Functional recovery after central infusion of alpha-melanocyte-stimulating hormone in rats with spinal cord contusion injury

Electrical stimulation affects the differentiation of transplanted regionally specific human spinal neural progenitor cells (sNPCs) after chronic spinal cord injury

BACKGROUND

There are currently no effective clinical therapies to ameliorate the loss of function that occurs after spinal cord injury. Electrical stimulation of the rat spinal cord through the rat tail has previously been described by our laboratory. We propose combinatorial treatment with human induced pluripotent stem cell-derived spinal neural progenitor cells (sNPCs) along with tail nerve electrical stimulation (TANES). The purpose of this study was to examine the influence of TANES on the differentiation of sNPCs with the hypothesis that the addition of TANES would affect incorporation of sNPCs into the injured spinal cord, which is our ultimate goal.

METHODS

Chronically injured athymic nude rats were allocated to one of three treatment groups: injury only, sNPC only, or sNPC + TANES. Rats were sacrificed at 16 weeks post-transplantation, and tissue was processed and analyzed utilizing standard histological and tissue clearing techniques. Functional testing was performed. All quantitative data were presented as mean ± standard error of the mean. Statistics were conducted using GraphPad Prism.

RESULTS

We found that sNPCs were multi-potent and retained the ability to differentiate into mainly neurons or oligodendrocytes after this transplantation paradigm. The addition of TANES resulted in more transplanted cells differentiating into oligodendrocytes compared with no TANES treatment, and more myelin was found. TANES not only promoted significantly higher numbers of sNPCs migrating away from the site of injection but also influenced long-distance axonal/dendritic projections especially in the rostral direction. Further, we observed localization of synaptophysin on SC121-positive cells, suggesting integration with host or surrounding neurons, and this finding was enhanced when TANES was applied. Also, rats that were transplanted with sNPCs in combination with TANES resulted in an increase in serotonergic fibers in the lumbar region. This suggests that TANES contributes to integration of sNPCs, as well as activity-dependent oligodendrocyte and myelin remodeling of the chronically injured spinal cord.

CONCLUSIONS

Together, the data suggest that the added electrical stimulation promoted cellular integration and influenced the fate of human induced pluripotent stem cell-derived sNPCs transplanted into the injured spinal cord.

The melanocortin receptor signaling system and its role in neuroprotection against neurodegeneration: Therapeutic insights

The melanocortin signaling system consists of the melanocortin peptides, their distinctive receptors, accessory proteins, and endogenous antagonists. Melanocortin peptides are small peptide hormones that have been studied in a variety of physiological and pathological conditions. There are five types of melanocortin receptors, and they are distributed within the central nervous system and in several tissues of the periphery. The G protein-coupled melanocortin receptors typically signal through adenylyl cyclase and other downstream signaling pathways. Depending on the ligand, surface expression of melanocortin receptor, receptor occupancy period, related proteins, the type of cell, and other parameters, the signaling pathways are complicated and pleiotropic. While it is known that all five melanocortin receptors are coupled to Gs, they can also occasionally couple to Gq or Gi. Both direct and indirect neuroprotection are induced by the melanocortin signaling system. Targeting several of the components of the melanocortin signaling system (ligands, receptors, accessory proteins, signaling effectors, and regulators) may provide therapeutic opportunities. Activation of the melanocortin system improves different functional traits in neurodegenerative diseases. There is a potential for additional melanocortin system interventions by interfering with dimerization or dissociation. This review aims to discuss the melanocortin receptor signaling system and its role in neuroprotection, as well as its therapeutic potential.

Human induced pluripotent stem cells integrate, create synapses and extend long axons after spinal cord injury

Numerous interventions have been explored in animal models using cells differentiated from human induced pluripotent stem cells (iPSCs) in the context of neural injury with some success. Our work seeks to transplant cells that are generated from hiPSCs into regionally specific spinal neural progenitor cells (sNPCs) utilizing a novel accelerated differentiation protocol designed for clinical translation. We chose a xenotransplantation model because our laboratory is focused on the behaviour of human cells in order to bring this potential therapy to translation. Cells were transplanted into adult immunodeficient rats after moderate contusion spinal cord injury (SCI). Twelve weeks later, cells derived from the transplanted sNPCs survived and differentiated into neurons and glia that filled the lesion cavity and produced a thoracic spinal cord transcriptional program in vivo. Furthermore, neurogenesis and ionic channel expression were promoted within the adjacent host spinal cord tissue. Transplanted cells displayed robust integration properties including synapse formation and myelination by host oligodendrocytes. Axons from transplanted hiPSC sNPC‐derived cells extended both rostrally and caudally from the SCI transplant site, rostrally approximately 6 cm into supraspinal structures. Thus, iPSC‐derived sNPCs may provide a patient‐specific cell source for patients with SCI that could provide a relay system across the site of injury.

Delayed administration of the human anti-RGMa monoclonal antibody elezanumab promotes functional recovery including spontaneous voiding after spinal cord injury in rats

Spinal cord injury (SCI) often results in permanent functional loss due to a series of degenerative events including cell death, axonal damage, and the upregulation of inhibitory proteins that impede regeneration. Repulsive Guidance Molecule A (RGMa) is a potent inhibitor of axonal growth that is rapidly upregulated following injury in both the rodent and human central nervous system (CNS). Previously, we showed that monoclonal antibodies that specifically block inhibitory RGMa signaling promote neuroprotective and regenerative effects when administered acutely in a clinically relevant rat model of thoracic SCI. However, it is unknown whether systemic administration of RGMa blocking antibodies are effective for SCI after delayed administration. Here, we administered elezanumab, a human monoclonal antibody targeting RGMa, intravenously either acutely or at 3 h or 24 h following thoracic clip impact-compression SCI. Rats treated with elezanumab acutely and at 3 h post-injury showed improvements in overground locomotion and fine motor function and gait. Rats treated 24 h post-SCI trended towards better recovery demonstrating significantly greater stride length and swing speed. Treated rats also showed greater tissue preservation with reduced lesion areas. As seen with acute treatment, delayed administration of elezanumab at 3 h post-SCI also increased perilesional neuronal sparing and serotonergic and corticospinal axonal plasticity. In addition, all elezanumab treated rats showed earlier spontaneous voiding ability and less post-trauma bladder wall hypertrophy. Together, our data demonstrate the therapeutic efficacy of delayed systemic administration of elezanumab in a rat model of SCI, and uncovers a new role for RGMa inhibition in bladder recovery following SCI.

Activation of Melanocortin-4 Receptor by a Synthetic Agonist Inhibits Ethanolinduced Neuroinflammation in Rats

BACKGROUND

High ethanol intake induces a neuroinflammatory response resulting in the subsequent maintenance of chronic alcohol consumption. The melanocortin system plays a pivotal role in the modulation of alcohol consumption. Interestingly, it has been shown that the activation of melanocortin-4 receptor (MC4R) in the brain decreases the neuroinflammatory response in models of brain damage other than alcohol consumption, such as LPS-induced neuroinflammation, cerebral ischemia, glutamate excitotoxicity, and spinal cord injury.

OBJECTIVES

In this work, we aimed to study whether MC4R activation by a synthetic MC4R-agonist peptide prevents ethanol-induced neuroinflammation, and if alcohol consumption produces changes in MC4R expression in the hippocampus and hypothalamus.

METHODS

Ethanol-preferring Sprague Dawley rats were selected offering access to 20% ethanol on alternate days for 4 weeks (intermittent access protocol). After this time, animals were i.p. administered an MC4R agonist peptide in the last 2 days of the protocol. Then, the expression of the proinflammatory cytokines interleukin 6 (IL-6), interleukin 1-beta (IL-1β), and tumor necrosis factor-alpha (TNF-α) were measured in the hippocampus, hypothalamus and prefrontal cortex. It was also evaluated if ethanol intake produces alterations in the expression of MC4R in the hippocampus and the hypothalamus.

RESULTS

Alcohol consumption increased the expression of MC4R in the hippocampus and the hypothalamus. The administration of the MC4R agonist reduced IL-6, IL-1β and TNF-α levels in hippocampus, hypothalamus and prefrontal cortex, to those observed in control rats that did not drink alcohol.

CONCLUSION

High ethanol consumption produces an increase in the expression of MC4R in the hippocampus and hypothalamus. The administration of a synthetic MC4R-agonist peptide prevents neuroinflammation induced by alcohol consumption in the hippocampus, hypothalamus, and prefrontal cortex. These results could explain the effect of α-MSH and other synthetic MC4R agonists in decreasing alcohol intake through the reduction of the ethanol-induced inflammatory response in the brain.

Activation of Melanocortin-4 Receptor Inhibits Both Neuroinflammation Induced by Early Exposure to Ethanol and Subsequent Voluntary Alcohol Intake in Adulthood in Animal Models: Is BDNF the Key Mediator?

The concept that neuroinflammation induced by excessive alcohol intake in adolescence triggers brain mechanisms that perpetuate consumption has strengthened in recent years. The melanocortin system, composed of the melanocortin 4 receptor (MC4R) and its ligand α-melanocyte-stimulating hormone (α-MSH), has been implicated both in modulation of alcohol consumption and in ethanol-induced neuroinflammation decrease. Chronic alcohol consumption in adolescent rats causes a decrease in an α-MSH release by the hypothalamus, while the administration of synthetic agonists of MC4R causes a decrease in neuroinflammation and a decrease in voluntary alcohol consumption. However, the mechanism that connects the activation of MC4R with the decrease of both neuroinflammation and voluntary alcohol consumption has not been elucidated. Brain-derived neurotrophic factor (BDNF) has been implicated in alcohol drinking motivation, dependence and withdrawal, and its levels are reduced in alcoholics. Deficiencies in BDNF levels increased ethanol self-administration in rats. Further, BDNF triggers important anti-inflammatory effects in the brain, and this could be one of the mechanisms by which BDNF reduces chronic alcohol intake. Interestingly, MC4R signaling induces BDNF expression through the activation of the cAMP-responsive element-binding protein (CREB). We hypothesize that ethanol exposure during adolescence decreases the expression of α-MSH and hence MC4R signaling in the hippocampus, leading to a lower BDNF activity that causes dramatic changes in the brain (e.g., neuroinflammation and decreased neurogenesis) that predispose to maintain alcohol abuse until adulthood. The activation of MC4R either by α-MSH or by synthetic agonist peptides can induce the expression of BDNF, which would trigger several processes that lead to lower alcohol consumption.

RGMa inhibition with human monoclonal antibodies promotes regeneration, plasticity and repair, and attenuates neuropathic pain after spinal cord injury

Traumatic spinal cord injury (SCI) causes a cascade of degenerative events including cell death, axonal damage, and the upregulation of inhibitory molecules which prevent regeneration and limit recovery. Repulsive guidance molecule A (RGMa) is a potent neurite growth inhibitor in the central nervous system, exerting its repulsive activity by binding the Neogenin receptor. Here, we show for the first time that inhibitory RGMa is markedly upregulated in multiple cell types after clinically relevant impact-compression SCI in rats, and importantly, also in the injured human spinal cord. To neutralize inhibitory RGMa, clinically relevant human monoclonal antibodies were systemically administered after acute SCI, and were detected in serum, cerebrospinal fluid, and in the injured tissue. Rats treated with RGMa blocking antibodies showed significantly improved recovery of motor function and gait. Furthermore, RGMa blocking antibodies promoted neuronal survival, and enhanced the plasticity of descending serotonergic pathways and corticospinal tract axonal regeneration. RGMa antibody also attenuated neuropathic pain responses, which was associated with fewer activated microglia and reduced CGRP expression in the dorsal horn caudal to the lesion. These results show the therapeutic potential of the first human RGMa antibody for SCI and uncovers a new role for the RGMa/Neogenin pathway on neuropathic pain.

Developments and new vistas in the field of melanocortins

Melanocortins play a fundamental role in several basic functions of the organism (sexual activity, feeding, inflammation and immune responses, pain sensitivity, response to stressful situations, motivation, attention, learning, and memory). Moreover, a large body of animal data, some of which were also confirmed in humans, unequivocally show that melanocortins also have impressive therapeutic effects in several pathological conditions that are the leading cause of mortality and disability worldwide (hemorrhagic, or anyway hypovolemic, shock; septic shock; respiratory arrest; cardiac arrest; ischemia- and ischemia/reperfusion-induced damage of the brain, heart, intestine, and other organs; traumatic injury of brain, spinal cord, and peripheral nerves; neuropathic pain; toxic neuropathies; gouty arthritis; etc.). Recent data obtained in animal models seem to moreover confirm previous hypotheses and preliminary data concerning the neurotrophic activity of melanocortins in neurodegenerative diseases, in particular Alzheimer's disease. Our aim was (i) to critically reconsider the established extrahormonal effects of melanocortins (on sexual activity, feeding, inflammation, tissue hypoperfusion, and traumatic damage of central and peripheral nervous system) at the light of recent findings, (ii) to review the most recent advancements, particularly on the effects of melanocortins in models of neurodegenerative diseases, (iii) to discuss the reasons that support the introduction into clinical practice of melanocortins as life-saving agents in shock conditions and that suggest to verify in clinical setting the impressive results steadily obtained with melanocortins in different animal models of tissue ischemia and ischemia/reperfusion, and finally, (iv) to mention the advisable developments, particularly in terms of selectivity of action and of effects.

Pharmacological Suppression of CNS Scarring by Deferoxamine Reduces Lesion Volume and Increases Regeneration in an In Vitro Model for Astroglial-Fibrotic Scarring and in Rat Spinal Cord Injury In Vivo

Lesion-induced scarring is a major impediment for regeneration of injured axons in the central nervous system (CNS). The collagen-rich glial-fibrous scar contains numerous axon growth inhibitory factors forming a regeneration-barrier for axons. We demonstrated previously that the combination of the iron chelator 2,2’-bipyridine-5,5’-decarboxylic acid (BPY-DCA) and 8-Br-cyclic AMP (cAMP) inhibits scar formation and collagen deposition, leading to enhanced axon regeneration and partial functional recovery after spinal cord injury. While BPY-DCA is not a clinical drug, the clinically approved iron chelator deferoxamine mesylate (DFO) may be a suitable alternative for anti-scarring treatment (AST). In order to prove the scar-suppressing efficacy of DFO we modified a recently published in vitro model for CNS scarring. The model comprises a co-culture system of cerebral astrocytes and meningeal fibroblasts, which form scar-like clusters when stimulated with transforming growth factor-β (TGF-β). We studied the mechanisms of TGF-β-induced CNS scarring and compared the efficiency of different putative pharmacological scar-reducing treatments, including BPY-DCA, DFO and cAMP as well as combinations thereof. We observed modulation of TGF-β-induced scarring at the level of fibroblast proliferation and contraction as well as specific changes in the expression of extracellular matrix molecules and axon growth inhibitory proteins. The individual and combinatorial pharmacological treatments had distinct effects on the cellular and molecular aspects of in vitro scarring. DFO could be identified as a putative anti-scarring treatment for CNS trauma. We subsequently validated this by local application of DFO to a dorsal hemisection in the rat thoracic spinal cord. DFO treatment led to significant reduction of scarring, slightly increased regeneration of corticospinal tract as well as ascending CGRP-positive axons and moderately improved locomotion. We conclude that the in vitro model for CNS scarring is suitable for efficient pre-screening and identification of putative scar-suppressing agents prior to in vivo application and validation, thus saving costs, time and laboratory animals.

Comparison of deferoxamine and methylprednisolone: protective effect of pharmacological agents on lipid peroxidation in spinal cord injury in rats

STUDY DESIGN

Experimental study.

OBJECTIVE

To investigate the protective effect of deferoxamine (DFO) administration in comparison with methylprednisolone (MP) on lipid peroxidation and antioxidants after spinal cord injury (SCI) in rats.

SUMMARY OF BACKGROUND DATA

DFO is used for treating an iron-chelating agent, which is also used in the treatment of iron poisoning and thalassaemia. The neuroprotective effect of DFO was evaulated as a therapeutic agent for SCI.

METHODS

Forty Wistar rats were randomly divided into 5 groups as sham laminectomy (n = 8), laminectomy with SCI (n = 8), laminectomy with SCI and 0.9% saline intraperitoneal (i.p.) (n = 8), laminectomy with SCI and 30 mg/kg MP i.p. (n = 8), and laminectomy with SCI and 30 mg/kg DFO i.p. (n = 8). Neurological deficits were examined 24 hours after trauma, and all rats were killed. Spinal cord segments were harvested for both biochemical and histopathological evaluation.

RESULTS

At 24 hours post-SCI, whereas malondialdehyde levels were increased, superoxide dismutase, catalase, and glutathione peroxidase levels were decreased in groups I, II, and III. MP and DFO treatment decreased MDA levels and increased superoxide dismutase CAT, and glutathione peroxidase levels in control and study groups. There was no statistically significant difference between treatment with MP and DFO (P> 0.05). All rats were paraplegic after SCI, except in the sham group. Histopathological improvement was observed in control and study groups.

CONCLUSION

This study indicates that beneficial effects may be provided and further studies need to investigate the dose-dependent beneficial and side effects of DFO in SCI.

LEVEL OF EVIDENCE

N/A.

Effect of alpha-melanocyte stimulating hormone on locomotor recovery following spinal cord injury in mice: Role of serotonergic system

The present study underscores the effect of serotonergic antagonist on alpha-melanocyte stimulating hormone (α-MSH) induced neuronal regeneration. Swiss-albino mice were subjected to experimental spinal cord injury (ESCI) and treated with serotonergic antagonist, ritanserin, alone or in combination with α-MSH, and the locomotor recovery was investigated. ESCI was induced at thoracic T(10-12) level by compression method. Motor function score (0-10) of each mouse was monitored prior to, and on days 1, 4, 7, 10 and 14 following ESCI. Untreated ESCI animals showed almost normal hind limb motor function by 14days. Similar degree of recovery was observed on day 10 in animals given α-MSH or ritanserin. However, in animals treated with both agents, comparable recovery was observed on day 4. While histological examination of the spinal cord following ESCI showed demyelination, necrosis and cyst formation, treatment with ritanserin, alone and in combination with α-MSH, significantly prevented the tissue damage. We suggest that early antagonism of serotonergic 5-HT(2a/2c) receptors may potentiate the neurotropic and locomotor recovery activity of α-MSH.

Extensive scarring induced by chronic intrathecal tubing augmented cord tissue damage and worsened functional recovery after rat spinal cord injury

Intrathecal infusion has been widely used to directly deliver drugs or neurotrophins to a lesion site following spinal cord injury. Evidence shows that intrathecal infusion is efficient for 7 days but is markedly reduced after 14 days, due to time dependent occlusion. In addition, extensive fibrotic scarring is commonly observed with intrathecal infusion. These anomalies need to be clearly elucidated in histology. In the present study, all adult Long-Evans rats received a 25 mm contusion injury on spinal cord T10 produced using the NYU impactor device. Immediately after injury, catheter tubing with an outer diameter of 0.38 mm was inserted through a small dural opening at L3 into the subdural space with the tubing tip positioned near the injury site. The tubing was connected to an Alzet mini pump, which was filled with saline solution and was placed subcutaneously. Injured rats without tubing served as control. Rats were behaviorally tested for 6 weeks using the BBB locomotor rating scale and histologically assessed for tissue scarring. Six weeks later, we found that the intrathecal tubing caused extensive scarring and inflammation, related to neutrophils, macrophages and plasma cells. The tubing's tip was occluded by scar tissue and inflammatory cells. The scar tissue surrounding the tubing consists of 20-70 layers of fibroblasts and densely compacted collagen fibers, seriously compressing and damaging the cord tissue. BBB scores of rats with intrathecal tubing were significantly lower than control rats (p<0.01) from 2 weeks after injury, implying serious impairment of functional recovery caused by the scarring.

Involvement of ERK2 in traumatic spinal cord injury

Activation of extracellular signal-regulated protein kinase 1/2 (ERK1/2) are implicated in the pathophysiology of spinal cord injury (SCI). However, the specific functions of individual ERK isoforms in neurodegeneration are largely unknown. We investigated the hypothesis that ERK2 activation may contribute to pathological and functional deficits following SCI and that ERK2 knockdown using RNA interference may provide a novel therapeutic strategy for SCI. Lentiviral ERK2 shRNA and siRNA were utilized to knockdown ERK2 expression in the spinal cord following SCI. Pre-injury intrathecal administration of ERK2 siRNA significantly reduced excitotoxic injury-induced activation of ERK2 (p < 0.001) and caspase 3 (p < 0.01) in spinal cord. Intraspinal administration of lentiviral ERK2 shRNA significantly reduced ERK2 expression in the spinal cord (p < 0.05), but did not alter ERK1 expression. Administration of the lentiviral ERK2 shRNA vector 1 week prior to severe spinal cord contusion injury resulted in a significant improvement in locomotor function (p < 0.05), total tissue sparing (p < 0.05), white matter sparing (p < 0.05), and gray matter sparing (p < 0.05) 6 weeks following severe contusive SCI. Our results suggest that ERK2 signaling is a novel target associated with the deleterious consequences of spinal injury.

Acute rolipram/thalidomide treatment improves tissue sparing and locomotion after experimental spinal cord injury

Traumatic spinal cord injury (SCI) causes severe and permanent functional deficits due to the primary mechanical insult followed by secondary tissue degeneration. The cascade of secondary degenerative events constitutes a range of therapeutic targets which, if successfully treated, could significantly ameliorate functional loss after traumatic SCI. During the early hours after injury, potent pro-inflammatory cytokines, including tumor necrosis factor alpha (TNF-alpha) and interleukin-1 beta (IL-1beta) are synthesized and released, playing key roles in secondary tissue degeneration. In the present investigation, the ability of rolipram and thalidomide (FDA approved drugs) to reduce secondary tissue degeneration and improve motor function was assessed in an experimental model of spinal cord contusion injury. The combined acute single intraperitoneal administration of both drugs attenuated TNF-alpha and IL-1beta production and improved white matter sparing at the lesion epicenter. This was accompanied by a significant (2.6 point) improvement in the BBB locomotor score by 6 weeks. There is, at present, no widely accepted intervention strategy that is appropriate for the early treatment of human SCI. The present data suggest that clinical trials for the acute combined application of rolipram and thalidomide may be warranted. The use of such "established drugs" could facilitate the early initiation of trials.

Neuroprotective effects of caspase-3 inhibition on functional recovery and tissue sparing after acute spinal cord injury

STUDY DESIGN

We used gene microarrays and found that caspase-related death genes were upregulated. We tested caspase inhibition and evaluated its effect on the spinal cord after traumatic injury.

OBJECTIVE

The logical extension of previous studies was to determine whether downstream CASP genes might also be involved and whether inhibition might prevent injury-induced cell death.

SUMMARY OF BACKGROUND DATA

Apoptotic cell death occurs in all endogenous cellular compartments of the spinal cord, peaking at 3 days after injury in neurons, astrocytes, and microglia. The downstream effector caspase-3 cleaves several important cellular sites after being activated by upstream initiator caspases. Along with others, we have previously identified caspase signature cleavage of PARP, alpha-fodrin, and DFF45/ICAD in the injured rat spinal cord. We also showed rapid upregulation of caspase-3 gene expression along with localization of active caspase-3 in neurons and activated microglia after SCI. Others have reported that a more general active-site mimetic peptide ketone, benzylocarbonyl-Val-Ala-Asp-fluromethylketone (zVAD-fmk) was neuroprotective after rat spinal cord injury (SCI).

METHODS

In this study, we administered the caspase-3 subfamily tetrapeptide cell permeable inhibitor Z-Asp(O-Me)-Glu(O-Me)-Val-Asp(O-Me) fluoromethyl ketone (DEVD-fmk) intraperitoneally 1 hour after laminectomy and moderate (25 g cm force) SCI in rats. RESULTS.: We used the open field locomotor rating (LRS) over a 14-day course and found statistically significant improvement in DEVD-fmk-treated rats, LRS, 9.8 +/- 0.93 SEM, compared with vehicle, 6.6 +/- 0.4 (P < 0.05). Histologic analysis of percent spinal cord tissue volume spared was 50% greater for DEVD-fmk versus control (P < 0.5).

CONCLUSION

These results indicate neuroprotection at both the cellular level and with substantial functional recovery, suggesting caspase-3 inhibition may be a viable therapy in the early hours after experimental SCI.

Alpha-melanocyte-stimulating hormone and related tripeptides: biochemistry, antiinflammatory and protective effects in vitro and in vivo, and future perspectives for the treatment of immune-mediated inflammatory diseases

Alpha-MSH is a tridecapeptide derived from proopiomelanocortin. Many studies over the last few years have provided evidence that alpha-MSH has potent protective and antiinflammatory effects. These effects can be elicited via centrally expressed melanocortin receptors that orchestrate descending neurogenic antiinflammatory pathways. alpha-MSH can also exert antiinflammatory and protective effects on cells of the immune system and on peripheral nonimmune cell types expressing melanocortin receptors. At the molecular level, alpha-MSH affects various pathways implicated in regulation of inflammation and protection, i.e., nuclear factor-kappaB activation, expression of adhesion molecules and chemokine receptors, production of proinflammatory cytokines and mediators, IL-10 synthesis, T cell proliferation and activity, inflammatory cell migration, expression of antioxidative enzymes, and apoptosis. The antiinflammatory effects of alpha-MSH have been validated in animal models of experimentally induced fever; irritant and allergic contact dermatitis, vasculitis, and fibrosis; ocular, gastrointestinal, brain, and allergic airway inflammation; and arthritis, but also in models of organ injury. One obstacle limiting the use of alpha-MSH in inflammatory disorders is its pigmentary effect. Due to its preserved antiinflammatory effect but lack of pigmentary action, the C-terminal tripeptide of alpha-MSH, KPV, has been delineated as an alternative for antiinflammatory therapy. KdPT, a derivative of KPV corresponding to amino acids 193-195 of IL-1beta, is also emerging as a tripeptide with antiinflammatory effects. The physiochemical properties and expected low costs of production render both agents suitable for the future treatment of immune-mediated inflammatory skin and bowel disease, fibrosis, allergic and inflammatory lung disease, ocular inflammation, and arthritis.

Melanocortin receptor 4 is induced in nerve‐injured motor and sensory neurons of mouse

We previously identified melanocortin receptor 4 (MC4R) in a search for genes associated with hypoglossal nerve regeneration. As melanocortins promote nerve regeneration after axonal injury, we investigated whether MC4R functions as a key receptor for peripheral nerve regeneration. In situ hybridization revealed that MC4R mRNA is induced in mouse hypoglossal motor neurons after axonal injury, whereas mRNAs for MC1R, MC2R, MC3R, and MC5R are not expressed either before or after nerve injury. This result was confirmed by RT-PCR. The level of MC4R mRNA expression increased significantly from day 3 after axotomy, reached a peak on day 5, and decreased to the control level on day 14. Similar induction of MC4R was observed in axotomized mouse dorsal root ganglia (DRGs). MC4R mRNA expression was induced exclusively among the MCR family in the L4-6 DRG after sciatic nerve injury. We further examined whether alpha-melanocortin stimulating hormone (alpha-MSH) promotes neurite elongation via MC4R. In mouse DRG neuron culture, alpha-MSH significantly promoted neurite outgrowth at a concentration of 10(-8) mol/L. This neurite-elongation effect was entirely inhibited by the addition of a selective MC4R blocker, JKC-363. Therefore, it is concluded that alpha-MSH could stimulate neurite elongation via MC4R in DRG neurons. The present results suggest that induction of MC4R is crucial for motor and sensory neurons to regenerate after axonal injury.

Neuroprotective effects of melanocortins in experimental spinal cord injury. An experimental study in the rat using topical application of compounds with varying affinity to melanocortin receptors

The possibility that local administration of low molecular weight non-peptide compounds with varying affinities at melanocortin receptors in the spinal cord will influence pathophysiological outcome of spinal cord injury (SCI) was examined in a rat model. Five new Melacure compounds ME10092, ME10354, ME10393, ME10431 and ME10501 were used in this investigation. Each compound was dissolved in saline and tested at 3 different doses, i.e. 1 microg, 5 microg and 10 microg total dose in 10 microl applied topically 5 min after SCI. The animals were allowed to survive 5 h and trauma induced edema formation, breakdown of the blood-spinal cord barrier (BSCB) and cell injuries were examined and compared with untreated injured rats. A focal SCI inflicted by an incision into the right dorsal horn of the T10-11 segments resulted in marked edema formation, breakdown of the BSCB to Evans blue albumin and caused profound nerve cell injury in the T9 and the T12 segments. Topical application of ME10501 (a compound with high affinity at melanocortin, MC-4 receptors) in high doses (10 microg) resulted in most marked neuroprotection in the perifocal spinal cord (T9 and T12) segments. On the other hand, only a mild or no effect on spinal cord pathology was observed in the traumatized animals that received ME10092, ME10354, ME10393 and ME10431 at 3 different doses. These observations suggest that non-peptide compounds with varying affinity to melanocortin receptors are able to influence the pathophysiology of SCI. Furthermore, compounds acting at melanocortin, MCR4 receptors are capable to induce neuroprotection in spinal cord following trauma.

Minocycline neuroprotects, reduces microgliosis, and inhibits caspase protease expression early after spinal cord injury

Minocycline, a clinically used tetracycline for over 40 years, crosses the blood-brain barrier and prevents caspase up-regulation. It reduces apoptosis in mouse models of Huntington's disease and familial amyotrophic lateral sclerosis (ALS) and is in clinical trial for sporadic ALS. Because apoptosis also occurs after brain and spinal cord (SCI) injury, its prevention may be useful in improving recovery. We analyzed minocycline's neuroprotective effects over 28 days following contusion SCI and found significant functional recovery compared to tetracycline. Histology, immunocytochemistry, and image analysis indicated statistically significant tissue sparing, reduced apoptosis and microgliosis, and less activated caspase-3 and substrate cleavage. Since our original report in abstract form, others have published both positive and negative effects of minocycline in various rodent models of SCI and with various routes of administration. We have since found decreased tumor necrosis factor-alpha, as well as caspase-3 mRNA expression, as possible mechanisms of action for minocycline's ameliorative action. These results support reports that modulating apoptosis, caspases, and microglia provide promising therapeutic targets for prevention and/or limiting the degree of functional loss after CNS trauma. Minocycline, and more potent chemically synthesized tetracyclines, may find a place in the therapeutic arsenal to promote recovery early after SCI in humans.

Neuroprotective Effects of Recombinant Thrombomodulin in Controlled Contusion Spinal Cord Injury Implicates Thrombin Signaling

Although the central nervous system (CNS) of mammals has had poor prospects for regeneration, recent studies suggest this might improve from blocking "secondary cell loss" or apoptosis. In this regard, intravenous activated protein C (aPC) improved neurologic outcomes in a rat compression spinal cord injury (SCI) model. Protein C activation occurs when the serine protease thrombin binds to the cell surface proteoglycan thrombomodulin (TM) forming a complex that halts coagulation. In culture, rTM blocks thrombin's activation of protease-activated receptors (PARs), that mediate thrombin killing of neurons and glial reactivity. Both PAR1 and prothrombin are rapidly upregulated after contusion SCI in rats, prior to peak apoptosis. We now report neuroprotective effects of intraperitoneal soluble recombinant human rTM on open-field locomotor rating scale (BBB) and spinal cord lesion volume when given 1 h after SCI. BBB scores from four separate experiments showed a 7.6 +/- 1.4 absolute score increase (p < 0.05) at 3 days, that lasted throughout the time course. Histological sections at 14 days were even more dramatic where a twofold reduction in lesion volume was quantified in rTM-treated rats. Thionin staining revealed significant preservation of motor neuronal profiles both at, and two segments below, the lesion epicenter. Activated caspase-3 immunocytochemistry indicated apoptosis was quite prominent in motor neurons in vehicle (saline) controls, but was dramatically reduced by rTM. Microglia, increased and activated after injury, were reduced with rTM treatment. Taken together, these and previous results support a prominent role for coagulation-inflammation signaling cascades in the subacute changes following SCI. They identify a neuroprotective role for rTM by its inhibition of thrombin generation and blockade of PAR activation.

Targeting Melanocortin Receptors as a Novel Strategy to Control Inflammation

Adrenocorticotropic hormone and alpha-, beta-, and gamma-melanocyte-stimulating hormones, collectively called melanocortin peptides, exert multiple effects upon the host. These effects range from modulation of fever and inflammation to control of food intake, autonomic functions, and exocrine secretions. Recognition and cloning of five melanocortin receptors (MCRs) has greatly improved understanding of peptide-target cell interactions. Preclinical investigations indicate that activation of certain MCR subtypes, primarily MC1R and MC3R, could be a novel strategy to control inflammatory disorders. As a consequence of reduced translocation of the nuclear factor kappaB to the nucleus, MCR activation causes a collective reduction of the major molecules involved in the inflammatory process. Therefore, anti-inflammatory influences are broad and are not restricted to a specific mediator. Short half-life and lack of selectivity could be an obstacle to the use of the natural melanocortins. However, design and synthesis of new MCR ligands with selective chemical properties are already in progress. This review examines how marshaling MCR could control inflammation.

Melanocortins applied intravitreally delay retinal dystrophy in Royal College of Surgeons rats

BACKGROUND

Alpha-melanocyte-stimulating hormone (MSH) is a neurotrophic agent. In Royal College of Surgeons (RCS) rats, the effects of an MSH analog (MA) were investigated on: (1) the preservation of photoreceptors in vivo following MA intravitreal injection; (2) whether MA is a mitogenic factor.

METHODS

The study comprised five RCS rat groups, two injected with different doses of MA, one injected with PBS, and two non-injected groups. A single injection of MA or PBS was applied intravitreally to RCS rats on postnatal day 20 (20p). Photoreceptor preservation on 40p was studied using light microscopy. Considering the mitogenic effect of MA, it was studied whether cell proliferation was induced by MA in cultured retinal pigment epithelium (RPE) using the thymidine uptake technique.

RESULTS

In degenerating untreated RCS retinae the number of photoreceptor rows on 40p was 60-70% lower than on 20p. Retinae treated with higher doses of MA revealed on 40p a localized significant photoreceptor rescue in the retinal hemisphere which had been injected. However, only a small area of photoreceptor preservation was noted in the injected hemisphere in retinae treated with the lower MA dose. MA showed no mitogenic effect in endothelial or RPE cell culture in vitro.

CONCLUSIONS

This study is the first to demonstrate that: (1) intravitreally injected MA promotes a dose-related localized rescue of photoreceptors in RCS retinae which may be related to the hormone's neurotrophic activity; (2) MA has no mitogenic or angiogenic properties; (3) MA, as a neuroprotective agent, might be considered for future treatment of retinal dystrophy.

Locomotor Recovery after Spinal Cord Contusion Injury in Rats Is Improved by Spontaneous Exercise

We have recently shown that enriched environment (EE) housing significantly enhances locomotor recovery following spinal cord contusion injury (SCI) in rats. As the type and intensity of locomotor training with EE housing are rather poorly characterized, we decided to compare the effectiveness of EE housing with that of voluntary wheel running, the latter of which is both well characterized and easily quantified. Female Wistar rats were made familiar with three types of housing conditions, social housing (nine together) in an EE (EHC), individual housing in a running wheel cage (RUN, n = 8), and standard housing two together (CON, n = 10). Subsequently, a 12.5 gcm SCI at Th8 was produced and animals were randomly divided over the three housing conditions. Locomotor function was measured regularly, once a week by means of the BBB score, BBB sub score, TLH test, Gridwalk test, and CatWalk test. In the RUN group, daily distance covered was also measured. Locomotor recovery in the EHC and the RUN groups was equal and significantly better than in the CON group. The extent of recovery at 8 weeks post injury in the RUN group did not correlate with distance covered. We conclude that locomotor training needs to exceed a given threshold in order to be effective in enhancing locomotor recovery in this experimental model, but that once this threshold is exceeded no further improvement occurs, and that the specificity of locomotor training plays little role.

Ex vivo adenoviral vector-mediated neurotrophin gene transfer to olfactory ensheathing glia: effects on rubrospinal tract regeneration, lesion size, and functional recovery after implantation in the injured rat spinal cord

The present study uniquely combines olfactory ensheathing glia (OEG) implantation with ex vivo adenoviral (AdV) vector-based neurotrophin gene therapy in an attempt to enhance regeneration after cervical spinal cord injury. Primary OEG were transduced with AdV vectors encoding rat brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or bacterial marker protein beta-galactosidase (LacZ) and subsequently implanted into adult Fischer rats directly after unilateral transection of the dorsolateral funiculus. Implanted animals received a total of 2 x 105 OEG that were subjected to transduction with neurotrophin-encoding AdV vector, AdV-LacZ, or no vector, respectively. At 4 months after injury, lesion volumes were smaller in all OEG implanted rats and significantly reduced in size after implantation of neurotrophin-encoding AdV vector-transduced OEG. All OEG grafts were filled with neurofilament-positive axons, and AdV vector-mediated expression of BDNF by implanted cells significantly enhanced regenerative sprouting of the rubrospinal tract. Behavioral analysis revealed that OEG-implanted rats displayed better locomotion during horizontal rope walking than unimplanted lesioned controls. Recovery of hind limb function was also improved after implantation of OEG that were transduced with a BDNF- or NT-3-encoding AdV vector. Hind limb performance during horizontal rope locomotion did directly correlate with lesion size, suggesting that neuroprotective effects of OEG implants contributed to the level of functional recovery. Thus, our results demonstrate that genetic engineering of OEG not only resulted in a cell that was more effective in promoting axonal outgrowth but could also lead to enhanced recovery after injury, possibly by sparing of spinal tissue.

The potent melanocortin receptor agonist melanotan-II promotes peripheral nerve regeneration and has neuroprotective properties in the rat

The neurotrophic and neuroprotective potential of the alpha-melanocyte-stimulating hormone (alpha-MSH) analog cyclo-[Ac-Nle(4),Asp(5),D-Phe(7),Lys(10)]alpha-MSH-(4-10) amide (melanotan-II), a potent melanocortin receptor agonist, was investigated. The sciatic nerve crush model was used as a paradigm to investigate the neurotrophic properties of melanotan-II. Melanotan-II significantly enhanced the recovery of sensory function following a crush lesion of the sciatic nerve in the rat at a dose of 20 microg kg(-1) per 48 h, s.c., but not at a dose of 2 or 50 microg kg(-1). In addition, we observed that melanotan-II also possesses neuroprotective properties, as it partially protected the nerve from a toxic neuropathy induced by cisplatin. Thus, the present data for the first time demonstrate the effectiveness of the potent alpha-MSH analog melanotan-II in nerve regeneration and neuroprotection.

Discovery and in vivo evaluation of new melanocortin-4 receptor-selective peptides

The melanocortin-4 receptor (MC4R) is involved in several physiological processes, including body weight regulation and grooming behaviour in rats. It has also been suggested that the MC4R mediates the effects of melanocortin ligands on neuropathic pain. Selective compounds are needed to study the exact role of the MC4R in these different processes. We describe here the development and evaluation of new melanocortin compounds that are selective for the MC4R as compared with the other centrally expressed receptors, MC3R and MC5R. First, a library of 18 peptides, in which a melanocortin-based sequence was systematically point-mutated, was screened for binding to and activity on the MC3R, MC4R and MC5R. Compound Ac-Nle-Gly-Lys-D-Phe-Arg-Trp-Gly-NH(2) (JK1) appeared to be the most selective MC4R compound, based on affinity. This compound is 90- and 110-fold selective for the MC4R as compared to the MC3R and MC5R, respectively. Subsequent modification of JK1 yielded compound Ac-Nle-Gly-Lys-D-Nal(2)-Arg-Trp-Gly-NH(2) (JK7)(,) a selective MC4R antagonist with 34-fold MC4R/MC3R and 109-fold MC4R/MC5R selectivity. The compounds were active in vivo as determined in a grooming assay and a model for neuropathic pain in rats. Intravenous (i.v.) injections suggested that they were able to pass the blood-brain barrier.The compounds identified here will be useful in further research on the physiological roles of the MC4R.

A statistical method for analyzing rating scale data: the BBB locomotor score

The Basso, Beattie and Bresnahan (BBB) locomotor rating scale is widely used to test behavioral consequences of spinal cord injury (SCI) to the rat. Sensitivity of this rating scale can differentiate hind limb locomotor skills over a wide range of injury severities. While the 21-point BBB scale is ordinal in nature, the present discussion recommends the use of parametric statistics to evaluate the locomotor results. Specifically, it defines appropriate statistical analysis of these data in order to facilitate interpretation of results between laboratories and to provide a common methodology for the correct interpretation of SCI behavioral data.

Injury-induced class 3 semaphorin expression in the rat spinal cord

In this study we evaluate the expression of all members of the class 3 semaphorins and their receptor components following complete transection and contusion lesions of the adult rat spinal cord. Following both types of lesions the expression of all class 3 semaphorins is induced in fibroblast in the neural scar. The distribution of semaphorin-positive fibroblasts differs markedly in scars formed after transection or contusion lesion. In contusion lesions semaphorin expression is restricted to fibroblasts of the meningeal sheet surrounding the lesion, while after transection semaphorin-positive fibroblast penetrate deep into the center of the lesion. Two major descending spinal cord motor pathways, the cortico- and rubrospinal tract, continue to express receptor components for class 3 semaphorins following injury, rendering them potentially sensitive to scar-derived semaphorins. In line with this we observed that most descending spinal cord fibers were not able to penetrate the semaphorin positive portion of the neural scar formed at the lesion site. These results suggest that the full range of secreted semaphorins contributes to the inhibitory nature of the neural scar and thereby may inhibit successful regeneration in the injured spinal cord. Future studies will focus on the neutralization of class 3 semaphorins, in order to reveal whether this creates a more permissive environment for regeneration of injured spinal cord axons.

Chronic blockade of melanocortin receptors alleviates allodynia in rats with neuropathic pain

We investigated the involvement of the spinal cord melanocortin (MC) system in neuropathic pain. Because we recently demonstrated that MC receptor ligands acutely alter nociception in an animal model of neuropathic pain, in this study we tested whether chronic administration was also effective. We hypothesized that chronic blockade of the spinal MC system might decrease sensory abnormalities associated with this condition. The effects of the MC receptor antagonist SHU9119 (0.5 microg/d) and agonist MTII (0.1 microg/d) were evaluated in rats with a chronic constriction injury of the sciatic nerve. Drugs were continuously infused into the cisterna magna. Antinociceptive effects were measured with tests involving temperature (10 degrees C or 47.5 degrees C) or mechanical (von Frey) stimulation. The administration of MTII increased mechanical allodynia, whereas SHU9119 produced a profound cold and mechanical antiallodynia, altering responses to control levels. The antiallodynic effects of SHU9119 were very similar to those produced by the alpha(2)-adrenergic agonist tizanidine (50 microg/d). The effects of SHU9119 and MTII are most likely mediated through the MC4 receptor, because this is the only MC-receptor subtype present in the spinal cord. We conclude that the chronic administration of MC4-receptor antagonists might provide a promising tool in the treatment of neuropathic pain.

IMPLICATIONS

In this study we demonstrated that continuous intrathecal infusion of the melanocortin-receptor antagonist SHU9119 reduces cold and mechanical allodynia in rats with a chronic constriction injury of the sciatic nerve, a lesion producing neuropathic pain.

Neuropathic pain: a possible role for the melanocortin system?

In humans, damage to the nervous system can lead to a pain state referred to as neuropathic pain. Here, we give a short overview of the clinical picture and classification of neuropathic pain and highlight some of the currently known pathophysiological mechanisms involved, with special emphasis on neuropeptide plasticity. In this context, we discuss a specific group of neuropeptides, the melanocortins. These peptides have been demonstrated to play a role in nociception and to functionally interact with the opiate system. Recently, we demonstrated that spinal melanocortin receptors are upregulated in a rat model of neuropathic pain and that blockade of the melanocortin MC(4) receptor has anti-allodynic effects in this condition, suggesting that the melanocortin system plays a role in neuropathic pain. A natural agonist of melanocortin receptors is alpha-melanocyte-stimulating hormone (alpha-MSH), derived from the precursor molecule pro-opiomelanocortin (POMC). Cleavage of this precursor also yields beta-endorphin, which is co-released with alpha-MSH in nociception-associated areas of the spinal cord. We hypothesise that melanocortin receptor blockade attenuates a tonic influence of alpha-MSH on nociception, thus allowing the analgesic effects of beta-endorphin to develop, resulting in the alleviation of allodynia. In this way, treatment with melanocortin receptor antagonists might enhance opioid efficacy in neuropathic pain, which would be of great benefit in clinical practice.

Effects of enriched housing on functional recovery after spinal cord contusive injury in the adult rat

To date, most research performed in the area of spinal cord injury focuses on treatments designed to either prevent spreading lesion (secondary injury) or to enhance outgrowth of long descending and ascending fiber tracts around or through the lesion. In the last decade, however, several authors have shown that it is possible to enhance locomotor function after spinal cord injury in both animals and patients using specific training paradigms. As a first step towards combining such training paradigms with pharmacotherapy, we evaluated recovery of function in adult rats sustaining a spinal cord contusion injury (MASCIS device, 12.5 mm at T8), either housed in an enriched environment or in standard cages (n = 15 in both groups). The animals in the enriched environment were stimulated to increase their locomotor activity by placing water and food on opposite sides of the cage. As extra stimuli, a running wheel and several other objects were added to the cage. We show that exposure to the enriched environment improves gross and fine locomotor recovery as measured by the Basso, Beattie, and Bresnahan (BBB) locomotor rating scale, the BBB subscale, the Gridwalk, and the Thoracolumbar height test. However, no group differences were found on our electrophysiological parameters nor on the amount of spared white matter. These data justify further studies on enriched housing and more controlled exercise training, with their use as potential additive to pharmacological intervention.

Basic fibroblast growth factor (bFGF) enhances functional recovery following severe spinal cord injury to the rat

We have recently demonstrated that following a moderate contusion spinal cord injury (SCI) to rats, subsequent administration of basic fibroblast growth factor (bFGF) significantly enhances functional recovery and tissue sparing. To further characterize the effects of bFGF, we evaluated its efficacy after a more severe contusion injury at T(10) using the NYU impactor. Immediately after SCI, osmotic minipumps were implanted into the lateral ventricle and lumbar thecal sac to deliver bFGF at 3 or 6 microg per day versus control vehicle for 1 week. Animals were behaviorally tested for 6 weeks before histological assessment of tissue sparing through the injured segment and glial reactivity distal to the lesion. Compared to moderate SCI, all rats had more prolonged and sustained functional deficits 6 weeks after severe contusion. Subjects treated with bFGF had pronounced recovery of hindlimb movements from 2 to 6 weeks compared to controls, manifested in significantly higher behavioral scores. Only marginal tissue sparing was seen rostral to the injury in bFGF-treated spinal cords versus controls. Optical density measurements of astrocyte and microglial cell immunoreactivity in bFGF-treated spinal cords showed that after 6 weeks they approximated controls, although astrocyte immunoreactivity remained higher in controls rostrally. In summary, intrathecal infusion of bFGF following severe SCI significantly restores gross hindlimb motor function that is not correlated with significant tissue sparing. In light of previous evidence that pharmacological intervention with bFGF after moderate SCI enhances tissue preservation, the current findings indicate that yet undefined mechanisms contribute to the enhanced functional recovery following bFGF treatment.

Combined treatment with alphaMSH and methylprednisolone fails to improve functional recovery after spinal injury in the rat

To date, relatively little progress has been made in the treatment of spinal cord injury (SCI)-related neurological impairments. Until now, methylprednisolone (MP) is the only agent with clinically proven beneficial effect on functional outcome after SCI. Although the mechanism of action is not completely clear, experimental data point to protection against membrane peroxidation and edema reduction. The melanocortin melanotropin is known to improve axonal regeneration following sciatic nerve injury, and to stimulate corticospinal outgrowth after partial spinal cord transection. Recently, we showed that intrathecally administered alphaMSH had beneficial effects on functional recovery after experimental SCI. Since both drugs have shown their value in intervention studies after (experimental) spinal cord injury (ESCI), we decided to study the effects of combined treatment. Our results again showed that alphaMSH enhances functional recovery after ESCI in the rat and that MP, although not affecting functional recovery adversely by itself, abolished the effects observed with alphaMSH when combined. Our data, thus, suggest that the mechanism of action of MP interferes with that of alphaMSH.

Pre- and postsynaptic localization of RC3/neurogranin in the adult rat spinal cord: an immunohistochemical study

RC3 (neurogranin; BICKS) is a neuron-specific calmodulin-binding protein kinase C substrate. Thus far, immunohistochemical studies on the localization of RC3 revealed its presence in all neuronal phenotypes, which were restricted to specific areas in the neostriatum, the neocortex, and the hippocampus. RC3 was mostly found in cell bodies and dendrites, with some infrequent presence in axonal profiles, i.e. in the internal capsule. Until now, RC3 expression was reported to be absent in the adult rat spinal cord. RC3 might, however, act as an intermediate of protein kinase C-mediated signaling pathways during synaptic development and plasticity. We hypothesized a role for this 78-amino-acid protein in dendritic plasticity occurring after spinal cord injury. To our surprise, an immunohistological analysis of the uninjured adult rat spinal cord revealed the presence of RC3-positive cell bodies and dendrites in specific regions in the gray matter. Interestingly, axon-containing structures, such as the dorsal and ventral corticospinal tract, were also found to be RC3-positive. This axonal labeling was confirmed by preembedding electron microscopy. In conclusion, we demonstrate here that RC3 is present in the adult rat spinal cord in pre- and postsynaptic structures.