Responses of spinal dorsal horn neurons to foot movements in rats with a sprained ankle

Misencoding of ankle joint angle control system via cutaneous afferents reflex pathway in chronic ankle instability

This study aimed to investigate how the cutaneous reflexes in the peroneus longus (PL) muscle are affected by changing the ankle joint position in patients with chronic ankle instability (CAI). We also investigated the correlation between the degree of reflex modulation and angle position sense of the ankle joint. The participants were 19 patients with CAI and 20 age-matched controls. Cutaneous reflexes were elicited by applying non-noxious electrical stimulation to the sural nerve at the ankle joint in the neutral standing and eversion/inversion standing positions. The suppressive middle latency cutaneous reflex (MLR; ~ 70-120 ms) and angle position sense of the ankle joint were assessed. During neutral standing, the gain of the suppressive MLR was more prominent in the CAI patients than in controls, although no significant difference was seen during 30° inversion standing. In addition, the ratios of the suppressive MLR and background electromyography in a neutral position were significantly larger than those at the 15°, 25°, and 30° inversion positions in CAI patients. No such difference was seen in control individuals. Furthermore, the correlations between reflex modulation degree and position sense error were quite different in CAI patients compared to controls. These findings suggest that the sensory-motor system was deteriorated in CAI patients due to changes in the PL cutaneous reflex pathway excitability and position sense of the ankle joint.

History-dependent changes in the recovery process of the middle latency cutaneous reflex gain after ankle sprain injury

PURPOSE

We previously reported that suppressive middle latency cutaneous reflexes (MLRs) in the peroneus longus (PL) are exaggerated in subjects with chronic ankle instability, and the changes are related to functional instability. However, the time-varying history of these neurophysiological changes after an ankle sprain is yet to be elucidated. Therefore, in the present study, we investigated the time course of the changes in the PL MLR after an ankle sprain in relation to the number of sprain recurrences.

METHODS

Twenty-three subjects with ankle sprain were classified into 3 groups according to their history of ankle sprain: first ankle sprain, 2-3 ankle sprains, and ≥4 ankle sprains. Twenty-three age-matched control subjects also participated. The PL MLRs were elicited by stimulating the sural nerve while the subjects performed different levels of isometric ankle eversion. Gain of MLR was estimated using linear regression analysis (slope value) of the amplitude modulation of MLRs obtained from graded isometric contractions.

RESULT

The gain of MLRs first increased 4 weeks after the injury. In subjects with their first ankle sprain, the MLRs returned to almost baseline levels after 3 months. In contrast, the increase in MLR gain persisted even after 3 months in subjects with recurrent ankle sprains. In addition, the MLR gains were closely related to functional recovery of the ankle joint.

CONCLUSIONS

Our findings suggest that the recovery process of MLR gains were strongly affected by the history of ankle sprains as well as the functional recovery of the ankle joint.

A preclinical physiological assay to test modulation of knee joint pain in the spinal cord: effects of oxycodone and naproxen

Sensory processing in the spinal cord during disease states can reveal mechanisms for novel treatments, yet very little is known about pain processing at this level in the most commonly used animal models of articular pain. Here we report a test of the prediction that two clinically effective compounds, naproxen (an NSAID) and oxycodone (an opiate), are efficacious in reducing the response of spinal dorsal horn neurons to noxious knee joint rotation in the monosodium iodoacetate (MIA) sensitized rat. The overall objective for these experiments was to develop a high quality in vivo electrophysiology assay to confidently test novel compounds for efficacy against pain. Given the recent calls for improved preclinical experimental quality we also developed and implemented an Assay Capability Tool to determine the quality of our assay and ensure the quality of our results. Spinal dorsal horn neurons receiving input from the hind limb knee joint were recorded in anesthetized rats 14 days after they were sensitized with 1 mg of MIA. Intravenous administered oxycodone and naproxen were each tested separately for their effects on phasic, tonic, ongoing and afterdischarge action potential counts in response to innocuous and noxious knee joint rotation. Oxycodone reduced tonic spike counts more than the other measures, doing so by up to 85%. Tonic counts were therefore designated the primary endpoint when testing naproxen which reduced counts by up to 81%. Both reductions occurred at doses consistent with clinically effective doses for osteoarthritis. These results demonstrate that clinically effective doses of standard treatments for osteoarthritis reduce pain processing measured at the level of the spinal cord for two different mechanisms. The Assay Capability Tool helped to guide experimental design leading to a high quality and robust preclinical assay to use in discovering novel treatments for pain.

Gain modulation of the middle latency cutaneous reflex in patients with chronic joint instability after ankle sprain

OBJECTIVE

To investigate the neural alteration of reflex pathways arising from cutaneous afferents in patients with chronic ankle instability.

METHODS

Cutaneous reflexes were elicited by applying non-noxious electrical stimulation to the sural nerve of subjects with chronic ankle instability (n=17) and control subjects (n=17) while sitting. Electromyographic (EMG) signals were recorded from each ankle and thigh muscle. The middle latency response (MLR; latency: 70-120 ms) component was analyzed.

RESULTS

In the peroneus longus (PL) and vastus lateralis (VL) muscles, linear regression analyses between the magnitude of the inhibitory MLR and background EMG activity showed that, compared to the uninjured side and the control subjects, the gain of the suppressive MLR was increased in the injured side. This was also confirmed by the pooled data for both groups. The degree of MLR alteration was significantly correlated to that of chronic ankle instability in the PL.

CONCLUSIONS

The excitability of middle latency cutaneous reflexes in the PL and VL is modulated in subjects with chronic ankle instability.

SIGNIFICANCE

Cutaneous reflexes may be potential tools to investigate the pathological state of the neural system that controls the lower limbs in subjects with chronic ankle instability.

Electroacupuncture reduces the evoked responses of the spinal dorsal horn neurons in ankle-sprained rats

Acupuncture is shown to be effective in producing analgesia in ankle sprain pain in humans and animals. To examine the underlying mechanisms of the acupuncture-induced analgesia, the effects of electroacupuncture (EA) on weight-bearing forces (WBR) of the affected foot and dorsal horn neuron activities were examined in a rat model of ankle sprain. Ankle sprain was induced manually by overextending ligaments of the left ankle in the rat. Dorsal horn neuron responses to ankle movements or compression were recorded from the lumbar spinal cord using an in vivo extracellular single unit recording setup 1 day after ankle sprain. EA was applied to the SI-6 acupoint on the right forelimb (contralateral to the sprained ankle) by trains of electrical pulses (10 Hz, 1-ms pulse width, 2-mA intensity) for 30 min. After EA, WBR of the sprained foot significantly recovered and dorsal horn neuron activities were significantly suppressed in ankle-sprained rats. However, EA produced no effect in normal rats. The inhibitory effect of EA on hyperactivities of dorsal horn neurons of ankle-sprained rats was blocked by the α-adrenoceptor antagonist phentolamine (5 mg/kg ip) but not by the opioid receptor antagonist naltrexone (10 mg/kg ip). These data suggest that EA-induced analgesia in ankle sprain pain is mediated mainly by suppressing dorsal horn neuron activities through α-adrenergic descending inhibitory systems at the spinal level.