Hormones of choice: the neuroendocrinology of partner preference in animals.

Partner preference behavior can be viewed as the outcome of a set of hierarchical choices made by an individual in anticipation of mating. The first choice involves approaching a conspecific verses an individual of another species. As a rule, a conspecific is picked as a mating partner, but early life experiences can alter that outcome. Within a species, an animal then has the choice between a member of the same sex or the opposite sex. The final choice is for a specific individual. This review will focus on the middle choice, the decision to mate with either a male or a female. Available data from rats, mice, and ferrets point to the importance of perinatal exposure to steroid hormones in the development of partner preferences, as well as the importance of activational effects in adulthood. However, the particular effects of this hormone exposure show species differences in both the specific steroid hormone responsible for the organization of behavior and the developmental period when it has its effect. Where these hormones have an effect in the brain is mostly unknown, but regions involved in olfaction and sexual behavior, as well as sexually dimorphic regions, seem to play a role. One limitation of the literature base is that many mate or 'partner preference studies' rely on preference for a specific stimulus (usually olfaction) but do not include an analysis of the relation, if any, that stimulus has to the choice of a particular sexual partner. A second limitation has been the almost total lack of attention to the type of behavior that is shown by the choosing animal once a 'partner' has been chosen, specifically, if the individual plays a mating role typical of its own sex or the opposite sex. Additional paradigms that address these questions are needed for better understanding of partner preferences in rodents.

Coarse-grained protein-protein stiffnesses and dynamics from all-atom simulations.

Large protein assemblies, such as virus capsids, may be coarse-grained as a set of rigid units linked by generalized (rotational and stretching) harmonic springs. We present an ab initio method to obtain the elastic parameters and overdamped dynamics for these springs from all-atom molecular-dynamics simulations of one pair of units at a time. The computed relaxation times of this pair give a consistency check for the simulation, and we can also find the corrective force needed to null systematic drifts. As a first application we predict the stiffness of an HIV capsid layer and the relaxation time for its breathing mode.

Exogenous androgen during development alters adult partner preference and mating behavior in gonadally intact male rats.

In the rat, neonatal administration of testosterone propionate to a castrated male causes masculinization of behavior. However, if an intact male is treated neonatally with testosterone (hyper-androgen condition), male sexual behavior in adulthood is disrupted. There is a possibility that the hyper-androgen treatment is suppressing male sexual behavior by altering the male's partner preference and thereby reducing his motivation to approach the female. If so, this would suggest that exposure to supra-physiological levels of androgen during development may result in the development of male-oriented partner preference in the male. To test this idea, male rats were treated either postnatally or prenatally with testosterone, and partner preference and sexual behavior were examined in adulthood. The principal finding of this study was that increased levels of testosterone during early postnatal life, but not prenatal, decreased male sexual behavior and increased the amount of time a male spent with a stimulus male, without affecting the amount of time spent with a stimulus female during partner preference tests. Thus, the reduction in male sexual behavior produced by early exposure to high levels of testosterone is not likely due to a reduction in the male's motivation to approach a receptive female.

Estrogen treatment during development alters adult partner preference and reproductive behavior in female laboratory rats.

There is broad acceptance for the idea that during development estradiol 'organizes' many aspects of reproductive behavior including partner preferences in the laboratory rat. With respect to partner preference, this idea is drawn from studies where estrogen action was in someway blocked, either through aromatase or estrogen receptor inhibition, during development in male rats. The lack of estrogens neonatally results in a decrease in the male rat's preference for females. In this study, the effect of early postnatal estradiol treatment on the partner preferences of female rats was examined as a further test of the hypothesis that male-typical partner preference is dependent upon early exposure to estrogens. Our principal finding was that increased postnatal estradiol exposure during development affected partner preference in the expected direction, and this effect was seen under several adult hormonal and behavioral testing conditions. Female rats that received exogenous estradiol during development spent more time with an estrous female and less time with a sexually active male than did cholesterol treated females. The estradiol treatment also disrupted normal female sexual behavior, receptivity, and proceptivity.

Irreversible growth model for virus capsid assembly.

We model the spontaneous assembly of a capsid (a virus' closed outer shell) from many copies of identical units, using entirely irreversible steps and only information local to the growing edge. Our model is formulated in terms of (i) an elastic Hamiltonian with stretching and bending stiffness and a spontaneous curvature, and (ii) a set of rate constants for the addition of new units or bonds. An ensemble of highly irregular capsids is generated, unlike the well-known icosahedrally symmetric viruses, but (we argue) plausible as a way to model the irregular capsids of retroviruses such as HIV. We found that (i) the probability of successful capsid completion decays exponentially with capsid size; (ii) capsid size depends strongly on spontaneous curvature and weakly on the ratio of the bending and stretching elastic stiffnesses of the shell; (iii) the degree of localization of Gaussian curvature (a measure of facetedness) depends heavily on the ratio of elastic stiffnesses.

Semiclassical ordering in the large-N pyrochlore antiferromagnet.

We study the semiclassical limit of the Sp(N) generalization of the pyrochlore lattice Heisenberg antiferromagnet by expanding about the N --> infinity saddlepoint in powers of a generalized inverse spin. To leading order, we write down an effective Hamiltonian as a series in loops on the lattice. Using this as a formula for calculating the energy of any classical ground state, we perform Monte Carlo simulations and find a unique collinear ground state. This state is not a ground state of linear spin-wave theory, and can therefore not be a physical (N = 1) semiclassical ground state.