Specific Respiratory Neuron Types Have Increased Excitability That Drive Presympathetic Neurones in Neurogenic Hypertension

A major aspect of hypertension is excessive sympathetic activity but the reasons for this remain elusive. Sympathetic tone is increased in the spontaneously hypertensive (SH) rat reflecting, in part, enhanced respiratory-sympathetic coupling. We aimed to identify which respiratory cells might have altered properties. Using the working heart-brain stem preparation, we monitored simultaneously sympathetic and respiratory nerve activity in combination with intracellular recordings of physiologically characterized medullary presympathetic or respiratory neurons. In SH rats, respiratory modulation of both inspiratory and postinspiratory phases of sympathetic activity was larger relative to Wistar rats. An additional burst of sympathetic activity in the preinspiratory phase was also present in SH rats. After synaptic isolation of rostral medullary presympathetic neurons, there was no difference in their excitability compared with neurons in Wistar rats. Rather, both pre-Botzinger preinspiratory and Botzinger postinspiratory neurons had increased neuronal excitability in SH rats relative to Wistar rats; this was attributed to higher input resistance/reduced leak current in preinspiratory neurons and reduced calcium activated potassium conductance in postinspiratory neurons. Thus, the respiratory network of the SH rat is reconfigured to a pattern dominated by heightened excitability of preinspiratory and postinspiratory neurons. These neurons both provide augmented excitatory synaptic drive to rostral medullary presympathetic neurons contributing to excessive sympathetic nerve activity associated with hypertension in the in situ SH rat. Our data indicate selective modulation of potassium conductances in 2 subsets of respiratory neurons contributing to neurogenic hypertension.