J. Mos

Discriminant analysis of the localization of aggression-inducing electrode placements in the hypothalamus of male rats.

Over 400 sites in the hypothalami of 270 male CPB/WE-zob rats were electrically stimulated in order to induce fights between males. The localization of electrodes inducing fights seems to differ from the localization of electrodes in which no fights can be induced. The differences in localization were detected and tested by a non-parametric discriminant analysis. The results were plotted by computer in a stereotaxic atlas of the hypothalamus of the CPB/WE strain. The method delimits areas within the hypothalamus where the probability to induce aggression is high, intermediate or low. Moreover, the procedure allows discrimination between areas where the thresholds for attack behaviour are generally lower than elsewhere and where the fiercest forms of attack are induced. None of the areas delimited coincide with a classical subdivision of the hypothalamus. Parts of the perifornical, anterior, lateral and ventromedial hypothalamus seem to be involved. The methods developed here may help to relate stimulation-induced aggression to other characteristics of the aggressive area which cannot be obtained directly from fighting rats such as cytological, endocrinological, biochemical or physiological data. In addition, the procedure may help to settle disputes on the specificity of the localization of neural substrates of other stimulation-induced behaviours. The methods to discriminate between overlapping 3-dimensional reconstructions validated here for aggressive responses, can also be applied to other types of stereotaxic data and other types of effects, such as electrical, hormonal or other physiological responses. They may be especially useful if the localization of the neural population involved is not yet known, and unknown current-spread or diffusion of substances complicates the interpretation of stereotaxic data.

Comparison of Aggressive Behaviour Induced by Electrical Stimulation in the Hypothalamus of Male and Female Rats

Publisher Summary Sex differences in aggressive behavior are present in many species, and the genders often become aggressive in different ways or in different conditions. This chapter describes and attempts to induce aggression by stimulation in the hypothalamus of female as well as male rats and compares behavior, histology, and current intensity required to induce attacks in the two sexes. The chapter also discusses the effects of ovariectomy and subsequent oestradiol replacement on attack thresholds in female rats. The results show that electrical stimulation in the hypothalamus elicits aggression in both male and female rats, which seems to be derived from the same neural systems and no sex differences could be detected in the sensitivity of these neural systems to stimulation. Slight differences are observed in the forms of attack behavior induced by hypothalamic stimulation: males tend to show more intense, females weaker forms of attack. These differences may be due to hormonal differences of the animals, the males having normal levels of testosterone. Neither ovariectomy nor subsequent oestrogen replacement affects thresholds for hypothalamic attack in a convincing way, which is in contrast with the effects of castration and testosterone replacement in male rats. However, the precise neural mechanisms of this behavioral facilitation are not known.

Maternal aggression in rats: Effects of chlordiazepoxide and fluprazine

Although maternal agression in rats is confined to a restricted post-partum period, the high and stable aggression level and the constancy of its behavioural structure make it an attractive experimental procedure for studying the behavioural effects of psychotropic drugs. Female rats were tested against naive male intruder rats for 5 or 10 min on post-partum days 3–9, during which aggression is stable.Chlordiazepoxide (CDP; 5, 10 and 20 mg/kg, orally) had a biphasic effect on aggression; it increased aggression considerably at 5 and (to a lesser extent) at 10 mg/kg. At 20 mg/kg aggression returned to control level. CDP shortened the latency to the first attack at 5 mg/kg, but not at higher dosages. CDP enhanced aggression, particularly in the first 2 min of an encounter. It did not change the structure of the aggressive behaviour, but did induce a dosedependent increase in feeding. Fluprazine (Flu; 5, 10 and 20 mg/kg IP), a specific antiaggressive (serenic) drug, induced a dose-dependent decrease in aggression and exerted its largest effect in the first 2 min of an encounter. In accordance with the reduced aggression, latencies to the first attack increased. Maternal aggression in rats represents an extension to other (male) aggression paradigms in psychopharmacology. First, it has no male counterpart. Secondly, the hormonal mechanisms underlying this behaviour differ from those of male aggression. Thirdly, the morphology of maternal aggression is different from that shown in male models of agonistic behaviour (e.g. resident-intruder). These features make maternal aggression an attractive paradigm for pharmacological studies of female behaviour.

Modulatory actions of benzodiazepine receptor ligands on agonistic behaviour

Several experiments were conducted to establish the role of benzodiazepine (BDZ) receptor ligands in aggressive behaviour in male and female rats. In particular, the pro-aggressive effects of BDZ agonists was subject of investigation. Predatory aggression (mouse killing) was facilitated by chlordiazepoxide (CDP) when tested in naive female rats, but CDP was unable to induce muricide in non-killing rats with extensive experience with mice. In experiments on maternal aggression in rats a post-hoc analysis revealed that the pro-aggressive action of CDP on maternal aggression was base line dependent: the increase in aggression in spontaneously low aggressive females was significantly higher than in females with a higher base line level. A further study aimed at unravelling the underlying factors contributing to this pro-aggressive action by determining the role of opponent size on the effects induced by CDP. Normally large opponents evoke less aggression from lactating females than smaller opponents and CDP exerted its pro-aggressive effect particularly strongly in the large opponent situation. An ethological analysis was made of lateral display--an ambivalent posture frequently occurring in agonistic behaviour--to establish whether CDP indirectly increases aggression by reducing fear by means of its anxiolytic properties. The data partly support this hypothesis. These findings stress the importance of environmental and experiential factors in the possible outcome of CDP effects on aggression. Moreover, they point to possible explanations of seemingly contradictory data. In two final experiments an inverse benzodiazepine receptor agonist (beta-CCE) was tested in maternal aggression. beta-CCE reduced aggression, although not in a completely specific way. A neutral BDZ-receptor antagonist (Ro 15-1788) was tried in an attempt to antagonize the pro-aggressive effects of CDP in maternal aggression. Ro 15-1788 did not counteract the pro-aggressive action of CDP, but antagonized CDP effects on exploration. The modulatory role of the benzodiazepine receptor complex in aggression remains an intriguing area of research in which many subtleties in testing conditions play a role and in which more BDZ agonists, inverse agonists and antagonists have to be tested.

Anti-aggressive effect of a new phenylpiperazine compound (DU27716) on hypothalamically induced behavioural activities

Using the same hypothalamic electrodes, the following behaviour was evoked in male rats by electrical stimulation at roughly equal current intensities: attacks on a partner, teeth-chattering, switch-off behaviour and locomotion. Current thresholds were determined for each behaviour following the intraperitoneal administration of saline or DU27716, a new phenylpiperazine compound with interesting inhibitory effects on territorial and intermale aggression. DU27716 raised current thresholds for attack and teeth-chattering beginning at the lowest dose (4 mg/kg), whereas there was no effect on switch-off behaviour, and only a slight but significant effect on locomotion thresholds at the highest dose (8 mg/kg). The results provide support for the hypothesis that DU27716 possesses behaviourally selective, anti-aggressive properties, and illustrate the usefulness of hypothalamically induced behaviours as a pharmacological model.

Different test situations for measuring offensive aggression in male rats do not result in the same wound patterns

It has been reported by the Blanchards, that the attacks of offensively motivated rats produce a very characteristic wound distribution on the body of opponents, the back region being the most prominent target area. We now report that the distribution of wounds on male intruders produced by male resident Wistar rats in a resident-intruder paradigm, depends on the experimental situation used. Rats solitarily housed in small cages (0.1 m2), produced most wounds (57%) on the upper back, about 14% on the lower back, whereas head and belly were much less bitten. Rats housed in larger cages (0.52 m2) with a female (territorial situation) produced about 39% of the wounds on the back, equally divided over the anterior and posterior parts. In this situation the head (21%) and the belly (23%) were more frequently bitten than in the other test situations. In a large colony (5.7 m2), practically all wounds were situated dorsally (81%), the majority (58.5%) on the lower part of the back. It was concluded that the distribution of wounds over the body of the intruder was at least partly determined by the possibility of escape.

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.

Dose-dependent discriminative stimulus properties of 8-OH-DPAT.

Separate groups of rats were trained to discriminate either 0.1 mg/kg (low dose; L) or 2.5 mg/kg (high dose; H) of 8-OH-DPAT from saline, in a standard operant task. Both cues were found to be dose, time and route dependent and generalized completely to the 5-HTu agonists ipsapirone and flesinoxan. Buspirone substituted completely for 8-OH-DPAT in L and partially in H, whereas the 5-HT]A,1B receptor agonist eltoprazine substituted completely for 8-OH-DPAT in H but only partially in L. The 5-HT1A/1B receptor agonist RU24969, the 5-HTIB/2chA receptor agonist TFMPP and the 5-HT reuptake blocker fluvoxamine did not completely mimic the effect of 8-OH-DPAT in either L or H and the 5-HT,A mixed agonists/antagonists BMY 7378 and NAN-190 produced partial generalization in L, but no generalization in H. In antagonism tests, NAN-190 and BMY 7378 only partially blocked the 8-OH-DPAT cue in both groups. The non-selective 5-HT receptor antagonist methysergide did not completely block the 8-OH-DPAT cue in L or H. However, in generalization studies, it completely mimicked the 8-OH-DPAT cue in L and produced partial generalization in H. The /8-adrenergic/5-HTIA/1B receptor antagonist pindolol completely blocked the 8-OH-DPAT cue in L and H and did not mimic the 8-OH-DPAT cue in either condition. The a2-adrenoceptor blocker yohimbine substituted fully for the 8-OH-DPAT cue in L and partially in H. Idazoxan did not substitute for the cue of 8-OH-DPAT in H, but produced nearly 80% generalization in L. The dopamine receptor antagonist pimozide neither blocked nor mimicked the cue of 8-OH-DPAT in either group. A number of other drugs (i.e. m-CPP, S (-propranolol, DOI, ketanserin, clonidine and apomorphine) were only tested in H. S( - (-Propranolol blocked the 8-OH-DPAT cue but the other compounds produced neither stimulus generalization nor antagonism. The present study demonstrates that the cues produced by the low and the high training dose of 8-OH-DPAT are quantitatively different and mediated by the agonistic activity of 8-OH-DPAT at 5-HT1A receptors. Although the results suggest that the 8-OH-DPAT cue (both L and H) is mediated via postsynaptic 5-HT,A receptors, the involvement of presynaptic 5-HT,A receptors cannot yet be ruled out.

A locked, non-rotating, completely embedded, moveable electrode for chronic brain stimulation studies in freely moving, fighting rats

A light-weight, yet rugged moveable electrode assembly is described for chronic brain stimulation studies in small-brained animals. The assembly can be completely embedded in a smooth, unobtrusive dental cement cap and is therefore suitable for use in fighting experiments, where collisions with partners and cage walls will limit the use of other assemblies. It permits a variable electrode distance penetration of 3 mm in 75 mu-steps by using a separate unlocking turning-key. This design excludes the possibility of inadvertent displacement of the electrode tips by the animal itself. Since the electrode itself does not rotate during displacement, extra damage arising from possible eccentricity is avoided. The assembly has been used in a number of hypothalamic penetrations, demonstrating its usefulness and reliability.