Serotonin enhances the resistance reflex of the locomotor network of the crayfish through multiple modulatory effects that act cooperatively.
Serotonin (5HT) is an endogenous amine that modifies posture in crustacea. Here, we examined the mechanisms of action of 5HT on the resistance reflex in crayfish legs. This reflex, which counteracts movements imposed on a limb, is based on a negative feedback system formed by proprioceptors that sense joint angle movements and activate opposing motoneurons. We performed intracellular recordings from depressor motoneurons while repetitively stretching and releasing a leg joint proprioceptor in a resting in vitro preparation (i.e., a preparation that lacks spontaneous rhythmic activity). 5HT increased the amplitude of the depolarization during the release phase of the proprioceptor (corresponding to an upward movement of the leg) and the discharge frequency of the motoneurons. The 5HT-induced increase in the resistance reflex is caused, to a large extent, by polysynaptic pathways because it was very attenuated in the presence of high divalent cation solution. In addition to this activation of the polysynaptic pathways, 5HT also has postsynaptic effects that enhance the resistance reflex. 5HT causes a tonic depolarization, as well as an increase in the time constant and input resistance of motoneurons. We developed a simple mathematical model to describe the integrative properties of the motoneurons. The conclusion of this study is that the input frequency and the decay time constant of the EPSPs interact in such a way that small simultaneous changes in these parameters can cause a large effect on summation. Therefore, the conjunction of presynaptic and postsynaptic changes produces a strong cooperative effect on the resistance reflex response.
Le Bon-Jego
Morgane
M
Laboratoire de Neurobiologie des Réseaux, Centre National de la Recherche Scientifique Unité Mixte de Recherche 5816, Biologie Animale, Bâtiment B2, 33405 Talence cedex, France.
Cattaert
Daniel
D
Pearlstein
Edouard
E
D000818:Animals; D003400:Astacoidea / Q000502:physiology*; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D005260:Female; D007700:Kinetics; D008297:Male; D009043:Motor Activity / Q000502:physiology*; D009046:Motor Neurons / Q000187:drug effects / Q000502:physiology; D009415:Nerve Net / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D009475:Neurons, Afferent / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D012026:Reflex, Stretch / Q000187:drug effects; D012701:Serotonin / Q000494:pharmacology*; D013569:Synapses / Q000502:physiology
D012701:Serotonin