108 John W. Everett 



in the usual amount was given at 1.30 p.m. and was followed about 15 min 

 later by a subcutaneous injection of atropine sulfate (350 mg/kg) in physio- 

 logical saline. Everett and Sawyer (14) had shown that this dosage of atropine 

 is uniformly effective in blocking spontaneous ovulation. We are indebted 

 to Dr. C. D. Christian for determining that the combination of pentobarbital 

 and atropine is non-lethal. 



The dual electrode assembly was employed and, again, the site of stimu- 

 lation was the preoptic region. Stimulus parameters were like those in Series 

 IV-A. Current varied from 50 to 150 fxA (Table 3). 



Table 3. Preoptic Stimulation under the Combined Influence of 

 Pentobarbital and Atropine 



* Each + and indicates a rat with or without tubal ova. 



The result was clear cut. All but two of the rats ovulated. Eight other 

 animals served as controls to show that the ovulations could not have been 

 the result of mutual counteraction of the drugs. All control animals were 

 treated with pentobarbital and atropine in the same manner as were the 

 exp( rimenta subjects ; two of the group were subjected to stimulation (65 /mA), 

 but 'he electrodes lay outside the intended location. 



DISCUSSION 



Without question the preoptic region of the rat remains sensitive to 

 electrical stimulation in spite of pentobarbital anesthesia, and as we have 

 just seen, in spite of the blocking action of atropine as well. Examination 

 of Critchlow's (7) diagram discloses three cases in which the electrode tips 

 were near the anterior end of the optic chiasma in a basal location; two were 

 positive. Although the electrode loci are represented in sagittal projection 

 within the chiasma itself, they presumably rested in the diagonal band on 

 either side of it. In many of our own cases the electrodes have also impinged 

 on the diagonal band. Yet they were often just as effective when placed more 

 dorsally. 



Pentobarbital is thought to affect chiefly the multisynaptic pathways 

 such as those within the reticular system (2, 17). Sawyer, Critchlow and 

 Barraclough (31) reported that pentobarbital, atropine and morphine, in 

 doses adequate for blocking ovulation in the rat, all have similar action in 



