Castration induces luteinizing hormone (LH) secretion in hypophysectomized pituitary-grafted rats receiving pulsatile LH-releasing hormone infusions.

An in vivo isolated pituitary paradigm was used to examine the extent to which negative feedback actions of testicular hormones are exerted directly at the level of the anterior pituitary gland. Hypophysectomized male rats received single anterior pituitary transplants under the kidney capsule. On the next day each hypophysectomized, graft-bearing (H/G) animal was fitted with a concentric atrial catheter system which allowed for intermittent infusions of LHRH (250 ng/5 min.h) and chronic blood sampling. On the fifth or sixth day of infusions, blood samples were obtained 2 h before sham-castration (n = 6) or castration (n = 5) and at every 2-h interval for 24 h thereafter. For comparison, blood samples were similarly obtained from a group of normal pituitary-intact male rats before and after sham-castration (n = 5) or castration (n = 5). Plasma LH and PRL levels in all animals were determined by RIA. In the H/G sham-castrate rats, LH levels remained constant throughout the 24-h postsurgery period. By contrast, plasma LH concentrations in the H/G castrate rats increased steadily for 18 h, reaching a plateau at levels 2- to 3-fold higher than pretreatment values. The absolute amounts of immunoreactive LH, and the trajectory of the LH rise in the H/G castrates closely resembled those in the normal castrates during the initial 20 h after castration; at subsequent time points, however, these similarities were not apparent, as LH levels in normal castrates continued to rise, while those in H/G castrates did not. PRL levels were not significantly different in H/G rats compared to those in their pituitary-intact counterparts. We conclude from these studies that most of the acute (less than 20 h) effects of castration on LH secretion can be accounted for by pituitary escape from direct negative feedback suppression. At longer times after orchidectomy, however, the continued postcastration rise in LH secretion may increasingly depend upon additional hypothalamic input. It is hypothesized that this added input consists of an acceleration of LHRH pulse frequency.