REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1133 



tered Dibenamine or Dibenzyline to rats for 

 12 days after 5:00 p.m. After this period 

 regular cycles were resumed. Injections of 

 eitiier drug at the "critical period" during 

 the proestrum failed to block ovulation in 

 80 per cent of the cases. ]\Ioore interpreted 

 the blockade of ovulation observed in rats 

 not pretreated with Dibenamine or Diben- 

 zyline as the result of a shift in the pituitary 

 from gonadotrophin to adrenocorticotrophin 

 production and not to a "neural blockade." 

 This interpretation would cast doubt on the 

 hypothesis that progesterone acts by way of 

 a neural mechanism were it not for rather 

 abundant supportive evidence that this is 

 indeed the case. 



Fraps and Case (1953) found that diallyl 

 barbituric acid (Dial), Nembutal, and cal- 

 cium ethylisopropylbarbiturate (Ipral) 

 cause premature ovulation of the follicle 

 when given 12 to 16 hours before the ex- 

 pected time of ovulation. The incidence of 

 premature ovulations was 15 to 30 per cent 

 compared with 95 per cent when progester- 

 one was given. Dial and Nembutal acted 

 synergistically with subovulatory doses of 

 progesterone to cause an incidence of 57 

 per cent and 30 per cent premature ovula- 

 tions, respectively; the same doses of pro- 

 gesterone alone were followed by 5.5 per 

 cent premature ovulations. These findings 

 can be interpreted (Fraps, 1955b) by assum- 

 ing that, after the period of depression, a 

 period of excitation follows which lowers the 

 threshold for the stimuli which cause the 

 release of gonadotrophin from the pituitary. 

 Fraps (1955b) demonstrated that pheno- 

 barbital administration blocks progester- 

 one-induced ovulations. No explanation can 

 be given for this opposite effect of pheno- 

 barbital unless it is that it is longer acting 

 than Dial or Nembutal. 



2. Lesions placed in the ventromedian re- 

 gion of the preoptic hypothalamus within 

 about 2 hours after the injection of proges- 

 terone prevent premature ovulation (Ralph 

 and Fraps, 1959) . 



3. Injections of small amounts of proges- 

 terone (5 to 10 fig.) into the diencephalon 

 result in premature ovulations only when 

 they are placed in the preoptic region of the 

 hypothalamus (Ralph and Fraps, 1960). In- 

 jections into the caudal extensions of the 



forebrain are also effective (Ralph and 

 Fraps, 1960). Systemic injections of 10 fxg. 

 progesterone were ineffective as were injec- 

 tions of 10 ^g. progesterone into the anterior 

 pituitary. Taken together, these observa- 

 tions indicate that progesterone causes 

 premature ovulation by way of a neural 

 mechanism, but the possibility that other 

 mechanisms are involved has not been ex- 

 cluded. 



5. Corticosteroids 



Studies on the effects of corticosteroid 

 administration on the avian ovary have 

 been rather limited. Fraps (1955b) reported 

 that DOCA was as effective as progesterone 

 in inducing premature ovulations in chick- 

 ens, and the effect of either can be blocked 

 effectively by phenobarbital. Daily injec- 

 tions of 5 mg. of DOCA caused inhibition of 

 egg production in chickens (Hohn, 1960). 

 Cortisone acetate (2.0 mg. per day) had no 

 effect on egg production. 



6. Epinephrine 



Perry (1941) found that injections of 

 epinephrine daily for 15 to 20 days after 

 sparrows had been exposed to 15 hours of 

 light for 30 days caused regression of the 

 slightly stimulated ovaries and oviduct. The 

 mechanism whereby this occurred was not 

 established. 



7. Thyroid Hormone 



Early thyroidectomy (Blivaiss, 1947) or 

 destruction of the thyroid by large doses of 

 P-^^ (Winchester, Comar and Davis, 1949) 

 results in total lack of ovarian development. 

 Replacement therapy with thyroxine of 

 such chickens results in follicular matura- 

 tion and egg laying (Winchester, Comar and 

 Davis, 1949). Greenwood and Chu (1939) 

 thyroidectomized 5 pullets and found that 

 2 started to lay eggs at the same average age 

 as the control flock. Winchester (1939) ob- 

 served that thyroidectomy decreased egg 

 l^roduction of 7 hens from 3.77 to 0.42 eggs 

 per week. Whether this difference in results 

 between the experiments of Blivaiss (1947) 

 and Winchester, Comar and Davis (1949) 

 on the one hand, and of Greenwood and 

 Chu (1939) and Winchester (1939) on the 



