Glucocorticoid actions on synaptic plasma membranes: modulation of [125I]calmodulin binding

Verteporfin Therapy and Triamcinolone Acetonide: Convergent Modes of Action for Treatment of Neovascular Age-Related Macular Degeneration

PURPOSE

Choroidal neovascularization associated with age-related macular degeneration is the primary cause of blindness in the elderly in developed countries, due to a number of pathogenic effects, including angiogenesis, cell-mediated inflammation, leukocyte adhesion and extravasation, and matrix remodeling.

METHODS

By producing photochemical effects at the site of target tissue (lesion), photodynamic therapy (PDT) can induce vascular damage and blood flow stasis, leading to occlusion of vascularization and lesion leakage.

RESULTS

PDT with verteporfin (Visudyne, Novartis) has been shown to be safe and effective in reducing the risk of vision loss in patients with classic containing subfoveal CNV and occult with no classic CNV. However, in predominantly occult CNV, the treatment may be most effective in smaller lesions, and less in larger lesions. Most important, visual acuity rarely is improved.

CONCLUSIONS

Pilot studies and large case series suggest that a combination of PDT and intravitreal triamcinolone acetonide has the potential to improve visual outcomes and reduce the need for additional treatments. Randomized, prospective clinical trials are underway to confirm the efficacy and safety of this novel treatment modality.

TIME COURSE OF CHANGES IN RETINAL THICKNESS AND VISUAL ACUITY AFTER INTRAVITREAL TRIAMCINOLONE ACETONIDE FOR DIFFUSE DIABETIC MACULAR EDEMA WITH AND WITHOUT PREVIOUS MACULAR LASER TREATMENT

PURPOSE

To describe the changes in retinal thickness (RT) and visual acuity over time in patients with clinically significant diffuse diabetic macular edema (DME) after intravitreal injection of triamcinolone acetonide (IVTA) and to compare patients with and without previous laser treatment.

METHODS

A total of 23 eyes with clinically significant DME received a 4-mg IVTA injection. Twelve eyes were refractory to macular laser treatment (group 1), and 11 eyes received IVTA as primary therapy (group 2). Visual acuity and changes in macular thickening shown by optical coherence tomography were evaluated 48 hours after injection, every 7 days for 1 month, and at 3 months and 6 months of follow-up.

RESULTS

RT decreased in all eyes in both groups. The reduction of edema was maximal in the first 7 days after IVTA and tended to remain stable for 3 months. The decrease in RT over time was significant in both groups (P < 0.001). At 6 months, RT had increased in almost all eyes. Visual acuity improved quickly, to a maximum at 2 weeks in both groups, after which it remained stable for 3 months and then decreased. Improvement in visual acuity over time was significant in both groups (P < 0.001). The temporal characteristics of the changes in RT and visual acuity were similar in the two groups (P < 0.05).

CONCLUSIONS

IVTA was effective in reducing clinically significant DME and improving visual acuity in eyes with and without previous laser treatment. Its action was maximal in the first week and lasted approximately 3 months in this study.

Presumed sterile endophthalmitis following intravitreal triamcinolone acetonide injection

BACKGROUND AND OBJECTIVE

To report acute postoperative, presumed sterile endophthalmitis following intravitreal injection of triamcinolone acetonide (IVTA).

PATIENTS AND METHODS

Retrospective, interventional, multicenter study of patients with acute sterile endophthalmitis following IVTA injection.

RESULTS

A total of 922 IVTA injections were performed. Eight eyes of 8 patients with presumed sterile endophthalmitis were identified. The incidence of endophthalmitis was 0.87% (95% confidence interval, 0.38% to 1.70%). Median time to presentation was 1.5 days (range, 1 to 7 days). Median presenting visual acuity was 20/563 (range, 20/80 to light perception). Initial treatment included vitreous tap and injection of antibiotics (n = 4), pars plana vitrectomy and injection of intravitreal antibiotics (n = 2), or systemic treatment alone with oral levofloxacin (n = 2). Six of 6 intraocular cultures were sterile. Median follow-up was 5.9 months (range, 4 to 9 months) with a median visual acuity at last follow-up of 20/75 (range, 20/40 to counting fingers).

CONCLUSIONS

Acute presumed sterile endophthalmitis following IVTA injection presents early in the postoperative period. Visual outcomes are generally good.

Posterior subtenon triamcinolone acetonide for refractory diabetic macular edema

PURPOSE

To evaluate posterior subtenon injection of triamcinolone acetonide for refractory diabetic macular edema (DME).

DESIGN

Retrospective, interventional, case series.

METHODS

SETTING

Clinical practice.

PATIENT POPULATION

Patients with persistent clinically significant DME involving the center of the fovea 3 or more months after one or more treatments of focal macular photocoagulation were included. Exclusion criteria were a history of corticosteroid-responsive intraocular pressure (IOP) rise, intraocular surgery within 3 months, and any laser treatment within 1 month.

PROCEDURE

All patients received an ophthalmic history and examination including best-corrected Snellen visual acuity, IOP measurement, anterior segment examination including evaluation of lens status with LOCS II criteria, dilated fundus examination, and a posterior subtenon injection of 40 mg triamcinolone acetonide at baseline. Patients were reevaluated at 1, 3, 6, and 12 months after injection.

RESULTS

Seventy-three injections were performed in 63 eyes of 50 patients. The mean baseline visual acuity was 20/80. Mean visual acuity significantly improved to 20/50 at 1 month, then stabilized to 20/65 at 3 months, 20/68 at 6 months, and 20/63 at 12 months. Complications were rare, with a transient significant rise in intraocular pressure at the 3-month evaluation and ptosis in two patients.

CONCLUSIONS

Visual acuities remained stable or improved over a 12-month period after posterior subtenon triamcinolone injections for refractory DME. There was a statistically significant improvement in visual acuity at 1 month.

Intravitreal triamcinolone for macular oedema: efficacy in relation to aetiology

PURPOSE

To evaluate the efficacy and safety of intravitreal triamcinolone in patients with macular oedema of varying aetiology.

METHODS

Two milligrams of intravitreal triamcinolone acetonide was injected into 34 eyes with persistent macular oedema (17 eyes with macula oedema secondary to posterior uveitis, 13 eyes with diabetic retinopathy, and four with pseudophakic macular oedema). Best corrected visual acuity was determined and transfoveal optical coherence tomography performed after 1 week, 1 month, 3 months and 6 months.

RESULTS

Treatment improved visual acuity and subjective visual quality, and reduced foveal thickness in eyes with posterior uveitis and eyes with macular oedema secondary to diabetic retinopathy. Eyes treated for pseudophakic cystoid macular oedema demonstrated no improvement. A total of 32% of patients experienced a significant post-injection increase in intraocular pressure. Endophthalmitis, rhegmatogenous retinal detachment and cataract were absent.

CONCLUSION

Intravitreal triamcinolone appears to induce marked a improvement in macular oedema secondary to non-infectious uveitis and diabetic retinopathy.

The consequences of imbibing alcohol in the absence of adequate nutrition: the salt and water hypothesis

Alcohol can have an effect on almost every cell in the human body and it is becoming increasingly clear that when alcohol is consumed the prior nutritional status of the individual may be an important factor for long-term health. The salt and water hypothesis integrates the biochemical findings from the current alcohol literature and proposes a mechanism by which alcohol consumption prior to food intake may cause a transient alteration in the functioning of the hypothalamic-pituitary-adrenal axis due to an alcohol-induced impairment in electrolyte regulation.

Nongenomic membrane actions of glucocorticoids in vertebrates

For decades, it was widely assumed that glucocorticoids (GCs) work solely through changes in gene expression to exert their physiological actions, a process that normally takes several hours to occur. However, recent evidence indicates that GCs might also act at the membrane through specific receptors to exert multiple rapid effects on various tissues and cells. GCs modulate hormone secretion, neuronal excitability, behavior, cell morphology, carbohydrate metabolism and other processes within seconds or minutes. These early actions occur independent of the genome and are transduced by the same biochemical effector pathways responsible for mediating rapid responses to neurotransmitters. The biological significance of most rapid GC effects are not well understood, but many might be related to the important functions that this hormone plays in modulating stress responses.

Steroid hormones use non-genomic mechanisms to control brain functions and behaviors: a review of evidence

Progestins, estrogens, androgens, and corticosteroids are capable of modifying brain functions and behaviors by mechanisms that involve the classic genomic model for steroid action. However, experimental evidence indicates that some responses to steroid hormones use non-classical, non-genomic mechanisms. This paper reviews the evidence that steroids can bind to receptors in the plasma membrane, activate cell signaling pathways, and regulate responses on a time scale of seconds or a few minutes. The existence of these alternative regulatory pathways for steroid hormones should make endocrinologists and neurobiologists change how they think about steroid hormones. It is no longer valid to assume that minute-to-minute changes in steroid concentrations are not regulating biologically important, short-term responses, or that the only steroids with biological functions are the ones that bind with high affinity to intracellular steroid receptors.

Pleiotropic signaling pathways in rapid, nongenomic action of glucocorticoid

The traditional genomic theory of steroid action does not fully explain the rapid effects of hormonal steroids, and it is thought that the nongenomic actions mediated by a putative membrane receptor may provide a plausible explanation. Although there is a rich body of evidence to substantiate the rapid, nongenomic effects of steroid hormones, the signal transduction pathways involved have proved to be complex and pleiotropic. Based on previous studies on the rapid, nongenomic actions of glucocorticoid (GC) and the G-protein-protein kinase pathways involved, including our own studies on PC12, SK-N-SH, BT-325 cells, and synaptosomes, in this review we will discuss the issue of multiple signal transduction pathways involved in the rapid, nongenomic effects of GC.

Purification of a cortisol binding protein from hepatic plasma membrane

A cortisol binding protein from rat liver plasma membranes has been solubilized in active form by using the zwitterionic detergent CHAPS. Two types of binding sites have been characterised in both native and solubilized membranes. The first is of high affinity and low binding capacity (12 nM; 946 fmol/mg) and the other one is of low affinity and high capacity of binding (344 nM; 12677 fmol/mg) for solubilized membranes. The purified material retained a binding activity comparable to that displayed by the original membrane. The specific binding activity was enriched about 12700-fold, with an 8% yield. Analysis of the purified preparation on sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed two protein subunits with molecular mass of 52000 and 57000 Da. The new cortisol-specific binding membrane protein could be related to the nongenomic effects previously described for this hormone.

Dexamethasone rapidly increases hypothalamic neuropeptide Y secretion in adrenalectomized ob/ob mice

A single intracerebroventricular injection of dexamethasone (DEX) rapidly (within 30 min) suppresses brown adipose tissue thermogenesis and increases plasma insulin concentrations in adrenal-ectomized (ADX) ob/ob mice but not in ADX lean mice. Intracerebroventricular neuropeptide Y (NPY) administered intracerebroventricularly causes these same metabolic changes within 30 min in both ob/ob and lean ADX mice. We therefore hypothesized that DEX exerts these rapid-onset metabolic actions in ob/ob mice via a phenotype-specific enhancement of NPY secretion within the central nervous system. In support of this hypothesis, DEX (a type II glucocorticoid receptor agonist) administered intracerebroventricularly selectively lowered NPY concentrations in the whole hypothalamus of ADX ob/ob mice by 35% and in the arcuate nucleus region by approximately 70% within 30 min but not in the brain stem or hippocampus or in any of these regions of lean mice. DEX also functioned in vitro to enhance depolarization-dependent release of NPY from hypothalamic blocks of ADX ob/ob mice but not of ADX lean mice. Thus DEX acts in the hypothalamus of ob/ob mice in a phenotype-specific manner to evoke rapid transport of NPY from cell bodies within the arcuate nucleus to terminal regions including the dorsomedial and ventromedial hypothalamic regions for release.

Glucocorticoid interactions with ethanol effects on depolarization-induced calcium influx in brain synaptosomes

Depolarization-induced Ca2+ influx in brain synaptosomes is known to be inhibited by ethanol and stimulated by glucocorticoids. The present study was undertaken to characterize the interactions of corticosterone (CORT) with ethanol effects on 45Ca2+ uptake in synaptosomes depolarized by high K+ (70 mM). CORT was shown to antagonize the inhibitory effects of ethanol on the fast-phase component of the K(+)-induced 45Ca2+ uptake (the initial 3 s following depolarization). Glucocorticoid antagonism of ethanol inhibition of the 45Ca2+ uptake exhibited a high degree of steroid specificity; steroids with glucocorticoid activity including cortisol, dexamethasone and triamcinolone were effective, whereas gonadal steroids and excitatory neuroactive steroid metabolites were ineffective. From the shift of concentration-response relationships when CORT and ethanol were present in combination, the interaction between steroid stimulation and ethanol inhibition of 45Ca2+ uptake occurred in an additive manner over the range of their effective concentrations. Parallel to 45Ca2+ uptake, the binding sites for [3H]PN 200-110 were reduced by ethanol and increased by CORT. These opposite effects on [3H]dihydropyridine labeled sites were found also to antagonize each other, and the antagonism again occurred in an additive relationship. These results demonstrate that glucocorticoids antagonized ethanol inhibition of voltage-dependent Ca2+ channel activity in brain synaptosomes, and support the notion that these steroids may be among the endogenous factors that modulate neuronal sensitivity to ethanol.

Glucocorticoid actions on synaptic plasma membranes: modulation of dihydropyridine-sensitive calcium channels

We have previously shown that glucocorticoids accelerate depolarization-induced 45Ca2+ influx in synaptosomes isolated from rat cerebral cortex, indicating that the steroids may modulate voltage-dependent Ca2+ channels. The present study was undertaken to characterize the biochemical action of glucocorticoids on dihydropyridine-sensitive voltage-dependent Ca2+ channels known to be present in brain synaptosomes. The [3H]dihydropyridine labeled site was used as a marker to determine the levels of functional Ca2+ channels. No effect on equilibrium binding of [3H]PN 200-110 was found when membranes from disrupted synaptosomes were incubated with corticosterone as high as 1 microM. However, when intact synaptosomes were first incubated with corticosterone at 37 degrees C and then disrupted, a significant increase in [3H]PN 200-110 binding was found. Steroid incubation of synaptosomes at 0 degree C was ineffective. It appears that metabolic processes requiring intracellular factors were involved in the steroid action. In examining this possibility, [3H]PN 200-110 binding was activated in disrupted membranes by MgATP and Ca(2+)-calmodulin, and corticosterone was found to enhance the activation in a concentration-dependent manner. [3H]PN 200-110 binding to membranes was also activated by incubation with MgATP and cAMP-dependent protein kinase, but this activation was not enhanced by the steroid. These findings are consistent with the interpretation that the steroid promotes Ca2+ channel activity by enhancing calmodulin-dependent activation of the channels. The action on voltage-dependent Ca2+ channels in synaptic terminals may well be a mechanism whereby glucocorticoids modulate neuronal activity.

Acute and chronic actions of ethanol on endogenous calmodulin content in synaptic plasma membranes from rat brain

The present study was designed to demonstrate that endogenous calmodulin (CaM) content in synaptic plasma membranes (SPM) is altered by acute and chronic administration of ethanol and is a sequel to the kinetic characterization of ethanol inhibition of [125I]CaM binding to SPM reported in our previous study. In rats, an acute ethanol injection (2 g/kg, i.p.) rapidly reduced CaM content in SPM from cerebral cortex, whereas chronic ethanol treatment [6% (w/v) in a liquid diet for 3 weeks] led to an up-regulation of the CaM content. In both cases, the alteration of CaM content in SPM occurred in the EGTA-dissociable pool of CaM (77% of total membrane CaM); the EGTA-nondissociable pool (23% of total CaM) was not affected. In animals receiving chronic ethanol treatment, CaM content in SPM was not altered significantly by the acute ethanol dose that produced rapid reduction of CaM content in control animals, indicating that resistance to ethanol develops. This resistance to ethanol can be attributed to alterations of membrane properties. In control SPM, ethanol at 50 mM markedly accelerated the temperature-dependent dissociation of endogenous CaM, whereas in SPM from animals chronically treated with ethanol, significant acceleration of CaM dissociation required ethanol concentrations as high as 150-200 mM. These findings on SPM in vitro were consistent with the data on CaM content obtained in vivo. Since CaM mediates a variety of biochemical processes in synaptic membranes, we hypothesize that the effects of ethanol in altering the content of membrane-bound CaM may lead to a cascade of consequences in synaptic membrane function.

Calmodulin: effects of cell stimuli and drugs on cellular activation

The activity, localization and cellular content of CaM can be regulated by drugs, hormones and neurotransmitters. Regulation of physiological responses of CaM can depend upon local Ca(2+)-entry domains in the cells and phosphorylation of CaM target proteins, which would either decrease responsiveness of CaM target enzymes or increase CaM availability for binding to other target proteins. Despite the abundance of CaM in many cells, persistent cellular activation by a variety of substances can lead to an increase in CaM, reflected both in the nucleus and other cellular compartments. Increases in CaM-binding proteins can accompany stimuli-induced increases in CaM. A role for CaM in vesicular or protein transport, cell morphology, secretion and other cytoskeletal processes is emerging through its binding to cytoskeletal proteins and myosins in addition to the more often investigated activation of target enzymes. More complete knowledge of the physiological regulation of CaM can lead to a greater understanding of its role in physiological processes and ways to alter its actions through pharmacology.

Correlation between [125I]calmodulin binding and lipid fluidity in synaptic plasma membranes: effects of ethanol and other short-chain alcohols

Ethanol inhibits [125I]calmodulin binding to synaptic plasma membranes from rat brain, and this inhibition is correlated in a concentration-dependent manner with the increase of membrane fluidity, as determined by diphenylhexatriene fluorescence polarization. Moreover, several short-chain alcohols that increase membrane fluidity are also effective inhibitors of [125I]calmodulin binding. These data support the notion that ethanol inhibits calmodulin binding by increasing lipid fluidity of the synaptic membranes.

Regulation of calmodulin content in synaptic plasma membranes by glucocorticoids

Synaptic plasma membranes (SPM) from the brain are known to have specific binding sites for several steroid hormones, but the mechanisms of membrane transduction of steroid signals is not understood. In this study, corticosterone was found to prevent temperature-dependent dissociation of endogenous calmodulin (CaM) from highly purified SPM from rat cerebral cortex. The steroid stabilizes Ca(2+)-dependent membrane binding of endogenous CaM (78% of total CaM), whereas Ca(2+)-independent binding of CaM (the other 22%) is not affected. The stabilization of membrane binding of endogenous CaM by corticosterone is concentration-dependent, with the maximal effect occurring at steroid concentration of 1 microM. The EC50 is estimated as 130 nM, which is almost identical to the Kd of specific binding of the steroid to SPM (120 nM) reported previously. The effect in stabilizing membrane binding of CaM is specific to corticosterone and other glucocorticoids (cortisol, dexamethasone and triamcinolone); gonadal steroids (17 (17 beta-estradiol, progesterone and testosterone) are ineffective. Furthermore, corticosterone administration in vivo (2 mg/kg, i.p.) produced a rapid increase of CaM content in SPM, occurring within 5 min after steroid injection and persisting for at least 20 min. Since CaM mediates a variety of biochemical processes in synaptic membranes, we hypothesize that the effect of glucocorticoids in promoting membrane binding of CaM may lead to a cascade of consequences in synaptic membrane function.