Chapter 11--novel mechanism for hyperreflexia and spasticity.

We established that hyperreflexia is delayed after spinal transection in the adult rat and that passive exercise could normalize low frequency-dependent depression of the H-reflex. We were also able to show that such passive exercise will normalize hyperreflexia in patients with spinal cord injury (SCI). Recent results demonstrate that spinal transection results in changes in the neuronal gap junction protein connexin 36 below the level of the lesion. Moreover, a drug known to increase electrical coupling was found to normalize hyperreflexia in the absence of passive exercise, suggesting that changes in electrical coupling may be involved in hyperreflexia. We also present results showing that a measure of spasticity, the stretch reflex, is rendered abnormal by transection and normalized by the same drug. These data suggest that electrical coupling may be dysregulated in SCI, leading to some of the symptoms observed. A novel therapy for hyperreflexia and spasticity may require modulation of electrical coupling.

Modafinil normalized hyperreflexia after spinal transection in adult rats.

STUDY DESIGN: Hyperreflexia occurs after spinal cord injury and can be assessed by measuring low frequency-dependent depression of the H-reflex in the anesthetized animal. OBJECTIVE: To determine the effects of Modafinil (MOD), given orally, following a complete SCI compared with animals receiving MBET and transected untreated animals and examine if changes exist in Connexin 36 (Cx-36) protein levels in the lumbar enlargement of animals for the groups described. SETTING: Center for Translational Neuroscience, Little Rock, AR, USA. METHODS: Adult female rats underwent complete transection (Tx) at T10 level. H-reflex testing was performed 30 days following Tx in one group, and after initiation of treatment with MOD in another group, and after MBET training in the third group. The Lumbar enlargement tissue was harvested and western blots were performed after immunoprecipitation techniques to compare Cx-36 protein levels. RESULTS: Statistically significant decreases in low frequency-dependent depression of the H-reflex were observed in animals that received MOD and those that were treated with MBET compared with the Tx, untreated group. Statistically significant changes in Cx-36 protein levels were not observed in animals treated with MOD compared with Tx, untreated animals. CONCLUSION: Normalization of the loss of low frequency -dependent depression of the H-reflex was demonstrated in the group receiving MOD and the group receiving MBET compared with the Tx, untreated group. Further work is needed to examine if Cx-36 protein changes occur in specific subregions of the spinal cord.

The onset of hyperreflexia in the rat following complete spinal cord transection.

STUDY DESIGN: Hyperreflexia occurs after spinal cord injury (SCI) and can be assessed by measuring low frequency-dependent depression of the H-reflex. Previous studies showed the time course for the onset of hyperreflexia to occur between 6-28 days in the contusion model of SCI. OBJECTIVE: To determine the time course of the onset of hyperreflexia in the transection model of SCI and examine changes in Connexin-36 (Cx-36) protein levels in the lumbar enlargement of animals. SETTING: Spinal Cord Injury Mobilization Program of the Center for Translational Neuroscience, the research arm of the Jackson T. Stephens Neuroscience Institute, Little Rock, AR, USA. METHODS: Adult female rats underwent transection at T10 level. Low frequency-dependent depression of the H-reflex was tested at 7, 14 and 30 days post-transection. Lumbar enlargement tissue was harvested following reflex testing and western blots were performed after immunoprecipitation to compare Cx-36 protein levels. RESULTS: Significant decreases in low frequency-dependent depression of the H-reflex were observed in animals tested 14 and 30 days post-transection compared with control animals, but it was not different from control animals at 7 days. Significant decreases in Cx-36 protein levels were observed in animals 7 days post-transection compared with controls. CONCLUSION: Rats transition to a state of hyperreflexia between 7 and 14 days post-transection. Cx-36 protein levels decreased at 7 days post-transection and gradually returned to control levels by 30 days post-transection. These data suggest there may be a relationship between changes in neuronal gap junction protein levels and the delayed onset of hyperreflexia.

Restoration of frequency-dependent depression of the H-reflex by passive exercise in spinal rats.

STUDY DESIGN: Hyper-reflexia, measured as a decrease of low frequency-dependent depression of the H-reflex, is known to occur in both humans and animals after spinal cord injury (SCI). Previous studies have shown that passive exercise for 3 months could be used to restore low frequency-dependent depression of the H-reflex after SCI. OBJECTIVE: To determine the effects of various periods of time on the ability of passive exercise to restore low frequency-dependent depression of the H-reflex. SETTING: Spinal Cord Injury Mobilization Program of the Center for Translational Neuroscience, the research arm of the Jackson T Stephens Spine and Neuroscience Institute, Little Rock, AR, USA. METHODS: Adult rats underwent complete spinal cord transection at the T10 level. The hindlimbs were passively exercised in different groups of rats for 1 h/day, 5 days/week for 15, 30, 45, 60, or 90 days, and low frequency-dependent depression of the H-reflex was tested. RESULTS: Statistically significant low frequency-dependent depression of the H-reflex was evident by 30 days of exercise, although numerical reductions were seen even at 15 days. There was a linear decrease in low frequency-dependent depression of the H-reflex with duration of passive exercise. CONCLUSIONS: Passive exercise can restore frequency-dependent depression of spinal reflexes in a time-dependent manner if used following complete spinal transection.

Midlatency auditory evoked potentials and the startle response in the rat.

The P13 midlatency auditory evoked potential in the rat is (i) sleep state dependent, (ii) undergoes rapid habituation and (iii) is blocked by the cholinergic antagonist scopolamine. As such, the P13 potential in the rat shows the same characteristics as the P1 (or P50) potential in the human. These potentials are thought to be mediated, at least in part, by the cholinergic arm of the reticular activating system. Previous studies have linked the reticular activating system with the startle response. The present study was undertaken to explore this relationship by simultaneously recording the P13 potential and the electromyographically recorded startle response using stimuli designed to elicit each response. Simultaneous recordings from the vertex and neck musculature following auditory click stimuli showed that: (i) the mean threshold of the P13 potential was 69.3 +/- 1.9 dB, while that for the startle response was 87.9 +/- 6.4 dB; (ii) the P13 potential was present during waking and paradoxical sleep, but absent during slow-wave sleep, while the startle response was present reliably only during waking; (iii) both responses habituated in response to paired stimuli, but the startle response was more habituated than the P13 potential; and (iv) both responses were facilitated by trains of stimuli in a similar manner. Recordings carried out from the auditory cortex verified that the primary cortical response had properties different from the P13 potential; i.e. it was present during all sleep-wake states, had a lower threshold and did not habituate rapidly. Finally, different patterns of startle responses were detected in the neck muscles. In every case, the P13 potential occurred during the middle, inhibitory phase of the startle response. These results suggest that the P13 potential and the startle response share response features, but the P13 potential appears to be more sensitive to auditory stimulation and to sleep-wake states. The startle response may be modulating descending systems by priming the spinal cord to respond in a "fight vs flight" fashion. On the other hand, the P13 response may be modulating ascending systems by triggering thalamocortical activity and resetting descending systems to allow novel motor strategies.

Effects of exercise and fetal spinal cord implants on the H-reflex in chronically spinalized adult rats.

This study investigated the modulation of hindlimb reflex excitability after transection of the spinal cord in adult rats. After transection, the H-reflex exhibited decreased depression at high stimulation frequencies compared to intact animals. Groups of animals which received a spinal cord transection followed by either an exercise regimen for the hindlimbs or a fetal spinal cord implant, showed high stimulation frequency depression similar to controls. This suggests that each of these palliative strategies helped to "normalize' the excitability of specific spinal reflexes.

The pedunculopontine nucleus--auditory input, arousal and pathophysiology.

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

A middle-latency auditory-evoked potential in the rat.

Previous studies have established the presence of a middle-latency auditory-evoked potential that is characterized by a) sleep-state dependence, b) low following frequency (i.e., rapid habituation to repetitive stimulation), and c) blockade by the cholinergic antagonist, scopolamine. A vertex-recorded evoked potential having these characteristics was described in humans at a 50-80 ms latency (termed the P1 or.P50 potential) and in the cat at a 20-25 ms latency (termed wave A). These studies were undertaken to determine if a click stimulus-evoked potential having the same characteristics was present in the intact rat. Vertex and auditory cortex recordings in intact rats studied in a sound-attenuating chamber and exposed to free-field click stimuli showed a) the presence of a vertex recorded potential at a 11-15 ms latency, termed P13, and of an auditory cortex recorded potential at a 7-11 ms latency, termed Pa; b) the P13 was present during waking and paradoxical sleep but absent in slow-wave sleep, while Pa was present in all sleep-wake states; c) the P13 habituated markedly at stimulation rates above 1 Hz while Pa did not; and d) the P13 was blocked by low doses of scopolamine while Pa was not. These studies demonstrate the presence of a P1-like potential in the rat at a 13 +/- 2 ms latency.

Auditory input to the pedunculopontine nucleus: I. Evoked potentials.

The pedunculopontine nucleus (PPN) has been implicated in sleep-wake control, arousal responses, and motor functions. The PPN also has been implicated in the generation of the P1 middle-latency auditory-evoked potential. The present study was undertaken to determine the topographical distribution, threshold, and response properties of depth-recorded potentials following auditory click stimulation. Experiments were conducted in both decerebrate cat and rat, with a view towards determining the presence of P1-like middle-latency auditory-evoked potentials in the midbrain of both species. These results demonstrate a) the presence in and around the PPN of a P1-like potential in the decerebrate rat similar to that described in the accompanying article as the P13 in the intact rat; b) the presence in and around the PPN of a P1-like potential in the decerebrate cat similar to that previously described by others as wave A in the intact cat; c) although thresholds for these potentials were similar to those of intact preparations, following frequencies were higher in the decerebrate preparations, i.e., responsiveness to repetitive stimulation was higher; and d) depth-recorded somatosensory-evoked potentials also were studied in the cat and found to show an evoked potential at a similar latency as middle-latency auditory depth-recorded potentials. These findings suggest that click stimulus-evoked, depth-recorded potentials are present in and around the PPN in the decerebrate rat and cat, i.e., in the absence of cortex, at a similar latency as in intact preparations.

Auditory input to the pedunculopontine nucleus: II. Unit responses.

The pedunculopontine nucleus (PPN) has been implicated in sleep-wake control, arousal responses, and motor functions. The PPN also has been implicated in the generation of the P1 middle-latency auditory-evoked potential. The present study was undertaken to determine the nature of the responsiveness of single neurons in and around the PPN following auditory stimulation. Somatosensory responsiveness also was tested in some cells. These results demonstrate a) the presence of a significant proportion of PPN neurons that respond to auditory click stimuli; b) two populations of neurons showing either low threshold/short latency/low habituation or high threshold/longer latency/high habituation; c) the responses of longer latency neurons precede the onset and peak of depth- and vertex-recorded middle-latency auditory-evoked potentials; d) thresholds of longer latency neurons similar to the threshold for wave A in the intact cat, the P13 potential in the intact rat, or the startle reflex; and e) convergent somatosensory and auditory responses at a similar latency in a number of PPN neurons. These findings suggest that neurons in and around the PPN may participate in auditory and somatosensory information processing related to arousal, and may contribute to the manifestation of the P1 auditory middle-latency evoked potential.

Modulation of locomotion by a nerve graft across a spinal transection in rat.

Adult rats received a complete mid-lower thoracic spinal cord transection and a peripheral nerve autograft was inserted across the transection site. Testing 3-4 months later showed that, after decerebration, stimulation of the mesencephalic locomotor region (MLR) induced forelimb but not hindlimb locomotion. However, in 5/7 animals, tail pinch interrupted MLR stimulation-induced forelimb stepping, while pinna pinch induced hindlimb muscle twitch. These effects were not present following complete section of the nerve graft or in 6 control animals which did not receive a graft. Exposure of the cut mid-portion of the grafts to DiI revealed the presence of labeled axons entering the spinal cord through both ends of the graft in those animals which showed the above effects. There was no transport in the 2 cases in which tail pinch interruption of MLR-induced stepping or pinna pinch-induced hindlimb muscle twitch did not occur. We conclude that non-specific information which can modulate locomotion may be flowing through the graft.