Central respiratory modulation of barosensitive neurones in rat caudal ventrolateral medulla

The sympathetic nerves that maintain blood pressure are modulated by the central respiratory generator. Neurones in the rostral ventrolateral medulla (RVLM) that drive this sympathetic nerve activity (SNA) also display central respiratory drive (CRD)-related activity, suggesting integration of respiratory and cardiovascular regulatory systems within the brainstem. Whether CRD-related activity in the RVLM is due to direct inputs from central respiratory neurones or modulation of cardiovascular-related neurones that influence the RVLM is not known. The caudal ventrolateral medulla (CVLM) contains GABAergic neurones that tonically inhibit presympathetic RVLM neurones and are essential for the production of numerous cardiovascular reflexes. The present study sought to determine whether cardiovascular-related GABAergic neurones in the CVLM display CRD-related activity. The firing patterns of individual barosensitive CVLM neurones were examined in relation to phrenic nerve activity in chloralose-anaesthetized, ventilated, neuromuscularly blocked, vagotomized rats. Histograms of phrenic-triggered CVLM neuronal activity showed that all baro-activated CVLM neurones displayed one of four patterns of CRD-related activity: (i) inspiratory peak (n = 15), (ii) inspiratory depression (n = 15), (iii) inspiratory peak with postinspiratory depression (n = 10), and (iv) postinspiratory peak (n = 9). A subset of each type of CVLM neurone was identified as GABAergic by individually filling the recorded neurone with biotinamide and observing expression of GAD67 mRNA by in situ hybridization (n = 10). These data suggest that the activity of GABAergic neurones in the CVLM is regulated by cardiovascular and respiratory inputs, and baro-activated GABAergic CVLM neurones may contribute to CRD-related modulation of presympathetic RVLM neurones and SNA.