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1 From the Departments of Pharmaceutical Chemistry and 2 Pharmaceutics, University of Kuopio, Kuopio, Finland; and 3 HortaPharm BV, Amsterdam, The Netherlands.
| Abstract |
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METHODS. AEA or APN were each formulated in aqueous 2-hydroxypropyl-ß-cyclodextrin (HP-ß-CD) solutions and administered unilaterally to the rabbit eye (dose, 62.5 µg per rabbit). To prevent the degradation of AEA or APN, the rabbits were pretreated with a subcutaneous (SC) PMSF injection (0.2222 mg/kg) 30 minutes before eye drop instillation. To determine whether the neuronal cannabinoid (CB1) receptor mediates the hypotensive IOP effects of undegraded AEA, the rabbits were pretreated with simultaneous SC injections of a CB1 receptor antagonist SR141716A (1.22.1 mg/kg) and PMSF (2.2 mg/kg) before the ocularly applied AEA.
RESULTS. In the absence of PMSF, the IOP profiles of AEA and APN showed a biphasic ocular effectthat is, an initial increase of IOP followed by IOP hypotension in the treated eye. In the presence of PMSF (2.2 mg/kg for AEA and 22 mg/kg for APN), IOP profiles showed immediate IOP reduction in the treated eye. SR141716A antagonized the IOP reduction caused by the undegraded AEA.
CONCLUSIONS. These results indicate that the apparently undegraded AEA and APN decrease IOP in normotensive rabbits. AEA-induced IOP reduction in the presence of PMSF is probably mediated through a CB1 receptor.
| Introduction |
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9-tetrahydrocannabinol,7
the
major psychoactive constituent of Cannabis. Other endogenous
cannabinoids, including 2-arachidonylglycerol, have been found in
various tissues,8
9
suggesting that this class of
compounds may form an entire family of chemical mediators in the body. AEA has a very short duration of action in vivo. Its inactivation occurs by facilitated transport through the cell membrane,10 11 12 followed by an enzymatic hydrolysis13 to arachidonic acid and ethanolamine by fatty acid amide hydrolase (FAAH)13 14 15 16 17 (Fig. 1) . Thus formed, the arachidonic acid can serve as a precursor for prostaglandin synthesis.18 19 The presence of FAAH has been reported in a variety of porcine ocular tissues,20 and the bovine retina has been shown to enzymatically release AEA.21 Discovery of CB1 receptor mRNA in the rat eye22 and, subsequently, CB1 receptor protein in the human eye,23 particularly in the ciliary body, provides physioanatomical support for a possible role of cannabinoid receptors in controlling IOP.
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-unsubstituted
anandamides, because their IOP effects can be eliminated with an
indomethacin pretreatment.
-Substituted anandamides and synthetic
cannabinoids, such as CP55940 and WIN55212-2, seem to act through CB1
receptors.27
28
29 In this study, we investigated whether undegraded AEA and its synthetic nitrile analogue arachidonyl propionitrileamide (APN) decrease IOP in normotensive rabbits. Enzymatic degradation of AEA and APN was prevented by phenylmethylsulfonyl fluoride (PMSF). PMSF is a nonselective serine protease inhibitor that blocks the activity of FAAH and prevents the hydrolysis of anandamides.14 16 We also investigated whether a CB1 receptor mediates the IOP responses of the undegraded AEA, through the application of a specific CB1 receptor antagonist SR141716A.
| Materials and Methods |
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Animals
The experimental animals used in this study were normotensive
Dutch belted rabbits of either gender (2.34.0 kg, n =
56). The rabbits were housed singly in cages under standard
laboratory conditions: 12-hour dark12-hour light cycle, 20.0 ±
0.5°C and 55% to 75% relative humidity. They were given water and
food ad libitum, except during the tests. All animals were treated in
accordance with the ARVO Statement for the Use of Animals in Ophthalmic
and Vision Research.
Eye Drop Formulation
An ethanol solution of AEA or APN was evaporated under a stream
of nitrogen and redissolved in aqueous 10% HP-ß-CD (AEA) or 15%
HP-ß-CD (APN) solutions. The vehicle controls contained 10% or 20%
HP-ß-CD. The pH of eye drop solutions was adjusted to 7.4 with sodium
hydroxide, and the solutions were made isotonic with sodium chloride.
Final drug concentration of the AEA or APN solutions was 2.5 mg/mL
(0.25%). Drug concentrations of the eye drop solutions were analyzed
by high-pressure liquid chromatography (HPLC).
Drug Analysis
The analytical HPLC system for determination of drug
concentrations in the eye drops was a commercial system (Merck Hitachi;
Hitachi, Tokyo, Japan), including a pump (model L-7100), an interface
module (model L-7000), a diode array detector (model L-7455; 200400
nm, set at 211 nm), and a programmable autosampler (model L-7250). An
end-capped reversed-phase column (Purospher RP-18; 125 x 4 mm,
5 µm, Merck kGaA, Darmstadt, Germany) was used for separations. A
mobile-phase mixture of acetonitrile and 0.02 M phosphate buffer
solution (pH 7.0) was used at a flow rate of 1.2 mL/min. The proportion
of acetonitrile in the mobile phase was increased linearly from 60% to
90% over 15 minutes, maintained for 5 minutes, and returned to the
initial condition over the course of 6 minutes. Quantitation of the
compounds was made from measurements of their peak areas in relation to
standards chromatographed under the same conditions.
Preparation and Administration of the PMSF, Indomethacin, and
SR141716A Solutions
PMSF was dissolved in glycerol formal to concentrations of 3,
30, and 150 mg/mL. Depending on the test procedure, each rabbit
received approximately 0.22, 2.2, and 22.0 mg/kg PMSF in a subcutaneous
(SC) injection, 30 minutes before application of ocular AEA or APN,
because of its low solubility in the ophthalmic vehicle.
Indomethacin was dissolved in an aqueous 20% HP-ß-CD solution to a concentration of 5 mg/mL. The solution pH was adjusted to 7.4 with aqueous sodium hydroxide, and the solution was made isotonic with sodium chloride. Each rabbit in the indomethacin study received 12.5 mg (3.15.4 mg/kg) of indomethacin by SC injection approximately 30 minutes before the ocular AEA treatment.
An SR141716A solution (0.3 mg/mL) was prepared by dissolving the compound in a 42% HP-ß-CD solution of phosphate buffer at pH 4.2. Each rabbit in the antagonism study received 4.8 mg (1.22.1 mg/kg) SR141716A by SC injection, 30 minutes before ocular AEA treatment. SC pretreatments with a 0.9% sodium chloride solution were used as control treatments. The single-dose levels of cannabinoid agonists and the antagonist used were selected from previous studies.24 27 28
IOP Measurements
To perform each test, a rabbit was placed in a plastic
restraining box located in a quiet room. A single drop (25 µL) of the
test solution or vehicle was instilled unilaterally into the left eye,
on the upper corneoscleral limbus. The contralateral eye was left
untreated. During instillation, the upper eyelid was pulled slightly
away from the globe. IOP was measured using a pneumatonometer (Digilab
Modular One; Bio-Rad, Cambridge, MA). Before each measurement, 1 or 2
drops of oxybuprocaine (0.06%) was applied to the cornea to eliminate
discomfort. The upper and lower eyelids were then gently retracted, and
the sensor was brought into contact with the center of the cornea. For
each determination, at least two readings were taken from each treated
(ipsilateral) and untreated (contralateral) eye, and the mean of these
readings was used. IOP of the rabbits was measured at 2, 1, and 0 hours
before, and at 0.5, 1, 2, 3, 4, and 5 hours after, eye drop
administration. IOP at the time of eye drop administration (0 hour) was
used as a baseline value. All studies were set up using a masked and
randomized crossover design. At least 72 hours of washout time was
allowed for each rabbit between doses.
Analysis of the Data
Results are presented as a change in IOP (in millimeters of
mercury) mean ± SE (n = 56). A one-factor
analysis of variance (ANOVA) for repeated measurements was used to test
the statistical significance of differences between groups.
Significance in the differences of the means was tested using the
Fishers protected least significant difference (PLSD) method at the
95% confidence level. Significance in the differences of the means
between the two treatments was tested using a two-tailed paired
Students t-test evaluated at the 95% confidence level.
| Results |
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To investigate whether the arachidonic acid-based COX products participate in the IOP-lowering effects of AEA in the presence of PMSF, the COX inhibitor indomethacin (12.5 mg/rabbit) was administered as a SC injection in conjunction with the ocular administration of AEA (62.5 µg) and SC PMSF (22 mg/kg). The indomethacin pretreatment did not inhibit the IOP reduction effects caused by AEA in the presence of PMSF (Table 2) .
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| Discussion |
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In this study, prevention of arachidonic acid formation from AEA and APN and subsequent synthesis of prostanoid derivatives was attempted through the use of the FAAH inhibitor PMSF. This was intended to evaluate whether undegraded AEA and its synthetic analogue APN decreases IOP in normotensive rabbits. In the presence of PMSF, the IOP profiles of AEA and APN exhibited only a hypotensive phase, suggesting that neither AEA nor APN was degraded. Indeed, prostaglandin synthesis through the COX pathway seems not to be involved in the IOP effects of test compounds in the presence of PMSF, because the COX inhibitor (indomethacin) pretreatment had no influence on the IOP reduction effects of AEA in the presence of PMSF (Table 2) . These results suggest that the ocular hypotension seen after ocular administration of AEA or APN, in the presence of the FAAH inhibitor PMSF, is due to unmetabolized AEA or APN. However, it should be noted that PMSF is also a potent inhibitor of cholinesterases and other serine proteases30 and thus is nonselective for FAAH.
A specific CB1 cannabinoid receptor antagonist, SR141716A,31 was used to examine whether the IOP reduction induced by AEA in the presence of PMSF is mediated through CB1 receptors. In earlier studies, SR141716A has failed to antagonize the ocular effects of AEA.28 In this study, in the presence of the FAAH inhibitor, PMSF, hypotensive IOP effects of AEA were inhibited by the CB1 receptor antagonist, and an IOP increase was observed in treated and untreated eyes. Similar bilateral hypertension was observed when the CB1 receptor antagonist and PMSF were administered in conjunction with the ocular vehicle. This observation is consistent with earlier studies28 29 and supports the hypothesis that the CB1 receptor may be involved with the physiological control of IOP. SR141716A may increase IOP by acting as an antagonist for the endogenous cannabinoid receptor agonist, blocking its tonic regulatory effects on IOP. SR141716A has also been reported to act as an inverse agonist under certain test conditions.32 33
In conclusion, this study reports for the first time that ocularly administered AEA, an endogenous ligand of the CB1 receptor, and its synthetic analogue APN reduced IOP without the initial IOP hypertensive phase if their enzymatic degradation was prevented by the enzyme inhibitor PMSF. The IOP reduction induced by AEA in the presence of PMSF was mediated through a CB1 receptor, suggesting that the endocannabinoid system may have a physiological role in the regulation of ocular tension.
| Acknowledgements |
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| Footnotes |
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Submitted for publication July 2, 2001; revised September 4, 2001; accepted September 19, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked
"advertisement" in accordance with 18 U.S.C.
1734
solely to indicate this fact.
Corresponding author: Krista Laine, Department of Pharmaceutical Chemistry, University of Kuopio, PO Box 1627, FIN-70211 Kuopio, Finland; krista.laine{at}uku.fi
| References |
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, L, Breuer, A, et al (1992) Isolation and structure of a brain constituent that binds to the cannabinoid receptor Science 258,1946-1949
, L, Gopher, A, Almog, S, et al (1993) Two new unsaturated fatty acid ethanolamides in brain that bind to the cannabinoid receptor J Med Chem 36,3032-3034[Medline][Order article via Infotrieve]
, L, et al (1995) Identification of an endogenous 2-monoglyceride, present in canine gut, that binds to cannabinoid receptors Biochem Pharmacol 50,83-90[Medline][Order article via Infotrieve]
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