Orexin A in the central control of cardiorespiratory regulation

Presympathetic neurons in the rostral ventrolateral medulla (RVLM) provide a monosynaptic excitatory drive to sympathetic preganglionic neurons (SPN) of the spinal cord and are involved in the differential regulation of sympathetic outflows to different target tissues. Neurons in the RVLM integrate information from the centre and periphery, including: respiration, and baro-, chemo- and somatosympathetic reflex afferent neurons. Metabotropic transmitters including neuropeptides, apart from basic neurotransmitters (e.g. glutamate and GABA), with long-term effects play important roles in the regulation of RVLM neurons. This thesis investigates the role of one such neuropeptide, orexin A (OX-A), in the control of blood pressure and breathing and to see if its role is altered in diseases such as hypertension. Results of this work will provide a better understanding of the way that the brain controls blood pressure and breathing, and may also lead to the development of new therapies for hypertension. In the first set of experiments (chapter 3, Shahid et al., 2011), the effects of OX-A in the spinal cord on vasomotor tone and adaptive reflexes were investigated. Intrathecal (i.t.) administration of OX-A caused a prolonged and dose-dependent increase in mean arterial pressure (MAP), heart rate (HR) and splanchnic sympathetic nerve activity (sSNA). OX-A also dose-dependently increased respiratory drive, as indicated by a rise in phrenic nerve amplitude (PNamp) and a fall in phrenic nerve frequency (PNf), an increase in neural minute ventilation, a lengthening of the expiratory period, and a shortening of the inspiratory period. All cardiorespiratory effects of OX-A were attenuated by the orexin 1 (OX1) receptor antagonist SB 334867. I.t. OX-A significantly reduced both the sympathoexcitatory peaks of somatosympathetic reflex but increased baroreflex sensitivity. In the second set of experiments (chapter 4, Shahid et al., 2012a), immunohistochemistry was performed to detect the presence of OX-A and it's receptors in the RVLM. Tyrosine hydroxylase immunoreactive (TH-ir) neurons in the RVLM frequently expressed OX1 and orexin 2 (OX2) receptors and closely apposed to OX-A-ir terminals. OX1 receptor immunoreactivity was found in 78 ± 2% of TH-ir neurons in the RVLM and OX2 receptor in 77 ± 3% of TH-ir neurons. In addition, within the RVLM, about 51% of OX1 receptors and 56% of OX2 receptors were expressed in non-TH-ir or non-C1 neurons, suggesting that both C1 and non-C1 neurons in the RVLM contain both OX receptors. In the third set of experiments (chapter 4, Shahid et al., 2012a), OX-A was injected bilaterally into the RVLM of normotensive Sprague-Dawley rats and the effects on cardiorespiratory function and sympathetic reflexes were observed. Microinjection of OX-A into the RVLM elicited a pressor and sympathoexcitatory response. Responses to OX-A were attenuated by the OX1 receptor antagonist, SB 334867, and reproduced by the OX2 receptor agonist, [Ala11, D-Leu15]orexin B indicating that both OX receptors are involved in OX-A mediated response in the RVLM. OX-A increased baroreflex sensitivity but attenuated the somatosympathetic reflex. OX-A injection into the RVLM also increased or reduced sympathoexcitation following hypoxia or hypercapnia respectively. A fourth set of experiments (chapter 5) were conducted to investigate whether OX-A responses in the RVLM of normotensive rats are exaggerated in hypertensive animal models. OX-A injected into the RVLM elicited hypertension, tachycardia and sympathoexcitation in both hypertensive and normotensive rat models. The pressor and sympathoexcitatory responses evoked by OX-A were exaggerated in spontaneously hypertensive rats (SHR). OX-A also increased PNamp in both strains. The sympathoexcitatory peaks of somatosympathetic reflex were attenuated following OX-A injection in both SHR and Wistar-Kyoto rats (WKY). The attenuation of the sympathoexcitatory peaks of the somatosympathetic reflex was potentiated in SHR. OX-A injection into the RVLM increased baroreflex sensitivity in both SHR and WKY. An interesting finding is that the extent of increase in the barosensitivity evoked by OX-A was reduced in SHR as compared to WKY. Taken together, the presence of OX receptors in C1 bulbospinal sympathoexcitatory neurons of the rostral RVLM and the effects of OX-A in the RVLM and spinal cord on basal cardiorespiratory parameters and adaptive reflexes suggest that OX receptor activation plays a key role in mediating the sympathoexcitatory responses. These data also suggest a role for OX-A in the development and maintenance of essential hypertension. The precise physiological circumstances in which OX-A is released to exert the effects described in this thesis remains to be established.