The purpose of this study was to investigate mechanism(s) and site(s) of action involved in 7-hydroxy-2-dipropylaminotetralin (7-OH-DPAT)-induced
ocular hypotension. As measured by pneumatonometry, the topical, unilateral application of
7-OH-DPAT (75 microg), a
dopamine D(3)-preferring receptor agonist, decreased the intraocular pressure (IOP) bilaterally. The ocular hypotensive activity of
7-OH-DPAT was diminished in sympathetically denervated rabbits. Pretreatment with
raclopride, a D(2)/D(3) receptor antagonist; UH232, a D(3) receptor antagonist; or
U-99194A, a D(3) receptor antagonist antagonized 7-OH-DPAT-induced
ocular hypotension. However, pretreatment with
spiperone, a D(2) receptor antagonist, did not affect the 7-OH-DPAT-induced
ocular hypotension. In addition, topically applied
7-OH-DPAT caused a reduction of aqueous humor flow rate. To examine sites of action, immunohistochemistry of D(3)
dopamine receptors was performed.
Dopamine D(3) receptors were found to be present on postganglionic sympathetic nerves in the ciliary body of normal rabbits but were virtually undetectable in the same tissue of sympathectomized rabbits. In summary, the IOP-lowering effect caused by
7-OH-DPAT was due, in part, to the suppression of aqueous humor flow. Immunohistochemical identification of D(3) receptors in the ciliary body, associated with the diminution of IOP-lowering effects by D(3) receptor agonist
7-OH-DPAT in sympathetically denervated rabbits provided evidence of neuronal site of action of
7-OH-DPAT. Suppression of 7-OH-DPAT-induced
ocular hypotension by D(3) receptor antagonists (
U-99194A and UH232) and
sympathectomy, coupled with the immunohistochemical data, suggested that the primary site of D(3) receptor-mediated action of
7-OH-DPAT is located on postganglionic sympathetic nerve endings in the ciliary body of rabbit.