title: Nonexocytotic serotonin release tonically suppresses serotonergic neuron activity.
abstract: The firing activity of serotonergic neurons in raphe nuclei is regulated by negative feedback exerted by extracellular serotonin (5-HT)o acting through somatodendritic 5-HT1A autoreceptors. The steady-state [5-HT]o, sensed by 5-HT1A autoreceptors, is determined by the balance between the rates of 5-HT release and reuptake. Although it is well established that reuptake of 5-HTo is mediated by 5-HT transporters (SERT), the release mechanism has remained unclear. It is also unclear how selective 5-HT reuptake inhibitor (SSRI) antidepressants increase the [5-HT]o in raphe nuclei and suppress serotonergic neuron activity, thereby potentially diminishing their own therapeutic effect. Using an electrophysiological approach in a slice preparation, we show that, in the dorsal raphe nucleus (DRN), continuous nonexocytotic 5-HT release is responsible for suppression of phenylephrine-facilitated serotonergic neuron firing under basal conditions as well as for autoinhibition induced by SSRI application. By using 5-HT1A autoreceptor-activated G protein-gated inwardly rectifying potassium channels of patched serotonergic neurons as 5-HTo sensors, we show substantial nonexocytotic 5-HT release under conditions of abolished firing activity, Ca(2+) influx, vesicular monoamine transporter 2-mediated vesicular accumulation of 5-HT, and SERT-mediated 5-HT transport. Our results reveal a cytosolic origin of 5-HTo in the DRN and suggest that 5-HTo may be supplied by simple diffusion across the plasma membrane, primarily from the dense network of neurites of serotonergic neurons surrounding the cell bodies. These findings indicate that the serotonergic system does not function as a sum of independently acting neurons but as a highly interdependent neuronal network, characterized by a shared neurotransmitter pool and the regulation of firing activity by an interneuronal, yet activity-independent, nonexocytotic mechanism.
authors.affiliation: Department of Neuroscience, Psychology, Drug Research and Child Health, University of Florence, 50121 Florence, Italy bmlinar@unifi.it.
authors.forename: Boris
authors.initials: B
authors.lastname: Mlinar
authors.affiliation: Department of Neuroscience, Psychology, Drug Research and Child Health, University of Florence, 50121 Florence, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Montalbano
authors.affiliation: Department of Neuroscience, Psychology, Drug Research and Child Health, University of Florence, 50121 Florence, Italy.
authors.forename: Gilda
authors.initials: G
authors.lastname: Baccini
authors.affiliation: Department of Neuroscience, Psychology, Drug Research and Child Health, University of Florence, 50121 Florence, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Tatini
authors.affiliation: Department of Neuroscience, Psychology, Drug Research and Child Health, University of Florence, 50121 Florence, Italy.
authors.forename: Rolando
authors.initials: R
authors.lastname: Berlinguer Palmini
authors.affiliation: Department of Neuroscience, Psychology, Drug Research and Child Health, University of Florence, 50121 Florence, Italy.
authors.forename: Renato
authors.initials: R
authors.lastname: Corradetti
mesh_terms: D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D005089:Exocytosis; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels / Q000378:metabolism; D008297:Male; D051381:Rats; D017208:Rats, Wistar; D059326:Serotonergic Neurons / Q000378:metabolism* / Q000502:physiology; D012701:Serotonin / Q000378:metabolism*; D050486:Serotonin Plasma Membrane Transport Proteins / Q000378:metabolism; D050493:Vesicular Monoamine Transport Proteins / Q000378:metabolism
chemical_list: D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D050486:Serotonin Plasma Membrane Transport Proteins; D050493:Vesicular Monoamine Transport Proteins; D012701:Serotonin; D002118:Calcium