Serotonin enhances the resistance reflex of the locomotor network of the crayfish through multiple modulatory effects that act cooperatively.

1. J Neurosci. 2004 Jan 14;24(2):398-411.

Serotonin enhances the resistance reflex of the locomotor network of the crayfish
through multiple modulatory effects that act cooperatively.

Le Bon-Jego M(1), Cattaert D, Pearlstein E.

Author information:
(1)Laboratoire de Neurobiologie des Réseaux, Centre National de la Recherche
Scientifique Unité Mixte de Recherche 5816, Biologie Animale, Bâtiment B2, 33405
Talence cedex, France.

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.

DOI: 10.1523/JNEUROSCI.4032-03.2004
PMID: 14724238 [Indexed for MEDLINE]

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