PD128,907 induces ocular hypotension in rabbits: involvement of D2/D3 dopamine receptors and brain natriuretic peptide.

The purpose of this study was to determine the potential role of brain natriuretic peptide (BNP) in the PD128,907 (a dopamine D2/D3 receptor agonist)-induced ocular hypotension in rabbits. The effects of topical application of PD128,907 (75, 250, 750 microg) on intraocular pressure (IOP) were investigated. The lowest dose (75 microg) did not alter IOP; while the higher doses (250 and 750 microg) reduced IOP bilaterally. The PD128,907 (250 microg)-induced ocular hypotension, which lasted 3 hours, could be blocked by raclopride (1000 microg), a dopamine D2/D3 receptor antagonist, as well as by sympathetic denervation. Aqueous humor inflow was reduced by intravitreal injection of PD128,907 (10 microg) by 67% at 1 and 2 hours, which then returned to baseline at 3 hours. Furthermore, topical application of PD128,907 (250 microg) elevated aqueous BNP levels by 3-fold at 30 minutes, 6-fold at 1 hour and 5-fold at 2 hours, which could be blocked by pretreatment with raclopride (250 microg). Taken together, PD128,907-induced ocular hypotension by activation of dopamine D2/D3 receptors. This action was associated with reduced aqueous humor inflow and increased aqueous BNP levels.

7-OH-DPAT-induced inhibition of norepinephrine release in PC12 cells.

The purpose of this study was to investigate mechanisms of suppression of norepinephrine release by 7-OH-DPAT, a dopamine D(2)/D(3) receptor agonist, in PC12 cells pretreated with nerve growth factor (NGF). 7-OH-DPAT caused inhibition of basal and K(+)-evoked norepinephrine release, which could be blocked by pretreatment with raclopride, a D(2)/D(3) receptor antagonist. Moreover, dopamine D(2) and D(3 )receptors were identified by immunocytochemistry. Expression of D(2), D(3), and D(4) mRNAs and their proteins were detected using RT-PCR and immunoblotting. Furthermore, 7-OH-DPAT produced no change in cGMP levels; however, 7-OH-DPAT inhibited forskolin-stimulated cAMP accumulation that was antagonized by pretreatment with raclopride. In addition, 7-OH-DPAT inhibited carbachol-induced Ca(2+) transient, conversely, 7-OH-DPAT had no effect on 4-aminopyridine-induced Ca(2+) transient. Taken together, suppression of cAMP accumulation and calcium mobilization by 7-OH-DPAT is involved in the inhibition of norepinephrine release through activation of dopamine D(2)/D(3) receptors.

CNP-induced changes in pHi, cGMP/cAMP and mRNA expression of natriuretic peptide receptors in human trabecular meshwork cells.

It has been demonstrated that natriuretic peptides lower intraocular pressure, however, the underlying cellular mechanism(s) mediating this response remain(s) to be determined. The purpose of this study was to investigate the effects of C-type natriuretic peptide (CNP) on pH(i), cGMP/cAMP and expression of atrial natriuretic peptide receptor (NPR-A), brain natriuretic peptide receptor (NPR-B) and C-type natriuretic peptide receptor (NPR-C), in HTM cells. At concentrations of 10(-7) M, CNP caused an acidification of pH(i). In addition, CNP caused a dose-dependent increase in cGMP formation and inhibition of forskolin-stimulated cAMP accumulation. These changes were not significantly altered in the absence of 10(-3) M isobutylmethylxanthine (IBMX). Treatment with the NPR-A antagonist, anantin, produced no influence on basal cGMP/cAMP levels, the CNP-stimulated cGMP accumulation and CNP-induced inhibition of forskolin-stimulated cAMP accumulation. However, CNP-induced reduction of forskolin-stimulated cAMP accumulation was inhibited by pretreatment with pertussis toxin (PTX). Furthermore, NPRB receptors were predominantly expressed and pretreatment with CNP (10(-7) M, 24hr) enhanced all NPR mRNAs expression which was not altered by higher concentrations or longer incubation. Results demonstrate that NPR-A, NPR-B and NPR-C receptors' expression can be up-regulated by CNP treatment. CNP activates NPR-B receptors preferentially to increase cGMP accumulation and acts through the PTX-sensitive cAMP-signaling pathway leading to a decrease in pH(i).

4-aminopyridine transiently increases intraocular pressure in rabbits.

The purpose of this study is to determine the mechanisms of action involved in the ocular hypertension induced by 4-aminopyridine (4-AP), a voltage-dependent potassium (K+) channel blocker, in rabbits. Topical application of 4-AP elevated intraocular pressure (IOP). This action caused increases in the aqueous flow rate as well as aqueous levels of protein and norepinephrine. In isolated iris-ciliary body preparations, 4-AP (0.01, 0.1, 1 mmol/l) caused dose-related increases in field-stimulated norepinephrine release by 43, 222 and 243%, respectively. Taken together, the IOP-elevating effect evoked by 4-AP was associated with enhancement of aqueous norepinephrine levels and norepinephrine release from sympathetic nerves of the iris-ciliary body. These results demonstrate that the 4-AP-sensitive K+ channels in sympathetic nerves and the ciliary epithelium are the potential sites of action of the 4-AP-induced ocular hypertension.

Ocular hypotension induced by electroacupuncture.

The purpose of this study was to investigate the effects of electroacupuncture (EA) on aqueous humor dynamics in rabbits. EA stimulation was performed through two acupuncture needles placed in close proximity to the sciatic nerve. The sites of needle entry were anesthetized. After 1 hr of EA stimulation, intraocular pressure (IOP) decreased and was accompanied by reductions of blood pressure and aqueous humor flow rate. The maximum reduction of IOP was 9 mmHg at 3 hr and decreases in norepinephrine and dopamine levels in aqueous humor occurred simultaneously. In addition, EA stimulation induced an 8-fold increase of endorphin levels in aqueous humor. Ocular hypotension induced by EA lasted for more than 9 hrs and was antagonized by naloxone pretreatment. Furthermore, the EA-induced ocular hypotension was reduced markedly in sympathetically denervated eyes compared with the response of intact, normal eyes. Antagonism of EA-induced ocular hypotension by naloxone, suppression of aqueous humor flow and catecholamine levels by EA and elevation of endorphin levels in aqueous humor by EA indicate that opioids/opiate receptors are involved in modulating ocular hydrodynamics in response to EA.

Naphazoline-induced neuroendocrine changes: increases in ANP and cGMP levels, but suppression of NE, 3H-NE, and cAMP levels in rabbit eyes.

The objective of this study was to determine whether naphazoline, an alpha 2 (alpha2)/imidazoline (I1) agonist, can alter endogenous levels of atrial natriuretic peptide (ANP) and norepinephrine (NE) in aqueous humor and cyclic nucleotide (cAMP, cGMP) accumulation and NE overflow in the iris-ciliary body (ICB) of the rabbit eye. Topical naphazoline (25, 75, and 250 microg) caused a dose-dependent elevation of the ANP levels (36, 54, and 137 pg/ml, respectively) in aqueous humor. This effect was antagonized by pretreatment with efaroxan, an antagonist of I1/alpha2 receptors. Another alpha2/I1 agonist, moxonidine (75 microg topically), caused significant increases in ANP levels in aqueous humor, whereas other relatively selective alpha2-adrenergic receptor agonists, brimonidine (50 microg topically) and oxymetazoline (75 microg topically), did not. In naphazoline (75 microg) pretreated eyes, the NE levels in aqueous humor were attenuated by 36% (from 6.0 to 3.8 pg/ml). Furthermore, naphazoline (1, 10, and 100 micromol/l) caused a dose-related inhibition of NE release from ICBs: 25, 45, and 80%, respectively. The isoproterenol (1 micromol/l) stimulated cAMP accumulation was inhibited 53% by naphazoline (100 micromol/l). In contrast, naphazoline significantly increased the cGMP levels in ICBs. These data demonstrate that naphazoline acts on I1 receptors to increase ANP and to reduce NE levels in aqueous humor. The former effect could also contribute to elevation of cGMP levels and inhibition of cAMP accumulation in the ICB. Further studies will be required to determine if elevation of ANP levels is a critical component of naphazoline-induced alteration of aqueous humor dynamics.

Biodegradable calcium phosphate nanoparticles as a new vehicle for delivery of a potential ocular hypotensive agent.

The purpose of this study was to determine the efficacy of a newly prepared formulation containing biodegradable calcium phosphate nanoparticles (CAP) and 7-hydroxy-2-dipropyl-aminotetralin (7-OH-DPAT) in pigmented and non-pigmented rabbits using the surrogate end points of intraocular pressure (IOP) and aqueous flow rate. IOP (mmHg) was measured by utilizing a manometrically calibrated Mentor pneumatonometer. Rates of aqueous humor flow were measured with a Fluorotron Master by estimating the dilution rate of fluorescein. In non-pigmented rabbits, the ocular hypotension induced by topical administration of 7-OH-DPAT (75 microg) with CAP (115 microg) was more pronounced and sustained than that of 7-OH-DPAT without CAP. Furthermore, IOP-lowering effects of topically administered 7-OH-DPAT (125 microg) alone were markedly diminished in pigmented rabbits compared to non-pigmented rabbits. However, topical application of 7-OH-DPAT formulated with CAP produced significant dose-related (37.5, 75, 125 microg) reductions of IOP accompanied by suppression of aqueous humor flow rates in pigmented rabbits. It is postulated that 7-OH-DPAT in vehicle without CAP binds to pigments in the anterior segment of the pigmented rabbit's eyes, and this binding limits the 7-OH-DPAT's action. Pretreatment with raclopride, a dopamine D2/D3 receptor antagonist, reduced the ocular hypotensive effect induced by 7-OH-DPAT in vehicle containing CAP thereby supporting the role for dopamine D2/D3 receptors in modulating IOP. It is concluded that CAP, as a delivery system, enhances activity by 7-OH-DPAT in pigmented rabbit eyes suggesting that CAP is potentially useful for achieving controlled and targeted drug delivery for treatment of ocular diseases.