Projection-specific serotonergic input to the caudal nucleus tractus solitarius is required for arterial blood pressure regulation in neonatal rats

The nucleus tractus solitarius (nTS) is a central brainstem integrative site for cardiovascular control that receives dense neuromodulatory input. Pharmacological manipulation of serotonin receptors within the nTS alters arterial blood pressure (ABP) in anesthetized animals. The nTS receives serotonergic input from multiple sources, including the caudal raphe and area postrema, as well as potentially from within the nTS itself; however, the functional contribution of these serotonergic inputs under physiological conditions remains unclear. We hypothesized that serotonergic inputs to the caudal nTS are functionally hierarchical in ABP regulation in neonatal rats, with extrinsic serotonergic signaling exerting dominant control over ABP relative to intrinsic serotonergic activity. Using viral signal amplification in Long-Evans Tph2-Cre rats at postnatal day 15–16 (P15–16, age at analysis), we identified a discrete population of intrinsic tryptophan hydroxylase-2 (Tph2) + neurons within the caudal nTS. To dissociate local from projection-derived serotonergic signaling, Cre-dependent inhibitory Gi-DREADD AAVs containing an mCherry reporter were delivered to the caudal nTS at P7 as either non-retrograde (n=4) or retrograde (n=5) constructs. Viral expression and serotonergic specificity were verified by mCherry, serotonin, and Tph2 immunoreactivity, with tyrosine hydroxylase (TH) included to assess serotonergic–catecholaminergic overlap during development. Both strategies produced comparable mCherry/serotonin/Tph2+ cells in the caudal nTS, with approximately 8% of these cells coexpressing TH. Retrograde delivery additionally labeled serotonergic neurons in neighboring nuclei, including the caudal raphe, area postrema, hypoglossal nucleus, and dorsal motor nucleus of the vagus. ABP and heart rate were recorded using tail-cuff plethysmography in conscious rat pups at P15–16 following baseline, vehicle, and Compound-21 (C21) administration to silence serotonergic neurons. Gi-DREADD–mediated silencing of serotonergic neurons extrinsic or intrinsic to the nTS had no influence on heart rate. In contrast, silencing extrinsic serotonergic neurons produced a significant increase in MAP (16.3 ± 6.8 mmHg; p=0.0447), whereas silencing intrinsic neurons had no influence on MAP. These findings demonstrate that during late neonatal development, projection-derived serotonergic input to the caudal nTS tonically mitigates increases in arterial blood pressure. Although both approaches identified local Tph2 + neurons within the caudal nTS, including a small TH-coexpressing population, these cells appear to make a limited contribution to arterial blood pressure regulation under the conditions tested. Funding sources: NIH R01-HL-136710 and R01- HL-098602, and University of Missouri Research Council Grants awarded to KJC. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.