In the male Brown-Norway (BN) male rat, reproductive aging is associated with decreased LH-pulse amplitude and area

The Brown-Norway (BN) rat has been proposed as a rodent model for the study of human male reproductive aging. As in man, reduction in serum or plasma testosterone (T) and both testicular (primary) and hypothalamic-pituitary (secondary) reproductive dysfunction have been associated with aging in male BN rats. However, the presence of secondary testicular failure in this rodent, as indicated by low serum luteinizing hormone (LH) levels, needs further corroboration. The present study was designed to determine whether age-related differences in the pulsatile patterns of pituitary LH and follicle-stimulating hormone (FSH) secretion occur in gonad-intact mate BN rats. Three age groups were examined: young (3-4 months), middle aged (12-13 months), and old (21-22 months). Using intra-atrial cannulae, serial 5-minute blood samples were with-drawn from conscious, unrestrained animals. Plasma LH concentrations were determined by a supersensitive immunofluorometric assay (FIA) and FSH and T by radioimmunoassay (RIA). Mean T levels were different among groups (young > middle age > old). In young rats, T levels were higher in the late morning/early afternoon than in the late afternoon; this variation was not found in older rats. Mean FSH concentrations were higher in the old than in the middle-aged and young rats. Significant differences in mean LH levels were not found among groups. Compared to young rats, shortened pulse interval and reduced area of pulses characterized the secretory pattern of both gonadotropins in old rats. In addition, LH-pulse amplitude and total area of LH pulses were also significantly lower in old than in young rats. Besides the well-recognized primary testicular failure that occurs in the old BN rat, this study confirms a hypothalamic-pituitary deficiency that makes this rodent model ideal for studying human male reproductive aging.