Ocular inhibitory effects of the dopamine DA2 agonist (Ha-118) in cats and rabbits

Effect of intravenous fenoldopam on intraocular pressure in ocular hypertension

Intravenous fenoldopam, a selective dopamine-1 receptor agonist, was compared with placebo in this randomized, double-blind, two-period crossover study to evaluate its effects on intraocular pressure, aqueous dynamics, and macular blood flow in patients with elevated intraocular pressure or primary open-angle glaucoma. Doses of fenoldopam were titrated up to a maximum of 0.5 microgram/kg/min. Intraocular pressure, measured by pneumotonometry, was the primary outcome variable. Other outcomes included macular blood flow assessed by blue field examination, visual field examined by automated perimetry, aqueous outflow facility measured by tonography, and aqueous humor production determined by fluorophotometry. During infusions of fenoldopam, intraocular pressure increased from a mean baseline level of 29.2 mmHg to a mean maximum level of 35.7 mmHg. During the placebo infusions, pressure increased from a mean baseline of 28.4 mmHg to a mean of 29.0 mmHg at the time point that corresponded to the mean maximum intraocular pressure on the day intravenous fenoldopam was administered, to yield a mean difference in pressure between study days of 6.7 mmHg (P < 0.05). There were no apparent changes in macular blood flow, visual fields, or production or outflow of aqueous humor associated with fenoldopam infusion. The increase in intraocular pressure seen in this population of patients with ocular hypertension during infusions of fenoldopam is consistent with fenoldopam-associated increases in intraocular pressure reported in previous studies of healthy volunteers and of patients with accelerated systemic hypertension. These results further suggest that dopamine-1 receptors play a role in the regulation of intraocular pressure.

Ocular actions of an octahydrobenzo[f]quinoline: Ha117

An octahydrobenzo[f]quinoline compound, Ha117, was examined for activity on: (1) intraocular pressure and in normal, sympathectomized and water loaded rabbits and aqueous flow rate in normal and sympathectomized rabbits, respectively; (2) contractions of the cat nictitating membrane elicited by electrical stimulation of cervical sympathetic nerves; (3) cAMP accumulation in the isolated rabbit iris root-ciliary body preparation. Ha117 lowered intraocular pressure and aqueous flow rate in normal but not in sympathectomized rabbits. Elevated intraocular pressure produced by water loading was suppressed by Ha117 and pretreatment with metoclopramide antagonized the inhibitory effect of Ha117. Neuronally mediated contractions of the nictitating membrane were inhibited in a dose-related fashion by Ha117, and inhibitory effects were antagonized by domperidone. Ha117 and an analog, Ha118, did not suppress isoproterenol- and vasoactive intestinal peptide-induced accumulation of cAMP in the rabbit iris root/ciliary body. In vivo results in rabbit and cat models suggest that the ocular hypotensive effect of Ha117 is mediated by an action on prejunctional dopamine (DA2) receptors. In vitro data in the rabbit iris root/ciliary body suggest that Ha117 and Ha118-induced lowering of intraocular pressure does not involve postjunctional suppression of cAMP accumulation.

Comparative effects of alpha-2 and DA-2 agonists on intraocular pressure in pigmented and nonpigmented rabbits

Alpha-2 and DA2 adrenoceptor agonists produce ocular hypotension in cats, rabbits, monkeys and humans. In this study, the effects of topical, unilateral administration of medetomidine (MED), an alpha-2 agonist, and Ha-118 (HA), a DA2 agonist, were compared on intraocular pressure (IOP) in normal, unilaterally sympathectomized (SX), and ocular hypertensive (oral water loaded) Dutch Belted (DB) and New Zealand White (NZW) rabbits. MED (75 micrograms) produced bilateral ocular hypotensive effects in both DB and NZW normal rabbits; however, HA (250 micrograms) lowered IOP unilaterally in NZW rabbits only. MED (25 micrograms) inhibited the rise in IOP caused by oral water loading in both DB and NZW rabbits; HA, on the other hand, was effective only in NZW rabbits. Topical bilateral pretreatment with metoclopramide, a DA2 antagonist, inhibited the ocular antihypertensive effect of HA in NZW rabbits. The IOP lowering effects of MED were absent in the SX eyes of DB and NZW rabbits, but hypotensive responses to MED were present in normal (contralateral) eyes of both strains. In contrast, HA was effective only in the treated, normal eyes of SX NZW rabbits. These data support pharmacodynamic roles for alpha-2 and DA2 receptors in modulating IOP. The lack of activity by HA in DB rabbit eyes suggests a possible absence of DA2 receptors or excessive binding of HA to pigment in the anterior segment of DB rabbit eyes.

Biphasic dose-dependent effects of dopamine and involvement of dopamine autoreceptors on intra-ocular pressure in the rabbit

1. This work was conducted to provide new data concerning the possible dose-dependent activity of dopamine (DA) after ocular instillation. Experiments were done in rabbits with normal intraocular pressure (IOP), or after transitory induced ocular hypertension in water-loaded animals. 2. In ocular normotensive animals, a biphasic dose-dependent activity is observed, with no significant effect for 0.001% and 0.003% DA, a decrease in IOP after 0.005% and 0.01% DA instillation, then an important increase in IOP at concentrations from 0.05% onwards. 3. During transitory ocular hypertension, this phenomenon was confirmed, with a marked ocular hypotensive activity for 0.01% DA, no effect after 0.005% DA, then an important ocular hypertension with 0.05% and 0.5% DA as compared to the control group (0.9% NaCl). 4. An immediate and similar ocular hypertensive effect with DA could be reproduced by a subsequent instillation at high concentration (1%), while the hypotensive activity induced at low concentration (0.01%) is followed by a long-lasting refractory period (about 18 h). 5. Such a dose-dependent biphasic effect was also observed with N-methyl-dopamine (NMDA) after ocular instillation. The effects of instilled dopaminergic compounds were tested and ocular hypotensive activities of the S(-)enantiomer of the DA analogues 3-(3-hydroxyphenyl)-N-propylpiperidine (3-PPP), and of thiothixene (TIX) were also demonstrated. 6. The possible relationships to DA1 and DA2 receptors of the dual effect is discussed.