GABAergic and glycinergic inputs modulate rhythmogenic mechanisms in the lamprey respiratory network

Abstract

Key pointsr In this study we investigated the role of GABA and glycine receptors within the respiratory central pattern generator, i.e. the paratrigeminal respiratory group (pTRG), and the vagal motoneuron region of the lamprey.r Only GABA-mediated inhibition modulates the pTRG both during apnoea induced by blockade of glutamatergic transmission and under basal conditions. r Both GABA-and glycine-mediated inhibition within the vagal region are involved in the regulation of respiratory frequency via ascending excitatory projections to the pTRG.r Projecting neurons are retrogradely labelled from the pTRG, and intense GABA immunoreactivity is present within the pTRG and the vagal motoneuron region.r Inhibitory mechanisms, which appear to be evolutionarily conserved, regulate network excitability and may provide an important contribution to rhythmic activities, such as respiration and locomotion.Abstract We have previously shown that GABA and glycine modulate respiratory activity in the in vitro brainstem preparations of the lamprey and that blockade of GABA A and glycine receptors restores the respiratory rhythm during apnoea caused by blockade of ionotropic glutamate receptors. However, the neural substrates involved in these effects are unknown. To address this issue, the role of GABA A , GABA B and glycine receptors within the paratrigeminal respiratory group (pTRG), the proposed respiratory central pattern generator, and the vagal motoneuron region was investigated both during apnoea induced by blockade of glutamatergic transmission and under basal conditions through microinjections of specific antagonists. The removal of GABAergic, but not glycinergic transmission within the pTRG, causes the resumption of rhythmic respiratory activity during apnoea, and reveals the presence of a modulatory control of the pTRG under basal conditions. A blockade of GABA A and glycine receptors within the vagal region strongly increases the respiratory frequency through disinhibition of neurons projecting to the pTRG from the vagal region. These neurons were retrogradely labelled (neurobiotin) from the pTRG. Intense GABA immunoreactivity is observed both within the pTRG and the vagal area, which corroborates present findings. The results confirm the pTRG as a primary site of respiratory rhythm generation, and suggest that inhibition modulates the activity of rhythm-generating neurons, without any direct role in burst formation and termination mechanisms.

Inhibitory control of ascending glutamatergic projections to the lamprey respiratory rhythm generator

“…Only GABAergic influences have a modulatory role at the pTRG level. On the other hand, GABAergic and glycinergic inputs to neurons within the vagal motoneuron region mediate changes in respiratory frequency through ascending excitatory projections to the pTRG (Cinelli et al, 2014). However, an extensive characterization of the facial, glossopharyngeal and vagal motoneuron regions is still lacking with regard to projections to the pTRG and related neurochemical markers such as glutamate, GABA and glycine.…”

“…For details on animal preparation see Cinelli et al (2013Cinelli et al ( , 2014. The animals were anaesthetized with tricaine methanesulphonate (100 mg/l; MS 222, Sigma-Aldrich, St Louis, MO, USA) and transected below the gills.…”

“…Endogenously released excitatory amino acids, but not GABA and glycine, have been shown to have an essential role in the respiratory rhythmogenesis (Rovainen, 1983;Martel et al, 2007;Bongianni et al, 1999Bongianni et al, , 2006Bongianni et al, , 2016Cinelli et al, 2013Cinelli et al, , 2014. Only GABAergic influences have a modulatory role at the pTRG level.…”

“…The isolated brainstem of the adult lamprey, a lower vertebrate that diverged from the main vertebrate line $560 million years ago (Kumar and Hedges, 1998), spontaneously generates a stable and regular respiratory neuronal activity in vitro (fictive respiration) for at least 12 h; this activity is very similar to that underlying respiration in intact animals (Rovainen, 1977(Rovainen, , 1983Thompson, 1985;Russell, 1986;Bongianni et al, 1999Bongianni et al, , 2002Bongianni et al, , 2006Mutolo et al, 2007Mutolo et al, , 2010Mutolo et al, , 2011Martel et al, 2007;Cinelli et al, 2013Cinelli et al, , 2014. The lateral walls of its central nervous system consist of two longitudinal zones or plates: the ventral primarily motor basal plate and the dorsal primarily sensory alar plate that comprises the octavolateral area (OLA; see e.g.…”

“…Nieuwenhuys, 1972;Villar-Cervin˜o et al, 2008). Respiratory motoneurons are located in the facial, glossopharyngeal and, especially, in the vagal nuclei (Rovainen, 1974(Rovainen, , 1977(Rovainen, , 1979Guimond et al, 2003), while the putative central neural mechanisms generating the respiratory rhythmic activity are located in the paratrigeminal respiratory group (pTRG), rostral to the trigeminal motor nucleus (Mutolo et al, 2007(Mutolo et al, , 2010(Mutolo et al, , 2011Cinelli et al, 2013Cinelli et al, , 2014Bongianni et al, 2016).…”