Activation of GABAA receptors on sensory axons produces a primary afferent depolarization (PAD) that modulates sensory transmission in the spinal cord. While axoaxonic synaptic contacts of GABAergic interneurons onto afferent terminals have been extensively studied, less is known about the function of extrasynaptic GABA receptors on afferents. Thus, we examined extrasynaptic α5GABAA receptors on low-threshold proprioceptive (group Ia) and cutaneous afferents. Afferents were impaled with intracellular electrodes and filled with neurobiotin in the sacrocaudal spinal cord of rats. Confocal microscopy was used to reconstruct the afferents and locate immunolabelled α5GABAA receptors. In all afferents α5GABAA receptors were found throughout the extensive central axon arbors. They were most densely located at branch points near sodium channel nodes, including in the dorsal horn. Unexpectedly, proprioceptive afferent terminals on motoneurons had a relative lack of α5GABAA receptors. When recording intracellularly from these afferents, blocking α5GABAA receptors (with L655708, gabazine, or bicuculline) hyperpolarized the afferents, as did blocking neuronal activity with tetrodotoxin, indicating a tonic GABA tone and tonic PAD. This tonic PAD was increased by repeatedly stimulating the dorsal root at low rates and remained elevated for many seconds after the stimulation. It is puzzling that tonic PAD arises from α5GABAA receptors located far from the afferent terminal where they can have relatively little effect on terminal presynaptic inhibition. However, consistent with the nodal location of α5GABAA receptors, we find tonic PAD helps produce sodium spikes that propagate antidromically out the dorsal roots, and we suggest that it may well be involved in assisting spike transmission in general. NEW & NOTEWORTHY GABAergic neurons are well known to form synaptic contacts on proprioceptive afferent terminals innervating motoneurons and to cause presynaptic inhibition. However, the particular GABA receptors involved are unknown. Here, we examined the distribution of extrasynaptic α5GABAA receptors on proprioceptive Ia afferents. Unexpectedly, these receptors were found preferentially near nodal sodium channels throughout the afferent and were largely absent from afferent terminals. These receptors produced a tonic afferent depolarization that modulated sodium spikes, consistent with their location.

Blood vessels in the central nervous system (CNS) are controlled by neuronal activity; for example, widespread vessel constriction (vessel tone) is induced by brainstem neurons that release the monoamines serotonin and noradrenaline, and local vessel dilation is induced by glutamatergic neuron activity. Here, we examined how vessel tone adapts to the loss of neuron-derived monoamines after spinal cord injury (SCI) in rats. We find that, months after the imposition of SCI, the spinal cord below the site of injury is in a chronic state of hypoxia, due to paradoxical excess activity of monoamine receptors (5-HT1) on pericytes, despite the absence of monoamines. This monoamine receptor activity causes pericytes to locally constrict capillaries, reducing blood flow to ischemic levels. Receptor activation in the absence of monoamines results from the production of trace amines (such as tryptamine) by pericytes that ectopically express the enzyme aromatic-l-amino-acid-decarboxylase (AADC), which synthesizes trace amines directly from dietary amino acids (such as tryptophan). Inhibition of monoamine receptors or of AADC, or even increased inhaled oxygen, produces substantial relief from hypoxia and improves motoneuron and locomotor function after SCI.

Movement and posture depend on sensory feedback that is regulated by specialized GABAergic neurons (GAD2 + ) that form axo-axonic contacts onto myelinated proprioceptive sensory axons and are thought to be inhibitory. However, we report here that activating GAD2 + neurons, directly with optogenetics or indirectly by cutaneous stimulation, facilitates sensory feedback to motoneurons in awake rodents and humans. GABAA receptors and GAD2 + contacts adjacent to nodes of Ranvier at branch points of sensory axons cause this facilitation, preventing spike propagation failure that is otherwise common without GABA. GABAA receptors are generally lacking from axon terminals (unlike GABAB) and do not inhibit transmitter release onto motoneurons, disproving the long-standing assumption that GABAA receptors cause presynaptic inhibition. GABAergic innervation of nodes near branch points allows individual branches to function autonomously, with GAD2 + neurons regulating which branches conduct, adding a computational layer to the neuronal networks generating movement and likely generalizing to other CNS axons. MainThe ease with which animals move defies the complexity of the underlying neuronal circuits, which include corticospinal tracts (CSTs) that coordinate skilled movement, spinal interneurons that form central patterns generators (CGPs) for walking, and motoneurons that ultimately drive the muscles 1 .Sensory feedback ensures the final precision of such motor acts, with proprioceptive feedback to motoneurons producing a major part of the muscle activity in routine movement and posture 2-4 , without which severe ataxia occurs 5 . Proprioceptive sensory feedback is regulated by specialized GABAergic neurons (GAD2 + ; abbreviated GABAaxo neurons) that form axo-axonic connections onto the sensory axon terminals [6][7][8] . These neurons are thought to produce presynaptic inhibition of sensory feedback to motoneurons 9-11 and possibly limit inappropriate sensory feedback 3,4,7 . However, during movement the CST, CPG and even sensory neurons all augment GABAaxo neuron activity [11][12][13][14][15][16] right at a time when sensory feedback is known to be increased to ensure precision and postural stability 2-4 , raising the question of whether GABAaxo neurons have a yet undescribed excitatory action.The long-standing view that GABAergic neurons produce presynaptic inhibition of proprioceptive sensory axon terminals in adult mammals actually lacks direct evidence, largely because of the difficulty in recording from these small terminals and the technical limitations of previously employed

A new method of estimating synaptic drive to multiple, simultaneously recorded motor units provides evidence that the portion of the depolarizing drive from persistent inward currents that contributes to self-sustained firing is similar across motoneurons of different sizes.

Key points• Chronic in vivo imaging of cellular interactions within the adult spinal cord with subcellular resolution is important for understanding cellular physiology and disease progression.• Previous approaches for chronic in vivo spinal cord microscopy have required surgery for each imaging session.• Here we describe a novel method for implanting glass windows over the exposed spinal cords of adult mice for repeated in vivo microscopy.• We show that the windows remain clear for many months after implantation, do not damage axons or blood vessels, and are useful for studying cellular dynamics after spinal cord injury.• Our method represents an original technical breakthrough for scientists involved in spinal cord research and in vivo imaging, and is a useful tool for studying cellular physiology and disease progression.Abstract Repeated in vivo two-photon imaging of adult mammalian spinal cords, with subcellular resolution, would be crucial for understanding cellular mechanisms under normal and pathological conditions. Current methods are limited because they require surgery for each imaging session. Here we report a simple glass window methodology avoiding repeated surgical procedures and subsequent inflammation. We applied this strategy to follow axon integrity and the inflammatory response over months by multicolour imaging of adult transgenic mice. We found that glass windows have no significant effect on axon number or structure, cause a transient inflammatory response, and dramatically increase the throughput of in vivo spinal imaging. Moreover, we used this technique to track retraction/degeneration and regeneration of cut axons after a 'pin-prick' spinal cord injury with high temporal fidelity. We showed that regenerating axons can cross an injury site within 4 days and that their terminals undergo dramatic morphological changes for weeks after injury. Overall the technique can potentially be adapted to evaluate cellular functions and therapeutic strategies in the normal and diseased spinal cord.

Secondary manifestations of spinal cord injury beyond motor and sensory dysfunction can negatively affect a person's quality of life. Spinal cord injury is associated with an increased incidence of depression and anxiety; however, the mechanisms of this relationship are currently not well understood. Human and animal studies suggest that changes in the composition of the intestinal microbiota (dysbiosis) are associated with mood disorders. The objective of the current study is to establish a model of anxiety following a cervical contusion spinal cord injury in rats and to determine whether the microbiota play a role in the observed behavioural changes. We found that spinal cord injury caused dysbiosis and increased symptoms of anxiety-like behaviour. Treatment with a fecal transplant prevented both spinal cord injury-induced dysbiosis as well as the development of anxiety-like behaviour. These results indicate that an incomplete unilateral cervical spinal cord injury can cause affective disorders and intestinal dysbiosis, and that both can be prevented by treatment with fecal transplant therapy.

The degenerative processes following peripheral nerve and central nerve injuries are similar in many respects but there are several intrinsic and environmental differences that distinguish the two processes. Neurons whose axons travel in the peripheral nerves regenerate their axons within the permissive growth environment of the Schwann cells. Nonetheless, neurons and the Schwann cells of the peripheral nervous system (PNS) progressively fail to sustain the regenerative response with time after injury. In the central nervous system (CNS), the intrinsic growth potential of injured neurons contrasts with that of injured PNS neurons in being insufficient even immediately after ABSTRACT: Injured nerves regenerate their axons in the peripheral (PNS) but not the central nervous system (CNS). The contrasting capacities have been attributed to the growth permissive Schwann cells in the PNS and the growth inhibitory environment of the oligodendrocytes in the CNS. In the current review, we first contrast the robust regenerative response of injured PNS neurons with the weak response of the CNS neurons, and the capacity of Schwann cells and not the oligodendrocytes to support axonal regeneration. We then consider the factors that limit axonal regeneration in both the PNS and CNS. Limiting factors in the PNS include slow regeneration of axons across the injury site, progressive decline in the regenerative capacity of axotomized neurons (chronic axotomy) and progressive failure of denervated Schwann cells to support axonal regeneration (chronic denervation). In the CNS on the other hand, it is the poor regenerative response of neurons, the inhibitory proteins that are expressed by oligodendrocytes and act via a common receptor on CNS neurons, and the formation of the glial scar that prevent axonal regeneration in the CNS. Strategies to overcome these limitations in the PNS are considered in detail and contrasted with strategies in the CNS. RÉSUMÉ: Régénérescence axonale dans le système nerveux périphérique et dans le système nerveux central -Questions actuelles et progrès. Revue émanant de l'Association canadienne des neurosciences:Les nerfs du système nerveux périphérique (SNP) qui ont subi une lésion régénèrent leurs axones alors que ceux du système nerveux central (SNC) ne le font pas. Cette différence a été attribuée à un environnement permissif conféré par les cellules de Schwann dans le SNP et à un environnement inhibiteur de la croissance conféré par les oligodendrocytes dans le SNC. Dans cette revue, nous opposons la réponse régénératrice robuste des neurones du SNP lésés à la réponse faible des neurones du SNC et la capacité des cellules de Schwann de supporter la régénérescence axonale à l'incapacité des oligodendrocytes de le faire. Nous considérons ensuite les facteurs qui limitent la régénérescence axonale dans le SNP et dans le SNC. Dans le SNP, ces facteurs sont la régénérescence lente des axones pour franchir la lésion, et l'axotomie et la dénervation chronique des cellules de Schwann qui diminuent progressive...

It is well established that long, descending axons of the adult mammalian spinal cord do not regenerate after a spinal cord injury (SCI). These axons do not regenerate because they do not mount an adequate regenerative response and growth is inhibited at the injury site by growth cone collapsing molecules, such as chondroitin sulfate proteoglycans (CSPGs). However, whether axons of axotomized spinal interneurons regenerate through the inhibitory environment of an SCI site remains unknown. Here, we show that cut axons from adult mammalian spinal interneurons can regenerate through an SCI site and form new synaptic connections in vivo. Using morphological and immunohistochemical analyses, we found that after a midsagittal transection of the adult feline spinal cord, axons of propriospinal commissural interneurons can grow across the lesion despite a close proximity of their growth cones to CSPGs. Furthermore, using immunohistochemical and electrophysiological analyses, we found that the regenerated axons conduct action potentials and form functional synaptic connections with motoneurons, thus providing new circuits that cross the transected commissures. Our results show that interneurons of the adult mammalian spinal cord are capable of spontaneous regeneration after injury and suggest that elucidating the mechanisms that allow these axons to regenerate may lead to useful new therapeutic strategies for restoring function after injury to the adult CNS.