AMBIENT LIGHT MODIFIES GONADOTROPIN-RELEASING-HORMONE PULSE-GENERATORFREQUENCY IN THE RHESUS-MONKEY

In the course of previous studies using continuous monitoring of the e lectrophysiological correlates of GnRH pulse generator activity, chara cterized by episodic increases in hypothalamic multiunit electrical ac tivity (MUA volley), it was noticed that the nocturnal slowing of puls e generator frequency was an acute phenomenon observable in the first MUA volleys after the lights were turned off, as was the increase in f requency when the lights were turned on in the morning. This suggested that the reduction in pulse generator frequency at night may not be t he consequence of an intrinsic diurnal rhythm, but an effect of light per se. Indeed, as reported herein, such an effect was observed when t he lights were turned on or off at times other than the normal illumin ation period (normal light schedule, lights on from 0700-1900 h). That this was not simply a response to arousal was shown by awakening the animals with loud recorded noises in total darkness at the same unaccu stomed times without a resulting change in frequency. This suggests th at the effect of light is specific, perhaps mediated by the retino-hyp othalamic tract. This direct action of light, however, is superimposed upon a diurnal rhythm, as shown by a reduction in pulse generator fre quency during the subjective night when the monkeys were kept in const ant light or constant darkness.