Hormonal Pathways Regulating Intermale And Interfemale Aggression

Overview of the Molecular Steps in Steroidogenesis of the GABAergic Neurosteroids Allopregnanolone and Pregnanolone

Allopregnanolone and pregnanolone-neurosteroids synthesized from progesterone in the brain, adrenal gland, ovary and testis-have been implicated in a range of neuropsychiatric conditions including seizure disorders, post-traumatic stress disorder, major depression, post-partum depression, pre-menstrual dysphoric disorder, chronic pain, Parkinson's disease, Alzheimer's disease, neurotrauma, and stroke. Allopregnanolone and pregnanolone equipotently facilitate the effects of gamma-amino-butyric acid (GABA) at GABAA receptors, and when sulfated, antagonize N-methyl-D-aspartate receptors. They play myriad roles in neurophysiological homeostasis and adaptation to stress while exerting anxiolytic, antidepressant, anti-nociceptive, anticonvulsant, anti-inflammatory, sleep promoting, memory stabilizing, neuroprotective, pro-myelinating, and neurogenic effects. Given that these neurosteroids are synthesized de novo on demand, this review details the molecular steps involved in the biochemical conversion of cholesterol to allopregnanolone and pregnanolone within steroidogenic cells. Although much is known about the early steps in neurosteroidogenesis, less is known about transcriptional, translational, and post-translational processes in allopregnanolone- and pregnanolone-specific synthesis. Further research to elucidate these mechanisms as well as to optimize the timing and dose of interventions aimed at altering the synthesis or levels of these neurosteroids is much needed. This should include the development of novel therapeutics for the many neuropsychiatric conditions to which dysregulation of these neurosteroids contributes.

Sex and species differences in plasma testosterone and in counts of androgen receptor-positive cells in key brain regions of Sceloporus lizard species that differ in aggression

We studied neuroendocrine correlates of aggression differences in adults of two Sceloporus lizard species. These species differ in the degree of sex difference in aggressive color signals (belly patches) and in aggression: Sceloporus undulatus (males blue, high aggression; females white, low aggression) and Sceloporus virgatus (both sexes white, lower aggression). We measured plasma testosterone and counted cells expressing androgen receptor-like immunoreactivity to the affinity-purified polyclonal AR antibody, PG-21, in three brain regions of breeding season adults. Male S. undulatus had the highest mean plasma testosterone and differed significantly from conspecific females. In contrast, there was no sex difference in plasma testosterone concentrations in S. virgatus. Male S. undulatus also had the highest mean number of AR-positive cells in the preoptic area: the sexes differed in S. undulatus but not in S. virgatus, and females of the two species did not differ. In the ventral medial hypothalamus, S. undulatus males had higher mean AR cell counts compared to females, but again there was no sex difference in S. virgatus. In the habenula, a control brain region, the sexes did not differ, and although the sex by species interaction significant was not significant, there was a trend (p=0.050) for S. virgatus to have higher mean AR cell counts than S. undulatus. Thus hypothalamic AR cell counts paralleled sex and species differences in aggression, as did mean plasma testosterone levels in these breeding-season animals.

Aggressive interactions differentially modulate local and systemic levels of corticosterone and DHEA in a wild songbird

During the nonbreeding season, when gonadal androgen synthesis is basal, recent evidence suggests that neurosteroids regulate the aggression of male song sparrows. In particular, dehydroepiandrosterone (DHEA) is rapidly converted in the brain to androgens in response to aggressive interactions. In other species, aggressive encounters increase systemic glucocorticoid levels. However, the relationship between aggression and local steroid levels is not well understood. Here, during the breeding and nonbreeding seasons, we tested the effects of a simulated territorial intrusion (STI) on DHEA and corticosterone levels in the brachial and jugular plasma. Jugular plasma is enriched with neurosteroids and provides an indirect index of brain steroid levels. Further, during the nonbreeding season, we directly measured steroid levels in the brain and peripheral tissues. Both breeding and nonbreeding males displayed robust aggressive responses to STI. During the breeding season, STI increased brachial and jugular corticosterone levels and jugular DHEA levels. During the nonbreeding season, STI did not affect plasma corticosterone levels, but increased jugular DHEA levels. During the nonbreeding season, STI did not affect brain levels of corticosterone or DHEA. However, STI did increase corticosterone and DHEA concentrations in the liver and corticosterone concentrations in the pectoral muscle. These data suggest that 1) aggressive social interactions affect neurosteroid levels in both seasons and 2) local steroid synthesis in peripheral tissues may mobilize energy reserves to fuel aggression in the nonbreeding season. Local steroid synthesis in brain, liver or muscle may serve to avoid the costs of systemic increases in corticosterone and testosterone.

Neuroendocrinology of coping styles: towards understanding the biology of individual variation

Individual variation in behavior and physiology is a widespread and ecologically functional phenomenon in nature in virtually all vertebrate species. Due to domestication of laboratory animals, studies may suffer from a strong selection bias. This paper summarizes behavioral, neuroendocrine and neurobiological studies using the natural individual variation in rats and mice. Individual behavioral characteristics appear to be consistent over time and across situations. The individual variation has at least two dimensions in which the quality of the response to a challenging condition (coping style) is independent from the quantity of that response (stress reactivity). The neurobiology reveals important differences in the homeostatic control of the serotonergic neuron and the neuropeptides vasopressin and oxytocin in relation to coping style. It is argued that a careful exploitation of the broad natural and biologically functional individual variation in behavior and physiology may help in developing better animal models for understanding individual disease vulnerability.

Ghrelin's quick inhibition of androgen-dependent behaviors of male house mice (Mus musculus)

Ghrelin is a peptide hormone released by the stomach that stimulates hunger. Ghrelin also suppresses reproductive physiology by inhibiting the HPG axis. However, to our knowledge, our results are the first to demonstrate ghrelin's quick suppression of sex-hormone-regulated behaviors. In experiment 1, 2 orexigenic i.p. ghrelin injections (0.165 mg/kg and 0.33 mg/kg) suppressed male courtship behavior (ultrasonic calling to a female) and intermale aggression (latency to attack a stimulus male) 20 min following administration. Experiment 2 (examining only the 0.33 mg/kg dose ) replicated ghrelin's suppression of ultrasonic calling and intermale aggression; however, a third behavior, preference for volatile female odors (20 min following administration), was not significantly inhibited. In experiment 2, ghrelin treatment did not affect general locomotor activity (distance traveled 20 min following injection) or seminal vesicle weight (measured 5 days after completing ghrelin injections). We hypothesize that ghrelin's quick suppression of male aggression and ultrasonic mating calls was mediated through its effects on the brain (rather than indirectly through inhibition of the HPG axis).