Functional recovery of stepping in rats after a complete neonatal spinal cord transection is not due to regrowth across the lesion site.

Rats receiving a complete spinal cord transection (ST) at a neonatal stage spontaneously can recover significant stepping ability, whereas minimal recovery is attained in rats transected as adults. In addition, neonatally spinal cord transected rats trained to step more readily improve their locomotor ability. We hypothesized that recovery of stepping in rats receiving a complete spinal cord transection at postnatal day 5 (P5) is attributable to changes in the lumbosacral neural circuitry and not to regeneration of axons across the lesion. As expected, stepping performance measured by several kinematics parameters was significantly better in ST (at P5) trained (treadmill stepping for 8 weeks) than age-matched non-trained spinal rats. Anterograde tracing with biotinylated dextran amine showed an absence of labeling of corticospinal or rubrospinal tract axons below the transection. Retrograde tracing with Fast Blue from the spinal cord below the transection showed no labeled neurons in the somatosensory motor cortex of the hindlimb area, red nucleus, spinal vestibular nucleus, and medullary reticular nucleus. Retrograde labeling transsynaptically via injection of pseudorabies virus (Bartha) into the soleus and tibialis anterior muscles showed no labeling in the same brain nuclei. Furthermore, re-transection of the spinal cord at or rostral to the original transection did not affect stepping ability. Combined, these results clearly indicate that there was no regeneration across the lesion after a complete spinal cord transection in neonatal rats and suggest that this is an important model to understand the higher level of locomotor recovery in rats attributable to lumbosacral mechanisms after receiving a complete ST at a neonatal compared to an adult stage.

Shock-induced hyperalgesia: evidence forebrain systems play an essential role.

Exposure to a few moderately intense (1-mA) tailshocks has opposite effects on two measures of pain reactivity in rats. Tail-withdrawal to radiant heat is inhibited (antinociception) while vocalization thresholds are lowered (hyperalgesia) to both heat and shock (King et al., 1996). Prior work indicates that this hyperalgesia represents an unconditioned response and that it enhances the acquisition of both conditioned freezing and an avoidance response to thermal pain. The present experiments begin to explore the neural mechanisms that underlie hyperalgesia. Experiments 1 and 2 demonstrated that hyperalgesia is eliminated by both decerebration and pentobarbital anesthesia. Lesions limited to the frontal pole had a similar effect (Experiment 3). Experiment 4 showed that lesioning the frontal pole also disrupted the acquisition of conditioned fear.

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.

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).

Evidence for spinal conditioning in intact rats.

Prior work suggests that spinal systems are sensitive to the stimulus relationships that underlie Pavlovian conditioning. We studied this phenomenon in Sprague-Dawley rats by pairing a vibrotactile conditioned stimulus (CS) with a tailshock unconditioned stimulus (US). Experiment 1 showed that spinal rats exhibit differential conditioning, having longer tail-flick latencies on the tail-flick test during a CS that was paired with the US (conditioned antinociception). Experiment 2 showed that rats trained with the cord intact still exhibit differential conditioning after the cord is cut. This suggests that spinal learning contributes to behavioral plasticity in intact subjects.

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).

Impact of shock on pain reactivity: II. Evidence for enhanced pain.

Shocked rats (Rattus norvegicus) often exhibit longer tail withdrawal latencies to radiant heat, which suggests that exposure to shock reduces pain. But at the same time, rats appear hyperreactive to shock, suggesting than pain is enhanced. Experiment 1 replicated these findings and showed that when tail movement was monitored, shocked rats were less responsive to heat and hyperreactive to shock even when the same behavioral criteria were used. When latency to vocalize was measured, shocked rats appeared hyperreactive to both test stimuli (Experiments 2 and 3). Prior exposure to shock also enhanced the acquisition of conditioned fear in a different context (Experiment 4) and the speed with which rats learned a response to avoid a thermal stimulus (Experiment 5). The results suggest that exposure to shock enhances pain.

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.

Response-specific inhibition during general facilitation of defensive responses in Aplysia.

Siphon responses of Aplysia have been used to examine the neural basis of nociceptive behavioral inhibition. The authors tested the response specificity and functional significance of this inhibition. Video analysis showed that strong tail-nerve shock decreased the duration of siphon constriction evoked by weak siphon shock. Tail-nerve shock also caused the appearance of a novel flaring response to the test stimulus, which resembled the siphon response to tail-nerve shock. Novel flaring responses were expressed to both mechanical and electrical siphon stimuli. Tailshock facilitated another defensive response, inking, during the period of inhibited siphon constriction. Tailshock also facilitated tail contractions evoked by weak contralateral tail stimulation during this period. These results indicate that inhibition is not generalized across defensive responses and is specific to siphon responses that interfere with directed ink ejection toward an injured site.