Raphe Chemosensory Amplifier: A Carbon Dioxide-Sensitive Brain Network

Abstract

Central chemosensitivity is the vital ability of the brain to detect and respond to changes in tissue CO<sub>2 /pH. Changing CO<sub> 2 /pH causes brainstem central chemoreceptors to modulate ventilation, but the cellular basis of this chemosensitivity is not well understood. When studied in vitro, neurons within the rat medullary raphe are intrinsically sensitive to changes in pH. Serotonin/ substance P (5-HT) synthesizing raphe neurons are stimulated, and gamma-aminobutyric acid (GABA) synthesizing neurons are inhibited by CO<sub>2 /acidosis. The contribution of these neurons to central chemosensitivity in vivo, however, is controversial. Also unknown is whether there are other types of chemosensitive cells in the raphe. Here I tested the hypothesis that rat medullary raphe neurons are responsive to CO<sub>2 in a relatively intact preparation, that raphe 5-I-IT neurons are CO<sub> 2-stimulated, and that CO<sub>2inhibited raphe neurons are GABAergic. I used extracellular recording of individual raphe neurons in the unanesthetized juvenile rat in situ perfused decerebrate brainstem preparation to assess chemosensitivity of raphe neurons. I subsequently used juxtacellular labeling, and immunohistochemistry for markers of 5-HT and GABA synthesis to identify neurotransmitter phenotype of individually recorded cells. Results demonstrate that the medullary raphe is heterogeneous and clearly contains at least three distinct classes of CO<sub>2-sensitive neurons: modestly CO<sub>2-stimulated 5-I-IT neurons, CO<sub>2-inhibited GABAergic neurons that possess this sensitivity independent of major fast synaptic inputs, and robustly CO<sub>2-stimulated non-5-HT neurons. The CO<sub>2-stimulated non-5-HT neurons constitute a previously unrecognized class of chemosensitive raphe neuron that express receptors for substance P and are dependent on network inputs from 5-HT and GABA raphe cells for chemoresponsiveness. Based on my identification of these three distinct types of chemosensitive raphe cells, I propose a new raphe chemosensory amplifier (RCA) network model to explain raphe contributions to central chemosensitivity. In this model the three cell types interact as a CO<sub> 2-sensing network that potentially amplifies the chemosensory responses to CO<sub>2 and may limit toxic over excitation of 5-HT neurons. In this way, the RCA network could integrate inputs and respond to changes in tissue CO<sub>2 with an appropriate modulation of sympathetic and/or parasympathetic outflow, consistent with the broad role that brainstem raphe nuclei play in maintaining homeostasis.