Activation patterns of vasopressinergic and oxytocinergic brain regions following social play exposure in juvenile male and female rats.

Social play is a highly rewarding and motivated behavior predominately displayed by juveniles and expressed by nearly all mammalian species. Prior work suggested that the vasopressin (AVP) and oxytocin (OT) systems can regulate the expression of social play in sex-specific ways. Here we investigated whether there are sex differences in the recruitment of vasopressinergic and oxytocinergic brain regions following social play exposure in juvenile rats. Single-housed rats were allowed to play, in their home cage, with an age- and sex-matched unfamiliar conspecific for 10 min, or received similar handling but no partner. Double-labeled fluorescent immunohistochemistry for Fos and either AVP or OT was completed in adjacent series of tissue to determine recruitment of AVP- and OT-immunoreactive neurons in response to social play. Exposure to social play did not increase recruitment of AVP or OT neurons in the supraoptic (SO) or paraventricular (PVH) hypothalamic nuclei of either sex compared to the no-play control condition. Interestingly, there was a robust sex difference in SO recruitment, irrespective of social play condition, with males exhibiting twice the recruitment of SO-AVP and SO-OT neurons compared to females. Lastly, exposure to social play increased recruitment of the posterior bed nuclei of the stria terminalis (pBST) and the posterodorsal medial amygdalar nucleus (MEApd) compared to the no-play control condition, and this effect was most pronounced in females. Our findings revealed sex differences in the recruitment of brain regions (i) independent of play condition (i.e., SO) possibly representing a sex difference in the baseline levels of AVP and OT signaling required for typical functioning and (ii) specific to play condition (i.e., pBST, MEApd). In sum, this study provides further evidence that the neural substrates underlying social play behavior are sex-specific. This article is protected by copyright. All rights reserved.

Dynamic changes in extracellular release of GABA and glutamate in the lateral septum during social play behavior in juvenile rats: Implications for sex-specific regulation of social play behavior.

Social play is a motivated and rewarding behavior that is displayed by nearly all mammals and peaks in the juvenile period. Moreover, social play is essential for the development of social skills and is impaired in social disorders like autism. We recently showed that the lateral septum (LS) is involved in the regulation of social play behavior in juvenile male and female rats. The LS is largely modulated by GABA and glutamate neurotransmission, but their role in social play behavior is unknown. Here, we determined whether social play behavior is associated with changes in the extracellular release of GABA and glutamate in the LS and to what extent such changes modulate social play behavior in male and female juvenile rats. Using intracerebral microdialysis in freely behaving rats, we found no sex difference in extracellular GABA concentrations, but extracellular glutamate concentrations are higher in males than in females under baseline conditions and during social play. This resulted in a higher glutamate/GABA concentration ratio in males vs. females and thus, an excitatory predominance in the LS of males. Furthermore, social play behavior in both sexes is associated with significant increases in extracellular release of GABA and glutamate in the LS. Pharmacological blockade of GABA-A receptors in the LS with bicuculline (100 ng/0.5 µl, 250 ng/0.5 µl) dose-dependently decreased the duration of social play behavior in both sexes. In contrast, pharmacological blockade of ionotropic glutamate receptors (NMDA and AMPA/kainate receptors) in the LS with AP-5+CNQX (2mM+0.4mM/0.5 µl, 30 mM+3mM/0.5 µl) dose-dependently decreased the duration of social play behavior in females, but did not alter social play behavior in males. Together, these data suggest a role for GABA neurotransmission in the LS in the regulation of juvenile social play behavior in both sexes, while glutamate neurotransmission in the LS is involved in the sex-specific regulation of juvenile social play behavior.

Social Novelty Investigation in the Juvenile Rat: Modulation by the µ-Opioid System.

The drive to approach and explore novel conspecifics is inherent to social animals and may promote optimal social functioning. Juvenile animals seek out interactions with novel peers more frequently and find these interactions to be more rewarding than their adult counterparts. In the present study, we aimed to establish a behavioural paradigm to measure social novelty-seeking in juvenile rats and to determine the involvement of the opioid, dopamine, oxytocin and vasopressin systems in this behaviour. To this end, we developed the social novelty preference test to assess the preference of a juvenile rat to investigate a novel over a familiar (cage mate) conspecific. We show that across the juvenile period both male and female rats spend more time investigating a novel conspecific than a cage mate, independent of subject sex or repeated exposure to the test. We hypothesised that brain systems subserving social information processing and social motivation/reward (i.e. the opioid, dopamine, oxytocin, vasopressin systems) might support social novelty preference. To test this, receptor antagonists of each of these systems were administered i.c.v. prior to exposure to the social novelty preference test and, subsequently, to the social preference test, to examine the specificity of these effects. We find that µ-opioid receptor antagonism reduces novel social investigation in both the social novelty preference and social preference tests while leaving the investigation of a cage mate (social novelty preference test) or an object (social preference test) unaffected. In contrast, central blockade of dopamine D2 receptors (with eticlopride), oxytocin receptors (with des-Gly-NH2,d(CH2)5[Tyr(Me)2,Thr4]OVT) or vasopressin V1a receptors [with (CH2)5Tyr(Me2)AVP] failed to alter social novelty preference or social preference. Overall, we have established a new behavioural test to study social novelty-seeking behaviour in the juvenile rat and show that the µ-opioid system facilitates this behaviour, possibly by reducing risk avoidance and enhancing the hedonic and/or motivational value of social novelty.

Vasopressin regulates social recognition in juvenile and adult rats of both sexes, but in sex- and age-specific ways.

In adult male rats, vasopressin (AVP) facilitates social recognition via activation of V1a receptors within the lateral septum. Much less is known about how AVP affects social recognition in adult females or in juvenile animals of either sex. We found that administration of the specific V1a receptor antagonist d(CH(2))(5)[Tyr(Me)(2)]AVP into the lateral septum of adult rats impaired, whereas AVP extended, social discrimination in both sexes. In juveniles, however, we detected a sex difference, such that males but not females showed social discrimination. Interestingly, administration of the V1a receptor antagonist to juveniles (either intracerebroventricularly or locally in the lateral septum) did not prevent social discrimination, but instead significantly decreased the investigation of a novel as opposed to a familiar animal in both sexes, with stronger effects in males. V1a receptors were found to be abundantly expressed in the lateral septum with higher binding density in females than in males. These findings demonstrate that activation of V1a receptors in the lateral septum is important for social recognition in both sexes, and that the roles of septal V1a receptors in social recognition change during development.

Maternal separation interferes with developmental changes in brain vasopressin and oxytocin receptor binding in male rats.

Brain vasopressin V(1A) receptors (V(1A)-R) and oxytocin receptors (OT-R) are important modulators of social behaviors. We recently showed that exposure to maternal separation (MS; 3 h daily, postnatal days 1-14) induces changes in social behaviors in juvenile and adult male rats. Here, we hypothesize that MS induces brain region-specific changes in V(1A)-R and OT-R across development, which in turn, may underlie MS-induced changes in social behaviors. We examined the effects of MS on V(1A)-R and OT-R binding in forebrain regions of juvenile (5 weeks), adolescent (8 weeks), and adult (16 weeks) male rats. Robust age-related changes were found for V(1A)-R and OT-R binding in several brain regions. For example, in the lateral septum V(1A)-R binding increased while OT-R binding decreased with age. Most notably, OT-R binding in the caudate putamen showed a 2-fold decrease while OT-R binding in the ventromedial hypothalamus showed a 4-fold increase with age. Importantly, exposure to MS interfered with these developmental changes in several brain regions. Specifically, MS significantly increased V(1A)-R binding in the piriform cortex (at adolescent and adult ages), the lateral septum (at juvenile age), the hypothalamic attack area (at adolescent age), and the dentate gyrus of the hippocampus (at adolescent age), and decreased V(1A)-R binding in the arcuate nucleus (at juvenile age). Moreover, OT-R binding was significantly lower in the agranular cortex (at juvenile and adolescent age), the lateral septum (at adult age) and the caudate putamen (at adult age), but higher in the medial preoptic area (at adolescent age) and ventromedial hypothalamus (at adult age) after exposure to MS. In conclusion, age-dependent changes in V(1A)-R and OT-R binding are likely associated with the maturation of behaviors, such as sexual and aggressive behaviors, while disruption of these changes by MS might contribute to previously observed changes in social behaviors after MS.

Differential effects of stress on adult hippocampal cell proliferation in low and high aggressive mice.

Male wild house mice selected for a long (LAL) or a short (SAL) latency to attack a male intruder generally show opposing behavioural coping responses to environmental challenges. LAL mice, unlike SAL mice, adapt to novel challenges with a highly reactive hypothalamic-pituitary-adrenal axis and show an enhanced expression of markers for hippocampal plasticity. The present study aimed to test the hypothesis that these features of the more reactive LAL mice are reflected in parameters of hippocampal cell proliferation. The data show that basal cell proliferation in the subgranular zone (SGZ) of the dentate gyrus, assessed by the endogenous proliferation marker Ki-67, is lower in LAL than in SAL mice. Furthermore, application of bromodeoxyuridine (BrdU) over 3 days showed an almost two-fold lower cell proliferation rate in the SGZ in LAL versus SAL mice. Exposure to forced swimming resulted, 24 h later, in a significant reduction in BrdU + cell numbers in LAL mice, whereas cell proliferation was unaffected by this stressor in SAL mice. Plasma corticosterone and dentate gyrus glucocorticoid receptor levels were higher in LAL than in SAL mice. However, no differences between the SAL and LAL lines were found for hippocampal NMDA receptor binding. In conclusion, the data suggest a relationship between coping responses and hippocampal cell proliferation, in which corticosterone may be one of the determinants of line differences in cell proliferation responses to environmental challenges.

Adrenal insufficiency and colonic inflammation after a novel chronic psycho-social stress paradigm in mice: implications and mechanisms.

We investigated chronic psycho-social stress effects on stress-related parameters and on pathohistological changes in the murine colon. Moreover, we aimed to reveal the involvement of adrenal glucocorticoids in chronic stress effects. Chronic subordinate colony housing (CSC, 19 d) resulted in reduced body weight gain, thymus atrophy, adrenal hypertrophy, increased plasma norepinephrine, and increased anxiety. With respect to the time course of CSC effects, CRH mRNA in the hypothalamic paraventricular nucleus, light phase corticosterone and tyrosine hydroxylase expression in colonic tissue were found to be increased, whereas tyrosine hydroxylase expression in the locus coeruleus was found to be decreased on d 2 of CSC; these parameters returned to control levels thereafter. Nevertheless, after 19 d of CSC exposure, the adrenal corticosterone responses in vivo and in vitro, and glucocorticoid sensitivity of isolated splenic cells were found to be decreased. Importantly, in CSC mice a significant histological damage of the colon was found beginning on d 14 of CSC exposure. Additionally, pro- and antiinflammatory cytokine secretion by mesenteric lymph node cells was increased after CSC exposure. Adrenalectomy before CSC at least partially prevented these chronic stress effects as reflected by less increase in proinflammatory cytokine secretion and an equal histological damage score in adrenalectomized compared with sham-operated CSC mice. In conclusion, chronic exposure to CSC alters relevant neuronal, neuroendocrine and immune functions that could be directly or indirectly involved in the damage of the histological integrity of the colon comparable with that seen during the development of colitis.

Genetic selection for coping style predicts stressor susceptibility.

Genetically selected aggressive (SAL) and nonaggressive (LAL) male wild house-mice which show distinctly different coping styles, also display a differential regulation of the hypothalamic-pituitary-adrenal axis after exposure to an acute stressor. To test the hypothesis that coping style predicts stressor susceptibility, the present study examined line differences in response to a chronic stressor. Chronic psychosocial stress was evoked using two paradigms. In the first paradigm, a SAL or LAL male was living in sensory contact (except tactile contact) with a dominant SAL male for 25 days (sensory contact stress). In the second paradigm, a SAL or LAL male was, in addition to the first paradigm, defeated by a SAL male for 21 consecutive days (defeat stress). The sensory contact stressor induced in LAL mice chronic body weight loss and increased plasma adrenocorticotropic hormone levels compared to SAL mice and increased corticosterone levels, thymus involution and lower hippocampal mineralocorticoid receptor (MR) : glucocorticoid receptor (GR) ratio compared to LAL controls. The defeat stressor increased corticosterone secretion and caused adrenal hypertrophy and thymus involution in both mouse lines. Defeated LAL mice showed long-lasting body weight loss and higher corticosterone concentrations than SAL mice and lower hippocampal MR : GR ratio and decreased immobility behaviour in the forced swimming test than LAL controls. Hypothalamic corticotropin-releasing hormone mRNA expression was higher in defeated SAL than in controls. The present data show that both stress paradigms induced line-dependent physiological and neuroendocrine changes, but that the sensory contact stressor produced chronic stress symptoms in LAL mice only. This latter stress paradigm therefore seems promising to analyse the role of genetic factors in the individual differences in stress-related psychopathology.

Hippocampal serotonin responses in short and long attack latency mice.

Short and long attack latency mice, which are selected based on their offensive behaviour in a resident-intruder model, differ in their neuroendocrine regulation as well as in aspects of their brain serotonin system. Previous studies showed that the binding capacity and expression of serotonin-1A receptors in the hippocampal CA1 field of long attack latency mice are significantly lower than that found in short attack latency mice. We tested whether the functional responses of CA1 hippocampal cells to serotonin are also reduced in long attack latency mice. To this end, serotonin-induced changes in the membrane potential and input resistance were recorded in vitro with microelectrodes in CA1 pyramidal neurones of long and short attack latency mice. The data show that in long attack latency mice, along with a reduction of the serotonin-1A receptor mRNA expression, the serotonin-induced membrane hyperpolarization and decrease in resistance are attenuated. Basal membrane properties of CA1 neurones in the two.mice lines were comparable. Plasma corticosterone levels in response to a novelty stress were elevated in long compared to short attack latency mice and inversely related to the serotonin-induced responses. We tentatively conclude that long attack latency mice show attenuated functional responses to serotonin in the hippocampus, possibly linked to a chronic perturbation of hormonal levels.

Chronic corticosterone administration dose-dependently modulates Abeta(1-42)- and NMDA-induced neurodegeneration in rat magnocellular nucleus basalis.

The impact of glucocorticoids on beta-amyloid(1-42) (Abeta(1-42)) and NMDA-induced neurodegeneration was investigated in vivo. Abeta(1-42) or NMDA was injected into the cholinergic magnocellular nucleus basalis in adrenalectomized (ADX) rats, ADX rats supplemented with 25%, 100%, 2x100% corticosterone pellets, or sham-ADX controls. Abeta(1-42)- or NMDA-induced damage of cholinergic nucleus basalis neurones was assessed by quantitative acetylcholinesterase histochemistry. Plasma concentrations of corticosterone and cholinergic fibre loss after Abeta(1-42) or NMDA injection showed a clear U-shaped dose-response relationship. ADX and subsequent loss of serum corticosterone potentiated both the Abeta(1-42) and NMDA-induced neurodegeneration. ADX+25% corticosterone resulted in a 10-90 nM plasma corticosterone concentration, which significantly attenuated the Abeta(1-42) and NMDA neurotoxicity. ADX+100% corticosterone (corticosterone concentrations of 110-270 nM) potently decreased both Abeta(1-42)- and NMDA-induced neurotoxic brain damage. In contrast, high corticosterone concentrations of 310-650 nM potentiated Abeta(1-42)- and NMDA-triggered neurodegeneration. In conclusion, chronic low or high corticosterone concentrations increase the vulnerability of cholinergic cells to neurotoxic insult, while slightly elevated corticosterone levels protect against neurotoxic injury. Enhanced neurotoxicity of NMDA in the presence of high concentrations of specific glucocorticoid receptor agonists suggests that the corticosterone effects are mediated by glucocorticoid receptors.

Effect of corticosterone and adrenalectomy on NMDA-induced cholinergic cell death in rat magnocellular nucleus basalis.

The present study demonstrates the effects of adrenalectomy and subcutaneously administered corticosterone on N-methyl-D-aspartate-induced neurodegeneration in the cholinergic magnocellular basal nucleus of the rat. NMDA was unilaterally injected into the nucleus basalis at different plasma corticosterone concentrations in adrenalectomized rats, in adrenalectomized animals with subcutaneously implanted cholesterol-corticosterone pellets containing 25% or 100% corticosterone, and in sham-adrenalectomized controls. The neurotoxic impact of the NMDA injection in the various experimental groups was assessed by the loss of cholinergic fibers stained with acetylcholinesterase histochemistry in the parietal neocortex. Reactive cortical astrocytes as a result of the treatments were detected by glial fibrillary acidic protein immunohistochemistry. Measurements of the densities of astrocytes and cholinergic fibers at the injected side of the brain were carried out by image analysis. Adrenalectomy significantly potentiated the NMDA-induced neurodegeneration by 50%, while chronic administration of corticosterone significantly attenuated the NMDA-neurotoxicity in a dose-dependent manner. Compared to the ADX group, 25% corticosterone application reduced the NMDA damage by 37%, whereas the 100% corticosterone pellet diminished NMDA neurotoxicity by 75%. Both ADX and ADX + corticosterone implantation enhanced the NMDA-induced GFAP immunoreactivity. The increase of GFAP immunoreactivity was most pronounced in the adrenalectomized rats supplied with the 100% corticosterone pellets. The results demonstrate that corticosterone exerts a potent neuroprotective effect on NMDA-induced neurotoxicity in the magnocellular nucleus basalis. The activated astroglia suggest that astrocytes may contribute to the beneficial effect of corticosterone in the neuroprotective mechanisms against excitotoxic neuronal injury.