From ultradian to infradian rhythms: LH release patterns in vitro

In the present study, we examined in vitro luteinizing hormone (LH) release patterns from pituitaries and from pituitary cell cultures (3 and 7 days i n culture) to elucidate the endogenous period generated by the gonadotroph cell population and to evaluate the relationship between the basic period g enerated at the cellular level and the output pattern observed at the organ level. In addition, we examined the effect of photic environmental signals perceived by the animals on LH release patterns from pituitaries in vitro. When the animals were exposed to circadian photoperiodic signals, the in v itro LH release pattern from the pituitaries exhibited ultradian, circadian , and infradian frequencies; When the animals were exposed to continuous :i llumination, the in vitro patterns exhibited only ultradian and infradian f requencies. Furthermore, free running is a process, not a state. This proce ss is driven by a change in the relative dominance of different frequencies that construct the pattern without changing the basic period length. Evalu ation of the relative dominance of the different frequencies that construct the pattern indicates that, although infradian oscillators may take part i n shaping the output pattern, the basic rhythm generated by the pituitary c ells is in the ultradian domain. The results obtained from the examined sys tem suggest that an endogenous oscillator is a cellular entity with ulltrad ian periodicity, and that the rhythmic output of many biological variables is structured by various ultradian components that construct the circadian and infradian output rhythms.