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.

Activation of 5-Hydroxytryptamine Type 3 Receptor-Expressing C-Fiber Vagal Afferents Inhibits Retrotrapezoid Nucleus Chemoreceptors in Rats

“…The present study does not permit to elaborate further on these ventrolateral relay neurons except to suggest that these cells are unlikely to be postinspiratory interneurons. Although such cells are highly represented in the rVRG of the rat (Schwarzacher et al 1991), consistently activated by stimulating cardiopulmonary C-fibers with PBG (Paton 1997;Wilson and Bonham 1997;present results), and inhibit numerous types of respiratory neurons (Ezure et al 2003;Richter et al 1987;Schwarzacher et al 1991), they are also vigorously activated by lung inflation. Thus if they were implicated in the effect of PBG on RTN neurons, they should also mediate some of the inhibitory effect of lung inflation on these neurons.…”

“…The postinspiratory interneurons located in the rVRG probably mediate a large part of the inhibitory effect of slowly adapting stretch receptor (SAR) activation on the central respiratory pattern generator (Ezure et al 2003;Hayashi et al 1996;Kubin et al 2006;Okazaki et al 2001;Richter et al 1987). Because postinspiratory neurons are also activated by PBG (Wilson and Bonham 1997;present results), these cells presumably also contribute to PND inhibition when PBGsensitive vagal afferents are activated.…”

Bötzinger Expiratory-Augmenting Neurons and the Parafacial Respiratory Group

“…Three reasons suggest that the brief but intense preinspiratory discharge of the E-AUG Bötzinger neurons could be attributable to their delayed release from an abnormally prolonged early expiratory inhibition. Bötzinger E-DEC and E-AUG neurons are mutually inhibitory (Bryant et al, 1993;Ezure et al, 2003a;Rybak et al, 2004). A subset of Bötzinger E-DEC neurons becomes E-throughout during hypoxia in vivo (Fig.…”

“…The Bötzinger E-AUG inhibitory neurons were identified presently by their discharge characteristics (Bryant et al, 1993;Kanjhan et al, 1995;Ezure et al, 2003b) and the fact that they contained GLYT2 mRNA, a diagnostic marker of glycinergic neurons (Schreihofer et al, 1999;Ezure et al, 2003a). Additional congruent evidence included the fact that these E-AUG/pre-I cells had large numbers of axonal collaterals within more caudal regions of the ipsilateral VRC and had an axonal branch crossing the midline within the dorsal tegmentum at midfacial level (Bryant et al, 1993;Ezure et al, 2003b).…”

Activation of Protein Kinase C (PKC)α or PKCε as an Approach to Increase Morphine Tolerance in Respiratory Depression and Lethal Overdose

“…BötC neurons are glycinergic and project to the ventral respiratory group and phrenic nucleus. The inhibitory input from BötC suppresses inspiratory neurons during expiration (Dobbins and Feldman, 1994;Ezure et al, 2003). pFRG/RTN is involved in both expiration and CO 2 chemosensation (Janczewski and Feldman, 2006).…”