The Preoptic Region of the Brain and its Relation to Ovulation 105 



diestrus would induce ovulation one day early, especially if the stimulus was 

 administered during the late afternoon. 



With techniques like those in Series I, 25 rats were stimulated on the third 

 day of diestrus during 5-day cycles (Series II). In fact these two sets of experi- 

 ments were carried out in parallel. Nine of the 25 rats ovulated during the 

 night following stimulation and, significantly, among them were all 6 rats 

 that had been stimulated in the preoptic region. When considered with the 

 results in the proestrus series, these observations demonstrated that in this 

 region one can work with a high degree of predictability. 



To supplement the preliminary findings of Series II, 9 rats were stimulated 

 on diestrus day 3 with dual concentric electrodes placed in the preoptic 

 area (Series III). Parameters of stimulation were modified as follows : pulses 

 of 1 msec, 100/sec, 30 sec on and off" for 10 min. Voltage dial readings ranged 

 from 1.5 to 4.0. Series resistors were not employed. Ovulation was induced 

 in the 5 rats that were stimulated with voltages of 2.8 or more. Negative 

 results were obtained with 2.5 volts or less. 



A normal number of eggs was shed in 8 of the 9 ovulated rats of Series II 

 and in all 5 of the ovulated rats of Series III. The exceptional animal produced 

 only one tubal ovum. The ovaries closely resembled those normally found 

 on the morning after spontaneous ovulation. The uteri were characteristically 

 contracted in the animals that had shed the full complement of ova, whereas 

 in the anovulatory specimens distended uteri were the rule. The vaginal 

 smears of all animals, whether ovulation had occurred or not, were of the 

 proestrous type. There was no evidence that the release of ovulating hormone 

 modified the vaginal sequence in any degree. 



Comparative Thresholds, Diestrus vs. Proestrus 



The next step was to determine, if possible, whether any difference in 

 threshold to electrical stimulation could be detected between late diestrus 

 and proestrus. It seemed especially appropriate to compare proestrus in the 

 4-day cycle with the third day of diestrus in the 5-day cycle. Earlier work 

 (12, 13) had suggested that in the latter type of animal a physiological block 

 delays the spontaneous ovulating stimulus to the pituitary 24 hr longer than 

 in the 4-day cyclic rat. 



It was decided to monitor all stimuli for current by use of the oscilloscope. 

 A further change in circuitry was the introduction of the 200 kQ resistors in 

 the electrode leads to give a rectangular pattern on the oscilloscope screen. 

 In the absence of the resistors electrode capacitance creates a wave form 

 that is difficult to evaluate, with a high initial spike that falls logarithmically 

 to the end of the pulse and a negative spike after the break. 



Two series of experiments were carried out, Series IV-A by James Harp 

 and Series IV-B by J. W. Everett. The two series will be presented in 

 parallel. 



