Abstract
Bull spermatozoa physiology may be modulated by melatonin. We washed ejaculated spermatozoa free of melatonin and incubated them (4 h, 38 °C) with 0-pM, 1-pM, 100-pM, 10-nM and 1-µM melatonin in TALP-HEPES (non-capacitating) and TALP-HEPES-heparin (capacitating). This range of concentrations encompassed the effects mediated by melatonin receptors (pM), intracellular targets (nM–µM) or antioxidant activity (µM). Treatment effects were assessed as motility changes by computer-assisted sperm analysis (CASA) of motility and physiological changes by flow cytometry. Melatonin effects were more evident in capacitating conditions, with 100 pM reducing motility and velocity (VCL) while increasing a “slow” subpopulation. All concentrations decreased apoptotic spermatozoa and stimulated mitochondrial activity in viable spermatozoa, with 100 pM–1 µM increasing acrosomal damage, 10 nM–1 µM increasing intracellular calcium and 1 pM reducing the response to a calcium-ionophore challenge. In non-capacitating media, 1 µM increased hyperactivation-related variables and decreased apoptotic spermatozoa; 100 pM–1 µM increased membrane disorders (related to capacitation); all concentrations decreased mitochondrial ROS production. Melatonin concentrations had a modal effect on bull spermatozoa, suggesting a capacitation-modulating role and protective effect at physiological concentrations (pM). Some effects may be of practical use, considering artificial reproductive techniques.
Highlights
IntroductionWhile mainly known for its hormonal activity, transducing photoperiodic cues, it is a multifunctional molecule
Melatonin is a ubiquitous molecule fundamental for animal physiology
We evaluated the changes on the motility pattern, viability, capacitation, mitochondrial activity, intracellular levels of reactive oxygen species (ROS) and other physiological variables, after incubating bull spermatozoa both in capacitating and non-capacitating conditions
Summary
While mainly known for its hormonal activity, transducing photoperiodic cues, it is a multifunctional molecule. It shows diverse effects both at tissue and cell levels [1]. These tissues would be able to respond to locally produced melatonin due to the presence of membrane receptors (MT1 and MT2) [8] It accumulates in secretions and other fluids such as the seminal plasma [9,10,11] and the follicular fluid [12], affecting the physiology of the spermatozoon both within the male and the female
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