Existence of a powerful inhibitory mechanism in the medial region of caudal medulla--with special reference to the paramedian reticular nucleus.

Inhibitory actions of the medial trigon of the caudal medulla (ITM) with special reference to the paramedian reticular nucleus (PRN) were explored in cats under chloralose (40 mg/kg) and urethane (400 mg/kg) anesthesia. Stimulation with square wave pulses (80 Hz, 1 msec, 100-200 microA) produced a reduction of mean systemic arterial blood pressure (MSAP) of 15-90 mmHg, and change in heart rate (HR) that varied from mild increase of 15 to reduction of 85 beats/min. These responses were not affected by mid-collicular decerebration nor by bilateral vagotomy. Destruction of PRN did not change the resting MSAP, HR or baroreceptor reflex responses. Stimulation of PRN suppressed the sympathetic pressor and cardioacceleratory and the vagal bradycardia responses resulting from activating cardiovascular (CV) regulatory mechanisms in the hypothalamus, midbrain and medulla, or from activating the somatic or the baroreceptive afferents. Activation of the PRN suppressed the MSAP-increase produced by direct stimulation of the stellate or celiac ganglion. PRN stimulation could eliminate the pronounced CV reactions consequent either to asphyxial anoxia during occlusion of the trachea or to cerebral ischemia following occlusion of vertebral and carotid arteries. Furthermore, PRN activation could stop the general convulsion of the animal induced by picrotoxin, 4 mg/kg, IV. Our findings suggest that in the trigon area especially in the PRN, there resides an independent mechanism which exerts very powerful and broad inhibitory actions on the autonomic as well as somatic nervous system.