Dopamine receptors and intraocular pressure

Postnatal control of ocular growth: dopaminergic mechanisms

A vision-dependent feedback mechanism contributes to the regulation of postnatal eye growth and refraction; this mechanism is located at least in part in the retina. In chicks and rhesus monkeys, visual deprivation leads to ocular enlargement and a myopic refractive error, and it also reduces the retinal concentration of dopamine. In neonatal chicks, local application of the dopamine agonist apomorphine limits the excessive axial elongation that is associated with visual deprivation. Both D1 and D2 dopamine receptor mechanisms may participate. Remarkably, apomorphine is geometrically selective, not limiting the exaggerated equatorial growth that occurs during visual deprivation. Preliminary results in neonatal rhesus monkeys indicate that apomorphine eye drops also suppress exaggerated axial growth and myopic refractive error during visual deprivation; equatorial diameters were not measured. Our observations are consistent with a retinal hypothesis for emmetropization and suggest that retinal dopamine participates in a mechanism linking ocular growth control to vision. In addition, we conclude that axial and equatorial dimensions are independently regulated in the chick.

A novel D2-dopaminergic and alpha2-adrenoceptor receptor agonist induces substantial and prolonged IOP decrease in normotensive rabbits

The effects of a novel and selective D2-dopaminergic/alpha2-adrenoceptor agonist, CHF1035, and its metabolite CHF1024 on intraocular pressure (IOP) were determined in rabbits. Because CHF1035 is a mixture of two enantiomers, CHF1800 (+) and CHF1810 (-), pure enantiomers were also studied to determine possible differences in IOP-decreasing ability depending on the stereochemistry of the molecule. CHF1035, CHF1800 (+), CHF1810 (-), CHF1024, brimonidine and 0.9% NaCl were administered topically to rabbits and IOP was then measured at fixed time intervals. The dose-response profile (0.01-1.0% w/v) was determined for CHF1035. CHF1035 and its metabolite CHF1024 significantly lowered IOP in the treated eyes. CHF1035 showed a maximum IOP decrease (7.6 +/- 1.5 mmHg) 5 h post-dosing, whereas the metabolite CHF1024 showed a maximum decrease in IOP (7.0 +/- 0.8 mmHg) 3 h post-dosing. The maximum IOP decrease produced by CHF1035 in the treated eye was comparable with that produced by brimonidine (7.8 +/- 0.9 mmHg), but CHF1035 had a significantly longer duration of action. Unlike brimonidine, CHF1035 and CHF1024 did not decrease IOP in the untreated eye. CHF1810 (-) lowered the IOP more than CHF1800 (+). No irritation, evaluated as eyelid closure, was observed after topical administration of any of the compounds. Only in the case of CHF1035 1% solution, two rabbits out of six closed the eye for 30-45 s. In conclusion, CHF1035 and its metabolite CHF1024 significantly decreased the IOP in rabbits, and are potential novel IOP lowering agents. Especially, CHF1035 produced a substantial decrease in IOP for a prolonged period of time, and thus may prove useful in glaucoma therapy.

Effects of topical natural ergot alkaloids on intraocular pressure and aqueous humor dynamics in ocular normotensive rabbits

Although it has been suggested that ergot derivatives may play a role in antiglaucoma therapy, little attention has been paid to the ocular hypotensive action of these drugs. Having previously reported in our laboratory that topical dihydroergocristine decreases intraocular pressure both in ocular normotensive and alpha-chymotrypsin-induced ocular hypertensive rabbits, the aim of the present work was to assess the effect of natural ergot alkaloids, ergocristine, alpha-ergocryptine and ergocornine, on the intraocular pressure and aqueous humor dynamics in ocular normotensive rabbits in order to further explore the ocular actions of these compounds. Intraocular pressure was measured with a pneumatonometer manometrically calibrated for the rabbit eye. Changes in tonographic facility of aqueous humor outflow and rate of aqueous humor inflow were evaluated in anesthetized rabbits. Natural ergot alkaloids were found to reduce intraocular pressure in ocular normotensive eyes in a dose-related fashion. These compounds decreased both tonographic outflow facility and, to a greater extent, aqueous humor inflow, which explains their final hypotensive effect.

Intraocular pressure and vascular effects of sodium azide in bovine perfused eye

The effects of the nitrovasodilator, sodium azide, on intraocular pressure (IOP) and ciliary vascular tone were compared. IOP was measured in the bovine isolated eye that was perfused via the ciliary artery. Separately, vasodilator effects were assessed after raising the vascular tone using noradrenaline (10 microM). Aqueous humor formation (AHF) rate was estimated by a fluorescein dilution method. Cyclic GMP in the ciliary processes was measured by radioimmunoassay. When compared with controls, sodium azide (10 nmole bolus dose) was found to lower IOP (2.2 +/- 0.3 mm Hg; P < 0.01) via a reduction in AHF (12.19 +/- 0.26 microl/min to 6.36 +/- 0.53 microl/min; P < 0.001). Azide (1 micromole) also reduced ciliary vascular resistance (81.0 +/- 5.5%; P < 0.01). However, the drug was 20x more potent as an ocular hypotensive than as a vasodilator (ED50 0.28 nmole on IOP, 5.55 nmole on vascular effect). Azide (10 nmole) also increased levels of ciliary cyclic GMP (127 +/- 17 fmol/mg protein to 233 +/- 27 fmol/mg protein; P < 0.01). The IOP-lowering effect of azide does not appear to depend on its ability to activate guanylyl cyclase (GC) in vascular smooth muscle, but rather is likely a consequence of direct activation of ciliary epithelial GC.

Effects of dopamine D-1 and D-2 receptors on intraocular pressure in conscious rabbits

This investigation was designed as a randomized, placebo-controlled, double-masked, crossover study in NZW rabbits with normal intraocular pressure (IOP) to investigate dopaminergic effects on IOP. SKF 38393, a selective D1-receptor agonist, increased, and SDZ PSD-958, a selective D1-receptor antagonist, decreased IOP, respectively. The selective D2-receptor agonist quinpirole decreased IOP, whereas the selective D2 receptor antagonist metoclopramide had no significant effect. Combinations of quinpirole with SDZ PSD-958 decreased IOP in an additive manner. SDZ GLC-756, a mixed D1-receptor antagonist/D2-receptor agonist, decreased IOP in a dose-dependent manner with a maximum effect greater than the maximum effects produced either by the D1-receptor antagonist SDZ PSD-958 and the D2-receptor agonist quinpirole. The effect of SDZ GLC-756 could only be partially blocked by the selective D2-receptor antagonist metoclopramide suggesting that both D1-receptor blockade and D2-receptor stimulation participate in its IOP-lowering effect. Tonography suggests that SDZ GLC-756 has no significant effect on outflow facility. Furthermore, the results suggest that both D1 and D2 receptors each play an independent role in the regulation of IOP in rabbits. Thus, simultaneous blockade of D1 receptors and stimulation of D2 receptors may provide a new pharmacological approach for the treatment of ocular hypertension frequently associated with glaucoma.

The novel dopamine D-1 antagonist and D-2 agonist, SDZ GLC-756, lowers intraocular pressure in healthy human volunteers and in patients with glaucoma

PURPOSE

To investigate the intraocular pressure (IOP)-reducing effect and side effects of SDZ GLC-756, a novel dopamine D-1 antagonist and D-2 agonist, in control subjects and patients with open-angle glaucoma or ocular hypertension.

METHODS

A single-application, randomized, placebo-controlled, double-masked study was performed. SDZ GLC-756 eye drops at a concentration of 0.01% (n = 6) or 0.1% (n = 6) were administered to one eye in control subjects, vehicle alone (control) to the contralateral eye. In glaucoma patients the trial was designed as a crossover study administering SDZ GLC-756 (0.1%) eye drops and vehicle alone (control) to one eye in 12 patients with open-angle glaucoma or ocular hypertension.

RESULTS

In control subjects, IOP decreased significantly by 2.0 +/- 0.4 mmHg (P < 0.01) after treatment with 0.01% SDZ GLC-756 and by 4.7 +/- 0.5 mmHg (P < 0.001) after treatment with 0.1% SDZ GLC-756. In the glaucoma group, IOP decreased by 6.8 +/- 0.6 mmHg (P < 0.001) (range, 4-11 mmHg). In both groups, treatment with 0.1% eye drops resulted in a significant IOP-lowering effect for 6 hours, and for 3 hours with treatment with 0.01% eye drops. A significant IOP-lowering effect (P < 0.05) also was found in the contralateral eye. Except for slight conjunctival hyperemia lasting up to 30 minutes after drug application, no ocular or systemic side effects were observed.

CONCLUSION

SDZ GLC-756 reduced IOP significantly for approximately 6 hours. However, due to the study design with repeated IOP measurements before drug administration, the clinical efficacy of the preparation used remains unclear. The only systemic or local side effect observed was slight conjunctival hyperemia. The simultaneous D-1 antagonistic and D-2 agonistic properties of SDZ GLC-756 may provide a new pharmacologic approach to treating glaucoma and ocular hypertension, which deserves further investigation.

Drug effects on intraocular pressure and vascular flow in the bovine perfused eye using radiolabelled microspheres

A novel technique is described in which the effect of the beta-adrenoceptor antagonists timolol and carteolol, and the vasodilators sodium nitroprusside (SNP) and verapamil on intraocular pressure (IOP) and the distribution of ocular flow in the bovine arterially perfused eye is investigated using radiolabelled microspheres. At maximum IOP-reducing dose timolol was found to significantly reduce perfusion in the choroid and, at higher dose, it was found to significantly reduce perfusion in the iris. By contrast, a maximal IOP-reducing dose of carteolol markedly reduced perfusion in the iris, ciliary body and choroid. Vasoconstriction induced by carteolol was not inhibited by the alpha-antagonist phentolamine. Against a background of vascular tone induced by noradrenaline, SNP and verapamil were found to significantly increase perfusion in the iris, ciliary body and choroid. The effects of these drugs upon the vasculature of the bovine perfused eye are varied and complex and may not bear a direct relationship to their ocular hypotensive effect.

Ocular hypotension induced by topical dopaminergic drugs and phosphodiesterase inhibitors

The aim of this work was to investigate the ocular hypotensive activity of some topically administered cAMP-phosphodiesterase inhibitors alone and in combination with dopaminergic compounds. Experiments were performed with ocular normotensive rabbits and during transitory induced ocular hyper- or hypotension. An ocular hypotensive effect was observed after instillation of aminophylline, dyphylline, pentoxifylline, caffeine, and iso-caffeine, but not following topical hydroxypropyl-1,3-dimethylxanthine. Dopaminergic compounds were also studied in order to be combined with phosphodiesterase inhibitors as ocular anti-hypertensive treatment. Significant ocular hypotensive activity was observed after topical application of trifluperidol, fluphenazine, thiothixene, and the S(-) enantiomer of 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP). Of the cAMP-phosphodiesterase inhibitors that were tested, pentoxifylline was the most interesting compound, with good ocular tolerance, significant reduction in intra-ocular pressure, and potential retinal microvascular benefits. After allowing adequate time for pentoxifylline to reach its maximal activity, trifluperidol or S(-)-3-PPP was also instilled. A more pronounced ocular hypotensive effect was then observed. The findings of this study may suggest that administration of eye-drops combining drugs acting by separate ways on second messengers involved in the regulation of intra-ocular pressure (e.g. cyclic AMP) could be used to reduce intra-ocular pressure during glaucoma.

Dopamine, dopaminergic drugs and ocular hypertension

The study refers to the clinical experiences performed with several D1 and D2 dopaminergic receptors agonists in 20 patients with high tension open angle glaucoma. The substances were administered topically as eye drops as well as an ocular eye bath. The parameter examined was intraocular pressure (IOP). The substances taken in consideration were: Dopamine, Ibopamine (dopamine analog), Fenoldopam and 3B90 (D1-receptor agonists) and Bromocriptine (dopaminergic agonist with higher affinity for D2 than for D1-receptors). It has been shown that all selective D1-receptors agonists induce a significant increase in IOP only in eyes with hydrodynamic disorders (p < 0.001). Such hypertensive effects could not be antagonized either by topically administered dopaminergic antagonists (Sulpiride, D2-receptors antagonist, and Haloperidol, non-selective dopaminergic antagonist) or by the pretreatment with the commonly used topical antiglaucomatous drugs. The only substance which proved able to inhibit the IOP increase induced by the D1-receptors agonists was the D1-selective antagonist SCH-23390, suggesting that IOP increase may be a result of a stimulation of the D1-receptors. The authors hypothesize that dopaminergic system may play a role in the regulation of aqueous humor hydrodynamics.

5 Centrally Acting Dopamine D2 Receptor Ligands: Agonists

The collected amount of research on D2 agonists is immense. The D2 research field has been, and still is, very dynamic. Despite this fact, only a few compounds have reached the clinic so far. At present there are clinical trials ongoing with partial D2 agonists possessing a range of intrinsic efficacies. Some of these agonists are tested for their potential effects in Parkinson's disease (high intrinsic efficacy), while others are tested for potential anti-psychotic effects (low intrinsic efficacy). An interesting possibility has arisen through the research on the synergism between D1 and D2 receptors. This could possibly be utilized in the alleviation of Parkinsonian symptomatology. The near future will show whether the compounds under evaluation hold promise for being new, valuable medicines for treating major diseases like Parkinson's disease and schizophrenia.