Normal Distribution of VGLUT1 Synapses on Spinal Motoneuron Dendrites and Their Reorganization after Nerve Injury

Abstract

Peripheral nerve injury induces permanent alterations in spinal cord circuitries that are not reversed by regeneration. Nerve injury provokes the loss of many proprioceptive IA afferent synapses (VGLUT1-IR boutons) from motoneurons, the reduction of IA EPSPs in motoneurons, and the disappearance of stretch reflexes. After motor and sensory axons successfully reinnervate muscle, lost IA VGLUT1 synapses are not re-established and the stretch reflex does not recover; however, electrically evoked EPSPs do recover. The reasons why remaining IA synapses can evoke EPSPs on motoneurons, but fail to transmit useful stretch signals are unknown. To better understand changes in the organization of VGLUT1 IA synapses that might influence their input strength, we analyzed their distribution over the entire dendritic arbor of motoneurons before and after nerve injury. Adult rats underwent complete tibial nerve transection followed by microsurgical reattachment and 1 year later motoneurons were intracellularly recorded and filled with neurobiotin to map the distribution of VGLUT1 synapses along their dendrites. We found in control motoneurons an average of 911 VGLUT1 synapses; ϳ62% of them were lost after injury. In controls, VGLUT1 synapses were focused to proximal dendrites where they were grouped in tight clusters. After injury, most synaptic loses occurred in the proximal dendrites and remaining synapses were declustered, smaller, and uniformly distributed throughout the dendritic arbor. We conclude that this loss and reorganization renders IA afferent synapses incompetent for efficient motoneuron synaptic depolarization in response to natural stretch, while still capable of eliciting EPSPs when synchronously fired by electrical volleys.

Delaying the onset of treadmill exercise following peripheral nerve injury has different effects on axon regeneration and motoneuron synaptic plasticity

“…This assumption has been supported by others. Immunoreactive structures identified using the same antibodies as used here have been shown to contain synaptic active zones and, after extensive high-magnification reconstruction, to be in very close contact to the somata of intracellularly filled motoneurons (Rotterman et al 2014). However, without higher resolution images available through electron microscopy, we acknowledge that we cannot rule out that at least some of the contacts we have studied might be separated from the motoneuron soma by ultrafine processes of glial cells.…”

“…A similar amount of withdrawal of these excitatory connections was found in exercised rats in our Delayed group. In a recent paper Rotterman and colleagues (Rotterman et al 2014) provide a compelling explanation for this modest reflex (or EPSP) restoration despite a striking loss of the synaptic inputs responsible for them. Using an elegant analysis of all of the VGLUT1-IR inputs onto motoneurons marked by intracellular injection of neurobiotin, they found a marked reduction in contacts in the somatodendritic regions of the neurons but only a sparse reduction of contacts made on dendrites.…”

“…Among the best documented of these changes is the withdrawal of synaptic inputs from the somata and proximal-most dendrites of motoneurons ). Many of these synaptic inputs are restored as axons regenerate and reinnervate their targets, but one population of inputs, those containing VGLUT1, is lost permanently following peripheral nerve transection, irrespective of whether the regeneration of motor axons is successful Rotterman et al 2014). Several cellular processes have been invoked to explain the transient withdrawal of most synaptic inputs (reviewed in Liu et al 2014), but this permanent loss of VGLUT1-IR synapses has been attributed to injury to the peripheral process of the axons of these dorsal root ganglion neurons .…”

“…Using an elegant analysis of all of the VGLUT1-IR inputs onto motoneurons marked by intracellular injection of neurobiotin, they found a marked reduction in contacts in the somatodendritic regions of the neurons but only a sparse reduction of contacts made on dendrites. These retained inputs are smaller, not clustered, and more heavily distributed in the pathway of any remaining arbors of afferent axons (Rotterman et al 2014). They argued that the responses of motoneurons to synchronous electrical stimulation of sensory axons in the periphery might make them capable of producing EPSPs in motoneurons through this reorganization of dendritic inputs, but that the temporally dispersed activation of different afferent axons anticipated during muscle stretch might not be sufficient.…”

“…The small number of these somatodendritic inputs that remain also are reorganized. They are smaller and no longer clustered around the proximalmost dendrites of the motoneurons (Rotterman et al 2014). This permanent stripping of the synaptic inputs arising from large, stretch-sensitive afferents and the reorganization of the small proportion that remain are major contributors to the permanent loss of the stretch reflex in self-reinnervated muscles Cope and Clark 1993).…”