A comparison of the effects of male pheromone priming and optogenetic inhibition of accessory olfactory bulb forebrain inputs on the sexual behavior of estrous female mice.

Previous research has shown that repeated testing with a stimulus male is required for ovariectomized, hormone-primed female mice to become sexually receptive (show maximal lordosis quotients; LQs) and that drug-induced, epigenetic enhancement of estradiol receptor function accelerated the improvement in LQs otherwise shown by estrous females with repeated testing. We asked whether pre-exposure to male pheromones ('pheromone priming') would also accelerate the improvement in LQs with repeated tests and whether optogenetic inhibition of accessory olfactory bulb (AOB) projection neurons could inhibit lordosis in sexually experienced estrous female mice. In Experiment 1, repeated priming with soiled male bedding failed to accelerate the progressive improvement in LQs shown by estrous female mice across 5 tests, although the duration of each lordosis response and females' investigation of male body parts during the first test was augmented by such priming. In Experiment 2, acute optogenetic inhibition of AOB inputs to the forebrain during freely moving behavioral tests significantly reduced LQs, suggesting that continued AOB signaling to the forebrain during mating is required for maximal lordotic responsiveness even in sexually experienced females. Our results also suggest that pheromonal stimulation, by itself, cannot substitute for the full complement of sensory stimulation received by estrous females from mounting males that normally leads to the progressive improvement in their LQs with repeated testing.

Social regulation of cortisol receptor gene expression.

In many social species, individuals influence the reproductive capacity of conspecifics. In a well-studied African cichlid fish species, Astatotilapia burtoni, males are either dominant (D) and reproductively competent or non-dominant (ND) and reproductively suppressed as evidenced by reduced gonadotropin releasing hormone (GnRH1) release, regressed gonads, lower levels of androgens and elevated levels of cortisol. Here, we asked whether androgen and cortisol levels might regulate this reproductive suppression. Astatotilapia burtoni has four glucocorticoid receptors (GR1a, GR1b, GR2 and MR), encoded by three genes, and two androgen receptors (ARα and ARß), encoded by two genes. We previously showed that ARα and ARß are expressed in GnRH1 neurons in the preoptic area (POA), which regulates reproduction, and that the mRNA levels of these receptors are regulated by social status. Here, we show that GR1, GR2 and MR mRNAs are also expressed in GnRH1 neurons in the POA, revealing potential mechanisms for both androgens and cortisol to influence reproductive capacity. We measured AR, MR and GR mRNA expression levels in a microdissected region of the POA containing GnRH1 neurons, comparing D and ND males. Using quantitative PCR (qPCR), we found D males had higher mRNA levels of ARα, MR, total GR1a and GR2 in the POA compared with ND males. In contrast, ND males had significantly higher levels of GR1b mRNA, a receptor subtype with a reduced transcriptional response to cortisol. Through this novel regulation of receptor type, neurons in the POA of an ND male will be less affected by the higher levels of cortisol typical of low status, suggesting GR receptor type change as a potential adaptive mechanism to mediate high cortisol levels during social suppression.

Detection and avoidance of a carnivore odor by prey.

Predator-prey relationships provide a classic paradigm for the study of innate animal behavior. Odors from carnivores elicit stereotyped fear and avoidance responses in rodents, although sensory mechanisms involved are largely unknown. Here, we identified a chemical produced by predators that activates a mouse olfactory receptor and produces an innate behavioral response. We purified this predator cue from bobcat urine and identified it to be a biogenic amine, 2-phenylethylamine. Quantitative HPLC analysis across 38 mammalian species indicates enriched 2-phenylethylamine production by numerous carnivores, with some producing >3,000-fold more than herbivores examined. Calcium imaging of neuronal responses in mouse olfactory tissue slices identified dispersed carnivore odor-selective sensory neurons that also responded to 2-phenylethylamine. Two prey species, rat and mouse, avoid a 2-phenylethylamine odor source, and loss-of-function studies involving enzymatic depletion of 2-phenylethylamine from a carnivore odor indicate it to be required for full avoidance behavior. Thus, rodent olfactory sensory neurons and chemosensory receptors have the capacity for recognizing interspecies odors. One such cue, carnivore-derived 2-phenylethylamine, is a key component of a predator odor blend that triggers hard-wired aversion circuits in the rodent brain. These data show how a single, volatile chemical detected in the environment can drive an elaborate danger-associated behavioral response in mammals.

Prior stress and vasopressin promote corticotropin-releasing factor inhibition of serotonin release in the central nucleus of the amygdala.

Corticotropin-releasing factor (CRF) is essential for coordinating endocrine and neural responses to stress, frequently facilitated by vasopressin (AVP). Previous work has linked CRF hypersecretion, binding site changes, and dysfunctional serotonergic transmission with anxiety and affective disorders, including clinical depression. Crucially, CRF can alter serotonergic activity. In the dorsal raphé nucleus and serotonin (5-HT) terminal regions, CRF effects can be stimulatory or inhibitory, depending on the dose, site, and receptor type activated. Prior stress alters CRF neurotransmission and CRF-mediated behaviors. Lateral, medial, and ventral subdivisions of the central nucleus of the amygdala (CeA) produce CRF and coordinate stress responsiveness. The purpose of these experiments was to determine the effect of intracerebroventricular (icv) administration of CRF and AVP on extracellular 5-HT as an index of 5-HT release in the CeA, using in vivo microdialysis in freely moving rats and high performance liquid chromatography (HPLC) analysis. We also examined the effect of prior stress (1 h restraint, 24 h prior) on CRF- and AVP-mediated release of 5-HT within the CeA. Our results show that icv CRF infusion in unstressed animals had no effect on 5-HT release in the CeA. Conversely, in rats with prior stress, CRF caused a profound dose-dependent decrease in 5-HT release within the CeA. This effect was long-lasting (240 min) and was mimicked by CRF plus AVP infusion without stress. Thus, prior stress and AVP functionally alter CRF-mediated neurotransmission and sensitize CRF-induced inhibition of 5-HT release, suggesting that this is a potential mechanism underlying stress-induced affective reactivity in humans.

Evolution of stress responses refine mechanisms of social rank.

Social rank functions to facilitate coping responses to socially stressful situations and conditions. The evolution of social status appears to be inseparably connected to the evolution of stress. Stress, aggression, reward, and decision-making neurocircuitries overlap and interact to produce status-linked relationships, which are common among both male and female populations. Behavioral consequences stemming from social status and rank relationships are molded by aggressive interactions, which are inherently stressful. It seems likely that the balance of regulatory elements in pro- and anti-stress neurocircuitries results in rapid but brief stress responses that are advantageous to social dominance. These systems further produce, in coordination with reward and aggression circuitries, rapid adaptive responding during opportunities that arise to acquire food, mates, perch sites, territorial space, shelter and other resources. Rapid acquisition of resources and aggressive postures produces dominant individuals, who temporarily have distinct fitness advantages. For these reasons also, change in social status can occur rapidly. Social subordination results in slower and more chronic neural and endocrine reactions, a suite of unique defensive behaviors, and an increased propensity for anxious and depressive behavior and affect. These two behavioral phenotypes are but distinct ends of a spectrum, however, they may give us insights into the troubling mechanisms underlying the myriad of stress-related disorders to which they appear to be evolutionarily linked.

Neural and endocrine responses to social stress differ during actual and virtual aggressive interactions or physiological sign stimuli.

Neural and endocrine responses provide quantitative measures that can be used for discriminating behavioral output analyses. Experimental design differences often make it difficult to compare results with respect to the mechanisms producing behavioral actions. We hypothesize that comparisons of distinctive behavioral paradigms or modification of social signals can aid in teasing apart the subtle differences in animal responses to social stress. Eyespots are a unique sympathetically activated sign stimulus of the lizard Anolis carolinensis that influence aggression and social dominance. Eyespot formation along with measurements of central and plasma monoamines enable comparison of paired male aggressive interactions with those provoked by a mirror image. The results suggest that experiments employing artificial application of sign stimuli in dyadic interactions amplify behavioral, neural and endocrine responses, and foreshorten behavioral interactions compared to those that develop among pairs naturally. While the use of mirrors to induce aggressive behavior produces simulated interactions that appear normal, some behavioral, neural, and endocrine responses are amplified in these experiments as well. In contrast, mirror image interactions also limit the level of certain behavioral and neuroendocrine responses. As true social communication does not occur during interaction with mirror images, rank relationships can never be established. Multiple experimental approaches, such as combining naturalistic social interactions with virtual exchanges and/or manipulation of sign stimuli, can often provide added depth to understanding the motivation, context, and mechanisms that produce specific behaviors. The addition of endocrine and neural measurements helps identify the contributions of specific behavioral elements to the social processes proceeding.

Social status regulates kisspeptin receptor mRNA in the brain of Astatotilapia burtoni.

The brain controls reproduction in response to relevant external and internal cues. Central to this process in vertebrates is gonadotropin-releasing hormone (GnRH1) produced in neurons of the hypothalamic-preoptic area (POA). GnRH1 released from the POA stimulates pituitary release of gonadotropins, which in males causes sperm production and concomitant steroid hormone release from the testes. Kisspeptin, a neuropeptide acting via the kisspeptin receptor (Kiss1r), increases GnRH1 release and is linked to development of the reproductive system in mammals and other vertebrates. In both fish and mammals, kiss1r mRNA levels increase in the brain around the time of puberty but the environmental and other stimuli regulating kisspeptin signaling to GnRH1 neurons remain unknown. To understand where kiss1r is expressed and how it is regulated in the brain of a cichlid fish, Astatotilapia burtoni, we measured expression of a kiss1r homolog mRNA by in situ hybridization and quantitative reverse transcription-PCR (qRT-PCR). We found kiss1r mRNA localized in the olfactory bulb, specific nuclei in the telencephalon, diencephalon, mesencephalon, and rhombencephalon, as well as in GnRH1 and GnRH3 neurons. Since males' sexual physiology and behavior depend on social status in A. burtoni, we also tested how status influenced kiss1r mRNA levels. We found higher kiss1r mRNA levels in whole brains of high status territorial males and lower levels in low status non-territorial males. Our results are consistent with the hypothesis that Kiss1r regulates many functions in the brain, making it a strong candidate for mediating differences in reproductive physiology between territorial and non-territorial phenotypes.

Social status differentiates rapid neuroendocrine responses to restraint stress.

Male Anolis carolinensis that win aggressive interactions mobilize neuroendocrine responses to social stress more rapidly than defeated lizards. We initially examined temporal patterns of neuroendocrine response to restraint stress in lizards of unknown status, and then investigated whether winning males respond more rapidly to this non-social stressor. Size-matched male pairs interacted to establish social status, and then were returned to individual home cages for 3 days. Plasma and brains were collected from non-restrained dominants and subordinates, and from a non-interacting control group. Additional groups of dominants and subordinates underwent 90 s restraint stress, with plasma and brains collected either immediately or 300 s after restraint. In lizards of unknown social status restraint stimulated rapid monoaminergic responses in nucleus accumbens, hippocampus, amygdala, and locus ceruleus, with delayed responses seen in VTA and raphé. Non-restrained dominants and subordinates had lower levels of raphé serotonergic activity and lower hippocampal dopaminergic activity 3 days after interacting, compared to controls. Dominants had higher corticosterone levels, both immediately and 300 s after restraint, than either non-restrained dominants or restrained subordinates. Restraint induced higher raphé serotonergic activity in dominants. However, subordinates also showed rapid responses to restraint; exhibiting lower hippocampal dopamine (DA) levels than non-restrained subordinates. At 300 s after the stress, amygdalar serotonin levels increased in dominants, while subordinates showed higher amygdalar DA levels. These results suggest that stressful aggressive interactions will not only alter basal neurochemical activity, but also influence neuroendocrine responses to non-social stressors according to individual social status.

Individual, temporal, and population-level variations in circulating 11-ketotestosterone and 17beta-estradiol concentrations in the oyster toadfish Opsanus tau.

Sex steroid hormones are important for reproduction in all vertebrates, but few studies examine inter-individual, temporal, and population-level variations, as well as environmental influences on circulating steroid levels within the same species. In this study we analyzed plasma 11-ketotoestosterone (11-KT) and 17beta-estradiol (E(2)) levels in the oyster toadfish to test for 1) individual and temporal variations by serially sampling the same individuals during the reproductive and post-reproductive period, 2) variations in steroid levels among toadfish obtained from different sources or maintained under different holding conditions, and 3) correlations with environmental parameters. Results from serial sampling showed marked inter-individual variations in male 11-KT levels in two separate groups of toadfish, but no temporal differences from June to September. Females also showed inter-individual variations in E(2) concentrations, but most had elevated levels late in the reproductive season coincident with oocyte growth prior to winter quiescence. E(2) concentration, but not 11-KT, was positively correlated with water temperature, and negatively correlated with daylength and lunar phase. Maricultured toadfish held under constant conditions had elevated levels of E(2) and 11-KT that should be considered when using these fish for experimentation. This study provides important comparative information on the relationship between individual variations in steroid levels, and how they relate to physiological and environmental correlates in a model marine teleost.

Color change as a potential behavioral strategy.

Within species, color morphs may enhance camouflage, improve communication and/or confer reproductive advantage. However, in the male cichlid Astatotilapia burtoni, body color may also signal a behavioral strategy. A. burtoni live in a lek-like social system in Lake Tanganyika, Africa where bright blue or yellow territorial (T) males (together ~10-30% of the population) are reproductively capable and defend territories containing food with a spawning site. In contrast, non-territorial (NT) males are smaller, cryptically colored, shoal with females and have regressed gonads. Importantly, males switch between these social states depending on their success in aggressive encounters. Yellow and blue morphs were thought to be adaptations to particular habitats, but they co-exist both in nature and in the laboratory. Importantly, individual males can switch colors so we asked whether color influences behavioral and hormonal profiles. When pairing territorial males with opposite colored fish, yellow males became dominant over blue males significantly more frequently. Moreover, yellow T males had significantly higher levels of 11-ketotosterone than blue T males while only blue NT males had higher levels of the stress hormone cortisol compared to the other groups. Thus color differences alone predict dominance status and hormone profiles in T males. Since T males can and do change color, this suggests that A. burtoni may use color as a flexible behavioral strategy.

Behavioral plasticity in rainbow trout (Oncorhynchus mykiss) with divergent coping styles: when doves become hawks.

Consistent and heritable individual differences in reaction to challenges, often referred to as stress coping styles, have been extensively documented in vertebrates. In fish, selection for divergent post-stress plasma cortisol levels in rainbow trout (Oncorhynchus mykiss) has yielded a low (LR) and a high responsive (HR) strain. A suite of behavioural traits is associated with this physiological difference, with LR (proactive) fish feeding more rapidly after transfer to a new environment and being socially dominant over HR (reactive) fish. Following transport from the UK to Norway, a switch in behavioural profile occurred in trout from the 3rd generation; HR fish regained feeding sooner than LR fish in a novel environment and became dominant in size-matched HR-LR pairs. One year after transport, HR fish still fed sooner, but no difference in social dominance was found. Among offspring of transported fish, no differences in feeding were observed, but as in pre-transported 3rd generation fish, HR fish lost fights for social dominance against size-matched LR opponents. Transported fish and their offspring retained their distinctive physiological profile throughout the study; HR fish showed consistently higher post-stress cortisol levels at all sampling points. Altered risk-taking and social dominance immediately after transport may be explained by the fact that HR fish lost more body mass during transport than did LR fish. These data demonstrate that some behavioural components of stress coping styles can be modified by experience, whereas behavioural plasticity is limited by genetic effects determining social position early in life story.

Memory of opponents is more potent than visual sign stimuli after social hierarchy has been established.

During agonistic interactions between male Anolis carolinensis, perception of a visual sign stimulus (darkened eyespots) not only inhibits aggression and promotes initial attainment of dominant social status, but also evokes distinct neuroendocrine responses in each opponent. This study was designed to examine the effect of eyespot manipulation on behavior and social rank during a second interaction between opponents that had previously established a natural dyadic social hierarchy. Prior to a second interaction, eyespots of familiar size-matched combatants were manipulated to reverse information conveyed by this visual signal. Eyespots on the previously dominant male were masked with green paint to indicate low aggression and social status. Previously subordinate males had their eyespots permanently marked with black paint to convey high aggression and status. Opponents were then re-paired for a second 10 min interaction following either 1 or 3 days of separation. Aggression was generally decreased and social status between pairs remained reasonably consistent. Unlike rapidly activated monoaminergic activity that occurs following the initial pairing, most brain areas sampled were not affected when animals were re-introduced, regardless of visual signal reversal or length of separation between interactions. However in males with "normal" eyespot color, dominant males had reduced serotonergic activity in CA(3) and raphé, while subordinate males exhibited elevated CA(3) dopaminergic activity. Reversing eyespot color also reversed serotonergic activity in raphé and dopaminergic activity in CA(3) after 3 days of separation. The results suggest that males remember previous opponents, and respond appropriately to their previous social rank in spite of eyespot color.

Rapid neuroendocrine responses evoked at the onset of social challenge.

At the onset of agonistic social challenge, individuals must assess the degree of threat the opponent represents in order to react appropriately. We aimed to characterize the neuroendocrine changes accompanying this period of initial social assessment using the lizard Anolis carolinensis. Conveyance of aggressive intent by male A. carolinensis is facilitated by rapid postorbital skin darkening (eyespot), whereas eyespot presence inhibits opponent aggression. By manipulating this visual signal, we also investigated whether differing neuroendocrine changes were evoked by initial presentation of varying levels of social threat. Subjects were painted postorbitally either with black paint (high threat level), green paint (low threat level) or water (controls). Painted animals were presented with a mirror and sampled immediately upon exhibiting aggressive intent towards the reflected simulated opponent, but before producing behaviors such as motor pattern-based displays. Control animals (blank surface presented) were sampled at times derived from averaging response times of painted subjects. Brains and plasma were analyzed for monoamine activity and catecholamine levels using electrochemical HPLC. Social threat evoked increases in plasma catecholamine levels indistinguishable from those caused by brief environmental disturbance. However, brief social challenge caused distinct rapid increases in amygdala and nucleus accumbens (NAc) dopamine and serotonin levels. Amygdalar changes were associated with general social threat presence, but NAc monoamines were affected by both threat level and subject motivation to engage in confrontation. This suggests that specific rapid activity changes in key forebrain limbic nuclei differ according to the degree of social threat perceived at the start of the interaction.

Behavioral diversity and neurochemical plasticity: selection of stress coping strategies that define social status.

Social interactions include a variety of stimulating but challenging factors that are the basis for strategies that allow individuals to cope with novel or familiar stressful situations. Evolutionarily conserved strategies have been identified that reflect specific behavioral and physiological identities. In this review we discuss a unique model for social stress in the lizard Anolis carolinensis, which has characteristics amenable to an investigation of individual differences in behavioral responses via central and sympathetic neurochemical adaptation. Profiles of proactive and reactive phenotypes of male A. carolinensis are relatively stable, yet retain limited flexibility that allows for the development of the social system over time. For male A. carolinensis, the celerity of social signal expression and response translate into future social standing. In addition, proactive aggressive, courtship, and feeding behaviors also predict social rank, but are not as important as prior interactions and memories of previous opponents to modify behavioral output and affect social status. The central neurotransmitters dopamine and serotonin, and the endocrine stress axis (HPA) appear to be the fundamental link to adaptive stress coping strategies during social interactions. Only small adaptations to these neural and endocrine systems are necessary to produce the variability measured in behavioral responses to stressful social interactions. These neuroendocrine factors are also manifest in responses to other stimuli and form the basis of heritable strategies for coping with stress.

Optogenetic Activation of Accessory Olfactory Bulb Input to the Forebrain Differentially Modulates Investigation of Opposite versus Same-Sex Urinary Chemosignals and Stimulates Mating in Male Mice.

Surgical or genetic disruption of vomeronasal organ (VNO)-accessory olfactory bulb (AOB) function previously eliminated the ability of male mice to processes pheromones that elicit territorial behavior and aggression. By contrast, neither disruption significantly affected mating behaviors, although VNO lesions reduced males' investigation of nonvolatile female pheromones. We explored the contribution of VNO-AOB pheromonal processing to male courtship using optogenetic activation of AOB projections to the forebrain. Protocadherin-Cre male transgenic mice received bilateral AOB infections with channelrhodopsin2 (ChR2) viral vectors, and an optical fiber was implanted above the AOB. In olfactory choice tests, males preferred estrous female urine (EFU) over water; however, this preference was eliminated when diluted (5%) EFU was substituted for 100% EFU. Optogenetic AOB activation concurrent with nasal contact significantly augmented males' investigation compared to 5% EFU alone. Conversely, concurrent optogenetic AOB activation significantly reduced males' nasal investigation of diluted urine from gonadally intact males (5% IMU) compared to 5% IMU alone. These divergent effects of AOB optogenetic activation were lost when males were prevented from making direct nasal contact. Optogenetic AOB stimulation also failed to augment males' nasal investigation of deionized water or of food odors. Finally, during mating tests, optogenetic AOB stimulation delivered for 30 s when the male was in physical contact with an estrous female significantly facilitated the occurrence of penile intromission. Our results suggest that VNO-AOB signaling differentially modifies males' motivation to seek out female vs male urinary pheromones while augmenting males' sexual arousal leading to intromission and improved reproductive performance.

Dopaminergic activity modulation via aggression, status, and a visual social signal.

Social interaction may elicit aggression, establish social rank, and be influenced by changes in central dopaminergic activity. In the lizard Anolis carolinensis, a sign stimulus (darkening of postorbital skin or eyespots) inhibits aggressive response from opponents, in part because it forms more rapidly in dominant males. The authors report that artificially hiding or darkening eyespots influences central dopaminergic activity, social status, and aggression during dyadic social interaction. All males that viewed an opponent with eyespots painted black became subordinate and exhibited elevated dopamine in raphe, lateral amygdala, and medial amygdala but decreased dopamine in septum and locus ceruleus. In contrast, males that viewed opponents with hidden eyespots (painted green) became dominant and had increased dopamine in striatum, nucleus accumbens, hypothalamus, and combined substantia nigra/ventral tegmental area.

Effects of L-DOPA on aggressive behavior and central monoaminergic activity in the lizard Anolis carolinensis, using a new method for drug delivery.

The dopamine (DA) precursor, L-DOPA (500 microg), was injected into living crickets, which were ingested (one each) by adult male Anolis carolinensis. This method of delivery elevated plasma L-DOPA and DA concentrations by approximately 1000-fold. In contrast, plasma epinephrine (Epi) and norepinephrine (NE) were not influenced by L-DOPA treatment, although they were elevated following the consumption of the cricket. Lizards that ingested L-DOPA treated crickets had elevated L-DOPA in all brain regions measured, with DA and/or DOPAC also increased significantly in most brain regions studied. Despite increased DA levels in the striatum and nucleus accumbens as a response to L-DOPA, the treatment had no influence on general motor activity. Central serotonin, NE, and Epi systems were not affected in any brain region by oral L-DOPA treatment. In addition, aggression was inhibited by this dose of L-DOPA, even though there was no effect on serotonergic systems. This is surprising because controlling aggressive behavior is usually considered the province of serotonergic activity. Aggression was measured before and after treatment, and while saline-treated lizards retained the full vigor of aggressive activity, those fed a cricket injected with L-DOPA were only one-third as aggressive after treatment. As L-DOPA treatment did not affect general motor activity, the effect appears to be directly associated with aggression. This is supported by the observation that L-DOPA treatment delayed latency to eyespot darkening, which predicts the latency to aggression.

Glucocorticoid interaction with aggression in non-mammalian vertebrates: reciprocal action.

Socially aggressive interaction is stressful, and as such, glucocorticoids are typically secreted during aggressive interaction in a variety of vertebrates, which may both potentiate and inhibit aggression. The behavioral relationship between corticosterone and/or cortisol in non-mammalian (as well as mammalian) vertebrates is dependent on timing, magnitude, context, and coordination of physiological and behavioral responses. Chronically elevated plasma glucocorticoids reliably inhibit aggressive behavior, consistent with an evolutionarily adaptive behavioral strategy among subordinate and submissive individuals. Acute elevation of plasma glucocorticoids may either promote an actively aggressive response via action in specialized local regions of the brain such as the anterior hypothalamus, or is permissive to escalated aggression and/or activity. Although the permissive effect of glucocorticoids on aggression does not suggest an active role for the hormone, the corticosteroids may be necessary for full expression of aggressive behavior, as in the lizard Anolis carolinensis. These effects suggest that short-term stress may generally be best counteracted by an actively aggressive response, at least for socially dominant proactive individuals. An acute and active response may be evolutionarily maladaptive under chronic, uncontrollable and unpredictable circumstances. It appears that subordinate reactive individuals often produce compulsorily chronic responses that inhibit aggression and promote submissive behavior.

Does serotonin influence aggression? comparing regional activity before and during social interaction.

Serotonin is widely believed to exert inhibitory control over aggressive behavior and intent. In addition, a number of studies of fish, reptiles, and mammals, including the lizard Anolis carolinensis, have demonstrated that serotonergic activity is stimulated by aggressive social interaction in both dominant and subordinate males. As serotonergic activity does not appear to inhibit agonistic behavior during combative social interaction, we investigated the possibility that the negative correlation between serotonergic activity and aggression exists before aggressive behavior begins. To do this, putatively dominant and more aggressive males were determined by their speed overcoming stress (latency to feeding after capture) and their celerity to court females. Serotonergic activities before aggression are differentiated by social rank in a region-specific manner. Among aggressive males baseline serotonergic activity is lower in the septum, nucleus accumbens, striatum, medial amygdala, anterior hypothalamus, raphe, and locus ceruleus but not in the hippocampus, lateral amygdala, preoptic area, substantia nigra, or ventral tegmental area. However, in regions such as the nucleus accumbens, where low serotonergic activity may help promote aggression, agonistic behavior also stimulates the greatest rise in serotonergic activity among the most aggressive males, most likely as a result of the stress associated with social interaction.

Either main or accessory olfactory system signaling can mediate the rewarding effects of estrous female chemosignals in sexually naive male mice.

A long-held view has been that interest of male mice in female body odors reflects an activation of reward circuits in the male brain following their detection by the vomeronasal organ (VNO) and processing via the accessory olfactory system. We found that adult, sexually naive male mice acquired a conditioned place preference (CPP) after repeatedly receiving estrous female urine on the nose and being placed in an initially nonpreferred chamber with soiled estrous bedding on the floor. CPP was not acquired in control mice that received saline on the nose before being placed in a nonpreferred chamber with clean bedding. Robust acquisition of a CPP using estrous female odors as the reward persisted in separate groups of mice in which VNO-accessory olfactory function was disrupted by bilateral lesioning of the accessory olfactory bulb (AOB) or in which main olfactory function was disrupted by zinc sulfate lesions of the main olfactory epithelium (MOE). By contrast, no CPP was acquired for estrous odors in males that received combined AOB and MOE lesions. Either the main or the accessory olfactory system suffices to mediate the rewarding effects of estrous female odors in the male mouse, even in the absence of prior mating experience. The main olfactory system is part of the circuitry that responds to chemosignals involved in motivated behavior, a role that may be particularly important for humans who lack a functional accessory olfactory system.