Effects of gonadal steroids on the opioid regulation of LH and prolactin release in ovariectomized pony mares

The aim of the present study was to investigate the role of ovarian steroids in the opioid regulation of LH and prolactin release in mares. Effects of the opioid antagonist naloxone on LH and prolactin secretion were determined in ovariectomized pony mares. The animals were pretreated with either progesterone (500 mu g kg(-1)) or oestradiol benzoate (10 mu g kg(-1)) for 8 days and subsequently with a combination of progesterone and oestradiol for an additional 8 days. Naloxone administration (0.5 mg kg(-1) i.v.) resulted in a significant release of LH as well as prolactin in mares after pretreatment with either oestradiol benzoate or progesterone plus oestradiol benzoate (P < 0.05). No significant changes in LH and prolactin secretion were detected in progesterone-treated and non-steroid-treated ovariectomized mares. These results indicate that a prolonged oestrogen influence activates the opioid inhibition of LH and prolactin release in mares. In contrast to other species, progesterone alone does not activate a tonic opioid inhibition of LH and prolactin secretion, but modulates the effect of oestrogens. The opioid systems therefore seem to be regulated by a sequence of different steroid environments, as found during the oestrous cycle. The parallel increases in prolactin and LH secretion in mares may indicate a common regulatory pathway for these two hormones

Evidence for opioidergic inhibition of oxytocin release in periparturient mares

In the horse mare, the onset of parturition is associated with an increase in oxytocin secretion, and it has been suggested that the onset of parturition may be triggered by endogenous oxytocin release. To test the hypothesis that oxytocin secretion is regulated by endogenous opioids in the periparturient period, we have 1) characterized oxytocin secretion in response to vaginocervical stimulation and 2) determined the effect of naloxone, an opioid antagonist, on oxytocin secretion induced by vaginocervical stimulation in prepartum mares and in postpartum mares at estrus and diestrus. During the last 2 months of pregnancy, the first diestrus and subsequent estrus post partum, a total of 66 vaginocervical stimulations were performed. Mares were pretreated with naloxone (0.5 mg/kg iv) or saline, administered 20 min before vaginocervical stimulation on subsequent days, using a randomized switchback design in which mares served as their own controls. Plasma was collected from 30 min before until 30 min after stimulation and was analyzed for oxytocin concentrations. Vaginocervical stimulation resulted in a significant increase in oxytocin secretion in all mares. Between Days 30 and 20 prepartum, the total amount of oxytocin secreted (calculated as area under the curve for 0 to 10 min after vaginocervical stimulation) was significantly greater in naloxone-treated than in saline-treated mares. From Day 20 prepartum until parturition, the differences between naloxone and saline-treated mares tended to decrease with approaching parturition, and were no longer statistically different. Peak plasma oxytocin concentrations were greater in naloxone-treated mares than in saline-treated mares during the entire prepartum period. During the postpartum period, total amount of oxytocin secreted following vaginocervical stimulation tended to be greater than during the prepartum period, and stimulated oxytocin secretion was significantly greater in naloxone-treated mares than in saline-treated mares. In conclusion these data suggest that endogenous opioids suppress oxytocin secretion pre and post partum. It appears that opioid inhibition is not limited to the prepartum period, tends to decrease gradually towards parturition and is reinstated after foaling

Simulation based compensation techniques to minimize distortion of thin-walled monolithic aluminum parts due to residual stresses

Thin-walled milled monolithic aluminum workpieces are common in the aerospace industry, because of their advantages such as favorable material properties, established design methods, good machinability, known performance and reliable inspection techniques. Machining-induced residual stresses (MIRS) and initial bulk residual stresses (IBRS) are known to be the key factor for causing distortion of those workpieces. In this study the effect of both types, the milling path strategy and the wall thickness on the distortion is analyzed. A developed 3D linear elastic finite element (FEM) model, considering both types of RS as input, and predicting the resulting part distortion, is extended by using the true milling path as well as all RS contained in the entire workpiece. The simulation outcome is validated by experiments and distortion minimization techniques are derived. The results show that the model predicts the distortion shape and level for different wall thicknesses and milling strategies with a high accuracy. The experiments indicated that the distortion of low IBRS parts with 3 mm wall-thickness could be minimized by 42% by only changing the milling path. The simulation highlighted, that for smaller wall thicknesses, the distortion minimization potential is even higher (e.g. 2 mm wall thickness: up to 69% distortion minimization). For high IBRS parts, milling the inverse form of the predicted shape on the backside of the workpiece represents an alternative, which led up to a reduction of the distortion by 77%. In general, three main categories for precontrol distortion techniques can be identified as the process parameters, the part topology and the process strategy

The Effect of Bulk Residual Stress on Milling-Induced Residual Stress and Distortion

Background: Distortion arises during machining of metallic parts from two main mechanisms: 1) release of bulk residual stress (BRS) in the pre-form, and 2) permanent deformation induced by cut tools. Interaction between these mechanisms is unexplored. Objective: Assess this interaction using aluminum samples that have a flat surface with variations of BRS, where that surface is subsequently milled, and we observe milling-induced residual stress (MIRS) and distortion. Methods: Plate samples are cut from two kinds of large blocks, one kind stress-relieved by stretching and a second kind solution heat treated, quenched and aged. The BRS field in the plates is known from a recent series of measurements, being small in the stress relieved plates (within ±20&nbsp;MPa) and large (±100&nbsp;MPa) in the quenched plates, varying from tension to compression over the surface that is milled. MIRS is measured following milling using hole-drilling. Distortions of thin wafers cut at the milled surfaces are used to elucidate BRS/MIRS interactions. A finite element (FE) model and a strength of materials model are each used to assess consistency between wafer distortion and measured MIRS. Results: Milling in samples with high BRS magnitude changes the directions of MIRS and distortion relative to the milling direction, with the direction of maximum curvature rotating toward or away from the milling direction depending on the sign and direction of BRS. High magnitude BRS was also found to increase the wafer peak arc height, nearly doubling the amount found in low BRS samples. Conclusion: Measured residual stress and observed wafer distortion both show interactions between MIRS and BRS. Stress analysis models show that the differences in measured MIRS are consistent with the differences in observed distortion