Role Of Posterior Hypothalamic Nucleus In The Cardiovascular Regulation Of Central CRFergic System And The Cardiovascular Function Of The Orexinergic System Arising From The Posterior Hypothalamic Nucleus

Corticotrophin releasing factor （CRF） and orexin are two kinds of important neuropeptides in the central nervous system and play critical roles in regulation of various physiological functions including cardiovascular activity. The hypothalamus, a higher center for cardiovascular and automomic regulation, includes neurons synthetising CRF or orexin and expressing CRF receptors or orexin receptors. Among the numerous nuclei in the hypothalamus, the posterior hypothalamic nucleus （PH） is one of structures responsible for cardiovascular regulation. Electrical or chemical stimulation of the PH increases arterial pressure （AP）, heart rate （HR） and sympathetic nerve activity. Recently, it has also been reported that in patients, chronic deep brain stimulation of the PH is associated with an enhanced sympathoexcitatory drive on the cardiovascular system. Thus, in this study, we investigated the contribution of PH in cardiovascular regulation of central CRFergic system and the cardiovascular function of the orexinergic system arising from the PH. The results may help to understand the roles of central CRFergic and orexinergic system in cardiovascular regulation as well as their underlying neural mechanisms.1. CRF excites PH neurons to produce tachycardia in ratsCRF, a peptide hormone involved in the stress response, holds a key position in cardiovascular regulation. By using quantitative real-time RT-PCR, immunofluorescence, in vivo and in vitro electrophysiological recordings, we examined the effect of CRF on neurons in the rat PH, an important structure responsible for stress-induced cardiovascular changes, and the underlying receptor mechanisms. The role of PH in the central effect of CRF on cardiovascular regulation was especially determined. Here, we reported that the CRF receptor type 1 （CRFR1） and CRF receptor type 2 （CRFR2） are both expressed in the rat PH and co-localized on the same PH neurons. Whole-cell patch-clamp recordings on brain slices containing the PH showed that CRF directly excited PH neurons in a concentration-dependent manner via both postsynaptic CRFR1 and CRFR2. Microinjections of CRF into the PH in vivo elicited a concentration-dependent increase in HR, but did not affect the arterial pressure and renal sympathetic nerve activity （RSNA）. Both CRFR1 and CRFR2 mediated this CRF induced tachycardia. Bilateral vagotomy did not block the tachycardia response to microinjection of CRF in the PH, while systemic administration of β receptor antagonist propranolol almost totally abolished the CRF-induced tachycardia response. Furthermore, microinjecting CRF into the PH primarily increased neuronal activity of the rostral ventrolateral medulla （RVLM） and rostral ventromedial medulla （RVMM）, but did not influence that of the dorsal motor nucleus of the vagus nerve （DMNV）. These findings suggest that the PH is a critical target for central CRF system in regulation of cardiac activity and the PH-RVLM/RVMM-cardiac sympathetic nerve pathways, rather than PH-DMNV-vagus pathway, may contribute to the CRF-induced tachycardia.2. Role of PH orexinergic neurons in central cardiovascular regulation and the underling mechanismIn addition to the lateral hypothalamic area and perifornical area, the PH is also one of the origins of central orexinergic system. As a general modulator for whole brain activity, the central orexinergic system is not only involved in many physiological states including sleep/wakefulness, energy homeostasis, reward, emotion and somatic motor control, but also plays an important roles in cardiovascular regulation. Since the PH itself is a structure responsible for cardiovascular regulation, the present study was designed to investigate the role of the PH orexinergic neurons in central cardiovascular regulation, and the underlying neural circuitry, receptor mechanism and ionic mechanisms. Here, we reported that microinjection of orexin 1 receptor （OXIR） antagonist SB334867 or orexin 2 receptor （OX2R） antagonist TCS OX2 29 into the forth ventricle significantly attenuated the pressor, tachycardia and renal sympathoexcitatory responses to disinhibition of PH by bicuculline, a GABAA receptor antagonist. Furthermore, anterograde tracing and immunofluorescence results showed that the orexinergic neurons located in the PH-sent medium density of projections to the RVMM but rarely to the RVLM, both of which are key centers for cardiovascular control. Interestingly, microinjection of SB334867 or TCS OX2 29 into the RVMM significantly reduced the pressor, tachycardia and renal sympathoexcitatory responses to microinjection of bicuculline into the PH, whereas microinjection of SB334867 or TCS OX2 29 into RVLM had no such effect. Whole-cell patch-clamp recordings on brain slices showed that orexin directly excited RVMM neurons, which was mediated by OX1R and OX2R as well as their downstream Na⁺-Ca²⁺ exchangers, inward rectifier K⁺ channels and nonselective cation channels. Thus, the results demonstrate that the orexinergic neurons in the PH may play a critical role in the central cardiovascular control, and the projections from PH orexinergic neurons to the RVMM rather than RVLM may be responsible for the cardiovascular function of the PH orexinergic system.