Long time molecular dynamics for enhanced conformational sampling in biomolecular systems.

Although molecular dynamics methods are commonly used to drive biomolecular simulations, the technique provides insufficient sampling to impact studies of the 200-300 residue proteins of greatest interest. One severe limitation of molecular dynamics is that the integrators are restricted by resonance phenomena to small time steps (Delta t<8 fs) much slower then the time scales of important structural and solvent rearrangements. Here, a novel set of equations of motion and a reversible, resonance-free, integrator are designed which permit step sizes on the order of 100 fs to be used.

Basal and PAF-, interleukin 1-, ether stress-induced hypothalamic pituitary adrenal secretion of conscious rat: modulation by PAF antagonists.

We have previously shown that exogenous (1 to 5 nmol i.c.v.) PAF induces a rapid increase in plasma ACTH and beta endorphin followed by an increase in plasma corticosterone in conscious rats. The stimulatory action of PAF on the secretion of hypothalamic-pituitary-adrenal (HPA) axis products is mediated at least partly by stimulating hypothalamic CRF release. In addition rat hypothalamic membranes have two populations of specific PAF binding sites. In order to clarify the mode of PAF action on the stress-related hormones, we have now investigated the effect of two PAF antagonists, BN 50739 and RP 52770, on basal and PAF-induced ACTH and corticosterone secretion by conscious rats and on PAF specific binding to rat hypothalamic membranes. The role of PAF as a mediator of neuroendocrine secretion in response to acute stress was examined by determining the effect of PAF antagonists on ether-stress inducing HPA activity. We have also investigated their effect on IL 1-induced HPA activity. The ability of BN 50739 and RP 52770 to displace 3H PAF from its hypothalamic binding sites was correlated with their ability to alter basal hormone secretion and to counteract the PAF-stimulated secretion of HPA axis hormones in vivo (P less than 0.05 by ANOVA). Pretreatment with BN 50739. (50 nmol i.c.v.) did not alter ACTH response to a 1 min ether exposure or to IL1 beta injection (2 nmol i.c.v.). In contrast, RP 52770 (55 nmol i.c.v.) significantly inhibited the ether stress-induced ACTH and corticosterone production by 50% (P less than 0.05). In parallel, pretreatment with RP 52770 (55 nmol i.c.v.) caused a significant inhibition of IL1 beta-induced ACTH secretion. These results suggest that PAF acts, in vivo, on ACTH and corticosterone secretion, through a centrally mediated CRF dependent mechanism involving PAF receptor sites. Additionally, the data also indicate that PAF could have a central role in mediating basal and stress-induced ACTH secretion and that IL 1-induced HPA secretion may be mediated at least in part through the production of PAF.

Intracerebroventricular injection of platelet-activating factor induces secretion of adrenocorticotropin, beta-endorphin and corticosterone in conscious rats: a possible link between the immune and nervous systems.

To investigate whether platelet-activating factor (PAF) exerts an indirect action on immune cells by altering the secretion of hypothalamic-pituitary-adrenal (HPA) axis products, the effects of intracerebroventricular (i.c.v.) PAF on adrenocorticotropic hormone (ACTH), beta-endorphin and corticosterone blood levels were examined in adult male rats. Hormones were radioimmunoassayed on blood samples from conscious or ether-anesthetized rats after i.c.v. injection of PAF or vehicle into the left lateral ventricle. PAF induced significant increases in these stress-related hormones under both, basal and ether-induced stress conditions. The analysis of the time course response to PAF of hormone release into the blood of unrestrained rats revealed that: i.c.v. injection of 5.4 nmol PAF resulted in rapid increases in ACTH and beta-endorphin, at the latest within 15 min after the onset of injection. The maximal response of both hormones was reached within 45 min after the onset of injection and was followed by an elevation of plasma corticosterone. Hormone release is related to the PAF dose infused, the lowest effective PAF concentration was 1 nmol. The stimulatory effect of PAF on ACTH and beta-endorphin secretion was strongly decreased in rats previously treated with purified anti-rat corticotropin-releasing factor (CRF) antibody. These results, associated with the in vitro demonstration that PAF increases CRF release from incubated rat median eminence, strongly support the hypothesis that the stimulatory action of PAF on the secretion of HPA axis products is mediated at least partly, by stimulating hypothalamic CRF release.(ABSTRACT TRUNCATED AT 250 WORDS)

Leukotriene C4-induced release of LHRH into the hypophyseal portal blood and of LH into the peripheral blood.

Intracerebroventricular (i.c.v.) administration of leukotriene (LT) C4 at doses of 2, 0.5 and 0.2 micrograms/rat significantly stimulated (3-12 fold) the release of LH into the peripheral blood of male rats. Injection of anti-LHRH serum had no effect on LTC4-stimulated LH release, but did block PGE2- stimulated LH release. I.c.v.- infused LTC4 also stimulated the release of LHRH into the hypophyseal portal blood. This is the first report of an in vivo action of LTC4 on the release of a hypothalamic releasing factor (LHRH) and a pituitary hormone (LH). These observations, plus in vitro results, clearly show that LTC4 stimulates LH release by acting on both the hypothalamus, causing LHRH release, and on the pituitary. Then the action of LTC4 on LH release in vivo is quite different from the indirect action of PGE2.