Acute reactive and regenerative changes in mature cortical axons following injury

From symmetry to chaos and back: Understanding and imaging the mechanisms of neural repair after stroke

Neuroscience has made strides in recent years allowing us insight into the workings of the brain - from the molecular to the regional anatomy. These insights have given researchers an advantage in seeking novel therapies for neurological disorders, specifically stroke. Yet despite these discoveries, many aspects of stroke remain poorly understood - specifically post-stroke recovery. This review article seeks to outline cutting-edge neuroimaging technologies, and the current level of understanding of neurological repair after stroke, with the main focus on the mechanism of axonal sprouting. Neuronal connectivity has varying levels of complexity that allow neuronal networks to process information and give rise to our day-to-day functioning. As stroke causes the death of groups of regional neurons, it is likely that the reestablishment of function seen in some stroke patients is related to shifting patterns of functional connectivity. This paper touches on the timeline and limits on the amount of functional recovery, as well as the differences in organization of neuronal networks in a healthy versus post stroke brain. Finally, we discuss how the previously mentioned methods of imaging are critical in understanding the mechanisms of functional recovery. The mechanism of axonal sprouting and its theorized different types are explained, along with potential ways of imaging them in rodents. The hope is that, with a better understanding of the mechanisms underlying brain recovery, researchers can apply this knowledge to better help stroke patients and be of use in clinical settings.

Traumatic brain injury augurs ill for prolonged deficits in the brain's structural and functional integrity following controlled cortical impact injury

Previous neuroimaging studies in rodents investigated effects of the controlled cortical impact (CCI) model of traumatic brain injury (TBI) within one-month post-TBI. This study extends this temporal window to monitor the structural–functional alterations from two hours to six months post-injury. Thirty-seven male Sprague–Dawley rats were randomly assigned to TBI and sham groups, which were scanned at two hours, 1, 3, 7, 14, 30, 60 days, and six months following CCI or sham surgery. Structural MRI, diffusion tensor imaging, and resting-state functional magnetic resonance imaging were acquired to assess the dynamic structural, microstructural, and functional connectivity alterations post-TBI. There was a progressive increase in lesion size associated with brain volume loss post-TBI. Furthermore, we observed reduced fractional anisotropy within 24 h and persisted to six months post-TBI, associated with acutely reduced axial diffusivity, and chronic increases in radial diffusivity post-TBI. Moreover, a time-dependent pattern of altered functional connectivity evolved over the six months’ follow-up post-TBI. This study extends the current understanding of the CCI model by confirming the long-term persistence of the altered microstructure and functional connectivity, which may hold a strong translational potential for understanding the long-term sequelae of TBI in humans.

Chronic, low-level oral exposure to marine toxin, domoic acid, alters whole brain morphometry in nonhuman primates

Domoic acid (DA) is an excitatory neurotoxin produced by marine algae and responsible for Amnesiac Shellfish Poisoning in humans. Current regulatory limits (˜0.075-0.1 mg/kg/day) protect against acute toxicity, but recent studies suggest that the chronic consumption of DA below the regulatory limit may produce subtle neurotoxicity in adults, including decrements in memory. As DA-algal blooms are increasing in both severity and frequency, we sought to better understand the effects of chronic DA exposure on reproductive and neurobehavioral endpoints in a preclinical nonhuman primate model. To this end, we initiated a long-term study using adult, female Macaca fascicularis monkeys exposed to daily, oral doses of 0.075 or 0.15 mg/kg of DA for a range of 321-381, and 346-554 days, respectively. This time period included a pre-pregnancy, pregnancy, and postpartum period. Throughout these times, trained data collectors observed intentional tremors in some exposed animals during biweekly clinical examinations. The present study explores the basis of this neurobehavioral finding with in vivo imaging techniques, including diffusion tensor magnetic resonance imaging and spectroscopy. Diffusion tensor analyses revealed that, while DA exposed macaques did not significantly differ from controls, increases in DA-related tremors were negatively correlated with fractional anisotropy, a measure of structural integrity, in the internal capsule, fornix, pons, and corpus callosum. Brain concentrations of lactate, a neurochemical closely linked with astrocytes, were also weakly, but positively associated with tremors. These findings are the first documented results suggesting that chronic oral exposure to DA at concentrations near the current human regulatory limit are related to structural and chemical changes in the adult primate brain.

Role of bone marrow derived mesenchymal stromal cells and Schwann-like cells transplantation on spinal cord injury in adult male albino rats

BACKGROUND

Spinal cord injury is a considerable health impact accompanied with physical, psychological and economic burden. Bone marrow derived mesenchymal stromal cells (BM-MSCs) transplantation was found to produce neuronal regenerative effects. Schwann-like cells differentiated from BM-MSCs have myelin-forming ability.

AIM OF THE WORK

To compare the ability of BM-MSCs versus Schwann like cells to promote recovery of spinal cord injury.

MATERIAL AND METHODS

Adult male albino rats were used throughout the study. BM-MSCs were harvested from femora of rats. Sciatic nerves were extracted and used in the preparation of the induction culture medium for differentiation of BM-MSCs into Schwann-like cells. Rats were divided into control, spinal cord injured (SCI), spinal cord injured plus BM-MSCs transplantation (BM-MSC) and spinal cord injured plus Schwann-like cells transplantation (Sn) groups. BBB scale assessment was performed before and after SCI in all rats. Rats were euthanized at the end of the 7^th week and spinal cords were dissected and processed for light and transmission electron microscopic examinations.

RESULTS

Spinal cord sections of SCI group revealed cavitation, necrosis and demyelination. BM-MSC and Sn groups showed both functional and structural improvement compared to SCI group with better BBB score and histopathological features in the BM-MSC group and more expression of S100 in the Sn group.

CONCLUSION

Transplantation of BM-MSCs and Schwann-like cells improved the structural and functional alterations of spinal cord injury with better improvement in BM-MSC group.

The Microtubule-Modulating Drug Epothilone D Alters Dendritic Spine Morphology in a Mouse Model of Mild Traumatic Brain Injury

Microtubule dynamics underpin a plethora of roles involved in the intricate development, structure, function, and maintenance of the central nervous system. Within the injured brain, microtubules are vulnerable to misalignment and dissolution in neurons and have been implicated in injury-induced glial responses and adaptive neuroplasticity in the aftermath of injury. Unfortunately, there is a current lack of therapeutic options for treating traumatic brain injury (TBI). Thus, using a clinically relevant model of mild TBI, lateral fluid percussion injury (FPI) in adult male Thy1-YFPH mice, we investigated the potential therapeutic effects of the brain-penetrant microtubule-stabilizing agent, epothilone D. At 7 days following a single mild lateral FPI the ipsilateral hemisphere was characterized by mild astroglial activation and a stereotypical and widespread pattern of axonal damage in the internal and external capsule white matter tracts. These alterations occurred in the absence of other overt signs of trauma: there were no alterations in cortical thickness or in the number of cortical projection neurons, axons or dendrites expressing YFP. Interestingly, a single low dose of epothilone D administered immediately following FPI (and sham-operation) caused significant alterations in the dendritic spines of layer 5 cortical projection neurons, while the astroglial response and axonal pathology were unaffected. Specifically, spine length was significantly decreased, whereas the density of mushroom spines was significantly increased following epothilone D treatment. Together, these findings have implications for the use of microtubule stabilizing agents in manipulating injury-induced synaptic plasticity and indicate that further study into the viability of microtubule stabilization as a therapeutic strategy in combating TBI is warranted.

Molecular, cellular and functional events in axonal sprouting after stroke

Stroke is the leading cause of adult disability. Yet there is a limited degree of recovery in this disease. One of the mechanisms of recovery is the formation of new connections in the brain and spinal cord after stroke: post-stroke axonal sprouting. Studies indicate that post-stroke axonal sprouting occurs in mice, rats, primates and humans. Inducing post-stroke axonal sprouting in specific connections enhances recovery; blocking axonal sprouting impairs recovery. Behavioral activity patterns after stroke modify the axonal sprouting response. A unique regenerative molecular program mediates this aspect of tissue repair in the CNS. The types of connections that are formed after stroke indicate three patterns of axonal sprouting after stroke: reactive, reparative and unbounded axonal sprouting. These differ in mechanism, location, relationship to behavioral recovery and, importantly, in their prospect for therapeutic manipulation to enhance tissue repair.

Increased Network Excitability Due to Altered Synaptic Inputs to Neocortical Layer V Intact and Axotomized Pyramidal Neurons after Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) can produce long lasting cognitive dysfunction. There is typically no cell death and only diffuse structural injury after mTBI. Thus, functional changes in intact neurons may contribute to symptoms. We have previously shown altered intrinsic properties of axotomized and intact neurons within 2 d after a central fluid percussion injury in mice expressing yellow fluorescent protein (YFP) that allow identification of axonal state prior to recording. Here, whole-cell patch clamp recordings were used to examine synaptic properties of YFP(+) layer V pyramidal neurons. An increased frequency of spontaneous and miniature excitatory postsynaptic currents (EPSCs) was recorded from axotomized neurons at 1 d and intact neurons at 2 d after injury, likely reflecting an increased number of afferents. This also was reflected in the increased amplitude of the EPSC evoked by local extracellular stimulation for all neurons from injured cortex and increased likelihood of producing an action potential for intact cells. Field potentials recorded in superficial layers after online deep layer stimulation contained a single negative peak in controls but multiple negative peaks in injured tissue. The amplitude of this evoked negativity was significantly larger than controls over a series of stimulus intensities at both the 1 d and 2 d survival times. Interictal-like spikes never occurred in the field potential recordings from controls but were observed in 20-80% of stimulus presentations in injured cortex. Together, these results suggest an overall increase in network excitability and the production of particularly powerful (intact) neurons that have both increased intrinsic and synaptic excitability.

Stem cell therapy in spinal trauma: Does it have scientific validity?

Stem cell-based interventions aim to use special regenerative cells (stem cells) to facilitate neuronal function beyond the site of the injury. Many studies involving animal models of spinal cord injury (SCI) suggest that certain stem cell-based therapies may restore function after SCI. Currently, in case of spinal cord injuries, new discoveries with clinical implications have been continuously made in basic stem cell research, and stem cell-based approaches are advancing rapidly toward application in patients. There is a huge base of preclinical evidence in vitro and in animal models which suggests the safety and clinical efficacy of cellular therapies after SCI. Despite this, data from clinical studies is not very encouraging and at times confounding. Here, we have attempted to cover preclinical and clinical evidence base dealing with safety, feasibility and efficacy of cell based interventions after SCI. The limitations of preclinical data and the reasons underlying its failure to translate in a clinical setting are also discussed. Based on the evidence base, it is suggested that a multifactorial approach is required to address this situation. Need for standardized, stringently designed multi-centric clinical trials for obtaining validated proof of evidence is also highlighted.

Long-Term Consequences of Traumatic Brain Injury: Current Status of Potential Mechanisms of Injury and Neurological Outcomes

Traumatic brain injury (TBI) is a significant clinical problem with few therapeutic interventions successfully translated to the clinic. Increased importance on the progressive, long-term consequences of TBI have been emphasized, both in the experimental and clinical literature. Thus, there is a need for a better understanding of the chronic consequences of TBI, with the ultimate goal of developing novel therapeutic interventions to treat the devastating consequences of brain injury. In models of mild, moderate, and severe TBI, histopathological and behavioral studies have emphasized the progressive nature of the initial traumatic insult and the involvement of multiple pathophysiological mechanisms, including sustained injury cascades leading to prolonged motor and cognitive deficits. Recently, the increased incidence in age-dependent neurodegenerative diseases in this patient population has also been emphasized. Pathomechanisms felt to be active in the acute and long-term consequences of TBI include excitotoxicity, apoptosis, inflammatory events, seizures, demyelination, white matter pathology, as well as decreased neurogenesis. The current article will review many of these pathophysiological mechanisms that may be important targets for limiting the chronic consequences of TBI.

Cell transplantation for spinal cord injury: a systematic review

Cell transplantation, as a therapeutic intervention for spinal cord injury (SCI), has been extensively studied by researchers in recent years. A number of different kinds of stem cells, neural progenitors, and glial cells have been tested in basic research, and most have been excluded from clinical studies because of a variety of reasons, including safety and efficacy. The signaling pathways, protein interactions, cellular behavior, and the differentiated fates of experimental cells have been studied in vitro in detail. Furthermore, the survival, proliferation, differentiation, and effects on promoting functional recovery of transplanted cells have also been examined in different animal SCI models. However, despite significant progress, a "bench to bedside" gap still exists. In this paper, we comprehensively cover publications in the field from the last years. The most commonly utilized cell lineages were covered in this paper and specific areas covered include survival of grafted cells, axonal regeneration and remyelination, sensory and motor functional recovery, and electrophysiological improvements. Finally we also review the literature on the in vivo tracking techniques for transplanted cells.

RNA interference against repulsive guidance molecule A improves axon sprout and neural function recovery of rats after MCAO/reperfusion

Repulsive guidance molecule a (RGMa) is a neurite growth inhibitor that is of great interest in the study of CNS neuronal regeneration. We adopted RNA interference (RNAi) as a means of suppressing the expression of RGMa and observed the improvement in axonal regeneration and neurological function of rats after cerebral ischemic injury. Recombinant adenovirus rAd5-shRNA-RGMa was constructed and prepared for animal experimentation. RGMa and neurofilament protein 200 (NF200) in the ischemic cortex and ipsilateral hippocampus were detected by reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry. The ischemic regions were examined by triphenyltetrazolium chloride (TTC) staining and the newborn neurite branches by Biotinylated Dextran Amine (BDA) neuronal tracing. Behavior tests were adopted to evaluate neurologic function recovery. Results showed RGMa was down-regulated and axonal growth was improved in the RNAi treated group (P<0.01). The number of axonal sprouts of corticospinal tract from the uninjured side to the ischemic side in the RNAi treated group was increased (P<0.01). Behavior test scores in the RNAi treated group were significantly better than other groups after 6 weeks (P<0.01). RGMa in rat brains after middle cerebral artery occlusion (MCAO) can be down-regulated by RNAi successfully, which may lead to improved axonal growth and neural anatomy plasticity, as well as neuron functional recovery.

The contribution of gliosis to diffusion tensor anisotropy and tractography following traumatic brain injury: validation in the rat using Fourier analysis of stained tissue sections

Diffusion tensor imaging is highly sensitive to the microstructural integrity of the brain and has uncovered significant abnormalities following traumatic brain injury not appreciated through other methods. It is hoped that this increased sensitivity will aid in the detection and prognostication in patients with traumatic injury. However, the pathological substrates of such changes are poorly understood. Specifically, decreases in fractional anisotropy derived from diffusion tensor imaging are consistent with axonal injury, myelin injury or both in white matter fibres. In contrast, in both humans and animal models, increases in fractional anisotropy have been suggested to reflect axonal regeneration and plasticity, but the direct histological evidence for such changes remains tenuous. We developed a method to quantify the anisotropy of stained histological sections using Fourier analysis, and applied the method to a rat controlled cortical impact model to identify the specific pathological features that give rise to the diffusion tensor imaging changes in subacute to chronic traumatic brain injury. A multiple linear regression was performed to relate the histological measurements to the measured diffusion tensor changes. The results show that anisotropy was significantly increased (P < 0.001) in the perilesioned cortex following injury. Cortical anisotropy was independently associated (standardized β = 0.62, P = 0.04) with the coherent organization of reactive astrocytes (i.e. gliosis) and was not attributed to axons. By comparison, a decrease in white matter anisotropy (P < 0.001) was significantly related to demyelination (β = 0.75, P = 0.0015) and to a lesser extent, axonal degeneration (β = -0.48, P = 0.043). Gliosis within the lesioned cortex also influenced diffusion tensor tractography, highlighting the fact that spurious tracts in the injured brain may not necessarily reflect continuous axons and may instead depict glial scarring. The current study demonstrates a novel method to relate pathology to diffusion tensor imaging findings, elucidates the underlying mechanisms of anisotropy changes following traumatic brain injury and significantly impacts the clinical interpretation of diffusion tensor imaging findings in the injured brain.

Focal damage to the adult rat neocortex induces wound healing accompanied by axonal sprouting and dendritic structural plasticity

Accumulating evidence indicates that damage to the adult mammalian brain evokes an array of adaptive cellular responses and may retain a capacity for structural plasticity. We have investigated the cellular and architectural alterations following focal experimental brain injury, as well as the specific capacity for structural remodeling of neuronal processes in a subset of cortical interneurons. Focal acute injury was induced by transient insertion of a needle into the neocortex of anesthetized adult male Hooded-Wistar rats and thy1 green fluorescent protein (GFP) mice. Immunohistochemical, electron microscopy, and bromodeoxyuridine cell proliferation studies demonstrated an active and evolving response of the brain to injury, indicating astrocytic but not neuronal proliferation. Immunolabeling for the neuron-specific markers phosphorylated neurofilaments, α-internexin and calretinin at 7 days post injury (DPI) indicated phosphorylated neurofilaments and α-internexin but not calretinin immunopositive axonal sprouts within the injury site. However, quantitative studies indicated a significant realignment of horizontally projecting dendrites of calretinin-labeled interneurons at 14 DPI. This remodeling was specific to calretinin immunopositive interneurons and did not occur in a subpopulation of pyramidal neurons expressing GFP in the injured mouse cortex. These data show that subclasses of cortical interneurons are capable of adaptive structural remodeling.

Initial calcium release from intracellular stores followed by calcium dysregulation is linked to secondary axotomy following transient axonal stretch injury

Acute axonal shear and stretch in the brain induces an evolving form of axonopathy and is a major cause of ongoing motor, cognitive and emotional dysfunction. We have utilized an in vitro model of mild axon bundle stretch injury, in cultured primary cortical neurons, to determine potential early critical cellular alterations leading to secondary axonal degeneration. We determined that transient axonal stretch injury induced an initial acute increase in intracellular calcium, principally derived from intracellular stores, which was followed by a delayed increase in calcium over 48 h post-injury (PI). This progressive and persistent increase in intracellular calcium was also associated with increased frequency of spontaneous calcium fluxes as well as cytoskeletal abnormalities. Additionally, at 48 h post-injury, stretch-injured axon bundles demonstrated filopodia-like sprout formation that preceded secondary axotomy and degeneration. Pharmacological inhibition of the calcium-activated phosphatase, calcineurin, resulted in reduced secondary axotomy (p < 0.05) and increased filopodial sprout length. In summary, these results demonstrate that stretch injury of axons induced an initial substantial release of calcium from intracellular stores with elevated intracellular calcium persisting over 2 days. These long-lasting calcium alterations may provide new insight into the earliest neuronal abnormalities that follow traumatic brain injury as well as the key cellular changes that lead to the development of diffuse axonal injury and secondary degeneration.

Axonal shearing in mature cortical neurons induces attempted regeneration and the reestablishment of neurite polarity

While functional recovery after injury is limited, it has become evident that the mature central nervous system does retain some ability to regenerate. This study investigated the intrinsic capacity of relatively mature cortical neurons (21 days in vitro) to respond to axonal loss. Neurons, growing as clusters on poly-L-lysine, were completely sheared of axons through chemical and mechanical disruption and transferred to either an intact astrocyte monolayer or a substrate of poly-L-lysine. Injured neurons exhibited a regenerative sprouting response that was independent of neuronal cell division or neural progenitors, as demonstrated by negative bromodeoxyuridine (BrdU) and the neuronal precursor intermediate filament nestin, labeling. At 24 h after injury, neurons had extended appropriately polarized neurites, demonstrated by compartmentalized microtubule-associated proteins MAP2 and tau immunolabeling. Newly sprouting axons were tipped by growth cones; however, growth cones on the tips of sprouting axons (mean area, 26.32 +/- 2.20 microm) were significantly (p<0.05) smaller than their developmental counterparts (mean area, 48.64 +/- 5.9 microm), independent of substrate. Furthermore, live imaging indicated that regenerating neurons exhibited distinct axonal dynamics, with a significant (p<0.05) reduction (70%) in pausing, considered vital for interstitial branching and pathfinding, relative to developmental growth cones. This study indicates that mature cultured cortical pyramidal and interneurons have the intrinsic potential to survive, extend processes, and reestablish neurite polarity following significant physical damage. These results may aid in defining the cellular basis of neuronal structural plasticity and defining the role of astrocyte reactivity in the response to trauma.

5-HT4 receptor-mediated neuroprotection and neurogenesis in the enteric nervous system of adult mice

Although the mature enteric nervous system (ENS) has been shown to retain stem cells, enteric neurogenesis has not previously been demonstrated in adults. The relative number of enteric neurons in wild-type (WT) mice and those lacking 5-HT(4) receptors [knock-out (KO)] was found to be similar at birth; however, the abundance of ENS neurons increased during the first 4 months after birth in WT but not KO littermates. Enteric neurons subsequently decreased in both WT and KO but at 12 months were significantly more numerous in WT. We tested the hypothesis that stimulation of the 5-HT(4) receptor promotes enteric neuron survival and/or neurogenesis. In vitro, 5-HT(4) agonists increased enteric neuronal development/survival, decreased apoptosis, and activated CREB (cAMP response element-binding protein). In vivo, in WT but not KO mice, 5-HT(4) agonists induced bromodeoxyuridine incorporation into cells that expressed markers of neurons (HuC/D, doublecortin), neural precursors (Sox10, nestin, Phox2b), or stem cells (Musashi-1). This is the first demonstration of adult enteric neurogenesis; our results suggest that 5-HT(4) receptors are required postnatally for ENS growth and maintenance.

Stimulating neuroregeneration as a therapeutic drug approach for traumatic brain injury

Traumatic brain injury, a silent epidemic of modern societies, is a largely neglected area in drug development and no drug is currently available for the treatment of patients suffering from brain trauma. Despite this grim situation, much progress has been made over the last two decades in closely related medical indications, such as spinal cord injury, giving rise to a more optimistic approach to drug development in brain trauma. Fundamental insights have been gained with animal models of central nervous system (CNS) trauma and spinal cord injury. Neuroregenerative drug candidates have been identified and two of these have progressed to clinical development for spinal cord injury patients. If successful, these drug candidates may be used to treat brain trauma patients. Significant progress has also been made in understanding the fundamental molecular mechanism underlying irreversible axonal growth arrest in the injured CNS of higher mammals. From these studies, we have learned that the axonal retraction bulb, previously regarded as a marker for failure of regenerative growth, is not static but dynamic and, therefore, amenable to pharmacotherapeutic approaches. With the development of modified magnetic resonance imaging methods, fibre tracts can be visualised in the living human brain and such imaging methods will soon be used to evaluate the neuroregenerative potential of drug candidates. These significant advances are expected to fundamentally change the often hopeless situation of brain trauma patients and will be the first step towards overcoming the silent epidemic of brain injury.

Cellular dynamics underlying regeneration of damaged axons differs from initial axon development

While long-distance regeneration may be limited in mammalian species, it is becoming apparent that damaged mature neurons retain some capacity for attempted regeneration and that the adult CNS is not entirely inhibitory to axon growth. Our investigations show that there are critical intrinsic features of postinjury axonal regeneration that differ from initial axon development, and that these distinct differences may account for the limited and inappropriate regenerative response that currently characterizes the mature CNS. We compared the neurochemical and dynamic characteristics of developing axons to relatively mature regenerating axons, utilizing an in vitro model of axonal transection to long-term cultured rat cortical neurons. Immunolabelling studies revealed that regenerating and developing axons have a similar localization of cytoskeletal proteins, but the tips of regenerating axons, although morphologically similar, were smaller with reduced fillopodial extension, relative to developmental growth cones. Live imaging demonstrated that regenerating axons exhibited significantly less outgrowth than developmental neurites. Furthermore, growth cones of regenerating axons had a significant reduction in pausing, considered vital for interstitial branching and pathfinding, than did developmental growth cones. In addition, unlike developing axons, the regenerating axons were unresponsive to the growth factors BDNF and GDNF. Thus, although similar in their cytoskeletal composition, the growth cones of regenerative sprouts differed from their developmental counterparts in their size, their dynamic behaviour and their ability to respond to critical growth factors. These intrinsic differences may account for the inability of post-traumatic locally sprouting axons to make accurate pathway decisions and successfully respond to trauma.

Lasting changes in a network of interneurons after synapse regeneration and delayed recovery of sensitization

Regeneration of neuronal circuits cannot be successful without restoration of full function, including recovery of behavioral plasticity, which we have found is delayed after regeneration of specific synapses. Experiments were designed to measure neuronal changes that may underlie recovery of function. Sensitization of the leech withdrawal reflex is a non-associative form of learning that depends on the S-interneuron. Cutting an S-cell axon in Faivre's nerve disrupted the capacity for sensitization. The S-cell axon regenerated its electrical synapse with its homologous cell after 3-4 weeks, but the capacity for sensitization was delayed for an additional 2-3 weeks. In the present experiments another form of non-associative conditioning, dishabituation, was also eliminated by S-cell axotomy; it returned following regeneration. Semi-intact preparations were made for behavioral studies, and chains of ganglia with some skin were used for intracellular recording and skin stimulation. In both preparations there was a similar time-course, during 6 weeks, of a lesion-induced decrease and delayed restoration of both S-cell action potential threshold to depolarizing pulses and S-cell firing in response to test stimuli. However, the ability of sensitizing stimuli to decrease S-cell threshold and enhance S-cell activity in response to test stimuli did not fully return after regeneration, indicating that there were lasting changes in the circuit extending beyond the period necessary for full recovery of behavior. Intracellular recordings from the axotomized S-cell revealed a shift in the usual balance of excitatory and inhibitory input, with inhibition enhanced. These results indicate that loss of behavioral plasticity of reflexive shortening following axotomy in the S-cell chain may be related to reduced S-cell activity, and that additional processes underlie full recovery of sensitization of the whole body shortening reflex.