title: Raphé neurons stimulate respiratory circuit activity by multiple mechanisms via endogenously released serotonin and substance P.
abstract: Brainstem serotonin (5-HT) neurons modulate activity of many neural circuits in the mammalian brain, but in many cases endogenous mechanisms have not been resolved. Here, we analyzed actions of raphé 5-HT neurons on respiratory network activity including at the level of the pre-Bötzinger complex (pre-BötC) in neonatal rat medullary slices in vitro, and in the more intact nervous system of juvenile rats in arterially perfused brainstem-spinal cord preparations in situ. At basal levels of activity, excitation of the respiratory network via simultaneous release of 5-HT and substance P (SP), acting at 5-HT(2A/2C), 5-HT(4), and/or neurokinin-1 receptors, was required to maintain inspiratory motor output in both the neonatal and juvenile systems. The midline raphé obscurus contained spontaneously active 5-HT neurons, some of which projected to the pre-BötC and hypoglossal motoneurons, colocalized 5-HT and SP, and received reciprocal excitatory connections from the pre-BötC. Experimentally augmenting raphé obscurus activity increased motor output by simultaneously exciting pre-BötC and motor neurons. Biophysical analyses in vitro demonstrated that 5-HT and SP modulated background cation conductances in pre-BötC and motor neurons, including a nonselective cation leak current that contributed to the resting potential, which explains the neuronal depolarization that augmented motor output. Furthermore, we found that 5-HT, but not SP, can transform the electrophysiological phenotype of some pre-BötC neurons to intrinsic bursters, providing 5-HT with an additional role in promoting rhythm generation. We conclude that raphé 5-HT neurons excite key circuit components required for generation of respiratory motor output.
authors.affiliation: Cellular and Systems Neurobiology Section, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke-National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Ptak
authors.forename: Tadashi
authors.initials: T
authors.lastname: Yamanishi
authors.forename: Jason
authors.initials: J
authors.lastname: Aungst
authors.forename: Lorin S
authors.initials: LS
authors.lastname: Milescu
authors.forename: Ruli
authors.initials: R
authors.lastname: Zhang
authors.forename: George B
authors.initials: GB
authors.lastname: Richerson
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Smith