Phrenic nerve responses to chemical stimulation of the subregions of ventral medullary respiratory neuronal group in the rat

Phrenic nerve (PN) responses to unilateral microinjections of L- glutamate (L-Glu, 5 mM) or N-methyl-D-aspartic acid (NMDA, 1 mM) into different subregions of ventral respiratory neuronal group (VRG) were studied in urethane-anesthetized, immobilized, and artificially ventilated, adult male Wistar rats. A 50-nl volume of microinjection was used in all the subregions of VRG except in Pre-Botzinger complex (Pre-BotC) where a 20-nl volume was used. Unilateral microinjections of L-Glu or NMDA into the Botzinger complex (BotC) and caudal VRG (cVRG), caused a transient cessation of phrenic nerve (PN) activity. Expiratory neurons, abundant in BotC and cVRG, were excited by stimulation of cardiopulmonary receptors while their responses to carotid chemoreceptor stimulation were variable. Microinjections of L-Glu or NMDA into the Pre-BotC caused an increase in the PN background discharge (this response was unique to Pre-BotC) superimposed on which was an increase in the PN burst frequency. Microinjections of L-Glu or NMDA into the rostral VRG (rVRG) caused an increase in the frequency and amplitude of PN bursts. Inspiratory neurons, abundant in Pre-BotC and rVRG, were excited and inhibited by activation of carotid chemoreceptors and cardiopulmonary receptors, respectively. The coordinates for the location of different subregions of VRG were as follows (reference points are listed in parentheses). BotC: 1.6-2.6 mm rostral (calamus scriptorius), 1.7-2.7 mm lateral (midline), and 2.3-2.8 mm deep (dorsal surface of medulla); Pre- BotC: 1.4-1.6 mm rostral, 1.8-2.5 mm lateral, and 2.3-2.8 mm deep; rVRG: 0.4- 1.4 mm rostral, 1.6-2.5 mm lateral, and 2.3-2.8 mm deep; and cVRG: 0.5 mm caudal to 0.5 mm rostral, 1.0-2.2 mm lateral, and 2.1-2.6 mm deep. A detailed map of the subregions of VRG, functionally identified by L-Glu and NMDA- microinjections, has been presented. These data are likely to prove useful in future studies on respiratory reflex mechanisms.