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1 Physiology, Pavlov Institute, St. Petersburg, Russian Federation; Physiological Science, University of California Los Angeles, Los Angeles, California, United States
2 Physiological Science, University of California Los Angeles, Los Angeles, California, United States
3 Physiological Science, University of California Los Angeles, Los Angeles, California, United States; Brain Research Institute, UCLA School of Medicine, Center for Health Sciences, Los Angeles, California, United States
4 Brain Research Institute, UCLA School of Medicine, Center for Health Sciences, Los Angeles, California, United States; Physiological Science, University of California Los Angeles, Los Angeles, California, United States
5 Departement of Physiological Science, University of California-Los Angeles, Los Angeles, California, United States; Brain Research Institute, UCLA School of Medicine, Center for Health Sciences, Los Angeles, California, United States
* To whom correspondence should be addressed. E-mail: vre{at}ucla.edu.
We hypothesized that epidural spinal cord stimulation (ES) and quipazine (a serotonergic agonist) modulates the excitability of flexor and extensor related intraspinal neural networks in qualitatively unique, but complementary, ways to facilitate locomotion in spinal cord injured rats. To test this hypothesis, we stimulated (40 Hz) the S1 spinal segment before and after quipazine administration (0.3 mg/kg, i.p.) in bipedally step-trained and non-trained, adult, complete spinal (mid-thoracic) rats. The stepping pattern of these rats was compared with control rats. At the stimulation levels used, stepping was elicited only when the hindlimbs were placed on a moving treadmill. In non-trained rats the stepping induced by ES and quipazine administration was non-weight bearing and the cycle period was shorter than in controls. In contrast, the stepping induced by ES and quipazine in step-trained rats was highly coordinated with clear plantar foot placement and partial weight bearing. The effect of ES and quipazine on EMG burst amplitude and duration was greater in flexor than extensor pools. Using Fast Fourier Transformation analysis of EMG bursts during ES, we observed one dominant peak at 40 Hz in the medial gastrocnemius (ankle extensor), whereas there was no dominant spectral peak in the tibialis anterior (ankle flexor). We suggest that these frequency distributions reflect amplitude modulation of predominantly monosynaptic potentials in the extensor and predominantly polysynaptic pathways in the flexor muscle. Quipazine potentiated the amplitude of these responses. The data suggest that there are fundamental differences in the circuitry that generates flexion and extension during locomotion.
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