Multiunit Activity in the Anterior Median Eminence and Adjacent Areas of the Hypothalamus of the Ewe in Relation to LH Secretion

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Abstract
Changes in multiunit activity (MUA) in the anterior median eminence and adjacent tissues just posterior to the optic chiasma were recorded in lightly anaesthetized, ovariectomized ewes after an i.m. injection either of oil (group I) or of 50 μg oestradiol benzoate (ODB) in oil (group II). Changes in plasma LH concentration that occurred during the recording were determined by radioimmunoassay. In both groups, when the electrodes reached the correct site, the MUA showed a bursting pattern of firing with a frequency of 0.5–1.5 bursts/s. In group I (10 ewes), the MUA showed circhoral variations at intervals of 55.1 ± 2.8 min similar to those for LH pulses. Plasma LH levels, measured every 10 min, for 2 h in 8 ewes, showed a total of 14 pulses which were always preceded by an increase in MUA. Hormonal values and bioelectrical activity were positively correlated (p < 0.01). When cumulated hourly, the mean MUA in group I remained stable, from 10.5 to 22.5 h after oil treatment. Then, intravenous injection of 100 μg of ODB induced a rapid increase in MUA. During the 2 h after this injection, we observed only two pulses of LH in the 8 studied ewes. In animals injected i.m. with ODB before the recording (group II, n = 16), the mean MUA (cumulated every hour) from 10.5 to 22.5 postinjection was significantly higher (p<0.001) than in animals from group I (control). Within group II, 5 ewes discharged an obvious peak of LH, 18–24 h after ODB injection and showed a shortening of the MUA circhoral period (which varied from 10 to 20 min, at the time of initiation of the surge) followed by an increase in the mean amplitude. These 5 ewes showed a significantly higher increase (p < 0.03) in the MUA per hour (from 10.5 to 22.5 h after ODB) than the 9 which did not show an LH peak. Thus, the changes in MUA were clearly related in time to changes in LH secretion whether occurring as spontaneous pulses or as peaks induced by ODB.