Hypothalamic Multiunit Activity and LH Secretion in Conscious Sheep

The relationship between the pulsatile secretion of luteinizing hormone (LH) and multiunit activity (MUA) in the median eminence and retrochiasmatic area (RCh-ME) of the hypothalamus was investigated within 13 conscious, ovariectomized ewes during the anoestrous season. Eight of the ewes had been treated with oestradiol to reduce their LH pulse frequency. To test whether the activity had been recorded from sites involved in the control of GnRH release, we electrically stimulated the sites studied with the recording electrode and used the LH responses to classify the animals retrospectively for analysis of the MUA data. Following stimulation, LH secretion was either stimulated (Group STIM, n = 5 ewes), inhibited (Group INHIB, n = 4) or showed no response (Group NR, n = 4). Statistical analysis of the MUA data revealed that the onset of LH pulses was associated with an increase in cell activity in STIM ewes and a decrease in activity in both Group INHIB and Group NR ewes. Histological examination revealed that the electrodes were located near the midline in Group STIM ewes, but 1.25 or 2 mm lateral of the midline in Groups NR and INHIB, respectively. We concluded that the MUA in the RCh-ME probably reflects the activity of cells with fibres or terminals involved in the control of GnRH release. Activity which increased at the onset of LH pulses was detected in medial areas of the RCh-ME and may reflect the activity of GnRH neurones. The activity which decreased at the onset of LH pulses in lateral areas of the RCh-ME suggests that both stimulatory and inhibitory inputs may be involved in the release of GnRH pulses.