Fourcaud-Trocme, N. | bioRxiv Science

bioRxiv2021

After-hyperpolarization promotes the firing of mitral cells through a voltage dependent modification of action potential threshold

In the olfactory bulb (OB), mitral cells (MCs) display a spontaneous firing that is characterized by bursts of action potentials intermixed with silent periods. Burst firing frequency and duration are heterogeneous among MCs and increase with membrane depolarization. By using patch clamp recording on rat slices, we dissected out the intrinsic properties responsible of this activity. We showed that the threshold of action potential (AP) generation dynamically changes as a function of the trajectory of the membrane potential; becoming more negative when the membrane was hyperpolarized and having a recovering rate, inversely proportional to the membrane repolarization rate. Such variations appeared to be produced by changes in the inactivation state of voltage dependent Na+ channels. Thus, the modification AP threshold favours the initiation of the burst following hyperpolarizing event such as negative membrane oscillations or inhibitory transmission. After the first AP, the following afterhyperpolarization (AHP) brought the threshold just below the membrane resting potential or within membrane oscillations and, as a consequence, the threshold was exceeded during the fast repolarization component of the AHP. In this way the fast AHP acts as a regenerative mechanism that sustains the firing. Bursts were stopped by the development of a slow repolarization component of the AHP. The AHP characteristics appeared as determining the bursting properties; AHP with larger amplitudes and faster repolarizations being associated with longer and higher frequency bursts. Thus, the increase of bursts length and frequency upon membrane depolarization would be attributable to the modifications of the AHP and of Na+ channels inactivation.

neuroscience↗

bioRxiv2020

The deep and slow breathing characterizing rest favors brain respiratory-drive

A respiration-locked activity in the olfactory brain, mainly originating in the mechano-sensitivity of olfactory sensory neurons to air pressure, propagates from the olfactory bulb to the rest of the brain. Interestingly, changes in nasal airflow rate result in reorganization of olfactory bulb response. Therefore, if the respiratory drive of the brain originates in nasal airflow movements, then it should vary with respiration dynamics that occur spontaneously during natural conditions. We took advantage of the spontaneous variations of respiration dynamics during the different waking and sleep states to explore respiratory drive in various brain regions. We analyzed their local field potential activity relative to respiratory signal. We showed that respiration regime was state-specific, and that quiet waking was the only vigilance state during which all the recorded structures can be respiration-driven whatever the respiration frequency. We used a CO2-enriched air to change the respiratory regime associated to each state and, using a respiratory cycle-by-cycle analysis, we evidenced that the large and strong brain entrainment during quiet waking was the consequence of its associated respiration regime consisting in an optimal trade-off between deepness and duration of inspiration. These results show for the first time that changes in respiration regime alter the cortical dynamics and that the respiratory regime associated with rest is optimal for respiration to drive the brain.

neuroscience↗