Effect of resiniferatoxin on the isolated rabbit iris sphincter muscle: comparison with capsaicin and bradykinin

Mechanisms underlying lipopolysaccharide-induced kinin B1 receptor up-regulation in the pig iris sphincter in vitro

Kinin B1 receptors are known to be highly induced after inflammatory stimuli in several biological systems. We report that incubation of pig iris sphincter with lipopolysaccharide from Escherichia coli caused a marked and time-related up-regulation of B1, accompanied by a reduction of B2 receptor-mediated contractile responses. The up-regulation of B1 receptors by lipopolysaccharide stimulation was decreased by the inhibitors of protein synthesis, cycloheximide and actinomycin D, and by dexamethasone and the nuclear factor-kappaB (NF-kappaB) inhibitor pyrrolidinedithiocarbamate (PDTC). In addition, lipopolysaccharide-induced up-regulation of B1 receptors in the pig iris sphincter was significantly reduced by the p38 inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole (SB203580) and to a lesser extent by the extracellular signal-regulated kinases 1 and 2 (ERK1/2) blocker 2'-amino-3'-methoxyflavone (PD98059). Molecular biology experiments demonstrated that in vitro incubation with lipopolysaccharide resulted in a time-dependent and remarkable activation of NF-kappaB and of p38 and ERK1/2 mitogen-activated protein (MAP) kinases, in pig iris sphincter preparations. While attempting to verify how MAP kinases are part of the B1 receptor-activated signaling transduction pathways, we observed that PD98059 was able to markedly reduce the contraction induced by B1 receptor activation in lipopolysaccharide-pretreated pig iris sphincter muscle but that this response was only partially decreased by SB203580. Our results extend the previous evidence on the mechanisms underlying the B1 receptor upregulation processes and demonstrate for the first time how this takes place in an ocular tissue, the pig iris sphincter. It is therefore possible to define B1 receptors as therapeutic targets for the treatment of infectious and inflammatory alterations of the eye.

Ocular inflammation induced by electroconvulsive treatment: contribution of nitric oxide and neuropeptides mobilized from C-fibres

1. Electroconvulsive treatment (ECT) of rabbits produced ocular inflammation consisting of conjunctival hyperaemia, miosis and protein extravasation into the aqueous humour, reflected by the so-called aqueous flare response (AFR): the maximal reduction in pupil size was 3.8 +/- 0.1 mm (s.e. of mean, n = 16) while the maximal AFR was 28.1 +/- 2.8 (arbitrary units). 2. ECT also caused release of substance P (SP), pituitary adenylate cyclase-activating peptide (PACAP)-27, -38 and calcitonin gene-related peptide (CGRP). The concentrations of SP and CGRP in the aqueous humour of normal, untreated eyes were 10.6 +/- 1.4 and 117.4 +/- 12.4 pmol l-1, respectively, while the concentrations of PACAP-27 and -38 were below the detection limit. After ECT the concentrations of SP, PACAP-27, -38 and CGRP were 65.0 +/- 9.6, 46.9 +/- 8.4, 50.2 +/- 5.4 and 1109.9 +/- 133.1 pmol l-1, respectively (s.e. of mean, n = 12). Conceivably, ECT evoked an antidromic activation of sensory neurones in the trigeminal ganglion with the consequent release of neuropeptides from C-fibres in the uvea and the development of neurogenic inflammation. 3. Rabbits received the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 200 mg kg 1, i.v.). This pretreatment inhibited the ECT-evoked conjunctival hyperaemia, miosis and AFR: under these circumstances the maximal reduction in pupil size was 1.9 +/- 0.1 mm while the maximal AFR was 2.7 +/- 0.9 (n = 16). L-NAME also inhibited the ECT-evoked release of SP, PACAP-27, -38 and CGRP into the aqueous humour; the concentrations of SP and CGRP were 13.2 +/- 1.5 and 204.8 +/- 33.5 pmol l-1, respectively, while PACAP-27 and -38 were below the detection limit (n = 12). 4. The ECT-evoked miosis was also inhibited by pretreatment with the tachykinin receptor antagonist D-Pal9 spantide 11 (90 nmol, intravitreal injection); under these circumstances the maximal reduction in pupil size was only 0.7 +/- 0.03 mm, indicating an important role for SP in the miotic response. Pretreatment of the eye with capsaicin, which is known to cause functional ablation of C-fibres, inhibited the conjunctival hyperaemia, miosis and AFR by 40-50%; the maximal reduction in pupil size being 2.2 +/- 0.2 mm and the maximal AFR 13.8 +/- 2.1 (arbitrary units) (n = 8). 5. The results suggest (1) that ECT evokes ocular inflammation through antidromic C-fibre activation; (2) that SP contributes to the ECT-evoked miosis; and (3) that NO contributes to the antidromic C-fibre activation and possibly to the vascular responses mediated by the C-fibre transmitters.

Effects of ruthenium red and capsazepine on C-fibres in the rabbit iris

1. We have investigated the effects of ruthenium red and capsazepine on a C-fibre-smooth muscle preparation (the rabbit isolated iris sphincter muscle). 2. Like capsaicin, ruthenium red and capsazepine were found to produce contractions in a concentration-dependent manner. C-fibre activation was held to be responsible since the contractions could be inhibited by tachykinin receptor blockade. 3. Both ruthenium red and capsazepine inhibited capsaicin-induced contractions concentration-dependently; the pIC50 values were 5.1 and 4.9, respectively. The contractions induced by bradykinin, which, like capsaicin, acts by releasing tachykinins from C-fibres, were also inhibited by ruthenium red and capsazepine in a concentration-dependent manner; the pIC50 values were 4.1 and 4.6, respectively. 4. Electrically evoked, tachykinin-mediated contractions were inhibited by ruthenium red and capsazepine in a concentration-dependent manner; the pIC50 values were 4.3 and 4.5, respectively. 5. The contractile response to neurokinin A (NKA) was inhibited by capsazepine (and by capsaicin), but not by ruthenium red, in a concentration-dependent manner; the pIC50 value was 4.3. 6. The results suggest that, besides their ability to antagonize capsaicin, ruthenium red and capsazepine possess a weak capsaicin-like effect. Conceivably, capsazepine interacts with binding sites for capsaicin, acting as a partial agonist/antagonist, while ruthenium red interacts with capsaicin-operated cation channels. The inhibition of electrically evoked- or bradykinin-induced responses by capsazepine and ruthenium red suggests that capsaicin/capsazepine binding sites and capsaicin-operated cation channels play a role in the process of transmitter release in response not only to capsaicin but also to other C-fibre stimuli. In addition, capsazepine (and capsaicin) may affect smooth muscle non-specifically since the response to NKA was also inhibited by this drug. The fact that ruthenium red did not affect the response to NKA provides further evidence that ruthenium red acts in a mode different from that ofcapsazepine.

Tachykinin receptors mediating responses to sensory nerve stimulation and exogenous tachykinins and analogues in the rabbit isolated iris sphincter

1. We have used selective tachykinin receptor agonists and antagonists to investigate the nature of the receptors mediating responses to endogenous and exogenous tachykinins in the rabbit iris sphincter preparation in vitro. 2. The NK1-selective agonist, substance P methyl ester, induced contraction with a pD2 of 9.16 indicating the presence of NK1 receptors. In confirmation, the NK1-selective antagonist, GR82334, competitively antagonized responses to substance P methyl ester with high affinity (pKB 7.46). 3. NK3 receptors also mediate contraction since NK3-selective agonists exhibited high potency, e.g. the pD2 of [Me-Phe7]-neurokinin B was 9.67, and their responses were not inhibited by GR82334 (10 microM). 4. NK2 receptor activation does not seem to contribute to contraction since the NK2-selective agonist [beta-Ala8]-neurokinin A(4-10) had relatively low potency (pD2 6.43), and the NK2-selective antagonists MEN10207 (1 microM) and L-659,877 (10 microM) were inactive or had low affinity, respectively. 5. GR82334 (1 microM) significantly inhibited responses to electrical field-stimulation of non-adrenergic non-cholinergic sensory nerves (3, 10 and 30 Hz), and caused a rightward shift of the log concentration-response curve to bradykinin (lateral shift ca. 1000 fold). Higher concentrations of GR82334 (10 microM) significantly attenuated responses to capsaicin (1-60 microM) whilst completely abolishing responses to field-stimulation (3, 10 and 30 Hz) and bradykinin (1 nM- 3 microM). 6. In conclusion, NK1 and NK3 receptor activation results in contraction of the rabbit iris sphincter. The contractile response following sensory nerve stimulation by bradykinin, capsaicin and electrical field stimulation results from NK1 receptor activation.

Characterization of sensory neurotransmission and its inhibition via alpha 2B-adrenoceptors and via non-alpha 2-receptors in rabbit iris

To find out whether, and which type of, adrenoceptors mediate prejunctional inhibition of sensory neurotransmitter release from trigeminal fibres, the modulation of twitch response to electrical field stimulation on rabbit isolated iris was investigated. Evoked iris sphincter contractions consisted of a minor fast cholinergic and a large slow component. The latter was unaffected by atropine and guanethidine, hence nonadrenergic noncholinergic in nature (NANC), but nearly completely abolished by capsaicin pretreatment and by the neurokinin receptor antagonist spantide. The response was probably not mediated by NK2 receptors as SR 48,968, an NK2 selective nonpeptide antagonist, failed to reduce the response to the release of the endogenous neurokinin(s) (and exogenous substance P), but in part due to NK1 receptor activation as shown by a reduction of response by CP 96,345, an NK1 selective non-peptide antagonist, and in part perhaps mediated by NK3 receptors. A small neurokinin receptor antagonist- and capsaicin-insensitive NANC contraction is probably not mediated by CGRP receptors. The alpha 2-adrenoceptor agonist oxymetazoline inhibited the evoked NANC response (22 nmol/l, IC20; about 40%, maximum inhibition) without affecting the cholinergic response (up to 1 mumol/l) or the postjunctional iris sensitivity to exogenous substance P. The inhibition was antagonized by rauwolscine (apparent -log KB 8.04) and by the relatively alpha 2B-adrenoceptor selective antagonist ARC-239 (-log KB 8.51). The alpha 2- and imidazoline receptor agonist aganodine inhibited the evoked NANC response (0.25 mumol/l, IC20; about 30%, maximum inhibition) without affecting the postjunctional substance P responses. Rauwolscine 0.3 mumol/l failed to antagonize this effect. It is concluded that the release of sensory neurotransmitter(s) from trigeminal fibres in the rabbit eye may be inhibited by alpha 2B-adrenoceptors and by a non-alpha 2-receptor, perhaps an imidazoline receptor.