Intrinsic properties and network configuration of sympathetic premotor neurons in the rostral ventrolateral medulla

Sympathetic premotor neurons within the rostral ventrolateral medulla (RVLM) exhibit tonic activity in vivo, which is thought to be critical for generating sympathetic nerve activity (SNA) and maintaining blood pressure. The mechanisms responsible for the generation of spontaneous activity in these neurons are therefore a topic of considerable research interest. In the current thesis we examine intrinsic and extrinsic mechanisms that contributeto the excitability of these neurons in vitro. First we investigated two candidate mechanisms for mediating the hypoxia-sensitivity of these cells. We initially examined the evidence for expression and function of the putative oxygen-sensor heme oxygenase 2 (HO-2) on bulbospinal neurons. We found no HO-2 immunoreactivity on spinally projecting or catecholaminergic RVLM neurons, and found no effect of HO-2 blockade on electrophysiological responses to acute cyanide hypoxia. We then demonstrated that extracellular ATP release is critically important for mediating responses to hypoxia, and that destruction of brainstem glia abolishes cyanide sensitivity, indicating a previous unrecognized role for glia in driving hypoxic responses. Second, we examined the role played by RVLM sympathetic premotor neurons in driving adrenaline release in response to acute glucoprivation. We found that RVLM bulbospinal neurons are not sensitive to changes in extracellular glucose concentration in vitro, but that RVLM neurons are activated by an orexinergic relay originating in the perifornical hypothalamus (PeH) in response to systemic or PeH glucoprivation in vivo. Third, we examined the cellular mechanisms responsible for the dramatic sympathoinhibition evoked by microinjection of somatostatin (Sst) into the RVLM in vivo, despite the limited expression of the SST2A receptor on putative sympathetic premotor neurons. Consistent with our previous anatomical data we found that Sst hyperpolarises only 50% of bulbospinal RVLM neurons (via an inwardly rectifying potassium channel). Given the profound effect of Sst receptor agonists in this region and the limited distribution of the SST2A-R and sensitivity of bulbospinal neurons to Sst, these findings support our previous suggestion that Sst-sensitive neurons may play a critical role in generating vasomotor tone. Finally, we examined the hypothesis that synaptic coupling between spinally projecting RVLM neurons contributes to the burst generation in sympathetic nerves in vivo. We did not find any evidence of monosynaptic connections between simultaneously recorded pairs of RVLM bulbospinal neurons and conclude that synaptic mechanisms does not contribute to the synchronisation of these neurons. Overall, these data extend our knowledge of the innate properties of bulbospinal RVLM neurons and the neuronal networks in which they reside.