Inhibitory mechanisms control active expiration by limiting parafacial expiratory drive

Activity of parafacial neurons that control active expiration are heavily dependent on tonic and CO2/H+-dependent excitatory and inhibitory inputs from yet poorly defined sources. Contrary to the idea that CO2/H+ disinhibits parafacial expiratory neurons, the recent work of J.D. Silva et al., 2020, suggests GABAergic raphe neurons preforentially limit expiratory activity during high CO2. Here I discuss these findings and propose a model where GABAergic raphe neurons functions as CO2/H+-dependent breaks on expiratory drive.

Hypocretinergic interactions with the serotonergic system regulate REM sleep and cataplexy

Loss of muscle tone triggered by emotions is called cataplexy and is the pathognomonic symptom of narcolepsy, which is caused by hypocretin deficiency. Cataplexy is classically considered to be an abnormal manifestation of REM sleep and is treated by selective serotonin (5HT) reuptake inhibitors. Here we show that deleting the 5HT transporter in hypocretin knockout mice suppressed cataplexy while dramatically increasing REM sleep. Additionally, double knockout mice showed a significant deficit in the buildup of sleep need. Deleting one allele of the 5HT transporter in hypocretin knockout mice strongly increased EEG theta power during REM sleep and theta and gamma powers during wakefulness. Deleting hypocretin receptors in the dorsal raphe neurons of adult mice did not induce cataplexy but consolidated REM sleep. Our results indicate that cataplexy and REM sleep are regulated by different mechanisms and both states and sleep need are regulated by the hypocretinergic input into 5HT neurons.

GABAergic neurons of the medullary raphe regulate active expiration during hypercapnia

Medullary raphe has been involved in the inspiratory response to central chemoreflex; however, these reports have never addressed the role of raphe neurons on active expiration induced by hypercapnia. Here, we showed that a subset of GABA cells within the medullary raphe directly project to the parafacial respiratory region, modulating active expiration under high levels of CO2.

0148 Serotonergic Dorsal Raphe Neurons Regulate Hypercapnia Induced Arousal Through 5HT2A Receptors on the Parabrachial Neurons

Introduction Serotoninergic dorsal raphe neurons (DRSert) are CO2 responsive, and mice lacking serotonin have impaired arousal to CO2. We showed that the neurons in external lateral parabrachial nucleus containing calcitonin gene related peptide (PBelCGRP), are required for CO2-arousal. PBelCGRP neurons also receive serotoninergic innervation from the DRSert. 5HT2A agonist restores CO2 responsiveness in mice lacking serotonin, suggesting that DRSert may modulate CO2 arousal by acting on 5HT2A receptors possibly on the PBel neurons. Methods We used serotonin transporter (Sert)-Cre mice to optogenetically inhibit DRSert neurons and their terminals in the PBel. We injected AAV-FLEX-ArchT into the DR and implanted an optical fiber just above it in one set of Sert-Cre mice and bilaterally in the PBel in another set. All mice were instrumented for sleep and optogenetics and were tested for EEG arousals to 10% CO2. Latencies of arousal were compared with optogenetic inhibition of either the DR neurons or their terminals in the PBel with a 593nm laser light. We further tested whether a 5HT2A agonist (TCB-2) can reverse blockade of CO2 arousal in mice where DRSert terminals in PBel were inhibited. Finally, TCB-2 was injected in mice with PBelCGRP deletions and arousal latency to CO2 was compared. Results Compared to the control (Laser-OFF) condition, arousal latency to CO2 was significantly increased by photoinhibition of either the DRSert neurons (n=6; latency- 40.9 ± 6.4 vs. 13.81± 0.69 sec; F3, 17= 11.5; P< 0.001) or their terminals in PBel (n=8; latency-34.9 ± 2.3 sec vs. 16.62 ± 0.97sec, F1, 14= 56.9; P< 0.001). This was reversed by the 5HT2A agonist TCB-2 (5mg/kg), as it reduced the latency to CO2 arousal in mice with photoinhibition of terminals in PBel from 35.48 ± 7.31 sec to 16.24 ± 1.06 sec (F3, 9= 8.05; P= 0.006), but had no effect in mice with PBelCGRP neurons deletions. Conclusion The serotonin system modulate CO2-arousals by the DRSert input to the PBel. TCB-2 reversed the effect of inhibition of DRSert terminals in the PBel, but not in mice with PBelCGRP deletions, suggests that DRSert modulate PBelCGRP neurons through 5HT2a receptors. Support NIH- 2P01 HL095491 and NS112175

Delta glutamate receptor conductance drives excitation of mouse dorsal raphe neurons

The dorsal raphe nucleus is the predominant source of central serotonin, where neuronal activity regulates complex emotional behaviors. Action potential firing of serotonin dorsal raphe neurons is driven via α1-adrenergic receptors (α1-AR) activation. Despite this crucial role, the ion channels responsible for α1-AR-mediated depolarization are unknown. Here, we show in mouse brain slices that α1-AR-mediated excitatory synaptic transmission is mediated by the ionotropic glutamate receptor homolog cation channel, delta glutamate receptor 1 (GluD1). GluD1R-channels are constitutively active under basal conditions carrying tonic inward current and synaptic activation of α1-ARs augments tonic GluD1R-channel current. Further, loss of dorsal raphe GluD1R-channels produces an anxiogenic phenotype. Thus, GluD1R-channels are responsible for α1-AR-dependent induction of persistent pacemaker-type firing of dorsal raphe neurons and regulate dorsal raphe-related behavior. Given the widespread distribution of these channels, ion channel function of GluD1R as a regulator of neuronal excitability is proposed to be widespread in the nervous system.