Contribution of the retrotrapezoid nucleus/parafacial respiratory region to the expiratory-sympathetic coupling in response to peripheral chemoreflex in rats

Moraes DJ, Dias MB, Cavalcanti-Kwiatkoski R, Machado BH, Zoccal DB. Contribution of retrotrapezoid nucleus/parafacial respiratory region to the expiratory-sympathetic coupling in response to peripheral chemoreflex in rats. J Neurophysiol 108: 882-890, 2012. First published May 16, 2012; doi:10.1152/jn.00193.2012.-Central mechanisms of coupling between respiratory and sympathetic systems are essential for the entrainment between the enhanced respiratory drive and sympathoexcitation in response to hypoxia. However, the brainstem nuclei and neuronal network involved in these respiratory-sympathetic interactions remain unclear. Here, we evaluated whether the increase in expiratory activity and expiratory-modulated sympathoexcitation produced by the peripheral chemoreflex activation involves the retrotrapezoid nucleus/parafacial respiratory region (RTN/pFRG). Using decerebrated arterially perfused in situ rat preparations (60-80 g), we recorded the activities of thoracic sympathetic (tSN), phrenic (PN), and abdominal nerves (AbN) as well as the extracellular activity of RTN/pFRG expiratory neurons, and reflex responses to chemoreflex activation were evaluated before and after inactivation of the RTN/pFRG region with muscimol (1 mM). In the RTN/pFRG, we identified late-expiratory (late-E) neurons (n = 5) that were silent at resting but fired coincidently with the emergence of late-E bursts in AbN after peripheral chemoreceptor activation. Bilateral muscimol microinjections into the RTN/pFRG region (n = 6) significantly reduced basal PN frequency, mean AbN activity, and the amplitude of respiratory modulation of tSN (P < 0.05). With respect to peripheral chemoreflex responses, muscimol microinjections in the RTN/pFRG enhanced the PN inspiratory response, abolished the evoked late-E activity of AbN, but did not alter either the magnitude or pattern of the tSN reflex response. These findings indicate that the RTN/pFRG region is critically involved in the processing of the active expiratory response but not of the expiratory-modulated sympathetic response to peripheral chemoreflex activation of rat in situ preparations.

P2Y1 receptors expressed by C1 neurons determine peripheral chemoreceptor modulation of breathing, sympathetic activity, and blood pressure

Catecholaminergic C1 cells of the rostral ventrolateral medulla (RVLM) are key determinants of the sympathoexcitatory response to peripheral chemoreceptor activation. Overactivation of this reflex is thought to contribute to increased sympathetic activity and hypertension; however, molecular mechanisms linking peripheral chemoreceptor drive to hypertension remain poorly understood. We have recently determined that activation of P2Y1 receptors in the RVLM mimicked effects of peripheral chemoreceptor activation. Therefore, we hypothesize that P2Y1 receptors regulate peripheral chemoreceptor drive in this region. Here, we determine whether P2Y1 receptors are expressed by C1 neurons in the RVLM and contribute to peripheral chemoreceptor control of breathing, sympathetic activity, and blood pressure. We found that injection of a specific P2Y1 receptor agonist (MRS2365) into the RVLM of anesthetized adult rats increased phrenic nerve activity (≈55%), sympathetic nerve activity (38±6%), and blood pressure (23±1 mm Hg), whereas application of a specific P2Y1 receptor antagonist (MRS2179) decreased peripheral chemoreceptor–mediated activation of phrenic nerve activity, sympathetic nerve activity, and blood pressure. To establish that P2Y1 receptors are expressed by C1 cells, we determine in the brain slice preparation using cell-attached recording techniques that cells responsive to MRS2365 are immunoreactive for tyrosine hydroxylase (a marker of C1 cells), and we determine in vivo that C1-lesioned animals do not respond to RVLM injection of MRS2365. These data identify P2Y1 receptors as key determinants of peripheral chemoreceptor regulation of breathing, sympathetic nerve activity, and blood pressure.