Modulation of respiratory-related sympathetic activity evoked by activation or disinhibition of neurons located in the pre-Bötzinger complex

Recently, we identified a region located in the pre-Bötzinger (pre-Böt) complex in which chemical activation of neurons and/or blockade of GABA_A receptors augments phrenic neurogram (PN) activity. Since respiratory-synchronous (R-S) sympathetic activity generally parallels PN output, we wanted to determine whether stimulation of neurons located in this region would also augment R-S sympathetic output. We therefore examined the R-S activity recorded from the preganglionic superior cervical sympathetic nerve in response to chemical stimulation and/or disinhibition of neurons located in the pre-Böt complex in chloralose- or chloralose/urethane-anesthetized cats. In all experiments, sites in the pre-Böt complex were selected on the basis that chemical stimulation using DL-homocysteic acid (DLH) produced either a gasp-like or tonic excitatory pattern in the PN. In 4 sites, unilateral microinjection of DLH (10 mM; 21 nl) produced a gasp-like pattern in both the phrenic and sympathetic neurogram outputs. In contrast, in 5 sites, unilateral microinjection of DLH produced a tonic excitatory pattern in the PN and suppressed R-S activity in the sympathetic output. Unilateral microinjection of bicuculline methiodide (4.5 mM; 21-42 nl) into 8 of these sites produced an augmented or gasp-like pattern in both the phrenic and sympathetic neurogram outputs. These findings demonstrate that while augmentation of phasic respiratory activity similarly augments sympathetic output, activation of tonic respiratory activity was not associated with an increase in sympathetic output. Taken together, these findings suggest that neurons located in the pre-Böt complex may provide a neural substrate for differential control of respiratory and sympathetic control.