The time course of serotonin 2C receptor expression after spinal transection of rats: an immunohistochemical study

Changes in 5-HT1F receptor expression in rats with spasticity following spinal cord injury

Spasticity is a common complication in patients with spinal cord injury (SCI) and adversely affects patients' quality of life. Little is known about the distribution of the serotonin 1F receptor (5-HT1FR) in the spinal cord, especially in relation to the spasticity caused by SCI. Adult male Wistar rats were divided into a sham-operation group and spinalized group. SCI-induced spasticity was caused by spinal transection at the second sacral segment. The spinal cord below the transection was obtained at the end of the experiment. The expression and distribution of 5-HT1FR in the spinal cord were analyzed. The results showed that the expression of 5-HT1FR (mRNA and protein) exhibited the same downward trend after spinal transection and reached the lowest expression level at 2 and 5 days, respectively. The expression of 5-HT1FR (mRNA and protein) thereafter gradually approached the levels in the sham-operation group after 60 days. Immunostaining suggested that 5-HT1FR showed particularly strong expression in the ventral horn (VH) region. The time course of 5-HT1FR mRNA downregulation is positively correlated with the development of tail spasticity after sacral spinal cord transection. There may be a connection between 5-HT1FR and the occurrence of spasticity, but elucidation of the specific mechanism needs further experimental verification.

The Development of Hindlimb Postural Asymmetry Induced by Focal Traumatic Brain Injury Is Not Related to Serotonin 2A/C Receptor Expression in the Spinal Cord

Brain injury and stroke are leading causes of adult disability. Motor deficits are common problems, and their underlying pathological mechanisms remain poorly understood. The serotoninergic system is implicated in both functional recovery from and the occurrence of spasticity after injuries to the central nervous system. This study, which was conducted on rats, investigated the development of limb postural changes and their relationship to the expression of serotonin (5-HT) 2A and 2C receptors in the spinal cord in the 4 weeks after focal traumatic brain injury (TBI) to the right hindlimb sensorimotor cortex. The limb motor deficits were assessed by measuring gait pattern changes during walking and hindlimb postural asymmetry at different time intervals (3−28 days) after surgery. The expressions of the 5-HT2A and 2C receptors in the lumbar spinal cord were investigated using immunohistochemistry. The results showed that all the rats with TBI, independently of the duration of the interval, displayed postural asymmetry with flexion on the contralateral (left) side (>2 mm), while the sham-operated rats showed no apparent postural asymmetry. The TBI rats also had longer stride lengths during walking in both their hindlimbs and their forelimbs compared with the sham rats. For both the TBI and the sham rats, the hind-paw placement angles were larger on the contralateral side in some of the groups. Compared to the sham-operated rats, the 5-HT2A and 2C receptor expression did not significantly change on either side of the lumbar spinal cords of the TBI rats in any of the groups. These results suggest that focal TBI can induce motor deficits lasting a relatively long time, and that these deficits are not related to the expression of the 5-HT2A and 2C receptors in the spinal cord.

Evidence of treating spasticity before it develops: a systematic review of spasticity outcomes in acute spinal cord injury interventional trials

Introduction: Spasticity is a common consequence of spinal cord injury (SCI), estimated to affect up to 93% of people living with SCI in the community. Problematic spasticity affects around 35% people with SCI spasticity. The early period after injury is believed to be the most opportune time for neural plasticity after SCI. We hypothesize that clinical interventions in the early period could reduce the incidence of spasticity. To address this, we evaluated the spasticity outcomes of clinical trials with interventions early after SCI.Methods: We performed a systematic review of the literature between January 2000 and May 2021 to identify control trials, in humans and animals, that were performed early after SCI that included measures of spasticity in accordance with PRISMA guidelines.Results: Our search yielded 1,463 records of which we reviewed 852 abstracts and included 8 human trial peer-reviewed publications and 9 animal studies. The 9 animal trials largely supported the hypothesis that early intervention can reduce spasticity, including evidence from electrophysiological, behavioral, and histologic measures. Of the 8 human trials, only one study measured spasticity as a primary outcome with a sample size sufficient to test the hypothesis. In this study, neuromodulation of the spinal cord using electric stimulation of the common peroneal nerve reduced spasticity in the lower extremities compared to controls.Conclusion: Given the prevalence of problematic spasticity, there is surprisingly little research being performed in the early period of SCI that includes spasticity measures, and even fewer studies that directly address spasticity. More research on the potential for early interventions to mitigate spasticity is needed.

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

Spinal Cord Injury Alters Spinal Shox2 Interneurons by Enhancing Excitatory Synaptic Input and Serotonergic Modulation While Maintaining Intrinsic Properties in Mouse

Neural circuitry generating locomotor rhythm and pattern is located in the spinal cord. Most spinal cord injuries (SCIs) occur above the level of spinal locomotor neurons; therefore, these circuits are a target for improving motor function after SCI. Despite being relatively intact below the injury, locomotor circuitry undergoes substantial plasticity with the loss of descending control. Information regarding cell type-specific plasticity within locomotor circuits is limited. Shox2 interneurons (INs) have been linked to locomotor rhythm generation and patterning, making them a potential therapeutic target for the restoration of locomotion after SCI. The goal of the present study was to identify SCI-induced plasticity at the level of Shox2 INs in a complete thoracic transection model in adult male and female mice. Whole-cell patch-clamp recordings of Shox2 INs revealed minimal changes in intrinsic excitability properties after SCI. However, afferent stimulation resulted in mixed excitatory and inhibitory input to Shox2 INs in uninjured mice which became predominantly excitatory after SCI. Shox2 INs were differentially modulated by serotonin (5-HT) in a concentration-dependent manner in uninjured conditions but following SCI, 5-HT predominantly depolarized Shox2 INs. 5-HT7 receptors mediated excitatory effects on Shox2 INs from both uninjured and SCI mice, but activation of 5-HT2B/2C receptors enhanced excitability of Shox2 INs only after SCI. Overall, SCI alters sensory afferent input pathways to Shox2 INs and 5-HT modulation of Shox2 INs to enhance excitatory responses. Our findings provide relevant information regarding the locomotor circuitry response to SCI that could benefit strategies to improve locomotion after SCI.SIGNIFICANCE STATEMENT Current therapies to gain locomotor control after spinal cord injury (SCI) target spinal locomotor circuitry. Improvements in therapeutic strategies will require a better understanding of the SCI-induced plasticity within specific locomotor elements and their controllers, including sensory afferents and serotonergic modulation. Here, we demonstrate that excitability and intrinsic properties of Shox2 interneurons, which contribute to the generation of the locomotor rhythm and pattering, remain intact after SCI. However, SCI induces plasticity in both sensory afferent pathways and serotonergic modulation, enhancing the activation and excitation of Shox2 interneurons. Our findings will impact future strategies looking to harness these changes with the ultimate goal of restoring functional locomotion after SCI.

Respiratory axon regeneration in the chronically injured spinal cord

Promoting the combination of robust regeneration of damaged axons and synaptic reconnection of these growing axon populations with appropriate neuronal targets represents a major therapeutic goal following spinal cord injury (SCI). A key impediment to achieving this important aim includes an intrinsic inability of neurons to extend axons in adult CNS, particularly in the context of the chronically-injured spinal cord. We tested whether an inhibitory peptide directed against phosphatase and tensin homolog (PTEN: a central inhibitor of neuron-intrinsic axon growth potential) could restore inspiratory diaphragm function by reconnecting critical respiratory neural circuitry in a rat model of chronic cervical level 2 (C2) hemisection SCI. We found that systemic delivery of PTEN antagonist peptide 4 (PAP4) starting at 8 weeks after C2 hemisection promoted substantial, long-distance regeneration of injured bulbospinal rostral Ventral Respiratory Group (rVRG) axons into and through the lesion and back toward phrenic motor neurons (PhMNs) located in intact caudal C3-C5 spinal cord. Despite this robust rVRG axon regeneration, PAP4 stimulated only minimal recovery of diaphragm function. Furthermore, re-lesion through the hemisection site completely removed PAP4-induced functional improvement, demonstrating that axon regeneration through the lesion was responsible for this partial functional recovery. Interestingly, there was minimal formation of putative excitatory monosynaptic connections between regrowing rVRG axons and PhMN targets, suggesting that (1) limited rVRG-PhMN synaptic reconnectivity was responsible at least in part for the lack of a significant functional effect, (2) chronically-injured spinal cord presents an obstacle to achieving synaptogenesis between regenerating axons and post-synaptic targets, and (3) addressing this challenge is a potentially-powerful strategy to enhance therapeutic efficacy in the chronic SCI setting. In conclusion, our study demonstrates a non-invasive and transient pharmacological approach in chronic SCI to repair the critically-important neural circuitry controlling diaphragmatic respiratory function, but also sheds light on obstacles to circuit plasticity presented by the chronically-injured spinal cord.

Exercise-Induced Plasticity in Signaling Pathways Involved in Motor Recovery after Spinal Cord Injury

Spinal cord injury (SCI) leads to numerous chronic and debilitating functional deficits that greatly affect quality of life. While many pharmacological interventions have been explored, the current unsurpassed therapy for most SCI sequalae is exercise. Exercise has an expansive influence on peripheral health and function, and by activating the relevant neural pathways, exercise also ameliorates numerous disorders of the central nervous system (CNS). While the exact mechanisms by which this occurs are still being delineated, major strides have been made in the past decade to understand the molecular underpinnings of this essential treatment. Exercise rapidly and prominently affects dendritic sprouting, synaptic connections, neurotransmitter production and regulation, and ionic homeostasis, with recent literature implicating an exercise-induced increase in neurotrophins as the cornerstone that binds many of these effects together. The field encompasses vast complexity, and as the data accumulate, disentangling these molecular pathways and how they interact will facilitate the optimization of intervention strategies and improve quality of life for individuals affected by SCI. This review describes the known molecular effects of exercise and how they alter the CNS to pacify the injury environment, increase neuronal survival and regeneration, restore normal neural excitability, create new functional circuits, and ultimately improve motor function following SCI.

Early escitalopram administration as a preemptive treatment strategy against spasticity after contusive spinal cord injury in rats

In the majority of spinal cord injury (SCI) patients, spasticity develops in the subacute phase and chronically persists with muscle hypertonia. Among various pathological conditions underlying spasticity, upregulated expression of 5-HT receptors (5-HTR) on the spinal motor neurons due to 5-HT denervation is considered one of crucial factors for hyperexcitability of the spinal circuit. As a 5-HT signal modulator, selective serotonin re-uptake inhibitors (SSRIs) are ordinarily prescribed for diseases associated with 5-HT in the CNS, and are known for their ability to increase 5-HT levels as well as to desensitize 5-HTR. Here, we hypothesized that early SSRI administration as a preemptive treatment strategy would effectively prevent the onset of spasticity. We used a rat model of contusive SCI and administered escitalopram during the first 4 weeks after injury, which is the period required for spasticity development in rodent models. We performed a swimming test to quantify spastic behaviors and conducted the Hoffman reflex test as well as histological analyses for 5-HT_2AR and KCC2 expressions. Four weeks of escitalopram administration suppressed spastic behaviors during the swimming test and reduced the population of spasticity-strong rats. Moreover, the treatment resulted in decreased immunoreactivity of 5-HT_2AR in the spinal motor neurons. Result of the H-reflex test and membrane expression of KCC2 were not significantly altered. In summary, early escitalopram administration could prevent the onset of spastic behaviors via regulation of 5-HT system after SCI, but could not modulate exaggerated spinal reflex. Our results suggest a novel application of SSRIs for preventative treatment of spasticity.

An overview of ongoing clinical trials assessing pharmacological therapeutic strategies to manage chemotherapy-induced peripheral neuropathy, based on preclinical studies in rodent models

Chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting side effect induced by a variety of chemotherapeutic agents. Symptoms are mainly sensory: pain, tingling, numbness, and temperature sensitivity. They may require the tapering of chemotherapy regimens or even their cessation; thus, the prevention/treatment of CIPN is critical to increase effectiveness of cancer treatment. However, CIPN management is mainly based on conventional neuropathic pain treatments, with poor clinical efficacy. Therefore, significant effort is made to identify new pharmacological targets to prevent/treat CIPN. Animal modeling is a key component in predicting human response to drugs and in understanding the pathophysiological mechanisms underlying CIPN. In fact, studies performed in rodents highlighted several pharmacological targets to treat/prevent CIPN. This review provides updated information about ongoing clinical trials testing drugs for the management of CIPN and presents some of their proof-of-concept studies conducted in rodent models. The presented drugs target oxidative stress, renin-angiotensin system, glutamatergic neurotransmission, sphingolipid metabolism, neuronal uptake transporters, nicotinamide adenine dinucleotide metabolism, endocannabinoid system, transient receptor potential channels, and serotoninergic receptors. As some clinical trials focus on the effect of the drugs on pain, others evaluate their efficacy by assessing general neuropathy. Moreover, based on studies conducted in rodent models, it remains unclear if some of the tested drugs act in an antinociceptive fashion or have neuroprotective properties. Thus, further investigations are needed to understand their mechanism of action, as well as a global standardization of the methods used to assess efficacy of new therapeutic strategies in the treatment of CIPN.

Role of 5-HT receptors in neuropathic pain: potential therapeutic implications

5-HT plays a crucial role in the progress and adjustment of pain both centrally and peripherally. The therapeutic action of the 5-HT receptors` agonist and antagonist in neuropathic pain have been widely reported in many studies. However, the specific roles of 5-HT subtype receptors have not been reviewed comprehensively. Therefore, we summarized the recent findings on multiple subtypes of 5-HT receptors in both central and peripheral nervous system in neuropathic pain, particularly, 5-HT1, 5-HT2, 5-HT3 and 5-HT7 receptors. In addition, 5-HT4, 5-HT5 and 5-HT6 receptors were also reviewed. Most of studies focused on the function of 5-HT subtype receptors in spinal level compared to brain areas. Based on these evidences, the pain process can be facilitated or inhibited that depending on the specific subtypes and the distribution of 5-HT receptors. Therefore, this review may provide potential therapeutic implications in treatment of neuropathic pain.

Serotonergic Modulation of Nociceptive Circuits in Spinal Cord Dorsal Horn

Background:

Despite the extensive number of studies performed in the last 50 years, aimed at describing the role of serotonin and its receptors in pain modulation at the spinal cord level, several aspects are still not entirely understood. The interpretation of these results is often complicated by the use of different pain models and animal species, together with the lack of highly selective agonists and antagonists binding to serotonin receptors.

Method:

In this review, a search has been conducted on studies investigating the modulatory action exerted by serotonin on specific neurons and circuits in the spinal cord dorsal horn. Particular attention has been paid to studies employing electro-physiological techniques, both in vivo and in vitro.

Conclusion:

The effects of serotonin on pain transmission in dorsal horn depend on several factors, including the type of re-ceptors activated and the populations of neurons involved. Recently, studies performed by activating and/or recording from identified neurons have importantly contributed to the understanding of serotonergic modulation on dorsal horn circuits.

Cervical spinal 5-HT2A and 5-HT2B receptors are both necessary for moderate acute intermittent hypoxia-induced phrenic long-term facilitation

Serotonin (5-HT) is a key regulator of spinal respiratory motor plasticity. For example, spinal 5-HT receptor activation is necessary for the induction of phrenic long-term facilitation (pLTF), a form of respiratory motor plasticity triggered by moderate acute intermittent hypoxia (mAIH). mAIH-induced pLTF is blocked by cervical spinal application of the broad-spectrum 5-HT-receptor antagonist, methysergide. However, methysergide does not allow distinctions between the relative contributions of different 5-HT receptor subtypes. Intravenous administration of the Gq protein-coupled 5-HT_2A/2C receptor antagonist ketanserin blocks mAIH-induced pLTF when administered before, but not after, mAIH; thus, 5-HT₂ receptor activation is necessary for the induction but not maintenance of mAIH-induced pLTF. However, systemic ketanserin administration does not identify the site of the relevant 5-HT_2A/2C receptors. Furthermore, this approach does not differentiate between the roles of 5-HT_2A versus 5-HT_2C receptors, nor does it preclude involvement of other Gq protein-coupled metabotropic 5-HT receptors capable of eliciting long-lasting phrenic motor facilitation, such as 5-HT_2B receptors. Here we tested the hypothesis that mAIH-induced pLTF requires cervical spinal 5-HT₂ receptor activation and determined which 5-HT₂ receptor subtypes are involved. Anesthetized, paralyzed, and ventilated adult male Sprague Dawley rats were pretreated intrathecally with cervical (~C3-C5) spinal injections of subtype selective 5-HT_2A/2C, 5-HT_2B, or 5-HT_2C receptor antagonists before mAIH. Whereas cervical spinal 5-HT_2C receptor inhibition had no impact on mAIH-induced pLTF, pLTF was no longer observed after pretreatment with either 5-HT_2A/2C or 5-HT_2B receptor antagonists. Furthermore, spinal pretreatment with an MEK/ERK MAPK inhibitor blocked phrenic motor facilitation elicited by intrathecal injections of 5-HT_2A but not 5-HT_2B receptor agonists. Thus, mAIH-induced pLTF requires concurrent cervical spinal activation of both 5-HT_2A and 5-HT_2B receptors. However, these distinct receptor subtypes contribute to phrenic motor facilitation via distinct downstream signaling cascades that differ in their requirement for ERK MAPK signaling. The demonstration that both 5-HT_2A and 5-HT_2B receptors make unique contributions to mAIH-induced pLTF advances our understanding of mechanisms that underlie 5-HT-induced phrenic motor plasticity.NEW & NOTEWORTHY Moderate acute intermittent hypoxia (mAIH) triggers a persistent enhancement in phrenic motor output, an effect termed phrenic long-term facilitation (pLTF). mAIH-induced pLTF is blocked by cervical spinal application of the broad-spectrum serotonin (5-HT) receptor antagonist methysergide, demonstrating the need for spinal 5-HT receptor activation. However, the exact type of 5-HT receptors required for initiation of pLTF remains unknown. To the best of out knowledge, the present study is the first to demonstrate that 1) spinal coactivation of two distinct Gq protein-coupled 5-HT₂ receptor subtypes is necessary for mAIH-induced pLTF, and 2) these receptors contribute to pLTF via cascades that differ in their requirement for ERK MAPK signaling.

The serotonin (5-hydroxytryptamine) 5-HT7 receptor is up-regulated in Onuf's nucleus in rats with chronic spinal cord injury

OBJECTIVES

To examine the effect of intrathecal (i.t.) serotonin (5-hydroxytryptamine) 5-HT₇ agonist administration on voiding function in the urethane-anesthetised rat, and the change in 5-HT₇ receptor (5-HT₇ R) expression in the lumbosacral cord Onuf's nucleus after spinal cord injury (SCI).

MATERIALS AND METHODS

In all, 32 female Sprague-Dawley (SD) rats were equally divided into a spinally intact (SI) group and SCI group (n = 16 each). At 8 weeks after transection, half of the rats underwent continuous cystometry under urethane anaesthesia, and the 5-HT₇ R-selective agonist LP44 was given (i.t.). The remaining rats were used for pseudorabies (PRV) retrograde tracing, immunofluorescence, and Western Blot.

RESULTS

LP44 administered i.t. had no effect in the SI rats. In SCI rats, LP44 (1-30 μg/kg) induced significant dose-dependent increases in micturition volume, voiding efficiency, number of high-frequency oscillations per micturition; and decreases in residual volume, bladder capacity, peak bladder pressure, threshold pressure and non-voiding contractions. The 5-HT₇ R antagonist, SB-269970 (10 μg/kg), partially reversed LP44-induced changes. Using PRV retrograde tracing and immunofluorescence, 5-HT₇ Rs were found in the L6-S1 spinal cord Onuf's nucleus in both SI and SCI rats, but the expression was significantly greater in the SCI rats. Western blot showed significantly more 5-HT₇ Rs in the ventral L6-S1 spinal cord in SCI rats.

CONCLUSION

A 5-HT₇ R agonist, given i.t., improved voiding efficiency in urethane-anesthetised SCI rats, and the 5-HT₇ R was significantly up-regulated in the lumbosacral cord Onuf's nucleus. If valid for humans, these findings suggest that the 5-HT₇ R could be a target for therapeutic interventions.

Heterogenic Distribution of Aromatic L-Amino Acid Decarboxylase Neurons in the Rat Spinal Cord

Aromatic L-amino acid decarboxylase (AADC) is an essential enzyme in the synthesis of serotonin, dopamine, and certain trace amines and is present in a variety of organs including the brain and spinal cord. It is previously reported that in mammalian spinal cord AADC cells (called D-cells) were largely confined to a region around the central canal and that they do not produce monoamines. To date, there has not been a detailed description of their distribution and morphology in mammals. In the present study this issue is systematically investigated using immunohistochemistry. We have found that AADC cells in the rat spinal cord are both more numerous and more widely distributed than previously reported. In the gray matter, AADC neurons immunolabeled for NeuN were not only found in the region around the central canal but also in the dorsal horn, intermediate zone, and ventral horn. In the white matter a large number of glial cells were AADC-immunopositive in different spinal segments and the vast majority of these cells expressed oligodendrocyte and radial glial phenotypes. Additionally, a small number of AADC neurons labeled for NeuN were found in the white matter along the ventral median fissure. The shapes and sizes of AADC neurons varied according to their location. For example, throughout cervical and lumbar segments AADC neurons in the intermediate zone and ventral horn tended to be rather large and weakly immunolabeled, whereas those in comparable regions of sacrocaudal segments were smaller and more densely immunolabeled. The diverse morphological characteristics of the AADC cells suggests that they could be further divided into several subtypes. These results indicate that AADC cells are heterogeneously distributed in the rat spinal cord and they may exert different functions in different physiological and pathological situations.

On Denny-Brown's 'spastic dystonia' - What is it and what causes it?

In this review, we will work around two simple definitions of two different entities, which most often co-exist in patients with lesions to central motor pathways: Spasticity is "Enhanced excitability of velocity-dependent responses to phasic stretch at rest", which will not be the subject of this review, while Spastic dystonia is tonic, chronic, involuntary muscle contraction in the absence of any stretch or any voluntary command (Gracies, 2005). Spastic dystonia is a much less well understood entity that will be the subject this review. Denny-Brown (1966) observed involuntary sustained muscle activity in monkeys with lesions restricted to the motor cortices . He further observed that such involuntary muscle activity persisted following abolition of sensory input to the spinal cord and concluded that a central mechanism rather than exaggerated stretch reflex activity had to be involved. He coined the term spastic dystonia to describe this involuntary tonic activity in the context of otherwise exaggerated stretch reflexes. Sustained involuntary muscle activity in the absence of any stretch or any voluntary command contributes to burdensome and disabling body deformities in patients with spastic paresis. Yet, little has been done since Denny-Brown's studies to determine the pathophysiology of this non- stretch or effort related sustained involuntary muscle activity following motor lesions and there is a clear need for research studies in order to improve current therapy. The purpose of the present review is to discuss some of the possible mechanisms that may be involved in the hope that this may guide future research. We discuss the existence of persistent inward currents in spinal motoneurones and present the evidence that the channels involved may be upregulated following central motor lesions. We also discuss a possible contribution from alterations in synaptic inputs from surviving or abnormally branched sensory and descending fibres leading to over-activity and lack of motor coordination. We finally discuss evidence of alterations in motor cortical representational maps and basal ganglia lesions.

The activity of the serotonin receptor 2C is regulated by alternative splicing

The central nervous system-specific serotonin receptor 2C (5HT2C) controls key physiological functions, such as food intake, anxiety, and motoneuron activity. Its deregulation is involved in depression, suicidal behavior, and spasticity, making it the target for antipsychotic drugs, appetite controlling substances, and possibly anti-spasm agents. Through alternative pre-mRNA splicing and RNA editing, the 5HT2C gene generates at least 33 mRNA isoforms encoding 25 proteins. The 5HT2C is a G-protein coupled receptor that signals through phospholipase C, influencing the expression of immediate/early genes like c-fos. Most 5HT2C isoforms show constitutive activity, i.e., signal without ligand binding. The constitutive activity of 5HT2C is decreased by pre-mRNA editing as well as alternative pre-mRNA splicing, which generates a truncated isoform that switches off 5HT2C receptor activity through heterodimerization; showing that RNA processing regulates the constitutive activity of the 5HT2C system. RNA processing events influencing the constitutive activity target exon Vb that forms a stable double stranded RNA structure with its downstream intron. This structure can be targeted by small molecules and oligonucleotides that change exon Vb alternative splicing and influence 5HT2C signaling in mouse models, leading to a reduction in food intake. Thus, the 5HT2C system is a candidate for RNA therapy in multiple models of CNS disorders.

Chronic spinal cord injury causes upregulation of serotonin (5-HT)2A and 5-HT2C receptors in lumbosacral cord motoneurons

OBJECTIVES

To investigate whether the voiding dysfunction caused by spinal cord injury (SCI) in rats can be improved by i.v. administration of the serotonin (5-HT)_2A/2C receptor agonist 2,5-dimethoxy-4-iodophenyl-2-aminopropanehydrochloride (DOI), and to discuss whether the mechanism can be ascribed to 5-HT_2A and 5-HT _2C receptor upregulation in lumbosacral cord motoneurons.

MATERIALS AND METHODS

Female Sprague-Dawley rats were divided into two groups (SCI group vs normal control [NC] group). Under urethane anaesthesia, cystometry was performed to examine the variation in urodynamic variables before and after successive intrathecal (i.t.) administration of various doses of DOI into the lumbosacral cord. Changes in 5-HT_2A and -_2C receptors in the lumbosacral cord were also investigated using immunohistochemical staining and Western blot analysis.

RESULTS

Compared with NC rats, the SCI rats had higher bladder capacity and post-void residual urine volume, and lower voiding efficiency. After SCI, DOI improved voiding efficiency, probably via external urethral sphincter (EUS) activity. Immunohistochemical staining and Western blot analysis showed that 5-HT_2A and -_2C receptors were upregulated in lumbosacral cord motoneurons.

CONCLUSION

In rats with SCI, DOI can improve voiding efficiency; this may be attributable to 5-HT_2A and -_2C receptor upregulation in lumbosacral cord motoneurons controlling EUS activity.

Agomelatine: a new opportunity to reduce neuropathic pain-preclinical evidence

Antidepressants are first-line treatments of neuropathic pain but not all these drugs are really effective. Agomelatine is an antidepressant with a novel mode of action, acting as an MT1/MT2 melatonergic receptor agonist and a 5-HT2C receptor antagonist that involves indirect norepinephrine release. Melatonin, serotonin, and norepinephrine have been involved in the pathophysiology of neuropathic pain. Yet, no study has been conducted to determine agomelatine effects on neuropathic pain in animal models. Using 3 rat models of neuropathic pain of toxic (oxaliplatin/OXA), metabolic (streptozocin/STZ), and traumatic (sciatic nerve ligation/CCI [chronic constriction nerve injury]) etiologies, we investigated the antihypersensitivity effect of acute and repeated agomelatine administration. We then determined the influence of melatonergic, 5-HT2C, α-2 and β-1/2 adrenergic receptor antagonists in the antihypersensitivity effect of agomelatine. The effect of the combination of agomelatine + gabapentin was evaluated using an isobolographic approach. In STZ and CCI models, single doses of agomelatine significantly and dose dependently reduced mechanical hypersensitivity. After daily administrations for 2 weeks, this effect was confirmed in the CCI model and agomelatine also displayed a marked antihypersensitivity effect in the OXA model. The antihypersensitivity effect of agomelatine involved melatonergic, 5-HT2C, and α-2 adrenergic receptors but not beta adrenoceptors. The isobolographic analysis demonstrated that the combination of agomelatine + gabapentin had additive effects. Agomelatine exerts a clear-cut antihypersensitivity effect in 3 different neuropathic pain models. Its effect is mediated by melatonergic and 5-HT2C receptors and, although agomelatine has no affinity, also by α-2 adrenergic receptors. Finally, agomelatine combined with gabapentin produces an additive antihypersensitivity effect.

Bursting deep dorsal horn neurons: the pharmacological target for the antispastic effects of zolmitriptan?

In a recent publication, Thaweerattanasinp et al. (J Neurophysiol 116: 1644-1653, 2016) investigated spinal cord injury and firing properties of deep dorsal horn neurons during NMDA or zolmitriptan application by employing electrophysiology in an in vitro spinal cord preparation. Deep dorsal horn neurons were classified into bursting, simple, or tonic firing groups,with bursting neurons showing NMDA and zolmitriptan sensitivity. We discuss the findings in a methodological framework and propose future experiments of importance for translating the results into physiological settings.

Production of Dopamine by Aromatic l-Amino Acid Decarboxylase Cells after Spinal Cord Injury

Aromatic l-amino acid decarboxylase (AADC) cells are widely distributed in the spinal cord, and their functions are largely unknown. We have previously found that AADC cells in the spinal cord could increase their ability to produce serotonin (5-hydroxytryptamine) from 5-hydroxytryptophan after spinal cord injury (SCI). Because AADC is a common enzyme catalyzing 5-hydroxytryptophan to serotonin and l-3,4-dihydroxyphenylalanine (l-dopa) to dopamine (DA), it seems likely that the ability of AADC cells using l-dopa to synthesize DA is also increased. To prove whether or not this is the case, a similar rat sacral SCI model and a similar experimental paradigm were adopted as that which we had used previously. In the chronic SCI rats (> 45 days), no AADC cells expressed DA if there was no exogenous l-dopa application. However, following administration of a peripheral AADC inhibitor (carbidopa) with or without a monoamine oxidase inhibitor (pargyline) co-application, systemic administration of l-dopa resulted in ∼94% of AADC cells becoming DA-immunopositive in the spinal cord below the lesion, whereas in normal or sham-operated rats none or very few of AADC cells became DA-immunopositive with the same treatment. Using tail electromyography, spontaneous tail muscle activity was increased nearly fivefold over the baseline level. When pretreated with a central AADC inhibitor (NSD-1015), further application of l-dopa failed to increase the motoneuron activity although the expression of DA in the AADC cells was not completely inhibited. These findings demonstrate that AADC cells in the spinal cord below the lesion gain the ability to produce DA from its precursor in response to SCI. This ability also enables the AADC cells to produce 5-HT and trace amines, and likely contributes to the development of hyperexcitability. These results might also be implicated for revealing the pathological mechanisms underlying l-dopa-induced dyskinesia in Parkinson's disease.

Preclinical models of muscle spasticity: valuable tools in the development of novel treatment for neurological diseases and conditions

Poor validity of preclinical animal models is one of the most commonly discussed explanations for the failures to develop novel drugs in general and in neuroscience in particular. However, there are several areas of neuroscience such as injury-induced spasticity where etiological factor can be adequately recreated and models can focus on specific pathophysiological mechanisms that likely contribute to spasticity syndrome in humans (such as motoneuron hyperexcitability and spinal hyperreflexia). Methods used to study spasticity in preclinical models are expected to have a high translational value (e.g., electromyogram (EMG)-based electrophysiological tools) and can efficiently assist clinical development programs. However, validation of these models is not complete yet. First, true predictive validity of these models is not established as clinically efficacious drugs have been used to reverse validate preclinical models while newly discovered mechanisms effective in preclinical models are yet to be fully explored in humans (e.g., 5-HT2C receptor inverse agonists, fatty acid amid hydrolase inhibitors). Second, further efforts need to be invested into cross-laboratory validation of study protocols and tools, adherence to the highest quality standards (blinding, randomization, pre-specified study endpoints, etc.), and systematic efforts to replicate key sets of data. These appear to be readily achievable tasks that will enable development not only of symptomatic but also of disease-modifying therapy of spasticity, an area that seems to be currently not in focus of research efforts.

Two-step production of monoamines in monoenzymatic cells in the spinal cord: a different control strategy of neurotransmitter supply?

Monoamine neurotransmitters play an important role in the modulation of sensory, motor and autonomic functions in the spinal cord. Although traditionally it is believed that in mammalian spinal cord, monoamine neurotransmitters mainly originate from the brain, accumulating evidence indicates that especially when the spinal cord is injured, they can also be produced in the spinal cord. In this review, I will present evidence for a possible pathway for two-step synthesis of dopamine and serotonin in the spinal cord. Published data from different sources and unpublished data from my own ongoing projects indicate that monoenzymatic cells expressing aromatic L-amino acid decarboxylase (AADC), tyrosine hydroxylase (TH) or tryptophan hydroxylase (TPH) are present in the spinal cord and that these TH and THP cells often lie in close proximity to AADC cells. Prompted by the above evidence, I hypothesize that dopamine and serotonin could be synthesized sequentially in two monoenzymatic cells in the spinal cord via a TH-AADC and a TPH-AADC cascade respectively. The monoamines synthesized through this pathway may compensate for lost neurotransmitters following spinal cord injury and also may play specific roles in the recovery of sensory, motor and autonomic functions.

Transplantation of erythropoietin gene-modified neural stem cells improves the repair of injured spinal cord

The protective effects of erythropoietin on spinal cord injury have not been well described. Here, the eukaryotic expression plasmid pcDNA3.1 human erythropoietin was transfected into rat neural stem cells cultured in vitro. A rat model of spinal cord injury was established using a free falling object. In the human erythropoietin-neural stem cells group, transfected neural stem cells were injected into the rat subarachnoid cavity, while the neural stem cells group was injected with non-transfected neural stem cells. Dulbecco's modified Eagle's medium/F12 medium was injected into the rats in the spinal cord injury group as a control. At 1–4 weeks post injury, the motor function in the rat lower limbs was best in the human erythropoietin-neural stem cells group, followed by the neural stem cells group, and lastly the spinal cord injury group. At 72 hours, compared with the spinal cord injury group, the apoptotic index and Caspase-3 gene and protein expressions were apparently decreased, and the bcl-2 gene and protein expressions were noticeably increased, in the tissues surrounding the injured region in the human erythropoietin-neural stem cells group. At 4 weeks, the cavities were clearly smaller and the motor and somatosensory evoked potential latencies were remarkably shorter in the human erythropoietin-neural stem cells group and neural stem cells group than those in the spinal cord injury group. These differences were particularly obvious in the human erythropoietin-neural stem cells group. More CM-Dil-positive cells and horseradish peroxidase-positive nerve fibers and larger amplitude motor and somatosensory evoked potentials were found in the human erythropoietin-neural stem cells group and neural stem cells group than in the spinal cord injury group. Again, these differences were particularly obvious in the human erythropoietin-neural stem cells group. These data indicate that transplantation of erythropoietin gene-modified neural stem cells into the subarachnoid cavity to help repair spinal cord injury and promote the recovery of spinal cord function better than neural stem cell transplantation alone. These findings may lead to significant improvements in the clinical treatment of spinal cord injuries.

Spinal Cord Hemisection Facilitates Aromatic L-Amino Acid Decarboxylase Cells to Produce Serotonin in the Subchronic but Not the Chronic Phase

Neuromodulators, such as serotonin (5-hydroxytryptamine, 5-HT) and noradrenalin, play an essential role in regulating the motor and sensory functions in the spinal cord. We have previously shown that in the rat spinal cord the activity of aromatic L-amino acid decarboxylase (AADC) cells to produce 5-HT from its precursor (5-hydroxytryptophan, 5-HTP) is dramatically increased following complete spinal cord transection. In this study, we investigated whether a partial loss of 5-HT innervation could similarly increase AADC activity. Adult rats with spinal cord hemisected at thoracic level (T11/T12) were used with a postoperation interval at 5 days or 60 days. Using immunohistochemistry, first, we observed a significant reduction in the density of 5-HT-immunoreactive fibers in the spinal cord below the lesion on the injured side for both groups. Second, we found that the AADC cells were similarly expressed on both injured and uninjured sides in both groups. Third, increased production of 5-HT in AADC cells following 5-HTP was seen in 5-day but not in 60-day postinjury group. These results suggest that plastic changes of the 5-HT system might happen primarily in the subchronic phase and for longer period its function could be compensated by plastic changes of other intrinsic and/or supraspinal modulation systems.

A consistent, quantifiable, and graded rat lumbosacral spinal cord injury model

The purpose of this study is to develop a rat lumbosacral spinal cord injury (SCI) model that causes consistent motoneuronal loss and behavior deficits. Most SCI models focus on the thoracic or cervical spinal cord. Lumbosacral SCI accounts for about one third of human SCI but no standardized lumbosacral model is available for evaluating therapies. Twenty-six adult female Sprague-Dawley rats were randomized to three groups: sham (n=9), 25 mm (n=8), and 50 mm (n=9). Sham rats had laminectomy only, while 25 mm and 50 mm rats were injured by dropping a 10 g rod from a height of 25 mm or 50 mm, respectively, onto the L4-5 spinal cord at the T13/L1 vertebral junction. We measured footprint length (FL), toe spreading (TS), intermediate toe spreading (ITS), and sciatic function index (SFI) from walking footprints, and static toe spreading (STS), static intermediate toe spreading (SITS), and static sciatic index (SSI) from standing footprints. At six weeks, we assessed neuronal and white matter loss, quantified axons, diameter, and myelin thickness in the peroneal and tibial nerves, and measured cross-sectional areas of tibialis anterior and gastrocnemius muscle fibers. The result shows that peroneal and tibial motoneurons were respectively distributed in 4.71 mm and 5.01 mm columns in the spinal cord. Dropping a 10-g weight from 25 mm or 50 mm caused 1.5 mm or 3.75 mm gaps in peroneal and tibial motoneuronal columns, respectively, and increased spinal cord white matter loss. Fifty millimeter contusions significantly increased FL and reduced TS, ITS, STS, SITS, SFI, and SSI more than 25 mm contusions, and resulted in smaller axon and myelinated axon diameters in tibial and peroneal nerves and greater atrophy of gastrocnemius and anterior tibialis muscles, than 25 mm contusions. This model of lumbosacral SCI produces consistent and graded loss of white matter, motoneuronal loss, peripheral nerve axonal changes, and anterior tibialis and gastrocnemius muscles atrophy in rats.

Serotonin receptor and KCC2 gene expression in lumbar flexor and extensor motoneurons posttransection with and without passive cycling

Sacrocaudal motoneuron gene expression is altered following a spinal transection. Of interest here is the regulation of serotonin (5-HT) receptors (R), glutamate receptor, metabotropic 1 (mGluR1), and potassium-chloride cotransporter (KCC2), which mediate motoneuron excitability, locomotor recovery, and spasticity posttransection. The examination of these genes in lumbar motoneurons posttransection has not been studied, which is necessary for developing potential pharmacological interventions aimed at restoring locomotion and/or reducing spasticity. Also, if activity is to be used to promote recovery or reduce spasticity postinjury, a further examination of neuromuscular activity on gene expression posttransection is warranted. The purpose of this study was to examine motoneuronal gene expression of 5-HT receptors, KCC2, and mGluR1 at 3 mo following a complete thoracic spinal cord transection, with and without the inclusion of daily passive cycling. Physiological hindlimb extensor and flexor motoneurons were differentially identified with two retrograde fluorescent tracers, allowing for the identification and separate harvesting of extensor and flexor motoneurons with laser capture microdissection and the subsequent examination of mRNA content using quantitative RT-PCR analysis. We demonstrate that posttransection 5-HT1AR, 5-HT2CR, and mGluR1 expression was downregulated, whereas the 5-HT2AR was upregulated. These alterations in gene expression were observed in both flexor and extensor motoneurons, whereas passive cycling influenced gene expression in extensor but not flexor motoneurons. Passive cycling in extensor motoneurons further enhanced 5-HT2AR expression and increased 5-HT7R and KCC2 expression. Our results demonstrate that passive cycling influences serotonin receptor and KCC2 gene expression and that extensor motoneurons compared with flexor motoneurons may be more plastic to activity-based interventions posttransection.

The role of the serotonergic system in locomotor recovery after spinal cord injury

Serotonin (5-HT), a monoamine neurotransmitter synthesized in various populations of brainstem neurons, plays an important role in modulating the activity of spinal networks involved in vertebrate locomotion. Following spinal cord injury (SCI) there is a disruption of descending serotonergic projections to spinal motor areas, which results in a subsequent depletion in 5-HT, the dysregulation of 5-HT transporters as well as the elevated expression, super-sensitivity and/or constitutive auto-activation of specific 5-HT receptors. These changes in the serotonergic system can produce varying degrees of locomotor dysfunction through to paralysis. To date, various approaches targeting the different components of the serotonergic system have been employed to restore limb coordination and improve locomotor function in experimental models of SCI. These strategies have included pharmacological modulation of serotonergic receptors, through the administration of specific 5-HT receptor agonists, or by elevating the 5-HT precursor 5-hydroxytryptophan, which produces a global activation of all classes of 5-HT receptors. Stimulation of these receptors leads to the activation of the locomotor central pattern generator (CPG) below the site of injury to facilitate or improve the quality and frequency of movements, particularly when used in concert with the activation of other monoaminergic systems or coupled with electrical stimulation. Another approach has been to employ cell therapeutics to replace the loss of descending serotonergic input to the CPG, either through transplanted fetal brainstem 5-HT neurons at the site of injury that can supply 5-HT to below the level of the lesion or by other cell types to provide a substrate at the injury site for encouraging serotonergic axon regrowth across the lesion to the caudal spinal cord for restoring locomotion.

Spinal cord injury enables aromatic L-amino acid decarboxylase cells to synthesize monoamines

Serotonin (5-HT), an important modulator of both sensory and motor functions in the mammalian spinal cord, originates mainly in the raphe nuclei of the brainstem. However, following complete transection of the spinal cord, small amounts of 5-HT remain detectable below the lesion. It has been suggested, but not proven, that this residual 5-HT is produced by intraspinal 5-HT neurons. Here, we show by immunohistochemical techniques that cells containing the enzyme aromatic l-amino acid decarboxylase (AADC) occur not only near the central canal, as reported by others, but also in the intermediate zone and dorsal horn of the spinal gray matter. We show that, following complete transection of the rat spinal cord at S2 level, AADC cells distal to the lesion acquire the ability to produce 5-HT from its immediate precursor, 5-hydroxytryptophan. Our results indicate that this phenotypic change in spinal AADC cells is initiated by the loss of descending 5-HT projections due to spinal cord injury (SCI). By in vivo and in vitro electrophysiology, we show that 5-HT produced by AADC cells increases the excitability of spinal motoneurons. The phenotypic change in AADC cells appears to result from a loss of inhibition by descending 5-HT neurons and to be mediated by 5-HT1B receptors expressed by AADC cells. These findings indicate that AADC cells are a potential source of 5-HT at spinal levels below an SCI. The production of 5-HT by AADC cells, together with an upregulation of 5-HT2 receptors, offers a partial explanation of hyperreflexia below a chronic SCI.

Traumatic injury induces changes in the expression of the serotonin 1A receptor in the spinal cord of lampreys

After spinal cord injury (SCI) in mammals, the loss of serotonin coming from the brainstem reduces the excitability of motor neurons and leads to a compensatory overexpression of serotonin receptors. Despite the key role of the serotonin receptor 1a in the control of locomotion, little attention has been put in the study of this receptor after SCI. In contrast to mammals, lampreys recover locomotion after a complete SCI, so, studies in this specie could help to understand events that lead to recovery of function. Here, we showed that in lampreys there is an acute increase in the expression of the serotonin 1A receptor transcript (5-ht1a) after SCI and a few weeks later expression levels go back to normal rostrally and caudally to the lesion. Overexpression of the 5-ht1a in rostral levels after SCI has not been reported in mammals, suggesting that this could be part of the plastic events that lead to the recovery of function in lampreys. The analysis of changes in 5-ht1a expression by zones (periventricular region and horizontally extended grey matter) showed that they followed the same pattern of changes detected in the spinal cord as a whole, with the exception of the caudal periventricular layer, where no significant differences were observed between control and experimental animals at any time post lesion. This suggests that different molecular signals act on the periventricular cells of the rostral and caudal regions to injury site and thus affecting their response to the injury in terms of expression of the 5-ht1a.

Quantitative expression profiling in mouse spinal cord reveals changing relationships among channel and receptor mRNA levels across postnatal maturation

Neural networks ultimately arrive at functional output via interaction of the excitability of individual neurons and their synaptic interactions. We investigated the relationships between voltage-gated ion channel and neurotransmitter receptor mRNA levels in mouse spinal cord at four different postnatal time points (P5, P11, P17, and adult) and three different adult cord levels (cervical, thoracic, and lumbosacral) using quantitative reverse-transcription polymerase chain reaction (qRT-PCR). Our analysis and data visualization are novel in that we chose a focal group of voltage-gated channel subunits and transmitter receptor subunits, performed absolute quantitation of mRNA copy number for each gene from a sample, and used multiple correlation analyses and correlation matrices to detect patterns in correlated mRNA levels across all genes of interest. These correlation profiles suggest that postnatal maturation of the spinal cord includes changes among channel and receptor subunits that proceed from widespread co-regulation to more refined and distinct functional relationships.

Recovery of neuronal and network excitability after spinal cord injury and implications for spasticity

The state of areflexia and muscle weakness that immediately follows a spinal cord injury (SCI) is gradually replaced by the recovery of neuronal and network excitability, leading to both improvements in residual motor function and the development of spasticity. In this review we summarize recent animal and human studies that describe how motoneurons and their activation by sensory pathways become hyperexcitable to compensate for the reduction of functional activation of the spinal cord and the eventual impact on the muscle. Specifically, decreases in the inhibitory control of sensory transmission and increases in intrinsic motoneuron excitability are described. We present the idea that replacing lost patterned activation of the spinal cord by activating synaptic inputs via assisted movements, pharmacology or electrical stimulation may help to recover lost spinal inhibition. This may lead to a reduction of uncontrolled activation of the spinal cord and thus, improve its controlled activation by synaptic inputs to ultimately normalize circuit function. Increasing the excitation of the spinal cord with spared descending and/or peripheral inputs by facilitating movement, instead of suppressing it pharmacologically, may provide the best avenue to improve residual motor function and manage spasticity after SCI.

Serotonergic modulation in neuropathy induced by oxaliplatin: effect on the 5HT2C receptor

Fluoxetine has been shown to be effective in clinical and experimental studies of neuropathic pain. Besides to increase serotonin levels in the synaptic cleft, fluoxetine is able to block the serotonergic 5-HT2C receptor subtype, which in turn has been involved in the modulation of neuropathic pain. This study investigated the effect of repeated treatments with fluoxetine on the neuropathic nociceptive response induced by oxaliplatin and the effects of both treatments on 5-HT2C receptor mRNA expression and protein levels in the rat spinal cord (SC), rostral ventral medulla (RVM), midbrain periaqueductal gray (PAG) and amygdala (Amy). Nociception was assessed by paw-pressure, cold plate and Von Frey tests. Fluoxetine prevented mechanical hypersensitivity and pain threshold alterations induced by oxaliplatin but did not prevent the impairment in weight gain induced by this anticancer drug. Ex vivo analysis revealed that oxaliplatin increased the 5-HT2C receptor mRNA expression and protein levels in the SC and PAG. Similar effects were observed in fluoxetine-treated animals but only within the PAG. While oxaliplatin decreased the 5-HT2C mRNA expression levels in the Amy, fluoxetine increased their protein levels in this area. Fluoxetine impaired the oxaliplatin effects on the 5-HT2C receptor mRNA expression in the SC and Amy and protein levels in the SC. All treatments increased of 5-HT2C receptor mRNA expression and protein levels in the PAG. These results suggest that the effects of fluoxetine on neuropathic pain induced by oxaliplatin are associated with quantitative changes in the 5-HT2C receptors located within important areas of the nociceptive system.

Overexpression of BDNF increases excitability of the lumbar spinal network and leads to robust early locomotor recovery in completely spinalized rats

Strategies to induce recovery from lesions of the spinal cord have not fully resulted in clinical applications. This is a consequence of a number of impediments that axons encounter when trying to regrow beyond the lesion site, and that intraspinal rearrangements are subjected to. In the present study we evaluated (1) the possibility to improve locomotor recovery after complete transection of the spinal cord by means of an adeno-associated (AAV) viral vector expressing the neurotrophin brain-derived neurotrophic factor (BDNF) in lumbar spinal neurons caudal to the lesion site and (2) how the spinal cord transection and BDNF treatment affected neurotransmission in the segments caudal to the lesion site. BDNF overexpression resulted in clear increases in expression levels of molecules involved in glutamatergic (VGluT2) and GABAergic (GABA, GAD65, GAD67) neurotransmission in parallel with a reduction of the potassium-chloride co-transporter (KCC2) which contributes to an inhibitory neurotransmission. BDNF treated animals showed significant improvements in assisted locomotor performance, and performed locomotor movements with body weight support and plantar foot placement on a moving treadmill. These positive effects of BDNF local overexpression were detectable as early as two weeks after spinal cord transection and viral vector application and lasted for at least 7 weeks. Gradually increasing frequencies of clonic movements at the end of the experiment attenuated the quality of treadmill walking. These data indicate that BDNF has the potential to enhance the functionality of isolated lumbar circuits, but also that BDNF levels have to be tightly controlled to prevent hyperexcitability.

Neural circuit interactions between the dorsal raphe nucleus and the lateral hypothalamus: an experimental and computational study

Orexinergic/hypocretinergic (Ox) neurotransmission plays an important role in regulating sleep, as well as in anxiety and depression, for which the serotonergic (5-HT) system is also involved in. However, little is known regarding the direct and indirect interactions between 5-HT in the dorsal raphe nucleus (DRN) and Ox neurons in the lateral hypothalamus (LHA). In this study, we report the additional presence of 5-HT1BR, 5-HT2AR, 5-HT2CR and fast ligand-gated 5-HT3AR subtypes on the Ox neurons of transgenic Ox-enhanced green fluorescent protein (Ox-EGFP) and wild type C57Bl/6 mice using single and double immunofluorescence (IF) staining, respectively, and quantify the colocalization for each 5-HT receptor subtype. We further reveal the presence of 5-HT3AR and 5-HT1AR on GABAergic neurons in LHA. We also identify NMDAR1, OX1R and OX2R on Ox neurons, but none on adjacent GABAergic neurons. This suggests a one-way relationship between LHA's GABAergic and Ox neurons, wherein GABAergic neurons exerts an inhibitory effect on Ox neurons under partial DRN's 5-HT control. We also show that Ox axonal projections receive glutamatergic (PSD-95 immunopositive) and GABAergic (Gephyrin immunopositive) inputs in the DRN. We consider these and other available findings into our computational model to explore possible effects of neural circuit connection types and timescales on the DRN-LHA system's dynamics. We find that if the connections from 5-HT to LHA's GABAergic neurons are weakly excitatory or inhibitory, the network exhibits slow oscillations; not observed when the connection is strongly excitatory. Furthermore, if Ox directly excites 5-HT neurons at a fast timescale, phasic Ox activation can lead to an increase in 5-HT activity; no significant effect with slower timescale. Overall, our experimental and computational approaches provide insights towards a more complete understanding of the complex relationship between 5-HT in the DRN and Ox in the LHA.