Soon Kwon Park

Reactive oxygen species (ROS) play an important role in a rat model of neuropathic pain

&NA; Reactive oxygen species (ROS) are free radicals produced in biological systems that are involved in various degenerative brain diseases. The present study tests the hypothesis that ROS also play an important role in neuropathic pain. In the rat spinal nerve ligation (SNL) model of neuropathic pain, mechanical allodynia develops fully 3 days after nerve ligation and persists for many weeks. Systemic injection of a ROS scavenger, phenyl‐N‐tert‐butylnitrone (PBN), relieves SNL‐induced mechanical allodynia in a dose‐dependent manner. Repeated injections cause no development of tolerance or no loss of potency. Preemptive treatment with PBN is also effective in preventing full development of neuropathic pain behavior. Systemic injection was mimicked by intrathecal injection with a little less efficacy, while intracerebroventricular administration produced a much smaller effect. These data suggest that PBN exerts its anti‐allodynic action mainly by spinal mechanisms. Systemic treatment with other spin‐trap reagents, 5,5‐dimethylpyrroline‐N‐oxide and nitrosobenzene, showed similar analgesic effects, suggesting that ROS are critically involved in the development and maintenance of neuropathic pain. Thus this study suggests that systemic administration of non‐toxic doses of free radical scavengers could be useful for treatment of neuropathic pain.

Low dose of tetrodotoxin reduces neuropathic pain behaviors in an animal model

We hypothesize that the accumulation of tetrodotoxin (TTX) sensitive sodium channels in injured dorsal root ganglion (DRG) neurons plays a critically important role in the generation of ectopic discharges and mechanical allodynia after peripheral nerve injury. Using the segmental spinal nerve (L5) ligation model of neuropathic pain, this hypothesis was tested by examining the effect of TTX on the mechanical sensitivity of the affected hind paw. Various concentrations of TTX were applied topically to the L5 DRG by using chronically implanted polyethylene tubing. The data showed that application of TTX at low doses (12.5-50 nM), which are far less than those needed for blocking action potential conduction, produced a significant elevation of mechanical threshold in the paw for foot withdrawals, a sign of reduced allodynic behaviors. The data suggest that TTX-sensitive subtypes of sodium channels play an important role in maintaining allodynic behaviors in an animal model of neuropathic pain.

The Medial Prefrontal Cortex Is Involved in Spatial Memory Retrieval under Partial-Cue Conditions

Brain circuits involved in pattern completion, or retrieval of memory from fragmented cues, were investigated. Using different versions of the Morris water maze, we explored the roles of the CA3 subregion of the hippocampus and the medial prefrontal cortex (mPFC) in spatial memory retrieval under various conditions. In a hidden platform task, both CA3 and mPFC lesions disrupted memory retrieval under partial-cue, but not under full-cue, conditions. For a delayed matching-to-place task, CA3 lesions produced a deficit in both forming and recalling spatial working memory regardless of extramaze cue conditions. In contrast, damage to mPFC impaired memory retrieval only when a fraction of cues was available. To corroborate the lesion study, we examined the expression of the immediate early gene c-fos in mPFC and the hippocampus. After training of spatial reference memory in full-cue conditions for 6 d, the same training procedure in the absence of all cues except one increased the number of Fos-immunoreactive cells in mPFC and CA3. Furthermore, mPFC inactivation with muscimol, a GABA agonist, blocked memory retrieval in the degraded-cue environment. However, mPFC-lesioned animals initially trained in a single-cue environment had no difficulty in retrieving spatial memory when the number of cues was increased, demonstrating that contextual change per se did not impair the behavioral performance of the mPFC-lesioned animals. Together, these findings strongly suggest that pattern completion requires interactions between mPFC and the hippocampus, in which mPFC plays significant roles in retrieving spatial information maintained in the hippocampus for efficient navigation.

Neuroprotective effect of topiramate on hypoxic ischemic brain injury in neonatal rats.

Perinatal hypoxia-ischemia is one of the most common risk factors for neonatal mortality and permanent neurodevelopmental disability. Topiramate [2,3:4,5-bis-o-(1-methylethylidene) beta-D-fructo-pyranose sulfamate; TPM] is widely used as an antiepileptic agent with multiple targets. In the present study, we found that treatment with TPM reduced the neuronal damage induced by oxygen-glucose deprivation in vitro with strong inhibition of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor. Because perinatal hypoxia is mediated, at least in part, by aberrant glutamatergic excitation, we tested whether treatment with TPM was effective against perinatal brain hypoxia-ischemia. Intraperitoneal or oral pretreatment with TPM was found to reduce the brain damage and subsequent cognitive impairments induced by transient hypoxia-ischemia in perinatal rats. A potent neuroprotective effect of TPM was also observed in a post-treatment regime although post-treatment window appears to be relatively narrow (<2 h). These results suggest that TPM treatment may be beneficial for perinatal hypoxia-ischemia and related damage.

Regulation of synaptic Rac1 activity, long-term potentiation maintenance, and learning and memory by BCR and ABR Rac GTPase-activating proteins

Rho family small GTPases are important regulators of neuronal development. Defective Rho regulation causes nervous system dysfunctions including mental retardation and Alzheimers disease. Rac1, a member of the Rho family, regulates dendritic spines and excitatory synapses, but relatively little is known about how synaptic Rac1 is negatively regulated. Breakpoint cluster region (BCR) is a Rac GTPase-activating protein known to form a fusion protein with the c-Abl tyrosine kinase in Philadelphia chromosome-positive chronic myelogenous leukemia. Despite the fact that BCR mRNAs are abundantly expressed in the brain, the neural functions of BCR protein have remained obscure. We report here that BCR and its close relative active BCR-related (ABR) localize at excitatory synapses and directly interact with PSD-95, an abundant postsynaptic scaffolding protein. Mice deficient for BCR or ABR show enhanced basal Rac1 activity but only a small increase in spine density. Importantly, mice lacking BCR or ABR exhibit a marked decrease in the maintenance, but not induction, of long-term potentiation, and show impaired spatial and object recognition memory. These results suggest that BCR and ABR have novel roles in the regulation of synaptic Rac1 signaling, synaptic plasticity, and learning and memory, and that excessive Rac1 activity negatively affects synaptic and cognitive functions.

Effects of purinergic and adrenergic antagonists in a rat model of painful peripheral neuropathy.

&NA; In previous studies, pain behaviors produced in the spinal nerve ligation rat model of neuropathic pain were partly reduced by surgical lumbar sympathectomy. However, systemic injection of phentolamine, an &agr;‐adrenoceptor blocker, was not effective in reducing pain behaviors, at least in the Sprague–Dawley strain of rats. This suggests that sympathectomy removes not only adrenoceptor function but also other factors that must contribute importantly to the generation of neuropathic pain behaviors. Since the purinergic substance adenosine 5′‐triphosphate (ATP) is known to be co‐released with norepinephrine (NE) from the sympathetic nerve terminals, we hypothesized that ATP might be involved in the sympathetic dependency of neuropathic pain. The present study tested this hypothesis by examining the effects of systemic injection of an adrenoceptor blocker (phentolamine), a purinoceptor blocker (suramin), and a combination of these two on behavioral signs of mechanical allodynia in the spinal nerve ligation model of neuropathic pain. The results of the present study showed two novel findings. First, the mechanical hypersensitivity (allodynia) resulting from the L5/6 spinal nerve ligation can be reduced either by sympathetic block accomplished by application of a local anesthetic or by surgical sympathectomy of the L2–L6 sympathetic ganglia. Second, suramin (at 100 mg/kg, i.p.) can reduce mechanical hypersensitivity in neuropathic rats when given in combination with 5 mg/kg of phentolamine. This effect was observed in a subset of neuropathic rats, and the drug responses were consistent in repeated treatments within the animal group. Neither phentolamine nor suramin changed the mechanical sensitivity of neuropathic rats when given alone. The data suggest that the purinergic substance ATP is co‐released with NE from sympathetic nerve terminals and these two are together involved, at least in part, in the maintenance of the sympathetically dependent component of pain behaviors in some neuropathic rats.

The maintenance of specific aspects of neuronal function and behavior is dependent on programmed cell death of adult‐generated neurons in the dentate gyrus

A considerable number of new neurons are generated daily in the dentate gyrus (DG) of the adult hippocampus, but only a subset of these survive, as many adult‐generated neurons undergo programmed cell death (PCD). However, the significance of PCD in the adult brain for the functionality of DG circuits is not known. Here, we examined the electrophysiological and behavioral characteristics of Bax‐knockout (Bax‐KO) mice in which PCD of post‐mitotic neurons is prevented. The continuous increase in DG cell numbers in Bax‐KO mice resulted in the readjustment of afferent and efferent synaptic connections, represented by age‐dependent reductions in the dendritic arborization of DG neurons and in the synaptic contact ratio of mossy fibers with CA3 dendritic spines. These neuroanatomical changes were associated with reductions in synaptic transmission and reduced performance in a contextual fear memory task in 6‐month‐old Bax‐KO mice. These results suggest that the elimination of excess DG neurons via Bax‐dependent PCD in the adult brain is required for the normal organization and function of the hippocampus.

Inositol 1,4,5-Trisphosphate 3-Kinase A Functions As a Scaffold for Synaptic Rac Signaling

Activity-dependent alterations of synaptic contacts are crucial for synaptic plasticity. The formation of new dendritic spines and synapses is known to require actin cytoskeletal reorganization specifically during neural activation phases. Yet the site-specific and time-dependent mechanisms modulating actin dynamics in mature neurons are not well understood. In this study, we show that actin dynamics in spines is regulated by a Rac anchoring and targeting function of inositol 1,4,5-trisphosphate 3-kinase A (IP3K-A), independent of its kinase activity. On neural activation, IP3K-A bound directly to activated Rac1 and recruited it to the actin cytoskeleton in the postsynaptic area. This focal targeting of activated Rac1 induced spine formation through actin dynamics downstream of Rac signaling. Consistent with the scaffolding role of IP3K-A, IP3K-A knock-out mice exhibited defects in accumulation of PAK1 by long-term potentiation-inducing stimulation. This deficiency resulted in a reduction in the reorganization of actin cytoskeletal structures in the synaptic area of dentate gyrus. Moreover, IP3K-A knock-out mice showed deficits of synaptic plasticity in perforant path and in hippocampal-dependent memory performances. These data support a novel model in which IP3K-A is critical for the spatial and temporal regulation of spine actin remodeling, synaptic plasticity, and learning and memory via an activity-dependent Rac scaffolding mechanism.

Specific plasticity of parallel fiber/Purkinje cell spine synapses by motor skill learning.

New synapse formation may underlie learning and memory. To examine specific synaptic plasticity by motor learning, we conducted quantitative analysis of synapses between parallel fibers and Purkinje cell dendritic spines in cerebella of rats trained to complete various obstacle courses. Synapses between parallel fibers and Purkinje cell spines were classified into single synapse boutons, multiple synapse boutons, and multiple synapse spines by their different contact features. Acrobat-trained animals had more single and multiple synaptic boutons, without change of multiple synapse spines, than motor control animals. These results may suggest that motor learning induces specific synaptogenesis and Purkinje cell spines are primary sites in motor learning-dependent cerebellar synaptic plasticity.

Repetitive transcranial magnetic stimulation protects hippocampal plasticity in an animal model of depression

Despite its therapeutic success in treating mood-related disorders, little is known about the mechanism by which repetitive transcranial magnetic stimulation (rTMS) alters physiological responses of neurons. Using the forced swim test (FST) in rats as a model of depression, we tested the protective effect of rTMS on synaptic plasticity, specifically, on the induction of hippocampal long-term potentiation (LTP). Male Sprague-Dawley rats were subjected to FST to induce immobility, a behavioral symptom of depression. They were subsequently treated with one of the three conditions: rTMS (rTMS: 1000 stimuli at 10Hz), sham rTMS (SHAM: acoustic stimulation only), or an antidepressant drug, fluoxetine (FLX: 10mg/kg, i.p.) for 7 days. There was a significant difference in immobility time between rTMS and SHAM groups after 7 days of treatment, but not after a single day. Following the second swim test on day 7, they were anesthetized and LTP was induced in vivo in the perforant path-dentate gyrus synapses. Another group (NAIVE) that had received no prior treatment was used as a control for LTP. The SHAM or FLX group exhibited little signs of LTP induction. On the contrary, the rTMS and NAIVE group showed a significant increase in field excitatory postsynaptic potentials after LTP induction. These results show that rTMS has an antidepressant-like effect after a relatively short period of treatment, and this effect might be mediated by a cellular process that can potentially reverse the impaired synaptic efficacy caused by the forced swim procedure.