Twenty-four-Hour Periodicity in the Mechanism of Pituitary Gonadotrophin Release for Follicular Maturation and Ovulation in the Chicken

Abstract

Coupled ovulation cycles include a single day of lapse between ovulations or sequences of 2 or more consecutive daily ovulations. The release of gonadotropic hormone (GTH) for ovulation of the terminal follicle of 1 cycle also leads to maturation of the 1st or C1 follicle of the succeeding cycle. As the number of members (n) in coupled cycles increases from 1 to about 7, the GTH release for maturation of the C1 follicle takes place at progressively later hours. A main factor in this relationship appears to be the decreasing interval (or decreasing lag) between consecutive daily GTH releases with increasing n. Uncoupled cycles are separated from preceding cycles by 2 days or more on which ovulation fails to occur. GTH release for maturation of the C1 follicle therefore takes place in the absence of those factors which appear to be associated with GTH release for maturation of C1 follicles in coupled cycles at progressively later hours as n increases from 1 to about 7. Theoretically, at least, it seemed possible that the uncoupled C1 maturation release might occur during a diurnally recurrent "critical period" in the GTH release mechanism, a period appearing, for example, at about the hour of the C1 maturation release in coupled cycles of some 7 or more members. If this were so, it was thought likely that 1st ovipositions would take place somewhat later in uncoupled cycles of low n than in coupled cycles of equal n. Times of ovipositions in coupled and uncoupled cycles of low n were accordingly compared. In one series of comparisons, uncoupled 1st ovipositions were later by 39 min. (p < 0.01) in single member cycles, 20 min. (p 0.1) in 2 member cycles and 2 min. (NS) in 3 member cycles. In a 2nd series, differences were 30 min. (p < 0.01) in the 1 member cycle and 20 min. (p < 0.01) in 2 member cycles. These differences, and more particularly the order of differences, are believed to be consistent with the view that the GTH release mechanism may exhibit a restricted 24-hr. periodicity which is masked in coupled cycles of low n. The postulated 24-hr. periodicity appears to be the means whereby the hen''s ovulation cycle is synchronized with environmental rhythms or periodicities (e.g., photoperiod).