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 subreg ions of ventral respiratory neuronal group (VRG) were studied in urethane a nesthetized, immobilized, and artificially ventilated, adult male Wistar ra ts. 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. U nilateral 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 s timulation of cardiopulmonary receptors while their responses to carotid ch emoreceptor stimulation were variable. Microinjections of L-Glu or NMDA int o the Pre-BotC caused an increase in the PN background discharge (this resp onse was unique to Pre-BotC) superimposed on which was an increase in the P N burst frequency. Microinjections of L-Glu or NMDA into the rostral VRG (r VRG) caused an increase in the frequency and amplitude of PN bursts. Inspir atory neurons, abundant in Pre-BotC and rVRG, were excited and inhibited by activation of carotid chemoreceptors and cardiopulmonary receptors, respec tively. The coordinates for the location of different subregions of VRG wer e 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 m m deep (dorsal surface of medulla); Pre-BotC: 1.4-1.6 mm rostral, 1.8-2.5 m m lateral, and 2.3-2.8 mm deep; rVRG: 0.4-1.4 mm rostral, 1.6-2.5 mm latera l, 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, f unctionally identified by L-Glu and NMDA-microinjections, has been presente d. These data are likely to prove useful in future studies on respiratory r eflex mechanisms. (C) 1999 Elsevier Science B.V. All rights reserved.