Effect of photoperiod, injection of pentobarbitone sodium or lesion of the suprachiasmatic nucleus on pre-partum decrease of blood progesterone concentrations or time of birth in the rat

Abstract

In Sprague-Dawley rats kept under 14L:10D (lights on 05:00-19:00 h), parturition occurred during the light phase on Day 23, and the pre-partum decrease in progesterone concentrations was observed between 07:00 and 15:00 h during the light period on Day 22. When the rats were transferred to reversed light-dark regimen (lights on 17:00-07:00 h) on Day 7, the progesterone decrease and parturition still occurred during the light period on Day 21 and 22-23, respectively. However, when rats were kept in constant darkness from Day 7, parturition occurred independently of the time of day between Day 22 and 24. A gradual decline of progesterone concentrations was randomly observed in individual rats. In Wistar rats kept under the usual light-dark regimen, parturitions were biphasic, occurring during the light periods on Day 22 and 23. The progesterone decrease occurred at the usual time even when the lighting regimen was changed only on the day of the expected progesterone decrease. However, treatment with pentobarbitone sodium at 15:00, 19:00 or 21:00 h, but not at 12:00 or 23:00 h, on Day 21 resulted in a delay of progesterone decrease and of parturition. Complete lesion of the suprachiasmatic nucleus on Day 13 or 14 led to advancement and random distribution of the time of birth. These results suggest that the time of parturition and of pre-partum progesterone decrease may be closely associated with an endogenous circadian system, and a luteolytic factor involving the nervous system may be present during a limited period before parturition.

A method for regulating the duration of pregnancy and the time of parturition in Sprague-Dawley rats (Charles River CD strain)

“…Dukes, Chester, and Atkinson (1974) Gilbert, Rosenwasser, and Adler (1985), who used a 14 : 10 photoperiod cycle (lights on at ), and found that 80% 0930 hr (23/29) of the Sprague-Dawley females gave birth between on Day 22. Also similar to our 1200 -2000 hr findings are those of Murakami et al (1987), who studied Sprague-Dawley and Wistar primiparous females that were pregnant under a 14 : 10 photoperiod cycle (lights on at ). Sprague-Dawley females 0500 hr had essentially a unimodal distribution of parturitions on Day 22 during the light phase between 0600 to Wistar females had a bimodal distribution of 1700 hr.…”

“…parturitions on Days 21 and 22 during the light phases between 1000 and with a small proportion of 1900 hr, parturitions occurring during the intervening dark phase. Breeding of Sprague-Dawley females under the conditions used by Gilbert et al (1985) and Murakami et al (1987) has the advantage that a shift in photoperiod is not required yet parturitions occur on Day 22 and are concentrated at reasonable hours. A disadvantage of both of these methods is that mating might be difficult to observe because females are most likely to be in heat during the night.…”

“…Closest to our own procedure, which was initiated in 1980 (Mayer & Rosenblatt, 1980), is the procedure employed by Murakami et al (1987). Sprague-Dawley females were mated under a 14 : 10 photoperiod (lights on ) by placing males in the female's cage 0500 hr overnight, and on Day 7 of pregnancy the photoperiod was reversed: Lights went on at and this pho-1900 hr, toperiod was maintained until parturition occurred.…”

“…However, the decline of plasma progesterone is only one of a series of endocrine events that culminate in parturition. These include an increase in which has luteolytic actions, increases in PGF , 2␣ plasma estradiol and relaxin that prepare the uterus for response to oxytocin, and the release of oxytocin that acts on the pregnant uterus (Dukes et al, 1974;Labhsetwar & Watson, 1974;Lincoln & Porter, 1979;Murakami et al, 1987). Timing of these endocrine events does not seem to be under the control of photoperiodic stimulation during the last few days before parturition because drastic changes in photoperiod, including constant light or darkness, do not appear to alter the time of parturition.…”

“…Timing of these endocrine events does not seem to be under the control of photoperiodic stimulation during the last few days before parturition because drastic changes in photoperiod, including constant light or darkness, do not appear to alter the time of parturition. It has been speculated that shifts in photoperiod during the 1st and 2nd weeks of pregnancy probably alter an endogenous biological rhythm involving the suprachiasmatic nucleus to which the endocrine events of parturition are linked (Bosc & Nicolle, 1980;Murakami et al 1987).…”