Pulsatile discharges of luteinizing hormone in the ovariectomized rat during the 24-hour day

Blood samples were taken every 5 min for a 2-h period commencing at 09.00 h or epsilon 13.00 h from long-term ovariectomized (OVX) rats under regular lighting conditions (lights on 05.00-19.00 h), and commencing at 09.30 h or 14.00 h from long-term OVX rats under reversed lighting conditions (lights on 18.00-08.00 h), the latter equivalent to starting sampling at 20.30 h or 01.00 h under regular lighting. Five rats were used in each group, and luteinizing hormone (LH) was measured in duplicate aliquots of each sample by radioimmunoassay. Pulsatile increases in plasma LH concentrations were observed in all 20 animals, and mean, peak, and nadir plasma LH concentrations for each group did not differ between sampling periods. Similarly, the time of day did not affect the frequency at which pulsatile discharges of LH occurred. These data suggest that pulsatile discharges of LH in long-term OVX rats occur throughout the 24-h day.