Microencapsulated rrbNGF as an Alternative Ovulation Induction Method in Rabbits

Background: Rabbits are a reflexive ovulatory species, and intramuscular administration of exogenous hormonal factors such as gonadotropin-releasing hormone (GnRH) analogues at artificial insemination (AI) are traditionally used to induce ovulation. To find more animal-friendly ovulation methods, the addition of a molecule naturally present in the seminal plasma, such as nerve growth factor (NGF) microencapsulated with chitosan for protection and administered by intravaginal via, has been studied. First, its biological activity in pheochromocytoma of the rat adrenal medulla (PC12) cell cultures and its effects in semen were tested. Subsequently, the ability of the intravaginal NGF-chitosan delivery system administered at AI time (NGFch-0) or 30 min before AI (NGFch-30) in eliciting ovulation estimating progesterone concentrations on Day 7 post AI, as well as fertility and prolificacy results in nulliparous and multiparous rabbit does, were compared with the treatment with an analogue of GnRH (GnRH), as well as with the insertion of an empty (C-e) or containing diluted semen (C-s) catheter. Results NGF-chitosan could promote PC12 differentiation to the same extent as free NGF without impairing cell viability. Microcapsules did not interfere with semen motility, viability or capacitation status. All groups of nulliparous rabbit does responded to all treatments reaching ovulation rates statistically similar between them (GnRH 90%; NGFch-30 100%; NGFch-0 66.7% and C-e 83.3%), with higher fertility rates in GnRH and NGFch-30 groups (90 and 100%, respectively) than in NGFch-0 (60%) and similar prolificacy results. However, multiparous females showed reduced ovulation and fertility rates upon NGF-chitosan exposure regardless of administration time or after the mechanical stimulus compared to the GnRH group. In both C-e and C-s groups was also observed a significant ovulatory response. Conclusions We have demonstrated that NGF-chitosan administration 30 min before AI-induced ovulation at a similar rate to GnRH injection in nulliparous but not in multiparous does. A better receptivity status of nulliparous females could be a determining factor of this response. Nonetheless, mechanical stimulation also determined a high ovulation induction rate, thus masking the NGF-derived effect.