Progesterone feedback effects on LH pulse and surge secretion in ovariectomized Bos taurus , Bos indicus and crossbred heifers

This study evaluated the effects of progesterone (P4) on LH pulse and surge secretion, and hypothalamic neuroendocrine differences between Bos indicus, Bos taurus, and crossbred heifers. Ovariectomized Brahman (n = 8), Hereford (n = 6), and Braford (F1; n = 7) heifers received estradiol (E2) implants to maintain low E2 concentrations. On d − 5, each heifer from the three genetic groups (Gen) was exposed to high P4 (HP4; two CIDRs for 5d), decreasing P4 (DP4; sequential CIDR removal from d − 1.5), and no P4 (NoP4) environments, creating a 3 × 3 crossover factorial design. Serial blood samples were collected for 8h on d − 0.5 (Pulse period) and for 6.5h on d0 (Surge period) following a GnRH challenge (100µg gonadorelin-acetate i.m.) to evaluate LH secretion. Subsequently, hypothalamic tissue was collected for dual-label immunofluorescence of kisspeptin and dynorphin in the arcuate nucleus. Circulating P4 differed among environments (P<0.01) but was not affected by Gen (P = 0.98), with no interaction. LH pulse frequency was reduced under HP4 and increased under DP4 compared to NoP4 (2.7±0.4, 6.7±0.5, and 5.0±0.6 pulses/8h; P<0.01), with no Gen effect (P=0.48). However, Bos indicus heifers exhibited lower LH pulse amplitude and peak than Bos taurus and crossbred (P<0.01). P4 environment did not affect GnRH-induced LH surge (P=0.96), but Bos taurus had greater surge than crossbred and Bos indicus (P<0.01). Additionally, Bos indicus had fewer kisspeptin- and dynorphin-immunopositive neurons in the arcuate nucleus. In conclusion, P4 suppressed LH pulse frequency without altering surge magnitude. Regardless of P4 environment, Bos indicus heifers exhibited reduced LH secretion and fewer kisspeptin/dynorphin neurons than Bos taurus.