Suppression of abnormally increased excitability of monosynaptic spinal reflex arcs by riluzole

We studied the effects of a neuroprotector, riluzole, on the evoked mass activity of spinal neuronal mechanisms and on action potentials (APs) recorded from the sciatic nerve in intact rats and rats with the manifestations of postdenervational and 4-aminopyridine (4-AP)-induced hyperreflexia, as well as in animals in the superreflexia state (induced by combined action of denervation and 4-AP). We measured the parameters of monosynaptic reflex discharges (monosynaptic reflexes, MRs) recorded from the ventral root (VR), of the spinal dorsal surface potential (DSPs), and of mass APs evoked in afferent and efferent fibers of the SN before and 10, 30, 60, and 120 min after injection of riluzole. It was found that in intact animals riluzole significantly (by 60–70%) decreased the amplitude of VR MRs and those of the afferent peak and N₁ component of DSPs. Riluzole exerted smaller suppressive effects on mass APs in the afferent fibers of the SN; the effect on APs in the SN efferent fibers was the minimum (a 4 to 5% decrease). Under conditions of increased sensitivity of the motoneuronal postsynaptic membrane to the transmitter (postdenervational hyperreflexia) and an increased release of glutamate from presynaptic elements (4-AP-induced hyperreflexia), as well as under superreflexia conditions, the dynamics of suppression of the evoked spinal activity by riluzole showed relatively moderate differences from those in intact animals. Under the above conditions, riluzole in the same manner decreased the amplitude of VR MRs. In the superreflexia state, the agent blocked the development of additional components of these dramatically increased potentials (in the above state, their amplitude increased by nearly nine times, on average, and this resulted in the generation of such components). We believe that the inhibitory effect of riluzole on glutamatergic neurotransmission in the spinal cord is based, first of all, on blocking of excitation in afferent presynaptic terminals. The possibility to use riluzole for correction of abnormally increased hyperexcitability of the spinal neuronal systems is discussed. Neirofiziologiya/Neurophysiology, Vol. 37, Nos. 5/6, pp. 416–423, September–December, 2005.