• View in gallery
    Figure 1—

    Schematic diagram (A) and representative UBM image (B) of the iridocorneal angle and CC region in a dog. The ciliary body (CB) splits into 2 portions to form the CC. The ciliary body musculature (CBM) of the outer leaflet is much smaller than that of the inner leaflet (the inner leaflet forms the inner border of the CC). CP = Ciliary process.

  • View in gallery
    Figure 2—

    An enlarged version of the representative UBM image in Figure 1. Measurement of the AOD, CCW, CCL, and CCA is indicated.

  • View in gallery
    Figure 3—

    Representative UBM images of the left (A, C, E, and G) and right (B, D, F, and H) eyes used to measure the CCW in the same dog during each of 4 phases. Latanoprost (phase L; B), pilocarpine (phase P; D), pilocarpine followed 5 minutes later by latanoprost (phase PL; F), and latanoprost followed 5 minutes later by pilocarpine (phase LP; H) were administered to the right eye of each dog. Artifcial tears (placebo treatment) were administered to the left eye (control eye) during each phase (A, C, E, and G, for phases L, P, PL, and LP, respectively). Washout period between treatments was ≥ 5 days.

  • 1. Toris CB, Gabelt BT, Kaufman PL. Update on the mechanism of action of topical prostaglandins for intraocular pressure reduction. Surv Ophthalmol 2008;53:S107S120.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2. Toris CB, Zhan G, Zhao J, et al. Potential mechanism for the additivity of pilocarpine and latanoprost. Am J Ophthalmol 2001;131:722728.

  • 3. Grierson I, Lee WR, Abraham S. Effects of pilocarpine on the morphology of the human outflow apparatus. Br J Ophthalmol 1978;62:302313.

  • 4. Toris CB, Alm A, Camras CB. Latanoprost and cholinergic agonists in combination. Surv Ophthalmol 2002;47:S141S147.

  • 5. Kent AR, Vroman DT, Thomas TJ, et al. Interaction of pilocarpine with latanoprost in patients with glaucoma and ocular hypertension. J Glaucoma 1999;8:257262.

    • Search Google Scholar
    • Export Citation
  • 6. Sarchahi AA, Abbasi N, Gholipour MA. Effects of an unfixed combination latanoprost and pilocarpine on the intraocular pressure and pupil size of normal dogs. Vet Ophthalmol 2012;15:6470.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 7. Plummer CE, Regnier A, Gelatt KN. The canine glaucomas. In: Gelatt KN, ed. Veterinary ophthalmology. 5th ed. Oxford, England: Wiley-Blackwell, 2013;10501145.

    • Search Google Scholar
    • Export Citation
  • 8. Yoshitomi T, Ito Y. Effects of indomethacin and prostaglandins on the dog iris sphincter and dilator muscles. Invest Ophthalmol Vis Sci 1988;29:127132.

    • Search Google Scholar
    • Export Citation
  • 9. Carreras FJ, Porcel D, Gonzalez-Caballero F. Expanding forces in aqueous outflow pathways of a nonaccommodating mammal: an approach via comparative dynamic morphology. Comp Biochem Physiol A Physiol 1997;117:197209.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Gum GG, Metzger KJ, Gelatt KJ, et al. Tonographic effects of pilocarpine and pilocarpine-epinephrine in dogs. J Small Anim Pract 1993;34:112116.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11. Morrison JC, Van Buskirk EM. The canine eye: pectinate ligaments and aqueous outflow resistance. Invest Ophthalmol Vis Sci 1982;23:726732.

    • Search Google Scholar
    • Export Citation
  • 12. Samuelson D, Streit A. Microanatomy of the anterior uveoscleral outflow pathway in normal and primary open-angle glaucomatous dogs. Vet Ophthalmol 2012;15:4753.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13. Park S, Kang S, Lee E, et al. Ultrasound biomicroscopic study of the effects of topical latanoprost on the anterior segment and ciliary body thickness in dogs. Vet Ophthalmol 2016;19:498503.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14. Overby DR, Bertrand J, Schicht M, et al. The structure of the trabecular meshwork, its connections to the ciliary muscle, and the effect of pilocarpine on outflow facility in mice. Invest Ophthalmol Vis Sci 2014;55:37273736.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 15. Miller PE, Bentley E. Clinical signs and diagnosis of the canine primary glaucomas. Vet Clin North Am Small Anim 2015;45:11831212.

  • 16. Alario AF, Strong TD, Pizzirani S. Medical treatment of primary canine glaucoma. Vet Clin North Am Small Anim 2015;45:12351259.

  • 17. Taniguchi T, Haque MSR, Sugiyama K, et al. Ocular hypotensive mechanism of topical isopropyl unoprostone, a novel prostaglandin metabolite-related drug, in rabbits. J Ocul Pharmacol Ther 1996;12:489498.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18. Richter M, Kraus AH, Woodward DF, et al. Morphological changes in the anterior eye segment after long-term treatment with different receptor selective prostaglandin agonists and a prostamide. Invest Ophthalmol Vis Sci 2003;44:44194426.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 19. Takagi Y, Nakajima T, Shimazaki A, et al. Pharmacological characteristics of AFP-168 (tafluprost), a new prostanoid FP receptor agonist, as an ocular hypotensive drug. Exp Eye Res 2004;78:767776.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20. Miller PE. Study design and methodologies for evaluation of anti-glaucoma drugs. In: Gilger B, ed. Ocular pharmacology and toxicology. Methods in pharmacology and toxicology. Totowa, NJ: Humana Press, 2013;205242.

    • Search Google Scholar
    • Export Citation
  • 21. Gwin RM, Gelatt KN, Gum GG, et al. The effect of topical pilocarpine on intraocular pressure and pupil size in the normotensive and glaucomatous beagle. Invest Ophthalmol Vis Sci 1977;16:11431148.

    • Search Google Scholar
    • Export Citation
  • 22. Kahane N, Raskansky H, Bdolah-Abram T, et al. The effects of topical parasympatholytic drugs on pupil diameter and intraocular pressure in healthy dogs treated with 0.005% latanoprost. Vet Ophthalmol 2016;19:464472.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 23. Smith LN, Miller PE, Felchle LM. Effects of topical administration of latanoprost, timolol, or a combination of latanoprost and timolol on intraocular pressure, pupil size, and heart rate in clinically normal dogs. Am J Vet Res 2010;71:10551061.

    • Crossref
    • Search Google Scholar
    • Export Citation

Advertisement

Effects of prostaglandin-mediated and cholinergic-mediated miosis on morphology of the ciliary cleft region in dogs

Sangwan ParkDepartment of Veterinary Clinical Sciences, College of Veterinary Medicine and Research Institute for Veterinary Science, Seoul National University, Seoul 08826, Republic of Korea.

Search for other papers by Sangwan Park in
Current site
Google Scholar
PubMed
Close
 DVM
,
Seonmi KangDepartment of Veterinary Clinical Sciences, College of Veterinary Medicine and Research Institute for Veterinary Science, Seoul National University, Seoul 08826, Republic of Korea.

Search for other papers by Seonmi Kang in
Current site
Google Scholar
PubMed
Close
 DVM, PhD
,
Jaegook LimDepartment of Veterinary Clinical Sciences, College of Veterinary Medicine and Research Institute for Veterinary Science, Seoul National University, Seoul 08826, Republic of Korea.

Search for other papers by Jaegook Lim in
Current site
Google Scholar
PubMed
Close
 DVM
,
Eunjin ParkDepartment of Veterinary Clinical Sciences, College of Veterinary Medicine and Research Institute for Veterinary Science, Seoul National University, Seoul 08826, Republic of Korea.

Search for other papers by Eunjin Park in
Current site
Google Scholar
PubMed
Close
 DVM
,
Taekjin NamDepartment of Veterinary Clinical Sciences, College of Veterinary Medicine and Research Institute for Veterinary Science, Seoul National University, Seoul 08826, Republic of Korea.

Search for other papers by Taekjin Nam in
Current site
Google Scholar
PubMed
Close
 DVM
,
Seowoo JeongDepartment of Veterinary Clinical Sciences, College of Veterinary Medicine and Research Institute for Veterinary Science, Seoul National University, Seoul 08826, Republic of Korea.

Search for other papers by Seowoo Jeong in
Current site
Google Scholar
PubMed
Close
 DVM
, and
Kangmoon SeoDepartment of Veterinary Clinical Sciences, College of Veterinary Medicine and Research Institute for Veterinary Science, Seoul National University, Seoul 08826, Republic of Korea.

Search for other papers by Kangmoon Seo in
Current site
Google Scholar
PubMed
Close
 DVM, PhD

Abstract

OBJECTIVE To compare morphology of the ciliary cleft (CC) region in dogs after topical administration of latanoprost, pilocarpine, or a combination of latanoprost and pilocarpine.

ANIMALS 6 Beagles.

PROCEDURES A prospective 4-phase crossover study with washout periods was performed. Latanoprost (phase L), pilocarpine (phase P), pilocarpine followed by latanoprost (phase PL), and latanoprost followed by pilocarpine (phase LP) were administered to the right eye. Artificial tears were administered to the left eye (control eye). For each phase, pupil diameter and intraocular pressure (IOP) were measured and ultrasonographic biomicroscopy was performed 2 hours after topical treatment. Angle opening distance (AOD), ciliary cleft width (CCW), ciliary cleft length (CCL), and ciliary cleft area (CCA) were evaluated.

RESULTS All treated eyes had marked miosis without significant differences in pupil diameter among phases. Significant IOP reductions were detected for all phases, except phase P. The AOD and CCA were significantly increased in all phases for treated eyes, compared with results for control eyes. The CCW was significantly increased in phases P, PL, and LP; CCL was significantly increased in phases PL and LP. Comparison of treated eyes among phases revealed that CCW differed significantly between phases L and P and between phases L and PL.

CONCLUSIONS AND CLINICAL RELEVANCE Prostaglandin-mediated and cholinergic-mediated miosis caused variations in CC configurations. When latanoprost and pilocarpine were used in combination, the first drug administered determined the cleft morphology, which was not fully reversed by the second drug. The CC morphology did not fully explain IOP reductions.

Abstract

OBJECTIVE To compare morphology of the ciliary cleft (CC) region in dogs after topical administration of latanoprost, pilocarpine, or a combination of latanoprost and pilocarpine.

ANIMALS 6 Beagles.

PROCEDURES A prospective 4-phase crossover study with washout periods was performed. Latanoprost (phase L), pilocarpine (phase P), pilocarpine followed by latanoprost (phase PL), and latanoprost followed by pilocarpine (phase LP) were administered to the right eye. Artificial tears were administered to the left eye (control eye). For each phase, pupil diameter and intraocular pressure (IOP) were measured and ultrasonographic biomicroscopy was performed 2 hours after topical treatment. Angle opening distance (AOD), ciliary cleft width (CCW), ciliary cleft length (CCL), and ciliary cleft area (CCA) were evaluated.

RESULTS All treated eyes had marked miosis without significant differences in pupil diameter among phases. Significant IOP reductions were detected for all phases, except phase P. The AOD and CCA were significantly increased in all phases for treated eyes, compared with results for control eyes. The CCW was significantly increased in phases P, PL, and LP; CCL was significantly increased in phases PL and LP. Comparison of treated eyes among phases revealed that CCW differed significantly between phases L and P and between phases L and PL.

CONCLUSIONS AND CLINICAL RELEVANCE Prostaglandin-mediated and cholinergic-mediated miosis caused variations in CC configurations. When latanoprost and pilocarpine were used in combination, the first drug administered determined the cleft morphology, which was not fully reversed by the second drug. The CC morphology did not fully explain IOP reductions.

Latanoprost and pilocarpine are effective ocular hypotensive drugs for dogs and humans. Latanoprost, a prostaglandin analog, reduces IOP in humans and other primates by initial relaxation of the ciliary muscle and remodeling of the extracellular matrix between the muscle bundles over time, thus facilitating uveoscleral outflow.1,2 Pilocarpine, a cholinergic receptor agonist, contracts the ciliary muscle bundles to pull on the scleral spur, thus opening fluid channels in the trabecular meshwork and increasing trabecular outflow.3 Despite the antagonistic mechanism of action on the ciliary muscle status, additive effects of latanoprost with pilocarpine have been suggested in human clinical studies.2,4,5 Slightly greater reductions in IOP, although not significantly different, were also detected after a combination of latanoprost and pilocarpine was administered to dogs.6

In contrast to effects in humans and other primates, prostaglandin-mediated miosis is commonly described in dogs, cats, and horses.7 Among the prostaglandins, prostaglandin F is the most potent for contracting the iris sphincter muscle of dogs.8 Miosis has been regarded as an important effect6,7; however, it has been suggesteda that latanoprost-induced miosis could play an important role in rapid reduction of IOP by resolving the pupillary block (contact of the pupillary margins with the lens) in dogs with primary closed-angle glaucoma. In addition, cholinergic miotics have been used for the long-term treatment of dogs with open-angle glaucoma.7 During cholinergic-mediated miosis, the iris reportedly pulls the inner leaflet of the ciliary body rostrally and centrally, which results in widening of the CC.9 However, few studies have been conducted to investigate differences between prostaglandin-mediated and cholinergic-mediated miosis.

The purpose of the study reported here was to investigate differences in morphology of the CC region between prostaglandin-mediated and cholinergic-mediated miosis in dogs. We hypothesized that there would be structural differences associated with their pharmacological action on ciliary muscle contractility. Ultrasonographic biomicroscopy was used to describe the dynamic morphology of the anterior ocular segment in vivo. Furthermore, we sought to identify possible additive effects of pilocarpine and latanoprost in dogs and the relationship between those effects and structural alterations.

Materials and Methods

Animals

Six laboratory Beagles were used in the study. All dog eyes were ophthalmologically normal, as determined on the basis of results of a complete ophthalmic examination, including slit-lamp biomicroscopy,b indirect ophthalmoscopy,c and rebound tonometry.d Experimental procedures were reviewed and approved by the Institutional Animal Care and Use Committee of the Seoul National University (SNU-160613-19).

Experimental design

All dogs were acclimatized to experimental procedures for 3 days before the start of the experiments. At the same time on each of those 3 days, rebound tonometry was performed 3 times; a topical anesthetic was then applied to each eye, and the iridocorneal angle was evaluated by use of UBM.

A prospective crossover study consisting of 4 phases was conducted; the washout period between phases was ≥ 5 days. For each phase, IOP was measured in both eyes of all dogs by use of a rebound tonometer.d Then, all dogs received 1 drop of 0.005% latanoproste (phase L), 1 drop of 2% pilocarpinef (phase P), 1 drop of pilocarpine followed 5 minutes later by 1 drop of latanoprost (phase PL), or 1 drop of latanoprost followed 5 minutes later by 1 drop of pilocarpine (phase LP) in the right eye (drug-treated eye). One drop of artificial tearsg (placebo treatment) was administered to the left eye (control eye) of each dog for phases L and P, and 2 drops of artificial tears were administered 5 minutes apart to the control eye in phases PL and LP.

Measurement of pupil diameter and IOP as well as UBM were performed on both eyes 2 hours after topical treatment. Three valid IOP measurements were obtained with the tonometer, and the mean value was calculated for each eye in each phase. A Jameson caliper was used to measure the maximal horizontal pupil diameter. Anesthetic (0.5% proparacaine hydrochlorideh) was applied topically. The eyelids were manually held open, and UBM was performed with a handheld 50-MHz transduceri placed perpendicular to the limbus at the 12 o'clock position. All experimental procedures were performed in the same quiet examination room by the same investigator (SP). Dogs were not sedated for experimental procedures. Although the light intensity was not measured, the dimmer switch in the examination room was set at the same intensity throughout the experiments.

UBM measurements

Only images with all the anatomic landmarks (corneoscleral limbus, iris root, CC, and anterior lens capsule) were selected for analyses (Figure 1). Considering the unmasked and nonrandomized nature of the study, each image was analyzed 5 separate times with an interval of at least 1 week between analyses to minimize operator bias and reduce subjectivity.

Figure 1—
Figure 1—

Schematic diagram (A) and representative UBM image (B) of the iridocorneal angle and CC region in a dog. The ciliary body (CB) splits into 2 portions to form the CC. The ciliary body musculature (CBM) of the outer leaflet is much smaller than that of the inner leaflet (the inner leaflet forms the inner border of the CC). CP = Ciliary process.

Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.980

Several variables were measured with the built-in caliper of the UBM software (Figure 2). These included the AOD (distance on a perpendicular line from the surface of the peripheral iris root to the inner surface of the cornea or sclera), CCW (distance from the internal border of the outer leaflet of the ciliary body to the internal border of the inner leaflet of the ciliary body at the caudal end of the CC), CCL (distance between the caudal end of the CC and the surface of the peripheral iris root), and CCA (cleft area surrounded by the internal border of the CC and AOD).

Figure 2—
Figure 2—

An enlarged version of the representative UBM image in Figure 1. Measurement of the AOD, CCW, CCL, and CCA is indicated.

Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.980

Statistical analysis

Mean and SD values were calculated for pupil diameter, IOP, and UBM measurements in each eye of each dog for each phase. Differences for the control eyes among the 4 phases were evaluated by use of the Kruskal-Wallis test. Within each phase, results for drug-treated eyes were compared with results for control eyes by use of the Wilcoxon signed rank test. Differences for each variable of the drug-treated eyes among the 4 phases were evaluated by use of the Kruskal-Wallis test. Statistical analyses were performed with commercially available software.j Values of P < 0.05 were considered significant.

Results

Mean ± SD pupil diameter of the control eyes was 6.7 ± 1.2 mm, 7.4 ± 0.9 mm, 7.0 ± 0.6 mm, and 7.4 ± 0.9 mm for phases L, P, PL, and LP, respectively; values did not differ significantly among the 4 phases. Mean ± SD pupil diameter of drug-treated eyes was 1.1 ± 0.2 mm, 1.4 ± 0.5 mm, 1.0 ± 0.0 mm, and 1.0 ± 0.0 mm for phases L, P, PL, and LP, respectively. All drug-treated eyes had marked miosis; however, pupil diameter of the drug-treated eyes did not differ significantly among the 4 phases.

The IOPs for the control and drug-treated eyes were determined for all phases (Table 1). No significant differences were identified for the control eyes among the 4 phases. Similarly, no significant differences were identified for the drug-treated eyes among the 4 phases. However, mean IOPs of the drug-treated eyes were significantly lower, compared with the mean IOPs of the control eyes, for all phases, except for phase P.

Table 1—

Mean ± SD values of IOP measured in eyes of 6 dogs during each of 4 experimental phases.

 IOP (mm Hg) 
PhaseControl eyeDrug-treated eyeΔIOP
L14.89 ± 2.4412.67 ± 3.52*2.22 ± 1.89 (14.9)
P13.22 ± 2.4711.33 ± 2.441.89 ± 3.61 (14.3)
PL11.72 ± 2.478.67 ± 1.74*3.06 ± 2.59 (26.1)
LP12.39 ± 2.529.22 ± 0.54*3.17 ± 2.54 (25.6)

Values in parentheses are percentages.

Latanoprost (phase L), pilocarpine (phase P), pilocarpine followed 5 minutes later by latanoprost (phase PL), and latanoprost followed 5 minutes later by pilocarpine (phase LP) were administered to the right eye of each dog. Artificial tears (placebo treatment) were administered to the left eye (control eye) during each phase. Washout period between treatments was ≥ 5 days.

Within a phase, the value differs significantly (P < 0.05) from the value for the control eye.

ΔIOP = Difference in IOP between control and drug-treated eyes.

Values of all UBM variables were determined for all 4 phases (Table 2). The AOD and CCA were significantly higher in the drug-treated eyes than in control eyes for all phases. The CCW was significantly higher in the drug-treated eyes than in the control eyes for phases P, PL, and LP. The CCL was significantly higher in the drug-treated eyes than in the control eyes only in phases PL and LP.

Table 2—

Mean ± SD values of UBM variables measured in eyes of 6 dogs during each of 4 phases.

PhaseEyeAOD (mm)CCW (mm)CCL (mm)CCA (mm2)
LControl0.36 ± 0.100.27 ± 0.071.93 ± 0.210.70 ± 0.17
 Drug-treated0.55 ± 0.04*0.24 ± 0.032.03 ± 0.200.92 ± 0.14*
PControl0.35 ± 0.080.29 ± 0.051.58 ± 0.430.58 ± 0.16
 Drug-treated0.60 ± 0.08*0.44 ± 0.09*1.64 ± 0.310.95 ± 0.22*
PLControl0.36 ± 0.090.27 ± 0.081.46 ± 0.230.53 ± 0.21
 Drug-treated0.59 ± 0.08*0.41 ± 0.08*1.88 ± 0.41*0.96 ± 0.21*
LPControl0.39 ± 0.060.28 ± 0.081.70 ± 0.240.63 ± 0.14
 Drug-treated0.57 ± 0.04*0.34 ± 0.09*1.83 ± 0.21*0.89 ± 0.16*

Within a variable within a phase, value differs significantly (P < 0.05) from the value for the control eye.

See Table 1 for remainder of key.

Comparison of results for drug-treated eyes among the 4 phases revealed that the CCW differed significantly between phases L and P and between phases L and PL (Figure 3). Other variables for the drug-treated eyes did not differ significantly among the phases.

Figure 3—
Figure 3—

Representative UBM images of the left (A, C, E, and G) and right (B, D, F, and H) eyes used to measure the CCW in the same dog during each of 4 phases. Latanoprost (phase L; B), pilocarpine (phase P; D), pilocarpine followed 5 minutes later by latanoprost (phase PL; F), and latanoprost followed 5 minutes later by pilocarpine (phase LP; H) were administered to the right eye of each dog. Artifcial tears (placebo treatment) were administered to the left eye (control eye) during each phase (A, C, E, and G, for phases L, P, PL, and LP, respectively). Washout period between treatments was ≥ 5 days.

Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.980

Discussion

In the study reported here, administration of both pilocarpine and latanoprost induced marked miosis in the eyes of dogs. Miosis has been described as an important effect caused by prostaglandin analogs in several studies of dogs.6–8 Although pilocarpine as a cholinergic receptor agonist is considered to be a traditional miotic agent,6,7 latanoprost-induced miosis was not significantly different from pilocarpine-induced miosis. There was no significant difference in pupil size among prostaglandin-mediated miosis, cholinergic-mediated miosis, and miosis caused by concurrent administration of both drugs in the present study.

Several UBM variables were used as indicators of iris or ciliary body movement to describe changes in the anterior segment after administration of the 2 classes of miotic agents. The increase in AOD and CCW were used as surrogate indicators of centripetal displacement of the iris root and inner leaflet of the ciliary body, respectively. The increase in CCL was considered to indicate axially oriented expansion of the CC likely attributable to rostral or caudal (or both) movement of the inner leaflet of the ciliary body.

Both pilocarpine and latanoprost expanded the cleft area to a similar degree, which appeared to be the result of centripetal traction of the iris root represented as the increase in AOD. The iris root was also used as one of the landmarks of CCL; however, the CCL did not differ significantly between the control and drug-treated eyes in phases L and P. This might imply that the inner leaflet of the ciliary body connected to the iris root did not stretch or move rostrally despite the latanoprost-induced and pilocarpine-induced miosis. The pectinate ligament apparently acts as a strut to prevent excessive widening and stretching of the cleft.10,11 However, significant increases in the CCL were identified in phases PL and LP (ie, when both drugs were used in combination). These changes appeared to be clinically unimportant, considering that the values of the drug-treated eyes in phases PL and LP were well within the range of measurements for control eyes across the 4 phases.

Latanoprost alone in phase L and the combination of both drugs in phases PL and LP induced a significant decrease in IOP. Although AOD and CCA were significantly increased after miosis in all 4 phases, it is unclear whether these changes were directly related to IOP reductions because pilocarpine alone (phase P) did not cause a significant reduction in IOP.6,10 In other words, we were unable to establish a causative link between CC configurations and IOP reductions in the present study. Further studies are needed to clarify the role of miosis and resultant CC expansion in the reduction of IOP. Clinically, dramatic IOP reduction was achieved by latanoprost-induced miosis in dogs with primary closed-angle glaucoma, which was attributable to reduced iridolenticular contact at the pupillary margin by miosis.a Because pilocarpine also induces miosis but does not decrease IOP as profoundly as latanoprost, despite their similar degree of CC expansion, we sought to identify differences in anterior ocular segments between prostaglandin-mediated and cholinergic-mediated miosis to provide evidence of another mechanism of action for reducing IOP.

An important finding in the study reported here was that the CCW differed between the 2 classes of miotics and also on the basis of the order of administration of those miotics. Although latanoprost alone did not modify the CCW, and even slightly decreased it in phase L, pilocarpine induced a significant increase in the CCW in phase P. Considering that the smooth muscle bundles of the outer leaflet of the ciliary body are much smaller than those of the inner leaflet of the ciliary body12 and that the inner leaflet of the ciliary body forms the inner border of the CC,9 the CCW could have been affected by the musculature of the inner leaflet of the ciliary body.

Latanoprost and pilocarpine have opposing actions on ciliary muscle contractility.1–4,7 In humans and other primates, latanoprost causes short-term relaxation of ciliary muscle bundles.1,2,4 Although the effect of latanoprost on ciliary muscle contractility of dogs has not yet been determined, it was suggested in a recent UBM study13 of dogs that a similar short-term relaxation is probable. In the present study, relaxation of the ciliary muscle 2 hours after latanoprost administration and the associated decrease in ciliary body thickness (similar to that reported in another study13) may have caused the nonsignificant reduction in CCW. On the other hand, pilocarpine is a cholinergic agent that causes contraction of ciliary muscle bundles,2–5 which could have caused a thickening of the inner leaflet of the ciliary body musculature and led to the increase in CCW identified in the present study. In addition, contraction of the ciliary muscle by pilocarpine can expand the anterior lamellated trabecular meshwork, which is equivalent to the CC in dogs.9,14 This also could have contributed to the increase in CCW.

Evaluation of the order of administration of pilocarpine and latanoprost (phases PL and LP) revealed contradictory results. Although significant increases in CCW were identified in drug-treated eyes in phases PL and LP, compared with results for the control eyes, the increase of the CCW in phase LP was less than that in phase PL. In addition, the CCW of phase LP was not significantly different from the CCW of phase L. On the basis of results of the present study, the order of pilocarpine and latanoprost administration affected ciliary muscle contractility such that the first drug determined the ciliary muscle contractility and associated cleft morphology, which was not fully reversed by the second drug.

Clinically, the progressive and irreversible closure of the CC is a main feature of primary closed-angle glaucoma in dogs.7,15 Because the CC of most glaucomatous dogs is already collapsed at the time of the initial examination, drugs that increase the CCW may not be able to reopen the CC; however, it may be worthwhile to administer such drugs because even minimal alterations in the CCW can improve aqueous humor outflow. Dogs for which the iridocorneal angle is partially occluded by pectinate ligament dysplasia or dogs that have repeated episodes of intermittent angle closure could benefit from a treatment regimen targeted to widen the CC. However, it should be mentioned that the SD values for the CCW were quite large (especially in phases PL and LP) in the present study. Because it sometimes was difficult to distinguish the caudal margin of the CC from echogenic fibrillar strands located inside the CC, and because the size of the built-in caliper cursor was quite large, there could have been some error in measurement of the CCW.

Latanoprost is used as the sole agent or in conjunction with other topical hypotensive drugs to treat dogs with glaucoma.16 However, administration of latanoprost in combination with pilocarpine did not result in a significant difference in IOP from administration of latanoprost alone in the present study. There was a nonsignificant additional reduction in IOP identified when both drugs were administered together. Further studies are necessary to determine the additive effects of pilocarpine and latanoprost in dogs.

Results of the present study might indicate that anterior segment morphology is not directly related to reductions in IOP. In addition, it is possible that the hypotensive action of latanoprost could not be attributed solely to enhancement of uveoscleral outflow. There is evidence that prostaglandin derivatives could also increase conventional trabecular outflow and reduce aqueous production in dogs.1,17–19,k However, results of a more recent studyl suggest that latanoprost does not alter aqueous humor production in dogs. The complexity of the mechanism of action of prostaglandin could be another reason that gross anatomic changes identified in the present study did not fully explain reductions in IOP.

The study reported here had several limitations. The small number of dogs used in the present study could have been a major reason that significant differences were not detected. The response rate to each drug must also be considered.20 There was 1 dog that did not respond to latanoprost and 2 dogs that did not respond to pilocarpine. This could have restricted our ability to detect differences between effects of pilocarpine and latanoprost. Moreover, it is also possible that the timing of IOP measurements in the present study did not reflect the maximal ocular hypotensive effects of the drugs. In dogs, the maximal IOP-lowering effects of 2% pilocarpine and latanoprost are evident at 3 and 5 to 6 hours after administration of a single dose, respectively.21–23 Additionally, over time, latanoprost enhances uveoscleral outflow through remodeling of the extracellular matrix between ciliary muscle bundles, reaching maximal reduction of IOP the fifth day of twice-daily administration to dogs.6 This chronic, hypotensive effect of latanoprost could not occur within 2 hours after administration of a single dose. Because most glaucomatous patients would likely receive once-daily (humans) or twice-daily (dogs) administration of latanoprost for an extended period, a multiple-dose study with a treatment duration of at least 5 days is needed to elucidate the relationship between structural changes and reductions in IOP.

In the present study, prostaglandin-mediated and cholinergic-mediated miosis in dogs resulted in differences in CC morphology that were related to the contractility of the ciliary musculature. When both drugs were administered in combination, the first drug administered determined the cleft morphology, which was not fully reversed by the second drug. Further studies are needed to clarify the relationship between anterior segment morphology and reductions in IOP.

Acknowledgments

Supported by the BK 21 PLUS Program for Creative Veterinary Science Research and the Research Institute for Veterinary Science (RIVS) of Seoul National University, Seoul, Republic of Korea.

Presented in part as an abstract at the Annual Meeting of the European College of Veterinary Ophthalmologists, Estoril, Portugal, May 2017.

ABBREVIATIONS

AOD

Angle opening distance

CC

Ciliary cleft

CCA

Ciliary cleft area

CCL

Ciliary cleft length

CCW

Ciliary cleft width

IOP

Intraocular pressure

UBM

Ultrasonographic biomicroscopy

Footnotes

a.

Miller PE, Bentley E, Diehl KA, et al. High-resolution ultrasound imaging of the anterior segment of dogs with primary glaucoma prior to and following the topical application of 0.005% latanoprost (abstr), in Proceedings. 34th Annu Meet Am Coll Vet Ophthalmol 2003;53.

b.

SL-D7, Topcon Corp, Tokyo, Japan.

c.

Vantage plus, Keeler, Windsor, England.

d.

TonoVet, iCare, Helsinki, Finland.

e.

Xalatan, Pfizer Inc, New York, NY.

f.

Isopto carpine, Alcon Korea, Seoul, Republic of Korea.

g.

Lacure, Samil Pharm Co Ltd, Seoul, Republic of Korea.

h.

Alcaine, Alcon Korea, Seoul, Republic of Korea.

i.

MD-320W, MEDA Co Ltd, Tianjin, People's Republic of China.

j.

SPSS, version 21, SPSS Inc, Chicago, Ill.

k.

Ward DA. Effects of latanoprost on aqueous humor flow rate in normal dogs (abstr), in Proceedings. 36th Annu Meet Am Coll Vet Ophthalmol 2005;1.

l.

Fentiman KE, Rankin AJ, Meekins JM. The effect of topical latanoprost on aqueous humor flow in normal dogs (abstr), in Proceedings. 48th Annu Meet Am Coll Vet Ophthalmol 2017;20:E3.

References

  • 1. Toris CB, Gabelt BT, Kaufman PL. Update on the mechanism of action of topical prostaglandins for intraocular pressure reduction. Surv Ophthalmol 2008;53:S107S120.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2. Toris CB, Zhan G, Zhao J, et al. Potential mechanism for the additivity of pilocarpine and latanoprost. Am J Ophthalmol 2001;131:722728.

  • 3. Grierson I, Lee WR, Abraham S. Effects of pilocarpine on the morphology of the human outflow apparatus. Br J Ophthalmol 1978;62:302313.

  • 4. Toris CB, Alm A, Camras CB. Latanoprost and cholinergic agonists in combination. Surv Ophthalmol 2002;47:S141S147.

  • 5. Kent AR, Vroman DT, Thomas TJ, et al. Interaction of pilocarpine with latanoprost in patients with glaucoma and ocular hypertension. J Glaucoma 1999;8:257262.

    • Search Google Scholar
    • Export Citation
  • 6. Sarchahi AA, Abbasi N, Gholipour MA. Effects of an unfixed combination latanoprost and pilocarpine on the intraocular pressure and pupil size of normal dogs. Vet Ophthalmol 2012;15:6470.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 7. Plummer CE, Regnier A, Gelatt KN. The canine glaucomas. In: Gelatt KN, ed. Veterinary ophthalmology. 5th ed. Oxford, England: Wiley-Blackwell, 2013;10501145.

    • Search Google Scholar
    • Export Citation
  • 8. Yoshitomi T, Ito Y. Effects of indomethacin and prostaglandins on the dog iris sphincter and dilator muscles. Invest Ophthalmol Vis Sci 1988;29:127132.

    • Search Google Scholar
    • Export Citation
  • 9. Carreras FJ, Porcel D, Gonzalez-Caballero F. Expanding forces in aqueous outflow pathways of a nonaccommodating mammal: an approach via comparative dynamic morphology. Comp Biochem Physiol A Physiol 1997;117:197209.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Gum GG, Metzger KJ, Gelatt KJ, et al. Tonographic effects of pilocarpine and pilocarpine-epinephrine in dogs. J Small Anim Pract 1993;34:112116.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11. Morrison JC, Van Buskirk EM. The canine eye: pectinate ligaments and aqueous outflow resistance. Invest Ophthalmol Vis Sci 1982;23:726732.

    • Search Google Scholar
    • Export Citation
  • 12. Samuelson D, Streit A. Microanatomy of the anterior uveoscleral outflow pathway in normal and primary open-angle glaucomatous dogs. Vet Ophthalmol 2012;15:4753.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13. Park S, Kang S, Lee E, et al. Ultrasound biomicroscopic study of the effects of topical latanoprost on the anterior segment and ciliary body thickness in dogs. Vet Ophthalmol 2016;19:498503.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14. Overby DR, Bertrand J, Schicht M, et al. The structure of the trabecular meshwork, its connections to the ciliary muscle, and the effect of pilocarpine on outflow facility in mice. Invest Ophthalmol Vis Sci 2014;55:37273736.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 15. Miller PE, Bentley E. Clinical signs and diagnosis of the canine primary glaucomas. Vet Clin North Am Small Anim 2015;45:11831212.

  • 16. Alario AF, Strong TD, Pizzirani S. Medical treatment of primary canine glaucoma. Vet Clin North Am Small Anim 2015;45:12351259.

  • 17. Taniguchi T, Haque MSR, Sugiyama K, et al. Ocular hypotensive mechanism of topical isopropyl unoprostone, a novel prostaglandin metabolite-related drug, in rabbits. J Ocul Pharmacol Ther 1996;12:489498.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18. Richter M, Kraus AH, Woodward DF, et al. Morphological changes in the anterior eye segment after long-term treatment with different receptor selective prostaglandin agonists and a prostamide. Invest Ophthalmol Vis Sci 2003;44:44194426.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 19. Takagi Y, Nakajima T, Shimazaki A, et al. Pharmacological characteristics of AFP-168 (tafluprost), a new prostanoid FP receptor agonist, as an ocular hypotensive drug. Exp Eye Res 2004;78:767776.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20. Miller PE. Study design and methodologies for evaluation of anti-glaucoma drugs. In: Gilger B, ed. Ocular pharmacology and toxicology. Methods in pharmacology and toxicology. Totowa, NJ: Humana Press, 2013;205242.

    • Search Google Scholar
    • Export Citation
  • 21. Gwin RM, Gelatt KN, Gum GG, et al. The effect of topical pilocarpine on intraocular pressure and pupil size in the normotensive and glaucomatous beagle. Invest Ophthalmol Vis Sci 1977;16:11431148.

    • Search Google Scholar
    • Export Citation
  • 22. Kahane N, Raskansky H, Bdolah-Abram T, et al. The effects of topical parasympatholytic drugs on pupil diameter and intraocular pressure in healthy dogs treated with 0.005% latanoprost. Vet Ophthalmol 2016;19:464472.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 23. Smith LN, Miller PE, Felchle LM. Effects of topical administration of latanoprost, timolol, or a combination of latanoprost and timolol on intraocular pressure, pupil size, and heart rate in clinically normal dogs. Am J Vet Res 2010;71:10551061.

    • Crossref
    • Search Google Scholar
    • Export Citation

Contributor Notes

Address correspondence to Dr. Seo (kmseo@snu.ac.kr).