Motoneurons In Spinal Cord Slices Research Articles

The influence of increased membrane conductance on response properties of spinal motoneurons

During functional spinal neural network activity motoneurons receive massive synaptic excitation and inhibition, and their membrane conductance increases considerably – they are switched to a high-conductance state. High-conductance states can substantially alter response properties of motoneurons. In the present study we investigated how an increase in membrane conductance affects spike frequency adaptation, the gain (i.e., the slope of the frequency-current relationship) and the threshold for action potential generation. We used intracellular recordings from adult turtle motoneurons in spinal cord slices. Membrane conductance was increased pharmacologically by extracellular application of the GABAA receptor agonist muscimol. Our findings suggest that an increase in membrane conductance of about 40–50% increases the magnitude of spike frequency adaptation, but does not change the threshold for action potential generation. Increased conductance causes a subtractive rather than a divisive effect on the initial and the early frequency-current relationships and may have not only a subtractive but also a divisive effect on the steady-state frequency-current relationship.

Muscarinic control of the excitability of hindlimb motoneurons in chronic spinal-transected salamanders

The excitability of spinal motoneurons (MNs) is regulated by acetylcholine via the activation of muscarinic receptors. The objective of the present study was to determine whether this cholinergic modulation of MN excitability is altered following a chronic spinal cord transection. Juvenile salamanders (Pleurodeles waltlii) were spinally transected at the mid-trunk level, and patch-clamp recordings from hindlimb MNs in spinal cord slices were performed 9-30 days after transection, with and without bath application of muscarine (20 mum). Our results showed that the input-output relationship was larger in MNs recorded 2 weeks after spinal transection than in MNs recorded 3-4 weeks after spinal transection. They further revealed that muscarine increased both the gain of MNs and the proportion of MNs that could exhibit plateau potentials and afterdischarges, whereas it decreased the amplitude of the medium afterhypolarizing potential. Moreover, muscarine had no effect on the hyperpolarization-activated cation current (I(h)), whereas it increased the inward rectifying K(+) current (I(Kir)) in MNs recorded > or = 2 weeks after spinal transection. We conclude that following chronic spinal cord injury, the muscarinic modulation of some intrinsic properties of MNs previously reported in acute spinal-transected animals [S. Chevallier et al. (2006)The Journal of Physiology, 570, 525-540] was preserved, whereas that of other intrinsic properties of MNs was suppressed, either transiently (I(Kir)) or definitively (I(h)). These alterations in muscarinic modulation of MN excitability may contribute to the spontaneous recovery of locomotion displayed in long-term chronic spinal-transected salamanders.

Spinal cholinergic interneurons regulate the excitability of motoneurons during locomotion

To effect movement, motoneurons must respond appropriately to motor commands. Their responsiveness to these inputs, or excitability, is regulated by neuromodulators. Possible sources of modulation include the abundant cholinergic "C boutons" that surround motoneuron somata. In the present study, recordings from motoneurons in spinal cord slices demonstrated that cholinergic activation of m2-type muscarinic receptors increases excitability by reducing the action potential afterhyperpolarization. Analyses of isolated spinal cord preparations in which fictive locomotion was elicited demonstrated that endogenous cholinergic inputs increase motoneuron excitability during locomotion. Anatomical data indicate that C boutons originate from a discrete group of interneurons lateral to the central canal, the medial partition neurons. These results highlight a unique component of spinal motor networks that is critical in ensuring that sufficient output is generated by motoneurons to drive motor behavior.

Metabotropic Modulation of Motoneurons by Scratch-Like Spinal Network Activity

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.

Axotomy-induced change in the properties of ( S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor channels in rat motoneurons

Properties of ( S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor channels were studied in fluorescence-labelled control and axotomized motoneurons in spinal cord slices using a patch-clamp technique. Axotomy performed on the third postnatal day resulted in motoneuron death. Application of AMPA or kainate induced large whole-cell currents, but outside-out patches isolated from control motoneurons were either unresponsive or displayed only single-channel activity in response to rapid application of AMPA. Measurement of AMPA receptor channel openings in outside-out patches revealed multiple single-channel conductance levels: 12.2±1.0, 21.9±1.5 and 32.6±3.2 pS. In control motoneurons dialysed with spermine, the current–voltage relationship of responses induced by activation of AMPA receptor channels exhibited various degrees of inward rectification. The rectification index, the ratio of responses at +40 and −60 mV, was used to compare the degree of inward rectification. The mean values of rectification index of responses to focal application of AMPA and AMPA receptor-mediated excitatory postsynaptic currents induced by focal electric stimulation were 0.64±0.17 and 0.50±0.27, respectively. In axotomized motoneurons, the degree of rectification was significantly less for both responses induced by application of AMPA and for excitatory postsynaptic currents (0.91±0.09 and 0.95±0.12, respectively). Deactivation of AMPA receptors assessed from motoneuron excitatory postsynaptic currents at −70 mV was independent of postnatal age, with τ fast=0.88±0.35 ms ( A fast=78.2±11.8%) and τ slow=6.3±3.2 ms. In axotomized motoneurons, the decay time constants of excitatory postsynaptic currents were similar, τ fast=0.91±0.42 ms ( A fast=85.8±12.6%) and τ slow=5.9±3.4 ms. However, the mean amplitude of excitatory postsynaptic currents was only 43% of the amplitude recorded in control motoneurons. The results show that the current induced by activation of AMPA receptors in neonatal motoneurons is mediated by opening of both Ca 2+-permeable and Ca 2+-impermeable channels. As a result of axotomy, an experimental model of neurodegeneration, AMPA receptor channels in injured motoneurons destined to die become predominantly Ca 2+ impermeable. These findings suggest phenotypic control of AMPA receptor channel properties, presumably by affecting their subunit composition.