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. 2025 Jul;343(6):677-692.
doi: 10.1002/jez.2918. Epub 2025 Apr 9.

Serotonin-Induced Sperm Hyper-Motility In Pacific Oyster (Crassostrea Gigas) Associates With K+ Efflux and Membrane Hyperpolarization

Sayyed Mohammad Hadi Alavi  1   2 , Sepideh Barzegar-Fallah  2   3 , Manabu Yoshida  4 , Ian A E Butts  3 , Makoto Osada  1   5
Affiliations

Serotonin-Induced Sperm Hyper-Motility In Pacific Oyster (Crassostrea Gigas) Associates With K+ Efflux and Membrane Hyperpolarization

Sayyed Mohammad Hadi Alavi et al. J Exp Zool A Ecol Integr Physiol. 2025 Jul.

Abstract

Serotonin (5-HT) induces sperm hyper-motility in bivalves. This process has been suggested to be associated with K+ efflux due to higher concentrations of K+ ions in testicular fluid compared to that of seawater. This hypothesis was supported by inhibition of 5-HT-induced sperm hyper-motility in artificial seawater (ASW) containing high extracellular K+ ions or in the presence of a voltage-dependent K+ channel blocker (4-AP). Here, we studied changes of sperm membrane potential to elucidate 5-HT-induced sperm hyper-motility signaling in Pacific oyster (Crassostrea gigas). Sperm motility was partially initiated (48.34 ± 7.80%) in ASW, and decreased at 5 min post-activation (p < 0.05). In the presence of 10-5 M 5-HT, sperm motility was recorded 81.63 ± 3.55%, which remained unchanged within 60 min post-activation. After sperm activation in ASW with or without 5-HT, fluorescence intensity of membrane potential-sensitive fluorescent (DiSC3(5)) was decreased to lower than that of the resting stage, indicating membrane hyperpolarization. Induction of membrane hyperpolarization, using valinomycin or in K+-free ASW (KF-ASW) could not trigger sperm hyper-motility, suggesting that hyperpolarization itself did not induce sperm hyper-motility. Next, we showed that membrane hyperpolarization was due to K+ efflux. The fluorescence intensity of DiSC3(5) was increased in ASW or KF-ASW containing 4-AP, suggesting membrane depolarization due to inhibition of K+ efflux. The valinomycin-induced membrane hyperpolarization was changed to depolarization by subsequent additions of KCl, suggesting that changes in the electrochemical gradient of K+ ions resulted in the retention of intracellular K+ ions. Observed membrane depolarization in the presence of 4-AP or high K+ ions was associated with inhibition of 5-HT-induced sperm hyper-motility. Taken together, this study shows that 5-HT-induced sperm hyper-motility was associated with membrane hyperpolarization due to K+ efflux.

Keywords: 4‐aminopyridine; K+ efflux; serotonin; sperm hyper‐motility; valinomycin.

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References

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