Melatonin Non-Linearly Modulates Bull Spermatozoa Motility and Physiology in Capacitating and Non-Capacitating Conditions

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.

Bos taurus and Cervus elaphus as Non-Seasonal/Seasonal Models for the Role of Melatonin Receptors in the Spermatozoon

“…Previously [ 18 ], we found that melatonin showed little effect on bull sperm motility and even decreased it at 100 nM. Melatonin agonists (PHE and 8M-PDOT) significantly increased total sperm motility ( Figure 5 a) concerning the CTL and CTLm, indicating an activator role for at least the MT 2 receptor (since 8M-PDOT, which is much more specific for MT 2 ).…”

“…This activity resembles results in somatic cells, in which melatonin membrane receptors activate inhibitory G proteins, reducing cAMP levels and decreasing PKA activity [ 27 , 29 ]. In a previous report, our group found an important differential response of bull spermatozoa to a range of melatonin concentrations (1 pM to 1 µM) relative to motility, cell apoptosis, and other physiological parameters, which is compatible with receptor activation due to the low range of concentrations used [ 18 ].…”

“…Once in the oviduct, the spermatozoon seems to be influenced by melatonin present in oviductal secretions but most relevantly in the follicular fluid [ 15 ]. Recent research has confirmed that melatonin could enhance sperm survival [ 16 , 17 , 18 ] and have a modulatory effect on sperm capacitation [ 18 , 19 , 20 , 21 , 22 ], a physiological process enabling the spermatozoon to fertilize the oocyte, eliciting its release from the oviductal epithelium, priming the acrosomal membrane for fusing with the plasma membrane (acrosomal reaction), and modifying the sperm membranes for fusion with the oolemma. These effects seem to be mediated by an action on signal transduction cascades by modifying the intracellular concentrations of second messengers and reactive oxygen species (ROS) [ 23 , 24 ].…”

Melatonin improves rate of monospermic fertilization and early embryo development in a bovine IVF system

“…However, it is necessary to consider that regardless of in vivo or in vitro conditions, a high concentration of capacitated spermatozoa at the fertilization site is considered to be involved in an increased proportion of polyspermy [ 38 ]. Under capacitating conditions bull spermatozoa exposed to melatonin had reduced progressive motility and curvilinear velocity (VCL), inhibiting sperm capacitation [ 39 ]. Modulation of sperm capacitation by melatonin has been linked to reduced cAMP levels [ 40 ] and presumably could at least in part explain the reduced percentage of polyspermic zygotes after melatonin treatment found in our study.…”

Melatonin in male and female fertility

“…A recent study done on bull spermatozoa suggested that melatonin has capacitation-modulating effect and protective action at physiological concentrations (44).…”