EFFECT OF SEASON ON LH-SECRETION IN OVARIECTOMIZED AUSTRALIAN CASHMERE DOES

An experiment was conducted to define the seasonal changes in LH secre tion in ovariectomized does with oestrogen implants and the effect of immunization against melatonin. Fifteen mature Australian cashmere goa ts were ovariectomized and given either no further treatment or one or two implants containing oestradiol; another similar group of 15 does were immunized against melatonin before ovariectomy and oestrogen trea tment. LH concentrations and livemasses were recorded every week for 2 years. Livemasses of both groups showed a distinct seasonal pattern w ith a summer maximum and a winter minimum irrespective of treatment. L H concentrations also showed distinct seasonal patterns with a signifi cant interaction between the number of implants and the time of the ye ar. In the nonimmunized does, the presence of a constant low dose of o estrogen (one implant) resulted in low concentrations of LH except fro m May to August, the normal period of spontaneous ovulatory activity i n intact does. In contrast, nonimmunized does receiving a high dose of oestrogen (two implants) showed a rise in LH concentrations in Februa ry, and concentrations remained high until August. Immunization agains t melatonin abolished this differential LH secretory pattern, and both doses of oestrogen were associated with a short period of high LH con centration between May and September. These results indicate that a ne gative feedback effect of oestrogen results in low LH secretion for mo st of the year and that hypothalamic sensitivity to LH decreases for o nly a short period between May and August. It is suggested that the ea rly increase in LH secretion in does with two oestrogen implants is du e to a positive oestrogen feedback mechanism that may depend on a phot operiodic signal for activation.