In healthy adults, activation of γ-aminobutyric acid (GABA) A and glycine receptors inhibits neurons as a result of low intracellular chloride concentration ([Cl -] i ), which is maintained by the potassium-chloride cotransporter KCC2. A reduction of KCC2 expression or function is implicated in the pathogenesis of several neurological disorders, including spasticity and chronic pain following spinal cord injury (SCI). Given the critical role of KCC2 in regulating the strength and robustness of inhibition, identifying tools that may increase KCC2 function and, hence, restore endogenous inhibition in pathological conditions is of particular importance. We show that activation of 5-hydroxytryptamine (5-HT) type 2A receptors to serotonin hyperpolarizes the reversal potential of inhibitory postsynaptic potentials (IPSPs), E IPSP , in spinal motoneurons, increases the cell membrane expression of KCC2 and both restores endogenous inhibition and reduces spasticity after SCI in rats. Up-regulation of KCC2 function by targeting 5-HT 2A receptors, therefore, has therapeutic potential in the treatment of neurological disorders involving altered chloride homeostasis. However, these receptors have been implicated in several psychiatric disorders, and their effects on pain processing are controversial, highlighting the need to further investigate the potential systemic effects of specific 5-HT 2A R agonists, such as (4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2). ] i (depolarizing shift of the chloride equilibrium potential, E Cl ) dramatically compromises the inhibitory control of firing rate and excitatory inputs (5-7). Given the role of KCC2 in regulating the strength of inhibitory synaptic transmission, identifying tools that may increase KCC2 function and, hence, restore endogenous inhibition in pathological conditions is of particular importance.Spasticity is a disabling complication affecting individuals with spinal cord injury (SCI) and is characterized by a velocity-dependent increase in muscle tone resulting from hyperexcitable stretch reflexes, spasms, and hypersensitivity to normally innocuous sensory stimulations (8, 9). Down-regulation of KCC2 after SCI in rats is implicated in the development of spasticity (10) and chronic pain (11,12). Notably, the expression of KCC2 in the motoneuron membrane is reduced, and, concomitantly, the density of cytoplasmic clusters is higher, suggesting that the surface stability of the transporter is reduced in these pathological conditions (10).Mounting evidence indicates that phosphorylation of KCC2 in the C-terminal intracellular domain dynamically regulates its activity and surface expression (1). In particular, phosphorylation by protein kinase (PK)C, enhances KCC2 activity and reduces endocytosis (13). Interestingly, activation of 5-hydroxytryptamine type 2 receptors (5-HT 2 Rs) to serotonin stimulates PKC and strengthens the left-right alternation of motor bursts observed during locomotion (14-16), which rely on reciprocal inhibition (17, 18). ...

Reactive cell proliferation occurs rapidly in the cat vestibular nuclei (VN) after unilateral vestibular neurectomy (UVN) and has been reported to facilitate the recovery of posturo-locomotor functions. Interestingly, whereas animals experience impairments for several weeks, extraordinary plasticity mechanisms take place in the local microenvironment of the VN: newborn cells survive and acquire different phenotypes, such as microglia, astrocytes, or GABAergic neurons, whereas animals eventually recover completely from their lesion-induced deficits. Because brain-derived neurotrophic factor (BDNF) can modulate vestibular functional recovery and neurogenesis in mammals, in this study, we examined the effect of BDNF chronic intracerebroventricular infusion versus K252a (a Trk receptor antagonist) in our UVN model. Results showed that long-term intracerebroventricular infusion of BDNF accelerated the restoration of vestibular functions and significantly increased UVN-induced neurogenesis, whereas K252a blocked that effect and drastically delayed and prevented the complete restoration of vestibular functions. Further, because the level of excitability in the deafferented VN is correlated with behavioral recovery, we examined the state of neuronal excitability using two specific markers: the cation-chloride cotransporter KCC2 (which determines the hyperpolarizing action of GABA) and GABA A receptors. We report for the first time that, during an early time window after UVN, significant BDNF-dependent remodeling of excitability markers occurs in the brainstem. These data suggest that GABA acquires a transient depolarizing action during recovery from UVN, which potentiates the observed reactive neurogenesis and accelerates vestibular functional recovery. These findings suggest that BDNF and/or KCC2 could represent novel treatment strategies for vestibular pathologies.

No abstract

Maturation of inhibitory postsynaptic transmission onto motoneurons in the rat occurs during the perinatal period, a time window during which pathways arising from the brainstem reach the lumbar enlargement of the spinal cord. There is a developmental switch in miniature IPSCs (mIPSCs) from predominantly long-duration GABAergic to short-duration glycinergic events. We investigated the effects of a complete neonatal [postnatal day 0 (P0)] spinal cord transection (SCT) on the expression of Glycine and GABA A receptor subunits (GlyR and GABA A R subunits) in lumbar motoneurons. In control rats, the density of GlyR increased from P1 to P7 to reach a plateau, whereas that of GABA A R subunits dropped during the same period. In P7 animals with neonatal SCT (SCT-P7), the GlyR densities were unchanged compared with controls of the same age, while the developmental downregulation of GABA A R was prevented. Whole-cell patch-clamp recordings of mIPSCs performed in lumbar motoneurons at P7 revealed that the decay time constant of miniature IPSCs and the proportion of GABAergic events significantly increased after SCT. After daily injections of the 5-HT 2 R agonist DOI, GABA A R immunolabeling on SCT-P7 motoneurons dropped down to values reported in control-P7, while GlyR labeling remained stable. A SCT made at P5 significantly upregulated the expression of GABA A R 1 week later with little, if any, influence on GlyR. We conclude that the plasticity of GlyR is independent of supraspinal influences whereas that of GABA A R is markedly influenced by descending pathways, in particular serotoninergic projections.

Rats with complete spinal cord transection (SCT) can recover hindlimb locomotor function under strategies combining exercise training and 5-HT agonist treatment. This recovery is expected to result from structural and functional re-organization within the spinal cord below the lesion. To begin to understand the nature of this reorganization, we examined synaptic changes to identified gastrocnemius (GS) or tibialis anterior (TA) motoneurons (MNs) in SCT rats after a schedule of early exercise training and delayed 5-HT agonist treatment. In addition, we analyzed changes in distribution and number of lumbar interneurons (INs) presynaptic to GS MNs using retrograde transneuronal transport of rabies virus. In SCT-untrained rats, we found few changes in the density and size of inhibitory and excitatory inputs impinging on cell bodies of TA and GS MNs compared to intact rats, whereas there was a marked trend for a reduction in the number of premotor INs connected to GS MNs. In contrast, after training of SCT rats, a significant increase of the density of GABAergic and glycinergic axon terminals was observed on both GS and TA motoneuronal cell bodies, as well as of presynaptic P-boutons on VGLUT1 afferents. Despite these changes in innervation the number of premotor INs connected to GS MNs was similar to control values although some new connections to MNs were observed. These results suggest that adaptation of gait patterns in SCT-trained rats was accompanied by changes in the innervation of lumbar MNs while the distribution of the spinal premotor circuitry was relatively preserved.

No abstract

No abstract

No abstract