Effects of aging and morphine administration on calmodulin and calmodulin-regulated enzymes in striata of mice

Synthesis of the Mechanisms of Opioid Tolerance: Do We Still Say NO?

The use of morphine as a first-line agent for moderate-to-severe pain is limited by the development of analgesic tolerance. Initially opioid receptor desensitization in response to repeated stimulation, thought to underpin the establishment of tolerance, was linked to a compensatory increase in adenylate cyclase responsiveness. The subsequent demonstration of cross-talk between N-methyl-D-aspartate (NMDA) glutamate receptors and opioid receptors led to the recognition of a role for nitric oxide (NO), wherein blockade of NO synthesis could prevent tolerance developing. Investigations of the link between NO levels and opioid receptor desensitization implicated a number of events including kinase recruitment and peroxynitrite-mediated protein regulation. Recent experimental advances and the identification of new cellular constituents have expanded the potential signaling candidates to include unexpected, intermediary compounds not previously linked to this process such as zinc, histidine triad nucleotide-binding protein 1 (HINT1), micro-ribonucleic acid (mi-RNA) and regulator of G protein signaling Z (RGSZ). A further complication is a lack of consistency in the protocols used to create tolerance, with some using acute methods measured in minutes to hours and others using days. There is also an emphasis on the cellular changes that are extant only after tolerance has been established. Although a review of the literature demonstrates a lack of spatio-temporal detail, there still appears to be a pivotal role for nitric oxide, as well as both intracellular and intercellular cross-talk. The use of more consistent approaches to verify these underlying mechanism(s) could provide an avenue for targeted drug development to rescue opioid efficacy.

Riboflavin and pyridoxine restore dopamine levels and reduce oxidative stress in brain of rats

BACKGROUND

Neurological disorders suggest that the excitotoxicity involves a drastic increase in intracellular Ca2+ concentrations and the formation of reactive oxygen species. The presence of these free radicals may also affect the dopaminergic system. The aim of this work was to determine if riboflavin (B2) and pyridoxine (B6) provide protection to the brain against free radicals generated by 3-nitropropionic acid (3-NPA) by measuring the levels of dopamine (DA) and selected oxidative stress markers.

METHODS

Male Fisher rats were grouped (n = 6) and treated as follows: group 1, control (NaCl 0.9%); group 2, 3-NPA (20 mg/kg); group 3, B2 (10 mg/kg); group 4, B2 (10 mg/kg) + 3-NPA (20 mg/kg); group 5, B6 (10 mg/kg) and group 6, B6 + 3-NPA. All treatments were administered every 24 h for 5 days by intraperitoneal route. After sacrifice, the brain was obtained to measure DA, GSH, and lipid peroxidation, Ca2+, Mg2+, ATPase and H2O2.

MAIN FINDINGS

Levels of dopamine increased in cortex, striatum and cerebellum/medulla oblongata of animals that received 3-NPA alone. The lipid peroxidation increased in cortex, striatum, and cerebellum/medulla oblongata, of animals treated with B2 vitamin alone. ATPase dependent on Ca+2, Mg+2 and H2O2 increased in all regions of animals that received 3-NPA alone.

CONCLUSION

The results confirm the capacity of 3-NPA to generate oxidative stress. Besides, the study suggests that B2 or B6 vitamins restored the levels of DA and reduced oxidative stress in brain of rats. We believe that these results would help in the study of neurodegenerative diseases.

Molecular and cellular mechanisms of the age-dependency of opioid analgesia and tolerance

The age-dependency of opioid analgesia and tolerance has been noticed in both clinical observation and laboratory studies. Evidence shows that many molecular and cellular events that play essential roles in opioid analgesia and tolerance are actually age-dependent. For example, the expression and functions of endogenous opioid peptides, multiple types of opioid receptors, G protein subunits that couple to opioid receptors, and regulators of G protein signaling (RGS proteins) change with development and age. Other signaling systems that are critical to opioid tolerance development, such as N-methyl-D-aspartic acid (NMDA) receptors, also undergo age-related changes. It is plausible that the age-dependent expression and functions of molecules within and related to the opioid signaling pathways, as well as age-dependent cellular activity such as agonist-induced opioid receptor internalization and desensitization, eventually lead to significant age-dependent changes in opioid analgesia and tolerance development.

Increase of calmodulin III gene expression by mu-opioid receptor stimulation in PC12 cells

Calmodulin (CaM) is a principal multifunctional mediator of Ca2+ signaling in cells. It is reported that morphine increases CaM contents in mouse brain. However, the precise mechanism of CaM induction by morphine is unknown. We investigated the changes of CaM by opioid receptor stimulation in mRNA and protein levels. Expression of CaM was increased in dose- and time-dependent manners by morphine with RT-PCR assay in PC12 cells, and naloxone inhibited the effect of morphine. The expression was also increased with DAMGO (mu-opioid agonist), but not by DPDPE (delta) and U50488 (kappa). Northern blot analysis revealed that the CaMIII gene was responsive to morphine or DAMGO. CaM protein increased by DAMGO were distributed in both soluble and membranous fractions in the cells. Taken together, the data suggest that morphine induces the expression of CaMIII gene through mu-opioid receptor stimulation.

Region-specific downregulation of free intracellular calcium in the aged rat brain

Age-related changes in resting levels of the free intracellular calcium concentration ([Ca2+]i) as well as alterations of the rise in [Ca2+]i following depolarization have been investigated in acutely isolated brain cells of various regions of the rat brain. Characterization of the Ca2+ responses following KCl depolarization in the hippocampus, cortex, striatum, and cerebellum of young rats revealed significant regional differences in the basal [Ca2+]i level as well as in the KCl-induced rise in [Ca2+]i. However, there was no correlation between both parameters. Resting [Ca2+]i as well as Ca2+ responses after depolarization were lower in the hippocampus and cortex of the aged animals, but not in the striatum or cerebellum. It is concluded that the Ca2+ homeostasis in the first two regions is specially susceptible to the aging process, resulting in a downregulation of [Ca2+]i, probably as a consequence of an enhanced sensitivity of mechanisms regulating transmembraneous Ca2+ fluxes. The cellular Ca2+ homeostasis was altered in a comparable way in rat spleenocytes. The rise in [Ca2+]i in the aged animals following stimulation of lymphocytes with the mitogen phytohemagglutinin (PHA) was significantly reduced in the plateau phase, which is maintained by Ca2+ influx mechanisms. The data indicate that age-related disturbances of the cellular Ca2+ homeostasis may be present in different cell types and seem to affect mainly transmembraneous Ca2+ flux much more than intracellular Ca2+ release.

Disturbances of the neuronal calcium homeostasis in the aging nervous system

Maintenance of the cellular calcium homeostasis plays an important role for neuronal cell function and interneuronal cell to cell communication. Therefore, alterations of the neuronal Ca2+ homeostasis may play a crucial role for brain aging in general and for age-related deficits in cognitive functions particularly. Numerous studies indicate various disturbances of the Ca2+ homeostasis on different levels like Ca2+ channel properties, 45Ca2+ uptake, or Ca2+ binding proteins. Investigations on alterations of the free intracellular calcium concentration ([Ca2+]i) in presynaptic synaptosomal preparations led to inconsistent results reporting increased or unchanged [Ca2+]i in aged animals. Postsynaptic alterations of [Ca2+]i have been investigated mainly indirectly by electrophysiological methods and revealed prolonged Ca(2+)-dependent afterhyperpolarization or prolonged Ca2+ spike duration. By using acutely dissociated mouse brain cells it was possible for the first time to evaluate age-dependent alterations of postsynaptic [Ca2+]i directly. In neurons of aged mice basal [Ca2+]i was reduced and depolarization-induced rise in [Ca2+]i was also reduced, probably as a result of increased activation of Ca(2+)-dependent mechanisms terminating Ca(2+)-influx. Depolarization-induced, Ca(2+)-mediated inositolphosphate accumulation was also increased in aged animals. This leads to the conclusion that Ca(2+)-dependent intracellular processes become more sensitive during aging. Investigations about the effect of beta-amyloid on the Ca2+ homeostasis in the same system revealed a small but consistent destabilizating effect of this peptide on K(+)-induced rise in [Ca2+]i which may result in chronically increased neuronal vulnerability. Together with increased Ca2+ sensitivity during aging this might be one of the reasons for the increasing prevalence of Alzheimer's disease (AD) with aging.

Effect of CDP-choline treatment on mitochondrial and synaptosomal protein composition in different brain regions during aging

Several age-dependent modifications of inner mitochondrial membrane and synaptosomal plasma membrane proteins from different brain regions of 4-, 12-, 18- and 24-month-old male Wistar rats, were observed. Some proteins, identified by immunoblotting assay as various subunits of mitochondrial respiratory chain complexes and calmodulin, were particularly impaired. Chronic treatment with CDP-choline at a dose of 20 mg/kg body weight per day for 28 days caused significant changes in the amounts of several of the above mentioned proteins. Most of the proteins, which decreased during aging, showed a significant increase after CDP-choline treatment compared with the corresponding control values at the same age. The effect of CDP-choline might be due to: the increased availability of cytidylic nucleotides, which in the brain are present in limited amounts compared to the other nucleotides; the increased content of total adenine nucleotides; the improvement of brain energy metabolism.

Changes in calcium's role as a messenger during aging in neuronal and nonneuronal cells

Alterations in calcium transport appear to be functionally significant. Treatment with drugs that promote calcium uptake partially reverse some of the age-related deficits in calcium-dependent processes. Thus, the relevance of decreased calcium coupled receptor binding is supported by the ability of 3,4-diaminopyridine to promote acetylcholine release by forebrain slices from aged mice. This drug also reduces the age-related depression in synaptosomal calcium uptake in aged rats and mice. 3,4-Diaminopyridine also reverses the age-related deficit in calcium transport, the age-related deficits in the tight rope test, and 8 arm maze performance. 3,4-Diaminopyridine is also effective in nonexcitable tissues, such as cultured skin fibroblasts; it increases the decreased cytosolic-free calcium. Depressed cell spreading of fibroblasts can be reversed by treatment of cells with the calcium ionophore A23187 which promotes calcium influx. 4-Aminopyridine, a similarly related compound, partially reverses short-term memory deficits in patients with Alzheimer's disease. Tetrahydroaminoacridine, an aminopyridine analog with anticholinesterase properties, produces clinical improvement in behavioral deficits due to Alzheimer's disease. Only recently has the aging brain become a subject of intense study. Evidently, the neurobiology of aging needs to develop its own theories to account for the unique aspects of brain aging as well as integrate them with the peripheral changes. An exciting but unexplored area of research in the aging brain concerns the coupling between calcium and the final end product, the induction of genes. Still unknown are the molecular events that set these processes in motion. In addition, whether conditions such as dietary restriction that increase longevity in certain rodents also retard age-related changes in calcium remains to be determined.

Modifications of synaptosomal plasma membrane protein composition in various brain regions during aging

The age-dependent modifications of synaptosomal plasma membrane protein composition in three different rat brain regions (cerebral cortex, cerebellum and striatum) at various ages (4, 12 and 24 months) were studied. The proteins were separated by gel-electrophoresis and the quantity of the different polypeptides was determined densitometrically from the stained gels. In the three brain regions examined several age-related modifications in the amount of the synaptosomal plasma membrane proteins were observed. In particular a significant decrease in the content of some synaptosomal plasma membrane proteins at 24 months of age was found. The age-related modifications in the protein composition of synaptosomal plasma membrane may cause changes in many brain functions, such as neurotransmission, ionic transport and enzyme activities. Particularly interesting is the decrease of a protein with 18 kDa mol. wt. This protein has been identified as calmodulin by immunoblotting assay. The decrease in the amount of this protein may be correlated to the impairment of several Ca(2+)-requiring processes in the aging brain.

Effect of hypoxia on protein composition of synaptic plasma membranes from cerebral cortex during aging

The effect of hypoxia on the protein composition of synaptic plasma membranes (SPM) isolated from cerebral cortex of rats at 4, 12, and 24 months of age was investigated. The proteins were separated by SDS polyacrylamide gel electrophoresis and the percent content was evaluated by measuring the optical density of the stained gels. After hypoxic treatment various proteins showed significant changes. Some proteins were only affected at 4 and 12 months of age and not at 24 months. The various modified proteins may be identified according to their molecular weight, as follows: the 18 kDa protein with calmodulin; the 23 kDa protein with D3 subunits; the 28 kDa protein could contain the delta subunit of the Ca2+ channel. The changes in the amount of some SPM proteins during hypoxia is consistent with the alteration in membrane polarization and neurotransmission observed in this condition. The effect of aging at the synaptosomal level seems to be a selective process; after hypoxia the age-related changes of many proteins are more pronounced.

The opioid system in neurologic and psychiatric disorders and in their experimental models

Evidence from experimental and clinical studies suggests the involvement of the endogenous opioid system in several neurologic and psychiatric disorders (Alzheimer's, Huntington's and Parkinson's diseases, drug-induced movement disorders, Gilles de la Tourette syndrome, stroke, ischemia, brain and spinal cord injury, epilepsy, schizophrenia and affective disorders). However, its involvement is rather a secondary one, perhaps being a severe consequence of a primary, nonopioid disturbance. Thus, treatment of an opioidergic manifestation of a disorder of nonopioidergic origin is necessarily symptomatic and targets only the restoration of the opioid system; such treatment may be beneficial in ameliorating the clinical symptoms of the disorder.

Aging does not alter cytosolic calcium levels of cortical synaptosomes in Fischer 344 rats

Several lines of experimental evidence support an association between altered Ca2+ regulation and aging. It has been supposed that free cytosolic Ca2+ concentrations ([Ca2+]i) may decrease or increase in aged animals. In this study, both resting and KCl-stimulated [Ca2+]i were measured in purified cortical synaptosomes from young (3 mo.), middle-aged (12 mo.), and old (24 mo.) Fischer 344 rats. Two additional groups of rats were included, one middle-aged and one old which were trained on a treadmill for 6 months prior to experimentation. The [Ca2+]i was determined using the fluorescent Ca2+ chelator fura-2. Net KCl-dependent changes (delta K) in [Ca2+]i were determined by the difference between stimulatory (100 microM Ca2+/60 mM KCl) and resting (100 microM Ca2+/5 mM KCl buffer) conditions among the 3 age groups. Significant increases in [Ca2+]i were observed in each age group upon depolarization with 60 mM KCl. However, there were no significant age-dependent differences in either resting [Ca2+]i or KCl-stimulated [Ca2+]i.

Age-induced differentiation of morphine's effect on cyclic nucleotide metabolism

Male ICR mice, immature (25 days old), mature adult (3 months old) and aged (22 months old), were injected with morphine sulfate (10 mg/kg, SC) or were implanted with morphine pellets (75 mg). Age-matched controls received saline injections or placebo pellets. One hour after injections and 72 hours after pellet implantation (when tolerance to morphine had occurred), the mice were decapitated and the frontal cortex and cerebellum were removed. Basal activities of adenylate cyclase, guanylate cyclase, cyclic AMP phosphodiesterase and cyclic GMP phosphodiesterase were determined in both brain regions. Results showed that there are age- and region-differentiated effects of morphine on these enzymes.

Calcium and neuronal cytoskeletal proteins: alterations with aging

Calcium plays a major role in regulating cellular function. Alterations in calcium systems may underlie some of the physiological changes associated with aging. Calcium activates calmodulin-dependent protein kinase, and this enzyme mediates some effects of calcium on cellular function. Calcium/calmodulin-dependent kinase II may play a significant role in specific cytoskeletal abnormalities of normal aging and selected neurodegenerative diseases.

Calcium and the aging nervous system

Many aspects of calcium homeostasis change with aging. Numerous calcium compartments complicate studies of altered calcium regulation. However, age-related decreases in calcium permeation across membranes and mobilization from organelles may be a common fundamental change. Deficits in ion movements appear to lead to altered coupling of calcium-dependent biochemical and neurophysiological processes and may lead to pathological and behavioral changes. The calcium-associated changes during aging probably do not occur with equal intensity in all cell types or in different parts of the same cell. Thus, cells or compartments with a high proportion of calcium activated processes would be more sensitive to diminished calcium availability. These age-related changes may predispose the brain to the development of age-related neurological disorders. The effects of decreased ion movement may be further aggravated by an age-related decline in other calcium-dependent processes. Depression of some of these calcium-dependent functions appears physiologically significant, since increasing calcium availability ameliorates age-related deficits in neurotransmission and behavior. A better understanding of the interactions between calcium homeostasis and calcium-dependent processes during aging will likely help in the design of more effective therapeutic strategies.

Compartmentalization of calmodulin and tubulin in the male C57BL/6J mouse brain: heterogeneity of age changes in calmodulin compartments

Calmodulin (CaM) and tubulin were analyzed by radioimmunoassay in subcellular fractions prepared from cerebral cortex and striatum of aging male C57BL/6J mice. Three fractions were prepared by a new procedure: cytosol (soluble); EGTA-releasable, membrane-bound; and detergent-extractable (Triton X-100), membrane-bound fractions. CaM concentration in all three fractions prepared from striatum showed small (10-15%, p less than 0.05) decreases with age (3-31 months). Cortical CaM concentrations were less affected by age, and only the EGTA-releasable fraction decreased. To compare functional activity and immunoreactivity of CaM, soluble CaM was also assayed by the activation of cyclic nucleotide phosphodiesterase (PDE). The radioimmunoassay and PDE activation assays gave equivalent results, suggesting that no alteration occurred with age in biological activity of CaM, via post-translational modification or other mechanisms. Soluble and particulate tubulin concentration did not change significantly with age in either brain region. The changes observed in striatal CaM, particularly in membrane-bound compartments, could contribute to the age-related decline in mammalian basal ganglial function.

Determination of calmodulin by competitive binding assay

Calmodulin levels in tissue or cellular extracts can be determined by competition with 125I-calmodulin in a filtration-based direct binding assay. The method is rapid, uses readily available stable components, and possesses a selectivity and sensitivity comparable to that observed with immunoassay and phosphodiesterase activation. This assay provides a tool to readily probe changes in calmodulin levels in cells and tissues as a function of pathophysiologic state.