Leucine at the carboxyl-terminal of endokinins C and D contributes to elicitation of the antagonistic effect on substance P in rat pain processing

Tachykinins, new players in the control of reproduction and food intake: A comparative review in mammals and teleosts

In vertebrates, the tachykinin system includes tachykinin genes, which encode one or two peptides each, and tachykinin receptors. The complexity of this system is reinforced by the massive conservation of gene duplicates after the whole-genome duplication events that occurred in vertebrates and furthermore in teleosts. Added to this, the expression of the tachykinin system is more widespread than first thought, being found beyond the brain and gut. The discovery of the co-expression of neurokinin B, encoded by the tachykinin 3 gene, and kisspeptin/dynorphin in neurons involved in the generation of GnRH pulse, in mammals, put a spotlight on the tachykinin system in vertebrate reproductive physiology. As food intake and reproduction are linked processes, and considering that hypothalamic hormones classically involved in the control of reproduction are reported to regulate also appetite and energy homeostasis, it is of interest to look at the potential involvement of tachykinins in these two major physiological functions. The purpose of this review is thus to provide first a general overview of the tachykinin system in mammals and teleosts, before giving a state of the art on the different levels of action of tachykinins in the control of reproduction and food intake. This work has been conducted with a comparative point of view, highlighting the major similarities and differences of tachykinin systems and actions between mammals and teleosts.

Novel Pituitary Actions of TAC4 Gene Products in Teleost

Tachykinin 4 (TAC4) is the latest member of the tachykinin family involved in several physiological functions in mammals. However, little information is available about TAC4 in teleost. In the present study, we firstly isolated TAC4 and six neurokinin receptors (NKRs) from grass carp brain and pituitary. Sequence analysis showed that grass carp TAC4 could encode two mature peptides (namely hemokinin 1 (HK1) and hemokinin 2 (HK2)), in which HK2 retained the typical FXGLM motif in C-terminal of tachyinin, while HK1 contained a mutant VFGLM motif. The ligand-receptor selectivity showed that HK2 could activate all 6 NKRs but with the highest activity for the neurokinin receptor 2 (NK2R). Interestingly, HK1 displayed a very weak activation for each NKR isoform. In grass carp pituitary cells, HK2 could induce prolactin (PRL), somatolactin α (SLα), urotensin 1 (UTS1), neuromedin-B 1 (NMB1), cocaine- and amphetamine-regulated transcript 2 (CART2) mRNA expression mediated by NK2R and neurokinin receptor 3 (NK3R) via activation cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA), phospholipase C (PLC)/inositol 1,4,5-triphosphate (IP3)/protein kinase C (PKC) and calcium²⁺ (Ca²⁺)/calmodulin (CaM)/calmodulin kinase-II (CaMK II) cascades. However, the corresponding stimulatory effects triggered by HK1 were found to be notably weaker. Furthermore, based on the structural base for HK1, our data suggested that a phenylalanine (F) to valine (V) substitution in the signature motif of HK1 might have contributed to its weak agonistic actions on NKRs and pituitary genes regulation.

Overview of Neurological Mechanism of Pain Profile Used for Animal "Pain-Like" Behavioral Study with Proposed Analgesic Pathways

Pain is the most common sensation installed in us naturally which plays a vital role in defending us against severe harm. This neurological mechanism pathway has been one of the most complex and comprehensive topics but there has never been an elaborate justification of the types of analgesics that used to reduce the pain sensation through which specific pathways. Of course, there have been some answers to curbing of pain which is a lifesaver in numerous situations-chronic and acute pain conditions alike. This has been explored by scientists using pain-like behavioral study methodologies in non-anesthetized animals since decades ago to characterize the analgesic profile such as centrally or peripherally acting drugs and allowing for the development of analgesics. However, widely the methodology is being practiced such as the tail flick/Hargreaves test and Von Frey/Randall-Selitto tests which are stimulus-evoked nociception studies, and there has rarely been a complete review of all these methodologies, their benefits and its downside coupled with the mechanism of the action that is involved. Thus, this review solely focused on the complete protocol that is being adapted in each behavioral study methods induced by different phlogogenic agents, the different assessment methods used for phasic, tonic and inflammatory pain studies and the proposed mechanism of action underlying each behavioral study methodology for analgesic drug profiling. It is our belief that this review could significantly provide a concise idea and improve our scientists' understanding towards pain management in future research.

Pharmacological characteristics of hemokinin-1-derived peptides in rat pruriceptive processing

Hemokinin-1 (HK-1) is a member of mammalian tachykinin peptide family, and [Leu¹¹]-HK-1 has an antagonistic effect on HK-1. The attenuation of pruritogen-induced scratching behavior by pretreatment with [Leu¹¹]-HK-1 indicates the involvement of HK-1 in pruriceptive processing. However, it remains unclear whether the intrathecal or intranasal administration of HK-1-derived peptides, such as [D-Trp^7,9]-[Leu¹¹]-HK-1 or [D-Trp⁷]-[Leu¹¹]-HK-1, elicits the effects different from [Leu¹¹]-HK-1. The induction of scratching by intrathecal administration of HK-1 was attenuated 30 min, 4 h and 24 h after pretreatment with [Leu¹¹]-HK-1, [D-Trp^7,9]-[Leu¹¹]-HK-1 and [D-Trp⁷]-[Leu¹¹]-HK-1 or [D-Trp⁹]-[Leu¹¹]-HK-1, respectively. Similarly, the scratching induced by subcutaneous injection of pruritogens as chloroquine and histamine was ameliorated 30 min and 24 h after pretreatment with [Leu¹¹]-HK-1 and these three HK-1-derived peptides, respectively. Moreover, the effective minimum concentrations of intrathecal administrations of [D-Trp⁹]-[Leu¹¹]-HK-1 on scratching induced by chloroquine and histamine were 10^-6 M, while the effective minimum concentrations of intranasal administration of this peptide on scratching induced by chloroquine and histamine were 10^-5 M and 10^-4 M, respectively. Thus, the present results indicate that the intrathecal administration of HK-1-derived peptides with D-Trp extends its effective time on scratching induced by intrathecal administration of HK-1 and pruritogens such as chloroquine and histamine. Similarly, the induction of scratching by pruritogens was attenuated by intranasal administration of HK-1-derived peptide, although the effective minimum concentration of this peptide was slightly lower than that of intrathecal administration, indicating that intranasal administration is an effective tool for carrying peptides into the brain.

Pain regulation of endokinin A/B or endokinin C/D on chimeric peptide MCRT in mice

The present study focused on the interactive pain regulation of endokinin A/B (EKA/B, the common C-terminal decapeptide in EKA and EKB) or endokinin C/D (EKC/D, the common C-terminal duodecapeptide in EKC and EKD) on chimeric peptide MCRT (YPFPFRTic-NH2, based on YPFP-NH2 and PFRTic-NH2) at the supraspinal level in mice. Results demonstrated that the co-injection of nanomolar EKA/B and MCRT showed moderate regulation, whereas 30 pmol EKA/B had no effect on MCRT. The combination of EKC/D and MCRT produced enhanced antinociception, which was nearly equal to the sum of the mathematical values of single EKC/D and MCRT. Mechanism studies revealed that pre-injected naloxone attenuated the combination significantly compared with the equivalent analgesic effects of EKC/D alone, suggesting that EKC/D and MCRT might act on two totally independent pathways. Moreover, based on the above results and previous reports, we made two reasonable hypotheses to explain the cocktail-induced analgesia, which may potentially pave the way to explore the respective regulatory mechanisms of EKA/B, EKC/D, and MCRT and to better understand the complicated pain regulation of NK1 and μ opioid receptors, as follows: (1) MCRT and endomorphin-1 possibly activated different μ subtypes; and (2) picomolar EKA/B might motivate the endogenous NPFF system after NK1 activation.

Effects of endokinin A/B, endokinin C/D, and endomorphin-1 on the regulation of mean arterial blood pressure in rats

Endokinins are four novel human tachykinins, including endokinins A (EKA), B (EKB), C (EKC), and D (EKD). Endokinin A/B (EKA/B) is the common C-terminal decapeptide in EKA and EKB, while endokinin C/D (EKC/D) is the common C-terminal duodecapeptide in EKC and EKD. In this study, we attempted to investigate the interactions between EKA/B, EKC/D, and endomorphin-1 (EM-1) on the depressor effect at peripheral level. The effects of EKA/B produced a U-shaped curve. The maximal effect was caused by 10 nmol/kg. EKC/D and EM-1 showed a dose-dependent relationship. Co-administration of EKA/B (0.1, 1, 10 nmol/kg) with EM-1 produced effects similar to those of EKA/B alone but slightly lower. Co-injection of EKA/B (100 nmol/kg) with EM-1 caused an effect stronger than any separate injection. Co-administration of EKC/D (10 nmol/kg) with EM-1 (30 nmol/kg) caused a depressor effect, which was one of the tradeoffs of EM-1 and EKC/D. Mechanism studies showed that SR140333B could block the depressor effects of EKA/B, EKC/D, EM-1, EKA/B+EM-1, and EKC/D+EM-1; SR48968C could block EM-1, EKA/B, EKC/D, and EKC/D+EM-1 and partially block EKA/B+EM-1; SR142801 could block EM-1, EKC/D, and EKC/D+EM-1 and partially block EKA/B and EKA/B+EM-1; naloxone could block EM-1, EKC/D, and EKC/D+EM-1 and partially block EKA/B and EKA/B+EM-1. Pretreatment with NG-nitro-l-arginine methyl ester partially decreased depressor intensity and half-recovery time of EKA/B and EKC/D.

Pharmacological characteristics of endokinin C/D-derived peptides in nociceptive and inflammatory processing in rats

Endokinins designated from the human TAC4 gene consist of endokinin A, endokinin B, endokinin C (EKC) and endokinin D (EKD). EKC/D is a peptide using the common carboxyl-terminal in EKC and EKD and consists of 12 amino acids, and exerts antagonistic effects on the induction of scratching behavior by substance P (SP). Some of SP-preferring receptor antagonists have several d-tryptophan (d-Trp); however, the pharmacological effect of EKC/D-derived peptides with d-Trp remains to be solved. Therefore, to clarify the pharmacological characteristics of EKC/D-derived peptides, effects of pretreatment with these peptides on SP-induced scratching and thermal hyperalgesia, formalin-induced flinching and carrageenan-induced inflammation were evaluated. Intrathecal administration of [d-Trp(8)]-EKC/D and [d-Trp(10)]-EKC/D showed a markedly long inhibitory effect, at least 14 h, whereas the antagonistic effects of [d-Trp(8,10)]-EKC/D and EKC/D without d-Trp disappeared after 1h. Furthermore, the inhibitory effect of [d-Trp(10)]-EKC/D-derived peptides was dependent on the number of amino acids from the amino-terminus, and the more numerous the amino acids, the more marked the antagonistic effect. Thus, these results indicate that the effective duration of EKC/D-derived peptides is dependent on the number of d-Trp in the carboxyl-terminal region and the amino-terminal region regulates the antagonistic effect of EKC/D.

Knockdown of the tachykinin neurokinin 1 receptor by intrathecal administration of small interfering RNA in rats

The contribution of tachykinin neurokinin 1 (NK₁) receptor to nociceptive processing in the dorsal horn has been evaluated by tachykinin NK₁ receptor antagonism and knockout or knockdown of tachykinin NK₁ receptor; however, these results have not always been consistent. Therefore, to reevaluate the role of tachykinin NK₁ receptor in the dorsal horn, a solution of hemagglutinating virus of the Japan envelope (HVJ-E) with small interfering RNA (siRNA) against tachykinin NK₁ receptor was administered intrathecally and then the effect of treatment on tachykinin NK₁ receptor immunohistochemistry and on the induction of inflammation, thermal hyperalgesia and scratching behavior was evaluated. This treatment resulted in marked reduction of tachykinin NK₁ receptor immunoreactivity through the spinal dorsal horn, and the induction of thermal hyperalgesia and scratching behavior by substance P was significantly attenuated in rats with tachykinin NK₁ receptor siRNA. In addition, only one intrathecal injection of tachykinin NK₁ receptor siRNA reduced carrageenan-induced inflammation and thermal hyperalgesia significantly and markedly attenuated the induction of flinching after formalin injection and c-Fos expression in the dorsal horn following formalin injection. The efficient down-regulation of tachykinin NK₁ receptor by intrathecal administration tachykinin NK₁ receptor siRNA suggests that this method may be a valuable tool for examining the function of genes expressed in the dorsal horn.

The N-methyl-D-aspartate-evoked cytoplasmic calcium increase in adult rat dorsal root ganglion neuronal somata was potentiated by substance P pretreatment in a protein kinase C-dependent manner

The involvement of substance P (SP) in neuronal sensitization through the activation of the neurokinin-1-receptor (NK1r) in postsynaptic dorsal horn neurons has been well established. In contrast, the role of SP and NK1r in primary sensory dorsal root ganglion (DRG) neurons, in particular in the soma, is not well understood. In this study, we evaluated whether SP modulated the NMDA-evoked transient increase in cytoplasmic Ca2+ ([Ca2+]cyt) in the soma of dissociated adult DRG neurons. Cultures were treated with nerve growth factor (NGF), prostaglandin E2 (PGE2) or both NGF+PGE2. Treatment with NGF+PGE2 increased the percentage of N-methyl-D-aspartate (NMDA) responsive neurons. There was no correlation between the percentage of NMDA responsive neurons and the level of expression of the NR1 and NR2B subunits of the NMDA receptor or of the NK1r. Pretreatment with SP did not alter the percentage of NMDA responsive neurons; while it potentiated the NMDA-evoked [Ca2+]cyt transient by increasing its magnitude and by prolonging the period during which small- and some medium-sized neurons remained NMDA responsive. The SP-mediated potentiation was blocked by the SP-antagonist ([D-Pro4, D-Trp7,9]-SP (4-11)) and by the protein kinase C (PKC) blocker bisindolylmaleimide I (BIM); and correlated with the phosphorylation of PKCε. The Nk1r agonist [Sar9, Met(O2)11]-SP (SarMet-SP) also potentiated the NMDA-evoked [Ca2+]cyt transient. Exposure to SP or SarMet-SP produced a rapid increase in the labeling of phosphorylated-PKCε. In none of the conditions we detected phosphorylation of the NR2B subunit at Ser-1303. Phosphorylation of the NR2B subunit at Tyr1472 was enhanced to a similar extent in cells exposed to NMDA, SP or NMDA+SP, and that enhancement was blocked by BIM. Our findings suggest that NGF and PGE2 may contribute to the injury-evoked sensitization of DRG neurons in part by enhancing their NMDA-evoked [Ca2+]cyt transient in all sized DRG neurons; and that SP may further contribute to the DRG sensitization by enhancing and prolonging the NMDA-evoked increase in [Ca2+]cyt in small- and medium-sized DRG neurons.

Subcutaneous injection of endokinin C/D attenuates carrageenan-induced inflammation

Endokinins, encoded by the human preprotachykinin C (PPT-C)/TAC4 gene, are peptides that consist of endokinin A (EKA), B (EKB), C (EKC) and D (EKD) and belong to the tachykinin family. Intrathecal injection of EKC/D (using the common carboxyl-terminal duodecapeptide in EKC and EKD) markedly attenuated the induction of thermal hyperalgesia and scratching behavior by intrathecal administration of substance P (SP), indicating that EKC/D has an antagonistic effect on the neurokinin 1 receptor (NK1R), SP-preferring receptor, at the spinal level; however, the pharmacological function of EKC/D at the periphery is not yet understood. Therefore, to clarify the effect of EKC/D on the peripheral tissue, the effect of subcutaneous injection of EKC/D on carrageenan-induced inflammation was examined. Subcutaneous injection of EKC/D attenuated an increase in paw volume following carrageenan-induced inflammation in a dose-dependent manner. Indeed, the increased paw volume was significantly decreased 40 min after treatment with 10(-4) M (10 nmol) and 10(-3) M (100 nmol) EKC/D (100 microl/rat). Similarly, injection of NK1R antagonists such as L-703,606 and Spantide I (10(-3) M) attenuated the increased paw volume following inflammation. Furthermore, the reduced withdrawal latency evoked by inflammation following subcutaneous injection of carrageenan was also dose-dependently attenuated by EKC/D administration. These results indicate that subcutaneous injection of EKC/D elicits an anti-inflammatory effect on carrageenan-induced inflammation.

The amino-terminal region of hemokinin-1 regulates the induction of thermal hyperalgesia in rats

It is known that intrathecal administration of substance P (SP) induces thermal hyperalgesia, whereas hemokinin-1 (HK-1), a member of the same tachykinin family as SP, hardly induces thermal hyperalgesia; however, the underlying mechanism remains to be elucidated. Therefore, we aimed to clarify which amino acid of these peptides contributes to the induction of thermal hyperalgesia. When two chimera peptides between the N-terminal region of SP and the C-terminal region of HK-1, and vice versa, SP (1-5)/HK-1 and HK-1 (1-5)/SP, were intrathecally administered, SP (1-5)/HK-1 induced thermal hyperalgesia whereas HK-1 (1-5)/SP had hardly any effect; furthermore, thermal hyperalgesia was induced by only C-terminal fragments of HK-1 and SP. These findings indicate that the N-terminal region of HK-1 is involved in the non-induction of thermal hyperalgesia. Next, we synthesized and intrathecally administered these chimera peptides in which part of the N-terminal region of HK-1 was replaced with that of SP, and vice versa, and all synthesized peptides induced thermal hyperalgesia. Both SP (1-2)/HK-1 and HK-1 (1-4)/SP certainly induced thermal hyperalgesia, although HK-1 and HK-1 (1-5)/SP had hardly any effect; therefore, it is probable that Ser at the 2nd position and Arg at the 5th position of HK-1 may be involved in the non-induction of thermal hyperalgesia. Furthermore, peptides in which amino acid at the 3rd and/or 4th positions of HK-1 was replaced with that of SP were synthesized. Intrathecal administration of HK-1 (1-2,4-5)/SP, but not HK-1 (1-2,5)/SP and HK-1 (1-3,5)/SP, hardly induced thermal hyperalgesia. These findings indicate that three amino acids, Ser, Thr and Arg at the 2nd, 4th and 5th positions of HK-1, respectively, regulate the induction of thermal hyperalgesia by HK-1.

Effects of Endokinin A/B and Endokinin C/D on the antinociception of Endomorphin-1 in mice

In our previous study, Endokinin A/B (EKA/B, the common C-terminal decapeptide in Endokinin A and Endokinin B) was found to induce analgesic effect at high dose and nociception at low dose, while Endokinin C/D (EKC/D, the common C-terminal duodecapeptide in Endokinin C and Endokinin D) has analgesic effect only. So in this study an attempt was undertaken to investigate the interaction of EKA/B and EKC/D with Endomorphin-1 (EM-1) on antinociceptive effect at supraspinal level. Results showed that the antinociceptive effect of EM-1 was enhanced by high dose of EKA/B and abolished by low dose of EKA/B, while EKC/D could only enhance the analgesic effect. Mechanism studies showed that EKA/B blocked the antinociception of EM-1 by activating neurokinin-1 receptor (NK(1)), whose specific antagonist, SR140333B could fully block EKA/B-induced attenuation on the analgesic response of EM-1. Surprisingly, EKC/D could also block the same EKA/B-induced attenuation. Taken together, the different effects of EKA/B and EKC/D on the antinociception of EM-1 may pave the way for a new strategy on investigating the interaction between tachykinins and opioids on pain modulation.

Differential effects of substance P or hemokinin-1 on transient receptor potential channels, TRPV1, TRPA1 and TRPM8, in the rat

Two tachykinin peptides, substance P (SP) and hemokinin-1 (HK-1), and three transient receptor potential (TRP) channels, TRPV1, TRPA1 and TRPM8, are similarly localized in the spinal dorsal horn and dorsal root ganglion, suggesting that TRP channels may be related or modulated by these tachykinin peptides. Thus, to clarify whether the responses of TRP channels are modulated by SP or HK-1, the effects of pretreatment with SP or HK-1 on the induction of scratching behavior by TRP channel agonists were examined. Pretreatment with SP or HK-1 enhanced the induction of scratching behavior by resiniferatoxin, a TRPV1 agonist, whereas scratching behavior induced by menthol, a TRPM8 agonist, was suppressed by pretreatment with these peptides. On the other hand, pretreatment with SP, but not HK-1, suppressed the induction of scratching behavior by cinnamaldehyde, a TRPA1 agonist. Taken together, the present results indicate that SP or HK-1 differentially modulated the response of TRPV1, TRPA1 or TRPM8 channel.

NMDA and AMPA receptors contribute to the maintenance of substance P-induced thermal hyperalgesia

It is well known that intrathecal administration of substance P (SP) induces thermal hyperalgesia, but the mechanisms underlying the maintenance of SP-induced thermal hyperalgesia remain to be clarified. Thus, to clarify the receptors involved in the maintenance of SP-induced thermal hyperalgesia, the effect of administering SP or glutamate receptor agonists, NMDA or AMPA, under SP-induced thermal hyperalgesia was investigated. Also, the effect of pretreatment with protein kinase inhibitors on scratching behavior by NMDA or AMPA under SP-induced thermal hyperalgesia was examined. Under SP-induced thermal hyperalgesia, the number of scratchings following SP administration was time-dependently suppressed, whereas the number of scratchings after NMDA or AMPA administration was markedly enhanced and SP-induced thermal hyperalgesia was attenuated by pretreatment with NMDA or AMPA receptor antagonist. Furthermore, pretreatment with kinase inhibitors significantly attenuated the enhancement of scratching behavior by NMDA or AMPA under SP-induced thermal hyperalgesia. These findings indicate that SP-induced thermal hyperalgesia may be maintained through the enhanced responsiveness of NMDA or AMPA receptors, but not the receptor of SP, mediated by kinases.

Effects of Endokinin A/B and Endokinin C/D on the modulation of pain in mice

Endokinins are novel tachykinins encoded on the human TAC4 and consist of Endokinin A (EKA), B (EKB), C (EKC) and D (EKD). To date, the function of Endokinins in pain processing was not fully understood. Therefore the aim of this study was to investigate the effects of Endokinin A/B (EKA/B, the common C-terminal decapeptide in EKA and EKB) and Endokinin C/D (EKC/D, the common C-terminal duodecapeptide in EKC and EKD) on pain modulation at supraspinal level in mice. Intracerebroventricular (i.c.v.) administration of EKA/B (1, 3, 12, 20nmol/mouse) dose dependently induced potent analgesic effect. This effect could be fully antagonized by SR140333B but not SR48968C or SR142801. Naloxone could also block the analgesic effect, suggesting that this analgesic effect is related to opioid receptors. However, i.c.v. administration of EKA/B (10, 30, 100pmol/mouse) caused hyperalgesic effect significantly, with a "U" shape curve. Interestingly, the hyperalgesic effect induced by EKA/B could be attenuated by SR140333B, SR142801 but not SR48968C. I.c.v. administration of EKC/D (1, 3, 12, 20nmol/mouse) also dose dependently induced analgesic effect, which could not be blocked by SR48968C or SR142801 or naloxone. But to our astonishment, it could be significantly enhanced by SR140333B. More interestingly, the hyperalgesic effect induced by EKA/B could be significantly attenuated by EKC/D. In addition, the analgesic effect induced by co-administration of EKA/B and EKC/D was much less stronger than the effect of either EKA/B or EKC/D.

In vitro characterization of the effects of rat/mouse hemokinin-1 on mouse colonic contractile activity: a comparison with substance P

Rat/mouse hemokinin-1 (r/m HK-1) has been identified as a member of the tachykinin family and its effect in colonic contractile activity remains unknown. We investigated the effects and mechanisms of actions of r/m HK-1 on the mouse colonic contractile activity in vitro by comparing it with that of substance P (SP). R/m HK-1 induced substantial contractions on the circular muscle of mouse colon. The maximal contractile responses to r/m HK-1 varied significantly among proximal-, mid- and distal-colon, suggesting that the action of r/m HK-1 was region-specific in mouse colon. The contractile response induced by r/m HK-1 is primarily via activation of tachykinin NK(1) receptors leading to activation of cholinergic excitatory pathways and with a minor contribution of NK(2) receptors, which may be on the smooth muscle itself. A direct action on colonic smooth muscles may be also involved. In contrast, SP induced biphasic colonic responses (contractile and relaxant responses) on the circular muscle, in which the contractile action of SP was equieffective with r/m HK-1. SP exerted its contractile effect predominantly through neural and muscular tachykinin NK(1) receptors, but unlike r/m HK-1 did not appear to act via NK(2) receptors. The relaxation induced by SP was largely due to release of nitric oxide (NO) produced via an action on neural NK(1) receptors. These results indicate that the receptors and the activation properties involved in r/m HK-1-induced mouse colonic contractile activity are different from those of SP.

Pharmacological characterization of desensitization in scratching behavior induced by intrathecal administration of hemokinin-1 in the rat

Desensitization is induced by the repeated administration of high doses of substance P (SP) or hemokinin-1 (HK-1). However, little information is available about the mechanisms involved in the induction of desensitization by these peptides. Thus, to characterize this desensitization, we examined the dose-dependent effect of these peptides, the effect of pretreatment with neurokinin 1(NK1) receptor antagonists, and the effect of pretreatment with inhibitors of protein kinases such as protein kinase A (PKA), protein kinase C (PKC), calcium/calmodulin kinase II (CaMKII) and mitogen-activated protein kinase kinase (MEK). The number of scratchings induced by 10(-3)M SP or HK-1 decreased following pretreatment with 10(-11)-10(-3)M SP or HK-1 with a marked reduction at 10(-3) and 10(-6)M SP or HK-1. The effect of NK1 receptor antagonists on desensitization induced by pretreatment with 10(-6)M SP was marked, whereas there was little effect of pretreatment with these antagonists on 10(-6)M HK-1-induced desensitization. Additionally, 10(-6)M SP- and HK-1-induced desensitization was attenuated by pretreatment with PKA, PKC and MEK inhibitors, except a CaMKII inhibitor that inhibited SP-induced desensitization. These results indicate that the receptor and kinases involved in HK-1-induced desensitization are partially different from those of SP.