Effects of the substance P antagonist, (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-SP on the miotic response to substance P, antidromic trigeminal nerve stimulation, capsaicin, prostaglandin E1, compound 48/80 and histamine.

The effects of the substance P analogue (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-SP on the ocular inflammatory responses (miosis, vasodilation, protein leakage into the aqueous humour and eye pressure rise) to antidromic trigeminal nerve stimulation (trigeminal stimulation), intracameral injections of substance P (SP), capsaicin, prostaglandin E1 (PGE1), compound 48/80 and histamine were investigated in albino rabbits. The effects of nerve blockade with tetrodotoxin and blockade of histamine receptors on the responses to compound 48/80 and histamine were also investigated. Histamine H1 receptors were blocked with clemastin and H2 receptors with cimetidin. Formation of endogenous prostaglandins was prevented with indomethacin. The pupil size and the eye pressure were measured. The aqueous humour was collected immediately after the animal was killed, and analyzed for protein concentration. (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-SP had no significant miotic effect, but tended to cause a break-down of the blood-aqueous barrier. Miosis caused by SP, trigeminal stimulation, capsaicin, PGE1, compound 48/80 or histamine was blocked by (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-SP. Histamine miosis was significantly reduced by blockade of nerve conduction or histamine receptors, while miosis caused by compound 48/80 was not. Nerve blockade abolished the rise in intraocular pressure caused by compound 48/80. Our results indicate that (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-SP is a specific SP blocker in the sphincter pupillae muscle. They are strong evidence for the hypothesis that trigeminal stimulation and capsaicin cause miosis by release of SP or a related substance (SPLI), and it seems likely that the miosis caused by PGE1 and compound 48/80 is also caused by SPLI release. Histamine miosis is probably mediated both by SP receptors and histamine receptors in the pupillary sphincter muscle.