Selective C1 neuron stimulation restores blood pressure (BP) and heart rate (HR) during decompensated hemorrhage in unanesthetized rats

Abstract

BackgroundDuring hemorrhage, BP is initially maintained near normal (compensated phase) by an increase in sympathetic nerve activity (SNA). However, further blood loss (>20–25% blood volume) causes an abrupt and severe BP drop (late or “decompensated” phase) associated with an unexplained reduction of SNA and HR. Under several circumstances (e.g. hypoxia, anesthesia) BP is largely determined by the level of activity of catecholaminergic presympathetic neurons (C1 neurons) located within the rostral ventrolateral medulla (RVLM).AimIn this study we sought to determine the contribution of the C1 neurons to BP and HR during the compensated and decompensated phases of hemorrhage in non‐anesthetized rats. Additionally, we tested whether C1 neuron stimulation can restore BP and HR during the decompensated phase.MethodsWe determined the contribution of the C1 neurons to mean arterial pressure (MAP) and HR by measuring how much these variables change when these neurons are selectively silenced or activated using loss‐ or gain‐of‐function optogenetics. To inhibit C1 neurons, they were transduced with an inhibitory opsin (Archaerhodopsin) by microinjecting a Cre‐dependent vector (AAV2‐DIO‐ArchT3.0‐eYFP) bilaterally into the RVLM of male and female adult TH‐Cre rats. To stimulate C1 neurons, they were transduced with an excitatory opsin (Channelrhodopsin, AAV2‐DIO‐ChR2‐eYFP) as described above. Four weeks later, C1 neurons were inhibited by delivering green light (532 nm, continuous pulse for 10 s) or stimulated by delivering blue light (473 nm, 5 ms pulse, 10 Hz, for 10 s) at regular intervals while the rats were subjected to a controlled hypotensive hemorrhage (18 ml/kg of blood withdrawal over 20 minutes, rate 0.3–0.5 ml/min). Data are expressed as mean ± SD and were analyzed using one‐way ANOVA for repeated measures. Differences were considered significant when p<0.05.ResultsDecompensated phase of hemorrhage was marked by a decrease in MAP (from 101 ± 8 to 53 ± 10 mmHg) and bradycardia (from 326 ± 20 to 245 ± 44 bpm). Bilateral C1 neurons inhibition produced a larger MAP reduction during the early compensated phase of hemorrhage than at rest (Δ MAP: −6 ± 2 vs. −23 ± 5 mmHg, n=9). However, the MAP drop elicited by C1 cell inhibition during the decompensated phase of hemorrhage (Δ MAP: −4 ± 4 mmHg) was no different from the drop at rest. Bilateral C1 neuron stimulation increased BP at rest (Δ MAP: 35 ± 6 mmHg) and raised BP to pre‐hemorrhage levels during the decompensated phase of hemorrhage (Δ MAP: 42 ± 5 mmHg, n=6). C1 stimulation, decreased HR at rest whereas it increased HR during the decompensated hemorrhage (Δ HR: −14 ± 36 vs. +58 ± 22 bpm).ConclusionsC1 neuron activation contributes to BP stability during the early compensated phase of hemorrhage. C1 neurons are probably inhibited during the decompensated phase of hemorrhage. Their re‐activation rescues BP and HR, indicating that these neurons remain potentially capable of maintaining BP despite the blood loss.Support or Funding InformationNational Institutes of Health, RO1 (HL028785) to P.G.G. and American Heart Association (19POST34430205) to G.M.P.R.S.