Miles Orchinik

Seasonal Regulation of Membrane and Intracellular Corticosteroid Receptors in the House Sparrow Brain

A number of studies have demonstrated seasonal regulation of the adrenocortical response to stress, or of corticosteroid binding globulins, but very few studies have examined seasonal regulation of corticosteroid receptor levels. As a result, there have been few attempts to produce an integrated picture of seasonal plasticity of the stress response. We measured baseline and stress‐induced corticosterone (CORT), corticosteroid binding globulin and neuronal cytosolic and membrane corticosteroid receptor levels in male and female, wild‐caught house sparrows (Passer domesticus) during three different seasons over the annual cycle (nesting, molting and winter). We identified three neuronal corticosteroid receptors in the house sparrow brain: two intracellular receptors and one membrane‐associated receptor. Little is known about corticosteroid receptors in neuronal membranes of avian and mammalian species, but we found that the levels of membrane corticosteroid receptors varied seasonally, being lowest during the nesting season. Cytosolic corticosteroid receptor numbers (both low and high affinity receptors) also varied seasonally. In contrast to the membrane bound receptors, however, the numbers of low and high affinity cytosolic receptors were lowest during winter. In addition, mean levels of total basal and stress‐induced CORT in the plasma varied seasonally. Both basal and stress‐induced levels of total CORT were significantly higher during nesting than during winter or molt. Finally, corticosteroid binding globulin levels in plasma were also seasonally regulated, in a pattern similar to total CORT, so that estimated free CORT levels did not vary between seasons. These data indicate that multiple components of the stress response are seasonally regulated in birds obtained from wild populations. Interactions between these regulated components provide a basis for seasonal differences in behavioural and physiological responses to stress.

Morphological changes in the hippocampal CA3 region induced by non-invasive glucocorticoid administration: a paradox.

Repeated stress induces atrophy, or remodeling, of apical dendrites in hippocampal CA3 pyramidal neurons. In rats, the stress effect is blocked by adrenal steroid synthesis inhibitors, and mimicked by daily injection of corticosterone. We report that non-invasive administration of corticosterone in the drinking water (400 micrograms/ml) also produced atrophy of apical dendrites in CA3. Unexpectedly, the combination of daily stress and oral corticosterone negated the effects of either treatment alone, and no changes in the apical dendritic length or branching pattern of CA3 pyramidal neurons were observed compared to control unstressed rats.

Corticosterone and corticosteroid binding globulin in birds: Relation to urbanization in a desert city

As cities expand worldwide, understanding how species adapt to novel urban habitats will become increasingly important to conservation. The adrenocortical stress response enables vertebrates to cope with novel environmental challenges to homeostasis. We examined total and estimates of free baseline and stress-induced corticosterone (CORT) concentrations and CORT binding globulin (CBG) levels in five passerine species within and around Phoenix, Arizona. We tested whether baseline and stress-induced CORT patterns differed among species living at varying densities in Phoenix and tested the hypothesis that, for species capable of successfully colonizing cities, individuals living in urban areas have a decreased acute stress response compared to individuals living in native desert. Baseline total CORT levels were generally similar in urban and rural birds. Capture and handling stress typically produced greater total CORT responses in urban birds than in rural birds, although these responses differed as a function of the life history stage (non-breeding, breeding or molt). CBG binding capacity did not change with life history stage or locality. Estimated free CORT concentrations differed less between groups than total CORT concentrations. Urban birds showed less variability in stress responses across life history stages than rural birds. We propose that more predictable resources in the city than in rural areas may decrease the need to vary stress responsiveness across life history stages. The results highlight the species-specific effects of urbanization on stress physiology and the difficulty to predict how urbanization impacts organisms.

Corticosterone-Sensitive Monoamine Transport in the Rat Dorsomedial Hypothalamus: Potential Role for Organic Cation Transporter 3 in Stress-Induced Modulation of Monoaminergic Neurotransmission

Glucocorticoid hormones act within the brain to alter physiological and behavioral responses to stress-related stimuli. Previous studies indicated that acute stressors can increase serotonin [5-hydroxytryptamine (5-HT)] concentrations in the dorsomedial hypothalamus (DMH), a midline hypothalamic structure involved in the integration of physiological and behavioral responses to stress. The current study tests the hypothesis that rapid, stress-induced accumulation of 5-HT is attributable to the inhibition of 5-HT transport via organic cation transporters (OCTs). OCTs are a family of high-capacity, bidirectional, multispecific transporters of organic cations (including 5-HT, dopamine, and norepinephrine) only recently described in brain. In peripheral tissues, organic cation transport via some OCTs is inhibited by corticosterone. We examined the expression and function of OCTs in the periventricular medial hypothalamus of male Sprague Dawley rats using reverse-transcriptase (RT)-PCR, immunohistochemistry, and in vitro transport assays. RT-PCR revealed expression of OCT3 mRNA, but not OCT1 or OCT2 mRNA, in the medial hypothalamus. OCT3-like immunoreactivity was observed in ependymal and glial-like cells in the DMH. Acutely prepared minces of rat medial hypothalamic tissue accumulated the OCT substrates [3H]-histamine and [3H]-N-methyl-4-phenylpyridinium ([3H]-MPP+). Consistent with the pharmacological profile of OCT3, corticosterone, 5-HT, estradiol, and the OCT inhibitor decynium22 dose-dependently inhibited histamine accumulation. Corticosterone and decynium22 also inhibited efflux of [3H]-MPP+ from hypothalamic minces. These data support the hypothesis that corticosterone-induced inhibition of OCT3 mediates stress-induced accumulation of 5-HT in the DMH and suggest that corticosterone may acutely modulate physiological and behavioral responses to stressors by altering serotonergic neurotransmission in this brain region.

Plasma steroid concentrations change in response to sexual behavior in Bufo marinus.

Steroid hormone concentrations change in response to social or environmental stimuli in many vertebrates. To test this phenomenon in an amphibian, we examined plasma androgen (A) and corticosterone (B) concentrations in male marine toads (Bufo marinus), a tropical species exhibiting intermale competition, amplectic clasping of females, and bouts of breeding behavior following rains. When males clasped females for 0, 1, 2, or 3 hr, plasma A concentrations were significantly and positively correlated with hours spent in amplexus. In field-sampled males, plasma A concentrations were higher in amplexing males than in single males. Among single males those found closer to breeding ponds had higher A concentrations than those more distant. These data support the hypothesis that sexual stimuli enhance plasma A concentrations in this amphibian. In 3-hr experimental tests and field-sampled males, B concentrations were higher in amplexing than in single males. Unlike some amphibians, short-term elevations of B apparently are not associated with decreased reproductive function. However, as in other amphibians in which high B concentrations are associated with stress-induced inhibition of reproduction, after 48-72 hr in captivity male toads showed high B concentrations and low plasma androgen concentrations. The bursts of sexual activities exhibited by B. marinus following heavy rains were associated with no changes in A concentration and with increased B concentration.

Distribution of organic cation transporter 3, a corticosterone-sensitive monoamine transporter, in the rat brain

Organic cation transporter 3 (OCT3) is a high‐capacity, low‐affinity transporter that mediates bidirectional, sodium‐independent transport of dopamine, norepinephrine, epinephrine, serotonin, and histamine. OCT3‐mediated transport is directly inhibited by corticosterone, suggesting a potential role for the transporter in mediating some of the effects of stress and glucocorticoids on monoaminergic neurotransmission. To elucidate the importance of OCT3 in clearance of extracellular monoamines in the brain, we used immunohistochemical techniques to describe the distribution of OCT3‐like‐immunoreactive (OCT3‐ir) cells throughout the rostrocaudal extent of adult male rat brains. OCT3‐ir cell bodies were widely distributed throughout the brain, with the highest densities observed in the superior and inferior colliculi, islands of Calleja, subiculum, lateral septum, lateral and dorsomedial hypothalamic nuclei, and granule cell layers of the main and accessory olfactory bulbs, the cerebellum, and the retrosplenial granular cortex. OCT3‐ir cells and/or fibers were also observed in circumventricular organs, and OCT3‐ir ependymal cells were observed in the linings of all cerebral ventricles. The widespread distribution of OCT3‐ir cell bodies, including regions receiving dense monoaminergic projections, suggests an important role for this transporter in regulating extracellular concentrations of monoamines in the rat brain and is consistent with the hypothesis that corticosterone‐induced inhibition of OCT3‐mediated transport may contribute to effects of acute stress or corticosterone on monoaminergic neurotransmission. J. Comp. Neurol. 512:529–555, 2009.

Heterogeneity of Hippocampal GABAA Receptors: Regulation by Corticosterone

Chronic stressors produce changes in hippocampal neurochemistry, neuronal morphology, and hippocampal-dependent learning and memory processes. In rats, stress-induced changes in CA3 apical dendritic structure are mediated by corticosterone (CORT) acting, in part, on excitatory amino acid neurotransmission. CORT also alters GABA-mediated inhibitory neurotransmission, so the GABAA receptor system may also contribute to dendritic remodeling and other stress-related changes in hippocampal function. A previous study indicated that chronic CORT treatment produces complex changes in GABAA receptor subunit mRNA levels, so we hypothesized that CORT alters the pharmacological properties of hippocampal GABAA receptors. To test this, adult male rats were treated with CORT or vehicle pellets for 10 d, after which we quantified [35S]t-butylbicyclophosphorothionate ([35S]TBPS) and [3H]flunitrazepam binding to GABAAreceptors using in vitro receptor autoradiography. Pharmacological properties of receptors were assessed by examining the allosteric regulation of binding at both sites by GABA and 5α-pregnane-3α,21-diol-20-one (THDOC), an endogenous anxiolytic steroid. We found striking regional differences in the modulation of [35S]TBPS binding, particularly between strata radiatum and strata oriens, suggesting a functional heterogeneity among hippocampal GABAA receptors even within the apical versus basal dendrites of pyramidal neurons. Furthermore, we found that CORT treatment decreased the negative modulation of hippocampal [35S]TBPS binding by both GABA and THDOC and increased the enhancement of [3H]flunitrazepam binding by GABA and THDOC in the dentate gyrus. Together, these data suggest that prolonged exposure to stress levels of corticosteroids may alter hippocampal inhibitory tone by regulating the pharmacological properties of GABAA receptors in discrete dendritic subfields.

Rapid changes in monoamine levels following administration of corticotropin-releasing factor or corticosterone are localized in the dorsomedial hypothalamus.

Monoaminergic systems are important modulators of the neuroendocrine, autonomic, and behavioral responses to stress-related stimuli. The male roughskin newt (Taricha granulosa) was used as a model system to investigate the effects of corticotropin-releasing factor (CRF) or corticosterone administration on tissue concentrations of norepinephrine, epinephrine, dopamine, 3,4-dihydroxyphenylacetic acid, serotonin, and 5-hydroxyindoleacetic acid (5-HIAA) in microdissected brain areas. Intracerebroventricular infusion of 25 or 50 ng of CRF increased locomotor activity and site-specifically increased dopamine concentrations within the dorsomedial hypothalamus 30 min after treatment when compared to vehicle-treated controls. In further studies, male newts were treated as follows: (1) no injection, no handling, (2) saline injection, or (3) 10 microg corticosterone and then placed in a novel environment. Monoamine and monoamine metabolite concentrations were similar in the unhandled and saline-injected controls 20 min after treatment. In contrast, corticosterone-injected newts had elevated concentrations of dopamine, serotonin, and 5-HIAA in the dorsomedial hypothalamus (a region that contains dopamine- and serotonin-accumulating neuronal cell bodies in representatives of all vertebrate classes) but not in several other regions studied. These site-specific neurochemical effects parallel neurochemical changes observed in the dorsomedial hypothalamic nucleus of mammals following exposure to a variety of physical and psychological stress-related stimuli. Therefore, these changes may reflect highly conserved, site-specific neurochemical responses to stress and stress-related neurochemicals in vertebrates. Given the important role of the dorsomedial hypothalamus in neuroendocrine, autonomic, and behavioral responses to stress, and a proposed role for this region in fast-feedback effects of glucocorticoids on the hypothalamo-pituitary-adrenal axis, these stress-related monoaminergic changes are likely to have important physiological or behavioral consequences.

A new function of the leptin receptor: mediation of the recovery from lipopolysaccharide-induced hypothermia

Obese (f/f) Koletsky rats lack the leptin receptor (LR), whereas their lean (F/?) counterparts bear a fully functional LR. By using f/f and F/? rats, we studied whether the LR is involved in lipopolysaccharide (LPS)‐induced fever and hypothermia. The body temperature responses to LPS (10 or 100 µg/kg iv) were measured in Koletsky rats exposed to a thermoneutral (28°C) or cool (22°C) environment. Rats of both genotypes responded to LPS with fever at 28°C and with dose‐dependent hypothermia at 22°C. The fever responses of the f/f and F/? rats were identical. The hypothermic response of the f/f rats was markedly prolonged compared with that of the F/? rats. The prolonged hypothermic response to LPS in the f/f rats was accompanied by enhanced NF‐κB signaling in the hypothalamus and an exaggerated rise in the plasma concentration of tumor necrosis factor (TNF)‐α. The f/f rats did not respond to LPS with an increase in the plasma concentration of corticosterone or adrenocorticotropic hormone, whereas their F/? counterparts did. The hypothermic response to TNF‐α (80 μg/kg iv) was markedly prolonged in the f/f rats. These data show that the LR is essential for the recovery from LPS hypothermia. LR‐dependent mechanisms of the recovery from LPS hypothermia include activation of the anti‐inflammatory hypothalamo‐pituitary‐adrenal axis, inhibition of both the production and hypothermic action of TNF‐α, and suppression of inflammatory (via NF‐κB) signaling in the hypothalamus.

Context-specific territorial behavior in urban birds: No evidence for involvement of testosterone or corticosterone

Testosterone produced by the gonads is a primary mediator of seasonal patterns of territoriality and may directly facilitate territorial behavior during an encounter with a potential intruder. Costs and benefits associated with territoriality can vary as a function of habitat, for example through differences in resource distribution between areas occupied by different individuals. We investigated behaviors in response to simulated territorial intrusions (hereafter territorial behaviors) in urban (Phoenix, Arizona) and nearby desert populations of two Sonoran Desert birds (Curve-billed Thrasher and Aberts Towhee). We also examined the degree to which these behaviors are mediated by testosterone (T) and the adrenal steroid, corticosterone (CORT), which can interact with T in territorial contexts. In both species, urban birds displayed more territorial behaviors than their desert conspecifics, but this difference was not associated with variation in either plasma total or in plasma free (i.e., unbound to binding globulins) T or CORT. In addition, neither plasma T nor plasma CORT changed as a function of duration of the simulated territorial intrusion. Urban Aberts Towhees displayed more territorial behaviors in areas where their population densities were high than in areas of low population densities. Urban Curve-billed Thrashers displayed more territorial behaviors in areas with a high proportion of desert-type vegetation, particularly in areas that differed in vegetation composition from nearby randomly sampled areas, than in areas with a high proportion of exotic or non-desert type vegetation. Associations between territorial behavior and habitat characteristics were not related to plasma T or CORT. Understanding the hormonal processes underlying these associations between behavior and habitat may provide insight into how free-ranging animals assess territorial quality and alter their defensive behavior accordingly.