Chronic administration of glucocorticoid receptor ligands increases anxiety-like behavior and selectively increase serotonin transporters in the ventral hippocampus

Organic cation transporter-3 (OCT3) is widely distributed in the brain with high expression in portions of the stress axis. These high capacity, polyspecific transporters function in monoamine clearance and are sensitive to the stress hormone corticosterone. In rats, withdrawal from chronic amphetamine increases OCT3 expression in specific limbic brain regions involved anxiety and stress responses, including the ventral hippocampus, central nucleus of amygdala (CeA) and dorsomedial hypothalamus. (DMH). Previous studies show that glucocorticoid receptor (GR) agonists increase OCT1 mRNA and OCT2 mRNA expression in non-neural tissues. Thus, we hypothesized that corticosterone increases OCT3 expression in the brain by activating GRs. Male Sprague-Dawley rats were pre-treated daily with the GR antagonist mifepristone (20 mg/kg; sc.) or vehicle followed 45 min later by injections of corticosterone or vehicle for 2 weeks. Corticosterone treatment significantly increased OCT3 expression in the ventral hippocampus and increased anxiety-like behavior. However, these effects were not blocked by mifepristone. Interestingly, treatment with mifepristone alone reduced plasma corticosterone levels and increased serotonin transporter and GR expression in the ventral hippocampus but did not significantly affect OCT3 expression or behavior. No treatment effects on OCT3, serotonin transporter or GR expression were observed in the DMH, CeA or dorsal hippocampus. Our findings suggest that corticosterone increases OCT3 expression in the ventral hippocampus by a mechanism independent of GRs, and that mifepristone and corticosterone can act in an independent manner to affect HPA axis-related physiological and behavioral parameters.

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.

Pleiotropic function of the oca2 gene underlies the evolution of sleep loss and albinism in cavefish

Adaptation to novel environments often involves the evolution of multiple morphological, physiological, and behavioral traits. One striking example of multi-trait evolution is the suite of traits that has evolved repeatedly in cave animals, including regression of eyes, loss of pigmentation, and enhancement of non-visual sensory systems.^1,2 The Mexican tetra, Astyanax mexicanus, consists of fish that inhabit at least 30 caves in Mexico and ancestral-like surface fish that inhabit the rivers of Mexico and southern Texas.³ Cave A. mexicanus are interfertile with surface fish and have evolved a number of traits, including reduced pigmentation, eye loss, and alterations to behavior.^4-6 To define relationships between different cave-evolved traits, we phenotyped 208 surface-cave F2 hybrid fish for numerous morphological and behavioral traits. We found differences in sleep between pigmented and albino hybrid fish, raising the possibility that these traits share a genetic basis. In cavefish and other species, mutations in oculocutaneous albinism 2 (oca2) cause albinism.^7-12 Surface fish with mutations in oca2 displayed both albinism and reduced sleep. Further, this mutation in oca2 fails to complement sleep loss when surface fish harboring this engineered mutation are crossed to independently evolved populations of albino cavefish with naturally occurring mutations in oca2. Analysis of the oca2 locus in wild-caught cave and surface fish suggests that oca2 is under positive selection in 3 cave populations. Taken together, these findings identify oca2 as a novel regulator of sleep and suggest that a pleiotropic function of oca2 underlies the adaptive evolution of albinism and sleep loss.

Phenotypic plasticity as a mechanism of cave colonization and adaptation

A widely accepted model for the evolution of cave animals posits colonization by surface ancestors followed by the acquisition of adaptations over many generations. However, the speed of cave adaptation in some species suggests mechanisms operating over shorter timescales. To address these mechanisms, we used Astyanax mexicanus, a teleost with ancestral surface morphs (surface fish, SF) and derived cave morphs (cavefish, CF). We exposed SF to completely dark conditions and identified numerous altered traits at both the gene expression and phenotypic levels. Remarkably, most of these alterations mimicked CF phenotypes. Our results indicate that many cave-related traits can appear within a single generation by phenotypic plasticity. In the next generation, plasticity can be further refined. The initial plastic responses are random in adaptive outcome but may determine the subsequent course of evolution. Our study suggests that phenotypic plasticity contributes to the rapid evolution of cave-related traits in A. mexicanus.

The Mexican tetra is a fish that has two forms: a surface-dwelling form, which has eyes and silvery grey appearance, and a cave-dwelling form, which is blind and has lost its pigmentation. Recent studies have shown that the cave-dwelling form evolved rapidly within the last 200,000 years from an ancestor that lived at the surface. The recent evolution of the cave-dwelling form of the tetra poses an interesting evolutionary question: how did the surface-dwelling ancestor of the tetra quickly adapt to the new and challenging environment found in the caves?

‘Phenotypic plasticity’ is a phenomenon through which a single set of genes can produce different observable traits depending on the environment. An example of phenotypic plasticity occurs in response to diet: in animals, poor diets can lead to an increase in the size of the digestive organs and to the animals eating more. To see if surface-dwelling tetras can quickly adapt to cave environments through phenotypic plasticity, Bilandžija et al. have exposed these fish to complete darkness (the major feature of the cave environment) for two years. After spending up to two years in the dark, these fish were compared to normal surface-dwelling and cave-dwelling tetras.

Results revealed that surface-dwelling tetras raised in the dark exhibited traits associated with cave-dwelling tetras. These traits included changes in the activity of many genes involved in diverse processes, resistance to starvation, metabolism, and levels of hormones and molecules involved in neural signaling, which could lead to changes in behavior. However, the fish also exhibited traits, including an increase in the cells responsible for pigmentation, that would have no obvious benefit in the darkness. Even though the changes observed require no genetic mutations, they can help or hinder the fish’s survival once they occur, possibly determining subsequent evolution. Thus, a trait beneficial for surviving in the dark that appears simply through phenotypic plasticity may eventually be selected for and genetic mutations that encode it more reliably may appear too.

These results shed light on how species may quickly adapt to new environments without accumulating genetic mutations, which can take hundreds of thousands of years. They also may help to explain how colonizer species succeed in challenging environments. The principles described by Bilandžija et al. can be applied to different organisms adapting to new environments, and may help understand the role of phenotypic plasticity in evolution.

The stalk-eyed fly as a model for aggression - is there a conserved role for 5-HT between vertebrates and invertebrates?

Serotonin (5-HT) has largely been accepted to be inhibitory to vertebrate aggression, whereas an opposing stimulatory role has been proposed for invertebrates. Herein, we argue that critical gaps in our understanding of the nuanced role of 5-HT in invertebrate systems drove this conclusion prematurely, and that emerging data suggest a previously unrecognized level of phylogenetic conservation with respect to neurochemical mechanisms regulating the expression of aggressive behaviors. This is especially apparent when considering the interplay among factors governing 5-HT activity, many of which share functional homology across taxa. We discuss recent findings using insect models, with an emphasis on the stalk-eyed fly, to demonstrate how particular 5-HT receptor subtypes mediate the intensity of aggression with respect to discrete stages of the interaction (initiation, escalation and termination), which mirrors the complex behavioral regulation currently recognized in vertebrates. Further similarities emerge when considering the contribution of neuropeptides, which interact with 5-HT to ultimately determine contest progression and outcome. Relative to knowledge in vertebrates, much less is known about the function of 5-HT receptors and neuropeptides in invertebrate aggression, particularly with respect to sex, species and context, prompting the need for further studies. Our Commentary highlights the need to consider multiple factors when determining potential taxonomic differences, and raises the possibility of more similarities than differences between vertebrates and invertebrates with regard to the modulatory effect of 5-HT on aggression.

Behavioural changes controlled by catecholaminergic systems explain recurrent loss of pigmentation in cavefish

Multiple cave populations of the teleost Astyanax mexicanus have repeatedly reduced or lost eye and body pigmentation during adaptation to dark caves. Albinism, the complete absence of melanin pigmentation, is controlled by loss-of-function mutations in the oca2 gene. The mutation is accompanied by an increase in the melanin synthesis precursor l-tyrosine, which is also a precursor for catecholamine synthesis. In this study, we show a relationship between pigmentation loss, enhanced catecholamine synthesis and responsiveness to anaesthesia, determined as a proxy for catecholamine-related behaviours. We demonstrate that anaesthesia resistance (AR) is enhanced in multiple depigmented and albino cavefish (CF), inversely proportional to the degree of pigmentation loss, controlled by the oca2 gene, and can be modulated by experimental manipulations of l-tyrosine or the catecholamine norepinephrine (NE). Moreover, NE is increased in the brains of multiple albino and depigmented CF relative to surface fish. The results provide new insights into the evolution of pigment modification because NE controls a suite of adaptive behaviours similar to AR that may represent a target of natural selection. Thus, understanding the relationship between loss of pigmentation and AR may provide insight into the role of natural selection in the evolution of albinism via a melanin-catecholamine trade-off.

Sex differences in aggression: Differential roles of 5-HT2, neuropeptide F and tachykinin

Despite the conserved function of aggression across taxa in obtaining critical resources such as food and mates, serotonin's (5-HT) modulatory role on aggressive behavior appears to be largely inhibitory for vertebrates but stimulatory for invertebrates. However, critical gaps exist in our knowledge of invertebrates that need to be addressed before definitively stating opposing roles for 5-HT and aggression. Specifically, the role of 5-HT receptor subtypes are largely unknown, as is the potential interactive role of 5-HT with other neurochemical systems known to play a critical role in aggression. Similarly, the influence of these systems in driving sex differences in aggressive behavior of invertebrates is not well understood. Here, we investigated these questions by employing complementary approaches in a novel invertebrate model of aggression, the stalk-eyed fly. A combination of altered social conditions, pharmacological manipulation and 5-HT2 receptor knockdown by siRNA revealed an inhibitory role of this receptor subtype on aggression. Additionally, we provide evidence for 5-HT2's involvement in regulating neuropeptide F activity, a suspected inhibitor of aggression. However, this function appears to be stage-specific, altering only the initiation stage of aggressive conflicts. Alternatively, pharmacologically increasing systemic concentrations of 5-HT significantly elevated the expression of the neuropeptide tachykinin, which did not affect contest initiation but instead promoted escalation via production of high intensity aggressive behaviors. Notably, these effects were limited solely to males, with female aggression and neuropeptide expression remaining unaltered by any manipulation that affected 5-HT. Together, these results demonstrate a more nuanced role for 5-HT in modulating aggression in invertebrates, revealing an important interactive role with neuropeptides that is more reminiscent of vertebrates. The sex-differences described here also provide valuable insight into the evolutionary contexts of this complex behavior.

Enhanced dopamine D2 autoreceptor function in the adult prefrontal cortex contributes to dopamine hypoactivity following adolescent social stress

Adult psychiatric disorders characterized by cognitive deficits reliant on prefrontal cortex (PFC) dopamine are promoted by teenage bullying. Similarly, male Sprague-Dawley rats exposed to social defeat in mid-adolescence (P35-39) show impaired working memory in adulthood (P56-70), along with decreased medial PFC (mPFC) dopamine activity that results in part from increased dopamine transporter-mediated clearance. Here, we determined if dopamine synthesis and D2 autoreceptor-mediated inhibition of dopamine release in the adult mPFC are also enhanced by adolescent defeat to contribute to later dopamine hypofunction. Control and previously defeated rats did not differ in either DOPA accumulation following amino acid decarboxylase inhibition (NSD-1015 100 mg/kg ip.) or total/phosphorylated tyrosine hydroxylase protein expression, suggesting dopamine synthesis in the adult mPFC is not altered by adolescent defeat. However, exposure to adolescent defeat caused greater decreases in extracellular dopamine release (measured using in vivo chronoamperometry) in the adult mPFC upon local infusion of the D2 receptor agonist quinpirole (3 nM), implying greater D2 autoreceptor function. Equally enhanced D2 autoreceptor-mediated inhibition of dopamine release is seen in the adolescent (P40 or P49) mPFC, which declines in control rats by adulthood. However, this developmental decrease in autoreceptor function is absent following adolescent defeat, suggesting retention of an adolescent-like phenotype into adulthood. Current and previous findings indicate adolescent defeat decreases extracellular dopamine availability in the adult mPFC via both enhanced inhibition of dopamine release and increased dopamine clearance, which may be viable targets for improving treatment of cognitive deficits seen in neuropsychiatric disorders promoted by adolescent stress.

Orphan receptor GPR158 controls stress-induced depression

Stress can be a motivational force for decisive action and adapting to novel environment; whereas, exposure to chronic stress contributes to the development of depression and anxiety. However, the molecular mechanisms underlying stress-responsive behaviors are not fully understood. Here, we identified the orphan receptor GPR158 as a novel regulator operating in the prefrontal cortex (PFC) that links chronic stress to depression. GPR158 is highly upregulated in the PFC of human subjects with major depressive disorder. Exposure of mice to chronic stress also increased GPR158 protein levels in the PFC in a glucocorticoid-dependent manner. Viral overexpression of GPR158 in the PFC induced depressive-like behaviors. In contrast GPR158 ablation, led to a prominent antidepressant-like phenotype and stress resiliency. We found that GPR158 exerts its effects via modulating synaptic strength altering AMPA receptor activity. Taken together, our findings identify a new player in mood regulation and introduce a pharmacological target for managing depression.

Activation of 5-HT1A receptors in the rat dorsomedial hypothalamus inhibits stress-induced activation of the hypothalamic-pituitary-adrenal axis

Acute activation of the hypothalamic-pituitary-adrenal (HPA) axis, leading to the release of corticosteroid hormones into the circulation, is an adaptive response to perceived threats. Persistent activation of the HPA axis can lead to impaired physiological or behavioral function with maladaptive consequences. Thus, efficient control and termination of stress responses is essential for well-being. However, inhibitory control mechanisms governing the HPA axis are poorly understood. Previous studies suggest that serotonergic systems, acting within the medial hypothalamus, play an important role in inhibitory control of stress-induced HPA axis activity. To test this hypothesis, we surgically implanted chronic jugular cannulae in adult male rats and conducted bilateral microinjection of vehicle or the 5-HT_1A receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT; 8 nmol, 0.2 μL, 0.1 μL/min, per side) into the dorsomedial hypothalamus (DMH) immediately prior to a 40 min period of restraint stress. Repeated blood sampling was conducted using an automated blood sampling system and plasma corticosterone concentrations were determined using enzyme-linked immunosorbent assay. Bilateral intra-DMH microinjections of 8-OH-DPAT suppressed stress-induced increases in plasma corticosterone within 10 min of the onset of handling prior to restraint and, as measured by area-under-the-curve analysis of plasma corticosterone concentrations, during the 40 min period of restraint. These data support an inhibitory role for serotonergic systems, acting within the DMH, on stress-induced activation of the HPA axis. Lay summary: Inhibitory control of the hypothalamic-pituitary-adrenal (HPA) stress hormone response is important for well-being. One neurochemical implicated in inhibitory control of the HPA axis is serotonin. In this study we show that activation of serotonin receptors, specifically inhibitory 5-HT_1A receptors in the dorsomedial hypothalamus, is sufficient to inhibit stress-induced HPA axis activity in rats.

Neuromodulation of Nestmate Recognition Decisions by Pavement Ants

Ant colonies are distributed systems that are regulated in a non-hierarchical manner. Without a central authority, individuals inform their decisions by comparing information in local cues to a set of inherent behavioral rules. Individual behavioral decisions collectively change colony behavior and lead to self-organization capable of solving complex problems such as the decision to engage in aggressive societal conflicts with neighbors. Despite the relevance to colony fitness, the mechanisms that drive individual decisions leading to cooperative behavior are not well understood. Here we show how sensory information, both tactile and chemical, and social context—isolation, nestmate interaction, or fighting non-nestmates—affects brain monoamine levels in pavement ants (Tetramorium caespitum). Our results provide evidence that changes in octopamine and serotonin in the brains of individuals are sufficient to alter the decision by pavement ants to be aggressive towards non-nestmate ants whereas increased brain levels of dopamine correlate to physical fighting. We propose a model in which the changes in brain states of many workers collectively lead to the self-organization of societal aggression between neighboring colonies of pavement ants.

Amphetamine Withdrawal Differentially Increases the Expression of Organic Cation Transporter 3 and Serotonin Transporter in Limbic Brain Regions

Amphetamine withdrawal increases anxiety and stress sensitivity related to blunted ventral hippocampus (vHipp) and enhances the central nucleus of the amygdala (CeA) serotonin responses. Extracellular serotonin levels are regulated by the serotonin transporter (SERT) and organic cation transporter 3 (OCT3), and vHipp OCT3 expression is enhanced during 24 hours of amphetamine withdrawal, while SERT expression is unaltered. Here, we tested whether OCT3 and SERT expression in the CeA is also affected during acute withdrawal to explain opposing regional alterations in limbic serotonergic neurotransmission and if respective changes continued with two weeks of withdrawal. We also determined whether changes in transporter expression were confined to these regions. Male rats received amphetamine or saline for two weeks followed by 24 hours or two weeks of withdrawal, with transporter expression measured using Western immunoblot. OCT3 and SERT expression increased in the CeA at both withdrawal timepoints. In the vHipp, OCT3 expression increased only at 24 hours of withdrawal, with an equivalent pattern seen in the dorsomedial hypothalamus. No changes were evident in any other regions sampled. These regionally specific changes in limbic OCT3 and SERT expression may partially contribute to the serotonergic imbalance and negative affect during amphetamine withdrawal.

Amphetamine withdrawal differentially affects hippocampal and peripheral corticosterone levels in response to stress

Amphetamine withdrawal is associated with heightened anxiety-like behavior, which is directly driven by blunted stress-induced glucocorticoid receptor-dependent serotonin release in the ventral hippocampus. This suggests that glucocorticoid availability in the ventral hippocampus during stress may be reduced during amphetamine withdrawal. Therefore, we tested whether amphetamine withdrawal alters either peripheral or hippocampal corticosterone stress responses. Adult male rats received amphetamine (2.5mg/kg, ip) or saline for 14 days followed by 2 weeks of withdrawal. Contrary to our prediction, microdialysis samples from freely-moving rats revealed that restraint stress-induced corticosterone levels in the ventral hippocampus are enhanced by amphetamine withdrawal relative to controls. In separate groups of rats, plasma corticosterone levels increased immediately after 20min of restraint and decreased to below stress-naïve levels after 1h, indicating negative feedback regulation of corticosterone following stress. However, plasma corticosterone responses were similar in amphetamine-withdrawn and control rats. Neither amphetamine nor stress exposure significantly altered protein expression or enzyme activity of the steroidogenic enzymes 11β-hydroxysteroid dehydrogenase (11β-HSD1) or hexose-6-phosphate dehydrogenase (H6PD) in the ventral hippocampus. Our findings demonstrate for the first time that amphetamine withdrawal potentiates stress-induced corticosterone in the ventral hippocampus, which may contribute to increased behavioral stress sensitivity previously observed during amphetamine withdrawal. However, this is not mediated by either changes in plasma corticosterone or hippocampal steroidogenic enzymes. Establishing enhanced ventral hippocampal corticosterone as a direct cause of greater stress sensitivity may identify the glucocorticoid system as a novel target for treating behavioral symptoms of amphetamine withdrawal.

Mild Traumatic Brain Injury with Social Defeat Stress Alters Anxiety, Contextual Fear Extinction, and Limbic Monoamines in Adult Rats

Mild traumatic brain injury (mTBI) produces symptoms similar to those typifying posttraumatic stress disorder (PTSD) in humans. We sought to determine whether a rodent model of stress concurrent with mTBI produces characteristics of PTSD such as impaired contextual fear extinction, while also examining concurrent alterations to limbic monoamine activity in brain regions relevant to fear and anxiety states. Male rats were exposed to social stress or control conditions immediately prior to mTBI induction, and 6 days later were tested either for anxiety-like behavior using the elevated plus maze (EPM), or for contextual fear conditioning and extinction. Brains were collected 24 h after EPM testing, and tissue from various limbic regions analyzed for content of monoamines, their precursors and metabolites using HPLC with electrochemical detection. Either social defeat or mTBI alone decreased time spent in open arms of the EPM, indicating greater anxiety-like behavior. However, this effect was enhanced by the combination of treatments. Further, rats exposed to both social defeat and mTBI exhibited greater freezing within extinction sessions compared to all other groups, suggesting impaired contextual fear extinction. Social defeat combined with mTBI also had greater effects on limbic monoamines than either insult alone, particularly with respect to serotonergic effects associated with anxiety and fear learning. The results suggest social stress concurrent with mTBI produces provides a relevant animal model for studying the prevention and treatment of post-concussive psychobiological outcomes.

The organization of societal conflicts by pavement ants Tetramorium caespitum: an agent-based model of amine-mediated decision making

Ant colonies self-organize to solve complex problems despite the simplicity of an individual ant’s brain. Pavement ant Tetramorium caespitum colonies must solve the problem of defending the territory that they patrol in search of energetically rich forage. When members of 2 colonies randomly interact at the territory boundary a decision to fight occurs when: 1) there is a mismatch in nestmate recognition cues and 2) each ant has a recent history of high interaction rates with nestmate ants. Instead of fighting, some ants will decide to recruit more workers from the nest to the fighting location, and in this way a positive feedback mediates the development of colony wide wars. In ants, the monoamines serotonin (5-HT) and octopamine (OA) modulate many behaviors associated with colony organization and in particular behaviors associated with nestmate recognition and aggression. In this article, we develop and explore an agent-based model that conceptualizes how individual changes in brain concentrations of 5-HT and OA, paired with a simple threshold-based decision rule, can lead to the development of colony wide warfare. Model simulations do lead to the development of warfare with 91% of ants fighting at the end of 1 h. When conducting a sensitivity analysis, we determined that uncertainty in monoamine concentration signal decay influences the behavior of the model more than uncertainty in the decision-making rule or density. We conclude that pavement ant behavior is consistent with the detection of interaction rate through a single timed interval rather than integration of multiple interactions.

Biparental behavior in the burying beetle Nicrophorus orbicollis: a role for dopamine?

Burying beetles Nicrophorus orbicollis exhibit facultative biparental care of young. To reproduce, a male–female burying beetle pair bury and prepare a small vertebrate carcass as food for its altricial young. During a breeding bout, male and female behavior changes synchronously at appropriate times and is coordinated to provide effective care for offspring. Although the ecological and evolutionary factors that shape this remarkable reproductive plasticity are well characterized, the neuromodulation of parental behavior is poorly understood. Juvenile hormone levels rise dramatically at the time beetle parents accept and feed larvae, remain highly elevated during the stages of most active care and fall abruptly when care is terminated. However, hormonal fluctuations alone cannot account for this elaborate control of reproduction. The biogenic amines octopamine (OA), dopamine (DA), and serotonin (5-HT) mediate a diversity of insect reproductive and social behaviors. In this study, we measured whole brain monoamine levels in individual male and female burying beetles and compared OA, DA, and 5-HT profiles between breeding (parental) and nonbreeding, unmated beetles. Remarkably, after 24 h of care, when parental feeding rates begin to peak, DA brain levels increase in breeding beetles when compared to nonbreeding controls. In contrast, brain OA and 5-HT levels did not change significantly. These results provide the first evidence for a potential role of DA in the modulation of burying beetle parental behavior.

Assessment strategies and fighting patterns in animal contests: a role for serotonin?

Accurate assessment of the probability of success in an aggressive confrontation with a conspecific is critical to the survival and fitness of the individuals. Various game theory models have examined these assessment strategies under the assumption that contests should favor the animal with the greater resource-holding potential (RHP), body size typically being the proxy. Mutual assessment asserts that an individual can assess their own RHP relative to their opponent, allowing the inferior animal the chance to flee before incurring unnecessary costs. The model of self-determined persistence, however, assumes that an individual will fight to a set personal threshold, independent of their opponent’s RHP. Both models have been repeatedly tested using size as a proxy for RHP, with neither receiving unambiguous support. Here we present both morphological and neurophysiological data from size-matched and mismatched stalk-eyed fly fights. We discovered differing fighting strategies between winners and losers. Winners readily escalated encounters to higher intensity and physical contact and engaged in less low-intensity, posturing behaviors compared with losers. Although these fighting strategies were largely independent of size, they were associated with elevated levels of 5-HT. Understanding the neurophysiological factors responsible for mediating the motivational state of opponents could help resolve the inconsistencies seen in current game theory models. Therefore, we contend that current studies using only size as a proxy for RHP may be inadequate in determining the intricacies of fighting ability and that future studies investigating assessment strategies and contest outcome should include neurophysiological data.

Increased dopamine transporter function as a mechanism for dopamine hypoactivity in the adult infralimbic medial prefrontal cortex following adolescent social stress

Being bullied during adolescence is associated with later mental illnesses characterized by deficits in cognitive tasks mediated by prefrontal cortex (PFC) dopamine (DA). Social defeat of adolescent male rats, as a model of teenage bullying victimization, results in medial PFC (mPFC) dopamine (DA) hypofunction in adulthood that is associated with increased drug seeking and working memory deficits. Increased expression of the DA transporter (DAT) is also seen in the adult infralimbic mPFC following adolescent defeat. We propose the functional consequence of this increased DAT expression is enhanced DA clearance and subsequently decreased infralimbic mPFC DA availability. To test this, in vivo chronoamperometry was used to measure changes in accumulation of the DA signal following DAT blockade, with increased DAT-mediated clearance being reflected by lower DA signal accumulation. Previously defeated rats and controls were pre-treated with the norepinephrine transporter (NET) inhibitor desipramine (20 mg/kg, ip.) to isolate infralimbic mPFC DA clearance to DAT, then administered the selective DAT inhibitor GBR-12909 (20 or 40 mg/kg, sc.). Sole NET inhibition with desipramine produced no differences in DA signal accumulation between defeated rats and controls. However, rats exposed to adolescent social defeat demonstrated decreased DA signal accumulation compared to controls in response to both doses of GBR-12909, indicating greater DAT-mediated clearance of infralimbic mPFC DA. These results suggest that protracted increases in infralimbic mPFC DAT function represent a mechanism by which adolescent social defeat stress produces deficits in adult mPFC DA activity and corresponding behavioral and cognitive dysfunction.

Role of the dorsomedial hypothalamus in glucocorticoid-mediated feedback inhibition of the hypothalamic-pituitary-adrenal axis

Previous studies suggest that multiple corticolimbic and hypothalamic structures are involved in glucocorticoid-mediated feedback inhibition of the hypothalamic-pituitary-adrenal (HPA) axis, including the dorsomedial hypothalamus (DMH), but a potential role of the DMH has not been directly tested. To investigate the role of the DMH in glucocorticoid-mediated negative feedback, adult male Sprague Dawley rats were implanted with jugular cannulae and bilateral guide cannulae directed at the DMH, and finally were either adrenalectomized (ADX) or were subjected to sham-ADX. ADX rats received corticosterone (CORT) replacement in the drinking water (25 μg/mL), which, based on initial studies, restored a rhythm of plasma CORT concentrations in ADX rats that was similar in period and amplitude to the diurnal rhythm of plasma CORT concentrations in sham-ADX rats, but with a significant phase delay. Following recovery from surgery, rats received microinjections of either CORT (10 ng, 0.5 μL, 0.25 μL/min, per side) or vehicle (aCSF containing 0.2% EtOH), bilaterally, directly into the DMH, prior to a 40-min period of restraint stress. In sham-ADX rats, bilateral intra-DMH microinjections of CORT, relative to bilateral intra-DMH microinjections of vehicle, decreased restraint stress-induced elevation of endogenous plasma CORT concentrations 60 min after the onset of intra-DMH injections. Intra-DMH CORT decreased the overall area under the curve for plasma CORT concentrations during the intermediate time frame of glucocorticoid negative feedback, from 0.5 to 2 h following injection. These data are consistent with the hypothesis that the DMH is involved in feedback inhibition of HPA axis activity at the intermediate time frame.

Neurochemistry as a bridge between morphology and behavior: Perspectives on aggression in insects

Aggression is a common behavioral trait shared in many animals, including both vertebrates and invertebrates. However, the type and intensity of agonistic encounters and displays can vary widely both across and within species, resulting in complicated or subjective interpretations that create difficulties in developing theoretical models that can be widely applied. The need to easily and objectively identify quantifiable behaviors and their associated morphologies becomes especially important when attempting to decipher the neurological mechanisms underlying this complex behavior. Monoamines, neuropeptides, and pheromones have been implicated as important neuromodulators for agonistic displays in both invertebrates and vertebrates. Additionally, recent breakthroughs in insect research have revealed exciting proximate mechanisms important in aggression that may be broadly relevant, due to the relatively high conservation of these neurochemical systems across animal taxa. In this review, we present the latest research demonstrating the importance of monoamines, neuropeptides, and pheromones as neuromodulators for aggression across a variety of insect species. Additionally, we describe the stalk-eyed fly as a model system for studying aggression, which integrates physiological, morphological, and neurochemical approaches in exploring detailed mechanisms responsible for this common yet complex behavior. We conclude with our perspective on the most promising lines of future research aimed at understanding the proximate and ultimate mechanisms underlying aggressive behaviors.

Serotonergic responses to stress are enhanced in the central amygdala and inhibited in the ventral hippocampus during amphetamine withdrawal

Withdrawal from amphetamine increases anxiety and reduces the ability to cope with stress, which are factors that are believed to contribute to drug relapse. Stress-induced serotonergic transmission in the central nucleus of the amygdala is associated with anxiety states and fear. Conversely, stress-induced increases in ventral hippocampal serotonin (5-HT) levels have been linked to coping mechanisms. The goal of this study was to investigate the neurobiological changes induced by amphetamine that contribute to stress sensitivity during withdrawal. We tested the hypothesis that limbic serotonergic responses to restraint stress would be altered in male Sprague-Dawley rats chronically pretreated with amphetamine (2.5 mg/kg, intraperitoneal) and then subjected to 2 weeks of withdrawal. Amphetamine withdrawal resulted in increased stress-induced behavioral arousal relative to control treatment, suggesting that drug withdrawal induced greater sensitivity to the stressor. When microdialysis was used to determine the effects of restraint on extracellular 5-HT, stress-induced increases in 5-HT levels were abolished in the ventral hippocampus and augmented in the central amygdala during amphetamine withdrawal. Reverse dialysis of the glucocorticoid receptor antagonist mifepristone into the ventral hippocampus blocked the stress-induced increase in 5-HT levels in saline-pretreated rats, suggesting that glucocorticoid receptors mediate stress-induced increases in 5-HT levels in the ventral hippocampus. However, mifepristone had no effect on stress-induced increases in 5-HT levels in the central amygdala, indicating that stress increases 5-HT levels in this region independently of glucocorticoid receptors. During amphetamine withdrawal, the absence of stress-induced increases in ventral hippocampal 5-HT levels combined with enhanced stress-induced serotonergic responses in the central amygdala may contribute to drug relapse by decreasing stress-coping ability and heightening stress responsiveness.

Serotonin in the ventral hippocampus modulates anxiety-like behavior during amphetamine withdrawal

Withdrawal from amphetamine is associated with increased anxiety and sensitivity to stressors which are thought to contribute to relapse. Rats undergoing amphetamine withdrawal fail to exhibit stress-induced increases in serotonin (5-HT) release in the ventral hippocampus and show heightened anxiety-like behaviors. Therefore, we tested the hypothesis that reducing 5-HT levels in the ventral hippocampus is a causal mechanism in increasing anxiety-like behaviors during amphetamine withdrawal. First, we tested whether reducing 5-HT levels in the ventral hippocampus directly increases anxiety behavior. Male rats were bilaterally infused with 5,7-dihydroxytryptamine (5,7-DHT) into the ventral hippocampus, which produced a 83% decrease in ventral hippocampus 5-HT content, and were tested on the elevated plus maze (EPM) for anxiety-like behavior. Reducing ventral hippocampus 5-HT levels decreased the time spent in the open arms of the maze, suggesting that diminished ventral hippocampus 5-HT levels increases anxiety-like behavior. Next, we tested whether increasing 5-HT levels in the ventral hippocampus reverses anxiety behavior exhibited by rats undergoing amphetamine withdrawal. Rats were treated daily with either amphetamine (2.5-mg/kg, i.p.) or saline for 2weeks, and at 2weeks withdrawal, were infused with the selective serotonin reuptake inhibitor paroxetine (0.5μM) bilaterally into the ventral hippocampus and tested for anxiety-like behavior on the EPM. Rats pre-treated with amphetamine exhibited increased anxiety-like behavior on the EPM. This effect was reversed by ventral hippocampus infusion of paroxetine. Our results suggest that 5-HT levels in the ventral hippocampus are critical for regulating anxiety behavior. Increasing 5-HT levels during withdrawal may be an effective strategy for reducing anxiety-induced drug relapse.

Decreased prefrontal cortex dopamine activity following adolescent social defeat in male rats: role of dopamine D2 receptors

RATIONALE

Adverse social experience in adolescence causes reduced medial prefrontal cortex (mPFC) dopamine (DA) and associated behavioral deficits in early adulthood.

OBJECTIVE

This study aims to determine whether mPFC DA hypofunction following social stress is specific to adolescent experience and if this results from stress-induced DA D2 receptor activation.

MATERIALS AND METHODS

Male rats exposed to repeated social defeat during adolescence or adulthood had mPFC DA activity sampled 17 days later. Separate experiments used freely moving microdialysis to measure mPFC DA release in response to adolescent defeat exposure. At P40, 49 and 56 mPFC DA turnover was assessed to identify when DA activity decreased in relation to the adolescent defeat experience. Finally, nondefeated adolescent rats received repeated intra-mPFC infusions of the D2 receptor agonist quinpirole, while another adolescent group received intra-mPFC infusions of the D2 antagonist amisulpride before defeat exposure.

RESULTS

Long-term decreases or increases in mPFC DA turnover were observed following adolescent or adult defeat, respectively. Adolescent defeat exposure elicits sustained increases in mPFC DA release, and DA turnover remains elevated beyond the stress experience before declining to levels below normal at P56. Activation of mPFC D2 receptors in nondefeated adolescents decreases DA activity in a similar manner to that caused by adolescent defeat, while defeat-induced reductions in mPFC DA activity are prevented by D2 receptor blockade.

CONCLUSIONS

Both the developing and mature PFC DA systems are vulnerable to social stress, but only adolescent defeat causes DA hypofunction. This appears to result in part from stress-induced activation of mPFC D2 autoreceptors.

Selection for increased voluntary wheel-running affects behavior and brain monoamines in mice

Selective-breeding of house mice for increased voluntary wheel-running has resulted in multiple physiological and behavioral changes. Characterizing these differences may lead to experimental models that can elucidate factors involved in human diseases and disorders associated with physical inactivity, or potentially treated by physical activity, such as diabetes, obesity, and depression. Herein, we present ethological data for adult males from a line of mice that has been selectively bred for high levels of voluntary wheel-running and from a non-selected control line, housed with or without wheels. Additionally, we present concentrations of central monoamines in limbic, striatal, and midbrain regions. We monitored wheel-running for 8 weeks, and observed home-cage behavior during the last 5 weeks of the study. Mice from the selected line accumulated more revolutions per day than controls due to increased speed and duration of running. Selected mice exhibited more active behaviors than controls, regardless of wheel access, and exhibited less inactivity and grooming than controls. Selective-breeding also influenced the longitudinal patterns of behavior. We found statistically significant differences in monoamine concentrations and associated metabolites in brain regions that influence exercise and motivational state. These results suggest underlying neurochemical differences between selected and control lines that may influence the observed differences in behavior. Our results bolster the argument that selected mice can provide a useful model of human psychological and physiological diseases and disorders.

Influence of chronic amphetamine treatment and acute withdrawal on serotonin synthesis and clearance mechanisms in the rat ventral hippocampus

Amphetamine withdrawal in both humans and rats is associated with increased anxiety states, which are thought to contribute to drug relapse. Serotonin in the ventral hippocampus mediates affective behaviors, and reduced serotonin levels in this region are observed in rat models of high anxiety, including during withdrawal from chronic amphetamine. This goal of this study was to understand the mechanisms by which reduced ventral hippocampus serotonergic neurotransmission occurs during amphetamine withdrawal. Serotonin synthesis (assessed by accumulation of serotonin precursor as a measure of the capacity of in vivo tryptophan hydroxylase activity), expression of serotonergic transporters, and in vivo serotonergic clearance using in vivo microdialysis were assessed in the ventral hippocampus in adult male Sprague Dawley rats at 24 h withdrawal from chronic amphetamine. Overall, results showed that diminished extracellular serotonin at 24 h withdrawal from chronic amphetamine was not accompanied by a change in capacity for serotonin synthesis (in vivo tryptophan hydroxylase activity), or serotonin transporter expression or function in the ventral hippocampus, but instead was associated with increased expression and function of organic cation transporters (low-affinity, high-capacity serotonin transporters). These findings suggest that 24 h withdrawal from chronic amphetamine reduces the availability of extracellular serotonin in the ventral hippocampus by increasing organic cation transporter-mediated serotonin clearance, which may represent a future pharmacological target for reversing anxiety states during drug withdrawal.

Gβ5-RGS complexes are gatekeepers of hyperactivity involved in control of multiple neurotransmitter systems

RATIONALE AND OBJECTIVES

Our knowledge about genes involved in the control of basal motor activity that may contribute to the pathology of the hyperactivity disorders, e.g., attention deficit hyperactivity disorder (ADHD), is limited. Disruption of monoamine neurotransmitter signaling through G protein-coupled receptors (GPCR) is considered to be a major contributing factor to the etiology of the ADHD. Genetic association evidence and functional data suggest that regulators of G protein signaling proteins of the R7 family (R7 RGS) that form obligatory complexes with type 5 G protein beta subunit (Gβ5) and negatively regulate signaling downstream from monoamine GPCRs may play a role in controlling hyperactivity.

METHODS

To test this hypothesis, we conducted behavioral, pharmacological, and neurochemical studies using a genetic mouse model that lacked Gβ5, a subunit essential for the expression of the entire R7 RGS family.

RESULTS

Elimination of Gβ5-RGS complexes led to a striking level of hyperactivity that far exceeds activity levels previously observed in animal models. This hyperactivity was accompanied by motor learning deficits and paradoxical behavioral sensitization to a novel environment. Neurochemical studies indicated that Gβ5-RGS-deficient mice had higher sensitivity of inhibitory GPCR signaling and deficits in basal levels, release, and reuptake of dopamine. Surprisingly, pharmacological treatment with monoamine reuptake inhibitors failed to alter hyperactivity. In contrast, blockade of NMDA receptors reversed the expression of hyperactivity in Gβ5-RGS-deficient mice.

CONCLUSIONS

These findings establish that Gβ5-RGS complexes are critical regulators of monoamine-NMDA receptor signaling cross-talk and link these complexes to disorders that manifest as hyperactivity, impaired learning, and motor dysfunctions.

Plasma cortisol and hypothalamic monoamine responses in yellow perch Perca flavescens after intraperitoneal injection of lipopolysaccharide

The concentrations of monoamines in the hypothalamus were determined in yellow perch Perca flavescens before and after injection with lipopolysaccharide (LPS; 3 mg kg⁻¹ fish weight) or saline to test for the presence of neurochemical changes potentially associated with changes in plasma cortisol characteristic of intraperitoneal (ip) challenge with LPS. In the first experiment, yellow perch were injected with saline or LPS and the hypothalamus removed and plasma sampled before and at 0.5, 1.5, 3.0, and 6 h after injection. Plasma cortisol was elevated in both saline- and LPS-injected fish through 1.5 h after injection and returned to levels resembling pre-injection by 3 h after injection. Significantly higher amounts of cortisol in plasma from LPS-injected relative to saline-injected fish were observed 6 h following injection. A significant decrease relative to levels observed 0.5-3 h after handling was observed in serotonin concentrations at 6 h following LPS and saline injection with a concomitant increase in the ratio of 5-hydroxyindoleacetic acid:serotonin. In the second experiment, hypothalamic monoamines were sampled before and at 3, 6, 9, 12, and 24 h after injection with LPS or saline. Significant increases from pre-injection levels were observed in the ratio 5-hydroxyindoleacetic acid:serotonin at 9, 12, and 24 h after injection, but no differences were detected between LPS- and saline-injected fish. These results support a model linking serotonergic system activation following handling stress, but no correlations with the sustained elevations of plasma cortisol associated with inflammatory challenge were observed.

Adolescent social defeat alters neural, endocrine and behavioral responses to amphetamine in adult male rats

The mesocorticolimbic dopamine system, which governs components of reward and goal-directed behaviors, undergoes final maturation during adolescence. Adolescent social stress contributes to adult behavioral dysfunction and is linked to adult psychiatric and addiction disorders. Here, behavioral, corticosterone and limbic dopamine responses to amphetamine were examined in adult male rats previously exposed to repeated social defeat stress during mid-adolescence. Amphetamine (2.5mg/kg, ip) was administered after a novel environment test, with behavior observed in the same context for 90min thereafter. Adult rats that had been defeated in adolescence showed increased locomotion in the novel environment but reduced amphetamine-induced locomotion relative to non-defeated age matched controls. Monoamine and corticosterone responses to amphetamine were examined following a second amphetamine injection 3 days later. In previously defeated rats, corticosterone and medial prefrontal cortex dopamine responses to amphetamine were blunted while dopamine responses in the nucleus accumbens core were elevated. Our results suggest that experience of social defeat stress during adolescent development can contribute to altered behavioral and endocrine responses to amphetamine in adulthood. Furthermore, these effects are paralleled by changes in amphetamine-induced dopamine responses in corticolimbic systems implicated in addiction disorders.

Central monoamine levels differ between rat strains used in studies of depressive behavior

Previous studies have shown that the Wistar-Kyoto (WKY) rat strain may be a genetic model of depression when their behaviors are compared to Sprague-Dawley (SD) or Wistar (WIS) rats. Significant differences in dopamine (DA), serotonin (5-HT), and norepinephrine (NE) transporter site densities have been reported when comparing WKY to both SD and WIS rats. Susceptibility of WKY rats to anxiety and depressive behavior may be related to underlying differences in monoamine levels in various regions of the brain. Levels of monoamines (DA, 5-HT and NE) and their metabolites were measured in monoaminergic cell body, cortical and limbic brain regions using HPLC with electrochemical detection and compared between WKY, WIS and SD rats. In regions where strain differences in monoamine levels were observed (the basolateral amygdala, subregions of the hippocampus and the nucleus accumbens shell), WKY rats consistently had lower levels than SD rats. Similarly, WKY rats had lower monoamine levels compared to WIS, although these differences were observed in a more restricted number of brain regions. Interestingly, WIS rats showed reduced levels of the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) in several regions including the prefrontal cortex, subregions of the hippocampus and subregions of the hypothalamus, suggesting decreased 5-HT turnover when compared to both WKY and SD rats. Overall, these results imply that decreased monoamine levels, combined with alterations in transporter sites, may be related to the predisposition of WKY rats towards depressive behavior.

Selection for intrinsic endurance modifies endocrine stress responsiveness

Physical exercise dampens an individual's stress response and decreases symptoms of anxiety and depression disorders. While the extrinsic relationship of exercise and psychological state is established, their intrinsic relationship is unresolved. We investigated the potential intrinsic relationship of exercise with stress responsiveness using NIH rats bidirectionally selected for intrinsic endurance capacity. Selection resulted in two populations, one with high intrinsic endurance (high capacity runners; HCR) and one with low intrinsic endurance (low capacity runners; LCR). Animals from these populations were subjected to the elevated plus maze (EPM) and novel environment to assess levels of anxiety-like behavior, and to restraint stress to determine stress responsiveness. Pre-test plasma corticosterone levels and the response of plasma corticosterone to exposure to the EPM and restraint were analyzed using ELISA. A dexamethasone suppression test was performed to assess negative feedback tone of corticosterone release. Pre-test plasma corticosterone levels were similar between LCR and HCR, and these populations had similar behavioral and corticosterone responses to the EPM. Following restraint, HCR animals exhibited more anxiotypic behavior than LCR animals on the EPM, and exhibited an increase in plasma corticosterone following EPM and restraint that was not observed in LCR animals. HCR animals also exhibited more anxiotypic behavior in the novel environment compared to LCR animals. Plasma corticosterone levels were equally reduced in both populations following dexamethasone administration. Overall, our data suggest a positive genetic relationship between exercise endurance and stress responsiveness, which is at odds with the established extrinsic relationship of these traits.

Phenotypic and evolutionary plasticity of body composition in rats selectively bred for high endurance capacity

We investigated the effects of genetic selection and prolonged wheel access (8 wk) on food consumption and body composition in lines of rats selected for high and low intrinsic (untrained) endurance running capacity (HCR and LCR, respectively) to test the generality of phenotypic correlations between physical activity levels, aerobic capacity, and body composition. HCR rats ran more minutes per day on activity wheels than LCR rats, supporting the hypothesis that voluntary activity and physiological capacity are genetically correlated (self-induced adaptive plasticity). Both treatments (selection and wheel access) significantly affected food consumption. HCR rats consumed and digested more food than LCR rats. Access to running wheels did not result in changes in overall body mass, but lean body mass increased and percent body fat decreased in both lines. Selection for high endurance capacity resulted in hypertrophy of the heart and kidneys and decreased long intestine length. We found significant phenotypic flexibility in a number of organ masses after wheel running. Specifically, access to running wheels resulted in hypertrophy of the heart, liver, kidney, stomach, and small and large intestines in LCR and HCR rats. The selected line×wheel access interaction was significantly greater in HCR rats in relative mass for the heart and lung. Compared with LCR rats, HCR rats fortify wheel running with increased food consumption along with greater hypertrophy of key organs for O2 transport.

Withdrawal from chronic amphetamine produces persistent anxiety-like behavior but temporally-limited reductions in monoamines and neurogenesis in the adult rat dentate gyrus

Acute amphetamine administration activates monoaminergic pathways and increases systemic corticosterone, both of which influence anxiety states and adult dentate gyrus neurogenesis. Chronic amphetamine increases anxiety states in rats when measured at 24 h and at 2 weeks of withdrawal. However, the effects of chronic amphetamine exposure and withdrawal on long term anxiety-like behavior and adult neurogenesis in the dentate gyrus are unknown. Adult male rats were administered amphetamine (2.5 mg/kg, ip.) daily for two weeks. Anxiety-like behaviors were increased markedly in amphetamine-treated rats following four weeks of withdrawal from amphetamine. Plasma corticosterone level was unaltered by amphetamine treatment or withdrawal. However, norepinephrine and serotonin concentrations were selectively reduced in the dentate gyrus 20 h following amphetamine treatment. This effect did not persist through the four week withdrawal period. In separate experiments, rats received bromodeoxyuridine to label cells in S-phase, prior to or immediately following amphetamine treatment. Newly generated cells were quantified to measure extent of progenitor cell proliferation and neurogenesis following treatment or withdrawal. Progenitor cell proliferation and neurogenesis were not significantly affected by amphetamine exposure when measured 20 h following the last amphetamine treatment. However, neurogenesis in the dentate gyrus was reduced after four weeks of withdrawal when compared to saline-pretreated rats. Overall, our findings indicate that withdrawal from chronic amphetamine leads to persistent anxiety-like behavior which may be maintained by reduced neurogenesis in the dentate gyrus at this protracted withdrawal time point. However, neurogenesis is unaffected at earlier withdrawal time points where anxiety states emerge, suggesting different mechanisms may underlie the emergence of anxiety states during amphetamine withdrawal.

Organic cation transporter inhibition increases medial hypothalamic serotonin under basal conditions and during mild restraint

The dorsomedial hypothalamus (DMH) has been implicated in the coordination of stress responses. Restraint stress or systemic corticosterone (CORT) treatment induces a rapid increase in tissue concentrations of serotonin (5-hydroxytryptamine; 5-HT) in the DMH. Although the mechanism for rapid changes in 5-HT concentrations in the DMH is not clear, earlier results suggest that stress-induced increases in CORT may inhibit 5-HT transport from the extracellular fluid by acting on corticosterone-sensitive organic cation transporters (OCTs). We tested the hypothesis that perfusion of the medial hypothalamus (MH), which includes the DMH, with the OCT blocker decynium 22 (D-22) would potentiate the effects of mild restraint on extracellular 5-HT. Male Sprague-Dawley rats, implanted with a microdialysis probe into the MH, were treated with reverse-dialysis of D-22 (20 microM; 40 min) or vehicle and subjected to either 40 min mild restraint or undisturbed control conditions. Perfusates collected from a separate group of rats were evaluated for the effect of restraint on extracellular CORT concentrations in the MH. Reverse-dialysis of D-22 induced an increase (200%) in extracellular 5-HT concentrations in the MH in undisturbed control rats. Restraint in the absence of D-22 did not significantly affect MH CORT or 5-HT concentrations. However, perfusion of the MH with D-22 during restraint led to an increased magnitude and duration of extracellular 5-HT concentrations, relative to D-22 by itself. These results are consistent with the hypothesis that OCTs in the DMH contribute to the clearance of 5-HT from the extracellular fluid under both baseline conditions and mild restraint.

Opponent recognition and social status differentiate rapid neuroendocrine responses to social challenge

Individual social status discriminates rapid neuroendocrine responses to non-social stress in male Anolis carolinensis, but whether such status-influenced reactions are retained in response to subsequent social stress is unknown. Dominant and subordinate males modify their behavioral responses to social challenge according to familiarity of the opponent, suggesting that accompanying neuroendocrine responses may differ according to opponent recognition despite social rank. We examined endocrine and neurochemical correlates of prior social status and opponent recognition during the opening stages of social challenge. Male pairs interacted and established dominant/subordinate status, followed by 3 days separation. Subsequently, subjects were paired with either the same opponent or an unfamiliar male according to rank (dominant with subordinate). After 90 s of social exposure, subjects were caught and brains and plasma collected for measurement of circulating corticosterone and limbic monoamines. Controls included pairs experiencing just one 90 s encounter plus a group of non-interacting subjects. Opponent recognition differentiated status-influenced responses, such that dominant lizards paired with familiar subordinate opponents had increased hippocampal dopamine and epinephrine, but showed increased plasma corticosterone and ventral tegmental area (VTA) norepinephrine when challenged with an unfamiliar opponent. Subordinate lizards encountering familiar opponents also had increased corticosterone, along with decreased hippocampal dopamine and increased VTA epinephrine, but showed no changes in response to an unfamiliar opponent. Such plasticity in status-influenced rapid neuroendocrine responses according to opponent recognition may be necessary for facilitating production of behavioral responses adaptive for particular social contexts, such as encountering a novel versus familiar opponent.

Adolescent male rats exposed to social defeat exhibit altered anxiety behavior and limbic monoamines as adults

Social stress in adolescence is correlated with emergence of psychopathologies during early adulthood. In this study, the authors investigated the impact of social defeat stress during mid-adolescence on adult male brain and behavior. Adolescent male Sprague-Dawley rats were exposed to repeated social defeat for 5 days while controls were placed in a novel empty cage. When exposed to defeat-associated cues as adults, previously defeated rats showed increased risk assessment and behavioral inhibition, demonstrating long-term memory for the defeat context. However, previously defeated rats exhibited increased locomotion in both elevated plus-maze and open field tests, suggesting heightened novelty-induced behavior. Adolescent defeat also affected adult monoamine levels in stress-responsive limbic regions, causing decreased medial prefrontal cortex dopamine, increased norepinephrine and serotonin in the ventral dentate gyrus, and decreased norepinephrine in the dorsal raphe. Our results suggest that adolescent social defeat produces both deficits in anxiety responses and altered monoaminergic function in adulthood. This model offers potential for identifying specific mechanisms induced by severe adolescent social stress that may contribute to increased adult male vulnerability to psychopathology.

Local perfusion of corticosterone in the rat medial hypothalamus potentiates D-fenfluramine-induced elevations of extracellular 5-HT concentrations

The dorsomedial hypothalamus (DMH) plays an important role in coordinating physiological and behavioral responses to stress-related stimuli. In vertebrates, DMH serotonin (5-HT) concentrations increase rapidly in response to acute stressors or corticosterone (CORT). Recent studies suggest that CORT inhibits postsynaptic clearance of 5-HT from the extracellular fluid in the DMH by blocking organic cation transporter 3 (OCT3), a polyspecific CORT-sensitive transport protein. Because OCTs are low-affinity, high-capacity transporters, we hypothesized that CORT effects on extracellular 5-HT are most pronounced in the presence of elevated 5-HT release. We predicted that local application of CORT into the DMH would potentiate the effects of d-fenfluramine, a 5-HT-releasing agent, on extracellular 5-HT. These experiments were conducted using in vivo microdialysis in freely-moving male Sprague-Dawley rats implanted with a microdialysis probe into the medial hypothalamus (MH), which includes the DMH. In Experiment 1, rats simultaneously received intraperitoneal (i.p.) injections of 1 mg/kg D-fenfluramine or saline and either 200 ng/mL CORT or dilute ethanol (EtOH) vehicle delivered to the MH by reverse-dialysis for 40 min. In Experiment 2, 5 microM D-fenfluramine and either 200 ng/mL CORT or EtOH vehicle were concurrently delivered to the MH for 40 min using reverse-dialysis. CORT potentiated the increases in extracellular 5-HT concentrations induced by either i.p. or intra-MH administration of D-fenfluramine. Furthermore, CORT and D-fenfluramine interacted to alter home cage behaviors. Our results support the hypothesis that CORT inhibition of OCT3-mediated 5-HT clearance from the extracellular fluid contributes to stress-induced increases in extracellular 5-HT and 5-HT signaling.

Fluoxetine potentiates the effects of corticotropin-releasing factor on locomotor activity and serotonergic systems in the roughskin newt, Taricha granulosa

The anxiety- and stress-related neuropeptide corticotropin-releasing factor (CRF) elicits behavioral changes in vertebrates including increases in behavioral arousal and locomotor activity. Intracerebroventricular injections of CRF in an amphibian, the roughskin newt (Taricha granulosa), induces rapid increases in locomotor activity in both intact and hypophysectomized animals. We hypothesized that this CRF-induced increase in locomotor activity involves a central effect of CRF on serotonergic neurons, based on known stimulatory actions of serotonin (5-hydroxytryptamine, 5-HT) on spinal motor neurons and the central pattern generator for locomotor activity in vertebrates. In Experiment 1, we found that neither intracerebroventricular injections of low doses of CRF (25 ng) nor the selective serotonin reuptake inhibitor fluoxetine (10, 100 ng), by themselves, altered locomotor activity. In contrast, newts treated concurrently with CRF and fluoxetine responded with marked increases in locomotor activity. In Experiment 2, we found that increases in locomotor activity following co-administration of CRF (25 ng) and fluoxetine (100 ng) were associated with decreased 5-HT concentrations in a number of forebrain structures involved in regulation of emotional behavior and emotional states, including the ventral striatum, amygdala pars lateralis, and dorsal hypothalamus, measured 37 min after treatment. These results are consistent with the hypothesis that CRF stimulates locomotor activity through activation of serotonergic systems.

Fluoxetine inhibits corticotropin-releasing factor (CRF)-induced behavioural responses in rats

Corticotropin-releasing factor (CRF) is a potent neuromodulator of stress-related behaviour but the neural mechanisms underlying these effects are not clear. Studies were designed to test the hypothesis that CRF-induced behavioural arousal involves interactions with brainstem serotonergic systems. To examine interactions between CRF and serotonergic systems in the regulation of behaviour, CRF (1 microg, intracerebroventricular (i.c.v.)) or vehicle was infused in the presence or absence of the selective serotonin re-uptake inhibitor fluoxetine (0, 0.1, 1 or 10 mg/kg, intravenous (i.v.)). Fluoxetine was used at these doses because it is known to decrease serotonin cell firing rates while increasing extracellular serotonin concentrations in select forebrain regions. We then measured behavioural, neurochemical and endocrine responses. CRF increased locomotion and spontaneous non-ambulatory motor activity (SNAMA) in the home cages. Fluoxetine decreased tissue 5-hydroxyindoleacetic acid concentrations, a measure of serotonin metabolism, in specific limbic brain regions of CRF-treated rats (nucleus accumbens shell region, entorhinal cortex, central nucleus of the amygdala). Furthermore, fluoxetine inhibited CRF-induced SNAMA. CRF and fluoxetine independently increased plasma corticosterone concentrations, but the responses had distinct temporal profiles. Overall, these data are consistent with the hypothesis that CRF-induced facilitation of behavioural activity is dependent on brainstem serotonergic systems. Therefore, fluoxetine may attenuate or alleviate some behavioural responses to stress by interfering with CRF-induced responses.

Individual differences in amphetamine sensitization, behavior and central monoamines

Repeated amphetamine treatment results in behavioral sensitization in a high percentage of rats. Alterations to plasma corticosterone, neural monoamines and stress behavior can accompany amphetamine sensitization. Whether these changes occur following repeated amphetamine treatment in the absence of behavioral sensitization is not known. Male Sprague-Dawley rats were treated with amphetamine (2.5 mg/kg, i.p.) or saline once daily for 6 days. Amphetamine-induced locomotion and stereotypy, open-field anxiety behavior, plasma corticosterone and limbic monoamines were measured during withdrawal. Sixty-two percent of amphetamine-treated rats showed behavioral sensitization over the test periods. Only amphetamine-sensitized rats showed increased latency to enter the center of the open-field, as well as increased plasma corticosterone when compared to saline-treated controls. Amphetamine-sensitized rats showed increased dopamine concentrations in the shell of the nucleus accumbens and increased serotonin concentrations in the dorsal hippocampus, which were not observed in amphetamine-treated non-sensitized rats. These findings suggest that anxiety behavior, plasma corticosterone and limbic monoamines concentrations are altered by repeated amphetamine (2.5 mg/kg) treatment, and that these neuroendocrine and behavioral changes are often associated with sensitization to the psychostimulant effects of amphetamine.

Early life social isolation alters corticotropin-releasing factor responses in adult rats

Stress induced by early life social isolation leads to long-lasting alterations in stress responses and serotonergic activity. Corticotropin-releasing factor (CRF) is a neurotransmitter that mediates stress responses and alters serotonergic activity. We tested the hypothesis that the stress of early life isolation enhances responses to CRF in adulthood by determining the effect of CRF infusions into the dorsal raphe nucleus (dRN) on 5-HT release in the nucleus accumbens (NAc) of adult rats using in vivo microdialysis. Juvenile male rats were either isolated or housed in groups of three for a 3-week period beginning on postnatal day 21 after which, all rats were group-reared for an additional 2 weeks. Following the isolation/re-socialization procedure, infusion of 100 ng CRF into the dRN decreased 5-HT release in the NAc of group-reared rats. This treatment did not significantly affect 5-HT release in the NAc of isolation-reared animals. In contrast, infusion of 500 ng CRF into the dRN transiently increased 5-HT release in the NAc of both group-reared and isolated animals with isolated animals showing a more prolonged serotonergic response. Western blot and immunofluorescent staining for CRF receptors in the dRN showed that CRF(2) receptor levels were increased in the dRN of isolation-reared animals when compared with group-reared rats. Taken together, the results suggest that isolation during the early part of development causes alterations in both CRF receptor levels and CRF-mediated serotonergic activity. These effects may underlie the increased sensitivity to stress observed in isolates.

Corticotropin-releasing factor in the dorsal raphe nucleus increases medial prefrontal cortical serotonin via type 2 receptors and median raphe nucleus activity

Interactions between central corticotropin-releasing factor (CRF) and serotonergic systems are believed to be important for mediating fear and anxiety behaviors. Recently we demonstrated that infusions of CRF into the rat dorsal raphe nucleus result in a delayed increase in serotonin release within the medial prefrontal cortex that coincided with a reduction in fear behavior. The current studies were designed to study the CRF receptor mechanisms and pathways involved in this serotonergic response. Infusions of CRF (0.5 microg/0.5 microL) were made into the dorsal raphe nucleus of urethane-anesthetized rats following either inactivation of the median raphe nucleus by muscimol (25 ng/0.25 microL) or antagonism of CRF receptor type 1 or CRF receptor type 2 in the dorsal raphe nucleus with antalarmin (25-50 ng/0.5 microL) or antisauvagine-30 (2 microg/0.5 microL), respectively. Medial prefrontal cortex serotonin levels were measured using in-vivo microdialysis and high-performance liquid chromatography with electrochemical detection. Increased medial prefrontal cortex serotonin release elicited by CRF infusion into the dorsal raphe nucleus was abolished by inactivation of the median raphe nucleus. Furthermore, antagonism of CRF receptor type 2 but not CRF receptor type 1 in the dorsal raphe nucleus abolished CRF-induced increases in medial prefrontal cortex serotonin. Follow-up studies involved electrical stimulation of the central nucleus of the amygdala, a source of CRF afferents to the dorsal raphe nucleus. Activation of the central nucleus increased medial prefrontal cortex serotonin release. This response was blocked by CRF receptor type 2 antagonism in the dorsal raphe. Overall, these results highlight complex CRF modulation of medial prefrontal cortex serotonergic activity at the level of the raphe nuclei.

Estrogen regulation of proteins in the rat ventromedial nucleus of the hypothalamus

The effects of estradiol (E2) on the expression of proteins in the pars lateralis of the ventromedial nucleus of the hypothalamus (VMNpl) in ovariectomized rats was studied using 2-dimensional gel electrophoresis followed by RPLC-nanoESI-MS/MS. E2 treatment resulted in the up-regulation of 29 identified proteins. Many of these proteins are implicated in the promotion of neuronal plasticity and signaling.

Corticotropin-releasing factor 1 and 2 receptors in the dorsal raphé differentially affect serotonin release in the nucleus accumbens

Corticotropin-releasing factor (CRF) is a neurohormone that mediates stress, anxiety, and affects serotonergic activity. Studies have shown that CRF has dose-dependent opposing effects on serotonergic activity. This effect has been hypothesized to be differentially mediated by CRF(1) and CRF(2) receptors in the dorsal raphé nucleus. We directly tested this hypothesis by using in vivo microdialysis to determine the effects of CRF and CRF receptor antagonists in the dorsal raphé nucleus on serotonin (5-HT) release in the nucleus accumbens, a brain region implicated in the neuropathology of stress-related psychiatric disorders. Male urethane-anesthetized rats were implanted with a microdialysis probe into the nucleus accumbens, and CRF (0, 100 or 500 ng) was infused into the dorsal raphé. Infusion of CRF into the dorsal raphé nucleus had dose-dependent opposite effects, with 100 ng of CRF significantly decreasing 5-HT levels in the nucleus accumbens and 500 ng CRF significantly increasing accumbal 5-HT levels. In subsequent experiments, the raphé was pre-treated with the CRF(1) receptor antagonist antalarmin (0.25 microg) or the CRF(2) receptor antagonist antisauvagine-30 (ASV-30; 2 microg) prior to CRF infusion. Antagonism of CRF(1) receptors in the dorsal raphé nucleus abolished the decrease in accumbal 5-HT levels elicited by 100 ng CRF, and CRF(2) receptor antagonism in the raphé blocked the increase in accumbal 5-HT levels elicited by 500 ng CRF. These results suggest that the opposing effects of dorsal raphé CRF on 5-HT release in the nucleus accumbens are dependent on differential activation of CRF(1) and CRF(2) receptors in the dorsal raphé nucleus.

Corticotropin releasing factor induces anxiogenic locomotion in trout and alters serotonergic and dopaminergic activity

Corticotropin releasing factor (CRF) and serotonin (5-HT) are strongly linked to stress and anxiety in vertebrates. As a neuromodulator in the brain, CRF has anxiogenic properties often characterized by increased locomotion and stereotyped behavior in familiar environments. We hypothesized that expression of anxiogenic behavior in response to CRF will also be exhibited in a teleost fish. Rainbow trout were treated with intracerebroventricular (icv) injections of artificial cerebrospinal fluid (aCSF), 500 or 2000 ng ovine CRF, or not injected. Treatment with either dose of CRF elicited greater locomotion and pronounced head shaking behavior but did not influence water column position. Locomotor and head shaking behaviors may be analogous to the increased stereotypy evoked by icv CRF in rats and may reflect the expression of stress/anxiety behavior. Injection with either aCSF or CRF produced significant increases in plasma cortisol. The absence of behavioral changes in aCSF-injected fish suggests that the behavioral responses following CRF were not due to cortisol. Treatment with 2000 ng CRF significantly increased serotonin, 5-HIAA and dopamine concentrations in the subpallium and raphé and increased 5-HIAA in the preoptic hypothalamus (POA). Concurrent effects of CRF on central monoamines, locomotion and head shaking in trout suggest that anxiogenic properties of CRF are evolutionarily conserved. In addition, positive linear correlations between locomotion and serotonergic and dopaminergic function in the subpallium, POA and raphé nuclei suggest a locomotory function for these monoamines.

Mice lacking central serotonergic neurons show enhanced inflammatory pain and an impaired analgesic response to antidepressant drugs

A large body of literature has implicated serotonin [5-hydroxytryptamine (5-HT)] in descending modulation of nociceptive transmission. Here, we have studied the pain behavior of Lmx1b conditional knock-out mice (Lmx1b(f/f/p)), which lack 5-HT neurons in the CNS. Lmx1b(f/f/p) mutant mice showed normal thermal and visceral pain responses but were less sensitive to mechanical stimuli and exhibited enhanced inflammatory pain compared with their littermate control mice. Importantly, the analgesic effect of several antidepressant drugs, including selective serotonin reuptake inhibitors (SSRIs), serotonin-norepinephrine reuptake inhibitors (SNRIs), and tricyclic antidepressants, was either abolished or greatly attenuated in Lmx1b(f/f/p) mice. Moreover, in the acute versus persistent pain settings, the analgesic actions of the SNRI duloxetine and the SSRI fluoxetine were differentially affected. Together, our results provide in vivo genetic evidence demonstrating that although the predominant role of the central 5-HT system in inflammatory pain is inhibitory, its role in acute mechanical pain is facilitatory. The findings that the analgesic effects of various antidepressant drugs are differentially dependent on the central 5-HT system should help us to understand the mechanism of the analgesic action of different classes of antidepressants in the management of persistent pain.

Effects of labor contractions on catecholamine release and breathing frequency in newborn rats

Plasma catecholamines in newborn rats (0-2 hr old) were analyzed following vaginal birth, cesarean section with simulated labor contractions, or cesarean section without labor contractions. Upon delivery, pups were exposed to key elements of the rat's natural birth process, that is, umbilical cord occlusion, tactile stimulation, and cooling. Only pups exposed to actual or simulated labor showed an immediate rise in norepinephrine and epinephrine. Initial postpartum respiratory frequencies were higher in vaginal than in cesarean delivered pups and, in all groups, inversely correlated with catecholamine titers, suggesting respiratory distress or transient tachypnea at lower catecholamine levels. These findings establish a rat model for analyzing effects of labor on neonatal adaptive response during the transition from prenatal to postnatal life.

Proteomic analysis of the ventromedial nucleus of the hypothalamus (pars lateralis) in the female rat

The use of proteomics to study changes in the expression of CNS proteins, which may underlie the regulation of physiological and/or behavioral responses, represents an emerging application of this technology. In the current study, the Palkovits' microdissection method was evaluated as a means of obtaining proteomic data from discrete brain nuclei. The pars lateralis of the ventromedial nucleus of the hypothalamus (VMN) was chosen for the initial studies because of its established role in the expression of gonadal hormone dependent female sexual behavior. The VMN from ovariectomized rats was microdissected from 300 microm frozen brain sections using a 500 microm punch. Total proteins were separated using 2-DE. A group consensus of 432 protein spots, visualized by SYPRO Ruby stain, was obtained from gels from four independent VMN samples. A low mean CV and high gel-to-gel correlation coefficients indicate that reproducible 2-DE gels can be generated from microdissected tissue samples. Proteins from the mediobasal hypothalamus (MBH) were also separated on 2-DE gels. Evaluation of the 2-DE maps from the VMN and the MBH revealed different protein profiles, and indicates that microdissection improves the detection of low-abundance proteins, and reduces the relative occurrence of abundant proteins on 2-DE maps.