Timekeeping in the hindbrain: a multi-oscillatory circadian centre in the mouse dorsal vagal complex

Abstract

Metabolic and cardiovascular processes controlled by the hindbrain exhibit 24 h rhythms, but the extent to which the hindbrain possesses endogenous circadian timekeeping is unresolved. Here we provide compelling evidence that genetic, neuronal, and vascular activities of the brainstem's dorsal vagal complex are subject to intrinsic circadian control with a crucial role for the connection between its components in regulating their rhythmic properties. Robust 24 h variation in clock gene expression in vivo and neuronal firing ex vivo were observed in the area postrema (AP) and nucleus of the solitary tract (NTS), together with enhanced nocturnal responsiveness to metabolic cues. Unexpectedly, we also find functional and molecular evidence for increased penetration of blood borne molecules into the NTS at night. Our findings reveal that the hindbrain houses a local network complex of neuronal and nonneuronal autonomous circadian oscillators, with clear implications for understanding local temporal control of physiology in the brainstem.

Phasic Neuronal Firing in the Rodent Nucleus of the Solitary Tract ex vivo

“…Neuronal activity in the murine DVC exhibits daily and circadian changes, with higher levels at late day/early night ( Chrobok et al, 2020 ). Thus, here we assessed if phasic neurons in the NTS are amongst these potential timekeeping neurons.…”

“…Intriguingly, we found that phasic activity pattern was not evenly distributed in the NTS – most of phasic neurons were localised in its dorsomedial part, in proximity to the AP. Our previous study found that in mice, this particular subdivision of the NTS has direct access to blood-borne information during the behaviourally active night, when the glial barrier between the AP and the NTS exhibits notable permeability ( Chrobok et al, 2020 ). Other studies localise aldosterone-sensing cells or neuropeptide Y-synthesising neurons in this NTS subdivision ( van den Pol et al, 2009 ; Resch et al, 2017 ).…”

“…Accumulating evidence place the DVC, including the AP and NTS, as an important hindbrain circadian timekeeping centre ( Kaneko et al, 2009 ; Ubaldo-Reyes et al, 2017 ; Chrobok et al, 2020 ). Interestingly, in our previous study, we identified an area of the NTS adjacent to the AP in which circadian rhythmicity in the Per2 clock gene expression was sustained in ex vivo brain slices for up to a week in culture.…”

“…Four locations in the central AP were then switched from recording to stimulation mode and electrically stimulated at these selective locations. The previously validated stimulation protocol ( Chrobok et al, 2020 ) was composed of trains of 10 negative voltage pulses (amplitude: 200 mV, duration: 5 ms, inter-stimulus interval: 5 s), repeated three times every 2 min (with 75 s stimulation break). Passive responses to stimulation in recording locations immediately adjacent to the stimulation sites were not recorded.…”

“…It consists of (1) the area postrema (AP) – the sensory circumventricular organ situated in the caudal floor of the fourth ventricle, (2) the subjacent nucleus of the solitary tract (NTS), and (3) the dorsal motor nucleus of the vagus (DMV; Grill and Hayes, 2012 ). Recently, robust circadian timekeeping properties have been documented in the rodent DVC components, with a clear daily and circadian variation in their molecular activities and neuronal firing rate ( Herichová et al, 2007 ; Kaneko et al, 2009 ; Chrobok et al, 2020 ; Paul et al, 2020 ). Although rhythmic neuronal bursting has been observed in the DVC ( Tell and Jean, 1991 ; Paton et al, 2000 ; Baird et al, 2015 ), its spontaneous generation and modulation across the daily cycle remain unknown.…”