Chronic treatment with eltoprazine does not lead to tolerance in its anti-aggressive action, in contrast to haloperidol

Serotonin: a never-ending story

The neurotransmitter serotonin is an evolutionary ancient molecule that has remarkable modulatory effects in almost all central nervous system integrative functions, such as mood, anxiety, stress, aggression, feeding, cognition and sexual behavior. After giving a short outline of the serotonergic system (anatomy, receptors, transporter) the author's contributions over the last 40 years in the role of serotonin in depression, aggression, anxiety, stress and sexual behavior is outlined. Each area delineates the work performed on animal model development, drug discovery and development. Most of the research work described has started from an industrial perspective, aimed at developing animals models for psychiatric diseases and leading to putative new innovative psychotropic drugs, like in the cases of the SSRI fluvoxamine, the serenic eltoprazine and the anxiolytic flesinoxan. Later this research work mainly focused on developing translational animal models for psychiatric diseases and implicating them in the search for mechanisms involved in normal and diseased brains and finding new concepts for appropriate drugs.

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 127,935 (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 microg) 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 microg) decreased the rate and accelerating pattern of responding during the FI 10 schedule; lower doses were less effective. GR 127,935 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.

Do similar neural systems subserve aggressive and sexual behaviour in male rats? Insights from c-Fos and pharmacological studies

It is a common belief that male aggressive and sexual behaviour share many of the underlying neurobiological, neurological, pharmacological and neuroendocrine mechanisms. Therefore, we studied brain activation patterns in male rat after performance of aggressive and sexual behaviour and compared serotonergic pharmacology in the same paradigms to delineate possible similarities and differences. Patterns of Fos-immunoreactivity induced by aggressive and sexual encounters of Wild-type male Brown Norway rats were studied to localise the commonly activated (functionally shared) parts of the circuitry, and the specific (functionally different) parts of the neuronal circuitry. Some brain areas (caudal medial preoptic area and medial amygdala) were commonly activated, but other areas (e.g. posterodorsal parts of the medial amygdala, rostral preoptic and premammillary hypothalamus) showed remarkably specific differences in neural activation. 5-HT(1A) receptor agonists inhibit aggressive, but stimulate male sexual behaviour, whereas 5-HT(1B) receptor agonists inhibit both types of behaviour. Selective serotonin reuptake inhibitors share comparable inhibitory effects in aggression and sexual behaviour, although only at relatively high doses. We propose that separate hard-wired neural systems exist in the brain for aggressive and sexual behaviours, modulated via hierarchically 'higher-level' brain areas that are involved in the integration (gating) of the behavioural outcome of an organism.

5-HT1B receptors and aggression: A review

The serotonergic (5-HT) system in the brain is involved in the modulation of offensive aggressive behavior. The dogma that activity of the 5-HT system is inversely related to aggression is obsolete now. Research on the status of the 5-HT system before, during and after the execution of aggression is ongoing but has not yet led to a clear picture about the actual functional role of the 5-HT system, the more because state versus trait aggression seems to play a pivotal role in the outcome. Pharmacological challenges pinpoint 5-HT(1A) and 5-HT(1B) receptors as key players in the modulation of offensive aggression. This review emphasizes in particular the role of postsynaptic 5-HT(1B) (hetero) receptors as a premier site to modulate offensive aggression. Modulation of the firing and 5-HT release of the serotonergic neuron, via presynaptic 5-HT(1A) (auto) receptors, presynaptic 5-HT(1B) (auto) receptors and serotonergic transporters, may also have striking influences on aggression under certain conditions. Therefore, it is hypothesized that postsynaptic 5-HT(1B) (hetero) receptors directly influence the executive, consummatory phases of agonistic behavior, whereas presynaptic serotonergic feedback systems are particularly useful in the introductory (appetitive) phases of the agonistic behavioral complex.

5-HT1A and 5-HT1B receptor agonists and aggression: a pharmacological challenge of the serotonin deficiency hypothesis

More than any other brain neurotransmitter system, the indolamine serotonin (5-HT) has been linked to aggression in a wide and diverse range of species, including humans. The nature of this linkage, however, is not simple and it has proven difficult to unravel the precise role of this amine in the predisposition for and execution of aggressive behavior. The dogmatic view that 5-HT inhibits aggression has dominated both pharmacological research strategies to develop specific and effective novel drug treatments that reduce aggressive behavior and the pharmacological mechanistic interpretation of putative serenic drug effects. Our studies on brain serotonin and aggression in feral wild-type rats using the resident-intruder paradigm have challenged this so-called serotonin deficiency hypothesis of aggressive behavior. The well-known fact that certain 5-HT(1A/1B) receptor agonists potently and specifically reduce aggressive behavior without motor slowing and sedative effects is only consistent with this hypothesis under the assumption that the agonist mainly acts on the postsynaptic 5-HT(1A/1B) receptor sites. However, systemic injections of anti-aggressive doses of 5-HT(1A) and (1B) agonists robustly decrease brain 5-HT release due to their inhibitory actions at somatodendritic and terminal autoreceptors, respectively. The availability of the novel benzodioxopiperazine compound S-15535, which acts in vivo as a preferential agonist of the somatodendritic 5-HT(1A) auto-receptor and as an antagonist (weak partial agonist) at postsynaptic 5-HT(1A) receptors, allows for a pharmacological analysis of the exact site of action of this anti-aggressive effect. It was found that, similar to other prototypical full and partial 5-HT(1A) and/or 5-HT(1B) receptor agonists like repinotan, 8-OHDPAT, ipsapirone, buspirone, alnespirone, eltoprazine, CGS-12066B and CP-93129, also S-15535 very effectively reduced offensive aggressive behavior. Unlike the other ligands, however, a remarkable degree of behavioral specificity was observed after treatment with S-15535, in that the anti-aggressive effects were not accompanied by inhibiting (like other 5-HT(1A) receptor agonist with moderate to high efficacy at postsynaptic 5-HT(1A) receptors) or enhancing (like agonists with activity at 5-HT(1B) receptors and alnespirone) non-aggressive motor behaviors (e.g., social exploration, ambulation, rearing, and grooming) beyond the range of undrugged animals with corresponding levels of aggression. The involvement of 5-HT(1A) and/or 5-HT(1B) receptors in the anti-aggressive actions of these drugs was convincingly confirmed by showing that the selective 5-HT(1A) receptor antagonist WAY-100635 and/or the 5-HT(1B) receptor antagonist GR-127935, while inactive when given alone, effectively attenuated/prevented these actions. Furthermore, combined administration of S-15535 with either alnespirone or CGS-42066B elicited a clear additive effect, indicated by a left-ward shift in their dose-effect curves, providing further support for presynaptic sites of action (i.e., inhibitory somatodendritic 5-HT(1A) and terminal 5-HT(1B) autoreceptors). These findings strongly suggest that the specific anti-aggressive effects of 5-HT(1A) and 5-HT(1B) receptor agonists are predominantly based on reduction rather than enhancement of 5-HT neurotransmission during the combative social interaction. Apparently, normal display of offensive aggressive behavior is positively related to brief spikes in serotonergic activity, whereas an inverse relationship probably exists between tonic 5-HT activity and abnormal forms of aggression only.

Effects of haloperidol on communicative and aggressive behavior in male mice with different experiences of aggression

Effects of two doses of haloperidol (0.1 and 0.4 mg/kg, 30 min and 24 h, IP) on communicative and aggressive behavior in C57BL/6J male mice have been studied. Some of the mice were without prior experience of aggression ("recruits"); the others had been victorious in 20 daily aggressive confrontations ("experienced winners"). Communicative behavior was estimated as the behavioral reaction to a standard tester (loser) in the partition test. Haloperidol in either dose significantly reduced communicative behavior in the "recruits." but not in the "experienced winners." Significantly fewer attacks, less total attacking time, and total time of aggressive behavior (aggressive grooming + attacks) were demonstrated by the "experiences winners," than by the "recruits," while the latency of the first attack, the number, the total and average duration of aggressive grooming events were significantly higher. In the "recruits," haloperidol dose dependently increased the latency and decreased the number of attacks, the total attacking time, and the total time of aggressive behavior 30 min and 24 h after injection. However, haloperidol did not affect the average or total time of aggressive grooming. Neither dose significantly affected any measure of aggressive behavior in the "experienced winners." It has been concluded that repeated aggression experience reduces the pharmacological sensitivity of the dopamine receptors.

Acute and subchronic effects of tiapride on isolation-induced aggression in male mice

Although the antiaggressive properties of several atypical neuroleptics are known, the actions of tiapride (a selective dopaminergic D2-receptor antagonist) on agonistic behavior have not been explored and there are no studies comparing acute and subchronic effects of this compound on aggression in rodents. In this work, the effects of tiapride (20-100 mg/kg, IP), administered acutely or subchronically for 10 days, on agonistic behaviour elicited by isolation in male mice were examined. Individually housed mice were exposed to anosmic "standard opponents" 30 min after drug administration, and the encounters were videotaped and evaluated using an ethologically based analysis. Tiapride decreased time spent in offenssive behaviors significantly, without an impairment of motor activity (60 and 80 mg/kg). Moreover, no tolerance to tiapride antiaggressive activity was observed after repeated administration of the drug. On the contrary, the action on immobility showed a clear tolerance development with repeated injections (100 mg/kg). The divergence found in the temporal course of tolerance to tiapride in its antiaggressive and motor effects is discussed.

Biological and psychopharmacological aspects of impulse-control disorders

Over the past decades extensive research has been performed on the presumed relationship between neurotransmitter functionality in the central nervous system and psychopathological states. Originally, in the mid-sixties it was hypothesized that depressive states may be associated with disorders in brain noradrenalin and/or serotonin (5-hydroxytryptamine; 5-HT) metabolism or function resulting in the socalled monoamine hypothesis of depression. Subsequent investigations revealed that altered 5-HT metabolism is not related to mood disorder per se, but to components of the depressive syndrome and more specifically to increased anxiety and/or signs of dysregulated aggression, including violent suicidal attempts. Concerning aggression and other types of disruptive or impulsive behavior, research on their neurobiological determinants has been focussed mainly on indicators of central 5-HT function.