Ovulation in the Domestic Fowl 145 



dependent upon some element which is responsive to the follicular (or 

 ovarian) stimulus and which also can activate the anterior pituitary. More- 

 over, the period of lapse appears in a relatively constant relationship with 

 certain phases of photoperiod; it is in fact one aspect of the diurnal rhyth- 

 micity so evident, under a wide range of photoperiods, in the restriction of 

 ovulation or OIH release to certain hours of the 24. Considering these 

 relationships together, one would almost inevitably have to conclude that 

 the suspected non-functional link seen during the period of lapse between 

 follicle and pituitary was nervous in nature. No other structure or entity 

 could be expected to possess the characteristics apparently required to pre- 

 vent OIH release during an interim so closely associated with photoperiod. 

 One might therefore conclude that OIH release, when it occurs naturally, 

 does so over nervous and neuroendocrine pathways, the nervous component 

 becoming operative in association with specific phases of photoperiod. How- 

 ever simple and attractive such an argument may seem in retrospect, it does 

 not afford rigorous experimental evidence that OIH release is in fact dependent 

 upon neural activation of the pituitary, nor could it tell us what nervous 

 structures are essential. 



Effects of Pharmacological Agents 



Presumptive evidence pointing to neural participation in the mechanism 

 of OIH release is based on effects of pharmacological agents believed to act 

 at a nervous level. Much of this work, it should be noted, was inspired by 

 results described by the Duke University investigators in the rabbit and the 

 rat. 



Progesterone has been used extensively in these investigations, and effects 

 on normally incident and progesterone-induced ovulation will be considered 

 together. 



Nembutal (pentobarbital sodium), amongst other barbiturates, was shown 

 by Everett and Sawj^er (11) to block LH release in the rat. Everett subse- 

 quently (10) demonstrated Nembutal blockade of the LH release induced by 

 progesterone in 5-day cycling rats. In the hen, however, Bastian and Zarrow (2) 

 were unable to prevent either normally incident or progesterone-induced 

 ovulation by the administration of Nembutal. It was later observed by 

 Fraps and Case (19) that Nembutal, Dial (diallylbarbituric acid) and Ipral 

 calcium (probarbital calcium), following administration at 4.00 p.m. for 

 effect on the Cj follicle, caused premature ovulation in low to moderate 

 (13 to 28) percentages of injected hens. Dial and Nembutal were found also 

 to act synergistically with low levels of progesterone in the induction of 

 premature ovulation, not to block its action. 



Fraps and Case (19) suggested that the ovulation-inducing effects of these 

 several barbiturates might result from neural excitation, secondary to depres- 

 sion, and consequent activation of the pituitary. But perhaps an alternative 



