Responses of functionally identified neurones in the dorsal horn of the cat spinal cord to substance P, neurokinin A and physalaemin
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The mammalian tachykinins, substance P and neurokinin A, and the non-mammalian tachykinin, physalaemin, were tested on functionally identified dorsal horn neurones in vivo. The experiments were done on cats which were anaesthetized with sodium pentobarbital or were anaemically decerebrated. Extracellular single-unit recordings were made in the lumbar spinal cord and the tachykinins were applied by iontophoresis. Each neurone was classified functionally as wide dynamic range, non-nociceptive, nociceptive specific or proprioceptive. The response to tachykinin application was determined for each neurone. Application of each of the tachykinins evoked a characteristic excitatory response which was delayed in onset, slow in developing and prolonged: physalaemin excited 99/131 neurones tested, neurokinin A excited 45/63 neurones and substance P excited 32/49 neurones. With two neurones physalaemin evoked a depression of the rate of firing, which may have been caused indirectly by excitation of a neighbouring neurone. Such depression was not elicited by either substance P or by neurokinin A. Physalaemin had a preferential excitatory effect on nociceptive neurones evoking excitation of 76/94 nociceptive neurones compared with 12/23 non-nociceptive neurones (chi 2 = 7.9, 1 d.f., P = 0.005). Substance P also caused a preferential excitation, with 30/40 nociceptive neurones being excited while all of the non-nociceptive neurones (n = 7) were unaffected (chi 2 = 11.5, 1 d.f., P = 0.0007). In contrast, neurokinin A failed to have a preferential effect; 32/46 nociceptive and 9/10 non-nociceptive neurones were excited (chi 2 = 1.0, 1 d.f., P = 0.40). Comparing the proportions of nociceptive neurones excited by the different tachykinins indicated that this type of neurone was not differently sensitive to any of the three peptides (chi 2 = 3.2, 2 d.f., P = 0.20). On the other hand, non-nociceptive neurones were preferentially excited by neurokinin A and physalaemin compared with substance P (chi 2 = 13.4, 2 d.f., P = 0.001). With regard to the endogenous tachykinins the results of this study may be interpreted in the following ways. The differential excitatory effect of substance P on nociceptive neurones supports the proposed role for this peptide in the transmission specifically of nociceptive inputs at the first afferent synapse. On the other hand, as neurokinin A excited non-nociceptive as well as nociceptive neurones, there may be a functional role for neurokinin A distinct from that of substance P.
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