Plasma concentrations of luteinizing hormone and follicle-stimulating hormone during anoestrus in ewes actively immunized against oestradiol-17 beta, oestrone or testosterone.

The concentrations of LH and FSH in jugular venous plasma were measured during anoestrus in control ewes and ewes actively immunized against 17 beta-oestradiol-6-(O-carboxymethyl)oxime--bovine serum albumin, oestrone-6-(O-carboxymethyl)oxime--bovine serum albumin or testosterone-3-(O-carboxymethyl)oxime--bovine serum albumin. During a 12 h sampling period, the basal level of LH and the frequency of LH pulses were significantly increased in all the immunized animals. The plasma FSH concentration was increased in two out of five oestradiol-immunized ewes and two out of three oestrone-immunized animals. The plasma FSH concentration in the testosterone-immunized animals was similar to the control values. Significant titres of antibodies to oestradiol-17 beta were found in all the immunized animals. It is suggested that binding of oestradiol-17 beta in the circulation by antibodies reduced the negative feedback action normally eserted by this steroid on LH secretion and, to a lesser extent, on FSH escretion. However, since the oestradiol antibody titres in the testosterone-immunized and oestrone-immunized ewes were considerably lower than those found in the oestradiol-immunized animals, it is possible that testosterone and oestrone, perhaps by acting as prehormones, could influence the negative feedback control by oestradiol-17 beta of gonadotrophin secretion in the anoestrous ewe.