Play fighting and corticotropin-releasing hormone in the lateral septum of golden hamsters.

This study was focused on determining the possible role of corticotropin-releasing hormone (CRH) on play fighting in juvenile golden hamsters. As no specific neural sites have been proposed, we looked for changes in CRH innervations at the peak of play-fighting activity on postnatal day 35 (P-35) from a week before on P-28. We noted that the increase in play-fighting activity between these two dates was associated with a 100% increase of the density of CRH fibers within the lateral septum. We, then, tested the possible role of CRH receptors on play fighting within the lateral septum through microinjections of alpha-helical CRH, a CRH receptor antagonist (either 0, 30, or 300 ng), directly into the area. The treatments inhibited play-fighting attacks and pins as well as reduced the duration of time that the resident hamsters spent in contact with the intruders, though locomotor activity remained unaffected. The possible source of CRH release in the lateral septum was addressed by quantification of CRH neurons also labeled with a marker of cellular activity, c-Fos, after consummation of play fighting. CRH neurons in the horizontal part of the diagonal band, an area reciprocally connected with the lateral septum, showed a 75% increase in double labeling with c-Fos as compared to controls. Together, these data show that CRH receptors in the lateral septum have a general role on play fighting, not just facilitating its consummation, but also likely enhancing appetitive aspects as well. In addition, this effect is associated with enhanced CRH availability in the area and enhanced neuronal activity within interconnected areas.

Risk assessment and avoidance in juvenile golden hamsters exposed to repeated stress.

Juvenile hamsters are typically less vulnerable to social subjugation than adults, although they will avoid aggressive individuals in some situations. The purpose of this study was to determine the extent to which social subjugation stimulates fear- or anxiety-like behavior in juvenile hamsters in both social and non-social contexts. Social context testing was conducted in a Y-maze while the non-social context apparatus consisted of an open field arena and a lat-maze. In the Y-maze, subjects were exposed to an unfamiliar aggressive adult hamster. Compared with non-subjugated controls, subjugated juveniles spent significantly more time in the area furthest from the aggressive adult stimulus. In addition, socially stressed animals were more likely to avoid the arm of the maze containing the social stimulus. When they did walk in the arm containing the social stimulus, subjugated individuals were more likely to ambulate slowly. Subjugated hamsters also performed fewer olfactory investigations in the proximity of the unfamiliar aggressive individual. Despite these behavioral differences detected between groups during testing in a social context, we observed no differences between groups in the open field and lat-maze. This suggests that the effects of subjugation observed in the Y-maze are specific to exposure to a social context and that social subjugation in juvenile hamsters does not result in a generalized state of fear. Instead, subjugated juveniles learned to avoid adult males and were otherwise behaviorally similar to non-subjugated controls.

Neural circuitry of play fighting in golden hamsters.

In hamsters, play fighting matures gradually into adult aggression. As these two behaviors share many similarities in this species, we predicted that a single neural circuitry controls their offensive component. The goal of the present study was to identify neural systems associated with offensive play fighting in male juvenile golden hamsters. The neural circuitry related to this behavior was identified through quantification of c-Fos immunolabeling. We also looked for vasopressin cells possibly associated with play fighting. We found that areas previously associated with offensive aggression in adult hamsters, including the ventrolateral hypothalamus, the medial amygdala, and the bed nucleus of the stria terminalis, also showed enhanced c-Fos expression after play fighting. In addition, vasopressin neurons in the nucleus circularis and the medial division of the supraoptic nucleus expressed enhanced c-Fos immunolabeling in juveniles after play fighting, as previously reported in adult hamsters after aggression. Finally, enhanced c-Fos expression associated with play fighting was also found in areas previously unexplored in adult hamsters, such as the prefrontal cortex. Together, our results support the hypothesis of a single core neural circuitry controlling the offensive components of play fighting and adult aggression throughout puberty in hamsters.

Individual differences in offensive aggression in golden hamsters: a model of reactive and impulsive aggression?

In humans, reactive aggression is associated with impulsivity. The purpose of this study is to relate reactive and impulsive aggression in humans with offensive aggression in animals and identify neurobiological correlates associated with certain forms of the behavior. We predicted that individual differences in offensive aggression are associated with individual differences in impulsivity. Adult male hamsters were repeatedly tested for offensive responses and divided into High-Aggression or Low-Aggression groups. They were then trained and tested under a delay-discounting paradigm to assess impulsivity. High-Aggression animals consistently attacked and bit more frequently and faster, and showed highly repetitive behavior, indicated by repeated attacks per contact bout. In addition, these animals engaged in more fragmented and shorter contact bouts. During impulsivity testing, High-Aggression animals preferred immediate smaller rewards over delayed larger rewards. Furthermore, 5-HT and vasopressin (AVP) innervation was compared between the groups. High-Aggression animals showed decreased 5-HT varicosities in several key brain areas involved in aggressive and/or impulsive behavior and decreased AVP fibers in the anterior hypothalamus. Together, these data show a convergence of behavioral phenotypes through individual differences in offensive aggression and impulsivity. As such, this association provides support for an animal model of reactive and impulsive aggression. Furthermore, this behavioral convergence is supported by a concurrent reduction in 5-HT innervation of brain areas controlling aggression and impulsivity, providing a common neural mechanism for this phenotype.

Glucocorticoids and the development of agonistic behaviour during puberty in male golden hamsters.

During puberty, the agonistic behaviour of male golden hamsters undergoes a transition from play fighting to adult aggression. Repeated exposure to social stress early in puberty accelerates this transition. The present study investigated the possible role of glucocorticoids on the maturation of agonistic behaviour. First, we compared serum cortisol levels following a 20-min restraint stress during early puberty, mid-puberty or adulthood. Across puberty, animals exhibited a two-fold increase in post-restraint cortisol levels. We also compared corticotrophin-releasing hormone (CRH) immunoreactive fibres projecting to the median eminence between animals in early puberty and adulthood. The CRH fibre density was two-fold greater in adults compared to juveniles. Furthermore, we investigated the effects of stress hormones on the maturation of agonistic behaviour. Male hamsters were injected daily with dexamethasone, a corticosteroid receptor type II agonist (0, 10 or 40 microg/100 g), early in puberty from postnatal day 31 (P-31) to P-36. When paired with a smaller and younger intruder on P-37, attack frequency did not differ between groups. However, dexamethasone-treated animals showed a dose-dependent decrease in the percentage of play-fighting attacks and an increase in the percentage of adult attacks. In summary, puberty can be described as a period of increasing hypothalamic-pituitary-adrenal activity in male golden hamsters. Moreover, increasing glucocorticoid levels influence the maturation of agonistic behaviour. These data shed new light on the neuroendocrine mechanisms that regulate the maturation of social behaviours during puberty.

Behavioural and neuroendocrine adaptations to repeated stress during puberty in male golden hamsters.

In adult animals, the consequences of stress are often severe and long lasting. Repeated subjugation in adult male golden hamsters inhibits aggression and increases submissive and avoidant behaviours. By contrast, subjugation during puberty enhances offensive aggression. The goals of this study were to characterize behavioural and neuroendocrine responses of naïve and repeatedly subjugated juveniles to social defeat and to assess potential recovery from social stress. From the onset of puberty on postnatal day 28 (P28) to mid puberty (P42), animals were either socially subjugated or placed in a clean and empty cage for 20 min daily. The subjugated and control groups were further divided into subgroups and sacrificed under basal conditions or after social defeat on P28, P35 (early puberty), P45 (mid puberty) and P70 (early adulthood). On P35 and P45, repeatedly subjugated juveniles showed a complete inhibition of olfactory investigation (i.e. risk assessment) towards aggressive adults. Repeatedly subjugated also animals had lower postdefeat cortisol levels than controls on P45. Interestingly, basal cortisol levels increased gradually during puberty but did not differ between treatment groups at any point. Repeated subjugation was also associated with increased tyrosine hydroxylase immunoreactivity (ir-TH) within the extended medial amygdala. After a 4-week recovery period, none of these variables differed between subjugated and control groups. In an additional experiment, subjugated adults also had increased ir-TH in the medial extended amygdala, suggesting that these neurones are particularly responsive to social stress. In conclusion, puberty may be a developmental period characterized by behavioural and neuroendocrine plasticity in stress responsiveness. Furthermore, peri-pubertal changes in stress hormones may explain why juvenile hamsters are more resilient to social stress than adults.

A neural network underlying individual differences in emotion and aggression in male golden hamsters.

In rodents, aggressive behavior can be altered by experimental manipulations of emotional responsiveness. The goal of this study was to identify characteristics of emotional reactivity associated with individual differences in aggressive behavior and their integration within a common neural network. Male golden hamsters were first screened for offensive aggression. Then, the animals were trained through immediate reinforcement and tested for their adaptation to a delayed reward. Similar protocols have been used to test behaviors associated with frustration. At first, all hamsters showed increased frequency of bar pressing per reward during delayed reinforcement. However, Low-Aggression animals were able to adapt to the delay and showed a decreased rate of bar pressing per reward within 5 days. In contrast, High-Aggression animals maintained a high rate of bar pressing per reward. In addition, brains were collected after immediate reward training or delayed reward testing, and labeled for pCREB-immunoreactivity as a marker of trans-synaptic activity. In High-Aggression individuals, elevated density of cyclic AMP response element binding protein, phosphorylated (pCREB) immunostaining was found within the anterior hypothalamus, an area critical to the control of aggression. Delayed reinforcement was associated with enhanced pCREB immunostaining within the central amygdala, medial amygdala and preoptic area/hypothalamus continuum. Further analysis of the data also showed a positive correlation in labeling density between the lateral septum and the anterior hypothalamus, specifically in Low-Aggression animals exposed to delayed reward. Therefore, as High-Aggression individuals lack control of their emotional reactivity, they are also characterized by a de-synchronization between the inhibitory output of the septum and the aggression areas of the hypothalamus. Finally, our data also show that frustration is associated with an extensive activation of the preoptic area/hypothalamus continuum and amygdala.

Adult levels of testosterone alter catecholamine innervation in an animal model of attention-deficit hyperactivity disorder.

The spontaneously hypertensive rat (SHR) has been used as an animal model of attention-deficit hyperactivity disorder (ADHD). This disorder, which is most prevalent in males during childhood, persists in adulthood more frequently in females. Since other work has shown that neonatal testosterone levels may be a contributing factor in the expression of ADHD-like behavior, the present study was designed to determine whether androgen levels also altered the neurobiology of adult SHRs compared to Wistar (WKY) controls. Males castrated on postnatal day 45 were implanted with testosterone, and the density of tyrosine-hydroxylase-immunoreactive (TH-ir) fibers (an indicator of catecholamine innervation) in the frontal cortex was compared between animals. The data show that testosterone-treated SHRs were associated with higher levels of TH immunoreactivity in the frontal cortex and hippocampus than WKY rats. These results may explain why high circulating levels of testosterone during adulthood do not support an increase in ADHD-like behavior in both the animal model and human males.

Neural connections of the anterior hypothalamus and agonistic behavior in golden hamsters.

In male golden hamsters, offensive aggression is regulated by an interaction between arginine-vasopressin and serotonin at the level of the anterior hypothalamus. The present studies were conducted to study a neural network underlying this interaction. The connections of the anterior hypothalamus were examined by retrograde and anterograde tracing in adult male hamsters. Several limbic areas were found to contain both types of tracing suggesting reciprocal connections with the anterior hypothalamus. Their functional significance relating to the consummation of aggression was tested by comparing neuronal activity (examined through quantification of c-Fos-immunolabeling) in two groups of animals. Experimental animals were sacrificed after attacking an intruder. Control animals were sacrificed after exposure to a woodblock carrying the odor of an intruder that elicited behaviors related to offensive aggression without its consummation. An increased density of Fos-immunoreactivity was found in experimental animals within the medial amygdaloid nucleus, ventrolateral hypothalamus, bed nucleus of the stria terminalis and dorsolateral part of the midbrain central gray. These data suggest that these areas are integrated in a neural network centered on the anterior hypothalamus and involved in the consummation of offensive aggression. Finally, c-Fos-immunoreactivity was combined with labeling of serotonin and vasopressin neurons to identify sub-populations particularly associated with offensive aggression. Vasopressin neurons in the nucleus circularis and medial division of the supraoptic nucleus showed increased neuronal activity in the fighters, supporting their role in the control of offensive aggression.

Quick sex determination of mouse fetuses.

We designed a rapid, simple and accurate PCR method to determine sexual identity of mouse fetuses collected on embryonic day 15. A multiplex PCR amplification was used to detect male-specific sequence (Sry) in DNA extracted from fetal livers through SDS denaturation followed by high salt extraction and precipitation. This extraction method resulted in sufficiently purified DNA in < 1 h and was suitable for PCR. The DNA obtained was amplified using a robot thermal cycler for 33 cycles. The reaction was performed in 50 microl, using two sets of primers specific for Sry gene (chromosome Y) and IL3 gene (chromosome 11). Amplification duration was 1.5 h. The assessment of the results was done by electrophoresis in 3% agarose run at high voltage. The 402 bp band (Sry) obtained identifies the male fetuses and the 544 bp product (IL3) confirms the correct amplification of the template DNA. The entire procedure took < 4 h. The specificity of the method was confirmed by fluorescent in situ hybridization using a specific male probe on cultured male and female neural stem cells. This method allowed the preparation and culture of pure male and female neural stem cells from fetal tissue.

Early androgen treatment decreases cognitive function and catecholamine innervation in an animal model of ADHD.

The spontaneously hypertensive rat (SHR) has been used as an animal model of attention deficit hyperactivity disorder (ADHD). The present study was designed to determine whether exposure to elevated androgen levels early in development demonstrated impairments in cognitive functioning, neuroendocrine control, and brain development parallel to those seen in ADHD children. The animals (SHR and Wistar (WKY) controls) were implanted with testosterone on postnatal day 10 and tested for behavior in a spatial cognition paradigm on postnatal day 45. Plasma samples were collected for determination of adrenocorticotrophin hormone (ACTH) and corticosterone levels as indicators of the basal tone of the pituitary-adrenal neuroendocrine axis. In addition, the density of tyrosine hydroxylase-immunoreactive fibers (an indicator of catecholamine innervation) in the frontal cortex was compared between animals. The current data show that early testosterone treatment in SHR animals resulted in additional deficits in spatial memory in the water maze, but was ineffective in altering the response of WKY animals. Furthermore, SHR rats had high basal ACTH and low corticosterone levels that may indicate a dysfunctional stress axis similar to other reports in humans with persistent ADHD. Finally, there was a further suppression of tyrosine hydroxylase-immunoreactivity in the frontal cortex of androgen-treated SHR rats. These results support the hypothesis that early androgen treatment may support the neurobiology of animals with genetic predisposition to hyperactivity, impulsivity and inattention in a manner consistent with the enhanced expression of ADHD-like behaviors.

Serotonin regulation of aggressive behavior in male golden hamsters (Mesocricetus auratus).

These studies examined the neurochemistry and neuroanatomy of the serotonin (5-HT) system innervating the anterior hypothalamus (AH) and the interaction of 5-HT receptor agonists with arginine vasopressin (AVP) in the regulation of offensive aggression in golden hamsters. Because specific 5-HT1A, 5-HT1B, and AVP V1A binding sites were observed within the AH by in vitro autoradiography, the hamsters were tested for offensive aggression after microinjections of AVP in combination with either the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetraline (DPAT) or the 5-HT1B agonist CGS-12066A (CGS) directly within the AH. Though treatment with DPAT resulted in a dose-dependent inhibition of AVP-facilitated offensive aggression, CGS was ineffective. In addition, a retrograde tracer was injected within the AH to localize the distribution of 5-HT neurons projecting to the area. Retrogradely labeled 5-HT neurons were found within the dorsal, median, and caudal linear raphe nuclei and are suspected to inhibit AVP-facilitated offensive aggression by an activation of 5-HT1A receptors in the AH.

Exposure to lead during development alters aggressive behavior in golden hamsters.

The present studies were designed to test the effects of early exposure to low doses of lead on aggressive behavior in male golden hamsters. Litters of hamsters were exposed to lead acetate (either 0 or 100 ppm) from embryonic day 8, through weaning on postnatal day 25 (P-25), until P-42. Play fighting behavior was tested on P-19 and P-20 around the developmental onset of the behavior. During the first day of testing, lead-exposed hamsters displayed less play fighting activity. However, this difference disappeared by P-20. Around the same time, lead-exposed animals were around 20% lighter than the controls, suggesting a delayed maturation in these hamsters. Blood lead levels assayed on P-42 ranged between 10 and 15 microg/dL. Aggressive behavior was tested in early adulthood (P-45) in a resident/intruder paradigm. Lead exposure affected aggressive behavior, because lead-exposed male hamsters were faster and more likely to attack and bite their intruders. These results support the possibility that early exposure to low doses of lead during development is capable of enhancing aggressive behavior in males.

Galanin antagonizes vasopressin-stimulated flank marking in male golden hamsters.

Microinjection of vasopressin (VP) into the anterior hypothalamus (AH) of golden hamsters induces a rapid bout of flank marking, a stereotyped scent marking behavior used for olfactory communication. In rats, VP is colocalized with galanin (GAL) in several brain regions. GAL has been shown to antagonize the postsynaptic actions of other cosecreted neurotransmitters including acetylcholine and norepinephrine; however, the ability of GAL to modulate the postsynaptic actions of VP has not been assessed. Here, we report that coadministration of GAL can block VP-induced flank marking in golden hamsters in a dose dependent manner. These findings provide the first evidence in any species that GAL can antagonize the central actions of VP. Using slice binding and receptor autoradiography, we have identified GAL binding sites in the AH and two other regions implicated in flank marking behavior (the lateral septum and central grey). These findings raise the possibility that endogenous GAL may function as an inhibitory modulator of this stereotypic scent marking behavior.

Behavioral and neurobiological consequences of social subjugation during puberty in golden hamsters.

In golden hamsters, offensive aggression is facilitated by vasopressin and inhibited by serotonin. We tested whether these neurotransmitter systems respond to modifications resulting from the stress of threat and attack (i.e., social subjugation) during puberty. Male golden hamsters were weaned at postnatal day 25 (P25), exposed daily to aggressive adults from P28 to P42, and tested for offensive aggression as young adults (P45). The results showed a context-dependent alteration in aggressive behavior. Subjugated animals were more likely to attack younger and weaker intruders than nonsubjugated controls. Conversely, subjugated animals were less likely to attack animals of similar size and age. After testing, the animals were killed, and their brains were collected to determine whether these behavioral changes are underlined by changes in the vasopressin and serotonin systems. Social subjugation resulted in a 50% decrease in vasopressin levels within the anterior hypothalamus, a site involved in the regulation of aggression. Furthermore, whereas the density of vasopressin-immunoreactive fibers within the area was not significantly altered in subjugated animals, the number of serotonin-immunoreactive varicosities within the anterior hypothalamus and lateral septum was 20% higher in subjugated animals than in their controls. These results establish puberty as a developmental period sensitive to environmental stressors. Furthermore, the results show that changes in the vasopressin and serotonin systems can correlate with behavioral alterations, supporting the role of these two neurotransmitters in the regulation of aggression.

Flank-marking behavior and the neural distribution of vasopressin innervation in golden hamsters with suprachiasmatic lesions.

In golden hamsters, microinjections of arginine-vasopressin (AVP) within the anterior hypothalamus trigger a stereotyped scent-marking behavior, flank marking. Our experiment was carried out to test the contribution of AVP neurons within the suprachiasmatic nucleus (SCN) in the control of this behavior. Our results suggest that the SCN does not contribute to flank-marking behavior. Whereas SCN lesions disrupted circadian rhythms of wheel running, the same lesions did not disrupt flank-marking. The results also suggest that neurons located outside the SCN contribute significantly to the vasopressinergic innervation of the brain and the expression of AVP-dependent behaviors, such as flank-marking behavior. Although AVP-immunoreactive fibers were severely (ca. 95%) depleted from several forebrain areas in SCN-lesioned hamsters, the effect of the lesions was much more limited within the forebrain areas involved in flank-marking behavior as well as within the midbrain and hindbrain.

Vasopressin/serotonin interactions in the anterior hypothalamus control aggressive behavior in golden hamsters.

Studies in several species of rodents show that arginine vasopressin (AVP) acting through a V1A receptor facilitates offensive aggression, i.e., the initiation of attacks and bites, whereas serotonin (5-HT) acting through a 5-HT1B receptor inhibits aggressive responding. One area of the CNS that seems critical for the organization of aggressive behavior is the basolateral hypothalamus, particularly the anterior hypothalamic region. The present studies examine the neuroanatomical and neurochemical interaction between AVP and 5-HT at the level of the anterior hypothalamus (AH) in the control of offensive aggression in Syrian golden hamsters. First, specific V1A and 5-HT1B binding sites in the AH are shown by in vitro receptor autoradiography. The binding for each neurotransmitter colocalizes with a dense field of immunoreactive AVP and 5-HT fibers and putative terminals. Putative 5-HT synapses on AVP neurons in the area of the AH are identified by double-staining immunocytochemistry and laser scanning confocal microscopy. These morphological data predispose a functional interaction between AVP and 5-HT at the level of the AH. When tested for offensive aggression in a resident/intruder paradigm, resident hamsters treated with fluoxetine, a selective 5-HT reuptake inhibitor, have significantly longer latencies to bite and bite fewer times than vehicle-treated controls. Conversely, AVP microinjections into the AH significantly shorten the latency to bite and increase biting attacks. The action of microinjected AVP to increase offensive aggression is blocked by the pretreatment of hamsters with fluoxetine. These data suggest that 5-HT inhibits fighting, in part, by antagonizing the aggression-promoting action of the AVP system.

Testosterone facilitates aggression by modulating vasopressin receptors in the hypothalamus.

In many species, testosterone treatment facilitates offensive aggression tested in resident-intruder models. As the mechanisms of action of testosterone remain unclear, we hypothesized that testosterone interacts with neurotransmitter systems involved in the regulation of offensive aggression. We tested this hypothesis with the vasopressinergic system in golden hamsters in three separate experiments. First, we compared the density of V1 vasopressin (VAP) receptor binding between castrated animals treated with testosterone and their untreated controls. The most noticeable difference was found within the ventrolateral hypothalamus (VLH), a site involved in the control of aggression in several species of mammals. Within this area, V1 AVP receptor binding disappeared after castration, while being maintained by testosterone-treatment. Second, we tested behavioral effects of AVP within the VLH. Microinjections of AVP (100 nl, 1 or 100 microM) within the VLH accelerated the onset of offensive aggression in testosterone-treated animals. However, AVP-injected animals did not bite more than their vehicle-injected controls. Third, microinjections of AVP failed to activate offensive aggression in animals deprived of testosterone. As AVP receptors appeared to overlay previously described distributions of androgen and estrogen receptors in golden hamsters, we propose that testosterone facilitates the onset of offensive aggression, at least partly, through an activation of AVP receptors within the VLH.

Vasopressin and developmental onset of flank marking behavior in golden hamsters.

Golden hamsters start displaying flank marking behavior (a form of scent marking) around postnatal day 20 (P-20). Because the behavior is dependent upon the central activity of arginine vasopressin (AVP), the present study was conducted to correlate this activation with changes in the vasopressinergic system. A first set of experiments was performed to compare flank marking activity between P-18 and P-22. A second set of experiments was performed to compare the density of AVP receptors between the age periods and assess responsiveness to AVP microinjection. Finally, a third set of experiments incorporated immunocytochemistry, radioimmunoassay, in situ hybridization, and Northern blot analysis to determine the location and numbers of AVP immunoreactive neurons and the level of mRNA correlating with the developmental onset of flank marking behavior. Our results show that flank marking develops between P-18 and P-22. Male and female hamsters do not display odor-induced flank marking anytime before P-19. However, all animals show odor-induced flank marking by P-22. The onset of flank marking does not appear to be associated with any change in AVP receptor binding in the anterior hypothalamus. Indeed, flank marking can be triggered in hamsters on P-18 by the microinjection of AVP in the anterior hypothalamus. This would suggest that the postsynaptic mechanisms contributing to the transduction of the AVP signal and the motor control of flank marking are intact prior to the onset of odor-induced flank marking. In contrast, AVP levels in the hypothalamus and pituitary increase by two to threefold between P-18 and P-22, suggesting that changes in AVP synthesis and release from presynaptic sites may contribute to the onset of flank marking. Interestingly, there is no change in AVP mRNA between P-18 and P-22, which raises questions about posttranslational processing during this developmental period. These results suggest that heightened synthesis and release of AVP between P-18 and P-22 may contribute to the developmental onset of flank marking.

Serotonin blocks vasopressin-facilitated offensive aggression: interactions within the ventrolateral hypothalamus of golden hamsters.

In golden hamsters, vasopressin (AVP) microinjected within the ventrolateral hypothalamus (VLH) facilitates offensive aggression. As serotonin is known to inhibit offensive aggression, we decided to test whether AVP-facilitated behavior is also inhibited by serotonin treatment. Testosterone-treated male golden hamsters received IP injections of fluoxetine, a serotonin reuptake inhibitor, or vehicle 1 h prior to AVP microinjections within the VLH. The animals were tested for offensive aggression in a resident-intruder model after the microinjections, and the results were compared between groups. Pretreatment with fluoxetine inhibited AVP-facilitated offensive aggression. Only one out of nine fluoxetine-treated animals attacked and bit the intruders, compared to six out of seven vehicle-treated animals. Furthermore, we also confirmed by in vitro autoradiography that the VLH contains vasopressin V(1) and serotonin 5-HT1B receptors. Therefore, it is possible that serotonin may inhibit AVP-facilitated offensive aggression by acting directly at the level of the VLH as well as at other sites.