title: Spatial segregation of excitatory and inhibitory effects of 5-HT on crayfish motoneurons.
abstract: Altering neuronal membrane properties, including input resistance, is a key modulatory mechanism for changing neural activity patterns. The effect of membrane currents generated by either synaptic or voltage-dependent channels directly depends on neuron input resistance. We found that local application of serotonin to different regions of identified motoneurons (MNs) of the postural/walking network of isolated crayfish produced different changes in input resistance. Puff-applied 5-HT in the periphery of the initial segment produced exclusively inhibitory responses. In contrast, when 5-HT was puff-applied on the central arbor of the same depressor (Dep) MN, exclusively depolarizing responses were obtained. Both inhibitory and excitatory responses were direct because they persisted in low-calcium saline. We found numerous close appositions between 5-HT-immunoreactive processes and the initial segment of dextran-rhodamine-filled Dep MNs. In contrast, almost no close apposition sites were found in Dep MN arbor. It seems that the 5-HT controls the level of excitability of postural network MNs by two mechanisms acting at two different sites: inhibitory responses (consistent with an action involving opening of K(+) channels) occur in the initial segment region and may involve classic synaptic transmission, whereas depolarizing responses (consistent with an action involving closing of K(+) channels) occur on MN branches via apparent paracrine effects.
authors.affiliation: Université de Bordeaux, Centre National de la Recherche Scientifique, Institut de Neurosciences Cognitives et Intégratives d'Aquitaine, Biologie Animale, Talence Cedex, France.
authors.forename: Julien
authors.initials: J
authors.lastname: Bacqué-Cazenave
authors.forename: Fadi A
authors.initials: FA
authors.lastname: Issa
authors.forename: Donald H
authors.initials: DH
authors.lastname: Edwards
authors.forename: Daniel
authors.initials: D
authors.lastname: Cattaert
mesh_terms: D000200:Action Potentials; D000818:Animals; D003400:Astacoidea; D002118:Calcium / Q000494:pharmacology; D017952:Ganglia, Invertebrate / Q000166:cytology / Q000502:physiology; D009046:Motor Neurons / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D009433:Neural Inhibition; D019899:Paracrine Communication; D015221:Potassium Channels / Q000378:metabolism; D012701:Serotonin / Q000494:pharmacology*
chemical_list: D015221:Potassium Channels; D012701:Serotonin; D002118:Calcium
keywords: 5-HT cell; Crustacea; crayfish; neuromodulation; paracrine; posture; serotonin; stretch reflex