Metabotropic Modulation of Motoneurons by Scratch-Like Spinal Network Activity

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Abstract
Glutamate is the main excitatory transmitter in the spinal motor network. The excitation is to a large extent mediated by ionotropic receptors, but glutamate also activates metabotropic receptors. In motoneurons in spinal cord slices the activation of group I metabotropic glutamate (mGlu1) receptors leads to facilitation of CaV1.3 L-type calcium channels. Here we investigate whether this pathway is activated by motor network activity induced by natural sensory stimuli. The lumbar carapace and spinal cord were isolated from adult turtles. In this preparation, mechanical stimulation in the receptive field for the scratch reflex induced episodes of rhythmic motor network activity. During an episode the excitability of coactivated motoneurons increased. This increase was associated with an increased persistent inward current and was abolished by local blockade of either mGlu1 receptors or CaV1.3 L-type calcium channels near the recording site. We conclude that glutamate released during spinal motor network activity excites motoneurons by parallel activation of ionotropic and mGlu1 receptors. The metabotropic facilitation of L-type calcium channels contributes significantly to this excitation. Our findings establish intrinsic modulation as an active component in the spinal motor network for limb movements.