Differences in forebrain activation in two strains of rat at rest and after spinal cord injury

Potentiation of the lateral habenula-ventral tegmental area pathway underlines the susceptibility to depression in mice with chronic pain

Chronic pain often develops severe mood changes such as depression. However, how chronic pain leads to depression remains elusive and the mechanisms determining individuals' responses to depression are largely unexplored. Here we found that depression-like behaviors could only be observed in 67.9% of mice with chronic neuropathic pain, leaving 32.1% of mice with depression resilience. We determined that the spike discharges of the ventral tegmental area (VTA)-projecting lateral habenula (LHb) glutamatergic (Glu) neurons were sequentially increased in sham, resilient and susceptible mice, which consequently inhibited VTA dopaminergic (DA) neurons through a LHbGlu-VTAGABA-VTADA circuit. Furthermore, the LHbGlu-VTADA excitatory inputs were dampened via GABAB receptors in a pre-synaptic manner. Regulation of LHb-VTA pathway largely affected the development of depressive symptoms caused by chronic pain. Our study thus identifies a pivotal role of the LHb-VTA pathway in coupling chronic pain with depression and highlights the activity-dependent contribution of LHbGlu-to-VTADA inhibition in depressive behavioral regulation.

Animal Models of Cerebral Changes Secondary to Spinal Cord Injury

Spinal cord injuries (SCIs) are difficult to treat. The first animal SCI model (featuring the dropping of a weight) was established by Allen in 1911, and other animal models have been developed since then. Most animal studies have focused only on the molecular features of SCIs, which remain disputed. Recently, it has become clear that SCI may trigger mental and cognitive disorders, however, and brain changes secondary to SCI are under investigation. No consensus on an optimal animal model for cerebral research has emerged. We discuss the appropriate SCI models for studying secondary brain changes.

Regional Hyperexcitability and Chronic Neuropathic Pain Following Spinal Cord Injury

Spinal cord injury (SCI) causes maladaptive changes to nociceptive synaptic circuits within the injured spinal cord. Changes also occur at remote regions including the brain stem, limbic system, cortex, and dorsal root ganglia. These maladaptive nociceptive synaptic circuits frequently cause neuronal hyperexcitability in the entire nervous system and enhance nociceptive transmission, resulting in chronic central neuropathic pain following SCI. The underlying mechanism of chronic neuropathic pain depends on the neuroanatomical structures and electrochemical communication between pre- and postsynaptic neuronal membranes, and propagation of synaptic transmission in the ascending pain pathways. In the nervous system, neurons are the only cell type that transmits nociceptive signals from peripheral receptors to supraspinal systems due to their neuroanatomical and electrophysiological properties. However, the entire range of nociceptive signaling is not mediated by any single neuron. Current literature describes regional studies of electrophysiological or neurochemical mechanisms for enhanced nociceptive transmission post-SCI, but few studies report the electrophysiological, neurochemical, and neuroanatomical changes across the entire nervous system following a regional SCI. We, along with others, have continuously described the enhanced nociceptive transmission in the spinal dorsal horn, brain stem, thalamus, and cortex in SCI-induced chronic central neuropathic pain condition, respectively. Thus, this review summarizes the current understanding of SCI-induced neuronal hyperexcitability and maladaptive nociceptive transmission in the entire nervous system that contributes to chronic central neuropathic pain.

[The roles of habenula and related neural circuits in neuropsychiatric diseases]

The habenula is a small and bilateral nucleus above dorsal thalamus, which contains several different types of neurons. The habenula has extensive connections with the forebrain, septum and monoaminergic nuclei in the midbrain and brainstem. Habenula is known as an 'anti-reward' nucleus, which can be activated by aversive stimulus and negative reward prediction errors. Accumulating researchs have implicated that the habenula is involved in several behaviors crucial to survival. Meanwhile, the roles of the habenula in neuropsychiatric diseases have received increasing attention. This review summaries the studies regarding the roles of habenula and the related circuits in neuropathic pain, depression, drug addiction and schizophrenia, and discusses the possibility to use the habenula as a treatment target.

Neuroplasticity of Supraspinal Structures Associated with Pathological Pain

Peripheral nerve and spinal cord injuries, along with other painful syndromes such as fibromyalgia, diabetic neuropathy, chemotherapeutic neuropathy, trigeminal neuralgia, complex regional pain syndrome, and/or irritable bowel syndrome, cause several neuroplasticity changes in the nervous system along its entire axis affecting the different neuronal nuclei. This paper reviews these changes, focusing on the supraspinal structures that are involved in the modulation and processing of pain, including the periaqueductal gray matter, red nucleus, locus coeruleus, rostral ventromedial medulla, thalamus, hypothalamus, basal ganglia, cerebellum, habenula, primary, and secondary somatosensory cortex, motor cortex, mammillary bodies, hippocampus, septum, amygdala, cingulated, and prefrontal cortex. Hyperexcitability caused by the modification of postsynaptic receptor expression, central sensitization, and potentiation of presynaptic delivery of neurotransmitters, as well as the reduction of inhibitory inputs, changes in dendritic spine, neural circuit remodeling, alteration of gray matter, and upregulation of proinflammatory mediators (e.g., cytokines) by reactivation of astrocytes and microglial cells are the main functional, structural, and molecular neuroplasticity changes observed in the above supraspinal structures, associated with pathological pain. Studying these changes in greater depth may lead to the implementation and improvement of new therapeutic strategies against pathological pain. Anat Rec, 300:1481-1501, 2017. © 2017 Wiley Periodicals, Inc.

Post-translational modification of cortical GluA receptors in rodents following spinal cord lesion

Previous studies investigating the pathophysiology of neuropathic pain caused by injury to the spinal cord suggest that pain may result, at least in part, from maladaptive plasticity in the somatosensory cortex and associated pain networks. However, little is known about the molecular and cellular mechanisms leading to maladaptive plasticity in the cortex and how they contribute to the development of neuropathic pain. AMPA-type glutamate receptors (GluARs) mediate fast excitatory synaptic transmission in the mammalian brain and play an important role in pain processing. Here we used an electrolytic lesion model of spinal cord injury in animals to study the expression and phosphorylation of GluA1 and 2 in the primary somatosensory cortex (S1). Experiments in rats and mice revealed that maladaptive plasticity and hypersensitivity after spinal cord lesion (SCL) are associated with a reduction in the fraction of GluA1 subunits that are phosphorylated at serine 831 (S831) in the hindlimb representation of S1 (S1HL). Manipulations that reduce the fraction of phosphorylated S831 in S1HL of non-lesioned animals, including low-frequency electrical stimulation and viral-mediated gene transfer of mutant S831, were associated with the development of hypersensitivity. Taken together, these findings suggest that phosphorylation of GluA1 at S831 plays an important role in the development of hypersensitivity after SCL.

Cortical reorganization after spinal cord injury: always for good?

Plasticity constitutes the basis of behavioral changes as a result of experience. It refers to neural network shaping and re-shaping at the global level and to synaptic contacts remodeling at the local level, either during learning or memory encoding, or as a result of acute or chronic pathological conditions. 'Plastic' brain reorganization after central nervous system lesions has a pivotal role in the recovery and rehabilitation of sensory and motor dysfunction, but can also be "maladaptive". Moreover, it is clear that brain reorganization is not a "static" phenomenon but rather a very dynamic process. Spinal cord injury immediately initiates a change in brain state and starts cortical reorganization. In the long term, the impact of injury - with or without accompanying therapy - on the brain is a complex balance between supraspinal reorganization and spinal recovery. The degree of cortical reorganization after spinal cord injury is highly variable, and can range from no reorganization (i.e. "silencing") to massive cortical remapping. This variability critically depends on the species, the age of the animal when the injury occurs, the time after the injury has occurred, and the behavioral activity and possible therapy regimes after the injury. We will briefly discuss these dependencies, trying to highlight their translational value. Overall, it is not only necessary to better understand how the brain can reorganize after injury with or without therapy, it is also necessary to clarify when and why brain reorganization can be either "good" or "bad" in terms of its clinical consequences. This information is critical in order to develop and optimize cost-effective therapies to maximize functional recovery while minimizing maladaptive states after spinal cord injury.

Thalamocortical asynchrony in conditions of spinal cord injury pain in rats

Spinal cord injury (SCI) pain is a debilitating chronic condition that is severe and unrelenting. Despite decades of extensive research, the neuropathological mechanisms responsible for the development of this devastating condition remain largely unknown, hindering our ability to develop effective treatments. Because several lines of evidence implicate abnormalities of the thalamus and cortex in the etiology of SCI pain, we hypothesized that SCI pain results from abnormal functional connectivity of brain areas heavily implicated in pain processing. We performed a longitudinal study in a rat model of SCI (SCI group, n = 8; sham-operated group, n = 6) and acquired resting-state functional magnetic resonance imaging scans before spinal surgery and 3, 7, 14, and 21 (SCI only) days after surgery in the same animals. Functional connectivity was decreased between the ventroposterior lateral thalamus (VPL) and primary somatosensory cortex (S1) 7 d after SCI. This reduction preceded an increase in connectivity between S1 and other cortical areas involved in nociceptive processing. In addition, VPL had increased connectivity to contralateral thalamus at 7 and 14 d after injury. The temporal pattern of the increase in functional connectivity within the thalamus and between cortical areas (particularly S1 and retrosplenial cortex) had a striking resemblance to the temporal pattern for the development of a "below-level" mechanical hypersensitivity in the same animals. Our findings suggest that below-level hypersensitivity is associated with functional disconnection (asynchrony) between the thalamus and cortical areas involved in nociceptive processing.

Unmasking the mysteries of the habenula in pain and analgesia

The habenula is a small bilateral structure in the posterior-medial aspect of the dorsal thalamus that has been implicated in a remarkably wide range of behaviors including olfaction, ingestion, mating, endocrine and reward function, pain and analgesia. Afferent connections from forebrain structures send inputs to the lateral and medial habenula where efferents are mainly projected to brainstem regions that include well-known pain modulatory regions such as the periaqueductal gray and raphe nuclei. A convergence of preclinical data implicates the region in multiple behaviors that may be considered part of the pain experience including a putative role in pain modulation, affective, and motivational processes. The habenula seems to play a role as an evaluator, acting as a major point of convergence where external stimuli is received, evaluated, and redirected for motivation of appropriate behavioral response. Here, we review the role of the habenula in pain and analgesia, consider its potential role in chronic pain, and review more recent clinical and functional imaging data of the habenula from animals and humans. Even through the habenula is a small brain structure, advances in structural and functional imaging in humans should allow for further advancement of our understanding of its role in pain and analgesia.

Strain-related differences after experimental traumatic brain injury in rats

The present study directly compares the effects of experimental brain injury in two commonly used rat strains: Fisher 344 and Sprague-Dawley. We previously found that Fisher rats have a higher mortality rate and more frequent seizure attacks at the same injury level than Sprague-Dawley rats. Although strain differences in rats are commonly accepted as contributing to variability among studies, there is a paucity of literature addressing strain influence in experimental neurotrauma. Therefore this study compares outcome measures in two rat strains following lateral fluid percussion injury. Fisher 344 and Sprague-Dawley rats were monitored for changes in physiological measurements, intracranial pressure, and electroencephalographic activity. We further analyzed neuronal degeneration and cell death in the injured brain using Fluoro-Jade-B (FJB) histochemistry and caspase-3 immunostaining. Behavioral studies using the beam walk and Morris water maze were conducted to characterize strain differences in both motor and cognitive functional recovery following injury. We found that Fisher rats had significantly higher intracranial pressure, prolonged seizure activity, increased FJB-positive staining in the injured cortex and thalamus, and increased caspase-3 expression than Sprague-Dawley rats. On average, Fisher rats displayed a greater amount of total recording time in seizure activity and had longer ictal durations. The Fisher rats also had increased motor deficits, correlating with the above results. In spite of these results, Fisher rats performed better on cognitive tests following injury. The results demonstrate that different rat strains respond to injury differently, and thus in preclinical neurotrauma studies strain influence is an important consideration when evaluating outcomes.

Strain differences in response to traumatic brain injury in Long-Evans compared to Sprague-Dawley rats

The selected strain of rodent used in experimental models of traumatic brain injury is typically dependent upon the experimental questions asked and the familiarity of the investigator with a specific rodent strain. This archival study compares the injury responsiveness and recovery profiles of two popular outbred strains, the Long-Evans (LE) and the Sprague-Dawley (SD), after brain injury induced by lateral fluid percussion injury (LFPI). General findings include a significantly longer duration of unconsciousness in LE rats, but similar durations of apnea. Both strains displayed the same level of initial FPI-induced behavioral deficits, followed by a more rapid rate of functional recovery in SD rats. Cortical volume loss was not significantly different, but close inspection of the data suggests the possibility that LE rats may be more susceptible to damage in the hemisphere contralateral to the injury site than are SD rats. It is hoped that the information provided here encourages greater attention to the subtle differences and similarities between strains in future pre-clinical efficacy studies of traumatic brain injury.

Animal models in spinal cord injury: a review

Multiple neuroprotective agents have shown benefit for the treatment of acute spinal cord injury (SCI) in animal studies. However, clinical trials have, thus far, been uniformly disappointing. This review explores reasons for discrepancies between promising animal studies and disappointing clinical trials and potential barriers to extrapolation of research results from animals to humans. The three major barriers disclosed are: differences in injury type between laboratory-induced SCI and clinical SCI, difficulties in interpreting functional outcome in animals, and inter-species and interstrain differences in pathophysiology of SCI. These barriers can impair the effectiveness of animal models of SCI to predict human outcomes. While some of these barriers can be overcome, others are inherent to the animal models.

Strain and age affect electroconvulsive seizure testing in rats

Electroconvulsive seizure thresholds were compared between adolescent and mature Sprague--Dawley, Wistar, and Fischer rats. All strains had similar hindbrain or forebrain seizure thresholds as adolescents. As adults, hindbrain or forebrain seizure thresholds were highest for Sprague--Dawley and lowest for Fischer rats. Conversely, limbic seizure thresholds during adolescence were highest for Fischer rats. Additional study is needed to better delineate strain and maturational effects on electroconvulsive seizure testing.

Concurrent activation of the somatosensory forebrain and deactivation of periaqueductal gray associated with diabetes-induced neuropathic pain

We combined behavioral testing with brain imaging using (99m)Tc-HMPAO (Amersham Health) to identify CNS structures reflecting alterations in pain perception in the streptozotocin (STZ) model of type I diabetes. We induced diabetic hyperglycemia (blood glucose >300 mg/dl) by injecting male Sprague-Dawley rats with STZ (45 mg/kg i.p.). Four weeks after STZ-diabetic rats exhibited behaviors indicative of neuropathic pain (hypersensitivity thermal stimuli) and this hypersensitivity persisted for up to 6 weeks. Imaging data in STZ-diabetic rats revealed significant increases in the activation of brain regions involved in pain processing after 6 weeks duration of diabetes. These regions included secondary somatosensory cortex, ventrobasal thalamic nuclei and the basolateral amygdala. In contrast, the activation in habenular nuclei and the midbrain periaqueductal gray were markedly decreased in STZ rats. These data suggest that pain in diabetic neuropathy may be due in part to hyperactivity in somatosensory structures coupled with a concurrent deactivation of structures mediating antinociception.