The role of substance P as a neurotransmitter in the reflexes of slow time courses in the neonatal rat spinal cord

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Abstract
1 In order to reveal the spinal reflexes involving the transmitter action of substance P (SP), the effects of capsaicin and an SP antagonist on the isolated spinal cord of the neonatal rat were studied. 2 When a single shock stimulus was given to a dorsal root (L3–L5) or a sciatic nerve, depolarizing responses of various time courses were recorded extracellularly from both ipsi- and contra-lateral ventral roots of the corresponding segments. The reflex response recorded from the contralateral ventral root consisted of fast and slow components, which will be referred to as contralateral fast and slow ventral root potentials (v.r.ps). The latter contralateral slow v.r.p. had a time-to-peak of 2–5 s and lasted 10–30 s. 3 The threshold for the contralateral slow v.r.p. was about two times higher than that for the monosynaptic reflex, and it coincided with the threshold for activating the slow-conducting afferent fibres. 4 The contralateral slow v.r.p. was abolished after the spinal cord was treated with capsaicin (1 μm for 30 min) in vitro. The contralateral slow v.r.p. was absent in the spinal cord derived from 4 day-old rats that had received capsaicin (50 mg kg−1, s.c.) on the 2nd day of life. The contralateral fast v.r.p. and other reflexes of fast time courses remained unaltered after treatment with capsaicin in vitro or in vivo. 5 Administration of an SP antagonist, [d-Arg1, d-Pro2, d-Trp7,9 Leu11]-SP in concentrations of 5–16 μm depressed the contralateral slow v.r.p., but did not affect the monosynaptic reflex, the dorsal root potential and the contralateral fast v.r.p. [d-Arg1, d-Pro2, d-Trp7,9, Leu11]-SP (5 μm) markedly depressed the SP-induced depolarizing response recorded from the ventral root whereas the responses to noradrenaline, 5-hydroxytryptamine, neurotensin and thyrotrophin releasing hormone (TRH) were unaffected by the SP antagonist. The response of the ventral root to acetylcholine was slightly depressed by the antagonist. The SP antagonist at 5–10 μm did not exert any agonist action on the motoneurones. 6 The present results in conjunction with those of previous studies support the hypothesis that SP released from certain primary afferent fibres acts as a neurotransmitter, producing in dorsal horn neurones slow excitatory postsynaptic potentials which lead to the generation of the contralateral slow v.r.p.