A phase plane graph based model of the ovulatory cycle lacking the "positive feedback" phenomenon

Abstract

When hormones during the ovulatory cycle are shown in phase plane graphs, reported FSH and estrogen values form a specific pattern that resembles the leaning “&" symbol, while LH and progesterone (Pg) values form a "boomerang" shape. Graphs in this paper were made using data reported by Stricker et al. [Clin Chem Lab Med 2006;44:883–887]. These patterns were used to construct a simplistic model of the ovulatory cycle without the conventional "positive feedback" phenomenon. The model is based on few well-established relations:hypothalamic GnRH secretion is increased under estrogen exposure during two weeks that start before the ovulatory surge and lasts till lutheolysis.the pituitary GnRH receptors are so prone to downregulation through ligand binding that this must be important for their function.in several estrogen target tissue progesterone receptor (PgR) expression depends on previous estrogen binding to functional estrogen receptors (ER), while Pg binding to the expressed PgRs reduces both ER and PgR expression.Some key features of the presented model are here listed:High GnRH secretion induced by the recovered estrogen exposure starts in the late follicular phase and lasts till lutheolysis. The LH and FSH surges start due to combination of accumulated pituitary GnRH receptors and increased GnRH secretion. The surges quickly end due to partial downregulation of the pituitary GnRH receptors (64% reduction of the follicular phase pituitary GnRH receptors is needed to explain the reported LH drop after the surge). A strong increase in the lutheal Pg blood level, despite modest decline in LH levels, is explained as delayed expression of pituitary PgRs. Postponed pituitary PgRs expression enforces a negative feedback loop between Pg levels and LH secretions not before the mid lutheal phase.Lutheolysis is explained as a consequence of Pg binding to hypothalamic and pituitary PgRs that reduces local ER expression. When hypothalamic sensitivity to estrogen is diminished due to lack of local ERs, hypothalamus switches back to the low GnRH secretion rate, leading to low secretion of gonadotropins and to lutheolysis. During low GnRH secretion rates, previously downregulated pituitary GnRH receptors recover to normal levels and thus allow the next cycle.Possible implications of the presented model on several topics related to reproductive physiology are shortly discussed with some evolutionary aspects including the emergence of menopause.

Dynamic Processes in Regulation and Some Implications for Biofeedback and Biobehavioral Interventions

“…A prime example is anxiety and depression’s propensity to amplify their inherent symptomology by triggering greater sensitivity to anxious or depressive thoughts, thus creating a vicious cycle. On the cellular level, positive feedback loops may be necessary to stimulate modulatory negative feedback loops (Kurbel, 2012; Nishi et al, 2000; Pomerening, Kim, & Ferrell, 2005; Prochazka, Gillard, & Bennett, 1997; Tsai et al, 2008). …”

“…Positive feedback loops have been shown to promote load compensation in control of movement (Prochazka et al, 1997), and to be necessary to prevent dampening of oscillations in cellular function (Pomerening et al, 2005). Positive feedback loops also are important for the maintenance of the ovulatory cycle (Kurbel, 2012), and for propagating dopaminergic signaling (Nishi et al, 2000). Without some form of regular perturbations, oscillations will gradually decline in amplitude and disappear.…”