Pharmacological properties of a C-fibre response evoked by saphenous nerve stimulation in an isolated spinal cord-nerve preparation of the newborn rat

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Abstract
1 An isolated spinal cord-peripheral nerve preparation of the newborn rat was developed. In this preparation it is possible to record spinal reflexes from a lumbar ventral root in response to stimulation of the ipsilateral saphenous or obturator nerve. 2 Single shock, weak intensity stimulation of the saphenous nerve induced a fast conducted compound action potential in the L3 dorsal root and a fast depolarizing response in the ipsilateral L3 ventral root. As a stronger stimulus was applied to the saphenous nerve, a slowly conducted compound action potential appeared in the dorsal root and a slow depolarizing ventral root potential (v.r.p.) in the L3 ventral root. 3 Single shock stimulation of the obturator nerve induced a rapidly conducted compound action potential in the L3 dorsal root and monosynaptic and polysynaptic reflexes, with a fast time course, in the ipsilateral L3 ventral root. 4 The slow v.r.p. evoked by saphenous nerve stimulation was depressed by the tachykinin antagonist, [D-Arg1, D-Trp7,9, Leu11] substance P (spantide), 4–16 μm. The response recovered its original shape and size 30–60 min after the removal of this antagonist. 5 The saphenous nerve-evoked slow v.r.p. was depressed by [Met5] enkephalin (0.1–1 μm), dynorphin (1–13)(0.2 μm) and morphine (1–2 μm), and these effects were reversed by naloxone (1 μm). 6 Two endogenous peptides, galanin (1–2 μm) and somatostatin (1–2.5 μm), inhibited the slow v.r.p. evoked by saphenous nerve stimulation, whereas another endogenous peptide, calcitonin gene-related peptide (0.1-0.5 μm), potentiated the slow v.r.p. The slow v.r.p. was also inhibited by γ-aminobutyric acid (GABA, 20 μm) and muscimol (0.2 μm), and their effects were antagonized by bicuculline (1 μm). 7 The present results suggest that substance P and neurokinin A are involved in the saphenous nerve-evoked C-fibre response in the spinal cord of the newborn rat.