Examination of the Role of the Commissural Nucleus of the Solitary Tract in the Maintenance of Hypertension in the SHR

Whilst the commissural NTS (cNTS) is known to play an important role in the maintenance of hypertension in spontaneously hypertensive rats (SHRs), the phenotype of cNTS neurons involved remains unknown. The aim of the present study was to identify the cNTS neurons responsible for the maintenance of hypertension in SH rats. We injected an adeno‐associated virus ‐ AAV‐CBA‐hM4DREADD‐mCherry ‐ into the cNTS. Three weeks later, injection of the ligand that activates the receptor to inhibit neuronal function, CNO (3 mg/kg i.v.), produced a decrease in blood pressure (BP) and heart rate (HR) in SHRs (Δ MAP: − 56.45 ± 5.6 vs. saline: −9.7 ± 6 mmHg, p<0.001); (Δ HR: − 66.1 ± 8.7 vs. saline: −17.8 ± 8.8 bpm, p<0.001). The injection of CNO did not affect MAP or HR in hM4DREADD‐expressing normotensive (NT) control rats. To our surprise, control, uninjected SHRs also showed a decrease in MAP and HR in response to CNO. This decrease was slower in onset, but, by 40 minutes post CNO injection, the magnitude of MAP decrease was similar between groups (Δ MAP: −60.3 ± 7.0 vs. hM4DREADD: −55.3 ± 5.8 mmHg; Δ HR: −60.7 ± 13.5 vs. hM4DREADD: −60.7 ± 13.1 bpm). CNO is a metabolite of clozapine, a drug with wide receptor affinities, including for α‐adrenergic receptors. Injection of clozapine i.v. in SHRs and NT rats also decreased MAP, with a greater magnitude in the SHRs (Δ MAP: −68.44 ± 1.4 vs. NT: −32.6 ± 4.1 mmHg, p<0.001). To determine whether CNO or clozapine interact with α1 adrenergic receptor we administered these drugs with the α1 adrenergic receptor agonist, phenylephrine. In the SHR the peak pressor response induced by phenylephrine was unchanged by CNO or clozapine injection, but the pressor response was sustained for a longer period. In NT rats CNO reduced the pressor response to phenylephrine by ~50%. We conclude that CNO is converted to clozapine in vivo and in the SHR this can lead to a sustained decrease in blood pressure that is slower in onset. We used immunohistochemistry to examine the phenotype of transduced neurons in cNTS and their projections. Around 40% of transduced neurons, determined by their expression of the fluorophore, mCherry, expressed the transcription factor PHOX2B. Some of these neurons also expressed the catecholamine synthetic enzyme, tyrosine hydroxylase. We observed a dense network of mCherry‐positive nerve terminal fields in cardiorespiratory nuclei, including the caudal and rostral ventrolateral medulla, the rostral ventral respiratory group and, although less dense, the Botzinger complex. We conclude that cNTS neurons contribute to the maintenance of hypertension through excitatory projections to pre‐motor sympathetic neurons in the rostral ventrolateral medulla and respiratory neurons. Support or Funding Information Supported by the Australian NHMRC and the Brazilian FAPESP. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .