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

Abstract

Abstract-Neurons within the vagal motoneuron region of the lamprey have been shown to modulate respiratory activity via ascending excitatory projections to the paratrigeminal respiratory group (pTRG), the proposed respiratory rhythm generator. The present study was performed on in vitro brainstem preparations of the lamprey to provide a characterization of ascending projections within the whole respiratory motoneuron column with regard to the distribution of neurons projecting to the pTRG and related neurochemical markers. Injections of Neurobiotin were performed into the pTRG and the presence of glutamate, GABA and glycine immunoreactivity was investigated by doublelabeling experiments. Interestingly, retrogradely labeled neurons were found not only in the vagal region, but also in the facial and glossopharyngeal motoneuron regions. They were also present within the sensory octavolateral area (OLA). The results show for the first time that neurons projecting to the pTRG are immunoreactive for glutamate, surrounded by GABA-immunoreactive structures and associated with the presence of glycinergic cells. Consistently, GABA A or glycine receptor blockade within the investigated regions increased the respiratory frequency. Furthermore, microinjections of agonists and antagonists of ionotropic glutamate receptors and of the GABA A receptor agonist muscimol showed that OLA neurons do not contribute to respiratory rhythm generation. The results provide evidence that glutamatergic ascending pathways to the pTRG are subject to a potent inhibitory control and suggest that disinhibition is one important mechanism subserving their function. The general characteristics of inhibitory control involved in rhythmic activities, such as respiration, appear to be highly conserved throughout vertebrate evolution.

Vertebrate Evolution Conserves Hindbrain Circuits despite Diverse Feeding and Breathing Modes

“…Second, inhibitory systems heavily modulate the frequency and amplitude of rhythm but may not be essential for rhythm generation per se. For example, respiratory rhythm still persists after blockade of GABAergic and glycinergic transmission in lampreys ( Bongianni et al, 2006 ; Cinelli et al, 2014 , 2016 ), in frogs (lung ventilation; Leclère et al, 2012 ; Baghdadwala et al, 2016 ), turtles ( Johnson et al, 2002 , 2007 ), and in mammals ( Janczewski et al, 2013 ; Sherman et al, 2015 ; Baertsch et al, 2018 ). However, in different species or different brainstem nuclei, inhibition has different effects.…”

Key role of 5‐HT1A receptors in the modulation of the neuronal network underlying the respiratory rhythm generation in lampreys

“…Animal preparation and experimental procedures were similar to those described in previous reports (Bongianni, Deliagina, & Grillner, 1999;Cinelli et al, 2013Cinelli et al, , 2014Cinelli et al, , 2016Cinelli et al, , 2017Mutolo et al, 2007Mutolo et al, , 2010Mutolo et al, , 2011. The animals were deeply anaesthetized with tricaine methanesulphonate (100 mg/L; MS 222, Sigma-Aldrich, St. Louis, MO, USA).…”

“…The isolated brainstem of the adult lamprey spontaneously generates respiratory neuronal activity in vitro. The putative respiratory central pattern generator is located in the paratrigeminal respiratory group (pTRG), a region rostral to the trigeminal motor nucleus (Mutolo, Bongianni, Einum, Dubuc, & Pantaleo, 2007; Mutolo, Bongianni, Cinelli, & Pantaleo, 2010; Mutolo, Cinelli, Bongianni, & Pantaleo, 2011; Cinelli, Iovino, & Mutolo, 2013; Cinelli, Mutolo, Contini, Pantaleo, & Bongianni, 2014; Cinelli et al., 2016; Cinelli et al., 2017; for review see Bongianni et al., 2016; Milsom, 2018). The pTRG shows many similarities with the mammalian pre‐BötC (see e.g.…”

Microcircuits in respiratory rhythm generation: commonalities with other rhythm generating networks and evolutionary perspectives

“…For the buccal rhythm generating network in bullfrogs, a sensitivity to changes in chloride-dependent conductances has been demonstrated [14 •• ], but the exact role of inhibition remains unclear. In lamprey, respiratory frequency is controlled by inhibition, but like mammalian respiratory microcircuits, inhibition is not necessary for rhythmogenesis [16]. …”

“…Thus, understanding rhythmogenesis is a core issue in neuroscience. Rhythmogenesis can be studied in microcircuits isolated from invertebrates [5,6], mammalian [7 • ,8 • ,9,10,11 • ,12], and non-mammalian vertebrates [13,14 •• ,15 • ,16] as well as fully integrated behavioral systems [17 •• ,18–20,21 • ,22,23]. Yet, the quest to unravel the mechanisms underlying rhythmogenesis has been a difficult journey.…”