Exercise therapy and recovery after SCI: evidence that shows early intervention improves recovery of function.

STUDY DESIGN: This was designed as an experimental study. OBJECTIVES: Locomotor training is one of the most effective strategies currently available for facilitating recovery of function after an incomplete spinal cord injury (SCI). However, there is still controversy regarding the timing of treatment initiation for maximal recovery benefits. To address this issue, the present study compares the effects of exercise initiated in the acute and secondary phase of SCI. SETTING: Texas A&M University, College Station, TX, USA. METHODS: Rats received a moderate spinal contusion injury and began an exercise program 1 (D1-EX) or 8 days (D8-EX) later. They were individually placed into transparent exercise balls for 60 min per day, for 14 consecutive days. Control rats were placed in exercise balls that were rendered immobile. Motor and sensory recovery was assessed for 28 days after injury. RESULTS: The D1-EX rats recovered significantly more locomotor function (BBB scale) than controls and D8-EX rats. Moreover, analyses revealed that rats in the D8-EX group had significantly lower tactile reactivity thresholds compared with control and D1-EX rats, and symptoms of allodynia were not reversed by exercise. Rats in the D8-EX group also had significantly larger areas of damage across spinal sections caudal to the injury center compared with the D1-EX group. CONCLUSION: These results indicate that implementing an exercise regimen in the acute phase of SCI maximizes the potential for recovery of function.

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.

BDNF and learning: Evidence that instrumental training promotes learning within the spinal cord by up-regulating BDNF expression.

We have previously shown that the spinal cord is capable of learning a sensorimotor task in the absence of supraspinal input. Given the action of brain-derived neurotrophic factor (BDNF) on hippocampal learning, the current studies examined the role of BDNF in spinal learning. BDNF is a strong synaptic facilitator and, in association with other molecular signals (e.g. cAMP-response element binding protein (CREB), calcium/calmodulin activated protein kinase II (CaMKII) and synapsin I), important for learning. Spinally transected rats given shock to one hind leg when the leg extended beyond a selected threshold exhibited a progressive increase in flexion duration that minimized shock exposure, a simple form of instrumental learning. Instrumental learning resulted in elevated mRNA levels of BDNF, CaMKII, CREB, and synapsin I in the lumbar spinal cord region. The increases in BDNF, CREB, and CaMKII were proportional to the learning performance. Prior work has shown that instrumental training facilitates learning when subjects are tested on the contralateral leg with a higher response criterion. Pretreatment with the BDNF inhibitor TrkB-IgG blocked this facilitatory effect, as did the CaMKII inhibitor AIP. Intrathecal administration of BDNF facilitated learning when subjects were tested with a high response criterion. The findings indicate that instrumental training enables learning and elevates BDNF mRNA levels within the lumbar spinal cord. BDNF is both necessary, and sufficient, to produce the enabling effect.

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.

The distinction between integral and separable dimensions: evidence for the integrality of pitch and loudness.

Six experiments are reported that investigated the reality and generality of dimensional integrality (Garner, 1974a) by evaluating whether the auditory dimensions of pitch and loudness are psychologically privileged and whether they combine in an integral fashion. In Experiment 1 we psychophysically scaled the dimensions to ensure that, within the stimulus range used, the perceived value on each dimension would remain constant in the face of variation on the other dimension. In Experiments 2 through 4 we assessed performance by using the converging operations by which Garner defined integrality and separability. Experiment 2 showed that in speeded classification, pitch and loudness lead to facilitation with redundant variation and interference with orthogonal variation. Experiment 3 showed that unspeeded classifications are guided predominantly by overall similarity. Experiment 4 established that the better-fitting metric by which multidimensional similarity is appreciated is Euclidean rather than city block. These results suggest that the dimensions of pitch and loudness combine in an integral fashion. In Experiments 5 and 6 we investigated whether the dimensions of pitch and loudness have a privileged status by assessing the impact of rotating the dimensional axes on performance in a speeded sorting task. Experiment 5 looked at six alternative dimensional orientations to pitch and loudness. If anything, rotating the dimensional axes increased the amount of interference in filtering. In Experiment 6 we assessed an alternative dimensional description of the stimuli based on the dimensions of volume and brightness. We found greater interference when the stimuli varied along the dimensions of volume and brightness than when they varied along the dimensions of pitch and loudness. The fact that the least interference is observed when the stimuli vary along the dimensions of pitch and loudness suggests that these dimensions are the more psychologically valid ones. These findings indicate that integrality is not a "myth," that is, merely a case of psychophysical mismatch. Instead, dimensions that are psychologically real are sometimes processed in a unitary fashion.

Monitoring recovery after injury: procedures for deriving the optimal test window.

Researchers studying the impact of treatments designed to facilitate recovery after neural injury face competing demands. On the one hand, because treatment effects often emerge slowly over days, and because researchers seek evidence of stable long-term effects, it is common practice to observe experimental subjects for many weeks after treatment. On the other hand, the cost of performing studies and the need to evaluate a multitude of alternative treatment procedures requires optimal efficiency, pushing researchers towards shorter test procedures. With these issues in mind, researchers have appeared to derive a test window based on previously published methodologies and inspection of their recovery curves, with testing terminated after the recovery curve reaches asymptote (approaches a slope of 0). An alternative procedure is introduced here that evaluates the stability of the data set over time. Using correlational techniques, researchers can determine whether (1) testing should be continued for additional days; or (2) equivalent statistical power can be achieved in fewer days. This provides a rational decision rule to help researchers balance competing demands. Applying these techniques to a procedure that evaluates the impact of acute treatments on recovery from spinal cord injury, it is shown that equal statistical power can be achieved in half the time, greatly increasing the efficiency with which alternative treatments can be evaluated.

The central representation of an aversive event maintains the opioid and nonopioid forms of analgesia.

Exposure to an aversive event, such as shock, can elicit either an opioid or nonopioid analgesia in rats. We suggest that the central representation of an aversive event in working memory activates both forms of analgesia. We formalize this basic hypothesis by coupling it with a current model of animal learning and memory, SOP (Wagner, 1981). SOP is designed to capture the standard operating procedures that govern memory systems. Our application of SOP suggests that manipulations which disrupt the maintenance of information in working memory should alter the magnitude and time course of analgesia. Three experiments are reported that support our proposal. Experiment 1 showed that analgesia decays more rapidly if the representation of the aversive event is displaced from working memory by presenting a postshock distractor. Experiment 2 demonstrated that the postshock distractor alters the magnitude and time course of both the opioid and nonopioid forms of analgesia. Experiment 3 demonstrated that pharmacologically disrupting working memory, by administering a high dose of pentobarbital, prevents mild shock from inducing a strong change in pain reactivity. Implications of the results are discussed.

Influence of naloxone on shock-induced freezing and analgesia.

Six experiments were designed to examine whether mild shock activates an opiate analgesia in rats. The first three experiments explored whether naloxone potentiates shock-induced freezing by blocking an opiate analgesia. In Experiment 1, subjects treated with either a low or a high dose of naloxone froze more following mild shock. Experiment 2 revealed that both dose levels of the drug increase pain reactivity. The results of Experiment 3 suggested that a naloxone-induced increase in pain reactivity accounts for the drug's effect on freezing. The last three experiments investigated the nature of the analgesia induced by mild shock. In Experiment 4, mild shock induced a profound analgesia as measured by the tail-flick test. Experiment 5 demonstrated that mild shock elicits a transient naloxone-insensitive analgesia which rapidly dissipates to reveal an analgesia that is reversed by a high dose of naloxone. This suggests that mild shock activates both the nonopiate and the opiate form of analgesia. Experiment 5 also showed that a low dose of naloxone potentiates shock-induced analgesia. Experiment 6 revealed that this potentiated analgesia is attenuated by a high dose of naloxone. Implications of the results are discussed.

Mechanisms of Pavlovian conditioning: role of protection from habituation in spinal conditioning.

Conditioned antinociception can be established in spinal rats by pairing stimulation to one hind leg (the conditioned stimulus [CS]) with an intense tailshock (the unconditioned stimulus [US]). After this training, the paired CS (CS+) elicits greater antinociception on the tail-flick test than a CS that was explicitly unpaired (CS-). Five experiments are reported that suggest that this effect reflects protection from habituation. Experiment 1 showed that the CS (legshock) induces antinociception before training. Presenting the CS alone weakened (habituated) its antinociceptive impact (Experiment 2). Less habituation was observed when the CS was paired with the US (Experiment 3). Decreasing habituation to the CS- (by increasing the interval between trials) and facilitating habituation to the CS+ (by increasing the number of trials) effectively eliminated the CS+/CS- difference (Experiments 4 and 5).

Role of supraspinal systems in environmentally induced antinociception: effect of spinalization and decerebration on brief shock-induced and long shock-induced antinociception.

Prior research suggests that afferent nociceptive information can directly activate the opioid and nonopioid brainstem antinociceptive systems. Grau (1987a) has hypothesized that direct activation occurs when an organism is exposed to severe aversive stimuli and that forebrain systems mediate the activation of the antinociception systems when mild aversive stimuli are used. The present experiments tested this hypothesis by examining the impact of spinalization and decerebration on the antinociception observed after mild (3 0.75-s 1.0-mA shocks) and vs. severe (3 25-s 1.0-mA shocks) tailshocks. It was found that spinal transection eliminated the antinociception observed after both shock schedules, whereas decerebration blocked mild shock-induced, but not severe shock-induced, antinociception. Surprisingly, decerebration potentiated severe shock-induced antinociception. The opioid antagonist naltrexone had no effect on the antinociception observed after severe shock in sham or decerebrate rats.

Frontal cortex lesions block the opioid and nonopioid hypoalgesia elicited by brief shocks but not the nonopioid hypoalgesia elicited by long shocks.

Previous research (Grau, 1987a, 1987b) suggests that forebrain systems play an essential role in the hypoalgesia observed after brief shock but not long shock. Additional research has shown that pentobarbital anesthesia and decerebration block the hypoalgesia observed after 3 brief (0.75-s) shocks but not the hypoalgesia observed after 3 long (25-s) shocks. This is a study of whether a specific forebrain lesion, a frontal cortex lesion, would have a similar impact on hypoalgesia induced by brief (0.75 s) and long (25-s) shocks. Frontal cortex lesions, like decerebration and pentobarbital anesthesia, eliminated the hypoalgesia observed after brief but not long shocks. Because other research suggests that the stress of surgery may influence whether the hypoalgesia elicited by shock is opioid or nonopioid, the 2nd experiment was to examine whether the sham operation per se alters the form of the hypoalgesia observed after brief shock. It does not; in the sham-treated subjects, brief shock induced the usual transient nonopioid hypoalgesia followed by prolonged opioid hypoalgesia. These data suggest that frontal cortex lesions block nonopioid and opioid hypoalgesia observed after brief shock.

Conditioned changes in pain reactivity: II. In search of the elusive phenomenon of conditioned hyperalgesia.

Previous research suggests that a stimulus that has been paired with an aversive event can elicit either an increase (hyperalgesia) or decrease (hypoalgesia) in pain reactivity in rats. Attempts have recently been made to isolate the variables that determine the direction of the conditioned response. Very little evidence has been found for conditioned hyperalgesia when a spinally mediated measure of pain reactivity (the tail-flick test) was used. In the present study, the impact of a conditioned stimulus was assessed using a procedure modeled after one in which conditioned hyperalgesia was obtained with the tail-flick test (Davis & Hendersen, 1985, Experiment 4). With this training paradigm, a discrete conditioned stimulus was found to elicit hypoalgesia but not hyperalgesia.

Tail-flick test: II. The role of supraspinal systems and avoidance learning.

It is held that the tail-flick test of pain depends on a spinal reflex because a similar response is observed in spinally transected rats. But when subjects were manually held and a cool heat setting was used, supraspinal systems facilitated the response (Experiment 1). This effect did not depend on the rate at which the tail was heated (Experiment 2) but rather on the co-occurrence of visual, auditory, and tactile cues that predict impending pain (Experiments 3 and 4). Subjects rapidly learned to exhibit a tail movement during these co-occurring cues, and this avoidance response was instrumental in nature (Experiment 5). Optimal learning was observed when the visual signal was presented 8-12 s before a heat-elicited response is normally observed (Experiment 6), and a low dose of morphine inhibited the performance of the instrumental response (Experiment 7).

Shock-induced hyperalgesia: II. Role of the dorsolateral periaqueductal gray.

Exposure to 3 moderately intense (1-mA) tailshocks has been shown to lower vocalization thresholds to both heat and shock. Previous shock exposure also facilitates the acquisition of conditioned fear as measured by freezing. These observations suggest that shock induces hyperalgesia (enhanced pain). This study explored whether shock-induced hyperalgesia depends on neurons within rostral or caudal portions of the dorsolateral periaqueductal gray (dlPAG). Experiment 1 examined the impact of dlPAG lesions on the acquisition of conditioned fear. Sham-operated rats demonstrated enhanced acquisition after shock exposure; both rostral and caudal lesions eliminated this effect. Experiment 2 showed that tailshock lowered vocalization thresholds to heat in sham-operated but not lesioned subjects. These results suggest that the dlPAG plays a critical role in the production of shock-induced hyperalgesia.

Instrumental learning within the spinal cord: I. Behavioral properties.

Four experiments are reported that explore whether spinal neurons can support instrumental learning. During training, one group of spinal rats (master) received legshock whenever one hindlimb was extended. Another group (yoked) received legshock independent of leg position. Master, but not yoked, rats learned to maintain their leg in a flexed position, exhibiting progressively longer flexions as a function of training (Experiment 1). All subjects were then tested by applying controllable shock to the same leg (Experiment 2). Master rats reacquired the instrumental response more rapidly (positive transfer), whereas yoked rats failed to learn (a learned helplessness-like effect). Disrupting response-outcome contiguity by delaying the onset and offset of shock by 100 ms eliminated learning (Experiment 3). Experiment 4 showed that shock onset contributes more to learning than does shock offset.

Associative learning and memory for an antinociceptive response in the spinalized rat.

Prior research suggests that associative and memorial processes can modulate the activation of the endogenous antinociceptive systems. It has been generally assumed that forebrain systems play an essential role in mediating the impact of these processes. The present experiments explored whether the behavioral effects indicative of associative and memorial processes can be obtained in spinalized rats. Experiment 1 demonstrated that a conditioned nonopioid antinociception can be established after rats have experienced a spinal transection at the level of the 2nd thoracic vertebrae. Experiment 2 showed that a postshock distractor can speed the decay of shock-induced antinociception in the spinalized rat. These findings suggest that the circuitry needed to obtain associative and memorylike effects is present within the spinal cord.

Activation of the opioid and nonopioid hypoalgesic systems at the level of the brainstem and spinal cord: does a coulometric relation predict the emergence or form of environmentally induced hypoalgesia?

Prior research suggests that a coulometric relation (Intensity x Duration) determines whether an opioid or nonopioid hypoalgesic system is activated by afferent nociceptive information. Using a paradigm that generates a brainstem-mediated hypoalgesia on the tail-flick test, we found that a coulometric relation does not predict either the emergence or the form of shock-induced hypoalgesia in decerebrate rats. In fact, no evidence was obtained that the brainstem's opioid hypoalgesic system can be activated by ascending neurons. More severe shocks elicited hypoalgesia in spinalized rats. Although a coulometric relation did not predict the emergence of hypoalgesia in spinalized rats, shock severity did predict the form of the hypoalgesia; the least severe shocks elicited an opioid hypoalgesia, and the more severe shocks generated a nonopioid hypoalgesia. A similar pattern of data was observed in intact rats exposed to the least severe shock parameters.

Role of cholinergic systems in pain modulation: I. Impact of scopolamine on environmentally induced hypoalgesia and pain reactivity.

Scopolamine was found to block both brief shock-induced (3 0.75-s, 1.0-mA shocks) and conditioned hypoalgesia on the tail-flick test in rats. The drug also produced a general increase in pain reactivity as measured by both the tail-flick test and shock-induced vocalization. It was shown that this hyperalgesia cannot account for the effect of the drug on brief-shock or conditioned hypoalgesia. Scopolamine did not block the nonopioid analgesia observed after long shock (3 25-s, 1.0-mA shocks). When the effect of the drug on baseline levels of pain reactivity was controlled, it potentiated long shock-induced hypoalgesia. Scopolamine also increased reactivity to tactile stimulation, which suggests the hyperalgesia reflects a general increase in arousal. None of these effects were observed with methylscopolamine, which suggests they are not peripherally mediated.

Impact of shock on pain reactivity: III. The magnitude of hypoalgesia observed depends on test location.

Pain reactivity is often assessed in rodents by measuring the latency of tail withdrawal from radiant heat (the tail-flick test). Using this test, the authors show that the magnitude of antinociception observed in spinal rats depends on test location; antinociception is observed at, and distal to, where shock is applied, but not at more proximal sites (Experiments 1 & 2). Experiment 3 evaluates the generality of this observation by testing 3 other shock schedules that are known to elicit distinct forms of antinociception. In all but 1 case, the magnitude of antinociception varied as a function of test location. Experiment 4 shows that morphine also has a greater impact at distal test locations. Experiment 5 assessed the impact of tailshock on reactivity to radiant heat applied to the foot. Of the 5 distinct forms of shock-induced antinociception studied, only 2 produce a robust antinociception at this test location.

Shock-induced hyperalgesia: III. Role of the bed nucleus of the stria terminalis and amygdaloid nuclei.

Rats exposed to a few moderately intense (1 mA) shocks subsequently exhibit lower vocalization thresholds to shock and thermal stimuli. They also exhibit facilitated learning in a Pavlovian conditioning paradigm. Together, these results suggest that shock exposure can enhance pain (hyperalgesia). The present study examined the role of the amygdala and bed nucleus of the stria terminalis (BNST), 2 systems that have been implicated in the induction and maintenance of negative affective states. Experiment 1 showed that lesions of the central, but not the basolateral, amygdala eliminate shock-induced hyperalgesia as measured by a decrease in vocalization thresholds to shock. Experiment 2 revealed that central nucleus lesions also prevent shock-induced sensitization of the vocalization response to heat. Anterior, but not posterior, BNST lesions had a similar effect.