The opioidergic system: potential roles and therapeutic indications in the eye

Delta Opioids: Neuroprotective Roles in Preclinical Studies

Since ancient times, opioids have been used clinically and abused recreationally. In the early stages (about 1,000 AD) of opium history, an Arab physician, Avicenna, administered opioids to control diarrhea and eye diseases. ¹ Opioids have very strong pain relieving properties and they also regulate numerous cellular responses. Opioid receptors are expressed throughout the body, including the nervous system, heart, lungs, liver, gastrointestinal tract, and retina. ^2-6 Delta opioid receptors (DORs) are a very attractive target from the perspective of both receptor function and their therapeutic potential. Due to a rapid progress in mouse mutagenesis and development of small molecules as DOR agonist, novel functions and roles of DORs have emerged in recent years. This review article focuses on the recent advances in the neuroprotective roles of DOR agonists in general and retina neuroprotection in particular. Rather than being exhaustive, this review highlights the selected studies of DOR function in neuroprotection. We also highlight our preclinical studies using rodent models to demonstrate the potentials of DOR agonists for retinal neuroprotection. Based on existing literature and our recently published data on the eye, DOR agonists possess therapeutic abilities that protect the retina and optic nerve injury against glaucoma and perhaps other retinopathies as well. This review also highlights the signaling events associated with DOR for neuroprotection in the eye. There is a need for translational research on DORs to recognize their potential for clinical application such as in glaucoma.

Neuroprotective Peptides in Retinal Disease

In the pathogenesis of many disorders, neuronal death plays a key role. It is now assumed that neurodegeneration is caused by multiple and somewhat converging/overlapping death mechanisms, and that neurons are sensitive to unique death styles. In this respect, major advances in the knowledge of different types, mechanisms, and roles of neurodegeneration are crucial to restore the neuronal functions involved in neuroprotection. Several novel concepts have emerged recently, suggesting that the modulation of the neuropeptide system may provide an entirely new set of pharmacological approaches. Neuropeptides and their receptors are expressed widely in mammalian retinas, where they exert neuromodulatory functions including the processing of visual information. In multiple models of retinal diseases, different peptidergic substances play neuroprotective actions. Herein, we describe the novel advances on the protective roles of neuropeptides in the retina. In particular, we focus on the mechanisms by which peptides affect neuronal death/survival and the vascular lesions commonly associated with retinal neurodegenerative pathologies. The goal is to highlight the therapeutic potential of neuropeptide systems as neuroprotectants in retinal diseases.

PI3K/Akt Pathway: A Role in δ-Opioid Receptor-Mediated RGC Neuroprotection

Purpose

This study examines the role of PI3K/Akt pathway in δ-opioid receptor agonist (SNC-121)-induced RGC neuroprotection in a chronic glaucoma rat model.

Methods

Injecting hypertonic saline into the limbal veins of Brown Norway rats elevated IOP. Rats were treated either with 1 mg/kg SNC-121 or 3 mg/kg 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY-294002; PI3K/Akt inhibitor) plus SNC-121 once daily for 7 days. Pattern ERGs were recorded in response to contrast reversal of patterned visual stimuli. Retinal ganglion cells (RGC) were visualized by Fluorogold retrograde labeling. Optic nerve head (ONH) astrocytes were pretreated with PI3K/Akt inhibitors for 30 minutes followed by 1-μM SNC-121 treatment. Changes in matrix metalloproteinases (MMP-1, -2, and -3) production and PI3K/Akt activation in optic nerve and TNF-α treated ONH astrocytes were measured by immunohistochemistry and Western blotting.

Results

SNC-121 activates the PI3K/Akt pathway in ONH astrocytes and the retina. In ONH astrocytes, SNC-121–induced Akt activation was fully inhibited by PI3K/Akt inhibitors. A sustained decline (7–42 days post injury) in Akt activation was seen in the ocular-hypertensive retina and optic nerve. This decline is reversed to normal levels by 1-mg/kg intraperitoneally (i.p.) SNC-121 treatment. Both pattern ERG amplitudes and RGC numbers were reduced in ocular hypertensive eyes, which were significantly increased in SNC-121–treated animals. Interestingly, SNC-121–induced increase in pattern-ERG amplitudes and RGC numbers were inhibited in LY-294002 pretreated animals. Additionally, SNC-121 treatment inhibited MMP-1, -2, and -3 production from the optic nerve of ocular hypertensive rats and TNF-α–treated ONH astrocytes.

Conclusions

PI3K/Akt pathway plays a crucial role in SNC-121–mediated RGC neuroprotection against glaucomatous injury.

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.

Delta-opioid agonist SNC-121 protects retinal ganglion cell function in a chronic ocular hypertensive rat model

PURPOSE

This study examined if the delta-opioid (δ-opioid) receptor agonist, SNC-121, can improve retinal function and retinal ganglion cell (RGC) survival during glaucomatous injury in a chronic ocular hypertensive rat model.

METHODS

IOP was raised in brown Norway rats by injecting hypertonic saline into the limbal venous system. Rats were treated with 1 mg/kg SNC-121 (intraperitoneally [IP]) once daily for 7 days. Pattern-electroretinograms (PERGs) were obtained in response to contrast reversal of patterned visual stimuli. RGCs were visualized by fluorogold retrograde labeling. Expression of TNF-α and p38 mitogen-activated protein (MAP) kinase was measured by immunohistochemistry and Western blotting.

RESULTS

PERG amplitudes in ocular hypertensive eyes were significantly reduced (14.3 ± 0.60 μvolts) when compared with healthy eyes (18.0 ± 0.62 μvolts). PERG loss in hypertensive eyes was inhibited by SNC-121 treatment (17.20 ± 0.1.3 μvolts; P < 0.05). There was a 29% loss of RGCs in the ocular hypertensive eye, which was inhibited in the presence of SNC-121. TNF-α production and activation of p38 MAP kinase in retinal sections and optic nerve samples were upregulated in ocular hypertensive eyes and inhibited in the presence of SNC-121. Furthermore, TNF-α induced increase in p38 MAP kinase activation in astrocytes was inhibited in the presence of SNC-121.

CONCLUSIONS

These data provide evidence that activation of δ-opioid receptors inhibited the loss of PERG amplitudes and rate of RGC loss during glaucomatous injury. Mechanistic data provided clues that TNF-α is mainly produced from glial cells and activates p38 MAP kinase, which was significantly inhibited by SNC-121 treatment. Overall, data indicate that enhancement of δ-opioidergic activity in the eye may provide retina neuroprotection against glaucoma.

Preservation of retina ganglion cell function by morphine in a chronic ocular-hypertensive rat model

PURPOSE

The current study examined if opioid-receptor-activation by morphine can improve retinal function and retinal ganglion cell (RGC) integrity in a chronic glaucoma rat model.

METHODS

IOP was raised in Brown Norway rats by injecting hypertonic saline into the limbal venous system. Rats were treated daily with 1 mg/kg morphine for 28 days at 24-hour intervals; animals were examined for changes in IOP by a TonoLab tonometer. Pattern-ERG (PERG) was obtained in response to contrast-reversal of patterned visual stimuli. RGCs were visualized by fluorogold retrograde-labeling. Changes in the expression pattern of TNF-α and caspases were measured by Western blotting.

RESULTS

A significant IOP elevation was seen as early as 7 days, and maintained for up to 8 weeks, after surgery. PERG amplitudes were significantly reduced in ocular-hypertensive eyes (15.84±0.74 μvolts) when compared with normal eyes (19±0.86 μvolts). PERG deficits in hypertensive eyes were reversed by morphine treatment (18.23±0.78 μvolts; P<0.05). In untreated rats, a 24% reduction in labeled RGCs was measured in the hypertensive eye compared with the normal eye. This reduction in RGC labeling was significantly ameliorated in the presence of morphine. In retinal samples, TNF-α, caspase-8, and caspase-3 expressions were significantly upregulated in ocular hypertensive eyes, but completely inhibited in the morphine-treated animals.

CONCLUSIONS

These data provide evidence that activation of opioid receptors can provide significant improvement in PERG and RGC integrity against glaucomatous injury. Mechanistic data provide clues that activation of one or more opioid receptors can reduce glaucomatous-injury via suppression of TNF-α and caspase activation.

Opioid receptor activation: suppression of ischemia/reperfusion-induced production of TNF-α in the retina

PURPOSE

The detrimental role of TNF-α in ischemia-induced tissue damage is known. The authors study examined whether opioid receptor activation alters TNF-α levels in the postischemic retina.

METHODS

Retinal ischemia was induced by raising the intraocular pressure above systolic blood pressure (155-160 mm Hg) for 45 minutes. Rats were pretreated with the opioid receptor agonist morphine (1 mg/kg; intraperitoneally) before injury. Selected animals were pretreated with the opioid antagonist naloxone (3 mg/kg; intraperitoneally). Human optic nerve head (ONH) astrocytes and rat microglial cells were treated with morphine (0.1-1 μM) for 24 hours and then treated with 10 μg/mL or 30 ng/mL lipopolysaccharide (LPS), respectively. TNF-α was measured by ELISA. Opioid receptor subtypes in astrocytes and microglia were determined by Western blot analysis.

RESULTS

There was a time-dependent increase in TNF-α production; the maximum production occurred at 4 hours after ischemia and localized to the inner retinal regions. Ischemia-induced TNF-α production was significantly inhibited by morphine. In astrocytes and microglia, LPS triggered a robust increase in the release of TNF-α, which was significantly inhibited (P < 0.05) by morphine. Naloxone reversed the morphine-induced suppression of TNF-α production in vivo and in vitro. Both ONH astrocytes and microglial cells expressed δ-, κ-, and μ-opioid receptor subtypes.

CONCLUSIONS

These data provide evidence that the production of TNF-α after ischemia/reperfusion injury is an early event and that opioid receptor activation reduces the production of TNF-α. Immunohistochemistry data and in vitro studies provide evidence that ONH astrocytes and microglial cells are the primary sources for the TNF-α production under ischemic/inflammatory conditions. Activation of one or more opioid receptors can reduce ischemic/reperfusion injury by the suppression of TNF-α production.