Alterations in the retinal dopaminergic neuronal system in rats with streptozotocin-induced diabetes

Impaired dopamine signaling in early diabetic retina: Insights from D1R and D4R agonist effects on whole retina responses

The retina has low dopamine levels early in diabetes. To determine how low dopamine levels affected dopamine signaling, the effects of dopamine receptor agonists and mRNA localization were measured after 6 weeks of diabetes. Whole retina ex vivo electroretinogram (ERG) recordings were used to analyze how dopamine type 1 receptor (D1R) and type 4 (D4R) agonists change the light-evoked retinal responses of non-diabetic and 6-week diabetic (STZ injected) mouse retinas. Fluorescence in situ hybridization was utilized to analyze D4R and D1R mRNA locations and expression levels. D4R activation reduced A- and B-wave ERG amplitudes and increased B-wave implicit time and rise-time in the non-diabetic group without a corresponding change in the diabetic group. D1R activation increased B-wave rise-time and oscillatory potential peak time in the non-diabetic group also with no change in the diabetic group. The lack of responsivity to D1R or D4R agonists shows an impairment of dopamine signaling in the diabetic retina. D4R mRNA was found primarily in the outer nuclear layer where photoreceptor cell bodies reside. D1R mRNA was found in the inner nuclear layer and ganglion cell layer that contain bipolar, amacrine, horizontal and ganglion cells. There was no change in D4R or D1R mRNA expression between the non-diabetic and diabetic retinas. This suggests that the significant dopamine signaling changes observed were not from lower receptor expression levels but could be due to changes in dopamine receptor activity or protein levels. These studies show that changes in retinal dopamine signaling could be an important mechanism of diabetic retinal dysfunction.

Visual Dysfunction in Diabetes

Although diabetic retinopathy (DR) is clinically diagnosed as a vascular disease, many studies find retinal neuronal and visual dysfunction before the onset of vascular DR. This suggests that DR should be viewed as a neurovascular disease. Prior to the onset of DR, human patients have compromised electroretinograms that indicate a disruption of normal function, particularly in the inner retina. They also exhibit reduced contrast sensitivity. These early changes, especially those due to dysfunction in the inner retina, are also seen in rodent models of diabetes in the early stages of the disease. Rodent models of diabetes exhibit several neuronal mechanisms, such as reduced evoked GABA release, increased excitatory glutamate signaling, and reduced dopamine signaling, that suggest specific neuronal deficits. This suggests that understanding neuronal deficits may lead to early diabetes treatments to ameliorate neuronal dysfunction.

Dopamine receptor-mediated roles on retinal ganglion cell hyperexcitability and injury in experimental glaucoma

Extraordinary excitability (hyperexcitability) is closely related to retinal ganglion cell (RGC) injury in glaucoma. Dopamine (DA) and its receptors are involved in modulating RGC excitability. We investigated how DA system affects RGC injury in chronic ocular hypertension (COH) experimental glaucoma model. Western blotting and immunohistochemistry results revealed that expression of DA D2-like receptor (D2R) in RGCs was increased in COH retinas. Patch-clamp recordings showed that outward K⁺ currents were downregulated, while Na⁺ currents and Na_V1.6 expression were upregulated in RGCs of COH retinas, which could be reversed by intravitreal pre-injection of the D2R antagonist sulpiride, but not by the D1-like receptor (D1R) antagonist SCH23390. However, pre-injection of the D1R agonist SKF81297 could partially reverse the increased expression of Na_V1.6 proteins. Consistently, the numbers of evoked action potentials induced by current injections were increased in RGCs of COH retinas, indicating that RGCs may be in a condition of hyperexcitability. The increased frequency of evoked action potentials could be partially block by pre-injection of sulpiride, SKF81297 or DA, respectively. Furthermore, the increased number of TUNEL-positive RGCs in COH retinas could be partially reduced by intravitreal pre-injection of sulpiride, but not by pre-injection of SCH23390. Moreover, pre-injection of SKF81297 or DA could reduce the number of TUNEL-positive RGCs in COH retinas. All these results indicate that in COH retina, activation of D2R enhances RGC hyperexcitability and injury, while activation of D1R results in the opposite effects. Selective inhibition of D2R or activation of D1R may be an effective strategy for treatment of glaucoma.

Preventable risk factors for type 2 diabetes can be detected using noninvasive spontaneous electroretinogram signals

Given the ever-increasing prevalence of type 2 diabetes and obesity, the pressure on global healthcare is expected to be colossal, especially in terms of blindness. Electroretinogram (ERG) has long been perceived as a first-use technique for diagnosing eye diseases, and some studies suggested its use for preventable risk factors of type 2 diabetes and thereby diabetic retinopathy (DR). Here, we show that in a non-evoked mode, ERG signals contain spontaneous oscillations that predict disease cases in rodent models of obesity and in people with overweight, obesity, and metabolic syndrome but not yet diabetes, using one single random forest-based model. Classification performance was both internally and externally validated, and correlation analysis showed that the spontaneous oscillations of the non-evoked ERG are altered before oscillatory potentials, which are the current gold-standard for early DR. Principal component and discriminant analysis suggested that the slow frequency (0.4-0.7 Hz) components are the main discriminators for our predictive model. In addition, we established that the optimal conditions to record these informative signals, are 5-minute duration recordings under daylight conditions, using any ERG sensors, including ones working with portative, non-mydriatic devices. Our study provides an early warning system with promising applications for prevention, monitoring and even the development of new therapies against type 2 diabetes.

18F-FP-CIT dopamine transporter PET findings in the striatum and retina of type 1 diabetic rats

PURPOSE

Noninvasive methods used in clinic to accurately detect DA neuron loss in diabetic brain injury and diabetic retinopathy have not been reported up to now. ¹⁸F-FP-CIT is a promising dopamine transporter (DAT) targeted probe. Our study first applies ¹⁸F-FP-CIT PET imaging to assess DA neuron loss in the striatum and retina of T1DM rat model.

METHODS

T1DM rat model was induced by a single intraperitoneal injection of streptozotocin (STZ) (65 mg kg^-1, ip). ¹⁸F-FP-CIT uptake in the striatum and retina was evaluated at 4 weeks, 8 weeks and 12 weeks after STZ injection. The mean standardized uptake value (SUVmean) and the maximum standardized uptake value (SUVmax) were analyzed. Western blot was performed to confirm the DAT protein levels in the striatum and retina.

RESULTS

PET/CT results showed that the SUV of ¹⁸F-FP-CIT was significantly reduced in the diabetic striatum and retina compared with the normal one from 4-week to 12-week (p < 0.0001). Western blots showed that DAT was significantly lower in the diabetic striatum and retina compared to the normal one for all three time points (p < 0.05). The results from Western blots confirmed the findings in PET imaging studies.

CONCLUSIONS

DA neuron loss in the striatum and retina of T1DM rat model can be non-invasively detected with PET imaging using ¹⁸F-FP-CIT targeting DAT. ¹⁸F-FP-CIT PET imaging may be a useful tool used in clinic for DR and diabetic brain injury diagnosis in future. The expression level of DAT in striatum and retina may act as a new biomarker for DR and diabetic brain injury diagnosis.

Thymoquinone Attenuates Retinal Expression of Mediators and Markers of Neurodegeneration in a Diabetic Animal Model

BACKGROUND

Diabetic retinopathy (DR) is a slow eye disease that affects the retina due to a long-standing uncontrolled diabetes mellitus. Hyperglycemia-induced oxidative stress can lead to neuronal damage leading to DR.

OBJECTIVE

The aim of the current investigation is to assess the protective effects of thymoquinone (TQ) as a potential compound for the treatment and/or prevention of neurovascular complications of diabetes, including DR.

METHODS

Diabetes was induced in rats by the administration of streptozotocin (55 mg/kg intraperitoneally, i.p.). Subsequently, diabetic rats were treated with either TQ (2 mg/kg i.p.) or vehicle on alternate days for three weeks. A healthy control group was also run in parallel. At the end of the treatment period, animals were euthanized, and the retinas were collected and analyzed for the expression levels of brain-derived neurotrophic factor (BDNF), tyrosine hydroxylase (TH), nerve growth factor receptor (NGFR), and caspase-3 using Western blotting techniques in the retina of diabetic rats and compared with the normal control rats. In addition, dichlorofluorescein (DCF) levels in the retina were assessed as a marker of reactive oxygen species (ROS), and blood-retinal barrier breakdown (BRB) was examined for vascular permeability. The systemic effects of TQ treatments on glycemic control, kidney and liver functions were also assessed in all groups.

RESULTS

Diabetic animals treated with TQ showed improvements in the liver and kidney functions compared with control diabetic rats. Normalization in the levels of neuroprotective factors, including BDNF, TH, and NGFR, was observed in the retina of diabetic rats treated with TQ. In addition, TQ ameliorated the levels of apoptosis regulatory protein caspase-3 in the retina of diabetic rats and reduced disruption of the blood-retinal barrier, possibly through a reduction in reactive oxygen species (ROS) generation.

CONCLUSION

These findings suggest that TQ harbors a significant potential to limit the neurodegeneration and retinal damage that can be provoked by hyperglycemia in vivo.

Time dependent effects of prolonged hyperglycemia in zebrafish brain and retina

Prolonged hyperglycemia causes long-term vision complications and an increased risk of cognitive deficits. High blood sugar also confers an osmotic load/stress to cells. We assessed behavioral and neurochemical changes in zebrafish brain and retina following prolonged hyperglycemia for 4-weeks or 8-weeks. At each time point, behavior was assessed using 3-chamber choice task and optomotor response; tissue was then collected and levels of inflammatory markers, tight junction proteins, and neurotransmitters determined using Western Blots. After 4-weeks, brain levels of v-rel reticuloendotheliosis viral oncogene homolog A (avian) (RelA; NF-kB subunit), IkB kinase (IKK), and glial fibrillary acidic protein (GFAP) were significantly elevated; differences in zonula occludens-1 (ZO-1), claudin-5, glutamic acid decarboxylase (GAD), and tyrosine hydroxylase (TH) were not significant. In retina, significant differences were observed only for TH (decreased), Rel A (increased), and GFAP (increased) levels. Glucose-specific differences in initial choice latency and discrimination ratios were also observed. After 8-weeks, RelA, GAD, and TH were significantly elevated in both tissues; IKK and GFAP levels were also elevated, though not significantly. ZO-1 and claudin-5 levels osmotically decreased in retina but displayed an increasing trend in glucose-treated brains. Differences in discrimination ratio were driven by osmotic load. OMRs increased in glucose-treated fish at both ages. In vivo analysis of retinal vasculature suggested thicker vessels after 4-weeks, but thinner vessels at 8-weeks. In vitro, glucose treatment reduced formation of nodes and meshes in 3B-11 endothelial cells, suggesting a reduced ability to form a vascular network. Overall, hyperglycemia triggered a strong inflammatory response causing initial trending changes in tight junction and neuronal markers. Most differences after 4-weeks of exposure were observed in glucose-treated fish suggesting effects on glucose metabolism independent of osmotic load. After 8-weeks, the inflammatory response remained and glucose-specific effects on neurotransmitter markers were observed. Osmotic differences impacted cognitive behavior and retinal protein levels; protein levels in brain displayed glucose-driven changes. Thus, we not only observed differential sensitivities of retina and brain to glucose-insult, but also different cellular responses, suggesting hyperglycemia causes complex effects at the cellular level and/or that zebrafish are able to compensate for the continued high blood glucose levels.

Insulin like growth factor-1 works synergistically with dopamine to attenuate diabetic retinopathy by downregulating vascular endothelial growth factor

AIM

Levels of Insulin-like growth factor-1 (IGF-1), a proangiogenic growth factor is elevated and dopamine downregulated in proliferative diabetic retinopathy (PDR). This study aims to investigate whether IGF-1 with dopamine can together modulate vascular endothelial growth factor (VEGF) to prevent proliferative diabetic retinopathy while also attenuating angiogenic effects of IGF-1.

METHODS

Effect of combination of levodopa L-Dopa with IGF-1 was tested on normal retinal pigment epithelium cells (ARPE-19) and human umbilical vein endothelial cells (HUVEC), followed by tube formation. Invivo analysis of anti-angiogenic potential assessed by chick chorioallantoic membrane (CAM) assay. Diabetes induction in wistar rats at two time points, 12 and 16 weeks, treated with L-Dopa+IGF-1 and analysed for morphological variations, serum and tissue dopamine levels, gene expression by real-time PCR and western blot assay.

RESULTS

L-Dopa+IGF-1 on ARPE-19 cells caused no toxicity and worked synergistically. Reduced number of vessels observed. Significant improvement in inner retina thickness (*p < 0.05) was observed when L-Dopa was given alone and/or with IGF-1. Dopamine levels improved significantly in both serum and tissue (*p < 0.05). Levels of VEGF and IGF-1 receptors reduced significantly in 12 weeks. Western studies suggest that L-Dopa+IGF-1 modulates its effects via Akt/ERK dependent pathway.

CONCLUSION

First ever report on synergistic effect of L-Dopa+IGF-1 in a rat model of diabetic retinopathy. Even though the effect of L-Dopa in combination with IGF-1 is comparable to levels of L-Dopa alone, this study presents an interesting finding of neuroprotective function of IGF-1, which has been studied in disease models of Parkinson's but not diabetes.

Impaired Light Adaptation of ON-Sustained Ganglion Cells in Early Diabetes Is Attributable to Diminished Response to Dopamine D4 Receptor Activation

Purpose

Retinal neuronal signaling is disrupted early in diabetes, before the onset of the vascular pathologies associated with diabetic retinopathy. There is also growing evidence that retinal dopamine, a neuromodulator that mediates light adaptation, is reduced in early diabetes. Previously, we have shown that after 6 weeks of diabetes, light adaptation is impaired in ON-sustained (ON-s) ganglion cells in the mouse retina. The purpose of this study was to determine whether changes in the response to dopamine receptor activation contribute to this dysfunction.

Methods

Single-cell retinal patch-clamp recordings from the mouse retina were used to determine how activating dopamine type D4 receptors (D4Rs) changes the light-evoked and spontaneous excitatory inputs to ON-s ganglion cells, in both control and 6-week diabetic (STZ-injected) animals. Fluorescence in situ hybridization was also used to assess whether D4R expression was affected by diabetes.

Results

D4R activation decreased light-evoked and spontaneous inputs to ON-s ganglion cells in control and diabetic retinas. However, D4R activation caused a smaller reduction in light-evoked excitatory inputs to ON-s ganglion cells in diabetic retinas compared to controls. This impaired D4R signaling is not attributable to a decline in D4R expression, as there was no change in D4R mRNA density in the diabetic retinas.

Conclusions

These results suggest that the cellular response to dopamine signaling is disrupted in early diabetes and may be amenable to chronic dopamine supplementation therapy.

Diabetic retinal neurodegeneration as a form of diabetic retinopathy

PURPOSE

To review the evidence supporting diabetic retinal neurodegeneration (DRN) as a form of diabetic retinopathy.

METHOD

Review of literature.

RESULTS

DRN is recognized to be a part of retinopathy in patients with diabetes mellitus (DM), in addition to the well-established diabetic retinal vasculopathy (DRV). DRN has been noted in the early stages of DM, before the onset of clinically evident diabetic retinopathy. The occurrence of DRN has been confirmed in animal models of DM, histopathological examination of donor's eyes from diabetic individuals and assessment of neural structure and function in humans. DRN involves alterations in retinal ganglion cells, photoreceptors, amacrine cells and bipolar cells, and is thought to be driven by glutamate, oxidative stress and dysregulation of neuroprotective factors in the retina. Potential therapeutic options for DRN are under evaluation.

CONCLUSIONS

Literature is divided on the temporal relation between DRN and DRV, with evidence of both precedence and simultaneous occurrence. The relationship between DRN and multi-system neuropathy in DM is yet to be evaluated critically.

Initiation of L-DOPA Treatment After Detection of Diabetes-Induced Retinal Dysfunction Reverses Retinopathy and Provides Neuroprotection in Rats

Purpose

L-DOPA treatment initiated at the start of hyperglycemia preserves retinal and visual function in diabetic rats. Here, we investigated a more clinically relevant treatment strategy in which retinal and visual dysfunction designated the beginning of the therapeutic window for L-DOPA treatment.

Methods

Spatial frequency thresholds using optomotor response and oscillatory potential (OP) delays using electroretinograms were compared at baseline, 3, 6, and 10 weeks after streptozotocin (STZ) between diabetic and control rats. L-DOPA/carbidopa treatment (DOPA) or vehicle was delivered orally 5 days per week beginning at 3 weeks after STZ, when significant retinal and visual deficits were measured. At 10 weeks after STZ, retinas were collected to measure L-DOPA, dopamine, and 3,4-dihydroxyphenylacetic acid (DOPAC) levels using high-performance liquid chromatography.

Results

Spatial frequency thresholds decreased at 6 weeks in diabetic vehicle rats (28%), whereas diabetic DOPA rats had stable thresholds (<1%) that maintained to 10 weeks, creating significantly higher thresholds compared with diabetic vehicle rats (P < 0.0001). OP2 implicit times in response to dim, rod-driven stimuli were decreased in diabetic compared with control rats (3 weeks, P < 0.0001; 10 weeks, P < 0.01). With L-DOPA treatment, OP2 implicit times recovered in diabetic rats to be indistinguishable from control rats by 10 weeks after STZ. Rats treated with L-DOPA showed significantly increased retinal L-DOPA (P < 0.001) and dopamine levels (P < 0.05).

Conclusions

L-DOPA treatment started after the detection of retinal and visual dysfunction showed protective effects in diabetic rats.

Translational Relevance

Early retinal functional deficits induced by diabetes can be used to identify an earlier therapeutic window for L-DOPA treatment which protects from further vision loss and restores retinal function.

The lack of Irs2 induces changes in the immunocytochemical expression of aromatase in the mouse retina

Insulin receptor substrate (Irs) belongs to a family of proteins that mediate the intracellular signaling of insulin and IGF-1. Insulin receptor substrate 2 (Irs2) is necessary for retinal function, since its failure in Irs2-deficient mice in hyperglycemic situation promotes photoreceptor degeneration and visual dysfunction, like in diabetic retinopathy. The expression of P450 aromatase, which catalyzes androgen aromatization to form 17ß-estradiol, increases in some neurodegenerative diseases thus promoting the local synthesis of neuroestrogens that exert relevant neuroprotective functions. Aromatase is also expressed in neurons and glial cells of the central nervous system (CNS), including the retina. To further understand the role of Irs2 at the retinal level, we performed an immunocytochemical study in adult normoglycemic Irs2-deficient mice. For this aim, the retinal immunoexpression of neuromodulators, such as aromatase, glutamine synthetase (GS), and tyrosine hydroxylase (TH) was analyzed, joint to a morphometric and planimetric study of the retinal layers. Comparing with wild-type (WT) control mice, the Irs2-knockout (Irs2-KO) animals showed a significant increase in the immunopositivity to aromatase in almost all of the retinal layers. Besides, Irs2-KO mice exhibited a decreased immunopositive reaction for GS and TH, in Müller and amacrine cells, respectively; morphological variations were also found in these retinal cell types. Furthermore, the retina of Irs2-KO mice displayed alterations in the structural organization, and a generalized decrease in the retinal thickness was observed in each of the layers, except for the inner nuclear layer. Our findings suggest that the absence of Irs2 induces retinal neurodegenerative changes in Müller and amacrine cells that are unrelated to hyperglycemia. Accordingly, in the Irs2-KO mice, the increased retinal immunocytochemical reactivity of aromatase could be associated with an attempt to repair such neural retina injuries by promoting local neuroprotective mediators.

The effects of early diabetes on inner retinal neurons

Diabetic retinopathy is now well understood as a neurovascular disease. Significant deficits early in diabetes are found in the inner retina that consists of bipolar cells that receive inputs from rod and cone photoreceptors, ganglion cells that receive inputs from bipolar cells, and amacrine cells that modulate these connections. These functional deficits can be measured in vivo in diabetic humans and animal models using the electroretinogram (ERG) and behavioral visual testing. Early effects of diabetes on both the human and animal model ERGs are changes to the oscillatory potentials that suggest dysfunctional communication between amacrine cells and bipolar cells as well as ERG measures that suggest ganglion cell dysfunction. These are coupled with changes in contrast sensitivity that suggest inner retinal changes. Mechanistic in vitro neuronal studies have suggested that these inner retinal changes are due to decreased inhibition in the retina, potentially due to decreased gamma aminobutyric acid (GABA) release, increased glutamate release, and increased excitation of retinal ganglion cells. Inner retinal deficits in dopamine levels have also been observed that can be reversed to limit inner retinal damage. Inner retinal targets present a promising new avenue for therapies for early-stage diabetic eye disease.

Early diabetes impairs ON sustained ganglion cell light responses and adaptation without cell death or dopamine insensitivity

Retinal signaling under dark-adapted conditions is perturbed during early diabetes. Additionally, dopamine, the main neuromodulator of retinal light adaptation, is diminished in diabetic retinas. However, it is not known if this dopamine deficiency changes how the retina responds to increased light or dopamine. Here we determine whether light adaptation is impaired in the diabetic retina, and investigate potential mechanism(s) of impairment. Diabetes was induced in C57BL/6J male mice via 3 intraperitoneal injections of streptozotocin (75 mg/kg) and confirmed by blood glucose levels more than 200 mg/dL. After 6 weeks, whole-cell recordings of light-evoked and spontaneous inhibitory postsynaptic currents (IPSCs) or excitatory postsynaptic currents (EPSCs) were made from rod bipolar cells and ON sustained ganglion cells, respectively. Light responses were recorded before and after D1 receptor (D1R) activation (SKF-38393, 20 μM) or light adaptation (background of 950 photons·μm^-2 ·s^-1). Retinal whole mounts were stained for either tyrosine hydroxylase and activated caspase-3 or GAD65/67, GlyT1 and RBPMS and imaged. D1R activation and light adaptation both decreased inhibition, but the disinhibition was not different between control and diabetic rod bipolar cells. However, diabetic ganglion cell light-evoked EPSCs were increased in the dark and showed reduced light adaptation. No differences were found in light adaptation of spontaneous EPSC parameters, suggesting upstream changes. No changes in cell density were found for dopaminergic, glycinergic or GABAergic amacrine cells, or ganglion cells. Thus, in early diabetes, ON sustained ganglion cells receive excessive excitation under dark- and light-adapted conditions. Our results show that this is not attributable to loss in number or dopamine sensitivity of inhibitory amacrine cells or loss of dopaminergic amacrine cells.

Plasma phenylalanine and tyrosine and their interactions with diabetic nephropathy for risk of diabetic retinopathy in type 2 diabetes

OBJECTIVE

Tight control of hyperglycemia reduces risk of diabetic retinopathy (DR), but the residual risk remains high. This study aimed to explore relationships between plasma phenylalanine and tyrosine with DR in type 2 diabetes (T2D) and interactions between the two amino acids, and their secondary interaction with renal dysfunction.

RESEARCH DESIGN AND METHODS

We extracted data of 1032 patients with T2D from tertiary hospital consecutively from May 2015 to August 2016. Binary logistic regression models with restricted cubic spline were used to check potential non-linear associations and to obtain ORs and 95% CIs of variables under study. Addictive interaction was estimated using relative excess risk due to interaction, attributable proportion due to interaction and synergy index. Area under the receiver operating characteristic curve was used to check increased predictive values.

RESULTS

Of 1032 patients, 162 suffered from DR. Copresence of low phenylalanine and low tyrosine increased DR risk (OR 6.01, 95% CI 1.35 to 26.8), while either of them alone did not have a significant effect with significant additive interaction. Presence of diabetic nephropathy further increased the OR of copresence of low phenylalanine and low tyrosine for DR to 25.9 (95% CI 8.71 to 76.9) with a significant additive interaction. Inclusion of phenylalanine and tyrosine in a traditional risk factor model significantly increased area under the curve from 0.81 to 0.83 (95% CI 0.80 to 0.86).

CONCLUSION

Plasma low phenylalanine and low tyrosine worked independently and synergistically to increase the risk of DR in T2D. Presence of renal dysfunction further amplified the effect of copresence of low phenylalanine and low tyrosine on DR risk.

Dopamine metabolite levels in the vitreous of diabetic and non-diabetic humans

Animal studies suggest that the retinal dysfunction in diabetic subjects that precedes overt clinical vasculopathy may be due to a retinal dopamine deficit. We analyzed levels of dopamine (DA) and its primary metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), in the vitreous of diabetic and non-diabetic human subjects. Adult patients undergoing pars plana vitrectomy for non-hemorrhagic indications were prospectively recruited from the Emory Eye Center in Atlanta, GA. Vitreous samples were analyzed using high performance liquid chromatography (HPLC) to measure levels of DOPAC and DA in the vitreous specimens. Vitreous samples from 9 diabetic patients and 20 from non-diabetic patients were analyzed. No eyes had apparent diabetic retinopathy. Mean normalized DA concentration in vitreous of diabetic subjects was 0.76 ± 0.12 pg/μL vs. 0.73 ± 0.08 pg/μL in non-diabetic vitreous (p = 0.849). DOPAC concentration was 8.84 ± 0.74 pg/μL in vitreous of diabetic subjects vs. 9.22 ± 0.56 pg/μL in vitreous of non-diabetic subjects (p = 0.691). No difference was observed in the concentrations of DA and DOPAC in the vitreous of people without diabetes compared to those with diabetes without retinopathy.

Dopamine D2 Receptor-Mediated Modulation of Rat Retinal Ganglion Cell Excitability

Ganglion cells (RGCs) are the sole output neurons of the retinal circuity. Here, we investigated whether and how dopamine D2 receptors modulate the excitability of dissociated rat RGCs. Application of the selective D2 receptor agonist quinpirole inhibited outward K⁺ currents, which were mainly mediated by glybenclamide- and 4-aminopyridine-sensitive channels, but not the tetraethylammonium-sensitive channel. In addition, quinpirole selectively enhanced Nav1.6 voltage-gated Na⁺ currents. The intracellular cAMP/protein kinase A, Ca²⁺/calmodulin-dependent protein kinase II, and mitogen-activated protein kinase/extracellular signal-regulated kinase signaling pathways were responsible for the effects of quinpirole on K⁺ and Na⁺ currents, while phospholipase C/protein kinase C signaling was not involved. Under current-clamp conditions, the number of action potentials evoked by positive current injection was increased by quinpirole. Our results suggest that D2 receptor activation increases RGC excitability by suppressing outward K⁺ currents and enhancing Nav1.6 currents, which may affect retinal visual information processing.

TRPV4 inhibition prevents increased water diffusion and blood-retina barrier breakdown in the retina of streptozotocin-induced diabetic mice

A better understanding of the molecular and cellular mechanisms involved in retinal hydro-mineral homeostasis imbalance during diabetic macular edema (DME) is needed to gain insights into retinal (patho-)physiology that will help elaborate innovative therapies with lower health care costs. Transient receptor potential cation channel subfamily vanilloid member 4 (TRPV4) plays an intricate role in homeostatic processes that needs to be deciphered in normal and diabetic retina. Based on previous findings showing that TRPV4 antagonists resolve blood-retina barrier (BRB) breakdown in diabetic rats, we evaluated whether TRPV4 channel inhibition prevents and reverts retinal edema in streptozotocin(STZ)-induced diabetic mice. We assessed retinal edema using common metrics, including retinal morphology/thickness (histology) and BRB integrity (albumin-associated tracer), and also by quantifying water mobility through apparent diffusion coefficient (ADC) measures. ADC was measured by diffusion-weighted magnetic resonance imaging (DW-MRI), acquired ex vivo at 4 weeks after STZ injection in diabetes and control groups. DWI images were also used to assess retinal thickness. TRPV4 was genetically ablated or pharmacologically inhibited as follows: left eyes were used as vehicle control and right eyes were intravitreally injected with TRPV4-selective antagonist GSK2193874, 24 h before the end of the 4 weeks of diabetes. Histological data show that retinal thickness was similar in nondiabetic and diabetic wt groups but increased in diabetic Trpv4^-/- mice. In contrast, DWI shows retinal thinning in diabetic wt mice that was absent in diabetic Trpv4^-/- mice. Disorganized outer nuclear layer was observed in diabetic wt but not in diabetic Trpv4^-/- retinas. We further demonstrate increased water diffusion, increased distances between photoreceptor nuclei, reduced nuclear area in all nuclear layers, and BRB hyperpermeability, in diabetic wt mice, effects that were absent in diabetic Trpv4^-/- mice. Retinas of diabetic mice treated with PBS showed increased water diffusion that was not normalized by GSK2193874. ADC maps in nondiabetic Trpv4^-/- mouse retinas showed restricted diffusion. Our data provide evidence that water diffusion is increased in diabetic mouse retinas and that TRPV4 function contributes to retinal hydro-mineral homeostasis and structure under control conditions, and to the development of BRB breakdown and increased water diffusion in the retina under diabetes conditions. A single intravitreous injection of TRPV4 antagonist is however not sufficient to revert these alterations in diabetic mouse retinas.

Dopamine D1 receptor activation reduces local inner retinal inhibition to light-adapted levels

During adaptation from dim to bright environments, changes in retinal signaling are mediated, in part, by dopamine. Dopamine is released with light and can modulate retinal receptive fields, neuronal coupling, inhibitory receptors, and rod pathway inhibition. However, it is unclear how dopamine affects inner retinal inhibition to cone bipolar cells, which relay visual information from photoreceptors to ganglion cells and are important signal processing sites. We tested the hypothesis that dopamine (D)1 receptor activation is sufficient to elicit light-adapted inhibitory changes. Local light-evoked inhibition and spontaneous activity were measured from OFF cone bipolar cells in dark-adapted mouse retinas while stimulating D1 receptors, which are located on bipolar, horizontal, and inhibitory amacrine cells. The D1 agonist SKF38393 reduced local inhibitory light-evoked response magnitude and increased response transience, which mimicked changes measured with light adaptation. D1-mediated reductions in local inhibition were more pronounced for glycinergic than GABAergic inputs, comparable with light adaptation. The effects of D1 receptors on light-evoked input were similar to the effects on spontaneous input. D1 receptor activation primarily decreased glycinergic spontaneous current frequency, similar to light adaptation, suggesting mainly a presynaptic amacrine cell site of action. These results expand the role of dopamine to include signal modulation of cone bipolar cell local inhibition. In this role, D1 receptor activation, acting primarily through glycinergic amacrine cells, may be an important mechanism for the light-adapted reduction in OFF bipolar cell inhibition since the actions are similar and dopamine is released during light adaptation. NEW & NOTEWORTHY Retinal adaptation to different luminance conditions requires the adjustment of local circuits for accurate signaling of visual scenes. Understanding mechanisms behind luminance adaptation at different retinal levels is important for understanding how the retina functions in a dynamic environment. In the mouse, we show that dopamine pathways reduce inner retinal inhibition similar to increased background luminance, suggesting the two are linked and highlighting a possible mechanism for light adaptation at an early retinal processing center.

Dopamine Deficiency Mediates Early Rod-Driven Inner Retinal Dysfunction in Diabetic Mice

Purpose

Electroretinograms (ERGs) are abnormal in diabetic retinas before the appearance of vascular lesions, providing a possible biomarker for diabetic vision loss. Previously, we reported that decreased retinal dopamine (DA) levels in diabetic rodents contributed to early visual and retinal dysfunction. In the current study, we examined whether oscillatory potentials (OPs) could serve as a potential marker for detecting early inner retinal dysfunction due to retinal DA deficiency.

Methods

Retinal function was tested with dark-adapted ERGs, taken at 3, 4, and 5 weeks after diabetes induction with streptozotocin. Electrical responses were analyzed and correlations were made with previously reported retinal DA levels. The effect of restoring systemic DA levels or removing DA from the retina in diabetic mice on OPs was assessed using L-3,4-dihydroxyphenylalanine (L-DOPA) treatments and retina-specific tyrosine hydroxylase (Th) knockout mice (rTHKO), respectively.

Results

Diabetic animals had significantly delayed OPs compared to control animals in response to dim, but not bright, flash stimuli. L-DOPA treatment preserved OP implicit time in diabetic mice. Diabetic rTHKO mice had further delayed OPs compared to diabetic mice with normal retinal Th, with L-DOPA treatment also providing benefit. Decreasing retinal DA levels significantly correlated with increasing OP delays mediated by rod pathways.

Conclusions

Our data suggest that inner retinal dysfunction in early-stage diabetes is mediated by rod-pathway deficits and DA deficiencies. OP delays may be used to determine the earliest functional deficits in diabetic retinopathy and to establish an early treatment window for DA therapies that may prevent progressive vision loss.

Involvement of ciliary neurotrophic factor in early diabetic retinal neuropathy in streptozotocin-induced diabetic rats

Objective

Ciliary neurotrophic factor (CNTF) has been evaluated as a candidate therapeutic agent for diabetes and its neural complications. However, its role in diabetic retinopathy has not been fully elucidated.

Methods

This is a randomized unblinded animal experiment. Wistar rats with streptozocin (STZ)-induced diabetes were regularly injected with CNTF or vehicle control in their vitreous bodies beginning at 2 weeks after STZ injection. A total of five injections were used. In diabetic rats, the levels of CNTF and neurotrophin-3 (NT-3) were evaluated by enzyme-linked immunosorbent assays (ELISA) and real-time PCR. The abundance of tyrosine hydroxylase (TH) and β-III tubulin was detected by western blot. Transferase-mediated dUTP nick-end labeling staining (TUNEL) was used to detect cell apoptosis in the retinal tissue. The activation of caspase-3 was also measured.

Results

The protein and mRNA levels of CNTF in diabetic rat retinas were reduced compared to control rats. In addition, retinal ganglion cells (RGCs) and dopaminergic amacrine cells appeared to undergo degeneration in diabetic rat retinas, as revealed by transferase-mediated dUTP nick-end labeling staining (TUNEL). Tyrosine hydroxylase (TH) and β-III tubulin protein levels also decreased significantly. Intraocular administration of CNTF rescued RGCs and dopaminergic amacrine cells from neurodegeneration and counteracted the downregulation of β-III tubulin and TH expression, thus demonstrating its therapeutic potential.

Conclusion

Our study suggests that early diabetic retinal neuropathy involves the reduced expression of CNTF and can be ameliorated by an exogenous supply of this neurotrophin.

Melanopsin-mediated pupil function is impaired in Parkinson's disease

Parkinson's disease (PD) is characterised by non-motor symptoms including sleep and circadian disruption. Melanopsin-expressing intrinsically photosensitive Retinal Ganglion Cells (ipRGC) transmit light signals to brain areas controlling circadian rhythms and the pupil light reflex. To determine if non-motor symptoms observed in PD are linked to ipRGC dysfunction, we evaluated melanopsin and rod/cone contributions to the pupil response in medicated participants with PD (n = 17) and controls (n = 12). Autonomic tone was evaluated by measuring pupillary unrest in darkness. In the PD group, there is evidence for an attenuated post-illumination pupil response (PIPR) amplitude and reduced pupil constriction amplitude, and PIPR amplitudes did not correlate with measures of sleep quality, retinal nerve fibre layer thickness, disease severity, or medication dosage. Both groups exhibited similar pupillary unrest. We show that melanopsin- and the rod/cone-photoreceptor contributions to the pupil control pathway are impaired in people with early-stage PD who have no clinically observable ophthalmic abnormalities. Given that ipRGCs project to brain targets involved in arousal, sleep and circadian rhythms, ipRGC dysfunction may underpin some of the non-motor symptoms observed in PD.

Neurodegeneration in diabetic retinopathy: Potential for novel therapies

The complex pathology of diabetic retinopathy (DR) affects both vascular and neural tissue. The characteristics of neurodegeneration are well-described in animal models but have more recently been confirmed in the clinical setting, mostly by using non-invasive imaging approaches such as spectral domain optical coherence tomography (SD-OCT). The most frequent observations report loss of tissue in the nerve fiber layer and inner plexiform layer, confirming earlier findings from animal models. In several cases the reduction in inner retinal layers is reported in patients with little evidence of vascular lesions or macular edema, suggesting that degenerative loss of neural tissue in the inner retina can occur after relatively short durations of diabetes. Animal studies also suggest that neurodegeneration leading to retinal thinning is not limited to cell death and tissue loss but also includes changes in neuronal morphology, reduced synaptic protein expression and alterations in neurotransmission, including changes in expression of neurotransmitter receptors as well as neurotransmitter release, reuptake and metabolism. The concept of neurodegeneration as an early component of DR introduces the possibility to explore alternative therapies to prevent the onset of vision loss, including neuroprotective therapies and drugs targeting individual neurotransmitter systems, as well as more general neuroprotective approaches to preserve the integrity of the neural retina. In this review we consider some of the evidence for progressive retinal neurodegeneration in diabetes, and explore potential neuroprotective therapies.

Dopamine deficiency contributes to early visual dysfunction in a rodent model of type 1 diabetes

Dopamine (DA) functions as an essential neuromodulator in the brain and retina such that disruptions in the dopaminergic system are associated with common neurologic disorders such as Parkinson's disease. Although a reduction in DA content has been observed in diabetes, its effects in the development of diabetes-induced neuropathy remains unknown. Because the retina is rich in DA and has a well known diabetes-induced pathology (diabetic retinopathy or DR), this study was designed to examine the role of retinal DA deficiency in early visual defects in DR. Using rodent models of type 1 diabetes mellitus, we investigated whether diabetes caused a reduction in retinal DA content in both rats and mice and determined whether restoring DA levels or activating specific DA receptor pathways could improve visual function (evaluated with optokinetic tracking response) of diabetic mice, potentially via improvement of retinal function (assessed with electroretinography). We found that diabetes significantly reduced DA levels by 4 weeks in rats and by 5 weeks in mice, coincident with the initial detection of visual deficits. Treatment with l-DOPA, a DA precursor, improved overall retinal and visual functions in diabetic mice and acute treatment with DA D1 or D4 receptor agonists improved spatial frequency threshold or contrast sensitivity, respectively. Together, our results indicate that retinal DA deficiency is an underlying mechanism for early, diabetes-induced visual dysfunction and suggest that therapies targeting the retinal dopaminergic system may be beneficial in early-stage DR.

Early visual deficits in streptozotocin-induced diabetic long evans rats

PURPOSE

Although diabetic retinopathy (DR) is clinically diagnosed based on vascular pathology, diabetic patients with angiographically normal retinas have been found to exhibit subtle defects in vision. This has led to the theory that diabetes-associated metabolic abnormalities directly impair neural retinal function before the development of vasculopathy, thereby resulting in visual deficits. In this study, we sought to delineate the temporal relationship between retinal dysfunction and visual deficits in a rat model of Type 1 diabetes. Moreover, we investigated the relative contribution of retinal dysfunction versus diabetes-induced lens opacity, to the visual deficits found in early-stage DR.

METHODS

Pigmented Long Evans rats were rendered diabetic with streptozotocin (STZ). Control and diabetic rats were assessed across 12 weeks of hyperglycemia for visual function with optokinetic tracking weekly visual acuity and monthly contrast sensitivity, retinal function with dark-adapted electroretinograms (monthly electroretinograms [ERGs]), and cataract formation with slit lamp exam (biweekly).

RESULTS

Diabetic rats exhibited significantly reduced visual function and delayed ERG responses by 1 month post-STZ. Significant cataracts did not develop until 6 weeks post-STZ. Moreover, increases in lens opacity (r = -0.728) and ERG implicit times (r = -0.615 for rod-dominated response and r = -0.322 for rod/cone mixed response) showed significant correlations with reductions in visual acuity in diabetic rats.

CONCLUSIONS

STZ-induced hyperglycemia reduces visual function, affecting both visual acuity and contrast sensitivity. The data suggest that visual defects found in early-stage DR may initially involve abnormalities of the neural retina and worsen with later development of cataracts.

Functional integrity and modification of retinal dopaminergic neurons in the rd1 mutant mouse: roles of melanopsin and GABA

The progressive loss of rod and cone photoreceptors in human subjects with retinitis pigmentosa causes a gradual decline in vision and can result in blindness. Current treatment strategies for the disease rely on the integrity of inner retinal neurons, such as amacrine cells, that are postsynaptic to photoreceptors. Previous work has demonstrated that a specialized subclass of retinal amacrine cell that synthesizes and releases the key neurotransmitter dopamine remains morphologically intact during the disease; however, the pathophysiological function of these neurons remains poorly understood. Here we examined spontaneous and light-evoked spike activity of genetically labeled dopamine neurons from the retinas of retinal degeneration 1 (rd1) mice. Our results indicated that rd1 dopamine neurons remained functionally intact with preserved spontaneous spiking activity and light-evoked responses. The light responses were mediated exclusively by melanopsin phototransduction, not by surviving cones. Our data also suggested that dopamine neurons were altered during photoreceptor loss, as evidenced by less spontaneous bursting activity and increased light-evoked responses with age. Further evidence showed that these alterations were attributed to enhanced GABA/melanopsin signaling to dopamine neurons during disease progression. Taken together, our studies provide valuable information regarding the preservation and functional modification of the retinal dopamine neuronal system in rd1; this information should be considered when designing treatment strategies for retinitis pigmentosa.

Corticosteroid use for diabetic macular edema: old fad or new trend?

Diabetic retinopathy is the leading cause of blindness in working age individuals in developed countries. Most cases of diabetes related vision loss result from breakdown of the blood-retinal barrier with resultant diabetic macular edema (DME). For over 30 years, laser photocoagulation has been the standard therapy for DME, but most eyes do not experience significant improvements in visual acuity. Intravitreal injections of drugs that inhibit the action of vascular endothelial growth factor (VEGF) lead to gains in vision, but can be expensive and need to be repeated frequently. In addition to VEGF-mediated breakdown of the blood-retinal barrier, recent evidence suggests that inflammation plays an important role in the development of DME. Recognizing this, physicians have injected steroids into the vitreous and developers have created sustained release implants. Intravitreal injections of triamcinolone acetonide lead to rapid resolution of macular edema and significant short-term improvements in visual acuity, but unfortunately, visual acuities diminish when treatment is continued through 2 years. However, intravitreal triamcinolone remains an attractive treatment option for eyes that are pseudophakic, scheduled to undergo cataract surgery, resistant to laser photocoagulation, or require urgent panretinal photocoagulation for proliferative retinopathy. In controlled trials, intraocular implants that slowly release dexamethasone and fluocinolone show promise in reducing macular edema and improving visual acuity. The high incidences of drug related cataracts and glaucoma, however, require that corticosteroids be used cautiously and that patients be selected carefully. The increasing number of patients with DME, the burgeoning cost of medical care and the continuing development of intravitreal steroids suggest that the use of these agents will likely increase in coming years.

Diabetic macular edema: pathogenesis and treatment

Diabetic macular edema is a major cause of visual impairment. The pathogenesis of macular edema appears to be multifactorial. Laser photocoagulation is the standard of care for macular edema. However, there are cases that are not responsive to laser therapy. Several therapeutic options have been proposed for the treatment of this condition. In this review we discuss several factors and mechanisms implicated in the etiology of macular edema (vasoactive factors, biochemical pathways, anatomical abnormalities). It seems that combined pharmacologic and surgical therapy may be the best approach for the management of macular edema in diabetic patients.

Changes of oscillatory potentials and photopic negative response in patients with early diabetic retinopathy

PURPOSE

To investigate the clinical significance of the oscillatory potentials (OPs) and photopic negative response (PhNR) of the electroretinogram (ERG) in patients with early diabetic retinopathy.

METHODS

One hundred two diabetic patients with diabetic retinopathy at different stages were examined. Thirty-two age-matched normal controls were also studied. Full-field maximal and photopic cone ERGs were recorded. The amplitudes and implicit times of the OPs, cone b wave, and PhNR were compared at the different stages of diabetic retinopathy.

RESULTS

The a and b wave amplitudes of the maximal scotopic ERGs remained unchanged despite advancing stages of retinopathy, but the OP amplitudes were significantly attenuated even at an early stage of diabetic retinopathy. The amplitudes of both the PhNR and cone b wave were reduced at an early stage of diabetic retinopathy. Analysis of the receiver operating characteristic curves demonstrated that the amplitudes and implicit times of the OPs were more sensitive and specific than those of the PhNR in detecting changes of retinal function in the early stages of diabetic retinopathy.

CONCLUSIONS

The amplitudes of the OPs and PhNR progressively decrease with the progression of diabetic retinopathy. The PhNR amplitudes were reduced along with the cone b wave, indicating that earlier change of the PhNR in diabetic patients reflects reduced input to the retinal ganglion cell from the distal retina. The amplitudes and implicit times of the OPs are better indicators than those of the PhNR in detecting functional decreases in patients with early diabetic retinopathy.

Involvement of brain-derived neurotrophic factor in early retinal neuropathy of streptozotocin-induced diabetes in rats: therapeutic potential of brain-derived neurotrophic factor for dopaminergic amacrine cells

Although neurotrophins have been assessed as candidate therapeutic agents for neural complications of diabetes, their involvement in diabetic retinopathy has not been fully characterized. We found that the protein and mRNA levels of brain-derived neurotrophic factor (BDNF) in streptozotocin-induced diabetic rat retinas were reduced to 49% (P < 0.005) and 74% (P < 0.05), respectively, of those of normal control animals. In addition, dopaminergic amacrine cells appeared to be degenerating in the diabetic rat retinas, as revealed by tyrosine hydroxylase (TH) immunoreactivity. Overall TH protein levels in the retina were decreased to one-half that of controls (P < 0.01), reflecting reductions in the density of dopaminergic amacrine cells and the intensity of TH immunoreactivity within them. To confirm the neuropathological implications of BDNF reduction, we administered BDNF protein into the vitreous cavities of diabetic rats. Intraocular administration of BDNF rescued dopaminergic amacrine cells from neurodegeneration and counteracted the downregulation of TH expression, demonstrating its therapeutic potential. These findings suggest that the early retinal neuropathy of diabetes involves the reduced expression of BDNF and can be ameliorated by an exogenous supply of this neurotrophin.

Electrical responses from diabetic retina

Diabetic retinopathy has long been considered to be a retinal manifestation of systemic diabetic angiopathy. Indeed, it is therapeutically true. However, the prolongation of OP peak latency in diabetic eyes without any angiographic evidence of angiopathy leads us to presume that certain neuronal disorders occur early in diabetic eyes. Even though we cannot neglect the possibility that the prolongation of the OP peak latency may derive from undetectable retinal hypoperfusion, it is still far from conventional diabetic angiopathy. Rather, the status should be properly termed "intraretinal diabetic neuropathy" in that the neurones are the disturbed cells to cause visual dysfunction. Thereafter, the OP amplitude diminishes as retinopathy advances, probably depending on the degree of retinal circulatory disturbance. Marked diminution of the OP amplitude predicts rapid progression and poor prognosis of retinopathy. Diabetic retinal pigment epitheliopathy as manifested by one of our non-photic EOG responses is another kind of early ocular involvement of diabetes. Because its mechanisms are not yet known, so far we have not succeeded in correlating it to any kind of subjective visual index. Routine fundus inspection or fluorescent fundus angiography is incapable of detecting the compromised neural retina and/or retinal pigment epithelial integrity and thus the electrophysiology of vision has the edge in ophthalmology.

Visual evoked potentials and optic nerve histopathology in normal and diabetic rats and effect of ginkgo biloba extract

The purpose of this study was to test the possible therapeutic role of ginkgo biloba extract on the impairment of visual function and pathological histology of the optic nerve caused by early diabetes. Ginkgo biloba extract entraps oxygenated free radicals and is also a strong inhibitor of the platelet activation factor (PAF). For this purpose, VEP recordings and optic nerve histopathology were studied on alloxan diabetic and normal Swiss albino rats in four experimental groups. The VEP recordings showed no statistical significance between diabetic and normal rats. However, the amplitudes were significantly increased in diabetic animals with ginkgo biloba extract compared with the diabetics, supposing an impression of axonal protection. But the amplitude values were decreased in normal rats treated with the same extract compared with normal animals, assuming a toxic activity. Optic nerve ultrastructural findings also confirmed these VEP changes. It was concluded that this extract could be encouraging for human clinical trials of diabetes.

Oxidative stress in diabetic retina

The authors describe the alterations usually associated with diabetic retinopathy. They concern the classical thickening of the basal membrane of retinal capillaries and the associated modification of retinal vessel permeability. These alterations correspond to the blood-retinal barrier disruption. The authors then discuss the participation of oxygenated free radicals in the pathogenesis of diabetic retinopathy. They report several experimental studies establishing such a participation and finally describe their own results obtained on a model of retinas isolated from alloxan-induced diabetic rats. After one month of evolution, the electroretinograms (ERG) recorded on isolated retinas from diabetic rats had an amplitude about 20% lower than the controls, whereas after two months of diabetes, this decrease was about 60%. Under these conditions, the authors tested the protective properties of Ginkgo biloba extract (EGb 761) on their model. They observed that in EGb-treated animals (100 mg/kg/day), the ERG had a significantly (p less than 0.001) greater amplitude than untreated animals after two months of diabetes evolution. In conclusion, the authors discuss the possible utilization of a free radical scavenger, such as EGb 761, in the prevention of the retinal impairment in diabetes.

Diabetes mellitus: stress, neurochemistry and behavior

Neurochemical alterations in several rodent models of insulin-dependent diabetes are compared and their relevance to behavioral and physiological pathology in the clinical disorder is discussed. In the majority of rodent models, reductions in metabolism of norepinephrine (NE), dopamine (DA) and serotonin (5HT) in the central nervous system (CNS) have been reported. While there are two reports of increased 5HT turnover in CSF or post-mortem brains of diabetic humans experiencing severe ketoacidosis, these patients were receiving insulin therapy. Insulin appears to have effects on monoamines opposite to that of chronic hyperglycemia. Both in rodent models and in clinical populations, there is widespread evidence of enhanced hormonal and behavioral responsiveness to stress. There are findings in rodent models indicating that hormonal responses to stress are related to CNS monoamine activity. The mechanisms responsible for both hormonal and CNS alterations in diabetes, as well as their involvement in behavioral pathology, can best be investigated further using animal models.

Muscarinic receptor-mediated regulation of OM-853-enhanced dopamine release in striatum of rat

The effect of OM-853, a new vincamine analogue, on cerebral dopaminergic neurons was investigated in male Wistar rats. The administration of OM-853 (200 mg/kg p.o.) induced facilitation of the metabolic turnover of dopamine (DA) in all brain areas except the cerebral cortex. Addition of OM-853 enhanced the release of [3H]DA from striatal slices; this release was antagonized by atropine (10(-7) M). However, pretreatment with scopolamine (0.5 mg/kg s.c.) inhibited the enhancement of striatal DA turnover induced by OM-853 administration. OM-853 (10(-4) M) inhibited [3H]quinuclidinyl benzilate binding to muscarinic cholinergic receptors in a striatal particulate fraction more potently than carbachol (10(-4) M). These results suggest that OM-853 may induce facilitation of striatal DA turnover by enhancing DA release via the stimulation of muscarinic cholinergic receptor.

Diurnal alterations in retinal tyrosine level and dopamine turnover in diabetic rats

Retinal tyrosine level is correlated not only to its serum concentration, but also to the serum tyrosine ratio (the ratio of serum tyrosine to the sum of 5 neutral amino acids, leucine, isoleucine, valine, tryptophan and phenylalanine which compete with it for tissue uptake). In control animals, serum tyrosine ratio, retinal tyrosine, dopamine (DA) and its metabolites, and DOPA synthesis exhibit roughly parallel daily rhythms with night-time lows and peak levels occurring in the early portion of the daily light phase (L + 4 HR). In rats made diabetic by streptozotocin injection serum leucine, isoleucine and valine are elevated, lowering the serum tyrosine ratio and retinal tyrosine level by about 50% throughout the daily cycle. Parameters of DA turnover are still highest in the light phase but normal peak levels are delayed by 4 h until the latter half of the light phase (L + 8 HR). In diabetic rats, the normal increase in retinal DA synthesis and release upon exposure to light may be compromised by the decreased availability of its precursor, tyrosine.

Effect of tyrosine administration on dopa accumulation in light- and dark-adapted retinas from normal and diabetic rats

The interaction of tyrosine concentration and lighting on in vivo dihydroxyphenylalanine (dopa) accumulation rate was studied in retinas of normal and diabetic rats. In both groups of rats, dopa accumulation and in vitro hydroxylase activity were higher in retinas exposed to light than in those adapted to darkness. In light-adapted diabetic rats, though, retinal tyrosine level, dopa accumulation, and in vitro tyrosine hydroxylase activity were all below normal. In both normal and diabetic rats exposed to light, tyrosine injection raised retinal tyrosine concentrations and stimulated dopa accumulation. Injection of tyrosine into dark-adapted rats raised retinal tyrosine level but did not enhance dopa accumulation. Together, these results suggest that in vivo retinal amacrine cells will vary their dopa accumulation rate as a function of substrate supply, but only in the light, when tyrosine hydroxylase is activated. They further indicate that dopa accumulation rate remains sensitive to tyrosine supply in the light-activated diabetic retina.