Co-localization of carnosine and glutamate in photoreceptors and bipolar cells of the frog retina

Carnosine as an effective neuroprotector in brain pathology and potential neuromodulator in normal conditions

Carnosine (b-alanyl-L-histidine) is an endogenous dipeptide widely distributed in excitable tissues, such as muscle and neural tissues-though in minor concentrations in the latter. Multiple benefits have been attributed to carnosine: direct and indirect antioxidant effect, antiglycating, metal-chelating, chaperone and pH-buffering activity. Thus, carnosine turns out to be a multipotent protector against oxidative damage. However, the role of carnosine in the brain remains unclear. The key aspects concerning carnosine in the brain reviewed are as follows: its concentration and bioavailability, mechanisms of action in neuronal and glial cells, beneficial effects in human studies. Recent literature data and the results of our own research are summarized here. This review covers studies of carnosine effects on both in vitro and in vivo models of cerebral damage, such as neurodegenerative disorders and ischemic injuries and the data on its physiological actions on neuronal signaling and cerebral functions. Besides its antioxidant and homeostatic properties, new potential roles of carnosine in the brain are discussed.

Recent advances in drug and nutrient transport across the blood-retinal barrier

INTRODUCTION

The blood-retinal barrier (BRB) is the barrier separating the blood and neural retina, and transport systems for low-weight molecules at the BRB are expected to be useful for developing drugs for the treatment of ocular neural disorders and maintaining a healthy retina. Areas covered: This review discusses blood-to-retina and retina-to-blood transport of drugs and nutrients at the BRB. In particular, P-gp (ABCB1/MDR1) has low impact on the transport of cationic drugs at the BRB, suggesting a significant role of novel organic cation transporters in influx and efflux transport of lipophilic cationic drugs between blood and the retina. The transport of pravastatin at the BRB involves transporters including organic anion transporting polypeptide 1a4 (Oatp1a4). Recent studies have shown the involvement of solute carrier transporters in the blood-to-retina transport of nutrients including riboflavin, L-ornithine, β-alanine, and L-histidine, implying that dipeptide transport at the BRB is minimal. Expert opinion: Novel organic cation transport systems and the elimination-dominant transport of pravastatin at the BRB are expected to be useful in systemic drug delivery to the neural retina without CNS side effects. The mechanism of nutrient transport at the BRB is expected to provide a new strategy for delivery of nutrient-mimetic drugs.

Effects of glutamate positive modulators on cognitive deficits in schizophrenia: a systematic review and meta-analysis of double-blind randomized controlled trials

Hypofunction of N-methyl-d-aspartate (NMDA) receptors has been proposed to have an important role in the cognitive impairments observed in schizophrenia. Although glutamate modulators may be effective in reversing such difficult-to-treat conditions, the results of individual studies thus far have been inconsistent. We conducted a systematic review and meta-analysis to examine whether glutamate positive modulators have beneficial effects on cognitive functions in patients with schizophrenia. A literature search was conducted to identify double-blind randomized placebo-controlled trials in schizophrenia or related disorders, using Embase, Medline, and PsycINFO (last search: February 2015). The effects of glutamate positive modulators on cognitive deficits were evaluated for overall cognitive function and eight cognitive domains by calculating standardized mean differences (SMDs) between active drugs and placebo added to antipsychotics. Seventeen studies (N=1391) were included. Glutamate positive modulators were not superior to placebo in terms of overall cognitive function (SMD=0.08, 95% confidence interval=-0.06 to 0.23) (11 studies, n=858) nor each of eight cognitive domains (SMDs=-0.03 to 0.11) (n=367-940) in this population. Subgroup analyses by diagnosis (schizophrenia only studies), concomitant antipsychotics, or pathway of drugs to enhance the glutamatergic neurotransmission (glycine allosteric site of NMDA receptors or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors) suggested no procognitive effect of glutamate positive modulators. Further, no effect was found in individual compounds on cognition. In conclusion, glutamate positive modulators may not be effective in reversing overall cognitive impairments in patients with schizophrenia as adjunctive therapies.

Physiology and pathophysiology of carnosine

Carnosine (β-alanyl-l-histidine) was discovered in 1900 as an abundant non-protein nitrogen-containing compound of meat. The dipeptide is not only found in skeletal muscle, but also in other excitable tissues. Most animals, except humans, also possess a methylated variant of carnosine, either anserine or ophidine/balenine, collectively called the histidine-containing dipeptides. This review aims to decipher the physiological roles of carnosine, based on its biochemical properties. The latter include pH-buffering, metal-ion chelation, and antioxidant capacity as well as the capacity to protect against formation of advanced glycation and lipoxidation end-products. For these reasons, the therapeutic potential of carnosine supplementation has been tested in numerous diseases in which ischemic or oxidative stress are involved. For several pathologies, such as diabetes and its complications, ocular disease, aging, and neurological disorders, promising preclinical and clinical results have been obtained. Also the pathophysiological relevance of serum carnosinase, the enzyme actively degrading carnosine into l-histidine and β-alanine, is discussed. The carnosine system has evolved as a pluripotent solution to a number of homeostatic challenges. l-Histidine, and more specifically its imidazole moiety, appears to be the prime bioactive component, whereas β-alanine is mainly regulating the synthesis of the dipeptide. This paper summarizes a century of scientific exploration on the (patho)physiological role of carnosine and related compounds. However, far more experiments in the fields of physiology and related disciplines (biology, pharmacology, genetics, molecular biology, etc.) are required to gain a full understanding of the function and applications of this intriguing molecule.

Inhibition of tumour cell growth by carnosine: some possible mechanisms

The naturally occurring dipeptide carnosine (β-alanyl-L-histidine) has been shown to inhibit, selectively, growth of transformed cells mediated, at least in part, by depleting glycolytic ATP levels. The mechanism(s) responsible has/have yet to be determined. Here, we discuss a number of probable and/or possible processes which could, theoretically, suppress glycolytic activity which would decrease ATP supply and generation of metabolic intermediates required for continued cell reproduction. Possibilities include effects on (i) glycolytic enzymes, (ii) metabolic regulatory activities, (iii) redox biology, (iv) protein glycation, (v) glyoxalase activity, (vi) apoptosis, (vii) gene expression and (viii) metastasis. It is possible, by acting at various sites that this pluripotent dipeptide may be an example of an endogenous "smart drug".

A preliminary, randomized, double-blind, placebo-controlled trial of L-carnosine to improve cognition in schizophrenia

BACKGROUND

Targeting glutamatergic dysfunction provides an exciting opportunity to improve cognitive impairment in schizophrenia. One treatment approach has targeted inadequate antioxidant defenses at glutamatergic synapses. Animal and human data suggest NMDA antagonists worsen executive cognitive controls--e.g. increase perseverative responses and impair set-shifting. We conducted a preliminary study to test the hypothesis that L-carnosine, an antioxidant and anti-glycation agent which is co-localized and released with glutamate would improve executive dysfunction, a cognitive domain associated with glutamate.

METHODS

Seventy-five symptomatically stable adults with chronic schizophrenia were randomly assigned to L-carnosine as adjunctive treatment (2 g/day) or a matched placebo in a double-blind manner for 3 months. Cognitive domains (executive dysfunction, memory, attention and motor speed) were assessed using a computerized battery at baseline, 4 and 12 weeks, along with psychopathology ratings and safety parameters.

RESULTS

The L-carnosine group performed significantly faster on non-reversal condition trials of the set-shifting test compared with placebo but reversal reaction times and errors were not significantly different between treatments. On the strategic target detection test, the L-carnosine group displayed significantly improved strategic efficiency and made fewer perseverative errors compared with placebo. Other cognitive tests showed no significant differences between treatments. Psychopathology scores remained stable. The carnosine group reported more adverse events (30%) compared with the placebo group (14%). Laboratory indices remained within acceptable ranges.

CONCLUSIONS

These preliminary findings suggest that L-carnosine merits further consideration as adjunctive treatment to improve executive dysfunction in persons with schizophrenia.

Distribution of carnosine-like peptides in the nervous system of developing and adult zebrafish (Danio rerio) and embryonic effects of chronic carnosine exposure

Carnosine-like peptides (carnosine-LP) are a family of histidine derivatives that are present in the nervous system of various species and that exhibit antioxidant, anti-matrix-metalloproteinase, anti-excitotoxic, and free-radical scavenging properties. They are also neuroprotective in animal models of cerebral ischemia. Although the function of carnosine-LP is largely unknown, the hypothesis has been advanced that they play a role in the developing nervous system. Since the zebrafish is an excellent vertebrate model for studying development and disease, we have examined the distribution pattern of carnosine-LP in the adult and developing zebrafish. In the adult, immunoreactivity for carnosine-LP is specifically concentrated in sensory neurons and non-sensory cells of the olfactory epithelium, the olfactory nerve, and the olfactory bulb. Robust staining has also been observed in the retinal outer nuclear layer and the corneal epithelium. Developmental studies have revealed immunostaining for carnosine-LP as early as 18 h, 24 h, and 7 days post-fertilization in, respectively, the olfactory, corneal, and retinal primordia. These data suggest that carnosine-LP are involved in olfactory and visual function. We have also investigated the effects of chronic (7 days) exposure to carnosine on embryonic development and show that 0.01 microM to 10 mM concentrations of carnosine do not elicit significant deleterious effects. Conversely, treatment with 100 mM carnosine results in developmental delay and compromised larval survival. These results indicate that, at lower concentrations, exogenously administered carnosine can be used to explore the role of carnosine in development and developmental disorders of the nervous system.

Structure and function of photoreceptor and second-order cell mosaics in the retina of Xenopus

The structure, physiology, synaptology, and neurochemistry of photoreceptors and second-order (horizontal and bipolar) cells of Xenopus laevis retina is reviewed. Rods represent 53% of the photoreceptors; the majority (97%) are green light-sensitive. Cones belong to large long-wavelength-sensitive (86%), large short-wavelength-sensitive (10%), and miniature ultraviolet wavelength-sensitive (4%) groups. Photoreceptors release glutamate tonically in darkness, hyperpolarize upon light stimulation and their transmitter release decreases. Photoreceptors form ribbon synapses with second-order cells where postsynaptic elements are organized into triads. Their overall adaptational status is regulated by ambient light conditions and set by the extracellular dopamine concentration. The activity of photoreceptors is under circadian control and is independent of the central body clock. Bipolar cell density is about 6000 cells/mm2 They receive mixed inputs from rods and cones. Some bipolar cell types violate the rule of ON-OFF segregation, giving off terminal branches in both sublayers of the inner plexiform layer. The majority of them contain glutamate, a small fraction is GABA-positive and accumulates serotonin. Luminosity-type horizontal cells are more frequent (approximately 1,000 cells/mm2) than chromaticity cells (approximately 450 cells/mm2). The dendritic field size of the latter type was threefold bigger than that of the former. Luminosity cells contact all photoreceptor types, whereas chromatic cells receive their inputs from the short-wavelength-sensitive cones and rods. Luminosity cells are involved in generating depolarizing responses in chromatic horizontal cells by red light stimulation which form multiple synapses with blue-light-sensitive cones. Calculations indicate that convergence ratios in Xenopus are similar to those in central retinal regions of mammals, predicting comparable spatial resolution.

Immunocytochemical and electrophysiological characterization of GABA receptors in the frog and turtle retina

The expression of GABA receptors (GABARs) was studied in frog and turtle retinae. Using immunocytochemical methods, GABA(A)Rs and GABA(C)Rs were preferentially localized to the inner plexiform layer (IPL). Label in the IPL was punctate indicating a synaptic clustering of GABARs. Distinct, but weaker label was also present in the outer plexiform layer. GABA(A)R and GABA(C)R mediated effects were studied by recording electroretinograms (ERGs) and by the application of specific antagonists. Bicuculline, the GABA(A)R antagonist, produced a significant increase of the ERG. Picrotoxin, when co-applied with saturating doses of bicuculline, caused a further increase of the ERG due to blocking of GABA(C)Rs. The putative GABA(C)R antagonist Imidazole-4-acidic acid (I4AA) failed to antagonize GABA(C)R mediated inhibition and, in contrast, appeared rather as an agonist of GABARs.

Retinal müller cell culture

A mini-review is presented of the current techniques for maintaining Müller cells in a culture. Within the retina, Müller cells are the predominant glial cells. These highly specialised cells extend over the entire neural retina. One of the most important of the various physiological functions of Müller cells is to regulate the balance of ions and neurotransmitters in the retina. Disturbance of these regulatory functions may lead to toxic effects on receptor and other neural cells in the neuroretina, and may be a common mechanism of clinical retinal neuropathy. The main excitatory neurotransmitter in the retina is glutamate. Müller cells regulate the amount of glutamate in the synaptic regions of the neural network in the retina. Accumulation of extra glutamate seems to be an important mechanism for initiating pathological changes leading to retinal damage. Many previous in vitro studies on the role of Müller cells in retinal toxicology have been based on the use of morphological and histochemical methods. In cell toxicology studies, it is important to develop culture techniques able to provide more cells for biochemical determinations.