Agonist-induced glycogenolysis in rabbit retinal slices and cultures

Lactate Transport and Receptor Actions in Retina: Potential Roles in Retinal Function and Disease

In retina, like in brain, lactate equilibrates across cell membranes via monocarboxylate transporters and in the extracellular space by diffusion, forming a basis for the action of lactate as a transmitter of metabolic signals. In the present paper, we argue that the lactate receptor GPR81, also known as HCAR1, may contribute importantly to the control of retinal cell functions in health and disease. GPR81, a G-protein coupled receptor, is known to downregulate cAMP both in adipose and nervous tissue. The receptor also acts through other down-stream mechanisms to control functions, such as excitability, metabolism and inflammation. Recent publications predict effects of the lactate receptor on neurodegeneration. Neurodegenerative diseases in retina, where the retinal ganglion cells die, notably glaucoma and diabetic retinopathy, may be linked to disturbed lactate homeostasis. Pilot studies reveal high GPR81 mRNA in retina and indicate GPR81 localization in Müller cells and retinal ganglion cells. Moreover, monocarboxylate transporters are expressed in retinal cells. We envision that lactate receptors and transporters could be useful future targets of novel therapeutic strategies to protect neurons and prevent or counteract glaucoma as well as other retinal diseases.

VIP down-regulates the inflammatory potential and promotes survival of dying (neural crest-derived) corneal endothelial cells ex vivo: necrosis to apoptosis switch and up-regulation of Bcl-2 and N-cadherin

The neuropeptide vasoactive intestinal peptide (VIP) is anti-inflammatory and protective in the immune and nervous systems, respectively. This study demonstrated in corneal endothelial (CE) cells injured by severe oxidative stress (1.4 mM H(2)O(2)) in bovine corneal organ cultures that VIP pre-treatment (0, 10(-10), 10(-8), and 10(-6) M; 15 min), in a VIP concentration-dependent manner, switched the inflammation-causing necrosis to inflammation-neutral apoptosis (showing annexin V-binding, chromatin condensation, and DNA fragmentation) and upheld ATP levels in a VIP antagonist (SN)VIPhyb-sensitive manner, while up-regulated mRNA levels of the anti-apoptotic Bcl-2 and the differentiation marker N-cadherin in a kinase A inhibitor-sensitive manner. As a result, VIP, in a concentration-dependent and VIP antagonist-sensitive manners, promoted long-term CE cell survival. ATP levels, a determining factor in the choice of apoptosis versus necrosis, measured after VIP pre-treatment and 0.5 min post-H(2)O(2) were 39.6 +/- 3.3, 50.8 +/- 6.2, 60.1 +/- 4.8, and 53.6 +/- 5.3 pmoles/microg protein (mean +/- SEM), respectively (p < 0.05, anova). VIP treatment alone concentration-dependently increased levels of N-cadherin (Koh et al. 2008), the phosphorylated cAMP-responsive-element binding protein and Bcl-2, while 10(-8) M VIP, in a VIP antagonist (SN)VIPhyb-sensitive manner, increased ATP level by 38% (p < 0.02) and decreased glycogen level by 32% (p < 0.02). VPAC1 (not VPAC2) receptor was expressed in CE cells. Thus, CE cell VIP/VPAC1 signaling is both anti-inflammatory and protective in the corneal endothelium.

Nitric oxide synthase in retina and optic nerve head of rat with increased intraocular pressure and effect of timolol

We investigated the expression of nitric oxide synthase (NOS) isoforms -1, -2 and -3 in the retina and optic nerve head (ONH) in an experimental rat model of elevated intraocular pressure (IOP) before and after treatment with timolol, to assess whether its neuroprotective action is associated with the activity of these enzymes. Episcleral vein cauterization in unilateral eyes of Wistar rats was performed to produce elevated IOP. Histological sections of retina and ONH from animals with normal IOP, with elevated IOP, and elevated IOP treated with timolol, were studied by immunohistochemistry with antibodies to NOS-1, NOS-2, and NOS-3. In the control rats, NOS-1 was localized to photoreceptor inner segments, amacrine cells and bipolar cells in the retina, and in astrocytes, pericytes and vascular nitrergic terminals in the ONH. NOS-3 immunostaining localized to the endothelial cells. The rats with elevated IOP showed increased expression of NOS-1 in the plexiform layers of the retina and reactive astrocytes in the ONH. These cells also showed NOS-2 positivity. The rats treated with timolol showed reduced expression of NOS-1 in the retina and ONH. NOS-2 was only detected in a few groups of astrocytes in the ONH. NOS-3 was unchanged in both elevated IOP and timolol-treated groups. These results show that excessive levels of NO synthesized by the NOS-1 and -2 isoforms, considered neurotoxic, might contribute to the progressive lesions of retinal ganglion cell axons. Their reduction after treatment suggests a possible neuroprotective effect of timolol in neurons exposed to excessive amounts of NO.

Beta-adrenergic receptor agonists and antagonists counteract LPS-induced neuronal death in retinal cultures by different mechanisms

Treatment with lipopolysaccharide (LPS) for 72 h was shown to dose-dependently increase nitric oxide production from 6-day-old retinal cultures. Cell death, as determined by lactate dehydrogenase (LDH) release and an increase in neuronal labelling for TUNEL, was elevated concurrently. During treatment there was an increase of both inducible nitric oxide synthase and glial fibrillary acidic protein labelling in glial cells and a reduction in the number of gamma-aminobutyric acid-positive neurones. The NOS inhibitors, N-nitro-L-arginine methyl ester, dexamethasone and indomethacin potently inhibited both nitric oxide stimulation and cell death caused by LPS. In this study, the beta(2)- (ICI-18551), beta(1)- (betaxolol) and mixed beta(1)/beta(2)- (timolol, metipranolol) adrenergic receptor antagonists were all shown to attenuate LPS-induced LDH release from these cultures, but to have no effect on LPS-stimulated nitric oxide production. This effect was mimicked by the calcium channel blocker, nifedipine. Interestingly, the beta-adrenergic receptor agonists, salbutamol, arterenol and isoproterenol were also able to attenuate cell death caused by LPS. Moreover, these compounds also inhibited LPS-stimulated nitric oxide release. These studies suggest that LPS stimulates nitric oxide release from cultured retinal glial cells and that this process leads to neurone death. beta-adrenergic receptor agonists prevent the effects of LPS by inhibiting the stimulation of nitric oxide production. The data also suggest that beta-adrenergic receptor antagonists can attenuate LPS-induced death of neurones, but that these compounds act in a manner that is neurone-dependent, is mimicked by blockade of calcium channels and is independent of the stimulation of nitric oxide release.

Different effects of visual deprivation on vasoactive intestinal polypeptide (VIP)-containing cells in the retinas of juvenile and adult rats

Vasoactive intestinal polypeptide (VIP) is a 28 amino acid peptide that has been shown to be present in a distinct subset of retinal amacrine cells (VIP+ cells). Previous work has suggested that the expression of retinal VIP might depend on the lighting conditions prevailing. Reasoning that a careful analysis of the exact conditions of this interaction and its time course might offer clues to the functional role of retinal VIP, we performed a quantitative immunohistochemical analysis of the effects of visual deprivation on the retinal VIP+ system in adult and in visually inexperienced neonatal rats. In adult rats visual deprivation caused a marked suppression of VIP-like immunoreactivity (IR) in both somata and processes of VIP+ cells which increased linearly over time, reducing the number of VIP+ cells to about 30% of the control value after approximately 50 days of deprivation. The observed changes were specific for VIP and were independent of the form of deprivation used (monocular lid suture versus keeping rats in constant darkness). However, the effects of visual deprivation were dependent on the developmental state of the rats, since juvenile rats without previous visual experience exhibited a decrease in VIP+ cells and fibers which was much smaller and occurred significantly slower than in adult rats. The suppression of VIP-like IR was completely reversible in both juvenile and adult rats when previously deprived rats were returned to a normal visual environment.