Effect of cimetidine and pyrilamine on histamine-induced ocular surface hyperemia

Histamine receptors and the conjunctiva

PURPOSE OF REVIEW

The purpose of this review is to evaluate the effect of histamine on various receptors in the conjunctiva. A Medline search from 1980 was performed on the histamine receptor subtypes H1, H2 and H3 in the human conjunctiva.

RECENT FINDINGS

In the conjunctiva, histamine has been shown to induce various physiological and immunological changes through both H1 and H2 receptor stimulation. Histamine binding to conjunctival H1 receptors through the phospholipase C-dependent inositol phosphate pathway leads to the symptom of pruritus while histamine stimulation of the conjunctival H2 receptors has been indirectly shown to cause vasodilation.

SUMMARY

The effect of histamine on conjunctival H1 receptors appears to be the primary target for ocular allergy treatment as it is primarily involved in ocular pruritus. The exact interaction of the conjunctival H2 receptors appears to work in a complementary fashion to the H1 receptor in controlling other features of ocular allergy such as vasodilation and injection. Thus, oral and topical antihistamines with multiple histamine receptor binding activities may provide an improved treatment paradigm for the various signs and symptoms of ocular allergy. The histamine H1, H2 and H3 receptor affinities of ketotifen, pyrilamine, and epinastine appear to have the strongest H1 and H2 affinities.

Vascular permeability in allergic conjunctivitis in mice lacking histamine H1 receptors

To clarify the role of histamine H1 receptors in allergic conjunctivitis, changes in vascular permeability of the conjunctiva were measured in histamine H1 receptor deficient mice. Wild-type mice showed a significant increase in vascular permeability of the conjunctiva induced by histamine. However, no such increase was found in histamine H1 receptor deficient mice. On the other hand, no differences were observed between wild-type and histamine H1 receptor deficient mice in response to serotonin. A significant increase in vascular permeability was observed in actively sensitized wild-type mice, whereas no increase was observed in histamine H1 receptor deficient mice. Similar findings were noted in passively sensitized animals. Histamine contents of the conjunctiva were significantly decreased by topical application of antigen in both wild-type and histamine H1 receptor deficient mice after active sensitization with antigen. These findings suggested that vascular permeability in the conjunctiva in allergic conjunctivitis is entirely regulated through histamine H1 receptor.

Interactive effects of peptidoleukotrienes and histamine on microvascular permeability and their involvement in experimental cutaneous and conjunctival immediate hypersensitivity

The involvement of peptidoleukotrienes (LTs) in mediating the increase in microvascular permeability associated with experimental cutaneous immediate hypersensitivity was studied by examining the effect of SK&F 104353, a potent and selective LT-antagonist, on the response evoked by graded, intradermal injections of antigen. SK&F 104353, employed at doses that profoundly blocked LTC4, LTD4 and LTE4 responses, significantly reduced the response produced by experimental cutaneous immediate hypersensitivity. The response to the lowest antigen dose (0.1 microgram) was, however, entirely insusceptible to SK&F 104353. The effect of SK&F 104353 was also examined in combination with a pyrilamine-cimetidine dosing regimen sufficient to remove the histaminergic component of cutaneous immediate hypersensitivity. The non-histaminergic component associated with higher antigen doses (10 and 100 micrograms) was significantly reduced but not abolished by SK&F 104353; the non-histaminergic component associated with low antigen doses (0.1 and 1 microgram) was not susceptible to SK&F 104353. Thus, the increase in cutaneous microvascular permeability evoked by immediate hypersensitivity appears to comprise three components: (1) A histaminergic response apparent for all antigen doses; (2) a LT-mediated component which is manifest in response to high antigen doses; (3) a third, unidentified component that is present for the entire antigen dose-range but contributes less to the overall response when high antigen doses are used. A distinct non-histaminergic, non-leukotriene mediated component was not a feature of conjunctival immediate hypersensitivity. SK&F 104353 administered in combinatio with pyrilamine-cimetidine virtually abolished the response with a small residual remaining only for the highest antigen dose. In further contrast to cutaneous immediate hypersensitivity, SK&F 104353 alone was comparatively ineffective in type 1 allergic conjunctivitis. This difference in susceptibility to SK&F 104353 appears to reflect the type of histamine-LTD4 interactive effect on microvascular permeability. Histamine and LTD4 were additive in terms of cutaneous microvascular permeability. In the conjunctiva, histamine and LTD4 appeared mutually exclusive in that the level of response produced by the combination tended not to exceed that of the single component which caused the greater effect.