i28 



PHYSIOLOGY OF GONADS 



2:00 



2.30 



3:00 



3:30 



4:00 



TIME 



Fig. 8.11. The "critical period" in certain rats on the afternoon of proestrum (Vanderbilt 

 strain, 4-day cycle, controlled lighting: 14 hours per day). Complete blockade of ovulation 

 (solid bars) is regularly found if either atropine or Nembutal is administered at 2:00 p.m. 

 Failure of blockade is usual when injections are made at 4:00 p.m. or later. Injection of 

 atropine at different times during the critical period results in jarogressive decline of per- 

 centage blocked and often accomplishes only partial blockade (cross-hatching). (From J. W. 

 Everett, Ciba Foundation Colloquia Endocrinol., 4, 172, 1952.) 



24-hour advancement of ovulation l)y i)ro- 

 gcsterone. Approximately 24 hours before 

 pituitary stimulation would occur spon- 

 taneously there is a period limited to a 

 few hours during which progesterone can 

 he effective (Everett and Sawyer, 1949). 

 Although these time relationships were first 

 recognized in a colony of inbred rats in 

 North Carolina, they have since been con- 

 firmed in commercial rats (Sprague-Daw- 

 ley ) kept under controlled lighting for only 

 a few weeks in southern California (Barra- 

 clough and Sawyer, 1955; Barraclough, 

 1955, 1956). In these studies morphine, 

 reserpine, and chlorj^romazine wcic con- 

 sistently effective when injected during pio- 

 estrus at 2 p.m., and ineffective at 4 p..m. 



Not only is the critical period pictlictablc 

 under controlled lighting, but it may be 

 readily changed by sliiftiiig the ligliting 

 schedule. For example, after an abrupt 3- 



liour advance of the time switch controlling 

 the lights, the animals slowly readjusted 

 over a 2- to 3-week period (Everett, 1952b ) ; 

 when the switch was later returned to the 

 original setting, they required again about 

 2 weeks to readjust. After a full 12-hour 

 change of lighting schedule, full reversal of 

 diurnal rhythms of activity and estrous 

 behavior similarly requires about 2 weeks 

 (Hemmingsen and Krarup, 1937). 



The duration of the process of ])ituitai'y 

 activation in rabbits and its time relation- 

 ship to release of LH and the subsequent 

 o\-uhiti()ii ha\'e l)een well defined (Table 

 8.1 I. In no other species is the available 

 iufoi'uiation so complete, but some advance 

 has been luadt' in the rat (also the hen; see 

 chapter l)y van Tienhoven). 



Diu'ation of the activating stimulus in the 

 rat has been estimated from the frequency 

 of partial blockade of ovulation after atro- 



