Effects of alpha 1 and alpha 2 activation of adrenergic receptors on aqueous humor dynamics

Melatonin and the control of intraocular pressure

Melatonin is not only synthesized by the pineal gland but by several ocular structures. This natural indoleamine is of great importance for regulating several eye processes, among which pressure homeostasis is included. Glaucoma, the most prevalent eye disease, also known as the silent thief of vision, is a multifactorial pathology that is associated to age and, often, to intraocular hypertension (IOP). Indeed IOP is the only modifiable risk factor and as such medications are available to control it; however, novel medications are sought to minimize undesirable side effects. Melatonin and analogues decrease IOP in both normotensive and hypertensive eyes. Melatonin activates its cognate membrane receptors, MT₁ and MT₂, which are present in numerous ocular tissues, including the aqueous-humor-producing ciliary processes. Melatonin receptors belong to the superfamily of G-protein-coupled receptors and their activation would lead to different signalling pathways depending on the tissue. This review describes the molecular mechanisms underlying differential functionalities that are attributed to melatonin receptors. Accordingly, the current work highlights the important role of melatonin and its analogues in the healthy and in the glaucomatous eyes, with special attention to the control of intraocular pressure.

Targeting Serotonin 2A and Adrenergic α1 Receptors for Ocular Antihypertensive Agents: Discovery of 3,4-Dihydropyrazino[1,2-b]indazol-1(2H)-one Derivatives

Glaucoma affects millions of people worldwide and causes optic nerve damage and blindness. The elevation of the intraocular pressure (IOP) is the main risk factor associated with this pathology, and decreasing IOP is the key therapeutic target of current pharmacological treatments. As potential ocular hypotensive agents, we studied compounds that act on two receptors (serotonin 2A and adrenergic α₁ ) linked to the regulation of aqueous humour dynamics. Herein we describe the design, synthesis, and pharmacological profiling of a series of novel bicyclic and tricyclic N2-alkyl-indazole-amide derivatives. This study identified a 3,4-dihydropyrazino[1,2-b]indazol-1(2H)-one derivative with potent serotonin 2A receptor antagonism, >100-fold selectivity over other serotonin subtype receptors, and high affinity for the α₁ receptor. Moreover, upon local administration, this compound showed superior ocular hypotensive action in vivo relative to the clinically used reference compound timolol.

A common humoral background of intraocular and arterial blood pressure dysregulation

BACKGROUND

It has been postulated that intraocular pressure, an important glaucoma risk factor, correlates positively with arterial blood pressure (blood pressure). However, results of experimental and clinical studies are often contradictory. It is hypothesized that, in some hypertensive patients, disturbances in intraocular pressure regulation may depend on biological effects of blood borne hormones underlying a particular type of hypertension, rather than on blood pressure level itself.

REVIEW

This review compares the effects of hormones on blood pressure and intraocular pressure, in order to identify a hormonal profile of hypertensive patients with an increased risk of intraocular pressure surge. The PUBMED database was searched to identify pre-clinical and clinical studies investigating the role of angiotensin II, vasopressin, adrenaline, noradrenaline, prostaglandins, and gaseous transmitters in the regulation of blood pressure and intraocular pressure.

RESULTS

Studies included in the review suggest that intraocular and blood pressures often follow a different pattern of response to the same hormone. For example, vasopressin increases blood pressure, but decreases intraocular pressure. In contrast, high level of nitric oxide decreases blood pressure, but increases intraocular pressure.

CONCLUSIONS

Arterial hypertension is associated with altered levels of blood borne hormones. Contradicting results of studies on the relationship between arterial hypertension and intraocular pressure might be partially explained by diverse effects of hormones on arterial and intraocular pressures. Further studies are needed to evaluate if hormonal profiling may help to identify glaucoma-prone patients.

Anterior segment optical coherence tomography changes with introduction and discontinuation of tamsulosin

Purpose

The aim of this study was to quantify changes and reversibility in pupil dilation and iris dilator muscle region thickness associated with introduction and subsequent discontinuation of tamsulosin in patients naïve to this drug with the aid of an anterior OCT system.

Methods

The study was carried out on 7 patients (14 eyes) naïve to tamsulosin and with benign prostatic hypertrophy (BHP). Measurements taken by Vistante OCT were done pre- and post-dilation of the following: pupil size, iris dilator muscle region (DMR) thickness, sphincter muscle region (SMR) thickness, and anterior chamber depth. These measurement were taken at Day 0 (tamsulosin naive), Day 30 (after one month of tamsulosin, the treatment period) and day 60 (after one month of no tamsulosin, the discontinuation period).

Results

Post-dilation pupil diameter significantly increased during the discontinuation period (P = 0.047). Iris DMR thickness measurements post-dilation significantly decreased during treatment (P = 0.00044), discontinuation (0.00011), and combined periods (P = 0.000050). Anterior chamber depth measurements in post-dilation were significantly decreased during treatment (P = 0.0016), discontinuation (P = 0.017), and combined periods (P = 0.00022).

Conclusion

Tamsulosin discontinuation effectively increases dilated pupil size, a measure that has been inversely linked to IFIS incidence pre-operatively. Decreased DMR thickness in this short term likely illustrates changes aside from atrophy, such as vascular changes. Decreased anterior chamber depths suggest aqueous humor production is decreased as well.

Gene expression-based comparison of the human secretory neuroepithelia of the brain choroid plexus and the ocular ciliary body: potential implications for glaucoma

Background

The neuroepithelia of the choroid plexus (CP) in the brain and the ciliary body (CB) of the eye have common embryological origins and share similar micro-structure and functions. The CP epithelium (CPE) and the non-pigmented epithelium (NPE) of the CB produce the cerebrospinal fluid (CSF) and the aqueous humor (AH) respectively. Production and outflow of the CSF determine the intracranial pressure (ICP); production and outflow of the AH determine the intraocular pressure (IOP). Together, the IOP and ICP determine the translaminar pressure on the optic disc which may be involved in the pathophysiology of primary open angle glaucoma (POAG). The aim of this study was to compare the molecular machinery of the secretory neuroepithelia of the CP and CB (CPE versus NPE) and to determine their potential role in POAG.

Methods

We compared the transcriptomes and functional annotations of healthy human CPE and NPE. Microarray and bioinformatic studies were performed using an Agilent platform and the Ingenuity Knowledge Database (IPA).

Results

Based on gene expression profiles, we found many similar functions for the CPE and NPE including molecular transport, neurological disease processes, and immunological functions. With commonly-used selection criteria (fold-change > 2.5, p-value < 0.05), 14% of the genes were expressed significantly differently between CPE and NPE. When we used stricter selection criteria (fold-change > 5, p-value < 0.001), still 4.5% of the genes were expressed differently, which yielded specific functions for the CPE (ciliary movement and angiogenesis/hematopoiesis) and for the NPE (neurodevelopmental properties). Apart from a few exceptions (e.g. SLC12A2, SLC4A4, SLC4A10, KCNA5, and SCN4B), all ion transport protein coding genes involved in CSF and AH production had similar expression profiles in CPE and NPE. Three POAG disease genes were expressed significantly higher in the CPE than the NPE, namely CDH1, CDKN2B and SIX1.

Conclusions

The transcriptomes of the CPE and NPE were less similar than we previously anticipated. High expression of CSF/AH production genes and candidate POAG disease genes in the CPE and NPE suggest that both might be involved in POAG.

α2-Adrenergic Receptors Are Present in Normal Human Conjunctiva

PURPOSE

Despite the passage of medications, including antiglaucoma drugs, through the ocular surface, and despite the increasing relevance of neurogenic inflammation in the ocular surface, the presence of some neuroreceptors in the conjunctiva has not been ascertained. This study describes the presence of alpha2-adrenergic receptors in normal human conjunctiva.

METHODS

Immunofluorescence microscopy, electrophoresis, and Western blot analyses were done in human conjunctival biopsies and rat control tissues. Antibodies against alpha2-adrenergic receptor subtypes alpha2A, alpha2B, and alpha2C were used.

RESULTS

Immunoreactivity for alpha2A- and alpha2B-, but not alpha2C-adrenergic receptors was evenly distributed in epithelial cells of human conjunctiva cryosections. Immunoreactive bands were detected for the three alpha2-adrenergic receptor subtypes: a major band of 48-50 kDa and fellow bands of 65-67 kDa.

CONCLUSIONS

Normal human conjunctival epithelial cells express alpha2A-, alpha2B-, and alpha2C-adrenergic receptors. Further studies to determine the functional implications of these receptors in ocular surface homeostasis are warranted.

Intravenous administration of clonidine reduces intraocular pressure and alters ocular blood flow

AIM

To evaluate the effect of intravenously administered clonidine on ocular blood flow in healthy volunteers.

METHODS

A randomised, double-masked, placebo-controlled, two-way crossover study was performed in 12 healthy young volunteers. Clonidine (0.2 microg/kg/min) or placebo was administered intravenously over 10 minutes. The effects of clonidine were studied at baseline and up to 150 minutes after infusion. Ocular haemodynamics were measured using laser Doppler flowmetry, laser Doppler velocimetry and a retinal vessel analyser.

RESULTS

Clonidine significantly decreased mean arterial pressure (MAP) and intraocular pressure (IOP). Calculated ocular perfusion pressure decreased significantly by -8.7+/-8.7% after infusion of clonidine (p<0.01 vs placebo). Retinal arterial diameters increased by +4.4+/-2.7% (p = 0.012 vs placebo), whereas no significant change was observed in retinal veins. Red blood cell velocity decreased by -16+/-14% (p<0.01 vs placebo) after infusion of clonidine. Hence, calculated retinal blood flow decreased by -14+/-12% (p = 0.033 vs placebo). Choroidal blood flow increased by +18+/-19% (p<0.01 vs placebo) and optic nerve head blood flow increased by +16+/-23% (p = 0.046 vs placebo) 30 minutes after administration of clonidine but both returned to baseline thereafter.

CONCLUSION

The short-time increase in choroidal and optic nerve head blood flow indicates a transient vasodilatory effect of clonidine due to an unknown mechanism. The decrease in retinal blood flow indicates clonidine-induced vasoconstriction in the retinal microvasculature.

Efficacy of Bunazosin Hydrochloride 0.01% as Adjunctive Therapy of Latanoprost or Timolol

PURPOSE

To evaluate the ocular hypotensive response of bunazosin hydrochloride 0.01% administered as adjunctive therapy in patients with glaucoma who were already receiving latanoprost 0.005% or timolol 0.5%.

METHODS

Patients with primary open angle glaucoma who had received latanoprost (n = 60) or timolol (n = 60) for 6 months or longer were enrolled and prospectively randomized to receive additional administration of bunazosin or placebo. One hundred twenty eyes of 120 patients were thus divided into 4 subgroups of 30 patients each. Bunazosin was administered twice daily, and timolol or latanoprost was administered per label. The patients were followed up for 3 months. Responders were defined as having a reduction in intraocular pressure of greater than 2 mm Hg from baseline.

RESULTS

Mean baseline intraocular pressure was 22.3 +/- 3.0 mm Hg in the bunazosin subgroup and 22.3 +/- 3.1 mm Hg in the placebo subgroup of the latanoprost arm, and 22.5 +/- 3.5 mm Hg in the bunazosin subgroup and 22.3 +/- 3.0 mm Hg in the placebo subgroup of the timolol arm. In the bunazosin subgroups of both arms, intraocular pressure was significantly reduced compared with baseline measurements (P < 0.05) with mean intraocular pressure measurement reductions of 2.1 +/- 2.4 mm Hg and 2.8 +/- 2.1 mm Hg in the latanoprost arm and 2.6 +/- 2.1 mm Hg and 2.8 +/- 2.1 mm Hg in the timolol arm at 6 and 12 weeks after the start of the follow-up, respectively. In the latanoprost group, bunazosin provided a further reduction of intraocular pressure (7.7%) at 12 weeks from that initially obtained at 2 weeks (P = 0.0377). In the placebo subgroups of the latanoprost and timolol arms, no significant change was found between at baseline and at any visit after the start of the follow-up. In the latanoprost and timolol arms, there was a significant difference in intraocular pressure and its change between the bunazosin subgroup and placebo subgroup at any visit after 4 weeks from the start of the follow-up (P < 0.01).

CONCLUSION

Bunazosin hydrochloride 0.01% may provide an additional intraocular pressure reduction in patients already receiving latanoprost or timolol. Since adding bunazosin to eyes treated with latanoprost caused a relatively small hypotensive response at 2 weeks and provided a further reduction from 2 weeks to 12 weeks, longer than 4 weeks may be required to evaluate a clinically meaningful response to treatment. Further investigation on more cases and longer follow-up are needed.

Effects of multiple dosing of epinephrine on aqueous humor dynamics in human eyes

Numerous studies have provided conflicting evidence to explain the ocular hypotensive mechanism of action of epinephrine. Although epinephrine has been shown consistently to increase outflow facility, its effects on aqueous flow and uveoscleral outflow are not as clear. The purpose of this study was to clarify the effects of multiple doses of topical epinephrine on aqueous humor dynamics in human eyes. This was done by evaluating the four main parameters that determine steady state intraocular pressure. These parameters were assessed at baseline and after a week of twice-daily treatment of epinephrine hydrochloride 2% to one eye. Twenty-six human volunteers were enrolled in the study. Intraocular pressure was measured by pneumatonometry, aqueous flow and trabecular outflow facility by fluorophotometry, episcleral venous pressure by venomanometry and uveoscleral outflow by mathematical calculation. In epinephrine-treated eyes compared to baseline, intraocular pressure and aqueous flow were reduced from 21.2 +/- 0.3 to 17.1 +/- 0.2 mmHg (19%, p = .01) and 3.3 +/- 0.2 to 2.9 +/- 0.2 microl/min (12%, p = .03), respectively. Trabecular outflow facility obtained by fluorophotometry was increased from 0.18 +/- 0.02 to 0.26 +/- 0.03 microl/min/mmHg (44%, p = .02). Topical epinephrine did not significantly affect uveoscleral outflow or episcleral venous pressure. In conclusion, multiple topical doses of epinephrine lowered intraocular pressure in human volunteers by reducing aqueous humor formation and increasing trabecular outflow facility. The increase in uveoscleral outflow suggested by other studies was not observed.

Adrenergic regulation of calcium-activated potassium current in cultured rabbit pigmented ciliary epithelial cells

1. The effects of adrenergic agonists on K+ currents were studied in cultured rabbit pigmented ciliary epithelial (PCE) cells. 2. Outward K+ current (IK) was reduced by tetraethylammonium chloride, the Ca2+-activated K+ (K(Ca)) channel blocker iberiotoxin (IbTX), or Ca2+-free external Ringer solution. The calcium ionophore ionomycin increased an IbTX-sensitive IK in PCE cells. 3. The adrenergic agonists adrenaline and phenylephrine increased IK in PCE cells. The induced current was blocked by IbTX and the alpha1-antagonist prazosin, suggesting that adrenergic agonists activate IK(Ca) via alpha1-adrenoreceptors. 4. Internal dialysis of D-myo-inositol 1,4, 5-trisphosphate (IP3) increased IK, whilst pre-incubation of PCE cells with thapsigargin or the phospholipase C (PLC) inhibitor U-73122 reduced phenylephrine-induced increases in IK(Ca). Adrenergic increases in IK(Ca) were mediated by a pertussis toxin-insensitive G protein. 5. These results demonstrate that IK(Ca) channels in rabbit PCE cells are coupled to alpha1-adrenergic receptors and a PLC/IP3 signalling pathway. Activation of these channels may modulate fluid secretion by the ciliary epithelium.

Bunazosin reduces intraocular pressure in rabbits by increasing uveoscleral outflow

The mechanism of the ocular hypotensive effect of bunazosin hydrochloride (an alpha1-adrenergic antagonist) and the possible intermediary role of prostaglandins were studied in New Zealand albino rabbits. Aqueous flow, outflow facility and uveoscleral outflow were determined by fluorophotometry, and intraocular pressure (IOP) was measured by pneumatonometry on the fourth day of twice daily topical treatment with 0.1% bunazosin. Uveoscleral outflow was measured with a tracer infusion technique at 1 to 2 hours after one dose of 0.1% bunazosin. Total outflow facility was measured by a two-level constant-pressure infusion method before and at one hour after one dose of 0.1% bunazosin. The effect of topically applied cyclooxygenase inhibitors, including 0.25% indomethacin and 0.03% flurbiprofen, on the IOP reduction after bunazosin was evaluated. At 3 hours after the seventh consecutive dose given twice-daily, bunazosin significantly (P<0.001) reduced IOP to 13.4+/-0.8 mm Hg (mean +/- SEM) from a baseline of 19.6+/-1.1 mm Hg. Indomethacin significantly inhibited the IOP reduction after one dose of bunazosin, whereas flurbiprofen did not (repeated measures ANOVA). Bunazosin significantly increased uveoscleral outflow (P<0.05) and total outflow facility (P<0.02), but not fluorophotometric outflow facility or aqueous flow. It is concluded that, in rabbits, 0.1% bunazosin reduces IOP predominantly by increasing uveoscleral outflow. The role of prostaglandins in this effect is equivocal.

Ocular responses in healthy subjects to topical bunazosin 0.3%--an alpha 1-adrenoceptor antagonist

The ocular effects of the alpha-adrenoceptor blocking drug bunazosin, administered as eyedrops, have been measured in a placebo-controlled double-blind single-dose study in 15 healthy volunteers. The drug significantly reduced intraocular pressure over 10 hours, and there was no associated change in pulsatile ocular blood flow. Characteristic effects of alpha-adrenoceptor blockade were observed--miosis, ptosis, and conjunctival hyperaemia. The miosis alone persisted for more than 24 hours in nine out of 15 subjects.

Corneal and conjunctival/scleral penetration of p-aminoclonidine, AGN 190342, and clonidine in rabbit eyes

The ocular penetration pathways of three alpha 2-adrenergic agents (p-aminoclonidine, AGN 190342, and clonidine) were investigated in rabbits both in vitro and in vivo. The corneal permeabilities of the compounds correlated positively with their octanol/water distribution coefficients. The ocular drug absorption via corneal and conjunctival/scleral penetration routes was evaluated separately after drug perfusion in vivo. In most cases, the corneal route was the major pathway for the intraocular drug absorption. However, the conjunctival/scleral penetration pathway was the predominant pathway for the delivery of p-aminoclonidine, the least lipophilic compound among the three drugs, to the ciliary body. The drug concentration in the iris was contributed mainly by the corneal route and correlated well with drug lipophilicity.

Development of D-timolol for the treatment of glaucoma and ocular hypertension

It has been found that D-timolol is equipotent or slightly less potent than L-timolol to lower the intraocular pressure (IOP) in normotensive rabbits, water loaded ocular hypertensive rabbits, alpha-chymotrypsin induced glaucoma rabbits, hypertonic saline infused IOP recovery model of rabbits, normotensive human volunteers, glaucoma patients and ocular hypertensive human individuals. Although L-timolol has been used widely for the treatment of glaucoma and ocular hypertension, it produces numerous side effects including cardiovascular disturbances, asthmatic attack, psychological depression, etc. Since D-timolol has much weaker affinity toward beta-adrenergic receptors, it was found to have 1/80-1/300 the beta-adrenergic blocking potency of L-timolol to block beta-adrenergic receptors in guinea pig tracheal preparations and 1/90 of L-timolol to block beta-adrenergic receptors in guinea pig atrial preparations. As a result, D-timolol showed no subjective nor objective side effects on pupil size, conjunctiva, cornea, blood pressure and pulse rate. Further, D-timolol was reported to increase retinal and choroid blood flow in rabbits without affecting overall ocular blood flow. On the contrary, L-timolol was found to significantly reduce the overall ocular blood flow and retinal and choroid blood flows in rabbits, although it might slightly increase the retinal blood flow in normotensive individuals. D-Timolol was well absorbed across the cornea as L-timolol and produced the duration of action as long as L-timolol. These results indicate that D-timolol could be a better agent than L-timolol for the treatment of glaucoma and ocular hypertension.

Effects of D-timolol and L-timolol on ocular blood flow and intraocular pressure

Effects of D-timolol and L-timolol on IOP were compared with two rabbit models. When the drug solution was injected into vortex vein, 1% D-timolol produced ocular hypotension just like 0.5% L-timolol except D-timolol was less potent than L-timolol to lower the IOP. On the other hand, when 0.5% of D-timolol and L-timolol were instilled into the rabbit eye on IOP recovery model both agents showed equipotency to delayed the IOP recovery. Effects of D-timolol and L-timolol on ocular blood flow were also studied with two rabbit models. D-Timolol at 0.5% did not affect the ocular pulsatile blood flow measured with Langham's pneumatonometer whereas 0.5% L-timolol significantly suppressed it. D-Timolol (0.5%) was found to increase retinal and choroidal blood flows measured with laser Doppler method whereas L-timolol suppressed it. These results indicate that D-timolol though less potent than L-timolol to lower IOP, is superior over L-timolol to improve the blood flow in retina and choroid.

Review of alpha adrenoceptor function in the eye

The location and function of alpha adrenoceptors in the eye are reviewed with emphasis on pharmacological agents and their role in the management of chronic simple glaucoma.

Neurotransmitters and intraocular pressure

The role of the ocular autonomic nervous system in IOP regulation has been well established. Pharmacological and autohistoradiographic studies confirmed the high density of beta 2 and alpha 2 receptors on ciliary processes and iris epithelium. Their respective pharmacological activation or blockade is discussed. The role of other ocular neurotransmitters is also complex, as shown by the paradoxical similar action of dopamine agonists and antagonists on IOP. Concerning the cholinergic system, ocular muscarinic receptors are pharmacologically not well documented. Numerous other neurotransmitters may modulate IOP without necessarily leading to the development of new drugs. Drugs of the future will probably concentrate on dopaminergic agonists, cAMP-stimulators such as forskolin, prostaglandins, and cannabinoids.

Alpha 1-adrenoceptors in the albino rabbit ciliary process

3H-Prazosin and 3H-rauwolscine binding sites were identified in a membrane suspension prepared from albino rabbit iris + ciliary body. Scatchard analysis of saturation binding experiments demonstrated that both 3H-prazosin and 3H-rauwolscine bind to a single population of binding sites with KD values of 0.87 nM and 5.33 nM, respectively. Bmax values of 65.7 and 198 fmol/mg protein were obtained for 3H-prazosin and 3H-rauwolscine, respectively. Displacement studies by several adrenergic agonists and antagonists indicated that 3H-prazosin and 3H-rauwolscine labelled alpha 1- and alpha 2-adrenoceptors, respectively, in the iris + ciliary body. Epinephrine, norepinephrine and phenylephrine were able to stimulate the synthesis of 3H-inositol phosphates in ciliary processes labelled with 3H-inositol, with EC50 values of 2.4, 12 and 10 microM, respectively. The corresponding maximum stimulations of basal activity were 433, 430 and 283%, respectively. Phenylephrine behaved like a partial agonist in this assay. The norepinephrine response could be potently antagonized by prazosin (Ki = 27 nM), with rauwolscine being 285-fold less potent. An epithelial cell suspension was prepared from the ciliary process. Stimulation of phosphatidylinositol turnover by norepinephrine (0.1 mM) was observed, and this could be blocked by prazosin (10 microM), thus, indicating the presence of alpha 1-adrenoceptors, coupled to phosphatidylinositol turnover, in epithelial cells of the rabbit ciliary process.

Alpha-2 adrenergic modulation of norepinephrine secretion in the perfused rabbit iris-ciliary body

Clonidine and other selective alpha-2 adrenergic agonists have been found to lower intraocular pressure in the eyes of rabbits and primates, including humans. It has been suggested that the ocular hypotensive response to alpha-2 agonists may be mediated, in part, by prejunctional inhibition of norepinephrine secretion at intraocular synapses. In this study, we have investigated the effects of adrenergic agonists and antagonists on field-stimulated, Ca++-dependent release of 3H-norepinephrine (3H-NE) from isolated, perfused rabbit iris-ciliary bodies and have utilized radioligand binding methods to identify prejunctional adrenoceptors in this tissue. Clonidine (10(-9)-10(-5) M) produced a dosage-dependent inhibition of stimulation-evoked 3H-NE secretion (EC50 approximately equal to 3 X 10(-8) M), but did not alter basal secretion. Other adrenergic agonists capable of activating alpha-2 adrenoceptors (e.g., epinephrine, norepinephrine and xylazine) also significantly depressed 3H-NE secretion, whereas selective alpha-1 adrenergic or beta adrenergic agonists were without effect. Clonidine-mediated inhibition of 3H-NE release was reversed by the selective alpha-2 antagonist yohimbine (10(-7) M), but was unaffected by prazosin or timolol. Yohimbine alone markedly enhanced 3H-NE secretion, indicating tonic activation of prejunctional alpha-2 adrenoceptors by endogenous released norepinephrine. Forskolin or 8-bromo-cAMP, which alone enhanced norepinephrine secretion, failed to attenuate the inhibitory responses to alpha-2 agonists. 3H-rauwolscine binding measurements showed a small decrease in alpha-2 receptor sites in iris-ciliary body membranes following surgical sympathetic denervation. It is concluded that the rabbit iris-ciliary body contains functional, prejunctional alpha-2 adrenoceptors which may play an autoregulatory role in vivo and contribute to the ocular effects of adrenergic drugs.

Alpha-adrenergic antagonists: correlation of the effect on intraocular pressure and on alpha 2-adrenergic receptor binding specificity in the rabbit eye

Six alpha-adrenergic antagonists, which have a range of selectivities for alpha 1- and alpha 2-adrenoreceptor subtypes, were compared with respect to their ability to reduce intraocular pressure (IOP) after topical application to the rabbit eye, and their affinity and selectivity for alpha 2-adrenoreceptors, as determined by binding to membranes prepared from rabbit iris-ciliary body. A routine assay for alpha 2-adrenoreceptors using [3H]-rauwolscine was developed for this purpose. ICB contained 200-300 fmol (mg protein)-1 alpha 2-adrenoreceptors which represents approximately two-thirds of the total number of alpha-adrenoreceptor sites present in this tissue. All six antagonists bound at alpha 2-adrenergic receptor sites in an apparently simple competitive manner. The Kd for three of the drugs was about 10 nM (rauwolscine, yohimbine, WB-4101) and the Kd for the other three was greater than 3500 nM (prazosin, corynanthine, thymoxamine). However, all six antagonists were effective ocular hypotensive agents when given topically in a 50 microliter dose of 1% (w/v) concentration. The ability of alpha-adrenergic antagonists to lower IOP in the rabbit did not correlate with a single alpha-receptor subtype and appears to involve at least two separate mechanisms, one mediated by alpha 2-adrenergic receptors and one mediated by alpha 1-adrenergic receptors.

A synopsis of recent developments in antiglaucoma drugs

Open-angle glaucoma is treated primarily with drugs, some of which have been used clinically for years. These drugs include: 1) cholinergic agonists that increase aqueous humor outflow, 2) adrenergic agonists and antagonist that affect both aqueous humor formation and outflow, and 3) carbonic anhydrase inhibitors that decrease aqueous humor formation. Several new classes of drugs are being tested for efficacy and mechanism of action. They include: 1) the D-isomer of timolol that reduces aqueous humor formation without producing adrenergic blockade, 2) dopaminergic agonists and antagonists, including bromocriptine and butyrophenones that reduce intraocular pressure, and 3) cannabinoids that reduce aqueous humor formation and increase outflow. In addition, several other types of drugs, such as prostaglandins, diuretics, Na+,K+-ATPase inhibitors, and adenyl cyclase stimulators are just now beginning to be studied.

Cellular mechanisms in the actions of antiglaucoma drugs

There are several classes of drugs currently in use for the therapeutic management of the glaucomas. Although the ocular hypotensive effects of these agents have been well characterized and described, little is known of their site of action and cellular mechanism. This review attempts to describe those cellular mechanisms that may be linked to the actions of several classes of antiglaucoma drugs. Special emphasis was placed on drug actions and 1) the adenylate cyclase system; 2) receptor-coupled phosphoinositide turnover; 3) prostaglandins and 4) ion transport processes. Models are presented depicting proposed cellular sites of the interaction of the antiglaucoma drugs with these cellular processes.

The ocular hypotensive action of SK&F 86466 in the conscious rabbit

SK&F 86466, 6-chloro-3-methyl-2, 3, 4, 5-tetrahydro-1H-3-benzazepine, a potent and selective competitive antagonist of the alpha 2-adrenoceptor is of interest as a potential antiglaucoma agent. Following instillation into the rabbit eye, SK&F 86466 produced concentration (0.25 to 10%) dependent reductions in intraocular pressure (IOP). The ocular hypotensive effect of SK&F 86466 was maximal 30-60 minutes post treatment and persisted for at least 90 minutes. The IOP of the contralateral (untreated) eye was significantly decreased only at the 10% concentration. A 10% solution of SK&F 86466 had no significant effect on mean arterial blood pressure (MAP) of rabbits during the time of maximal bilateral ocular hypotensive activity. These results suggest: (1) a selective alpha 2-adrenoceptor antagonist can reduce IOP when applied topically to rabbit eyes and (2) the ocular hypotensive effect of SK&F 86466 is not secondary to a reduction in systemic blood pressure.

Increases in volume, fluid content, and lens weight of eyes following systemic administration of melatonin

Melatonin's effects were studied in male golden hamsters (Mesocricetus auratus) distributed among five surgical groups (nonoperated, sham-pinealectomized, sham-pinealectomized plus black plastic shielding of the pineal region, pinealectomized, and pinealectomized plus black plastic shielding of the pineal region) and three injection groups (vehicle only, 25 micrograms melatonin, and 2,500 micrograms melatonin). Injections (s.c.) were daily for 28 d at L11 to L11.75 in a (light:dark) L:D 14:10 artificial photoperiod. Animals (N = 112) were killed and dissected on the day after the last injection (at 55-65 d of age). None of the surgical procedures affected weights of eyes or their parts, nor did they influence the effects of administered melatonin on the eyes. Melatonin caused an increase in absolute and relative eye weight and an increase in fluid content of intraocular space. The magnitudes of these effects were positively related to melatonin dose. These same eyes had a progressively lower weight of nonlenticular tissues with low to high doses of melatonin, probably in relation to greater fluid content, and suspected increase in intraocular pressure. Lens wet and dry weights were significantly greater in animals receiving melatonin, but only at the high dose. These actions of melatonin are likely to be direct and are shown to not require the presence of the pineal. Experiments of other designs are suggested in order to determine whether the effects of the low, near physiological, dose of melatonin represent physiological actions of endogenous melatonin, synthesized and released within the eye. However, effects of large doses of melatonin on the eye are still noteworthy in relation to interpretation of experiments employing such dosages, and of disease states involving changes in intraocular pressure.