Kappa opioid agonist-induced changes in IOP: correlation with 3H-NE release and cAMP accumulation

Opioid receptors: methods for detection and their modes of actions in the eye

This study provides evidence for the presence of opioid-receptors in the retina, optic nerve, and optic nerve head astrocytes. These receptors were measured by more than one technique including Western blotting, immunohistochemistry, and functional assays such as scotopic electroretinogram (ERG) and Pattern ERG. I also have provided evidence in recently published work from my laboratory that opioid receptors, more specifically δ-opioid receptors, play crucial roles in retina neuroprotection against ischemic and glaucomatous injuries. This chapter provides detailed procedures to measure opioid receptor activation and their roles in retina neuroprotection using functional assays such as scotopic ERG and pattern ERG.

Experimental Tibetan monkey domestication and its application for intraocular pressure measurement

AIM

To train Tibetan monkey (Macaca thibetana) for intraocular pressure (IOP) measurement in conscious state and obtain normal IOP in conscious Tibetan Macaque.

METHODS

The training was based on award-conditioned behavior. Food stimulation and human-animal interaction were used in this training.

RESULTS

Trained Tibetan monkeys calmly accepted IOP measurement by the TonoVet® rebound tonometer without sedation or anesthesia and their IOP values were similar to other primates.

CONCLUSION

Human-cultivated Thibetan monkeys are tamable, and can be used for biomedical research such as ophthalmic research without anesthesia.

Morphine-induced nitric oxide production in isolated, iris-ciliary bodies

Considerable evidence suggests that the nitric oxide (NO)/cGMP signaling pathway plays an integral role in opioid receptor-mediated responses in the cardiovascular and immune systems. Previous studies in our laboratory and others have shown that nitric oxide (NO) plays a role in morphine-induced reduction of intraocular pressure (IOP) and pupil diameter (PD) in the New Zealand white (NZW) rabbit. The present study is designed to determine the effect of morphine on NO production in the isolated, iris-ciliary body (ICB), site of aqueous humor production, as this effect could be associated with morphine-stimulated changes in aqueous humor dynamics and iris function. ICBs obtained from normal NZW rabbits were utilized in these experiments. In some experiments, ICB samples were treated with morphine (1, 10 and 100 microM) for 1 h and later examined for changes in NO levels using a NO detection kit. In other experiments, tissue samples were pretreated with naloxone (non-selective opioid receptor antagonist), L-NAME (non-selective NO-synthase inhibitor) or GSH (sulfhydryl reagent) for 30 min, followed by treatment with morphine (10 muM). Morphine caused a concentration-dependent increase in the release of NO from ICBs. Levels of NO detected in the incubation medium of ICB samples increased from 1.49 +/- 0.19 (control) to 8.81 +/- 2.20 microM/mg protein (morphine-treated; 100 microM). Morphine-stimulated release of NO was significantly inhibited in tissues pretreated with 10 microM naloxone, L-NAME, or GSH. Results obtained from this study suggest that morphine stimulates NO release from the ICB through a mechanism that involves activation of NO-releasing opioid receptors. These results support the in vivo effects of morphine demonstrated in previous studies.

Delta-opioid agonist-stimulated inositol phosphate formation in isolated, rabbit iris-ciliary bodies: role of Gi/o proteins and Gβγ-subunits

Our laboratory has previously demonstrated the ability of kappa- and delta-opioid agonists to decrease aqueous flow rates and intraocular pressure of rabbits. The mechanisms by which these agents act in the ciliary body of the rabbit could involve inhibition of cAMP production, as well as increased generation of inositol phosphates (IPs). With regard to enhanced production of IPs, it has been suggested that their levels can be augmented by stimulation of phospholipase C via opioid receptors linked to either Galpha-subunits derived from Gq proteins or Gbetagamma-subunits derived from G(i/o)-proteins. The aim of the current study is to investigate the role of pertussis toxin (PTX)-sensitive G-proteins and Gbetagamma-subunits in delta-opioid agonist-mediated changes in IP production in the isolated, rabbit iris-ciliary body (ICB). In one set of experiments, ICB segments were treated with the delta agonist, SNC80 (10(-12)-10(-7) mol/l) alone. Other experiments were conducted utilizing SNC80 following pretreatment with either phosducin (Gbetagamma-subunit scavenger), PTX, or the delta antagonist, naltrindole. IP production was measured by ion exchange chromatography. Basal levels of IPs in the rabbit ICB were 58,287 +/- 2162, 15,218 +/- 969 and 2083 +/- 367 dpm/mg protein for IP1, IP2 and IP3, respectively. The highly selective delta-opioid receptor agonist, SNC80, produced concentration-dependent increases in the levels of the IPs in the ICB, which were diminished in the presence of the delta antagonist, naltrindole, indicating the effect was mediated via activation of delta-opioid receptors. Pretreatment of tissues with PTX (75 ng/ml), completely abolished the concentration-dependent production of IPs generated by SNC80 (10(-11)-10(-7) mol/l). In addition, pretreatment with phosducin (1 nmol/l) ablated the SNC80 (1 nmol/l)-induced increase in the formation of all three inositol phosphates. Results from this study indicate that the delta-opioid receptor-mediated increase in IP production is a PTX-sensitive G(i/o) response that involves participation of Gbetagamma-subunits. Thus, delta-opioid receptor activation by SNC80 in the ICB could be responsible, in part, for suppression of aqueous humor dynamics.

Ocular hypotensive action of topical flunarizine in the rabbit: role of sigma 1 recognition sites

In a previous study we ascertained the presence of sigma1 and sigma2 recognition sites in the rabbit iris-ciliary body, an ocular structure involved in aqueous humor production and drainage. We characterized the sigma1 sites using the preferential ligand (+)-pentazocine, which caused a significant reduction of intraocular pressure (IOP). In the present study, flunarizine, a calcium channel blocker with a complex pharmacological profile, bound to sigma1 sites expressed in the iris-ciliary body with moderate affinity (K(i) = 68 nM). Unilateral topical flunarizine (0.01-0.1%) caused a dose-related reduction of IOP in ocular normotensive rabbits and in the alpha-chymotrypsin model of ocular hypertension, without altering the IOP of the contralateral eye. This activity was blocked by the sigma1 site antagonist NE-100 [N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]ethylamine HCl] which, by itself, had no effect on IOP. Detection of flunarizine in rabbit iris-ciliary body homogenates, after topical instillation, showed that it adequately penetrates the rabbit eye. To investigate mechanisms that may contribute to ocular hypotension induced by sigma1 agonists, we carried out in vitro studies on the isolated rabbit iris-ciliary body. Flunarizine (IC50 = 5. 96 nM) and (+)-pentazocine (IC50 = 3. 81 nM) inhibited [3H]norepinephrine release. Moreover, flunarizine (IC50 = 6.34 nM) and (+)-pentazocine (IC50 = 27.26 nM) also antagonized isoproterenol-induced cAMP accumulation. The action of flunarizine and (+)-pentazocine was sensitive to NE-100 antagonism; however, this latter compound partially prevented their effect on [3H]norepinephrine and cAMP accumulation. These findings indicate that flunarizine and (+)-pentazocine interact with ocular sigma1 sites and may prove effective in the control of ocular hypertension.

Inhibition of cAMP accumulation by kappa-receptor activation in isolated iris-ciliary bodies: role of phosphodiesterase and protein kinase C

The present study was designed to examine the roles of protein kinase C (PKC) and phosphodiesterase (PDE) in modulating the action of kappa receptor stimulation on cAMP accumulation in isolated iris-ciliary bodies (ICBs) of New Zealand White rabbits. The kappa receptor agonist, (+/-)-1-(3,4-dichlorophenyl)acetyl-2-(1-pyrrolidinyl)methylpiperidine (BRL-52537) (BRL), and the PKC activator, phorbol 12,13-dibutyrate (PDBu), both caused a concentration-dependent inhibition of forskolin-stimulated cAMP production. The inhibitory effect of BRL on cAMP levels was significantly reduced in the presence of the selective kappa receptor antagonist, norbinaltorphimine (10(-6) M), but the effect of PDBu was not, thus supporting the involvement of kappa-opioid receptors in the response to BRL. In the presence of 3-isobutyl-1-methylxanthine or rolipram (10(-5) M), the inhibitory effect of BRL or PDBu (10(-6) M) on cyclic AMP accumulation was abolished. In the presence of the selective PKC antagonist, chelerythrine (10(-6) M), the inhibitory effect of PDBu or BRL (10(-6) M) was significantly reduced. Direct measurement of PDE activity demonstrated the ability of BRL and PDBu (10(-6) M) to augment the activity of these enzymes. Preincubation of ICBs with rolipram (10(-5) M) or chelerythrine (10(-6) M) caused significant reversal of both BRL- and PDBu-induced increases in PDE activity. These results indicate that stimulation of PKC and PDE4 activity is part of the complex mechanism whereby kappa-opioid receptor agonists reduce levels of cAMP in the rabbit ICB. This mechanism of action could contribute to the ability of kappa-opioid agonists to suppress aqueous flow rate and to lower intraocular pressure.

Elevation of atrial natriuretic peptide levels in aqueous humor of the rabbit by kappa opioid receptor agonists

Following the topical administration of three kappa agonists (bremazocine, spiradoline and ICI 204448) to rabbit eyes, aqueous humor samples were analyzed for levels of atrial natriuretic peptide (ANP). Bremazocine (BRE) and spiradoline (SPR) elevated aqueous ANP levels in a dose-dependent manner. In contrast, ICI had no significant effect on ANP levels in aqueous humor. Nor-binaltorphimine (nor-BNI), a selective kappa opioid receptor antagonist, was used to assess kappa opioid receptor involvement and glibenclamide, an ATP-sensitive K+ channel blocker, was used to test for the role of ATP-sensitive potassium channels in ANP release. Pretreatment with nor-BNI antagonized the increases in ANP levels observed with both BRE and SPR. Likewise, glibenclamide suppressed the stimulation of ANP secretion by bremazocine. In summary, BRE and SPR increased ANP levels in aqueous humor of rabbits, in part, via activation of K+(ATP) channels that are assumed to be associated with kappa opioid receptors.