Naloxone supplementation during vitrification of equine in vitro matured oocytes after overnight holding: insights from a comparative study with the bovine model.

Naloxone supplementation was tested for its ability to improve cryopreservation outcomes of equine mature oocytes held overnight before vitrification, using bovine oocytes as a comparative model. The study found that while naloxone showed antioxidant activity, it did not enhance equine oocyte survival or development post-vitrification, and bovine oocytes were not reliable predictors of equine responses to naloxone.

Background and Purpose

• Cryopreservation of equine mature oocytes is challenging due to limited sample availability and the need for overnight holding before vitrification.
• Naloxone (NX) is an opioid receptor antagonist with reported antioxidant effects, which might protect oocytes during vitrification.
• The study aimed to evaluate if naloxone supplementation improves vitrification outcomes of equine oocytes held overnight and to test if bovine oocytes could serve as a reliable preliminary model for this research.

Experimental Design

• Experiment 1 (Bovine oocytes):
  • Immature (bGV) and mature (bMII) bovine oocytes were vitrified with or without naloxone (VIT vs. VIT-NX).
  • Assessments included protocol efficiency and potential to model equine oocytes without overnight holding.
• Experiment 2 (Equine oocytes):
  • In vitro matured equine oocytes held overnight (eMII) were vitrified with or without naloxone.
  • Post-warming analyses measured viability, reactive oxygen species (ROS), glutathione (GSH), mitochondrial membrane potential (HMMP), and developmental competence after intracytoplasmic sperm injection (ICSI).
  • Expression of apoptosis-related genes (BCL2, BAX, p53, survivin) by qRT-PCR was also evaluated.

Key Findings in Bovine Oocytes

• Naloxone supplementation did not affect vitrification efficiency or maturation rates in bovine oocytes.
• Vitrification at the mature MII stage resulted in a significantly higher proportion of viable oocytes post-warming compared to immature (bGV stage) vitrification (46.1% vs 13.5%).
• Findings indicated that bovine MII oocytes without overnight holding might serve as a reasonable model for comparing vitrification protocols.

Key Findings in Equine Oocytes

• Naloxone negatively affected the viability of equine oocytes post-warming compared to non-vitrified controls; viability was highest without naloxone supplementation (74.9% without NX vs. 64.7% with NX; control 91.1%).
• Although naloxone reduced the intracellular ROS increase induced by vitrification, it did not significantly affect glutathione (GSH) levels or mitochondrial membrane potential (HMMP).
• Vitrification overall reduced the cleavage rates and increased degeneration after fertilization regardless of naloxone supplementation.
• Gene expression analysis showed stable expression of anti-apoptotic BCL2, inconsistent detection of pro-apoptotic genes, and no significant change in the BAX:BCL2 ratio, suggesting limited apoptosis activation.

Conclusions and Implications

• Naloxone had antioxidant properties by preventing ROS increase but did not improve survival or developmental potential of equine oocytes after vitrification.
• The negative impact on viability suggests naloxone supplementation is not beneficial for equine oocyte vitrification protocols involving overnight holding.
• Bovine oocytes provided valuable information about vitrification efficiency and stage dependence but did not reliably predict effects of naloxone seen in equine oocytes.
• Future work should consider alternative antioxidants or protocol modifications for improving equine oocyte cryopreservation outcomes.