Effects of Experimentally Elevated Testosterone on Plasma Corticosteroid Binding Globulin in Dark-Eyed Juncos (\u3cem\u3eJunco hyemalis\u3c/em\u3e)

An earlier study of free-living male dark-eyed juncos found an increase in plasma corticosterone (B) in response to experimental elevation of plasma testosterone (T) (E. D. Ketterson et al., 1991, Horm. Behav. 25, 489-503). To investigate whether the increase was caused by enhanced secretion of corticosterone or by a slower clearance rate, or both, we exposed 52 captive yearling male dark-eyed juncos (Junco hyemalis) to day lengths corresponding to those of spring and implanted them with one or two testosterone-filled or sham implants (10 T-I, 22 T-II, and 20 C-males). We then examined the effect of experimentally elevated testosterone on plasma corticosterone and on corticosteroid-binding globulin (CBG), as measured by the ability of steroid-stripped plasma to bind labeled corticosterone. Plasma samples were taken five times, 2 weeks before experimental prolongation of day length and approximately every 3 weeks thereafter. Treatment with testosterone increased both plasma testosterone and plasma corticosterone two to three times above control levels, and the degree of elevation was dose-dependent. Only when all treatment groups were pooled, however, were plasma testosterone and corticosterone significantly correlated. The relationship between plasma corticosterone and time required to bleed the birds was similar for all three treatment groups, suggesting that there was no effect of treatment on the stress response. Testosterone significantly increased the capacity of the plasma to bind corticosterone, presumably because it contained more CBG, when compared to the plasma of controls. However, treatment with testosterone did not affect the affinity of the plasma for corticosterone. It seems likely that exogenous testosterone elevated corticosterone by slowing the corticosterone clearance rate via an increase in CBG. It is not clear what the net effect of chronic elevation of testosterone would be on the availability of corticosterone to target tissues

Relating endocrinology, physiology and behaviour using species with alternative mating strategies

1. Recent reviews demonstrate that genetically determined alternative mating strategies (AMS) are widespread and typically consist of morphs that are recognized by morphological or colour traits. Despite well-established behavioural differences associated with each morph, and evidence that androgens are involved in the induction of morphs, few studies have examined whether morphs also vary in whole-organismal performance traits, which may affect dominance status, resource holding potential (RHP) or mate attraction. 2. Our survey revealed a link between androgens and physiological performance traits that are associated with territorial or courtship displays across vertebrate taxa, although the number of species in the sample is limited. Experimental elevation of testosterone alters muscular contractile properties, swimming performance, sprint speed and endurance in males. Whether morphs differ in physiological capacities is relatively unexplored, although recent studies have found that males with high dominance status also exhibit greater physiological capacities (locomotor performance, call duration). 3. Multiple studies support the hypothesis that elevated testosterone results in fitness trade-offs. Potential costs of testosterone include impaired immune function, higher parasite loads, greater energetic requirements and ultimately reduced survival. Long term studies of Uta stansburiana highlight the trade-offs among life-history traits induced by variation in testosterone. Circumstantial evidence suggests a role of testosterone in depressing immune function in species with AMS. 4. We argue that hypotheses regarding the role of trade-offs in shaping selection on functional modules, which are involved in sexual selection, are best developed by manipulative studies on discrete morphs. Our review highlights the need to measure multiple traits to provide additional insights into the roles of sexual selection and physiological epistasis in maintaining intraspecific variation in reproductive phenotypes. The integration of endocrine control of behaviour, physiology and performance is rarely attempted in most studies and may be facilitated by analyses that focus on estimating correlational selection

An associativity requirement for locus coeruleus-induced long-term potentiation in the dentate gyrus of the urethane-anesthetized rat.

Contains fulltext : 88103reid.pdf (publisher's version ) (Closed access)Norepinephrine has been hypothesized to provide a learning and memory signal. Norepinephrine long-term potentiation of perforant path input to the dentate gyrus of the hippocampus provides a model for norepinephrine initiated memory processes. However, in vitro, the pairing of perforant path stimulation and norepinephrine is not required for the occurrence of norepinephrine-dependent long-term potentiation. Since bath application of norepinephrine induces long-term changes in 2nd messenger signalling and differs in a number of ways from physiological norepinephrine release, the present study is an in vivo test of the associative requirement for the pairing of perforant path input with norepinephrine to induce long-term potentiation. Phasic activation of the locus coeruleus is provided by glutamate infusion into the locus coeruleus to initiate transient norepinephrine release in the hippocampus of urethane-anesthetized Sprague-Dawley rats. Perforant path stimulation (0.067 Hz) was given throughout the experiment in the paired condition. In the unpaired condition perforant path stimulation was interrupted 10 min prior to locus coeruleus activation and resumed 10 min after locus coeruleus activation. Locus coeruleus-induced long-term potentiation of both EPSP slope and population spike only occurred in the pairing condition. This result argues that, in vivo, temporal proximity of locus coeruleus-associated norepinephrine release and perforant path stimulation are required to induce long-term plasticity. The associativity requirement for locus coeruleus activation and perforant path stimulation in vivo is consistent with the hypothesis that norepinephrine can initiate circuit changes supporting learning and memory.1 januari 201