Journal of neurophysiology
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Comparative Study
On the potential role of the corticospinal tract in the control and progressive adaptation of the soleus h-reflex during backward walking.
When untrained subjects walk backward on a treadmill, an unexpectedly large amplitude soleus H-reflex occurs in the midswing phase of backward walking. We hypothesized that activity in the corticospinal tract (CST) during midswing depolarizes the soleus alpha-motoneurons subliminally and thus brings them closer to threshold. To test this hypothesis, transcranial magnetic stimulation (TMS) was applied to the leg area of the motor cortex (MCx) during backward walking. ⋯ By contrast, no changes were observed in the timing or amplitude of soleus MEPs with training. Taken together, these observations make it unlikely that the motor cortex via the CST is involved in control of the H-reflex during the backward step cycle of untrained subjects nor in its progressive adaptation with training. Our observations raise the possibility that the large amplitude of H-reflex in untrained subjects and its adaptation with training are mainly due to control of presynaptic inhibition of Ia-afferents by other descending tracts.
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Comparative Study
Activation of spinal d1/d5 receptors induces late-phase LTP of C-fiber-evoked field potentials in rat spinal dorsal horn.
Long-term potentiation (LTP) of C-fiber-evoked field potentials in spinal dorsal horn may be relevant to pathological pain. Our previous work has shown that the late phase of the spinal LTP is protein synthesis-dependent. ⋯ We found the following. 1) Spinal application of SKF 38393, a D1/D5 receptor agonist, induced a slowly developed LTP of C-fiber-evoked field potentials, lasting for >10 h, and the effect was blocked by the D1/D5 antagonist SCH 23390, whereas a D2 receptor agonist (quinpirole) induced depression of C-fiber responses, lasting for 2 h. 2) The potentiation produced by D1/D5 receptor agonist occluded the late phase but not the early phase of the spinal LTP produced by tetanic stimulation. 3) SCH 23390 selectively depressed the late-phase LTP, when applied 40 min before tetanic stimulation. 4) The D1/D5 agonist-induced potentiation is blocked by the protein synthesis inhibitor anisomycin. 5) Activation of protein kinase A by spinal application of 8-Br-cAMP also induced spinal LTP, and the action occluded the potentiation induced by the D1/D5 receptor agonist. These results suggest that the spinal D1/D5 receptors participate in the protein synthesis-dependent late-phase LTP of C-fiber-evoked field potentials in spinal dorsal horn through the cAMP signaling pathway.