Pharmacological evidence for heterogeneity of ocular alpha 2 adrenoceptors.

Previous studies have shown that ocular alpha 2 adrenoceptors are located prejunctionally on sympathetic neurons and postjunctionally on cells in the iris/ciliary body. While the activation of alpha 2 adrenoceptors at each site has been postulated to alter aqueous humor dynamics, little is known about the pharmacological characteristics of these receptors or their role in the modulation of anterior segment function. The purpose of the current study was to determine the possible heterogeneity of ocular alpha 2 adrenoceptors using relatively selective alpha 2 adrenoceptor agonists and antagonists to examine ocular pre- and postjunctional alpha 2 adrenoceptors. Prejunctional alpha 2 effects were evaluated by means of the cat nictitating membrane (CNM) preparation. Postjunctional alpha 2 effects were evaluated by means of the cAMP assay in rabbit iris root/ciliary body. In the CNM, the administration of UK-14, 304 (UK) produced a dose-related inhibition of neuronally mediated contractions. Pretreatment with the alpha 2 antagonist rauwolscine caused a 1 to 2 log unit right shift in the dose-response curve of UK in the CNM. However, pretreatment with alpha 2 antagonist SKF 104078 had no demonstrable effect on UK-induced inhibition of neuronally mediated contractions of the CNM. In the rabbit iris root/ciliary body, UK produced a concentration-dependent inhibition of cAMP accumulation on isoproterenol- and VIP-induced cAMP production. Pretreatment of iris root/ciliary bodies with SKF 104078 or rauwolscine reversed the inhibitory effect of UK on isoproterenol- and VIP-induced accumulation of cAMP. These data provide the first evidence that the pre- and postjunctional alpha 2 adrenoceptors represent pharmacologically distinct subpopulations of receptors in the eye.(ABSTRACT TRUNCATED AT 250 WORDS)     

A cyclic AMP phosphodiesterase inhibitor, 8'-pivaloyloxymethyl ester (POM-ester) of griseolic acid, lowers rabbit intraocular pressure.

The effects of griseolic acid (GA), a cyclic-AMP phosphodiesterase (PDE) inhibitor, and its 8'-pivaloyloxymethyl (POM) ester on intraocular pressure (IOP) in rabbits were investigated. When 50 microliters of 1 and 2% GA POM ester solutions were topically applied to one eye in normal rabbits, significant IOP decreases were detected at 2 hrs and at 1 to 5 hrs, respectively. Other than ocular hypotension, no other ocular effects were detected locally even after administration of 2% GA POM ester. A more marked reduction in IOP occurred after the intravitreal injection of the GA POM ester. IOP was also reduced when GA was used in an intravitreal injection but not when it was topically applied. The difference in permeability between GA and GA POM ester across the corneal epithelium may explain why GA failed to reduced IOP following topical administration. GA and the GA POM ester inhibited cAMP PDE in rabbit ciliary body at low concentrations, the I50 being 0.075 microM and 2.4 microM, respectively, with 0.25 microM cAMP as substrate. GA and the GA POM ester markedly increased cAMP levels in vitro in iris-ciliary body specimens. Possibly, GA POM ester or its analogues may represent a new mechanistic class of ocular hypotensive agents.     

Naphazoline-induced suppression of aqueous humor pressure and flow: involvement of central and peripheral alpha(2)/I(1) receptors

The objective of this study was to examine the ocular hydrodynamic effects of topically and centrally administered naphazoline, alone and following pretreatment with pertussis toxin (PTX) and alpha(2)/I(1)receptor antagonists. Topically and intracisternally administered naphazoline was examined for its ability to alter intraocular pressure (IOP) of rabbits in the absence and presence of receptor antagonists (rauwolscine, efaroxan) and a G(i/o)ribosylating agent PTX. In addition, the topical effects of naphazoline on pupil diameter and aqueous humor flow rate were evaluated. Topical unilateral application of naphazoline (7.5, 25 and 75 micro g; 25 micro l) elicited an ipsilateral dose-dependent mydriasis (2, 4 and 5.5 mm) that peaked at 2 hr with a duration of up to 5 hr. The IOP decreases induced by naphazoline were bilateral and dose-dependent (3, 6 and 10 mmHg); the response peaked at 1 hr and lasted for up to 5 hr. Pretreatment with efaroxan (250 micro g) elicited significantly greater antagonism of the ocular hypotensive response to naphazoline than did rauwolscine (250 micro g) suggesting an involvement of imidazoline (I(1)) receptors. Intracisternal application of naphazoline (3.3 micro g) also produced bilateral reductions (6 mmHg) of IOP that were immediate (10 min post drug) and lasted for approximately 2 hr. In PTX-pretreated (2.5 micro g kg(-1), i.a.) rabbits, the ocular hypotensive effects of naphazoline by both routes (topically and centrally) were attenuated by 50--65%. In addition to producing ocular hypotension, topical application of naphazoline (75 micro g; 25 micro l) caused significant reduction, from 2.8 to 1.5 micro l min(-1), in aqueous humor flow. These in vivo data indicate that, regardless of route of administration, alteration of aqueous humor flow by naphazoline was induced by the activation of alpha(2)and I(1)receptors. The ocular hypotensive effects produced by central administration did not result in sedation, therefore, there is the suggestion that central alpha(2)adrenergic receptors were stimulated minimally by naphazoline. Thus, these data suggest that ocular hypotensive effects and suppression of aqueous humor flow rate by naphazoline are mediated, in part, by alpha(2)and/or central I(1)at both central (brain) and peripheral (eye) sites. Moreover, these data indicate that the receptors are linked to PTX-sensitive G((i/o))proteins.     

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.     

Eicosanoids as a new class of ocular hypotensive agents. 1. The apparent therapeutic advantages of derived prostaglandins of the A and B type as compared with primary prostaglandins of the E, F and D type

The classic primary prostaglandins (PGs), as well as some of their analogs and derivatives, are potent ocular hypotensive agents. The present studies show that A and B PGs, which are derived from PGs of the E type by dehydration and isomerization, have a much greater ocular hypotensive potency than the primary PGs of the E, F or D type. A single application of 5 micrograms of PGA2 to the cat eye in a 25-microliters volume of aqueous vehicle solution yielded a greater and more prolonged ocular hypotensive effect than as much as 100 micrograms of topically applied PGF2 alpha. As little as 1 microgram of PGA2 had a significant ocular hypotensive effect that was enhanced by three or more consecutive daily applications of the same dose. This IOP reduction, which remained significant for several days after the last of 10 daily treatments, was not associated with biomicroscopically detectable flare or invasion of the anterior chamber by cells. Although PGF2 alpha and, to a much lesser extent, PGE2 have a miotic effect in cats, PGs of the A and B type did not cause significant miosis even at doses 50- to 100-fold greater than the minimum dose required to yield significant ocular hypotension. PGA2 retained its ocular hypotensive potency when stored in an aqueous solution at room temperature for four months. The conjunctival hyperemia caused by 5 micrograms or 10 micrograms of A or B type PGs on rabbit eyes was milder and shorter in duration than that caused by the same doses of PGE2 or PGF2 alpha. These findings suggest that derived PGs, especially PGs of the A type, may have a therapeutic advantage over primary PGs for the treatment of ocular hypertension and glaucoma.     

The 5-HT(1A)Receptor agonist 8-OH-DPAT lowers intraocular pressure in normotensive NZW rabbits

The aims of this study were first to investigate the effect of topical instillation of the 5-HT(1A)receptor agonist 8-OH-DPAT on intraocular pressure (IOP) in normotensive rabbits and second to establish whether the drug reaches the aqueous humour of treated and contralateral eyes at concentrations sufficient to activate ciliary epithelial 5-HT(1A)receptors. Following topical unilateral instillation of (+/-), (+) or (-)8-OH-DPAT, the IOP of rabbits was measured using an applanation tonometer. For the penetration study, [(3)H]8-OH-DPAT was instilled into one eye of each rabbit. Animals were killed after 30 min and the radioactive content of treated and contralateral ocular tissues was assessed. Administration of (+/-)8-OH-DPAT caused dose-dependent decreases in IOP in treated eyes of rabbits during the light and dark. The full 5-HT(1A)agonist (+)8-OH-DPAT was shown to be a more effective hypotensive agent than the partial agonist (-)8-OH-DPAT. The effect of (+/-)8-OH-DPAT on IOP was blocked by pretreatment with pindolol, a mixed 5-HT(1A)antagonist/beta-blocker, but not by the specific beta-blocker betaxolol. After instillation of [(3)H]8-OH-DPAT, peak levels of radioactivity were found in the cornea, followed by similar amounts in the iris-ciliary body and aqueous. There was negligible radioactivity present in tissues of the contralateral eye. This study demonstrates that topical administration of the 5-HT(1A)agonist 8-OH-DPAT dose-dependently decreases IOP in normotensive rabbits during the light and dark. The action of 8-OH-DPAT is presumably local to the anterior uvea as IOP was reduced only in treated eyes and [(3)H]8-OH-DPAT failed to reach the contralateral eye after unilateral instillation. Moreover, since 5-HT(1A)receptors are located in the rabbit ciliary epithelium and the effect of 8-OH-DPAT is blocked by a recognized 5-HT(1A)antagonist, the mechanisms of action of 8-OH-DPAT may well involve a decreased secretion of aqueous humour.     

Ocular effects of a relatively selective alpha 2 agonist (UK-14, 304-18) in cats, rabbits and monkeys

UK-14, 304-18 (UK), a relatively selective alpha 2-agonist, was examined for its effects on intraocular pressure (IOP) and pupil diameter (PD) in rabbits, cats and monkeys and on noradrenergic function in the cat nictitating membrane (CNM) preparation. Topical, unilateral administration of UK (0.0005-0.5 mg) produced dose-dependent decreases in IOP and pupil size in normal, unanesthetized rabbits, cats and monkeys. The ocular hypotensive effect of UK in the ipsilateral eye was delayed relative to the contralateral eye in all three species; UK produced an initial transient ocular hypertension in rabbits which was abolished by surgical transection of three major extraocular muscles. Mean arterial blood pressure in rabbits was not affected by 0.005 mg UK topically. The ocular hypotensive and miotic effects of UK were attenuated in superior cervical ganglionectomized (SX) cats and rabbits. Intra-arterially administered UK (0.33, 1.0, 3.3 and 10 micrograms) produced dose-related systemic hypotension and inhibition of contractions of the CNM elicited by electrically stimulating the pre- and postganglionic sympathetic trunks in the urethane/chloralose anesthetized cat. This inhibition was reversed and prevented by 300 micrograms rauwolscine but not by 300 micrograms domperidone. UK also enhanced the contractile response of the CNM to injected norepinephrine (10 micrograms). UK suppressed ocular hypertension induced by water loading and IOP recovery rate following hypertonic saline infusion in rabbits suggesting that aqueous flow was inhibited. These results indicate that UK lowers IOP, in part, by suppressing sympathetic neuronal function which causes a reduction in aqueous flow.     

Stimulatory effects of prostaglandin D2 analogues on adenylate cyclase in rabbit iris-ciliary body membrane fractions

Effects of prostaglandin (PG) D2 analogues on the adenylate cyclase activity in membrane fractions of the iris-ciliary body complex were studied. PGD2 dose-dependently activated the adenylate cyclase with a maximal activity increase of about 60%. The concentration required to cause a half-maximal stimulation (EC50) was about 5 x 10(-7) M. The stimulatory effect of PGD2 was totally dependent on GTP with EC50 for GTP at about 10(-7) M. The rank order of potency of PGD2 analogues for stimulating the adenylate cyclase and BW245C (a selective PGD2 agonist) greater than PGD3 greater than PGD2 greater than 9 beta-PGD2. PGD2 metabolites and PGD2 analogues which have little hypotensive activity were essentially ineffective in stimulating the adenylate cyclase. This rank order was strikingly similar to that reported previously for their intraocular pressure-lowering effects. One exception was PGD2 methylester. This compound, though reportedly effective in reducing IOP, failed to activate the adenylate cyclase by itself, presumably because its hypotensive effect is due to its hydrolyzed product, PGD2. These results indicate that the abilities of PGD2 analogues to stimulate the adenylate cyclase of the iris-ciliary body complex in GTP-dependent manner are highly correlated with their ocular hypotensive activities, and suggest that a PGD2 receptor-stimulatory GTP-binding protein-adenylate cyclase complex is involved in the PGD2-induced ocular hypotension.     

Studies on the ocular pharmacology of prostaglandin D2

Prostaglandin D2 (PGD2) exerts a variety of biologic actions in the eye; these include ocular hypotension and inflammatory effects on the conjunctiva. The profile of activity of PGD2 in ocular tissues was compared to that of BW 245C, a selective agonist for the PGD2-sensitive (DP) receptor, and to that of the biologically active metabolites of PGD2, 9 alpha,11 beta-prostaglandin F2 (9 alpha,11 beta-PGF2) and prostaglandin J2 (PGJ2). PGD2 produced a dose-dependent decrease in intraocular pressure and in the conjunctiva it caused increased conjunctival microvascular permeability, eosinophil infiltration and goblet cell depletion. Although BW 245C was equipotent to PGD2 as an ocular hypotensive agent, it did not cause pathological effects in the conjunctiva. Thus, the ocular hypotensive effect of PGD2 may be separated from inflammatory effects on the conjunctiva by employing a selective DP-receptor agonist such as BW 245C. 9 alpha,11 beta-PGF2 was a weak ocular hypotensive and did not cause conjunctival inflammation. PGJ2 produced no significant effect on intraocular pressure. PGJ2 did not elicit a microvascular permeability response in the conjunctiva, but was inflammatory in other respects and caused eosinophil infiltration and goblet cell depletion similar to PGD2. Thus, both the ocular hypotensive actions and the conjunctival pathology of PGD2 may be replicated individually by employing PGD2 analogues and metabolites.     

Central imidazoline (I(1)) receptors modulate aqueous hydrodynamics

The purpose of this work is to determine the relative contributions of central imidazoline (I(1)) receptors to the ocular hydrodynamic action of moxonidine. Moxonidine (MOX), an alpha(2) and I(1) receptor agonist, and efaroxan (EFA), a relatively selective I(1) antagonist, were utilized to study alterations in intraocular pressure (IOP) and aqueous flow in New Zealand white rabbits subjected to intracerebroventricular (i.c.v.) cannulation and sympathectomy. Intracerebroventricular administration of MOX (0.033, 0.33 and 3.33 microg) to normal rabbits produced dose-dependent, bilateral IOP decreases of 3, 6, and 8 mmHg, respectively. The ocular hypotensive response to MOX was immediate (10 min. post drug), lasted for one hour, and was inhibited by prior administration of efaroxan (3.33 microg i.c.v.). In unilaterally sympathectomized (SX) rabbits, the ocular hypotensive response induced by i.c.v MOX in the denervated eye was attenuated approximately 50%, but the duration of ocular hypotension in the surgically altered eye was longer than that of the normal eye. MOX (0.33 microg i.c.v.), caused a statistically significant decrease (2.24 to 1.59 ml/min.) in aqueous flow in normal eyes. In SX eyes, there was no change in aqueous flow by MOX, suggesting that IOP effect in i.c.v. MOX observed in the SX eye might be mediated by changes in outflow resistance. Sedation was observed in all the rabbits treated with MOX (i.c.v.) and was dose-dependent. These in vivo data support the suggestion that centrally located I(1) receptors modulate the early contralateral response to topically administered MOX and are involved in lowering of IOP and aqueous flow in rabbit. In addition, expression of the full ocular hypotensive effect of centrally applied MOX depends on intact sympathetic innervation. Ocular hypotension induced by MOX in the SX eye may involve an effect on uveoscleral outflow.     

Eicosanoids as a new class of ocular hypotensive agents. 3. Prostaglandin A2-1-isopropyl ester is the most potent reported hypotensive agent on feline eyes

It has been shown that prostaglandin A2 (PGA2) is a more potent ocular hypotensive agent in cats than other PG free acids. We report here that significant IOP reduction can be achieved in normotensive cat eyes with the use of even lower doses of PGA2-1-isopropyl ester (PGA2-IE) than with PGA2, PGF2 alpha-1-isopropyl ester (PGF2 alpha-IE), or any other known ocular hypotensive agent. Furthermore, single applications of 0.5 microgram of PGA2-IE maintain significant IOP reductions for at least 24 hr. This hypotensive effect is enhanced during the first 3-5 days of daily treatment. Significant IOP reductions were maintained for several months as long as PGA2-IE was applied daily or at least once every 48 hr. None of the cats manifested signs of discomfort in response to treatment with doses ranging from 0.10 to 1.25 micrograms of PGA2-IE. Moreover, the extent of anterior chamber flare was less than that typically observed after the topical application of hypotensive doses of PGE2, PGD2, PGF2 alpha, or the esters or tromethamine salt of PGF2 alpha. Although it is possible that the human eye would respond differently to PGs of the A type, the results of these studies suggests that PGA2-IE or other esters of derived PGs of the A type, and probably the B type, may offer significant therapeutic advantages over the PGF2 alpha tromethamine salt and PGF2 alpha-IE, which have been shown to exert significant hypotensive effects on normal and glaucomatous human eyes.     

Mechanism related to reduction of intraocular pressure by melanocortins in rabbits

AIM: To investigate whether the ocular hypotensive effect of alpha melanocyte stimulating hormone (MSH) is related to eicosanoids or cyclic AMP (cAMP). METHODS: Intraocular pressure (IOP) readings were taken at a similar time on the day before and after a single dose of topical MSH. Changes in the levels of prostaglandin E(2) (PGE(2)) and prostacyclin in incubated iris ciliary body (ICB) explants were measured by specific radioimmunoassay (RIA). Incubated ICB explants were exposed to MSH or adrenaline (epinephrine) for a week. In addition, cAMP levels in the medium were determined following short term incubation using RIA. RESULTS: A significant dose related reduction in IOP was noted with topical MSH (mean (SD) maximal effect 4.5 (0.1) mm Hg (21%); p<0.001 v appropriate baseline) which persisted up to 6 hours (p=0.05). MSH treated ICB explants showed a 1.5-fold increase in PGE(2) and prostacyclin levels (p<0.001 for each parameter) while cAMP levels were increased twofold (p<0.001). CONCLUSIONS: A single application of MSH caused a sustained dose related ocular hypotensive effect with no side effects. An increase in eicosanoid and cAMP levels following ICB exposure to MSH indicated their involvement in MSH induced ocular hypotension. MSH and its analogues might have clinical relevance as antiglaucoma drugs with fewer side effects because of their antiallergic and anti-inflammatory properties.     

M-7 lowers rabbit intraocular pressure

Changes in intraocular pressure (IOP) and pupil diameter were studied in albino rabbits following topical administration of TL-99, M-7 (N,N-dimethyl analogues of 2-aminotetralin), DiPr-5,6-ADTN, DiPr-6,7-ADTN (dipropyl analogues of aminotetralin) and DPDA (N,N-di-n-propyl-dopamine), known as mixed alpha-adrenoceptor/dopamine receptor agonists with variable alpha 1/alpha 2-adrenoceptor/selectivity. The potencies of those compounds to decrease IOP were compared with each other. We found that M-7 has the most potent IOP-lowering properties, caused minimal ocular discomfort and is of interest as a potential antiglaucoma agent. The ocular hypotension was completely inhibited by rauwolscine and slightly antagonized by various other blocking agents. The results strongly suggest that the IOP-lowering effect of M-7 is mediated by the stimulation of alpha 2-adrenoceptors.     

Flesinoxan, a 5-HT1A receptor agonist/alpha 1-adrenoceptor antagonist, lowers intraocular pressure in NZW rabbits

PURPOSE: alpha1-Adrenoceptor antagonists and 5-HT1A receptor agonists reduce intraocular pressure (IOP) in the rabbit. The aims of this study were firstly, to determine the IOP-lowering effects of flesinoxan and selected other hybrid 5-HT1A receptor agonists/alpha1-adrenoceptor antagonists, and secondly, to investigate the mechanism of action of the IOP response to flesinoxan. METHODS: IOP and total outflow facility were measured in rabbits after administration of hybrid drugs. Inositol phosphates accumulation assays were performed using standard methodologies. RESULTS: Topical unilateral instillation of the drugs caused dose-related reductions of IOP. Comparison of the compounds tested revealed a potency order of WB 4101 > flesinoxan > 5-methyl-urapidil > or = BMY7378 > urapidil. WB-4101 caused a small increase in total outflow facility whereas flesinoxan had no effect. Measurement of the IC50 values for inhibition of phenylephrine-stimulated inositol phosphates accumulation in rabbit iris-ciliary body revealed a potency order of WB 4101 > 5-methyl-urapidil > flesinoxan > BMY 7378 = urapidil. Topical flesinoxan was ineffective in reversing phenylephrine-induced mydriasis, yet, pretreatment with the 5-HT1A receptor antagonists MDL 73005EF and pindolol only partially blocked the hypotensive effect of topical flesinoxan. CONCLUSIONS: The present studies indicate the potent and efficacious IOP-lowering capabilities of flesinoxan and certain other ligands with affinity for 5-HT1A receptors/alpha1-adrenoceptors. The exact mechanisms by which these drugs lower IOP in the rabbit are complex but our results indicate that flesinoxan likely reduces aqueous secretion.     

Potentiation of sympathetic neurosecretion by forskolin and cyclic AMP in the rabbit iris-ciliary body

Forskolin has been reported to stimulate cAMP formation and reduce intraocular pressure in rabbit and primate eyes. In view of recent evidence for the involvement of cAMP in modulation of transmitter release at adrenergic synapses, we have investigated the presynaptic effects of forskolin and other cAMP activators on field-stimulated secretion of 3H-norepinephrine (3H-NE) in the isolated, perfused rabbit iris-ciliary body. Forskolin (10(-7)-10(-5) M) was found to markedly enhance stimulation-evoked 3H-NE release without affecting basal (spontaneous) release. The response to forskolin was potentiated by the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX; 0.5 mM) and was mimicked by the cell-permeant cyclic nucleotide analog 8-bromo-cAMP. 8-bromo-cGMP also produce a small enhancement of stimulus-evoked 3H-NE secretion, whereas IBMX alone had little effect on either stimulated or basal secretion. These results suggest that cAMP may play an important neuromodulatory role in regulation of norepinephrine release at intraocular synapses, and raise the possibility that the ocular hypotensive response to forskolin in rabbit eyes may be mediated, in part, by enhanced adrenergic neurosecretion.     

Ocular hypotension in the rabbit. Receptor mechanisms of pirbuterol and nylidrin

Pirbuterol and nylidrin, both purported sympathomimetic amines, reduced intraocular pressure (IOP) when given topically (50 microliter, 0.1%) to albino rabbits. Pirbuterol increased the cyclic-AMP concentration in aqueous humor by a factor of 3.25, while nylidrin had no effect on aqueous cyclic-AMP nor on adenylate cyclase activity of iris-ciliary body membranes assayed in vitro. Studies of the receptor affinity of pirbuterol, timolol and nylidrin were carried out on iris-ciliary body membranes by competition binding with radioactive ligands. Four ligands were used that appear to label separate subpopulations of adrenergic receptors; dihydroalprenolol (beta-receptors), WB-4101 (alpha 1-receptors) prazosin (alpha 1-receptor subpopulation) and yohimbine (alpha 2-receptors). Pirbuterol and timolol showed exclusive selectivity for beta-receptors with high affinities (Kd 12.6 and 0.48 nM, respectively) compared with other adrenergic receptor populations in iris-ciliary body. Nylidrin had high affinities for beta-receptors (Kd 22 nM) and for the subpopulation of alpha 1-receptors labelled by prazosin (Kd 6.5 nM), but showed 100-fold lower affinity and complex binding characteristics to the two other classes of alpha-adrenergic receptor sites labelled by WB-4101 and yohimbine, respectively. The results show that pirbuterol and timolol are highly beta-receptor selective and that hypotensive responses to these drugs are not mediated by the other classes of alpha-adrenergic receptor determined in this study. However, the hypotensive response to nylidrin may be related to its prazosin-like (alpha 1-receptor) antagonist properties with additional activity at beta-receptors.     

Effects of B-HT 920 in the eye and on regional blood flows in anaesthetized and conscious rabbits

The influence of unilateral ocular instillation of B-HT 920 (50 micrograms) on regional ocular, cerebral and peripheral blood flows was investigated with the labelled microsphere method in conscious and anaesthetized albino rabbits. In urethane-anaesthetized rabbits the intraocular pressure (IOP) fell during 1 hr following topical B-HT 920 whereas no changes in regional blood flows were observed. Only in conscious rabbits was a decrease in regional blood flows found. B-HT 920 caused a short-term reduction in choroidal blood flow by about 20%. Transient vasoconstrictor effects, due to systemic absorption, were also seen in some extraocular tissues. Concomitantly, B-HT 920 reduced the total cerebral blood flow (CBF) by 23%. In the grey matter and hypothalamic region the decrease in flow was about 20%, while in the hippocampal region, thalamic region, collicles and pons-mesencephalon it was about 10%. In experiments with direct blood flow determination from an opened vortex vein, there was no consistent change of uveal vascular resistance, while IOP and mean arterial pressure (MAP) fell dose-dependently following cumulative intravenously administered bolus doses (10 and 50 micrograms/kg) of B-HT 920. Unilateral loss of the mediated sympathetic tone seemed to increase the ocular responses to B-HT 920, unmasking a vasoconstrictor effect. Additional systemic pretreatment with the selective blocking agents rauwolscine and sulpiride suggests that B-HT 920 produces its ocular hypotensive effect, predominantly by acting on dopamine (DA2) receptors in the eye rather than on alpha 2-adrenoceptors, and its ocular vasoconstrictor effects via both receptor types.     

Alpha2-adrenergic receptors in rat and rabbit eye: a tritium-sensitive film autoradiography

Using a tritium-sensitive film and [3H]-clonidine, alpha 2-adrenergic receptors were localized in entire eye sections of rabbits and rats. This radioactive agonist specifically labelled three structures in albino animals: ocular muscles, ciliary processes and retina. In pigmented animals, uvea with melanin was intensively but nonspecifically labelled. Microscopic analysis of autoradiograms obtained with iris-ciliary bodies, revealed that the major localization of binding sites was in the ciliary process epithelium. In displacement studies, [3H]-clonidine binding was inhibited by alpha 2-adrenergic compounds while alpha 1- or beta-selective drugs had no inhibitory effects.     

Effects of 0.2% brimonidine on ocular anterior structures.

The aim of the study was to evaluate the effect of an alpha2-adrenergic agonist, 0.2% brimonidine, on a number of echobiometry and ultrasound biomicroscopy ocular parameters. Ten healthy subjects ranging in age from 20 to 40 years (mean age 29+/-3.39) were recruited into this prospective open-label trial. After instillation of 0.2% brimonidine eye drops, the following parameters were assessed: refraction, visual acuity, pupil diameter, intraocular pressure, five A-scan echobiometric parameters and 15 ultrasound biomicroscopy parameters. As early as the first hour after administration of the drug, a marked ocular hypotensive effect was detected associated with a miotic effect, without any refractive or visual acuity alterations. The A-scan echobiometry parameters were unchanged, while, as far as the ultrasound biomicroscopy variables were concerned, we observed an increase in iris-lens contact distance and a reduction in iris root thickness with a resulting increase in posterior chamber depth and in iris-ciliary process distances. No changes were observed in either the anterior chamber or the anterior iris profile. Brimonidine 0.2% proved to be an ocular hypotensive agent which is also endowed with a mild miotic effect, without giving rise to refractive or visual acuity alterations. The drug does not alter the thickness and position of the lens and does not facilitate pupil block; it reduces the iris thickness with an increase in posterior chamber depth and in iris-ciliary process distance but with no changes in anterior chamber depth or chamber angle width.