Regular paperComplete inhibition of purinoceptor agonist-induced nociception by spinorphin, but not by morphine
Introduction
Spinorphin has been discovered as an endogenous factor that inhibits enkephalin-degrading enzymes from bovine spinal cords [5], [15], [16]. Structural analysis revealed that the factor is Leu-Val-Val-Tyr-Pro-Trp-Thr, and it inhibits a neural endopeptidase (EC 3.4.24.11; NEP) as well as aminopeptidase (EC 3.4.1; AP), dipeptidyl aminopeptidase (EC 3.4.14.1; DPP), and angiotensin-conversing enzyme (EC 3.4.15.1; ACE) from monkey brain [15]. Like LVV-hemorphin-6 [3] and valorphin [2], spinorphin is related to hemoglobin beta chain. Human polymorphonuclear neutrophil (PMN) induces some inflammatory responses, such as chemotaxis, O2− generation, and exocytosis stimulated by N-formylmethionyl-leucyl-phenylalanine (FMLP) [1], [12]. Spinorphin also inhibits PMN function by suppressing FMLP binding to its receptor on PMNs [21]. In addition, spinorphin when administered intracerebroventricularly (i.c.v.) shows analgesic activity in the tail-pinch method [17], possibly through an inhibition of brain enkephalin degrading enzymes. Recently, we developed a series of new strategies to characterize sensory neurons stimulated by pain-producing substances given into the intraplantar (i.pl.) space of hind paw of mouse [7], [8], [9], [10], [18]. Here we further attempted to characterize nociceptive sensory fibers into three types, and examined the mode of action of spinorphin-induced analgesia, in comparison to morphine-induced one.
Section snippets
Animals
Male ddY mice weighing 20 to 22 g were used in all experiments. Procedures were approved by Nagasaki University Animal Care Committee and complied with the recommendations of International Association for the Study of Pain [22].
Drugs
The following drugs were used: bradykinin (BK; Sigma, St. Louis, MO, USA), 2-metylthioadenosine triphosphate tetrasodium (2-MeS ATP; Research Biochemicals International, Wayland, MA, USA), morphine hydrochloride (Takeda Pharmac. Co. Ltd., Japan), pertussis toxin (PTX;
Pharmacological identification of three different kinds of nociceptive sensory neurons
The local application of BK at 2 pmol into the planta of hind limb (i.pl.) produced a nociceptive flexor response, and there were stable responses in amplitude upon repeated applications every 5 min, as previously reported [8], [18]. The mean ± SEM of BK responses (2 pmol) corresponds to the force of 6.86 ± 0.25 g (n = 50). BK in ranges of 0.002 to 2 pmol (i.pl.) showed such responses in a dose-dependent manner (Fig. 1 A), and the average of ND50, nociceptive dose ( ± SEM) showing 50% of
Discussion
The major findings in the present study have two issues. Firstly, peripherally stimulated nociceptive or sensory stimulations could be divided into three types from pharmacological characterizations. Type I fibers are stimulated by BK, type II by 2-MeS ATP and type III by PGI2 agonist. The response induced by BK was completely abolished by capsaicin treatment that had been carried out to neonatal mice. As it is well accepted that capsaicin treatment degenerates primary afferent unmyelinated C
Acknowledgements
Parts of this work were supported by Grants-in-Aid from the Ministry of Education, Science, Culture and Sports of Japan, by research grants from The Naito Foundation and Ono Medical Research Foundation.
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