The effects of idazoxan and 8-OH-DPAT on sexual behaviour and associated ultrasonic vocalizations in the rat

Two experiments were performed studying the effects of 8-OH-DPAT and idazoxan on sexual behaviour and ultrasonic communication of male rats. In addition, the reactions of the females towards drug-treated males were studied. 8-OH-DPAT (a very specific 5-HT1A agonist) and idazoxan (an alpha 2-adrenergic antagonist) differentially affected sexual behaviour: 8-OH-DPAT (0.1 and 0.4 mg/kg IP) markedly facilitated ejaculations, a feature indicated by decreased numbers of mounts and intromissions preceding ejaculation and a reduction in ejaculation latency. This drug concomitantly reduced the postejaculatory refractory period. Idazoxan reduced the number of intromissions before ejaculation only at the highest dose (10 mg/kg IP), but did not markedly facilitate other parameters. Both drugs markedly and dose-dependently suppressed the postejaculatory 22 kHz ultrasounds normally recorded during the postejaculatory refractory period. Ultrasound frequencies above 30 kHz first appear at the end of the absolute refractory period, even when the refractory period is shortened by 8-OH-DPAT. Idazoxan increased the number of these 30 kHz ultrasounds, whereas 8-OH-DPAT had no effect on them. No effects were observed on ultrasound production (either 22 kHz or above 30 kHz) before an ejaculation. The behaviour of the females towards 8-OH-DPAT-treated males was also affected, with the females showing more darting and lordosis before and after ejaculation, but less sitting after ejaculation. Idazoxan treatment of the males resulted in more hopping and earwiggling of the females before ejaculation. Following ejaculation, females treated with the antagonist showed more darting, hopping, earwiggling and lordosis, but sitting was decreased. It has been suggested in the rat that the emergence of ultrasounds higher than 30 kHz indicates the end of the absolute refractory period and signals to the female that the male is capable of resuming sexual activity. The significance of 22 kHz ultrasound in sexual behaviour remains puzzling because these vocalizations could be easily uncoupled from the refractory period by drugs acting via different receptor mechanisms without disturbing sexual behaviour per se. A failure to produce postejaculatory sounds appears to disinhibit (proceptive) behaviour by the females.

Sex behavior of male and female Wistar rats affected by the serotonin agonist 8-OH-DPAT

Four experiments were carried out to test the stimulatory effects of 8-OH-DPAT on various aspects of "masculine" sexual behavior of male and female rats and on the sexual attractivity of male rats. In Experiment 1 8-OH-DPAT (0.2 mg/kg) stimulated ejaculation frequency in middle-aged (approx. 15 months old) males, both sexually inactive and active subjects. There was a coinciding decrease in total number of mounts, intromissions, intromissions to first ejaculation and latency to first ejaculation. In Experiment 2 the effects of two doses (0.2 and 0.4 mg/kg) 8-OH-DPAT on the first ejaculation cycle were investigated. Especially, the higher dose made a high percentage (45-55%) of males to ejaculate "prematurely," i.e., at the first or second intromission. Latency to ejaculation decreased. With the higher dose, 25-35% of the males ejaculated extravaginally. In Experiment 3 8-OH-DPAT did not make males more attractive for an estrous female than saline-treated males, as judged by the time spent in their vicinity. However, estrous females received much more ejaculations from the tethered 8-OH-DPAT males, with the lowest latencies to first ejaculation, than from the saline-treated males. In Experiment 4 8-OH-DPAT stimulated mounting behavior in female rats only when they were long-term treated with testosterone. In that condition also shortest latencies to first mount were found with 8-OH-DPAT treatment.

Ethopharmacological studies of anxiolytics and aggression

This paper presents examples of the application of ethopharmacology to the study of aggression. Low doses of benzodiazepines may increase aggression under appropriate conditions. In various animal models in male and female rats and mice the aggression enhancing effects are particularly marked when aggression is inhibited by internal or external events. It is therefore suggested that benzodiazepines have no direct effect on aggression, but modulate inhibitory factors which regulate aggression.

Postsynaptic 5-HT1 receptors and offensive aggression in rats: a combined behavioural and autoradiographic study with eltoprazine

The present study was designed to assess whether the antiaggressive effects of eltoprazine are mediated via presynaptic and/or postsynaptic 5-HT1 receptors. We describe the effects of central 5-HT depletion 1) on the behaviour of resident TMD-S3 rats in a territorial situation, 2) on the efficacy of eltoprazine to inhibit offensive aggression, and 3) on the 5-HT1A, 5-HT1B and 5-HT1C receptor binding in brains of rats previously used in behavioural studies. Male resident rats were given combined 5,7-dihydroxytryptamine (5,7-DHT) injections into the dorsal and median raphe nuclei. Two to four weeks after the lesions, rats were confronted with an intruder Wiser rat in their home cage for a 10-min period. The 5,7-DHT treatment resulted in a modest reduction of offensive behaviour, while having no effects on other social and nonsocial behaviours. Oral administration of eltoprazine (1 mg/kg) specifically reduced offensive aggression in both sham- and 5,7-DHT-lesioned animals, leaving social interest and exploration intact or even increasing it. A low dose (0.3 mg/kg) of eltoprazine did not affect the behavioural repertoire of sham-operated rats, whereas this dose significantly reduced offense behaviours in the 5,7-DHT-lesioned residents. Quantitative autoradiographic studies 5 weeks after 5,7-DHT treatment revealed a significant increase in radioligand binding to 5-HT1A, 5-HT1B and 5-HT1C sites in many brain regions studied, except for the raphe nuclei where [3H]8-OH-DPAT binding to 5-HT1A sites was markedly reduced. The concentrations of 5-HT and 5-HIAA in frontal cortex were reduced to approximately 10% of controls. The results indicate that serotonin has a stimulatory rather than an inhibitory influence on offensive aggressive behaviour. Central 5-HT depletion does not prevent the antiaggressive effects of eltoprazine, indicating a role for postsynaptic 5-HT1 receptors in the modulation of offensive aggression. The 5,7-DHT-induced overall upregulation of 5-HT1A, 5-HT1B and 5-HT1C binding sites suggests that these three receptor subtypes receive a tonic serotonergic influence. It is conceivable that this postsynaptic 5-HT1 receptor supersensitivity is reflected by the increased efficacy of eltoprazine to inhibit offensive aggression.

The actions of the novel anti-aggressive drug eltoprazine on central neurones in the anaesthetised rat

5-Hydroxytryptamine (5-HT) and the novel anti-aggressive drug eltoprazine (1-(2,3-dihydro-1,4-benzodioxin-5-yl) piperazine hydrochloride) were applied by microiontophoresis to spinal motorneurones and also to neurones in the brainstem which gave two distinctly different responses to 5-HT. In vitro microiontophoretic release studies showed that the electrophoretic mobility of eltoprazine and 5-HT were similar and that similar amounts of each drug would be applied by similar iontophoretic currents. Cells in the brainstem have been shown previously to be excited by 5-HT, acting at a 5-HT2 receptor. Eltoprazine only occasionally and weakly mimicked the excitatory effect of 5-HT on these cells. Although a potent antagonism of the 5-HT excitation by eltoprazine was observed, this was a non-selective effect, as responses to glutamate and D,L-homocysteic acid were also reduced. Cells in the lateral brainstem are depressed by 5-HT, acting on a receptor which has previously been shown to be of the 5-HT1-like group. At this receptor, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)-tetralin) and 5-carboxamidotryptamine, are potent agonists. Eltoprazine was a more potent depressant agonist than 5-HT on these brainstem neurones. The antagonist metergoline did not antagonise responses to either 5-HT or eltoprazine. It is suggested however that both drugs act at the same receptor to depress these cells because desensitizing the receptor by repeated, frequent applications of 5-HT abolished responses to 5-HT and eltoprazine, without altering responses to GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus properties of flesinoxan

Different groups of rats were trained to discriminate either 0.3 mg/kg of flesinoxan (N = 13) or 0.1 mg/kg of 8-OH-DPAT (N = 7) from saline in a two-lever operant drug discrimination task using a fixed ratio 10 schedule of reinforcement. Once trained, animals in both groups displayed a dose-related decrease in discriminative performance upon administration of lower doses of the drug used in training. In generalization tests, flesinoxan generalized to 8-OH-DPAT in 8-OH-DPAT-trained animals and 8-OH-DPAT substituted for flesinoxan in flesinoxan-trained animals. Buspirone substituted partially for both the flesinoxan and the 8-OH-DPAT cue. The results of the present study indicate similarity between the discriminative stimulus effects of flesinoxan and the stimulus produced by the 5-HT1A agonist 8-OH-DPAT. These results, coupled with the finding that flesinoxan has a significant affinity and selectivity for 5-HT1A binding sites, suggest that the stimulus effects of flesinoxan are mediated by a 5-HT1A mechanism.

Behavioural pharmacology of the serenic, eltoprazine

In this paper the effects of serenics (eltoprazine and fluprazine) are described in several animal models for offensive agonistic, defensive agonistic and predatory behaviour. They are compared with the effects of a number of other putative anti-aggressive compounds or drugs used clinically in order to ameliorate aggressive behaviour of psychiatric patients. In isolation-induced offensive aggression in mice, eltoprazine has a marked and potent anti-aggressive activity, although numerous other psychoactive drugs also exert anti-aggressive effects. The behavioural specificity of this anti-aggressive profile was investigated using an ethologically derived animal model, social interaction in male mice. In this model, eltoprazine has a very specific anti-aggressive (serenic) profile, inhibiting aggression while social interaction and exploration are not decreased but even enhanced; inactivity, a measure for sedation, is not affected. Such a profile contrasts sharply with that of neuroleptics (chlorpromazine, haloperidol), psychostimulants (d-amphetamine) or benzodiazepines (chlordiazepoxide), which exert severe sedation (neuroleptics) or even aggression-enhancing effects (BDZ). After subchronic treatment no tolerance for the anti-aggressive effects of eltoprazine occurred. The specific anti-aggressive effects of eltoprazine were also found in rat models of offensive agonistic behaviour. In one such model - resident-intruder aggression - eltoprazine reduced offensive behaviour specifically, leaving social interactions and exploration intact, and did not induce sedation or other unwanted side-effects. The neuroleptic haloperidol was very sedative in this model, as was the 5-HT1A-agonist buspirone. Benzodiazepines (chlordiazepoxide) have a biphasic effect in this paradigm, enhancing offence at low doses and decreasing it at higher doses, due to muscle relaxation. In another offensive model, colony-aggression, in which a dominant and subordinate male in a colony are confronted with a male intruder, eltoprazine reduced offensive behaviour of both the dominant and the subordinate against the intruder. In contrast, chlordiazepoxide enhanced aggression, at least at lower doses, whereas alcohol had, up to very high doses, no effect on the offensive behaviour. In a brain-stimulation induced offensive model--hypothalamically-induced aggression in rats--eltoprazine specifically reduces offence. Locomotion, a measure for sedation, was either unaffected or even somewhat enhanced, indicating the absence of any sedatory activity of this serenic compound. In contrast, haloperidol heavily sedated animals, making them incapable of aggression.(ABSTRACT TRUNCATED AT 400 WORDS)

Ethopharmacology: a creative approach to identification and characterisation of novel psychotropics

The present contribution describes the basic fundamentals of animal models in ethopharmacology. After defining the role of ethopharmacology in the development of animal models of relevant human diseases, this methodology is used to classify different categories of aggression. Furthermore, the behavioural aspects of agonistic (aggressive) modelling are outlined and the various models used to describe offensive and defensive behaviours, and some miscellaneous models are summarized. Finally, some remarks on the new class of psychoactive drugs, serenics, are given.

Serotonergic modulation of social interactions in isolated male mice

Several serotonergic drugs were tested in isolation-induced aggressive behavior in male mice using ethological methodology. Eltoprazine, a mixed 5-HT1 agonist, reduced aggression but enhanced social interest and exploration. Several 5-HT1A agonists (8-OH-DPAT, ipsapirone, buspirone, 5-Me-ODMT) and a 5-HT uptake blocker (fluvoxamine) also reduced aggression. Although these drugs somewhat differentially affect aggressive behavior, the isolation-induced paradigm alone is not sensitive enough to successfully differentiate and screen the various serotonergic drugs with regard to their influence on social behavior in mice. It is argued that various animal paradigms in several species are necessary to describe specific effects of serotonergic drugs.

Quantitative and comparative analyses of pro-aggressive actions of benzodiazepines in maternal aggression of rats

The pro-aggressive effects of low doses of benzodiazepines on maternal aggression in rats were studied. Chlordiazepoxide, diazepam, oxazepam and alprazolam produced bell-shaped dose-response curves, with increased aggression at low doses. Only alprazolam significantly reduced aggression at higher doses. A comparison of the drug effects on different aggressive elements revealed that chlordiazepoxide and oxazepam increased the frequency of more elements of the aggressive repertoire than diazepam or alprazolam. Thus, although all benzodiazepine receptor agonists increased aggression, there were significant quantitative differences in their effects.

Maternal aggression towards different sized male opponents: effect of chlordiazepoxide treatment of the mothers and d-amphetamine treatment of the intruders

Lactating female rats vigorously attack equally sized conspecific males introduced into their home cage. Under conditions of such high aggression, the previously reported pro-aggressive action of a low (5 mg/kg) dosage of chlordiazepoxide (CDP) is hardly detectable. When opponents are large, the intensity of the aggression is less than what is seen with small ones. In this situation treatment of the females with CDP increases aggression levels substantially. The importance of intruders evoking aggression was further investigated by treating different sized opponents with d-amphetamine. d-Amphetamine treatment did not lead to major changes in the defensive capacities of either types of intruder. The data demonstrate that drug effects, such as pro-aggressive actions, may be observed using larger sized opponents that are not so easily defeated and show more adequate defense than small ones. The subtleness of the dyadic interactions in maternal aggression indicates that drug effects should be considered carefully before extrapolation to other conditions.

Postpartum aggression in rats does not influence threshold currents for EBS-induced aggression

Female Wistar rats were tested for aggressive behaviour induced by electrical brain stimulation (EBS) in the lateral hypothalamus. Threshold currents for the induction of aggression were determined on several days before the females were paired with experienced breeder males. Beginning in the second week of pregnancy threshold current values were measured once or twice weekly. No change in thresholds was observed either during pregnancy, the early postpartum period or after weaning. Lactation was the only period during which the females were spontaneously aggressive towards male intruders in their home cage, but not in the EBS cage. Analysis of bite targets revealed no difference between the bite patterns in the postpartum maternal aggression test and the EBS-induced attacks. The results demonstrate that the change in physiological and hormonal status in pregnant and lactating females has no influence on the propensity to attack during EBS. The similarity in wound patterns does not advocate a major difference in the types of aggression studied. We speculate upon the nature of EBS-induced attacks as the activation of a rigid, final pathway of aggression which is rather insensitive to mild modulations.