Mapping of afferent and efferent connections of phenylethanolamine N-methyltransferase-expressing neurons in the nucleus tractus solitarii.

OBJECTIVE: Phenylethanolamine N-methyltransferase (PNMT)-expressing neurons in the nucleus tractus solitarii (NTS) contribute to the regulation of autonomic functions. However, the neural circuits linking these neurons to other brain regions remain unclear. This study aims to investigate the connectivity mechanisms of the PNMT-expressing neurons in the NTS (NTSPNMT neurons). METHODS: The methodologies employed in this study included a modified rabies virus-based retrograde neural tracing technique, conventional viral anterograde tracing, and immunohistochemical staining procedures. RESULTS: A total of 43 upstream nuclei projecting to NTSPNMT neurons were identified, spanning several key brain regions including the medulla oblongata, pons, midbrain, cerebellum, diencephalon, and telencephalon. Notably, dense projections to the NTSPNMT neurons were observed from the central amygdaloid nucleus, paraventricular nucleus of the hypothalamus, area postrema, and the gigantocellular reticular nucleus. In contrast, the ventrolateral medulla, lateral parabrachial nucleus, and lateral hypothalamic area were identified as the primary destinations for axon terminals originating from NTSPNMT neurons. Additionally, reciprocal projections were evident among 21 nuclei, primarily situated within the medulla oblongata. CONCLUSION: Our research findings demonstrate that NTSPNMT neurons form extensive connections with numerous nuclei, emphasizing their essential role in the homeostatic regulation of vital autonomic functions.

Circuit-Specific Control of Blood Pressure by PNMT-Expressing Nucleus Tractus Solitarii Neurons.

The nucleus tractus solitarii (NTS) is one of the morphologically and functionally defined centers that engage in the autonomic regulation of cardiovascular activity. Phenotypically-characterized NTS neurons have been implicated in the differential regulation of blood pressure (BP). Here, we investigated whether phenylethanolamine N-methyltransferase (PNMT)-expressing NTS (NTSPNMT) neurons contribute to the control of BP. We demonstrate that photostimulation of NTSPNMT neurons has variable effects on BP. A depressor response was produced during optogenetic stimulation of NTSPNMT neurons projecting to the paraventricular nucleus of the hypothalamus, lateral parabrachial nucleus, and caudal ventrolateral medulla. Conversely, photostimulation of NTSPNMT neurons projecting to the rostral ventrolateral medulla produced a robust pressor response and bradycardia. In addition, genetic ablation of both NTSPNMT neurons and those projecting to the rostral ventrolateral medulla impaired the arterial baroreflex. Overall, we revealed the neuronal phenotype- and circuit-specific mechanisms underlying the contribution of NTSPNMT neurons to the regulation of BP.

A Neural Circuit Mechanism Controlling Breathing by Leptin in the Nucleus Tractus Solitarii.

Leptin, an adipocyte-derived peptide hormone, has been shown to facilitate breathing. However, the central sites and circuit mechanisms underlying the respiratory effects of leptin remain incompletely understood. The present study aimed to address whether neurons expressing leptin receptor b (LepRb) in the nucleus tractus solitarii (NTS) contribute to respiratory control. Both chemogenetic and optogenetic stimulation of LepRb-expressing NTS (NTSLepRb) neurons notably activated breathing. Moreover, stimulation of NTSLepRb neurons projecting to the lateral parabrachial nucleus (LPBN) not only remarkably increased basal ventilation to a level similar to that of the stimulation of all NTSLepRb neurons, but also activated LPBN neurons projecting to the preBötzinger complex (preBötC). By contrast, ablation of NTSLepRb neurons projecting to the LPBN notably eliminated the enhanced respiratory effect induced by NTSLepRb neuron stimulation. In brainstem slices, bath application of leptin rapidly depolarized the membrane potential, increased the spontaneous firing rate, and accelerated the Ca2+ transients in most NTSLepRb neurons. Therefore, leptin potentiates breathing in the NTS most likely via an NTS-LPBN-preBötC circuit.

Contribution of retrotrapezoid nucleus neurons to CO2 -amplified cardiorespiratory activity in spontaneously hypertensive rats.

KEY POINTS: This study demonstrates that both CO2 -induced respiratory and cardiovascular responses are augmented in spontaneously hypertensive rats (SHRs). Genetic ablation of the retrotrapezoid nucleus (RTN) neurons depresses enhanced hypercapnic ventilatory response and eliminates CO2 -stimulated increase in arterial pressure and heart rate in SHRs. SHRs have a high protein level of pH-sensitive channels in the RTN, including the TASK-2 channel, Kv12.1 channel and acid-sensing ion channel 3. The inhibition of putative TASK-2 channel activity by clofilium diminishes amplified hypercapnic ventilatory and cardiovascular responses, and reduces the number of CO2 -activated RTN neurons in SHRs. These results indicate that RTN neurons contribute to enhanced CO2 -stimulated respiratory and cardiovascular responses in SHRs. ABSTRACT: The respiratory regulation of cardiovascular activity is essential for maintaining an efficient ventilation and perfusion ratio. Activation of central respiratory chemoreceptors not only elicits a ventilatory response but also regulates sympathetic nerve activity and arterial blood pressure (ABP). The retrotrapezoid nucleus (RTN) is the most completely characterized cluster of central respiratory chemoreceptors. We hypothesize that RTN neurons contribute to augmented CO2 -stimulated respiratory and cardiovascular responses in adult spontaneously hypertensive rats (SHRs). Our findings indicate that SHRs exhibit more enhanced hypercapnic cardiorespiratory responses than age-matched normotensive Wistar-Kyoto rats. Genetic ablation of RTN neurons notably depresses an enhanced hypercapnic ventilatory response (HCVR) and eliminates a CO2 -stimulated greater increase in ABP and heart rate in SHRs. In addition, SHRs have a higher protein level of pH-sensitive channels in the RTN, including TASK-2 channels, Kv12.1 channels and acid-sensing ion channel 3. Administration of clofilium (i.p.), an unselective inhibitor of TASK-2 channels, not only significantly reduces the enhanced HCVR but also inhibits CO2 -amplified increases in ABP and heart rate in SHRs. Moreover, clofilium significantly decreases the number of CO2 -activated RTN neurons in SHRs. Taken together, we suggest that RTN neurons play an important role in enhanced hypercapnic ventilatory and cardiovascular responses in SHRs and the putative mechanism involved is associated with TASK-2 channel activity in the RTN.