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The Y1 and Y5 receptors for neuropeptide Y have overlapping functions in regulating anxiety. We previously demonstrated that conditional removal of the Y1 receptor in the Y5 receptor expressing neurons in juvenile Npy1r Y5R−/− mice leads to higher anxiety but no changes in hypothalamus-pituitary-adrenocortical axis activity, under basal conditions or after acute restraint stress. In the present study, we used the same conditional system to analyze the specific contribution of limbic neurons coexpressing Y1 and Y5 receptors on the emotional and neuroendocrine responses to social chronic stress, using different housing conditions (isolation vs. group-housing) as a model. We demonstrated that control Npy1r 2lox male mice housed in groups show increased anxiety and hypothalamus-pituitary-adrenocortical axis activity compared with Npy1r 2lox mice isolated for six weeks immediately after weaning. Conversely, Npy1r Y5R−/− conditional mutants display an anxious-like behavior but no changes in hypothalamus-pituitary-adrenocortical axis activity as compared with their control littermates, independently of housing conditions. These results suggest that group housing constitutes a mild social stress for our B6129S mouse strain and they confirm that the conditional inactivation of Y1 receptors specifically in Y5 receptor containing neurons increases stress-related anxiety without affecting endocrine stress responses. [ABSTRACT FROM AUTHOR]

Bisphenol A (BPA) is a well-known plastic-derived pollutant that can bind to oestrogen receptors and is considered an endocrine-disrupting chemical. Its impact on different behaviours in rodents has been largely investigated, however, only a few data are available on its effects upon neural circuits. In the present study, we investigated the long-term effects of early exposure of mice of both sexes to BPA on the nitrinergic system, one of the neural systems involved in the control of sexual behaviour and under the control of gonadal hormones. Mice of both sexes were exposed for eight prenatal and eight postnatal days to BPA that was administered to the mothers. The maternally-exposed mice were sacrificed at the age of 2 months and their brains were sectioned and immunohistochemically treated for the detection of neuronal nitric oxide synthase (nNOS). Significant effects of BPA exposure were detected for the number of immunoreactive cells in the medial preoptic nucleus and in the ventromedial subdivision of the bed nucleus of the stria terminalis, in a sex-oriented and dose-dependent way. These results indicate that BPA has a powerful effect on specific portions of the nNOS-immunoreactive system belonging to the accessory olfactory system that are particularly important for the control of sexual behaviour. In addition, they confirm that perinatal exposure to endocrine-disrupting chemicals, in particular to BPA, may have a high impact on the organisation of specific neural pathways that can later affect complex behaviours and functions. [ABSTRACT FROM AUTHOR]

Cross fostering is a widely used laboratory practice. However, relatively few studies have directly investigated the carry-over effects of this procedure in adult animals. The aim of the present study is to investigate the late effects of cross fostering (CF) at birth (in litters composed of no siblings) on adult mice. When adults, cross-fostered male and female mice were examined for intrasex aggression, and levels of emotionality, exploration and anxiety. In addition, body weight was monitored, several internal organs were weighed and plasma corticosterone levels were measured. When compared to controls, body weight of CF male and female mice was increased, at least after early puberty. CF males showed smaller preputial glands, while basal corticosterone level was not affected by cross fostering. In the free-exploratory test, CF males, but not females, showed a behavioral profile suggestive of lower anxiety. These effects in adulthood cannot be ascribed to differences in the maternal care received, which was not affected by cross fostering. In conclusion, cross fostering at birth induced a number of behavioral and physiological alterations in mice, particularly in males. These findings should be carefully evaluated when applying cross fostering procedure to laboratory animals. [ABSTRACT FROM AUTHOR]

Group housing of male mice has been described as a stressful condition. In particular, subordinate male mice have been reported to show increased levels of stress-related behavioural and physiological parameters. In many studies, however, males are housed in groups of adult unfamiliar and genetically unrelated individuals. We review here a series of recent studies investigating the behaviour and the physiology of group housed male sibling Swiss CD1 mice. As adults, Dominants (D) and Subordinates (S) were subjected to the Open Field Test and the Free Exploratory Paradigm. Body-weights were monitored and relevant organs (adrenals, testis, spleen, thymus, preputials) weighed. Basal corticosterone level and immune competence were also determined. Results showed no difference between D and S in any parameters measured. Our data provide evidence that social status in a sibling context is not stressful for male mice because no differences were detected between dominants and subordinates. From a review of the literature, it was possible to conclude that what is stressful for a mouse is not the group-housing condition in itself but the lack of familiarity/ relatedness with respect to the cage mates. Furthermore, the majority of studies on social stress, are performed with animals having either no or limited experience of aggression, i.e. they lack a natural component of the social behaviour in this species. In contrast, we suggest that group housed siblings, living in a non-stressful condition and experiencing a non-deprived social setting, can be regarded as a valid control group. It is also suggested that living in a nonstressful condition, mice housed in same-sex sibling groups since birth, also provides the ideal social environment for the welfare of the laboratory housed mice. [ABSTRACT FROM PUBLISHER]

Swiss CD-1 lactating mice show a different pattern of attack toward intruders of differing sex, displaying defensive attack against the male (bites on the head and ventrum associated with fear) and offensive attack against the female (bites on the back and flanks with no elicitation of fear). This dichotomy may reflect diverse functions of maternal aggression: the attack toward males (the more infanticidal gender in laboratory strains) has been interpreted as a counterstrategy to infanticide, whereas the attack toward females may serve to establish a social hierarchy or to space rivals of the same sex. In terms of proximal mechanisms, fear may be a key factor involved in the modulation of the different patterns of attack. In Experiment 1 we compared the pattern of attack of lactating females in Swiss CD-1 and Wild mice toward male and female intruders in relation to fear components of behavior of the attacking dams. Results showed that in Swiss mice, male intruders were attacked with a defensive type of attack accompanied by high levels of fear, whereas female intruders did not elicit fear in the attacking animal but were attacked with an offensive pattern. In Wild mice, both types of intruders were attacked with a defensive pattern; notwithstanding, fear was evident only toward male intruders. This suggests that fear is not totally responsible for the expression of the defensive type of attack. To test the hypothesis that defensive attack toward male and female intruders may be related to the infanticidal potential of the intruder, Experiment 2 examined levels of infanticide in both male and female Swiss CD-1 and Wild mice. Swiss female mice showed virtually no infanticidal behavior, whereas Swiss males and both sexes of Wild mice showed similarly high levels of infanticide (55%–75%). From a game theory perspective, the defensive pattern of maternal attack toward female intruders in Wild mice is discussed as “extreme” defense of a high value resource and thus, functionally, a competitive form of aggression. Aggr. Behav. 26:193–203, 2000. © 2000 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Abstract The present study examined the influence of prior social experience on the effects of chlordiazepoxide (CDP: 5.0, 10.0 and 20.0 mg/kg) on intrasexual aggression in male mice. Prior to drug testing, animals were either individually housed or screened in dyadic encounters in a neutral cage. This novel method yielded four experimental groups comprising animals with different social experiences and different aggressive/defensive characteristics: 1) individually-housed males (I): 2) aggressive males (A); 3) counter-attacking males (C), which actively responded to but did not initiate attack; and 4) defeated males (D). Twenty-four hours after screening, animals were treated with CDP and subjected to a resident-intruder test with untreated intruders. Results indicated that the lowest dose of CDP (5 mg/kg) increased aggressive behaviour but only in A males. At higher doses (10-20 mg/kg), CDP reduced attacks towards intruders in A, C and I, but not D, males. In A and C males, the antiaggressive action of CDP was associated with a prosocial effect (increased social investigation), whereas in I males, reduced aggression was associated with an increase in fear-related behaviours. As these differential effects of CDP on intermale aggression cannot be fully explained by differences in behavioural baselines, present data highlight the importance of experiential background as a powerful variable in determining behavioural responses to benzodiazepines. Present findings therefore suggest that an understanding of drug effects on social behaviour demands consideration of biological variability in phenotype. [ABSTRACT FROM AUTHOR]

A series of experiments was carried out to assess whether the «defensive» pattern of female parental attack towards sexually naive male conspecific intruders, in which there is no inhibition against biting vulnerable areas of the intruder's body, could function as a deterrent to infanticide. Primiparous lactating mice (Mus domesticus) which had not previously displayed parental attack showed clearly differentiated patterns of biting attack when confronting sexually naive male and female conspecific intruders, respectively. Males intruders were more prone to kill pups than were female counterparts, and this behaviour was facilitated in the absence of the mother. Remarkably, lactating females simultaneously confronting a virgin female and a sexually naive male intruder attacked only the latter animal. These data suggest that lactating female aggression serves as a counterstrategy to male infanticide. Nevertheless the mother's aggressive behaviour delayed but rarely thwarted the killing of pups by males that had dominated the female and that were characterized by high levels of social aggression in previous intermale aggression tests. Maternal attack was successful in defending the litter from those males that were incapable of defeating the lactating female and that were characterized by low levels of intermale aggression. Consequently, maternal aggression might also function, as a by-product, to prevent reproduction by males of poor fighting ability. The hypothesis that in this context female aggressive behaviour towards the interloper male could also be involved in intersexual selection is discussed. [ABSTRACT FROM PUBLISHER]

Lactating mice respond differently to male and female conspecific intruders, displaying defensive attack towards the former and offensive attack towards the latter. Two studies are reported in which the effects of naloxone on this differential response pattern were assessed. In the first study, lactating residents were pretreated with saline or naloxone (0.5, 2.5 mg/kg) and consecutively confronted with intruders of differing sex. Results suggested that attack (offensive) against female conspecific is more sensitive to the inhibitory effects of opiate-receptor blockade than attack (defensive) on males. This conclusion was strengthened in a follow-up study, in which independent groups of lactating residents were used to further examine the effects of naloxone (0.5 mg/kg) on response to male and female intrusion. Analysis indicated that this low dose of naloxone significantly inhibited attack on female, but not on male, intruders. Indeed, in response to male, but not to female, intrusion, naloxone-treated residents showed a significant increase in fear-related behaviour. Findings are discussed in relation to the motivational differences in the response of lactating mice to intruders of differing sex and to possible mechanisms underlying the reported differential effects of opiate-receptor blockade. [ABSTRACT FROM AUTHOR]