Endocrine and ovarian response after a 2-day controlled suckling and eCG treatment in lactating rabbit does

Abstract

Synchronization methods are used to obtain higher fertility when artificial insemination (AI) is applied to lactating rabbit does. The most common methods are eCG administration or temporary doe–litter separation. Nevertheless, drawbacks have been reported, such as negative side effects of hormonal treatment in the doe and low litter growth due to absence of suckling, respectively. Recently, improved reproductive performance (without visible consequences on young rabbit growth), has been obtained by applying a 2-day controlled nursing method before AI, by allowing for a 10 min nursing of the litter 24 h of separation. The present study was undertaken to examine the pituitary (PRL, LH, FSH) and the ovarian response (follicle size and number) to those methods. A total of 442 lactating does inseminated on day 11 post-partum were distributed in three experimental groups: 2CN (closing of nest box on day 9, controlled nursing on days 10 and 11), eCG (20 IU administered on day 9 post-partum) and CONTROL (untreated). Blood samples were obtained from 10 does per group at 48, 24 and 0 h before AI, and 1 h after AI. Both 2CN and eCG treatments similarly improved sexual receptivity (76.3, 77.5 and 58.2%, respectively; P < 0.001) and fertility (63.1, 64.1 and 48.4%, respectively; P < 0.05) in lactating does, compared to the CONTROL group. Similar plasma FSH levels in all groups of does and sampling times were observed. Due to the absence of suckling, plasma concentration of PRL on day 10 post-partum in the 2CN group was lower than in the CONTROL group ( P < 0.05); this endocrine change in PRL levels could explain the better reproductive performances obtained with 2CN treatment. At 1 h after exogenous administration of GnRH (at the moment of AI) a high LH response was observed in all groups ( P < 0.001). Ovaries from 20 rabbits treated in the same way but uninseminated (2CN, n = 10; eCG, n = 5; CONTROL, n = 5 does) were obtained on day 11 post-partum in order to check the morphometric status (weight, width and height) and to make histological and immunohistochemical studies to detect growth hormone receptor (GH-R). As a result, synchronization methods did not show any significant difference in relation to the CONTROL group. However, a small increase in the number of primary follicles was evidenced in the 2CN group with respect to the eCG group, similarly to the CONTROL group (23.0 ± 3.7, 9.4 ± 4.9 and 14.8 ± 4.92 primary follicles, respectively; P = 0.1). GH-R immunostaining-presence was more evident in the 2CN and the eCG groups, including primordial follicles and oocytes themselves. Thus, there could have been some direct effects of GH on follicular development, as described in other species. Some ovarian parameters described open new ways to study intra-ovarian mechanism of follicular development in the post-partum period of rabbit does.