Knockdown of 5‐HT1A Receptors in the Nucleus Tractus Solitarius Attenuates Respiratory‐Related Oscillations of Sympathetic Activity in Unanesthetized Male Rats

Abstract

Our prior studies demonstrate that systemic administration of 5‐HT1A receptor (5‐HT1AR) agonists protect against metabolic acidosis in male rats subjected to acute hypotensive blood loss as well as profound hemorrhagic shock. Protection is dependent on activation of 5‐HT1AR in the hindbrain that causes increases in sympathetic‐mediated venoconstriction, cardiac output and tissue perfusion. This phenomenon is dependent on sensory afferent input carried by sinoaortic nerves. However, the central mechanism of 5‐HT1AR‐mediated protection is unknown. Knockdown of 5‐HT1AR in the dorsomedial brainstem region that includes the nucleus tractus solitarius (NTS) delays recovery of sympathetic activity and exacerbates lactic acidosis following hypotensive hemorrhage. Here, we investigated the effect of 5‐HT1AR knockdown in the dorsomedial brainstem on frequency oscillations of renal sympathetic activity (RSNA) before and during hypotensive hemorrhage, and subsequent 5‐HT1AR agonist administration in the freely moving, unanesthetized male rat. Adeno‐associated viral vectors harboring genes encoding either shRNA targeting rat 5‐HT1AR mRNA (n= 6) or scrambled shRNA (n=7) were bilaterally injected into the NTS under ketamine anesthetic. Four weeks later renal sympathetic recording electrodes and vascular catheters were implanted under ketamine. The next day, RSNA, blood pressure (BP) and heart rate (HR) were recorded during withdrawal of blood from the femoral artery (14.5 mls in 10 min) of freely moving unanesthetized rats. 8‐OH‐DPAT (30 nmol/kg, iv) was given 7 minutes after start of blood withdraw. 5‐HT1AR knockdown had no effect on baseline BP and HR, but reduced respiratory‐related oscillations in RSNA (−54%). Knockdown did not affect the rate of decline in RSNA, BP or HR during hemorrhage. In control rats subjected to hemorrhage, 8‐OH‐DPAT increased BP (+45±4 mmHg), and HR (+98± 8 bpm) as well as total RSNA (75 ± 20% baseline) and power density of RSNA in the respiratory‐related frequency (+34 ± 5% of baseline) within 2 minute of injection. Rats subjected to 5‐HT1AR knockdown showed an attenuated recovery of BP (+24 mmHg, p<0.01), and trends for reduced recovery of HR (+75±17 bpm) and total RSNA (+47±20%). Knockdown completely prevented drug‐induced increases in the power density of RSNA in the respiratory‐related frequency. In situ hybridization studies showed 5‐HT1AR gene expression density in the NTS was greatest in large GABAergic cells found on the lateral edge of the interstitial subnucleus of the NTS at the levels just caudal to area postrema. Previous studies suggest these cells maybe the pump cells that receive excitatory input from slowly adapting pulmonary stretch receptors and inhibit sympathetic activity. Together, these findings suggest that activation of 5‐HT1ARs on GABAergic cells of the NTS during hemorrhage may disinhibit reflex sympathetic activation. This phenomenon may contribute to hemodynamic recovery following blood loss.