Alteration in ocular function induced by phenylethylamine analogs of dopamine

Role of dopaminergic receptors in glaucomatous disease modulation

Both studies on animals and humans suggest the presence of dopamine (DA) receptors in the anterior segment of the eye. Their role in the dynamics of intraocular pressure (IOP) is not yet clear. DA₂ and DA₃ receptors are mainly located on postganglionic sympathetic nerve endings. Their stimulation reduces the release of norepinephrine and suppresses the production of aqueous humor. DA₁ receptors seem to be more expressed by the ciliary body and the outflow pathway of aqueous humor. The administration of DA₁-selective agonists stimulates the production of aqueous humor, increasing IOP, whereas DA₂- and DA₃-selective agonists could reduce IOP and, therefore, the risk to develop a glaucoma (GL). GL is a broad spectrum of eye diseases which have in common the damage to the optic nerve and the progressive loss of the visual field. Further studies are desirable to clarify the role of the dopaminergic system and the usefulness of DA₂ and DA₃ agonists in reducing IOP.

Paradoxical effect of phentolamine on aqueous flow in the rabbit

PURPOSE

The aim of this study was to assess the effects of acute systemic, nonselective alpha-adrenergic blockade on aqueous flow.

METHODS

This study used pentobarbital-anesthetized rabbits (n=7), in which the following parameters were measured: mean arterial pressure, carotid blood flow, heart rate, intraocular pressure (IOP), orbital venous pressure (OVP), ciliary blood flow, and aqueous flow (AqFlow). Measurements were made before and after an intravenous administration of phentolamine (0.1 mg/kg).

RESULTS

Phentolamine caused significant decreases in IOP -23%+/-2%; P<0.01), OVP (-28%+/-12%; P<0.05), and AqFlow (-33%+/-6%; P<0.01). The other parameters were not significantly altered. The rapidity of the OVP and IOP responses were noteworthy, being essentially complete 60 s after the phentolamine injection.

CONCLUSIONS

A subpressor dose of phentolamine has complex effects on ocular hydrodynamics. The initial IOP decrease is too fast to be explained by aqueous dynamics or ocular rigidity, and so is most likely a result of the disgorgement of choroidal blood volume caused by decreased venous pressure outside the eye. The more prolonged ocular hypotensive effect is explained by the decrease in AqFlow, and perhaps a decrease in episcleral venous pressure or increase in uveoscleral outflow. However, the inhibition of aqueous production is odd, as lost prejunctional inhibition of norepinephrine release and unopposed beta-receptor activation should have increased aqueous production.

Motion sickness is linked to nystagmus-related trigeminal brain stem input: a new hypothesis

Motion sickness is a common and distressing but poorly understood syndrome associated with nausea/vomiting and autonomic nervous system accompaniments that develops in the air or space as well as on sea or land. A bidirectional aetiologic link prevails between migraine and motion-sickness. Motion sickness provokes jerk nystagmus induced by both optokinetic and vestibular stimulation. Fixation of gaze or closure of eyes generally prevents motion sickness while vestibular otolithic function is eliminated in microgravity of space, indicating a predominant pathogenetic role for visuo-sensory input. Scopolamine, dimenhydrinate, and promethazine reduce motion-related nystagmus. Contraction of extraocular muscles generates proprioceptive neural traffic and can provoke an ocular hypertensive response. It is proposed that repetitive contractions of the extraocular muscles during motion-related jerk nystagmus rapidly augment brain stem afferent input by increasing proprioceptive neural traffic through connections of the oculomotor nerves with the ophthalmic nerve in the lateral wall of the cavernous sinus as well as by raising the intraocular pressure thereby stimulating anterior segment ocular trigeminal nerve fibers. This verifiable hypothesis defines the pathophysiological basis of individual susceptibility to motion sickness, elucidates the preventive mechanism of gaze fixation or ocular closure, advances the aetiologic link between MS and migraine, rationalizes the mechanism of known preventive drugs, and explores new therapeutic possibilities.

Determination of dopamine D1 receptors in the human uveo scleral tissue by light microscope autoradiography

The aim of this paper is to clarify the distribution of Dopamine D1 (DA D1) receptors in the uveo-scleral tissue of human eyes with or without elevated intraocular pressure (IOP) and to study the relationships between DA D1 receptors and uveo-scleral tissue. Samples of human uveo-scleral tissue were taken from seven men undergoing eye surgery for a traumatic lesion of the anterior segment of the eye, without involvement of the iris-corneal angle and /or from eye donors. The subjects (in whom one eye bulb had been surgically enucleated) had been previously enrolled in our medical protocols because they suffered for increased IOP, while the eye donors (of both eye bulbs) had a normal IOP. Frozen sections from the uveo-scleral tissue were submitted to biochemical characterization and to morphological autoradiographic techniques for detection of DA-D1 receptors. [3H]-SCH-23390 was used as a ligand of Dopamine D1 receptors. [3H]-SCH 23390 was bound by sections of the human uveo-scleral tissue. The pharmacological profile of the binding was consistent with the labeling of D1 receptors. Light microscope analysis was used for localization of D1 receptors and revealed an accumulation of the radioligand in the human uveo-scleral tissue. In eyes with normal IOP there is a high reaction. The Bmax of radioligand decreases in the eyes with increased IOP. The possibility that dopaminergic receptors play a role in the controlling uveo-scleral tissue functions is suggested.

Comparison of effects of topical ibopamine and epinephrine on the circadian rhythm of intraocular pressure of the rabbit eye as measured by telemetry

The purpose of this study was to measure the effect of topical ibopamine and epinephrine on intraocular pressure in undisturbed rabbits. Six pigmented rabbits were outfitted with a telemetric transducer system connected via catheter to the vitreous cavity of one eye for continuous monitoring of intraocular pressure. After entrainment to a 12 hour light, 12 hour dark cycle and after stabilization of the circadian rhythm of intraocular pressure, the animals were tested with 2% ibopamine and 2% epinephrine. Each drug was instilled during the light phase and during the dark phase of the circadian cycle. When administered during the light phase, both drugs caused a transient pressure rise followed by prolonged hypotension. When administered during the dark phase, neither drug caused a pressure rise but both drugs caused prolonged hypotension. It was concluded that ibopamine and epinephrine cause identical intraocular pressure changes in the normal rabbit eye. The effect was dependent on the timing of administration during the circadian cycle. Since ibopamine is a pro-drug of deoxyepinephrine (N-methyl dopamine, epinine), its effects are assumed to be due to this metabolite, a metabolite that is structurally similar to epinephrine.

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.

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.

Ocular and systemic disposition of the dopamine agonist N-0437 in monkeys after ocular administration

The ocular and systemic disposition of the new dopamine agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437) was investigated in conscious monkeys after ocular administration of 0.56 mg N-0437. HCl (corresponding to 0.50 mg free base), containing 200 microCi [3H]N-0437, into the right eye. After killing the animal, the eyes were removed and several eye tissues were dissected and assessed for their content of radioactivity. In the treated eye, the iris contained by far the highest concentration of radioactivity, while in the untreated eye the lower conjunctiva, the iris the ciliary body and the choroid possessed the highest levels of radioactivity. In all dissected eye tissues, a substantially higher concentration of radioactivity was established for the treated eye compared to the untreated eye, except for the vitreous in which for both eyes about an equal concentration was measured. This result suggests the presence of a systemic transport, which was confirmed by the occurrence of bradycardia, starting immediately after ocular application of the drug and lasting for about 1.5 hr. As the total amount of radioactivity in both eyes 7 hr after ocular dosing is very low (0.3-0.5% of the dose), one can conclude that N-0437 is almost completely taken up into the general circulation. The radioactive measurements of bile and urine samples collected up to 7 hr after administration revealed that the elimination of N-0437 and its metabolites is very fast, with the urinary excretion (35% of the dose) slightly higher than to the biliary one (31% of the dose).

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

1. Topical administration of the dopamine (DA2) receptor agonist, Ha-118, produced unilateral ocular hypotension and miosis in normal cats. 2. The ocular hypotensive and mitotic effects of Ha-118 were not observed in surgically sympathectomized cats indicating that an intact sympathetic pathway is necessary to demonstrate activity. 3. Ha-118 caused dose-dependent suppression of contractions of the cat nictitating membrane (CNM) elicited by electrical stimulation of the pre- and postganglionic sympathetic nerve trunks but not by exogenously administered noradrenaline suggesting that Ha-118 affected prejunctional but not ganglionic or postjunctional receptors. 4. Sulpiride antagonized Ha-118-induced inhibition of neuronally mediated contractions in the CNM suggesting an interaction at DA2 receptors. 5. Topical administration of Ha-118 inhibited the rise in intraocular pressure induced by oral water loading in rabbits. 6. Topical pretreatment with metoclopramide, a DA2 antagonist, inhibited the ocular antihypertensive effect of Ha-118 in rabbits. 7. These studies demonstrate that Ha-118 decreased intraocular pressure (IOP) and pupil diameter (PD) in the cat and that this activity can be correlated with suppression of peripheral sympathetic tone in the CNM.

Review: alpha 2 and DA2 agonists as antiglaucoma agents: comparative pharmacology and clinical potential

Alpha-2 (alpha 2) and DA2 agonists lower intraocular pressure (IOP) in laboratory animals and man. Like beta-blockers, alpha 2 and DA2 agonists appear to lower IOP by reducing aqueous inflow. These agents share a common mode of action on sympathetic nerve terminals, where they modulate the release of neurotransmitters. However, one can demonstrate that peripheral prejunctional alpha 2 and DA2 receptors on sympathetic neurons are separate entities by utilizing selective agonists and antagonists. In addition to their prejunctional actions, alpha 2 agonists act postjunctionally in the iris root/ciliary body (IRCB). Moreover, utilizing selective postjunctional alpha 2 adrenoceptor antagonists, heterogeneity can be demonstrated between ocular pre- and postjunctional adrenoceptors. Stimulation of postjunctional alpha 2 adrenoceptors in the IRCB can inhibit the cellular responses to endogenous neurotransmitters and hormones that are coupled positively to adenylate cyclase. Based upon these observations, one can predict that alpha 2 agonists should have a broader spectrum of action in the eye than beta-receptor antagonists. Three bioassays were used in the activity analysis of alpha 2 and DA2 agonists. Prejunctional (neuronal) activity was determined in the cat nictitating membrane preparation in which frequency-related (2-8 Hz), neuronally induced contractions were inhibited by these compounds. Postjunctional activity was assayed on isolated rabbit IRCB tissue where cAMP levels were stimulated by either isoproterenol or VIP in the absence and presence of the test agonist (alpha 2 or DA2). In this system, it has been demonstrated that alpha 2 agonists have inhibitory properties, but DA2 agonists are inactive.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of unique ADTN-catecholamine binding sites in the iris root-ciliary body of rabbits

A binding site for tritiated 2-amino-6, 7-dihydroxy-1, 2,3,4-tetrahydronaphthalene (ADTN) has been partially characterized in the rabbit iris root-ciliary body. Binding of ADTN is proportional to protein content and requires at least 60 minutes to reach equilibrium. Binding is saturable, with a Kd of 27 +/- 1 nM and a Bmax of 2.1 +/- .3 pmol/mg protein (mean +/- SEM). Dopamine competes for this site with a Ki of 100 nM and apomorphine with a Ki of 180 nM. This site is not blocked by L-timolol, phenoxybenzamine, or by several DA1 and DA2 antagonists. It appears to be a new type of catecholamine binding site, of a type not observed outside the anterior eye. It is possible that some of the effects of dopamine on intraocular pressure are mediated through this binding site.

Effects of dopamine antagonists injected through vortex veins on intraocular pressure

In order to avoid the factors of the corneal barrier and tearing washout on absorption of eye drops, dopamine and antagonists were injected into rabbit eyeballs through vortex veins. Many dopamine antagonists, such as haloperidol, moperone, metoclopramide, trifluperidol, lenperone, and fluoropipamide, lowered the intraocular pressure (IOP), whereas a few of them, such as chlofluperol and trifluoperazine, raised the IOP. Furthermore, it was found that dopamine agonists also could either increase or decrease the IOP. The ocular hypertensive effect of dopamine was blocked by haloperidol as expected. However, when the hypotension-inducing dopamine agonist, bromocriptine, and antagonist, haloperidol, were combined, the IOP remained unchanged. It was concluded that dopamine agonists and antagonists act at different sites and changes in IOP resulted from a combined summary of these complex effects. As a result, dopamine agonists and antagonists, while they may both lower the IOP when given alone, can antagonize each other through different sites and produce no net change in IOP.

Dopamine receptors in rabbit and rat eye: characterization and localization of DA1 and DA2 binding sites

In vitro autoradiography is used to characterize and localize DA1 and DA2 receptors in rabbit and rat eyes. The bindings of 125I-SCH 23982 (DA1 antagonist) and of 125I-Iodosulpride (DA2 antagonist) to slide mounted tissue sections take place with characteristics expected of substances that recognize DA1 and DA2 receptors respectively. They are saturable, have high affinity and exhibit an appropriate pharmacology. The regional distribution of these receptors indicates that they are mainly present in retina. Precise cellular localizations are visualized in retinal structures by means of histoautoradiography: DA1 receptors are found in inner plexiform layer, outer plexiform layer and inner nuclear layer and DA2 binding sites are present in the two plexiform layers.

LY141865: a relatively selective DA2 agonist with complex ocular activity

LY141865 (LY), an ergoline derivative, was investigated for ocular effects in rabbits and monkeys. LY lowered intraocular pressure (IOP) bilaterally in monkeys and rabbits, prevented the bilateral rise in IOP caused by oral waterloading in rabbits and slowed the rate of IOP recovery bilaterally in rabbits. The ocular hypotensive activity of LY in rabbits was inhibited by sympathectomy and pretreatment with sulpiride. Since LY has cardiovascular effects that are centrally mediated, ocular responses may also be mediated, in part, by an action in the CNS.

Ocular effects of a N,N-disubstituted 5-OH aminotetralin (N-0437): evidence for a dual mechanism of action

The selective DA2 agonist, N-0437, produced an acute reduction in intraocular pressure (IOP) and pupil diameter (PD) when applied topically to the eyes of normal monkeys. Ocular hypotension and miosis were primarily unilateral in nature, dose-related and lasted 3 to 5 hours following drug instillation. In normal monkeys, 24 to 48 hours following drug administration, a secondary chronic (greater than 24 hrs) reduction in pressure was observed. Once-a-day topical administration of N-0437 (250 micrograms), to normal monkeys, for 4 days produced a chronic unilateral reduction in IOP that persisted for 18 days. Associated with this reduction in pressure on day 2 through 6 were miosis and ptosis of the treated eye. Although topical administration did not lower IOP in rabbits, intracameral injection of N-0437 significantly depressed IOP for 3 days when compared to control injected eyes. Evaluation of ocular sympathetic innervation in N-0437 treated rabbits indicated that these fibers were not functional. In rabbits, intracameral administration of the active (S,-) and inactive (R,+) enantiomer of N-0437 produced equivalent reductions in pressure. These data provide further support for the hypothesis that DA2 receptor agonists can produce acute reductions in IOP. In addition, N-0437 appears to have a second non-receptor mediated mechanism of action that produces a chronic reduction in IOP. This chronic reduction in pressure appears, in part, to result from an interruption of ocular sympathetic nerves function.

Aporphine derivatives affect ocular function in diverse ways

The sympathetic nervous system has been implicated in the ocular effects of several classes of dopamine receptor agonists. Two agonists of the aporphine class, (-) N-propylnorapomorphine [-)NPA) and norapomorphine (NA), were evaluated for effects on intraocular pressure (IOP) and pupil diameter (PD) in cats and normal (NL) and sympathectomized (SX) rabbits and on contractions of the cat nictitating membrane (CNM). Topically administered (-)NPA produced a monophasic IOP drop in the ipsilateral eye and a biphasic (increase, decrease) IOP response in the contralateral eye of normal rabbits. Pupil diameter increased bilaterally. In NL cats, the ipsilateral IOP and PD response to (-)NPA was biphasic (increase, decrease) and the contralateral IOP response was monophasic (increase). (-)NPA produced an increase in spontaneous motor activity (SMA) in rabbits and cats. NA lowered IOP unilaterally in NL rabbits. In SX rabbit eyes, (-)NPA lowered IOP as much as in NL eyes but with less mydriasis. NA did not lower IOP in SX rabbit eyes. Both (-)NPA and NA inhibited contractions of the CNM elicited by electrical stimulation of the pre- and post-superior cervical ganglionic nerves. (-)NPA, but not NA, inhibited contractions elicited by exogenous norepinephrine. These results suggest that aporphine derivatives produced two diametrically opposed effects on ocular function; 1) a centrally mediated effect that enhanced noradrenergic activity to elicit mydriasis, SMA and ocular hypertension, and 2) a peripherally mediated effect to produce miosis and ocular hypotension.

Effect of topical pergolide on aqueous dynamics in normal and glaucomatous monkeys

The effects of pergolide mesylate, an ergoline derivative, were studied on intraocular pressure (IOP), outflow facility, aqueous humor flow, and pupil size in monkeys. Unilateral topical administration of two 20-microliters drops of 0.1% pergolide significantly lowered IOP in the treated- and contralateral eye in both normal- and glaucomatous monkeys. In 12 normal monkeys, the baseline IOP of 18.3 +/- 0.4 mmHg [mean +/- S.E.(M.)] was maximally reduced to 14.4 +/- 0.7 mmHg in the treated eye (P less than 0.001) and 14.6 +/- 0.6 mmHg in the contralateral eye (P less than 0.001) at 2 hr after drug administration. In 10 monkeys made bilaterally glaucomatous by argon laser treatment of the trabecular meshwork, the baseline IOP of 33.9 +/- 3.0 mmHg [mean +/- S.E.(M.)] in the treated eyes and 31.7 +/- 3.3 mmHg in the untreated eyes maximally decreased to 23.9 +/- 2.2 mmHg (P less than 0.05) and 26.2 +/- 3.3 mmHg (P less than 0.005), respectively, at 5 hr. No significant change (P greater than 0.7) in outflow facility occurred in either eye of 11 normal monkeys 2 hr after unilateral 0.1% pergolide treatment. In six normal monkeys, the baseline aqueous humor flow of 1.58 +/- 0.20 microliter min-1 in treated eyes and 1.44 +/- 0.18 microliter min-1 in untreated eyes was reduced to 0.92 +/- 0.08 microliter min-1 (P less than 0.02) and 1.09 +/- 0.11 microliter min-1 (P greater than 0.10), respectively, from 0.5- to 3.5 hr after drug administration. A mydriatic response was observed in both eyes after unilateral treatment from 1- to 2 hr in eight normal monkeys. By the third day of treatment, bilateral twice a day 0.1% pergolide drops in eight glaucomatous monkey eyes no longer significantly (P greater than 0.05) decreased IOP.

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.

Stimulation of endogenous cyclic AMP levels in ciliary body by SK&F 82526, a novel dopamine receptor agonist

The effect of dopamine and SK&F 82526 on cyclic AMP metabolism in ocular tissues has been examined. The in vitro incubation of human and bovine ciliary body with these agonists produced a dose-dependent increase in endogenous levels of cyclic AMP. This stimulation was blocked by the selective DA1 receptor antagonist SK&F 83566, but not by the beta-receptor antagonist propranolol. The response to SK&F 82526 was stereoselective, with SK&F R-82526 being approximately 100 times more potent than SK&F S-82526 in this preparation. This stimulation of ciliary body cyclic AMP content was not observed in the rabbit or cat, nor was it seen in human, bovine or rabbit iris tissue. These data suggest that adenylate cyclase linked dopamine receptors are present in both human and bovine ciliary body.

Does B-HT 920 lower intraocular pressure in cats by interacting with alpha 2- and/or DA2 adrenoceptors?

Topical administration of B-HT 920 (50 micrograms) to the eyes of normal unanesthetized cats produced decreases in intraocular pressure and pupil diameter. The ocular hypotensive effect of B-HT 920 was eliminated by sympathectomy and pretreatment with sulpiride (2 mg/kg, s.c.). B-HT 920 also produced dose-related inhibition of contractions of the cat nictitans elicited by stimulating the pre- and postganglionic sympathetic trunks. B-HT 920-induced suppression of the contracting nictitans was antagonized more effectively by relatively selective DA2 antagonists, sulpiride and domperidone, than by rauwolscine, a relatively selective alpha 2-adrenoceptor antagonist. These data suggest that B-HT 920 produces ocular hypotension in the cat by interacting predominantly with DA2 receptor on peripheral sympathetic nerves.

An in vivo model for dissociating alpha 2-and DA2-adrenoceptor activity in an ocular adnexa: utility of the cat nictitating membrane preparation

Recent reports from this laboratory indicate that dopamine (DA2) agonists can lower intraocular pressure (IOP) in rabbits, monkeys and rats. This communication suggests that the cat nictitating membrane (CNM) preparation is a useful model for localizing the site(s) and mechanism(s) of action of dopamine (DA2) agonists and that this model can be used to discriminate between alpha 2 and DA2 activity. This in vivo preparation consists of indirect (neuronal)- and direct (agonist)-induced activation of the CNM by: 1) pre- and postganglionic stimulation of sympathetic nerves and 2) injection of norepinephrine (NEPI) into the external carotid artery (i.a.). A variety of DA2-agonists (ergolines, azepines, aminotetralins, aporphines) can cause dose-dependent depression of neuronally mediated contractions with minimal effects on contractions elicited by NEPI i.a. Characteristically, DA2-agonists depressed contractions elicited by low frequency (2 & 4 Hz) stimulation more than high frequency (6 & 8 Hz) stimulation. For example, the inhibitory effects of an ergoline derivative LY141865 on neuronally mediated contractions of the CNM could be reversed (or prevented) by relatively selective DA2 antagonists, such as sulpiride and domperidone, but not by relatively selective alpha 2-antagonists, such as yohimbine and rauwolscine. Interestingly, alpha 2-adrenoceptor agonists, such as clonidine and xylazine, caused less depression of nerve-stimulated responses than did DA2-agonists at the same dose. This modest suppression by alpha 2-agonists could be inhibited by yohimbine but not by sulpiride or domperidone. These results with the CNM document the presence of DA2 receptors on sympathetic neurons innervating an ocular adnexa and demonstrate a model for dissociating DA2 from alpha 2 activity.