Glutamatergic neuronal projections from the marginal layer of the rostral ventral medulla to the respiratory centers in rats

Abstract

The marginal layer (ML) that lines the ventral surface of the medulla oblongata (VMS) contains neurons thought to contribute to central chemoreception, the process by which systemic hypercapnia activates respiration. The transmitters and connectivity of ML neurons are poorly known. The present study focuses on a group of nonserotonergic ML neurons, often located in close proximity to the entry point of penetrating blood vessels. These neurons (approximately 300/brain) contain vesicular glutamate transporter2 (VGLUT2) mRNA and are thus probably glutamatergic. They cluster below the caudal half of the facial motor nucleus, lateral to the serotonergic cells of the ML. The projections of serotonergic and nonserotonergic ML neurons were investigated by retrograde labeling with Fluoro-Gold. ML VGLUT2 mRNA-expressing neurons lack spinal projections and innervate the dorsolateral pons and the ipsilateral ventral respiratory column (VRC), most particularly, the region of the pre-Bötzinger complex and rVRG. The latter two regions receive a very small input from ML serotonergic neurons which, instead, heavily innervate the spinal cord. In conclusion, a small region of the VMS marginal layer contains glutamatergic neurons that innervate the main respiratory centers of the medulla oblongata and pons. These glutamatergic neurons are located in a chemosensitive region of the ML and their projections are consistent with a role in central chemoreception. The serotonergic neurons of the ML, though known to be activated by CO(2), probably do not contribute to central chemoreception, given that they innervate sympathetic efferents and project at best very lightly to the VRC.

TASK Channels Determine pH Sensitivity in Select Respiratory Neurons But Do Not Contribute to Central Respiratory Chemosensitivity

“…The neurons we identified are robustly chemosensitive, even in the absence of excitatory synaptic transmission, and they selectively innervate components of the central respiratory pattern generator (Mulkey et al, 2004;Rosin et al, 2006;Takakura et al, 2006). In vitro, we identified a corresponding subset of chemosensitive RTN neurons; furthermore, we showed that a pH-sensitive and K ϩ -selective background current contributes to this chemosensitivity, suggesting that it could be carried by TASK channels (Mulkey et al, 2004;Weston et al, 2004;Guyenet et al, 2005a,b;Stornetta et al, 2006).…”

“…Historically, the region of the RTN in the rostroventrolateral medulla has been implicated in central chemoreception (Nattie, 1999;Feldman et al, 2003;Putnam et al, 2004;Nattie and Li, 2006), and our recent in vivo experiments have uncovered a population of pH-sensitive RTN neurons that represent excellent candidate neuronal substrates; those cells are glutamatergic (Mulkey et al, 2004;Weston et al, 2004;Guyenet et al, 2005a,b;Stornetta et al, 2006), and they express Phox2b , a transcription factor mutated in patients with congenital central hypoventilation syndrome in which chemical drive for breathing is selectively blunted (Amiel et al, 2003;Weese-Mayer et al, 2005a,b). In the present work, we used single cell RT-PCR to demonstrate that pH-sensitive RTN neurons recorded in vitro represent the cellular correlate of the glutamatergic and Phox2b-expressing chemoreceptors characterized in vivo (Mulkey et al, 2004;Weston et al, 2004;Guyenet et al, 2005a,b;Stornetta et al, 2006). Importantly, this identification allows direct experimental tests of the ionic basis for pH sensitivity in RTN chemoreceptor neurons in reduced preparations, and our characterization of phenotypic markers for these cells in mice will allow additional genetic exploration of the cellular and molecular bases for chemical control of breathing.…”

“…In separate work, we have amassed substantial evidence from in vivo experiments that implicates a subpopulation of chemosensitive neurons within the RTN as prime cellular substrates for central respiratory chemoreception (Mulkey et al, 2004;Weston et al, 2004;Guyenet et al, 2005a,b;Stornetta et al, 2006). This area of the rostroventrolateral medulla has long been implicated in central respiratory chemosensitivity (Nattie, 1999;Feldman et al, 2003;Putnam et al, 2004;Nattie and Li, 2006).…”

Task2 potassium channels set central respiratory CO 2 and O 2 sensitivity

“…RTN neurons are responsive to CO 2 , are glutamatergic, and have axonal projections anatomically appropriate for driving the respiratory network (43,44). Analysis of the hypercapnic response at different CO 2 concentrations revealed that Task2 −/− mice were hypersensitive to low CO 2 concentrations and showed an attenuated response at high CO 2 values.…”

Neural control of breathing: insights from genetic mouse models

“…They exhibited decreased ventilatory responses to hypercapnia on days 14 -15, suggesting a transient role for Kir2.2 in central chemosensitivity during postnatal development. Finally, null GLS1 mutant newborn mice had severe alterations in chemosensitivity to carbon dioxide (50), possibly related to dysfunction of glutaminergic neurons in central chemosensory areas (85).…”