Temporal pattern of LH secretion: Regulation by multiple ultradian oscillators versus a single circadian oscillator

The possibility that the 24h rhythm output is the composite expression of u ltradian oscillators of varying periodicities was examined by assessing the effect of external continuously or pulsed (20-minute) Gonadotropin-releasi ng hormone (GnRH) infusions on in vitro luteinizing hormone (LH) release pa tterns from female mouse pituitaries during 38h study spans. Applying stepw ise analyses (spectral, cosine fit, best-fit curve, and peak detection anal yses) revealed the waveform shape of LH release output patterns over time i s composed of several ultradian oscillations of different periods. The resu lts further substantiated previous observations indicating the pituitary fu nctions as an autonomous clock. The GnRH oscillator functions as a pulse ge nerator and amplitude regulator, but it is not the oscillator that drives t he ultradian LH release rhythms. At different stages of the estrus cycle, t he effect of GnRH on the expression of ultradian periodicities varies, resu lting in the modification of their amplitudes but not their periods. The fu nctional output from the system of ultradian oscillators may superimpose a "circadian or infradian phenotype" on the observed secretion pattern. An "a mplitude control" hypothesis is proposed: The temporal pattern of LH releas e is governed by several oscillators that function in conjunction with one another and are regulated by an amplitude-controlled mechanism. Simulated m odel:, show that such a mechanism results in better adaptive response to en vironmental requirements than does a single circadian oscillator.