LAUSR

The locus coeruleus (LC) is a cluster of neurons in the midbrain that, among other roles, participates in the control of breathing by sensing CO2 and evoking hyperventilation, which is observed in mammals, amphibians, and reptiles. The LC is considered the major noradrenergic nucleus of the brain, but in addition to norepinephrine, the mammalian LC also produces glutamate and GABA. Yet, little is known about the neurotransmitter phenotypes of chemosensitive LC neurons in other vertebrates. Thus, we investigated the neurotransmitter profile of neurons responsible for CO2/pH chemoreception in American bullfrogs. For that, 19 LC neurons from 5 American bullfrogs ( Lithobates catesbeianus) brains were analyzed. Chemosensitivity was ensured by monitoring the increase of firing in response to hypercapnia (switching from 1.5% to 5% CO2) in whole-cell patch clamp. A hypercapnic response was observed in all neurons recorded, which were then aspirated into the recording pipette and preserved. Single-cell quantitative PCR was performed to measure the mRNA copy number of markers for glutamatergic (glutamate transporter, VGLUT2), GABAergic (GABA catalytic enzyme, GAD1), and norepinephrinergic (norepinephrine biosynthetic enzyme, DBH) neurons. In addition, we measured the expression of genes with a potential role in pH/CO2 sensitivity; KCNK5 that encodes TASK-2, an alkaline-activated K+ channel; ASIC2 that encodes an acid-sensing cation channel; and KCNJ16 that encodes Kir 5.1, a K+ channel inhibited by acidosis. All LC neurons expressed both VGLUT2 and DBH, while GAD1 had a minimal expression, supporting the role of cotransmission using glutamate and norepinephrine in chemosensitive neurons. KCNK5 and ASIC2 were expressed in most neurons, while KCNJ16 was not. This result suggests that TASK-2 and ASIC2 proteins may contribute to CO2/pH chemosensitivity of LC neurons. Moreover, the expression of these pH sensing related genes ( KCNK5 and ASIC2) had expression linearly correlated to the noradrenergic marker DBH ( e.g., when DHB was high, KCNK5 was high and vice versa) but were not correlated to VGLUT2. Our results point to a transcriptional program that links the noradrenergic cell identity, but not the glutamatergic identity, with the capacity to sense CO2/pH. Given the comparable role of LC in regulating blood gases across vertebrates, our results suggest that the cotransmission of norepinephrine and glutamate from chemosensitive LC neurons plays a role in CO2/pH sensing. This research was funded by the Department of Defense (W911NF2010275) and NIH (R01NS114514) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.