibition by 35.5% of control). [±]8-OH-DPAT, a 5-HT_1A receptor agonist, [+/–]DOI, a 5-HT₂ receptor agonist, and 2-methyl-5-hydroxytryptamine, a 5-HT₃ receptor agonist, at concentrations ranging from 0.01 μM to 100 μM did not show any significant effect. Neither ketanserin (1 μM and 5 μM), a selective 5-HT₂/5-HT_1D receptor antagonist, nor ondansetron (1 μM), a 5-HT₃ receptor antagonist, changed the inhibitory effect of CP93,129. SB224289, GR55562, GR127935, isamoltane and metergoline, selective and non-selective 5-HT_1B receptor antagonists, in contrast, used at a concentration of 1 μM, antagonized the inhibitory effect of CP93,129 (3 μM and 10 μM). SB224289, a selective 5-HT_1B receptor antagonist, inhibited the effect of CP93,129 in a concentration-dependent manner; the calculated K _i value was 1.8 nM. Our results indicate that in rat striatal axon terminals the K⁺-evoked release of dopamine is regulated by the presynaptic 5-HT_1B heteroreceptors. Received: 7 September 1998 / Accepted: 2 November 1998     

Autoreceptors can modulate 5-hydroxytryptamine release from porcine and human small intestine in vitro

The effects of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists were studied on the release of 5-HT from enterochromaffin cells of incubated strips of porcine and human small intestine. Tetrodotoxin (1 μmol/l) was present in the incubation medium to block neuronally mediated inputs to the enterochromaffin cells. The 5-HT_1A receptor agonist (+)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT, 1 μmol/l) and the 5-HT₂ receptor agonist α-methyl-5-HT (1 μmol/l) increased 5-HT release by 40% in about 60% of the human preparations.These agonists showed no effect on 5-HT release in porcine intestinal mucosa. The 5-HT₃ receptor agonist 2-methyl-5-HT (3–100 μmol/l) increased 5-HT release in both species by 60% (pig) and 90% (man), respectively. These stimulatory effects were antagonized by tropisetron (10 nmol/l). The 5-HT₄ receptor agonist 5-methoxytryptamine (0.3–30 μmol/l) reduced 5-HT release by about 50% in both species. These inhibitory effects were antagonized by tropisetron (3 μmol/l). The basal outflow of 5-HT from the intestinal mucosa was not significantly affected by tropisetron (10 nmol/l; 3 μmol/l). The specific 5-HT₄ receptor antagonist GR 113808 ((1-[2-methylsulphonyl)amino]ethyl]-4-piperidinyl]methyl-1-methyl-1H-indole-3-carboxylate) (0.1 μmol/l) which by itself did not significantly affect 5-HT release from human duodenal specimens blocked the inhibitory effect of 5-methoxytryptamine (30 μmol/l). These findings indicate that stimulatory 5-HT₃ and inhibitory 5-HT₄ receptors are present on enterochromaffin cells of the porcine and human intestinal mucosa. Under the present experimental conditions endogenous 5-HT does not significantly activate these receptors. Stimulatory 5-HT_1A and 5-HT₂ receptors may additionally be present on human enterochromaffin cells. Received: 19 September 1997/ Accepted: 29 January 1998     

An examination of the behavioural specificity of hypophagia induced by 5-HT1B, 5-HT1C and 5-HT2 receptor agonists using the post-prandial satiety sequence in rats

Previous studies have shown that administration of 5-HT_1B, 5-HT_1C or 5-HT₂ agonists decreases food intake in rats. However, it has not been established whether these drugs induce satiety or decrease feeding by a non-specific mechanism. In the present study the post-prandial satiety sequence was used to characterise the actions of the 5-HT₂ receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), the 5-HT_1B/5-HT_1C receptor agonists, 1-(3-chorophenyl) piperazine (mCPP) and 1-[3-(trifluoromethyl)phenyl] piperazine (TFMPP), and the 5-HT_1B agonist, 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)H-indole (RU 24969), on feeding in rats. All four compounds reduced food intake in rats that had been food deprived overnight. The 5-HT_1B/5-HT_1C agonists, TFMPP (at a dose of 1.0 mg/kg) and mCPP (at a dose of 3.0 mg/kg), appeared to produce satiety as their effects on the satiety sequence were similar to those induced by a food pre-load. In contrast, the 5-HT_1B agonist RU 24969 and the 5-HT₂ agonist DOI did not produce behavioural profiles that resembled satiety. Thus, RU 24969 elevated active behaviours and did not accelerate resting whereas DOI appeared to induce hypophagia by a non-specific fragmentation of behaviour. The results suggest that simultaneous activation of 5-HT_1B and 5-HT_1C receptors may be sufficient to elicit behaviourally specific satiety in the rat. In contrast, selective activation of 5-HT₂ receptors does not induce satiety but elicits active behaviours and decreases feeding by response competition.     

Effects of selective h5-HT1B (SB-216641) and h5-HT1D (BRL-15572) receptor ligands on guinea-pig and human 5-HT auto- and heteroreceptors

Human cerebral cortical slices and synaptosomes, guinea-pig cerebral cortical slices and human right atrial appendages were used to study the effects of SB-216641, a preferential h5-HT_1B receptor ligand, and of BRL-15572, a preferential h5-HT_1D receptor ligand, on the presynaptic h5-HT_1B and h5-HT_1B-like autoreceptors in the human and guinea-pig brain preparations, respectively, and on the presynaptic h5-HT_1D heteroreceptors in the human atrium. The brain preparations, preincubated with [³H]serotonin ([³H]5-HT), and the segments of atrial appendages, preincubated with [³H]noradrenaline, were superfused with modified Krebs’ solution and tritium overflow was evoked electrically (human and guinea-pig cerebral cortex slices and human atrial appendages) or by high K⁺ (human cerebral cortex synaptosomes). The electrically evoked tritium overflow from guinea-pig cerebral cortex slices was reduced by the 5-HT receptor agonist 5-carboxamidotryptamine (5-CT). This effect was not modified by BRL-15572 (2μM; concentration 154 times higher than its K_i at h5-HT_1D receptors) but was antagonized by SB-216641 (0.1μM; concentration 100 times higher than its K_i at h5-HT_1B receptors; apparent pA₂ 8.45). SB-216641 (0.1μM) by itself facilitated, whereas BRL-15572 (2μM) did not affect, the evoked overflow. In human cerebral cortex slices SB-216641 (0.1μM) also facilitated, and BRL-15572 (2μM) again failed to affect, the electrically evoked tritium overflow. In human cerebral cortical synaptosomes, 5-CT reduced the K⁺-evoked tritium overflow. This response was unaffected by BRL-15572 (300nM) but antagonized by SB-216641 (15nM; drug concentrations 23 and 15 times higher than their K_i at h5-HT_1D and h5-HT_1B receptors, respectively). Both drugs, given alone, did not modify the K⁺-evoked tritium overflow. In human atrial appendages, the electrically evoked tritium overflow was inhibited by 5-HT in a manner susceptible to antagonism by BRL-15572 (300nM; 23 times K_i at h5-HT_1D receptors) but not by SB-216641 (30nM; 30 times K_i at h5-HT_1B receptors). Both drugs by themselves did not change the electrically evoked tritium overflow. In conclusion, SB-216641 behaves as a preferential antagonist at native human 5-HT_1B receptors and BRL-15572 as a preferential antagonist at native human 5-HT_1D receptors. These compounds are clearly useful tools for the differentiation between human 5-HT_1B and 5-HT_1D receptors in functional studies. Received: 14 March 1997 / Accepted: 18 May 1997     

Effect of 5-HT1A receptor agonists in two models of anxiety after dorsal raphe injection

The purpose of the present study was two-fold. Firstly, to present a more comprehensive analysis of the disinhibitory effects of 5-HT_1A receptor agonists after discrete dorsal raphe (DRN) injections (Higgins et al. 1988). Secondly, the effects of the 5-HT_1B receptor agonist CGS12066B and the 5-HT_1B/1C agonist mCPP were examined following injection into this nucleus. The increases in social interaction (SI) induced by intra-raphe injections of 8-OH DPAT (0.02–1 μg), buspirone (0.04–0.2 μg), ipsapirone (0.2 μg) and gepirone (0.2–1 μg) under a high light unfamiliar paradigm (HLU) were typically due to increased bout frequency, duration and a higher incidence of sniff, follow, allogroom behaviour. These increases were qualitatively similar to those seen in control animals tested under low light/familiar (LLF) conditions, thus supporting the belief that the drug-induced increases in SI reflected decreases in anxiety. Furthermore, at doses effective under the HLU condition, 8-OH DPAT, buspirone and gepirone failed to modify SI under conditions of minimal suppression (LLF paradigm). At doses which significantly increased punished responding in a water-lick conflict test 8-OH DPAT, ipsapirone and gepirone tended to also increase unpunished rates of drinking. However, in drug untreated rats, prior habituation to the test apparatus also increased unpunished drinking, suggesting some neophobia-induced suppression. At a comparatively high dose, the 5-HT_1B agonist CGS12066B (2.5 μg), but not the putative 5-HT_1B/1c agonist mCPP (0.5–12.5 μg), increased SI under the HLU condition. Considered along-side the other compounds described in this report, the relative potency of CGS12066B may be reflective of a 5-HT_1A receptor interaction. Together, these data support the proposal that the DRN is an important site through wich 5-HT_1A receptor agonists express their anxiolytic actions.     

Serotonin receptors in the caudal brainstem are necessary and sufficient for the anorectic effect of peripherally administered mCPP

 The role of caudal brainstem 5-HT receptors in mediating the anorectic effect of the direct 5-HT_2C/1B agonist, mCPP [1-(3-chlorophenyl)piperazine dihydrochloride], was evaluated. We demonstrated, first, that systemic injections of mCPP yielded a dose-related suppression of intra-oral intake of 12.5% glucose in intact rats and in chronically maintained supracollicular decerebrate rats. The results of the decerebrate experiment suggest that 5-HT receptors in the caudal brainstem are sufficient for mediating the drug’s intake effect. We also showed a dose-related intake suppression when mCPP was delivered to the fourth ventricle of intact rats, with potent suppression obtained at doses well below threshold for systemic administration. Whether and to what extent the 5-HT_2C/2A antagonist, mesulergine reverses the intake suppression that follows systemic or 4th ICV injection of mCPP was examined. Fourth ICV co-administration of mesulergine (60 μg) and mCPP (40 μg) eliminated the approximately 50% intake suppression observed when mCPP was delivered alone, a result that affirms the receptor selectivity of the 4th ICV agonist effect. We showed, further, that 4th ICV mesulergine (60 μg) completely reversed the intake suppression produced by systemic mCPP (2 mg/kg). The latter result indicates that stimulation of 5-HT receptors in the caudal brainstem is necessary for the intake suppression produced by systemic administration of this 5-HT agonist in the intact rat. Received: 24 June 1997 / Final version: 13 November 1997