Angiotensin II increases respiratory rhythmic activity in the preBötzinger complex without inducing astroglial calcium signaling

Abstract

Angiotensin II (Ang II) is the primary modulator of the renin-angiotensin system and has been widely studied for its effect on the cardiovascular system. While a few studies have also indicated an involvement of Ang II in the regulation of breathing, very little is known in this regard and its effect on brainstem respiratory regions such as the preBötzinger complex (preBötC), the kernel for inspiratory rhythm generation, has not been investigated yet. This study reports that Ang II temporarily increases phrenic nerve activity in the working heart-brainstem preparation, indicating higher central respiratory drive. Previous studies have shown that the carotid body is involved in mediating this effect and we revealed that the preBötC also plays a part, using acute slices of the brainstem. It appears that Ang II is increasing the respiratory drive in an AT1R-dependent manner by optimizing the interaction of inhibitory and excitatory neurons of the preBötC. Thus, Ang II-mediated effects on the preBötC are potentially involved in dysregulating breathing in patients with acute lung injury.

Fiber optical imaging of astroglial calcium signaling in the respiratory network in the working heart brainstem preparation

“…Activation of arterial baroreceptors by the elevated aPP or directly by NE ( Holmes and Ledsome, 1984 ), would negatively modulate the network in the VRC ( Simms et al, 2007 ; Barnett et al, 2021 ). After adaptation of the baroreflex, the neuron-mediated stimulation of the network by NE will become dominant ( Zanella et al, 2014 ; Tacke et al, 2023 ). Further experiments are required to test if the depression is eventually a response of the network to the astrocytic calcium rise and release of an inhibitory gliotransmitter ( Gourine et al, 2010 ; Forsberg et al, 2017 ).…”

“…Since at its isosbestic point the emission of GCaMP6s (Ca 2+ -free or Ca 2+ -saturated) is below 10% of the maximal fluorescence of the Ca 2+ -saturated GCaMP6 at 470 nm ( Dana et al, 2019 ), low baseline calcium levels might not be detectable. Thus, it can be concluded that the fiber imaging setup requires a higher signal to noise ratio for the detection of (baseline) calcium levels as e.g., two-photon imaging, where most astrocytes are visible at rest ( Tacke et al, 2023 ). One possible source of noise is autofluorescence of the imaging fiber ( Udovich et al, 2008 ).…”

“…For calcium imaging, we used Aldh1l1-GCaMP6s mice [Tg (Aldhll1-cre/ERT2)02Kan; 129S-B6J.Cg-Gt (ROSA)26SorTM96(CAG−GCaMP6s)He/MwarJ], which allow for conditional and tamoxifen-inducible expression of GCaMP6s in astrocytes ( Madisen et al, 2015 ; Winchenbach et al, 2016 ; Tacke et al, 2023 ). These mice were crossbred to receive double transgenic offsprings.…”

“…Respiratory neurons in the ventrolateral medulla are closely intertwined with astrocytes that play an important role in stabilizing the autonomous activity of this network ( Hulsmann et al, 2000 ; Grass et al, 2004 ; Schnell et al, 2011 ; Sheikhbahaei et al, 2018 ; Tacke et al, 2023 ). Astrocytes provide essential support to the neuronal network by controlling neurotransmitter homeostasis, ensuring proper neurotransmission ( Gomeza et al, 2003 ; Szoke et al, 2006 ; Eulenburg and Hülsmann, 2022 ).…”

“…Furthermore, various neuromodulators evoke intracellular calcium elevations in astrocytes, which may impact respiratory activity in turn ( Funk et al, 2008 ; Härtel et al, 2009 ; Huxtable et al, 2010 ; Schnell et al, 2016 ). However, whether spontaneous calcium activity influences or drives respiratory neuronal activity is still a subject of ongoing discussion ( Schnell et al, 2011 ; Okada et al, 2012 ; Oku et al, 2016 ; Tacke et al, 2023 ). Here, we built and tested a fiber optical setup to record and analyze astrocytic calcium signaling in the intact respiratory network of the working heart brainstem preparation.…”