Regulation of the Estrous Cycle by Neutrophils via Opioid Peptides

Abstract

We found previously that neutrophil-depleted mice exhibited significant blockading of both the regular estrous cycle and cyclic changes of steroid hormone levels. In this study, we aimed at elucidation of the underlying mechanism. To examine the possibility that an increase in bacteria in the vaginal vault of neutrophil-depleted mice causes blockading of the estrous cycle, we treated neutrophil-depleted mice with antibiotics but failed to restore the estrous cycle. We then examined another possibility that neutrophils regulate the estrous cycle via opioid peptides, because opioid peptides regulate steroidogenesis in theca and granulosa cells in the ovaries, and because neutrophils contain opioid peptides. In support of this possibility, naloxone, an opioid antagonist, blocked the estrous cycle and a μ opioid receptor agonist restored the estrous cycle in neutrophil-depleted mice. Pro-opiomelanocortin was immunohistochemically detected in peripheral blood neutrophils but not in ones that had infiltrated into the ovaries. i.v. injection of anti–MIP-2 polyclonal Ab caused blockading of the estrous cycle, whereas MIP-2 was detected in the ovaries, suggesting a role of MIP-2 in the regulation of the estrous cycle. Moreover, i.v. injection of MIP-2 decreased the pro-opiomelanocortin signal in peripheral blood neutrophils and caused blockading of the estrous cycle. Together, these results suggest that neutrophils maintain the estrous cycle via opioid peptides.

The Novel Roles of Neutrophils Via Opioid Peptides: Regulation of the Estrous Cycle and Pain

“…The results were exactly as we predicted (Sasaki et al 2011). The estrous cycle length in ICR mice was 4.7 ± 0.8 days (n = 54), whereas those in anti-Gr-1 mAb-injected mice and naloxone-treated ones were prolonged to 7.7 ± 2.3 (n = 67) and 7.9 ± 2.0 days (n = 7), respectively.…”

“…The backup mechanism is perhaps the same as a central mechanism described above (see ''Summary and Future Direction''), although it is not known at present. Our subsequent study provided several lines of evidence supporting the notion that upon the entry of neutrophils into the ovaries, they secrete opioid peptides to regulate the estrous cycle (Sasaki et al 2011).…”

“…Additionally, the estrogen and progesterone levels in plasma were affected in naloxone-treated mice in the same way as in anti-Gr-1 mAb-injected ones (Sasaki et al 2011). It is noteworthy that when naloxone was administered only once at estrus, but not other phases, the estrous cycle was blocked (Sasaki et al 2011), confirming the previous finding with anti-Gr-1 mAb that the estrus phase is the most critical among the four phases for neutrophil-dependent regulation of the estrous cycle (Sasaki et al 2009). These results are consistent with the hypothesis that neutrophils may secrete opioid peptides in the ovaries at estrus, thereby regulating the estrous cycle.…”

“…We then examined from which blood vessels neutrophils enter the female genital organs but we failed to obtain definitive results (Sasaki et al unpublished). In the meantime, we came across publications reporting that opioid peptides regulate steroidogenesis in theca cells and granulose cells in the ovaries (Kaminski et al 2003(Kaminski et al , 2004, and we finally hypothesized that neutrophils may secrete opioid peptides in the ovaries, thereby regulating the estrous cycle (Sasaki et al 2011). …”

“…We thus employed type-specific receptor agonists to examine whether or not they shorten (normalize) the estrous cycle length in anti-Gr-1 mAb-injected mice. The administration of [DAla 2 , N-MePhe 4 , Gly-ol 5 ]-enkephalin (DAMGO), a l opioid receptor agonist, to neutrophil-depleted mice shortened the cycle length to 6.0 ± 0.7 days (n = 5), the difference between anti-Gr-1 mAb-injected mice and DAMGO-administered, anti-Gr-1 mAb-injected ones being statistically significant (Sasaki et al 2011). Such an effect was not observed with (5a, 7a, 8b)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1-oxaspiro(4,5)dec-8-yl)benzenacetamide (U -69593), a j opioid receptor agonist, or [D-Pen 2,5 ]-enkephalin (DPDPE), a d opioid receptor agonist (Sasaki et al 2011).…”