Ethopharmacological studies differentiate the effects of various serotonergic compounds on aggression in rats

Effect of Hallucinogens on Unconditioned Behavior

Because of the ethical and regulatory hurdles associated with human studies, much of what is known about the psychopharmacology of hallucinogens has been derived from animal models. However, developing reliable animal models has proven to be a challenging task due to the complexity and variability of hallucinogen effects in humans. This chapter focuses on three animal models that are frequently used to test the effects of hallucinogens on unconditioned behavior: head twitch response (HTR), prepulse inhibition of startle (PPI), and exploratory behavior. The HTR has demonstrated considerable utility in the neurochemical actions of hallucinogens. However, the latter two models have clearer conceptual bridges to human phenomenology. Consistent with the known mechanism of action of hallucinogens in humans, the behavioral effects of hallucinogens in rodents are mediated primarily by activation of 5-HT2A receptors. There is evidence, however, that other receptors may play secondary roles. The structure-activity relationships (SAR) of hallucinogens are reviewed in relation to each model, with a focus on the HTR in rats and mice.

Multiple receptors contribute to the behavioral effects of indoleamine hallucinogens

Serotonergic hallucinogens produce profound changes in perception, mood, and cognition. These drugs include phenylalkylamines such as mescaline and 2,5-dimethoxy-4-methylamphetamine (DOM), and indoleamines such as (+)-lysergic acid diethylamide (LSD) and psilocybin. Despite their differences in chemical structure, the two classes of hallucinogens produce remarkably similar subjective effects in humans, and induce cross-tolerance. The phenylalkylamine hallucinogens are selective 5-HT(2) receptor agonists, whereas the indoleamines are relatively non-selective for serotonin (5-HT) receptors. There is extensive evidence, from both animal and human studies, that the characteristic effects of hallucinogens are mediated by interactions with the 5-HT(2A) receptor. Nevertheless, there is also evidence that interactions with other receptor sites contribute to the psychopharmacological and behavioral effects of the indoleamine hallucinogens. This article reviews the evidence demonstrating that the effects of indoleamine hallucinogens in a variety of animal behavioral paradigms are mediated by both 5-HT(2) and non-5-HT(2) receptors.

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.

The serotonin 5-HT(2A) receptor subtype does not mediate apomorphine-induced aggressive behaviour in male Wistar rats

We have studied the effect of the 5-HT(2A) receptor antagonists on apomorphine-induced aggressive behaviour in male Wistar rats. In acute behavioural experiments with apomorphine-pretreated (1.0 mg/kg, s.c., once daily, 2 weeks) animals, risperidone (0.5 and 1.0 mg/kg) inhibited aggressive behaviour, but ketanserin and ritanserin (0.5-5. 0 mg/kg) had no effect on the latency and intensity of aggressive behaviour. Concomitant risperidone (0.5 mg/kg) and haloperidol (0.03 and 0.3 mg/kg) administration blocked aggressive behaviour completely. In conclusion, our experiments confirm that inhibition of the apomorphine-induced aggressive behaviour is elicited by drugs with dopamine (DA) but not with 5-HT(2A) antagonistic activity. Moreover, it may be concluded that the serotonin 5-HT(2A) receptor subtype does not alter the DA-mediated behaviour.

Insights into the neurobiology of impulsive behavior from serotonin receptor knockout mice

Although the concept of impulsivity has proved useful in human and animal studies of addiction, violent aggression, and violent suicide, and has been recognized as an important component of human behavior, little research has been done to understand the underlying psychobiological mechanisms. We explore the concept of impulsivity and its relation with the neurotransmitter serotonin in the context of aggressive behavior and behavior associated with positive reinforcement using a knockout mouse that lacks one of the serotonin receptors, the 5-HT1B receptor. This knockout mouse shows more impulsive aggression, acquires cocaine self-administration faster, and drinks more alcohol than the corresponding wild-type control. We show that the impulsive characteristics of these mice are not due to change in cognitive functions: in a cognitive task involving a choice between a small immediate one and a larger more delayed reward, knockout mice showed intact choice and timing capabilities and good discrimination of reward amounts. Thus, this mouse may prove an animal model of addiction and motor impulsivity.

Regulation of 5-HT receptors and the hypothalamic-pituitary-adrenal axis. Implications for the neurobiology of suicide

Disturbances in the serotonin (5-HT) system is the neurobiological abnormality most consistently associated with suicide. Hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis is also described in suicide victims. The HPA axis is the classical neuroendocrine system that responds to stress and whose final product, corticosteroids, targets components of the limbic system, particularly the hippocampus. We will review results from animal studies that point to the possibility that many of the 5-HT receptor changes observed in suicide brains may be a result of, or may be worsened by, the HPA overactivity that may be present in some suicide victims. The results of these studies can be summarized as follows: (1) chronic unpredictable stress produces high corticosteroid levels in rats; (2) chronic stress also results in changes in specific 5-HT receptors (increases in cortical 5-HT2A and decreases in hipocampal 5-HT1A and 5-HT1B); (3) chronic antidepressant administration prevents many of the 5-HT receptor changes observed after stress; and (4) chronic antidepressant administration reverses the overactivity of the HPA axis. If indeed 5-HT receptors have a partial role in controlling affective states, then their modulation by corticosteroids provides a potential mechanism by which these hormones may regulate mood. These data may also provide a biological understanding of how stressful events may increase the risk for suicide in vulnerable individuals and may help us elucidate the neurobiological underpinnings of treatment resistance.

Assessment of relative efficacies of 5-HT1A receptor ligands by means of in vivo animal models

We have evaluated the effects of ligands with varying efficacies at beta-adrenoceptors and 5-HT1A receptors in three in vivo models reflecting pre- and/or postsynaptic 5-HT1A receptor activation. Forepaw treading in rats is mediated by postsynaptic 5-HT1A receptors, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamin)tetralin)-induced discriminative stimulus is predominantly mediated by postsynaptic, but presynaptic 5-HT1A receptors might also be involved, and footshock-induced ultrasonic vocalization involves predominantly presynaptic 5-HT1A receptors. In vitro receptor binding studies demonstrated high beta-adrenoceptor and 5-HT1A receptor affinity of (-)-penbutolol, high beta-adrenoceptor and 60 times lower 5-HT1A receptor affinity of (+)-penbutolol, high beta-adrenoceptor affinity and about 100 times lower 5-HT1A receptor affinity of pindolol and (-)-tertatolol, only affinity for beta-adrenoceptors of metoprolol and ICI 118,551 (erythro-D,L-1-(7-methylindan-4-yloxy)-3-isopropylamine-b utan-2-ol, and only affinity for 5-HT1A receptors of WAY 100.635 ((N-[2-[4- (2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexane-carboxamide). (-)-Penbutolol, (-)-tertatolol, pindolol and WAY 100.635 antagonized 5-MeODMT-induced (5-methoxy-N, N-dimethyltryptamine) forepaw treading in rats, and (+)-penbutolol, ICI 118,551 and metoprolol were inactive. (-)-Penbutolol, WAY 100.635 and (-)-tertatolol antagonized 8-OH-DPAT-induced discriminative stimulus in rats, pindolol and metoprolol showed a mixed antagonistic and agonistic profile. Pindolol antagonized footshock-induced ultrasonic vocalization in rats, tertatolol inhibited maximum 36% and WAY 100.635, (-)-penbutolol, (+)-penbutolol, metroprolol and ICI 118,551 were inactive. (-)-Penbutolol and WAY 100.635 reversed 8-OH-DPAT-induced inhibition of ultrasonic vocalization completely, (-)-tertatolol reversed maximum 52% and (+)-penbutolol and pindolol were inactive. It is concluded, that efficacies at 5-HT1A receptors can be estimated by applying a battery of in vivo test models that involve post- and presynaptic receptors to a variable degree. The in vivo ranking order of efficacy at 5-HT1A receptors was: WAY 100.635 = (-)-penbutolol < (-)-tertatolol < pindolol.

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

The behavioral effects of eltoprazine and haloperidol during a 4 week treatment period were studied in the resident-intruder model of aggression in male rats. Eltoprazine, a serotonergic (5-HT1A/1B) agonist with specific anti-aggressive actions in animals, was compared to haloperidol, a neuroleptic often used to control behavioral disorders. Eltoprazine (1 or 3 mg/kg p.o.) and haloperidol (2 mg/kg p.o.) were given 60 min before a 10 min aggression test. Acutely, eltoprazine reduced aggression, without adversely affecting other behaviors. Eltoprazine (1 or 3 mg/kg p.o.) was subsequently given daily for 4 weeks and aggression tests were performed each week. The anti-aggressive effects of eltoprazine remained stable over the period of 4 weeks whereas exploration was increased. After a wash-out period of 1 week aggression had returned to baseline levels. Acutely given, haloperidol (2 mg/kg p.o.) completely reduced aggression concomitant with massive sedation. Significant tolerance developed to the sedatory actions of haloperidol over the 4 week treatment period. Aggression returned slowly, but remained below baseline values. One week after wash-out a new challenge with haloperidol (2 mg/kg p.o.) revealed significant tolerance. After 2 weeks wash-out aggression had returned to baseline. The data demonstrate persistent and specific anti-aggressive effects after eltoprazine showing no tolerance. In contrast, haloperidol showed tolerance and rebound effects for aggression. The development of tolerance after haloperidol has a different course for sedation than for the anti-aggressive action.

The antiaggressive potency of (-)-penbutolol involves both 5-HT1A and 5-HT1B receptors and beta-adrenoceptors

The relative importance of 5-HT1A and beta-adrenergic activities in the antiaggressive effects of (-)-penbutolol was studied in male mice. (-)-Penbutolol had high affinity for 5-HT1A receptors and beta-adrenoceptors, and antagonized the 5-methoxy-N,N-dimethyltryptamine (5-MeODMT)-induced 5-HT syndrome and the 8-hydroxy-2-(di-n-propylamin)tetralin (8-OH-DPAT)-induced discriminatory stimulus in rats. (-)-Penbutolol abolished aggressive behaviour (ED50 = 56 mumol/kg), and reversed the antiaggressive effects of 8-OH-DPAT and 1-(3-trifluoromethylphenyl)piperazine (TFMPP) (ED50 = 8.1 and 2.1 mumol/kg, respectively). (N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]ethyl-N-(2-pyridinyl) cyclohexanecarboxamide (WAY 100635) reversed the antiaggressive effects of 8-OH-DPAT (ED50 = 0.012 mumol/kg), but did not affect the antiaggressive effects of TFMPP. The antiaggressive effect of a submaximal dose of 8-OH-DPAT was markedly potentiated by beta-adrenoceptor antagonists without 5-HT1A receptor affinity, whereas (-)-penbutolol was effective at only one dose (4.5 mumol/kg). In conclusion, the 5-HT1A receptor antagonistic potency of (-)-penbutolol in aggressive mice is attenuated by beta-adrenoceptor-induced facilitation of serotonergic neurotransmission.

5-HT1B receptor knock out--behavioral consequences

Serotonin is a neuromodulator that is involved in a number of mood disorders such as depression, anxiety and impulsive violence. In an attempt to dissect the contribution of individual 5-HT receptor subtypes to behavior, we have generated by homologous recombination, mutant mice lacking the 5-HT1B receptor. These mice did not exhibit any obvious developmental or behavioral defect. However, the hyperlocomotor effect of the 5-HT1A/1B agonist, RU 24969 was completely absent in mutant mice, indicating that this effect is mediated by 5-HT1B receptors. Moreover, when confronted with an intruder, isolated mutant mice attacked the intruder faster and more intensely than wild-type mice, suggesting an involvement of 5-HT1B receptors in the modulation of aggressive behavior. These data might be related to the fact that a class of 5-HT1 agonists, termed serenics, have anti-aggressive properties, and with the findings that certain impulsive aggressive behaviors are associated with deficits in central serotonin.

Aggression, anxiety and vocalizations in animals: GABAA and 5-HT anxiolytics

A continuing challenge for preclinical research on anxiolytic drugs is to capture the affective dimension that characterizes anxiety and aggression, either in their adaptive forms or when they become of clinical concern. Experimental protocols for the preclinical study of anxiolytic drugs typically involve the suppression of conditioned or unconditioned social and exploratory behavior (e.g., punished drinking or social interactions) and demonstrate the reversal of this behavioral suppression by drugs acting on the benzodiazepine-GABAA complex. Less frequently, aversive events engender increases in conditioned or unconditioned behavior that are reversed by anxiolytic drugs (e.g., fear-potentiated startle). More recently, putative anxiolytics which target 5-HT receptor subtypes produced effects in these traditional protocols that often are not systematic and robust. We propose ethological studies of vocal expressions in rodents and primates during social confrontations, separation from social companions, or exposure to aversive environmental events as promising sources of information on the affective features of behavior. This approach focuses on vocal and other display behavior with clear functional validity and homology. Drugs with anxiolytic effects that act on the benzodiazepine-GABAA receptor complex and on 5-HT1A receptors systematically and potently alter specific vocalizations in rodents and primates in a pharmacologically reversible manner; the specificity of these effects on vocalizations is evident due to the effectiveness of low doses that do not compromise other physiological and behavioral processes. Antagonists at the benzodiazepine receptor reverse the effects of full agonists on vocalizations, particularly when these occur in threatening, startling and distressing contexts. With the development of antagonists at 5-HT receptor subtypes, it can be anticipated that similar receptor-specificity can be established for the effects of 5-HT anxiolytics.

Induction of muricidal behavior by ACTH or adrenalectomy in young male Wistar rats

ACTH administered IP induces a muricidal behavior in 52% of male Wistar rats that do not express it before; such behavior is also observed spontaneously in 68% of adrenalectomized animals. This change in behavior is characterized by an exceptionally long duration (several months in some animals), and by its prevention by previous or substitutive treatments with dexamethasone. Data obtained using hypophysectomized or adrenalectomized animals with or without ACTH treatments suggest that the muricidal-inducing effect of ACTH might have, at least partially, a central origin.

Enhanced aggressive behavior in mice lacking 5-HT1B receptor

The neuromodulator serotonin (5-hydroxytryptamine, 5-HT) has been associated with mood disorders such as depression, anxiety, and impulsive violence. To define the contribution of 5-HT receptor subtypes to behavior, mutant mice lacking the 5-HT1B receptor were generated by homologous recombination. These mice did not exhibit any obvious developmental or behavioral defects. However, the hyperlocomotor effect of the 5-HT1A/1B agonist RU24969 was absent in mutant mice, indicating that this effect is mediated by 5-HT1B receptors. Moreover, when confronted with an intruder, mutant mice attacked the intruder faster and more intensely than did wild-type mice, suggesting the participation of 5-HT1B receptors in aggressive behavior.

Neuropharmacology of 5-hydroxytryptamine1B/D receptor ligands

In spite of a lack of compounds acting selectively at the 5-hydroxytryptamine (5-HT)1B and 5-HT1D receptor subtypes, by cross-relating the available data, this review attempts to tentatively assign behavioural and other in vivo correlates of these receptor subtypes. In addition, a summary of data from microdialysis studies is included to develop an integrated view. Finally, a suggestion is made as to the possible pathophysiological consequences of 5-HT1D receptor dysfunction in man.

The effects of dorsal raphe administration of eltoprazine, TFMPP and 8-OH-DPAT on resident intruder aggression in the rat

To evaluate the role of somatodendritic 5-HT1A receptors in the mediation of aggressive behaviour, eltoprazine, TFMPP (1-(3-trifluoromethylphenyl)piperazine hydrochloride) and 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) were administered locally into the dorsal raphe nucleus of rats. 8-OH-DPAT (1 and 10 micrograms) and eltoprazine (10 and 30 micrograms) reduced aggression, but concomitantly reduced social interest and increased inactivity. TFMPP (1 and 10 micrograms) did not reduce aggression. As 8-OH-DPAT and to a lesser extent eltoprazine affect 5-HT1A receptors, it is proposed that a general reduction of serotonergic neurotransmission by activation of somatodendritic serotonergic autoreceptor leads to a non-specific reduction of aggression. As TFMPP has a significantly lower affinity for 5-HT1A receptors than 8-OH-DPAT or eltoprazine, the lack of effect of TFMPP supports this view.