Glycine Is Used as a Transmitter by Decrementing Expiratory Neurons of the Ventrolateral Medulla in the Rat

Abstract

The medullary respiratory network involves various types of respiratory neurons. The present study focused on possible inhibitory neurons called decrementing expiratory (E-DEC) neurons and aimed to determine whether their transmitter is glycine or GABA. In Nembutal-anesthetized, neuromuscularly blocked, and artificially ventilated rats we labeled E-DEC neurons with Neurobiotin and processed the tissues for detection of mRNA encoding either glycine transporter 2 (GLYT2) as a marker for glycinergic neurons or glutamic acid decarboxylase isoform 67 (GAD67) as a marker for GABAergic neurons, using in situ hybridization. Of 38 E-DEC neurons that were labeled, cranial motoneurons (n = 14), which were labeled as control, were negative for either GLYT2 mRNA (n = 10) or GAD67 mRNA (n = 4). The other E-DEC neurons (n = 24) were non-motoneurons. Sixteen of them were examined for GLYT2 mRNA, and the majority (11 of 16) was GLYT2 mRNA-positive. The remaining E-DEC neurons (n = 8) were examined for GAD67 mRNA, and all of them were GAD67 mRNA-negative. The GLYT2 mRNA-positive E-DEC neurons were located in the ventrolateral medulla spanning the Bötzinger complex (BOT), the rostral ventral respiratory group (VRG), and the caudal VRG. We conclude that not only E-DEC neurons of the BOT but also many E-DEC neurons of the VRG are inhibitory and use glycine as a transmitter. Although the present negative data cannot rule out completely the release of GABA or co-release of glycine and GABA from E-DEC neurons, several lines of evidence suggest that the glycinergic process is primarily responsible for the phasic inhibition of the respiratory network during the expiratory phase.