Vasodepressor effects obtained by drugs acting on the ventral surface of the brain stem.

1. In cats anaesthetized with I.P. pentobarbitone sodium and atropinized with intravenous atropine methyl nitrate, the effects on arterial blood pressure were examined of nicotine, physostigmine, carbachol, glycine and pentobarbitone sodium applied to the exposed ventral surface of the brain stem by means of paired Perspex rings placed across the medulla. The drugs were placed inside each ring in a volume of 10mul. 2. Nicotine (0-5-6mg/ml.) produced a fall in blood pressure when the upper limit of the areas covered by the rings was about 5-6 mm caudal, but not when it was just caudal to the trapezoid bodies. The depressor effect was obtained both on bilateral and unilateral application. After application the nicotine sensitive area became for several minutes insensitive to its renewed application. 3. The depressor effect of nicotine was sensitive to hexamethonium (50 mg/ml.) but resistant to atropine (50 mg/ml.) similarly applied. 4. When the nicotine sensitive areas had become insensitive to nicotine, bilateral carotid occulusion produced its normal sustained pressor response. 5. By applying the nicotine through a single Perspex ring which could be moved stepwise along and across the medulla, the nicotine sensitive area was localized and the highest sensitivity was found in a region around and a little caudal to the rootlets of the XIIth cranial nerve. 6. Physostigmine (25 and 50 mg/ml.) and carbachol (6 mg/ml.) produced a fall in blood pressure when the uppermost limits of the areas covered by the paired rings were 5-6 mm caudal to and also when they were just caudal to the trapezoid bodies. From both regions the effects were obtained on bilateral and unilateral application. Their depressor effects were sensitive to atropine but resistant to hexamethonium similarly applied. 7. Glycine (200 mg/ml.) and pentobarbitone sodium (100 mg/ml.) produced scarcely any blood pressure effects when applied bilaterally to the nicotine sensitive areas but produced strong depressor effects on more rostral application, i.e. when the uppermost limit of the areas covered by the rings were just caudal to the trapezoid bodies. 8. It is concluded that the ventral surface of the medulla contains at least two bilateral areas from which vasodepressor effects are obtained on topical application of drugs: a more caudally situated one which is sensitive to nicotine, but insensitive to glycine and pentobarbitone sodium; a more rostrally situated one which is insensitive to nicotine but sensitive to glycine and pentobarbitone sodium; and both areas are sensitive to carbachol and physostigmine.