Search Results

The effects of melatonin on reproductive function were examined using hamster spermatozoa. When 1 pM to 1 μM melatonin was added to the mTALP medium, hyperactivation was significantly enhanced. Antagonists and agonists of the melatonin receptor (i.e., MT1 and MT2) were added to the medium. Luzindole, an MT1 and MT2 competitive antagonist, significantly inhibited melatonin-induced hyperactivation, whereas the MT2-specific antagonists, 4-phenyl-2-propionamidotetralin and N-pentanoyl-2-benzyltryptamine, had no effect. Moreover, hyperactivation was significantly enhanced when non-specific agonists, such as 6-chloromelatonin and 2-iodomelatonin, were added to the medium. 8-Methoxy-2-propionamidotetralin, which is a strong MT2 agonist and a weak MT1 agonist, significantly increased hyperactivation, although the effect was weak. Therefore, it is likely that melatonin enhances sperm hyperactivation via the MT1 receptor.

The effects of serotonin on reproductive function were examined using hamster spermatozoa. When serotonin at concentrations from 1 fmol/l to 1 μmol/l was added to modified Tyrode's albumin lactate pyruvate (mTALP) medium, hyperactivation was significantly enhanced. Agonists and antagonists of 5-hydroxytryptamine hydrochloride (5-HT) receptors (5-HT₂ and 5-HT₄ receptors) were added to the medium. Both 5-HT₂ and 5-HT₄ receptor agonists significantly enhanced hyperactivation, although the effect was greater than the former. However, both 5-HT₂ and 5-HT₄ receptor antagonists significantly suppressed serotonin-enhanced hyperactivation, with the former suppressing stimulation by a lower concentration of serotonin than the latter. These results indicate that serotonin enhances hyperactivation via 5-HT₂ and/or 5-HT₄ receptors in a dose-dependent manner.

In this study, I examined whether sperm hyperactivation in hamster is regulated by steroid hormones such as estrogen (estradiol, E₂) and progesterone. Although sperm hyperactivation was enhanced by progesterone, 17β-estradiol (17βE₂) itself did not affect sperm hyperactivation. However, 17βE₂ suppressed progesterone-enhanced hyperactivation in a concentration-dependent manner through non-genomic pathways when spermatozoa were exposed to 17βE₂ at the same time or before exposure to progesterone. When spermatozoa were exposed to 17βE₂ after exposure to progesterone, 17βE₂ did not suppress progesterone-enhanced hyperactivation. Moreover, 17α-estradiol, an inactive isomer of E₂, did not suppress progesterone-enhanced hyperactivation. Observations using a FITC-conjugated 17βE₂ showed that it binds to the acrosome region of the sperm head. Binding of 17βE₂ to spermatozoa was not inhibited by progesterone, although 17βE₂ did not suppress progesterone-enhanced hyperactivation when spermatozoa were exposed to 17βE₂ after exposure to progesterone. On the other hand, binding of progesterone to spermatozoa was also not inhibited by 17βE₂ even if progesterone-enhanced hyperactivation was suppressed by 17βE₂. Although tyrosine phosphorylations of sperm proteins were enhanced by progesterone, enhancement of tyrosine phosphorylations by progesterone was suppressed by 17βE₂. Moreover, tyrosine phosphorylations were inhibited by 17βE₂ when only 17βE₂ was added to the medium. From these results, it is likely that 17βE₂ competitively suppresses progesterone-enhanced hyperactivation through the inhibition of tyrosine phosphorylations via non-genomic pathways.

Abstract

Mammalian sperm motility has to be hyperactivated to be fertilization-competent. Hyperactivation is regulated by extracellular environment. Osmolality of mammalian semen is higher than that in female reproductive tract; however, the effect of them on hyperactivation has not been investigated. So we investigated the effect of osmotic environment on hyperactivation using hamster spermatozoa at first. Increase in the osmolality of the media (∼370 mOsm) by increasing the concentration of NaCl (∼150 mmol/L) caused the delay of the expression of hyperactivation. When NaCl concentration varied in the same range (75–150 mmol/L) whereas the osmolality was fixed at 370 mOsm by adding mannitol, the delay of hyperactivation occurred dependent on NaCl concentration. Increase in NaCl concentration also caused suppression of curvilinear velocity, bend angle, and sliding velocity of the flagellum at the onset of incubation, suggesting that NaCl concentration affect both activation and hyperactivation in hamster spermatozoa. Hamster sperm intracellular Ca²⁺ concentration decreased as extracellular NaCl concentration increased, whereas membrane potential and intracellular pH were unaffected by extracellular NaCl concentration. SN-6 and SEA0400, inhibitors of Na⁺-Ca²⁺ exchanger (NCX), increased intracellular Ca²⁺ and accelerated hyperactivation in the presence of 150 mmol/L NaCl. Tyrosine phosphorylation on fibrous sheath proteins was unaffected by extracellular NaCl concentration. These results suggest that extracellular Na⁺ suppresses hamster sperm hyperactivation by reducing intracellular Ca²⁺ via an action of NCX in a tyrosine phosphorylation-independent manner. It seems that the removal of suppression by extracellular Na⁺ leads to the expression of hyperactivated motility.

It has been widely accepted that serine/threonine protein phosphatases (PPPs) are associated with the regulation of sperm hyperactivation. In the present study, we examined the types of PPPs associated with the regulation of hamster sperm hyperactivation. Protein phosphatases PPP1CA, PPP1CC, PPP2, and PPP3 are present in hamster sperm. In the experiments using several inhibitors, sperm hyperactivation was enhanced when PPP2 was inhibited at least, although inhibition of PPP1 also enhanced sperm hyperactivation. Interestingly, sperm were hyperactivated after PPP2 became an inactive form. And then, PPP1CA became an active form after sperm were hyperactivated. It has also been widely accepted that tyrosine phosphorylation is closely associated with the regulation of sperm hyperactivation. When PPP2 was inhibited, tyrosine phosphorylation was not enhanced at all. On the other hand, inhibition of PPP1 enhanced tyrosine phosphorylation. From the results, it is likely that PPP2 is closely associated with the regulation of sperm hyperactivation, although it is not associated with the regulation of tyrosine phosphorylation.