Joshua P. Klein

Altered Sodium Channel Expression in Second-Order Spinal Sensory Neurons Contributes to Pain after Peripheral Nerve Injury

Peripheral nerve injury is known to upregulate the rapidly repriming Nav1.3 sodium channel within first-order spinal sensory neurons. In this study, we hypothesized that (1) after peripheral nerve injury, second-order dorsal horn neurons abnormally express Nav1.3, which (2) contributes to the responsiveness of these dorsal horn neurons and to pain-related behaviors. To test these hypotheses, adult rats underwent chronic constriction injury (CCI) of the sciatic nerve. Ten days after CCI, allodynia and hyperalgesia were evident. In situ hybridization, quantitative reverse transcription-PCR, and immunocytochemical analysis revealed upregulation of Nav1.3 in dorsal horn nociceptive neurons but not in astrocytes or microglia, and unit recordings demonstrated hyperresponsiveness of dorsal horn sensory neurons. Intrathecal antisense oligodeoxynucleotides targeting Nav1.3 decreased the expression of Nav1.3 mRNA and protein, reduced the hyperresponsiveness of dorsal horn neurons, and attenuated pain-related behaviors after CCI, all of which returned after cessation of antisense delivery. These results demonstrate for the first time that sodium channel expression is altered within higher-order spinal sensory neurons after peripheral nerve injury and suggest a link between misexpression of the Nav1.3 sodium channel and central mechanisms that contribute to neuropathic pain after peripheral nerve injury.

Changes of sodium channel expression in experimental painful diabetic neuropathy.

Although pain is experienced by many patients with diabetic neuropathy, the pathophysiology of painful diabetic neuropathy is not understood. Substantial evidence indicates that dysregulated sodium channel gene transcription contributes to hyperexcitability of dorsal root ganglion neurons, which may produce neuropathic pain after axonal transection. In this study, we examined sodium channel mRNA and protein expression in dorsal root ganglion neurons in rats with streptozotocin‐induced diabetes and tactile allodynia, using in situ hybridization and immunocytochemistry for sodium channels Nav1.1, Nav1.3, Nav1.6, Nav1.7, Nav1.8, and Nav1.9. Our results show that, in rats with experimental diabetes, there is a significant upregulation of mRNA for the Nav1.3, Nav1.6, and Nav1.9 sodium channels and a downregulation of Nav1.8 mRNA 1 and 8 weeks after onset of allodynia. Channel protein levels display parallel changes. Our results demonstrate dysregulated expression of the genes for sodium channels Nav1.3, Nav1.6, Nav1.8, and Nav1.9 in dorsal root ganglion neurons in experimental diabetes and suggest that misexpression of sodium channels contributes to neuropathic pain associated with diabetic neuropathy.

Early treatment suppresses the development of spike-wave epilepsy in a rat model

Purpose: Current treatments for epilepsy may control seizures, but have no known effects on the underlying disease. We sought to determine whether early treatment in a model of genetic epilepsy would reduce the severity of the epilepsy phenotype in adulthood.

Dysregulation of sodium channel expression in cortical neurons in a rodent model of absence epilepsy

Due to the involvement of cortical neurons in spike-wave discharge (SWD) initiation, and the contribution of voltage-gated sodium channels (VGSCs) to neuronal firing, we examined alterations in the expression of VGSC mRNA and protein in cortical neurons in the WAG/Rij absence epileptic rat. WAG/Rij rats were compared to age-matched Wistar control rats at 2, 4, and 6 months. Continuous EEG data was recorded, and percent time in SWD was determined. Tissue from different cortical locations from WAG/Rij and Wistar rats was analyzed for VGSC mRNA (by quantitative PCR) and protein (by immunocytochemistry). SWDs increased with age in WAG/Rij rats. mRNA levels for sodium channels Nav1.1 and Nav1.6, but not Nav1.2, were found to be up-regulated selectively within the facial somatosensory cortex (at AP +0.0, ML +6.0 mm). Protein levels for Nav1.1 and Nav1.6 were up-regulated in layer II-IV cortical neurons in this region of cortex. No significant changes were seen in adjacent regions or other brain areas, including the pre-frontal and occipital cortex. In the WAG/Rij model of absence epilepsy, we identified a specific region of cortex, in layer II-IV neurons on the lateral convexity of the cortex in the facial somatosensory area, where mRNA and protein expression of sodium channel genes Nav1.1 and Nav1.6 are up-regulated. This region of cortex approximately matches the electrophysiologically determined region of seizure onset. Changes in the expression of Nav1.1 and Nav1.6 parallel age-dependent increases in seizure frequency and duration.

The brain in diabetes: molecular changes in neurons and their implications for end-organ damage

Although secondary end-organ damage in diabetes has generally been thought to result from long-term passive shunting of excess glucose through alternative metabolic pathways, recent studies have elucidated a second mechanism of pathogenesis that involves active changes in gene expression in neurons of the CNS. These changes in gene expression result in molecular and functional changes that can become maladaptive over time. In this review, we examine two neuronal populations in the brain that have been studied in human beings and animal models of diabetes. First, we discuss overactivation of magnocellular neurosecretory cells within the hypothalamus and how it relates to the development of diabetic nephropathy. And second, we describe how changes in hippocampal synaptic plasticity can lead to cognitive and behavioural deficits in chronic diabetes. Changes in neuronal gene expression in diabetes represent a new pathway for diabetic pathogenesis. This pathway may hold clues for the development of therapies that, via the targeting of neurons, can slow or prevent the development of diabetic end-organ damage.

Role of hippocampal sodium channel Nav1.6 in kindling epileptogenesis

Purpose:  Central nervous system plasticity is essential for normal function, but can also reinforce abnormal network behavior, leading to epilepsy and other disorders. The role of altered ion channel expression in abnormal plasticity has not been thoroughly investigated. Nav1.6 is the most abundantly expressed sodium channel in the nervous system. Because of its distribution in the cell body and axon initial segment, Nav1.6 is crucial for action potential generation. The goal of the present study was to investigate the possible role of changes in Nav1.6 expression in abnormal, activity‐dependent plasticity of hippocampal circuits.

The relationships among MRI-defined spinal cord involvement, brain involvement, and disability in multiple sclerosis

To determine the interrelationships between MRI‐defined lesion and atrophy measures of spinal cord involvement and brain involvement and their relationships to disability in a small cohort of patients with multiple sclerosis (MS).

Preferential expression of IGF-I in small DRG neurons and down-regulation following injury

In this study, we examined the expression of insulin-like growth factor I (IGF-I) and its receptor (IGF-IR) in dorsal root ganglia (DRG) neurons in two rodent models of nerve injury: sciatic nerve axotomy and streptozotocin-induced (STZ) painful diabetic neuropathy. We demonstrate that IGF-I and its receptor are preferentially expressed in small (< 25 μm diameter) DRG neurons. There is a significant down-regulation in the expression of IGF-I and IGF-IR in the small DRG neurons of STZ rats by 59% and 71%, respectively. A parallel reduction in expression is shown in axotomized < 25 μm diameter DRG neurons for IGF-I (47%) but not for IGF-IR. The loss of IGF-I support to a population of predominantly nociceptive neurons may contribute to neuropathic pain observed in these models.

Patterned electrical activity modulates sodium channel expression in sensory neurons

Peripheral nerve injury induces changes in the level of gene expression for sodium channels Nav1.3, Nav1.8, and Nav1.9 within dorsal root ganglion (DRG) neurons, which may contribute to the development of hyperexcitability, ectopic neuronal discharge, and neuropathic pain. The mechanism of this change in sodium channel expression is unclear. Decreased availability of neurotrophic factors following axotomy contributes to these changes in gene transcription, but the question of whether changes in intrinsic neuronal activity levels alone can trigger changes in the expression of these sodium channels has not been addressed. We examined the effect of electrical stimulation on the expression of Nav1.3, Nav1.8, and Nav1.9 by using cultured embryonic mouse sensory neurons under conditions in which nerve growth factor (NGF) was not limiting. Expression of Nav1.3 was not significantly changed following stimulation. In contrast, we observed activity‐dependent down‐regulation of Nav1.8 and Nav1.9 mRNA and protein levels after stimulation, as demonstrated by quantitative polymerase chain reaction and immunocytochemistry. These results show that a change in neuronal activity can alter the expression of sodium channel genes in a subtype‐specific manner, via a mechanism independent of NGF withdrawal.

A biomolecular implementation of logically reversible computation with minimal energy dissipation

Abstract Energy dissipation associated with logic operations imposes a fundamental physical limit on computation and is generated by the entropic cost of information erasure, which is a consequence of irreversible logic elements. We show how to encode information in DNA and use DNA amplification to implement a logically reversible gate that comprises a complete set of operators capable of universal computation. We also propose a method using this design to connect, or ‘wire’, these gates together in a biochemical fashion to create a logic network, allowing complex parallel computations to be executed. The architecture of the system permits highly parallel operations and has properties that resemble well known genetic regulatory systems.