Noradrenergic modulation of chemosensitive neurons in the retrotrapezoid nucleus

Chemosensitive neurons in the retrotrapezoid nucleus (RTN) regulate breathing in response to CO 2 /H + changes, and serve as an integration center for other autonomic centers including brainstem noradrenergic neurons. Considering that application of norepinephrine (NE) into the RTN strongly modulates breathing (see abstract by Oliveira et al.) and disruption of NE signaling has been implicated in breathing disorders associated with Rett syndrome, sleep‐disordered breathing and multiple system atrophy, we sought to characterize the effects of NE on chemosensitive RTN neurons. Cell‐attached recordings were used to identify RTN chemoreceptors in brainstem slices (300 μm thick) isolated from rat pups (P7–11 days postnatal) by their characteristic response to CO 2 ; i.e., low basal activity in 5% CO 2 (0.2 ± 0.1 Hz) and high activity in 10% (1.8 ± 0.1 Hz) or 15% CO 2 (2.1 ± 0.1 Hz ). We found that bath application of NE (1 μM) increased activity by 1.1 ± 0.1 Hz in 48 of 66 cells (73%), decreased activity by 1.0 ± 0.1 Hz in 6 of 66 cells (9%), or had no effect on activity in 12 of 66 (18%) of cells. Interestingly, basal activity but not CO 2 /H + ‐sensitivity differed between each group based on their NE response; e.g., basal activity and firing response to 10% CO 2 was 0.4 ± 0.1 and 1.2 ± 0.1 Hz (NE‐activated), 1.1 ± 0.3. and 1.1 ± 0.1 Hz (NE‐inhibited) and 0.03 ± 0.01 and 1.1 ± 0.01 Hz (NE‐insensitive). The excitatory effect of NE on RTN chemoreceptors was dose dependent (EC 50 of 235 nM), retained during synaptic blockade (high Mg +2 /low Ca 2+ ) and could be mimicked by an α1‐receptor agonist (phenylephrine; 10 μM) and blocked with a α1‐receptor antagonist (prazosin; 1 μM). The inhibitory effect of NE on RTN chemoreceptors was retained during synaptic blockade and blunted by a α2‐receptor antagonist (idazoxan; 1 μM). A subset of NE‐activated cells were inhibited by NE in the presence of prazosin, whereas some NE‐inhibited cells showed an excitatory response to NE in the presence of idazoxan. A third group of RTN chemoreceptors did not respond to NE under control conditions or during hypercapnia, and these cells also did not respond to phenylephrine. In addition, whole‐cell voltage clamp (Vhold= −80 mV; TTX) recordings from CO 2 /H + ‐sensitive RTN astrocytes show the absence of an NE‐sensitive current. These results indicate that i) RTN chemoreceptors are intrinsically sensitive to NE; ii) NE can increase or decrease chemoreceptor activity by α1‐ and α2‐receptor dependent mechanisms, respectively; and iii) NE modulates discrete subsets of RTN chemoreceptors based on basal activity and the differential contributions of α1‐α2‐receptors. Support or Funding Information NIH Grant :HL104101