Chemosensitivity of medullary inspiratory neurones: a role for GABA(A) receptors?

Somatostatin-expressing parafacial neurons are CO2/H+ sensitive and regulate baseline breathing

Glutamatergic neurons in the retrotrapezoid nucleus (RTN) function as respiratory chemoreceptors by regulating breathing in response to tissue CO2/H+. The RTN and greater parafacial region may also function as a chemosensing network composed of CO2/H+-sensitive excitatory and inhibitory synaptic interactions. In the context of disease, we showed that loss of inhibitory neural activity in a mouse model of Dravet syndrome disinhibited RTN chemoreceptors and destabilized breathing (Kuo et al., 2019). Despite this, contributions of parafacial inhibitory neurons to control of breathing are unknown, and synaptic properties of RTN neurons have not been characterized. Here, we show the parafacial region contains a limited diversity of inhibitory neurons including somatostatin (Sst)-, parvalbumin (Pvalb)-, and cholecystokinin (Cck)-expressing neurons. Of these, Sst-expressing interneurons appear uniquely inhibited by CO2/H+. We also show RTN chemoreceptors receive inhibitory input that is withdrawn in a CO2/H+-dependent manner, and chemogenetic suppression of Sst+ parafacial neurons, but not Pvalb+ or Cck+ neurons, increases baseline breathing. These results suggest Sst-expressing parafacial neurons contribute to RTN chemoreception and respiratory activity.

CO2-inhibited neurons in the medullary raphé are GABAergic

Previous studies have reported subsets of medullary raphé neurons that are either stimulated or inhibited by CO2/pH in vitro, in situ, and in vivo. We tested the hypothesis that medullary raphé CO2-inhibited neurons are GABAergic. Extracellular recordings in unanesthetized juvenile in situ rat preparations showed reversible hypercapnia-induced suppression of 19% (63/323) of medullary raphé neurons, and this suppression persisted after antagonism of NMDA, AMPA/kainate, and GABAA receptors. We stained a subset of CO2-inhibited cells and found that most (11/12) had glutamic acid decarboxylase 67 immunoreactivity (GAD67-ir). These data indicate that the majority of acidosis-inhibited medullary raphé neurons are GABAergic, and that their chemosensitivity is independent of major fast synaptic inputs. Thus, CO2-sensitive GABAergic neurons may play a role in central CO2/pH chemoreception.

GABA-A receptors and the response to CO(2) inhalation - a translational trans-species model of anxiety?

The mechanisms by which the inhalation of carbon dioxide (CO(2)) produces anxiety and panic are not fully understood, although more recently there is evidence to suggest the involvement of a neural 'fear circuit'. We have suggested that this neural fear circuit is partly mediated by the brain noradrenaline network [Bailey, J.E., Argyropoulos, S.V., Lightman, S.L. and Nutt, D.J., (2003) Does the brain noradrenaline network mediate the effects of the CO(2) challenge? J Psychopharmacol 17(3): 252-259.]. However, we now review evidence that GABA-A may also play an important role in the modulation of CO(2)-induced anxiety. The review of this evidence starts with a key publication showing that 1 min of 35% CO(2)/65% air produced anxiogenic effects in a rat model of anxiety, to a similar extent to the anxiogenic betacarboline derivative FG7142, a benzodiazepine receptor inverse agonist. The effects of both anxiogenic stimuli were abolished with pre-treatment with alprazolam (0.5 mg/kg), but only those of FG7142, not CO(2), was blocked by a benzodiazepine antagonist [Cuccheddu, T., Floris, S., Serra, M., Porceddu, L., Sanna, E., Biggio, G., (1995) Proconflict effect of carbon dioxide inhalation in rats. Life Sci 56: PL 321-324.]. Although the evidence from this study did not conclusively prove that CO(2) had an action to reduce GABA function, it was an experiment designed to be translational to compare what was known about CO(2)-induced anxiety in patients, and to also to explore if GABA mechanisms are involved. Additional evidence from the literature is found in the association between GABA and chemoreceptors, both in laboratory and human studies and GABA and anxiety disorders. Evidence of this association is found across species from stress-induced change in GABA levels in plants and insects to humans, where there is now much evidence of abnormalities in GABA/benzodiazepine receptors in anxiety and other psychiatric disorders. This paper reviews some of the evidence and attempts to relate and compare these findings across species from the human to the Drosophila.

CO2-sensitivity of GABAergic neurons in the ventral medullary surface of GAD67-GFP knock-in neonatal mice

We investigated the CO2 responsiveness of GABAergic neurons in the ventral medullary surface (VMS), a putative chemoreceptive area using a 67-kDa isoform of GABA-synthesizing enzyme (GAD67)-green fluorescence protein (GFP) knock-in neonatal mouse, in which GFP is specifically expressed in GABAergic neurons. The slice was prepared by transversely sectioning at the level of the rostral rootlet of the XII nerve and the rostral end of the inferior olive in mock cerebrospinal fluid (CSF). Each medullary slice was continuously superfused with hypocapnic CSF. GFP-positive neurons in the VMS were selected by using fluorescent optics and their membrane potentials and firing activities were analyzed with a perforated patch recording technique. Thereafter, superfusion was changed from hypocapnic to hypercapnic CSF. In 4 out of 8 GABAergic neurons in the VMS, perfusion with hypercapnic CSF induced more than a 20% decrease in the discharge frequency and hyperpolarized the neurons. The remaining 4 GFP-positive neurons were CO2-insensitive. GABAergic neurons in the VMS have chemosensitivity. Inhibition of chemosensitive GABAergic neural activity in the VMS may induce increases in respiratory output in response to hypercapnia.

Differential control of central cardiorespiratory interactions by hypercapnia and the effect of prenatal nicotine

Hypercapnia evokes a strong cardiorespiratory response including gasping and a pronounced bradycardia; however, the mechanism responsible for these survival responses initiated in the brainstem is unknown. To examine the effects of hypercapnia on the central cardiorespiratory network, we used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and inhibitory synaptic neurotransmission to cardioinhibitory vagal neurons (CVNs). Hypercapnia differentially modulated inhibitory neurotransmission to CVNs; whereas hypercapnia selectively depressed spontaneous glycinergic IPSCs in CVNs without altering respiratory-related increases in glycinergic neurotransmission, it decreased both spontaneous and inspiratory-associated GABAergic IPSCs. Because maternal smoking is the highest risk factor for sudden infant death syndrome (SIDS) and prenatal nicotine exposure is proposed to be the link between maternal smoking and SIDS, we examined the cardiorespiratory responses to hypercapnia in animals exposed to nicotine in the prenatal and perinatal period. In animals exposed to prenatal nicotine, hypercapnia evoked an exaggerated depression of GABAergic IPSCs in CVNs with no significant change in glycinergic neurotransmission. Hypercapnia altered inhibitory neurotransmission to CVNs at both presynaptic and postsynaptic sites. Although the results obtained in this study in vitro cannot be extrapolated with certainty to in vivo responses, the results of this study provide a likely neurochemical mechanism for hypercapnia-evoked bradycardia and the dysregulation of this response with exposure to prenatal nicotine, creating a higher risk for SIDS.

CO2 central chemosensitivity: why are there so many sensing molecules?

CO2 central chemoreceptors (CCRs) play a critical role in respiratory and cardiovascular controls. Although the primary sensory cells and their neuronal networks remain elusive, recent studies have begun to shed insight into the molecular mechanisms of several pH sensitive proteins. These putative CO2/pH-sensing molecules are expressed in the brainstem, detect P(CO2) at physiological levels, and couple the P(CO2) to membrane excitability. Functional analysis suggests that multiple CO2/pH-sensing molecules are needed to achieve high sensitivity and broad bandwidth of the CCRs. In contrast to the diversity of pH sensitive molecules, molecular mechanisms for CO2 sensing are rather general. The sensing molecules detect pH changes rather than molecular CO2. One or a few titratable amino acid residues in these proteins are usually involved. Protonation of these residues may lead to a change in protein conformation that is coupled to a change in channel activity. Depending on the location of the protonation sites, a membrane protein can detect extra- and/or intracellular pH.