Electrophysiological approaches to gonadotrophin releasing hormone pulse generator activity in the rhesus monkey

Precipitous increases in multi-unit electrical activity (MUA volleys) can be recorded from the mediobasal hypothalamus of the rhesus monkey that are invariably synchronous with the initiation of luteinizing hormone (LH) pulses and thus serve as markers of gonadotrophin releasing hormone (GnRH) pulse generator activity. Continuous radiotelemetric monitoring of this activity throughout the menstrual cycle confirmed a lower pulse generator frequency during the luteal phase than in the follicular phase. Unexpectedly, however, just before the initiation of the LH surge an abrupt reduction in frequency was observed, followed, on occasion, by a total arrest of GnRH pulse generator activity for a day or two. This monitoring technique also revealed a reduction of pulse generator frequency at night. This appeared to be an acute phenomenon observable in the first MUA volleys after the lights are turned off, as is the increase in frequency when the lights are turned on in the morning suggesting a direct action of light, a conclusion that was verified using lighting periods other than the customary ones. This action of light, however, is superimposed upon a diurnal rhythm, as shown by a reduction in pulse generator frequency during the subjective night when the monkeys are kept in constant light or darkness. The duration of the MUA volley in the intact monkey is 1-3 min, compared with some 20 min in the ovariectomized female. While oestradiol reduces the volley duration in the ovariectomized female to that of intact monkeys within 6 h, it takes some 6 weeks following ovariectomy for the duration characteristic of castrates to be re-established.(ABSTRACT TRUNCATED AT 250 WORDS)