Demonstration of a 'septide-sensitive' inflammatory response in rat skin.

1. Measurement of plasma protein extravasation induced by the natural tachykinins following intradermal administration in rat skin indicated equipotency between substance P (SP), neurokinin A (NKA) and neurokinin B (NKB). The selective NK1 receptor agonist, [Sar9]SP sulphone was 10-100 times more potent than SP. The synthetic hexapeptide, septide, [pGlu6, Pro9]SP-(6-11), which has been proposed to act on a distinct NK1 receptor subtype/binding site was equipotent with [Sar9]SP sulphone. 2. The selective NK2 receptor agonist [beta Ala8]NKA(4-10) (0.1-1 nmol) and the selective NK3 receptor agonist, senktide (0.1-1 nmol) were both ineffective in producing oedema. The selective NK2 receptor antagonist, SR 48, 968 (0.3 mumol kg-1) had no significant inhibitory effects upon oedema induced by approximately equiactive doses of SP (0.2 nmol), septide (0.002 nmol), [Sar9]SP sulphone (0.002 nmol), or NKB (0.3 nmol). These results together suggest that neither NK2 nor NK3 receptors are involved in oedema formation in rat skin. 3. The non-peptide tachykinin NK1 receptor antagonist, RP 67,580 (1-3 mumol kg-1), inhibited plasma protein extravasation induced by septide (0.002 nmol) to a greater extent than that to SP (0.2 nmol). RP 67,580 (1 mumol kg-1) produced a significant inhibition of approximately 66% of the response to septide (0.002 nmol) only. Increasing the dose of RP 67,580 3 fold resulted in inhibition of the response to SP (0.2 nmol) and [Sar9]SP sulphone (0.002 nmol) by approximately 66% and 64% respectively with the response to septide being inhibited by approximately 70%. 4. Co-administration of the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME)(0.1 micromol) with the relevant tachykinin, resulted in a significant attenuation of the oedemaresponse to septide (0.1 nmol) producing only an approximate 56% inhibition of the response. The response to 0.2 nmol SP was unaffected whereas the response to a higher dose of 1 nmol was lowered byL-NAME but this did not reach significance.5. Degranulation of mast cells, achieved by pretreatment with compound 48/80 (5 mg kg-1) for 3 consecutive days, significantly inhibited the oedema responses to only high dose SP (1 nmol) and[Sar9SP sulphone (0.002 nmol). SP (0.2 nmol), septide (0.002 nmol), NKA (0.2 nmol) and NKB(0.3 nmol) were unaffected by this treatment.6. RP 67,580 (0.3-3 microM kg-1) inhibited oedema induced by both 0.002 nmol and 0.1 nmol of septide.When using equiactive doses of SP only the response to the lower dose of 0.2 nmol SP was significantly inhibited, while RP 67,580 (3 micromol kg1) did not affect the response to 1 nmol SP.7 These results suggest distinct mechanisms of action for SP and septide in producing plasma protein extravasation in rat skin. The response induced by septide is blocked by RP 67,580 and is both NO dependent and mast-cell independent. In contrast the response to SP is only partially blocked by RP67,580 and is NO-independent. These data support the existence of a distinct 'septide-sensitive' receptor/binding site and suggest that this site is involved in tachykinin-induced oedema formation in rat skin.