Neurons in the medullary gigantocellular reticular nucleus mediate cardioinhibition in cats

The cardioinhibitory mechanisms (bradycardia) of the gigantocellular reticular nucleus (GRN) of medulla was studied in chloralose-urethane anesthetized cats by means of microinjection of sodium glutamate (0.5 M, 100 nl) and single neuron recording. Microinjection of glutamate excited the GRN neurons and produced the bradycardiac responses. The rostral GRN (rostral to the caudal superior olivary nucleus) had higher proportion of bradycardiac loci than the caudal GRN. This suggests that the density of cardioinhibitory neurons in GRN is higher in the rostral than the caudal level. Seventy-eight GRN neurons were recorded extracellularly. Activities of 60 neurons were tested if they were correlated with cardiac rhythm or systemic arterial blood pressure (SAP) changes following intravenous norepinephrine or nitroglycerin. Among these neurons 45% (27/60) exhibited changes related to SAP and 25% (15/60) related to cardiac rhythm. Sixty-one GRN neurons were tested by antidromic activation to determine whether they have axonal projection to the dorsal motor nucleus of vagus and nucleus tractus solitarius (DMN/NTS). Eighteen percent (11/61) of them had axonal projections to the DMN/NTS. These findings suggest that: (i) In GRN there are neurons which mediate cardioinhibition. (ii) These neurons may receive baroreceptor inputs. (iii) They may decrease heart rate through the DMN/NTX.