The behavioral deficit observed following noncontingent shock in spinalized rats is prevented by the protein synthesis inhibitor cycloheximide

Updating perspectives on spinal cord function: motor coordination, timing, relational processing, and memory below the brain

Those studying neural systems within the brain have historically assumed that lower-level processes in the spinal cord act in a mechanical manner, to relay afferent signals and execute motor commands. From this view, abstracting temporal and environmental relations is the province of the brain. Here we review work conducted over the last 50 years that challenges this perspective, demonstrating that mechanisms within the spinal cord can organize coordinated behavior (stepping), induce a lasting change in how pain (nociceptive) signals are processed, abstract stimulus-stimulus (Pavlovian) and response-outcome (instrumental) relations, and infer whether stimuli occur in a random or regular manner. The mechanisms that underlie these processes depend upon signal pathways (e.g., NMDA receptor mediated plasticity) analogous to those implicated in brain-dependent learning and memory. New data show that spinal cord injury (SCI) can enable plasticity within the spinal cord by reducing the inhibitory effect of GABA. It is suggested that the signals relayed to the brain may contain information about environmental relations and that spinal cord systems can coordinate action in response to descending signals from the brain. We further suggest that the study of stimulus processing, learning, memory, and cognitive-like processing in the spinal cord can inform our views of brain function, providing an attractive model system. Most importantly, the work has revealed new avenues of treatment for those that have suffered a SCI.

The Spinal Cord, Not to Be Forgotten: the Final Common Path for Development, Training and Recovery of Motor Function

Research on learning, memory, and neural plasticity has long focused on the brain. However, the spinal cord also exhibits these phenomena to a remarkable degree. Following a spinal cord injury, the isolated spinal cord in vivo can adapt to the environment and benefit from training. The amount of plasticity or recovery of function following a spinal injury often depends on the age at which the injury occurs. In this overview, we discuss learning in the spinal cord, including associative conditioning, neural mechanisms, development, and applications to clinical populations. We take an integrated approach to the spinal cord, one that combines basic and experimental information about experience-dependent learning in animal models to clinical treatment of spinal cord injuries in humans. From such an approach, an important goal is to better inform therapeutic treatments for individuals with spinal cord injuries, as well as develop a more accurate and complete account of spinal cord and behavioral functioning.

Metaplasticity within the spinal cord: Evidence brain-derived neurotrophic factor (BDNF), tumor necrosis factor (TNF), and alterations in GABA function (ionic plasticity) modulate pain and the capacity to learn

Evidence is reviewed that behavioral training and neural injury can engage metaplastic processes that regulate adaptive potential. This issue is explored within a model system that examines how training affects the capacity to learn within the lower (lumbosacral) spinal cord. Response-contingent (controllable) stimulation applied caudal to a spinal transection induces a behavioral modification indicative of learning. This behavioral change is not observed in animals that receive stimulation in an uncontrollable manner. Exposure to uncontrollable stimulation also engages a process that disables spinal learning for 24-48 h. Controllable stimulation has the opposite effect; it engages a process that enables learning and prevents/reverses the learning deficit induced by uncontrollable stimulation. These observations suggest that a learning episode can impact the capacity to learn in future situations, providing an example of behavioral metaplasticity. The protective/restorative effect of controllable stimulation has been linked to an up-regulation of brain-derived neurotrophic factor (BDNF). The disruption of learning has been linked to the sensitization of pain (nociceptive) circuits, which is enabled by a reduction in GABA-dependent inhibition. After spinal cord injury (SCI), the co-transporter (KCC2) that regulates the outward flow of Cl^- is down-regulated. This causes the intracellular concentration of Cl^- to increase, reducing (and potentially reversing) the inward flow of Cl^- through the GABA-A receptor. The shift in GABA function (ionic plasticity) increases neural excitability caudal to injury and sets the stage for nociceptive sensitization. The injury-induced shift in KCC2 is related to the loss of descending serotonergic (5HT) fibers that regulate plasticity within the spinal cord dorsal horn through the 5HT-1A receptor. Evidence is presented that these alterations in spinal plasticity impact pain in a brain-dependent task (place conditioning). The findings suggest that ionic plasticity can affect learning potential, shifting a neural circuit from dampened/hard-wired to excitable/plastic.

Electrical Stimulation and Motor Recovery

In recent years, several investigators have successfully regenerated axons in animal spinal cords without locomotor recovery. One explanation is that the animals were not trained to use the regenerated connections. Intensive locomotor training improves walking recovery after spinal cord injury (SCI) in people, and >90% of people with incomplete SCI recover walking with training. Although the optimal timing, duration, intensity, and type of locomotor training are still controversial, many investigators have reported beneficial effects of training on locomotor function. The mechanisms by which training improves recovery are not clear, but an attractive theory is available. In 1949, Donald Hebb proposed a famous rule that has been paraphrased as “neurons that fire together, wire together.” This rule provided a theoretical basis for a widely accepted theory that homosynaptic and heterosynaptic activity facilitate synaptic formation and consolidation. In addition, the lumbar spinal cord has a locomotor center, called the central pattern generator (CPG), which can be activated nonspecifically with electrical stimulation or neurotransmitters to produce walking. The CPG is an obvious target to reconnect after SCI. Stimulating motor cortex, spinal cord, or peripheral nerves can modulate lumbar spinal cord excitability. Motor cortex stimulation causes long-term changes in spinal reflexes and synapses, increases sprouting of the corticospinal tract, and restores skilled forelimb function in rats. Long used to treat chronic pain, motor cortex stimuli modify lumbar spinal network excitability and improve lower extremity motor scores in humans. Similarly, epidural spinal cord stimulation has long been used to treat pain and spasticity. Subthreshold epidural stimulation reduces the threshold for locomotor activity. In 2011, Harkema et al. reported lumbosacral epidural stimulation restores motor control in chronic motor complete patients. Peripheral nerve or functional electrical stimulation (FES) has long been used to activate sacral nerves to treat bladder and pelvic dysfunction and to augment motor function. In theory, FES should facilitate synaptic formation and motor recovery after regenerative therapies. Upcoming clinical trials provide unique opportunities to test the theory.

Metaplasticity and behavior: how training and inflammation affect plastic potential within the spinal cord and recovery after injury

Research has shown that spinal circuits have the capacity to adapt in response to training, nociceptive stimulation and peripheral inflammation. These changes in neural function are mediated by physiological and neurochemical systems analogous to those that support plasticity within the hippocampus (e.g., long-term potentiation and the NMDA receptor). As observed in the hippocampus, engaging spinal circuits can have a lasting impact on plastic potential, enabling or inhibiting the capacity to learn. These effects are related to the concept of metaplasticity. Behavioral paradigms are described that induce metaplastic effects within the spinal cord. Uncontrollable/unpredictable stimulation, and peripheral inflammation, induce a form of maladaptive plasticity that inhibits spinal learning. Conversely, exposure to controllable or predictable stimulation engages a form of adaptive plasticity that counters these maladaptive effects and enables learning. Adaptive plasticity is tied to an up-regulation of brain derived neurotrophic factor (BDNF). Maladaptive plasticity is linked to processes that involve kappa opioids, the metabotropic glutamate (mGlu) receptor, glia, and the cytokine tumor necrosis factor (TNF). Uncontrollable nociceptive stimulation also impairs recovery after a spinal contusion injury and fosters the development of pain (allodynia). These adverse effects are related to an up-regulation of TNF and a down-regulation of BDNF and its receptor (TrkB). In the absence of injury, brain systems quell the sensitization of spinal circuits through descending serotonergic fibers and the serotonin 1A (5HT 1A) receptor. This protective effect is blocked by surgical anesthesia. Disconnected from the brain, intracellular Cl^- concentrations increase (due to a down-regulation of the cotransporter KCC2), which causes GABA to have an excitatory effect. It is suggested that BDNF has a restorative effect because it up-regulates KCC2 and re-establishes GABA-mediated inhibition.

Learning from the spinal cord: how the study of spinal cord plasticity informs our view of learning

The paper reviews research examining whether and how training can induce a lasting change in spinal cord function. A framework for the study of learning, and some essential issues in experimental design, are discussed. A core element involves delayed assessment under common conditions. Research has shown that brain systems can induce a lasting (memory-like) alteration in spinal function. Neurons within the lower (lumbosacral) spinal cord can also adapt when isolated from the brain by means of a thoracic transection. Using traditional learning paradigms, evidence suggests that spinal neurons support habituation and sensitization as well as Pavlovian and instrumental conditioning. At a neurobiological level, spinal systems support phenomena (e.g., long-term potentiation), and involve mechanisms (e.g., NMDA mediated plasticity, protein synthesis) implicated in brain-dependent learning and memory. Spinal learning also induces modulatory effects that alter the capacity for learning. Uncontrollable/unpredictable stimulation disables the capacity for instrumental learning and this effect has been linked to the cytokine tumor necrosis factor (TNF). Predictable/controllable stimulation enables learning and counters the adverse effects of uncontrollable stimulation through a process that depends upon brain-derived neurotrophic factor (BDNF). Finally, uncontrollable, but not controllable, nociceptive stimulation impairs recovery after a contusion injury. A process-oriented approach (neurofunctionalism) is outlined that encourages a broader view of learning phenomena.

Maladaptive spinal plasticity opposes spinal learning and recovery in spinal cord injury

Synaptic plasticity within the spinal cord has great potential to facilitate recovery of function after spinal cord injury (SCI). Spinal plasticity can be induced in an activity-dependent manner even without input from the brain after complete SCI. A mechanistic basis for these effects is provided by research demonstrating that spinal synapses have many of the same plasticity mechanisms that are known to underlie learning and memory in the brain. In addition, the lumbar spinal cord can sustain several forms of learning and memory, including limb-position training. However, not all spinal plasticity promotes recovery of function. Central sensitization of nociceptive (pain) pathways in the spinal cord may emerge in response to various noxious inputs, demonstrating that plasticity within the spinal cord may contribute to maladaptive pain states. In this review we discuss interactions between adaptive and maladaptive forms of activity-dependent plasticity in the spinal cord below the level of SCI. The literature demonstrates that activity-dependent plasticity within the spinal cord must be carefully tuned to promote adaptive spinal training. Prior work from our group has shown that stimulation that is delivered in a limb position-dependent manner or on a fixed interval can induce adaptive plasticity that promotes future spinal cord learning and reduces nociceptive hyper-reactivity. On the other hand, stimulation that is delivered in an unsynchronized fashion, such as randomized electrical stimulation or peripheral skin injuries, can generate maladaptive spinal plasticity that undermines future spinal cord learning, reduces recovery of locomotor function, and promotes nociceptive hyper-reactivity after SCI. We review these basic phenomena, how these findings relate to the broader spinal plasticity literature, discuss the cellular and molecular mechanisms, and finally discuss implications of these and other findings for improved rehabilitative therapies after SCI.

Impact of behavioral control on the processing of nociceptive stimulation

How nociceptive signals are processed within the spinal cord, and whether these signals lead to behavioral signs of neuropathic pain, depends upon their relation to other events and behavior. Our work shows that these relations can have a lasting effect on spinal plasticity, inducing a form of learning that alters the effect of subsequent nociceptive stimuli. The capacity of lower spinal systems to adapt, in the absence of brain input, is examined in spinally transected rats that receive a nociceptive shock to the tibialis anterior muscle of one hind leg. If shock is delivered whenever the leg is extended (controllable stimulation), it induces an increase in flexion duration that minimizes net shock exposure. This learning is not observed in subjects that receive the same amount of shock independent of leg position (uncontrollable stimulation). These two forms of stimulation have a lasting, and divergent, effect on subsequent learning: controllable stimulation enables learning whereas uncontrollable stimulation disables it (learning deficit). Uncontrollable stimulation also enhances mechanical reactivity. We review evidence that training with controllable stimulation engages a brain-derived neurotrophic factor (BDNF)-dependent process that can both prevent and reverse the consequences of uncontrollable shock. We relate these effects to changes in BDNF protein and TrkB signaling. Controllable stimulation is also shown to counter the effects of peripheral inflammation (from intradermal capsaicin). A model is proposed that assumes nociceptive input is gated at an early sensory stage. This gate is sensitive to current environmental relations (between proprioceptive and nociceptive input), allowing stimulation to be classified as controllable or uncontrollable. We further propose that the status of this gate is affected by past experience and that a history of uncontrollable stimulation will promote the development of neuropathic pain.

Glial tumor necrosis factor alpha (TNFα) generates metaplastic inhibition of spinal learning

Injury-induced overexpression of tumor necrosis factor alpha (TNFα) in the spinal cord can induce chronic neuroinflammation and excitotoxicity that ultimately undermines functional recovery. Here we investigate how TNFα might also act to upset spinal function by modulating spinal plasticity. Using a model of instrumental learning in the injured spinal cord, we have previously shown that peripheral intermittent stimulation can produce a plastic change in spinal plasticity (metaplasticity), resulting in the prolonged inhibition of spinal learning. We hypothesized that spinal metaplasticity may be mediated by TNFα. We found that intermittent stimulation increased protein levels in the spinal cord. Using intrathecal pharmacological manipulations, we showed TNFα to be both necessary and sufficient for the long-term inhibition of a spinal instrumental learning task. These effects were found to be dependent on glial production of TNFα and involved downstream alterations in calcium-permeable AMPA receptors. These findings suggest a crucial role for glial TNFα in undermining spinal learning, and demonstrate the therapeutic potential of inhibiting TNFα activity to rescue and restore adaptive spinal plasticity to the injured spinal cord. TNFα modulation represents a novel therapeutic target for improving rehabilitation after spinal cord injury.

Hindlimb immobilization in a wheelchair alters functional recovery following contusive spinal cord injury in the adult rat

BACKGROUND

Locomotor training of rats with thoracic contusion spinal cord injuries can induce task-specific changes in stepping but rarely results in improved overground locomotion, possibly due to a ceiling effect. Thus, the authors hypothesize that incompletely injured rats maximally retrain themselves while moving about in their cages over the first few weeks postinjury.

OBJECTIVE

To test the hypothesis using hindlimb immobilization after mild thoracic contusion spinal cord injury in adult female rats. A passive stretch protocol was included as an independent treatment.

METHODS

Wheelchairs were used to hold the hindlimbs stationary in an extended position leaving the forelimbs free. The wheelchairs were used for 15 to 18 hours per day, 5 days per week for 8 weeks, beginning at 4 days postinjury. A 20-minute passive hindlimb stretch therapy was applied to half of the animals.

RESULTS

Hindlimb locomotor function of the wheelchair group was not different from controls at 1 week postinjury but declined significantly over the next 4 weeks. Passive stretch had no influence on wheelchair animals but limited functional recovery of normally housed animals, preventing them from regaining forelimb-hindlimb coordination. Following 8 weeks of wheelchair immobilization and stretch therapy, only the wheelchair group displayed an improvement in function when returned to normal housing but retained significant deficits in stepping and coordination out to 16 weeks.

CONCLUSION

Hindlimb immobilization and passive stretch may hinder or conceal the normal course of functional recovery of spinal cord injured rats. These observations have implications for the management of acute clinical spinal cord injuries.

Timing in the absence of supraspinal input II: regularly spaced stimulation induces a lasting alteration in spinal function that depends on the NMDA receptor, BDNF release, and protein synthesis

The detection of temporal regularity allows organisms to predict the occurrence of future events. When events occur in an irregular manner, uncertainty is increased, and negative outcomes can ensue (e.g., stress). The present study shows that spinal neurons can discriminate between variable- and fixed-spaced stimulation and that the detection of regularity requires training and engages a form of NMDA receptor-mediated plasticity. The impact of stimulus exposure was assessed using a spinally mediated instrumental response, wherein spinally transected rats are given legshock whenever one hindlimb is extended. Over time, they learn to maintain the leg in a flexed position that minimizes net shock exposure. Prior exposure to 180-900 tailshocks given in a variable (unpredictable) manner inhibited this learning. A learning deficit was not observed when 900 tailshocks were applied using a fixed (predictable) spacing. Fixed-spaced stimulation did not have a divergent effect when fewer (180) shocks were presented, implying that the abstraction of temporal regularity required repeated exposure (training). Moreover, fixed-spaced stimulation both prevented and reversed the learning deficit. The protective effect of fixed-spaced shock lasted 48 h, and was prevented by pretreatment with the NMDA receptor antagonist MK-801. Administration of the protein synthesis inhibitor cycloheximide after training blocked the long-term effect. Inhibiting BDNF function, using TrkB-IgG, also eliminated the beneficial effect of fixed-spaced stimulation. The results suggest that spinal systems can detect regularity and that this type of stimulation promotes adaptive plasticity, which may foster recovery after spinal injury.

Pain and learning in a spinal system: contradictory outcomes from common origins

The long-standing belief that the spinal cord serves merely as a conduit for information traveling to and from the brain is changing. Over the past decade, research has shown that the spinal cord is sensitive to response-outcome contingencies, demonstrating that spinal circuits have the capacity to modify behavior in response to differential environmental cues. If spinally transected rats are administered shock contingent on leg extension (controllable shock), they will maintain a flexion response that minimizes shock exposure. If, however, this contingency is broken, and shock is administered irrespective of limb position (uncontrollable shock), subjects cannot acquire the same flexion response. Interestingly, each of these treatments has a lasting effect on behavior; controllable shock enables future learning, while uncontrollable shock produces a long-lasting learning deficit. Here we suggest that the mechanisms underlying learning and the deficit may have evolved from machinery responsible for the spinal processing of noxious information. Experiments have shown that learning and the deficit require receptors and signaling cascades shown to be involved in central sensitization, including activation of NMDA and neurokinin receptors, as well as CaMKII. Further supporting this link between pain and learning, research has also shown that uncontrollable stimulation results in allodynia. Moreover, systemic inflammation and neonatal hindpaw injury each facilitate pain responding and undermine the ability of the spinal cord to support learning. These results suggest that the plasticity associated with learning and pain must be placed in a balance in order for adaptive outcomes to be observed.

Group I metabotropic glutamate receptors control metaplasticity of spinal cord learning through a protein kinase C-dependent mechanism

Neurons within the spinal cord can support several forms of plasticity, including response-outcome (instrumental) learning. After a complete spinal transection, experimental subjects are capable of learning to hold the hindlimb in a flexed position (response) if shock (outcome) is delivered to the tibialis anterior muscle when the limb is extended. This response-contingent shock produces a robust learning that is mediated by ionotropic glutamate receptors (iGluRs). Exposure to nociceptive stimuli that are independent of limb position (e.g., uncontrollable shock; peripheral inflammation) produces a long-term (>24 h) inhibition of spinal learning. This inhibition of plasticity in spinal learning is itself a form of plasticity that requires iGluR activation and protein synthesis. Plasticity of plasticity (metaplasticity) in the CNS has been linked to group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5) and activation of protein kinase C (PKC). The present study explores the role of mGluRs and PKC in the metaplastic inhibition of spinal cord learning using a combination of behavioral, pharmacological, and biochemical techniques. Activation of group I mGluRs was found to be both necessary and sufficient for metaplastic inhibition of spinal learning. PKC was activated by stimuli that inhibit spinal learning, and inhibiting PKC activity restored the capacity for spinal learning. Finally, a PKC inhibitor blocked the metaplastic inhibition of spinal learning produced by a group I mGluR agonist. The data strongly suggest that group I mGluRs control metaplasticity of spinal learning through a PKC-dependent mechanism, providing a potential therapeutic target for promoting use-dependent plasticity after spinal cord injury.

Timing in the absence of supraspinal input I: variable, but not fixed, spaced stimulation of the sciatic nerve undermines spinally-mediated instrumental learning

Rats with complete spinal transections are capable of acquiring a simple instrumentally trained response. If rats receive shock to one hind limb when the limb is extended (controllable shock), the spinal cord will learn to hold the leg in a flexed position that minimizes shock exposure. If shock is delivered irrespective of leg position, subjects do not exhibit an increase in flexion duration and subsequently fail to learn when tested with controllable shock (learning deficit). Just 6 min of variable intermittent shock produces a learning deficit that lasts 24 h. Evidence suggests that the neural mechanisms underlying the learning deficit may be related to those involved in other instances of spinal plasticity (e.g. windup, long-term potentiation). The present paper begins to explore these relations by demonstrating that direct stimulation of the sciatic nerve also impairs instrumental learning. Six minutes of electrical stimulation (mono- or biphasic direct current [DC]) of the sciatic nerve in spinally transected rats produced a voltage-dependent learning deficit that persisted for 24 h (experiments 1-2) and was dependent on C-fiber activation (experiment 7). Exposure to continuous stimulation did not produce a deficit, but intermittent burst or single pulse (as short as 0.1 ms) stimulation (delivered at a frequency of 0.5 Hz) did, irrespective of the pattern (fixed or variable) of stimulus delivery (experiments 3-6, 8). When the duration of stimulation was extended from 6 to 30 min, a surprising result emerged; shocks applied in a random (variable) fashion impaired subsequent learning whereas shocks given in a regular pattern (fixed spacing) did not (experiments 9-10). The results imply that spinal neurons are sensitive to temporal relations and that stimulation at regular intervals can have a restorative effect.

Peripheral inflammation undermines the plasticity of the isolated spinal cord

Peripheral capsaicin treatment induces molecular changes that sensitize the responses of nociceptive neurons in the spinal dorsal horn. The current studies demonstrate that capsaicin also undermines the adaptive plasticity of the spinal cord, rendering the system incapable of learning a simple instrumental task. In these studies, male rats are transected at the second thoracic vertebra and are tested 24 to 48 hours later. During testing, subjects receive shock to one hindleg when it is extended (controllable stimulation). Rats quickly learn to maintain the leg in a flexed position. Rats that have been injected with capsaicin (1% or 3%) in the hindpaw fail to learn, even when tested on the leg contralateral to the injection. This learning deficit lasts at least 24 hours. Interestingly, training with controllable electrical stimulation prior to capsaicin administration protects the spinal cord against the maladaptive effects. Rats pretrained with controllable stimulation do not display a learning deficit or tactile allodynia. Moreover, controllable stimulation, combined with naltrexone, reverses the capsaicin-induced deficit. These data suggest that peripheral inflammation, accompanying spinal cord injuries, might have an adverse effect on recovery.

Lipopolysaccharide induces a spinal learning deficit that is blocked by IL-1 receptor antagonism

Previous studies have shown that spinal neurons are capable of supporting a form of instrumental conditioning. Subjects receiving a spinal transection will learn to maintain a flexion response after exposure to shock contingent on leg position. In contrast, subjects receiving shock irrespective of leg position will not show increased flexion duration. Activation of the immune system has deleterious effects on learning in intact animals, but the impact of immune system activation on learning spinal animals is not known. We found that a large dose of i.p. LPS (1.0mg/kg) significantly disrupted the acquisition of the instrumental flexion response. The LPS-induced learning deficit was not prevented by preexposure to contingent shock (i.e. immunization) (Experiment 2). Co-administration of the iNOS inhibitor L-NIL (0.1, 1.0 and 10.0 microg/microL) failed to block the deficit (Experiment 3). Co-administration of an IL-1 receptor antagonist (r-metHuIL-1ra [10.0, 30.0 and 100.0 microg/microL) prevented the LPS-induced learning deficit when given in a dose of 100.0 microg/microL(i.t.) only (Experiment 4). Findings indicate a role for spinal IL-1 in the decreased plasticity following LPS administration.

Reconsolidation of a long-term spatial memory is impaired by cycloheximide when reactivated with a contextual latent learning trial in male and female rats

Reconsolidation of long-term memory has become a topic of great interest in recent years, and has the potential to provide important information regarding memory processes and the treatment of memory-related disorders. The present study examined the role of systemic protein synthesis inhibition in reconsolidation of a long-term spatial memory reactivated by a contextual latent learning trial in male and female rats. Using the Morris water maze, we demonstrate that: 1) a contextual latent reactivation treatment enhances memory, 2) systemic protein synthesis inhibition selectively impairs test performance when administered in conjunction with a memory reactivation treatment, and 3) that these effects are more pronounced in female rats. These findings indicate a role for protein synthesis in the reconsolidation of a contextually reactivated long-term spatial memory using the water maze, and a potential differential effect of sex in this apparatus. The role of the strength of the memory trace is discussed and the relevance of these findings to theories of reconsolidation and therapeutic treatment of post-traumatic stress disorder is discussed.

Instrumental learning within the spinal cord: underlying mechanisms and implications for recovery after injury

Using spinally transected rats, research has shown that neurons within the L4-S2 spinal cord are sensitive to response-outcome (instrumental) relations. This learning depends on a form of N-methyl-D-aspartate (NMDA)-mediated plasticity. Instrumental training enables subsequent learning, and this effect has been linked to the expression of brain-derived neurotrophic factor. Rats given uncontrollable stimulation later exhibit impaired instrumental learning, and this deficit lasts up to 48 hr. The induction of the deficit can be blocked by prior training with controllable shock, the concurrent presentation of a tonic stimulus that induces antinociception, or pretreatment with an NMDA or gamma-aminobutyric acid-A antagonist. The expression of the deficit depends on a kappa opioid. Uncontrollable stimulation enhances mechanical reactivity (allodynia), and treatments that induce allodynia (e.g., inflammation) inhibit learning. In intact animals, descending serotonergic neurons exert a protective effect that blocks the adverse consequences of uncontrollable stimulation. Uncontrollable, but not controllable, stimulation impairs the recovery of function after a contusion injury.

Cycloheximide impairs acquisition but not extinction of cocaine self-administration

The aim of the present study was to assess the role of de novo protein synthesis in the acquisition and extinction of cocaine self-administration. In a first experiment, rats were trained to respond for intravenous cocaine infusions (0.3 mg/kg) and a protein synthesis inhibitor, cycloheximide (CHX; 3 mg/kg, s.c.) was injected immediately after each self-administration session. In a second experiment, rats were allowed to acquire cocaine self-administration and CHX was injected immediately after subsequent extinction sessions. CHX impaired the acquisition, but not extinction, of cocaine self-administration. In control experiments, CHX (3 mg/kg) blocked c-Fos protein expression after foot-shock stress and impaired the acquisition of conditioned freezing but did not inhibit spontaneous locomotor activity and sucrose drinking. Our results suggest that: i) the acquisition and extinction of cocaine-reinforced behaviour have a different molecular basis; and ii) only the former process requires de novo protein synthesis.

Nociceptive plasticity inhibits adaptive learning in the spinal cord

Spinal plasticity is known to play a role in central neurogenic pain. Over the last 100 years researchers have found that the spinal cord is also capable of supporting other forms of plasticity including several forms of learning. To study instrumental (response-outcome) learning in the spinal cord, we use a preparation in which spinally transected rats are given shock to the hind leg when the leg is extended. The spinal cord rapidly learns to hold the leg in a flexed position when given this controllable shock. However, if shock is independent of leg position (uncontrollable shock), subjects fail to learn. Uncontrollable shock also impairs future learning. As little as 6 min of uncontrollable shock to either the leg or the tail generates a learning deficit that lasts up to 48 h. Recent data suggest links between the learning deficit and the sensitization of pain circuits associated with inflammation or injury (central sensitization). Here, we explored whether central sensitization and the spinal learning deficit share pharmacological and behavioral features. Central sensitization enhances reactivity to mechanical stimulation (allodynia) and depends on the N-methyl-d-aspartate receptor (NMDAR). The uncontrollable shock stimulus that generates a learning deficit produced a tactile allodynia (Exp. 1) and administration of the NMDAR antagonist MK-801 blocked induction of the learning deficit (Exp. 2). Finally, a treatment known to induce central sensitization, intradermal carrageenan, produced a spinal learning deficit (Exp. 3). The findings suggest that the induction of central sensitization inhibits selective response modifications.