Adenosine A(2A) receptor antagonist treatment of Parkinson's disease.

BACKGROUND: Observations in animal models suggest that A(2A) antagonists confer benefit by modulating dopaminergic effects on the striatal dysfunction associated with motor disability. This double-blind, placebo-controlled, proof-of-principle study evaluated the pathogenic contribution and therapeutic potential of adenosine A(2A) receptor-mediated mechanisms in Parkinson disease (PD) and levodopa-induced motor complications. METHODS: Fifteen patients with moderate to advanced PD consented to participate. All were randomized to either the selective A(2A) antagonist KW-6002 or matching placebo capsules in a 6-week dose-rising design (40 and 80 mg/day). Motor function was rated on the Unified PD Rating Scale. RESULTS: KW-6002 alone or in combination with a steady-state IV infusion of each patient's optimal levodopa dose had no effect on parkinsonian severity. At a low dose of levodopa, however, KW-6002 (80 mg) potentiated the antiparkinsonian response by 36% (p < 0.02), but with 45% less dyskinesia compared with that induced by optimal dose levodopa alone (p < 0.05). All cardinal parkinsonian signs improved, especially resting tremor. In addition, KW-6002 prolonged the efficacy half-time of levodopa by an average of 47 minutes (76%; p < 0.05). No medically important drug toxicity occurred. CONCLUSIONS: The results support the hypothesis that A(2A) receptor mechanisms contribute to symptom production in PD and that drugs able to selectively block these receptors may help palliate symptoms in levodopa-treated patients with this disorder.

Prevention of beta-amyloid neurotoxicity by blockade of the ubiquitin-proteasome proteolytic pathway.

In many neurodegenerative disorders, such as Alzheimer's disease, inclusions containing ubiquitinated proteins have been found in the brain, suggesting a pathophysiological role for ubiquitin-mediated proteasomal degradation of neuronal proteins. Here we show for the first time that the beta-amyloid fragment 1-40, which in micromolar levels causes the death of cortical neurons, also induces the ubiquitination of several neuronal proteins. Prevention of ubiquitination and inhibition of proteasome activity block the neurotoxic effect of beta-amyloid. These data suggest that beta-amyloid neurotoxicity may cause toxicity through the activation of protein degradation via the ubiquitin-proteasome pathway. These findings suggest possible new pharmacological targets for the prophylaxis and/or treatment of Alzheimer's disease and possibly for other related neurodegenerative disorders.

Poly(ADP-ribosyl)ation basally activated by DNA strand breaks reflects glutamate-nitric oxide neurotransmission.

Poly(ADP-ribose) polymerase (PARP) transfers ADP ribose groups from NAD(+) to nuclear proteins after activation by DNA strand breaks. PARP overactivation by massive DNA damage causes cell death via NAD(+) and ATP depletion. Heretofore, PARP has been thought to be inactive under basal physiologic conditions. We now report high basal levels of PARP activity and DNA strand breaks in discrete neuronal populations of the brain, in ventricular ependymal and subependymal cells and in peripheral tissues. In some peripheral tissues, such as skeletal muscle, spleen, heart, and kidney, PARP activity is reduced only partially in mice with PARP-1 gene deletion (PARP-1(-/-)), implicating activity of alternative forms of PARP. Glutamate neurotransmission involving N-methyl-d-aspartate (NMDA) receptors and neuronal nitric oxide synthase (nNOS) activity in part mediates neuronal DNA strand breaks and PARP activity, which are diminished by NMDA antagonists and NOS inhibitors and also diminished in mice with targeted deletion of nNOS gene (nNOS(-/-)). An increase in NAD(+) levels after treatment with NMDA antagonists or NOS inhibitors, as well as in nNOS(-/-) mice, indicates that basal glutamate-PARP activity regulates neuronal energy dynamics.

cAMP-induced cytoskeleton rearrangement increases calcium transients through the enhancement of capacitative calcium entry.

In this report we investigated the correlation between cell morphology and regulation of cytosolic calcium homeostasis. Type I astrocytes were differentiated to stellate process-bearing cells by a 100-min exposure to cAMP. Differentiation of cortical astrocytes increased the magnitude and duration of calcium transients elicited by phospholipase C-activating agents as measured by single cell Fura-2-based imaging. Calcium imaging showed differences in the spatial pattern of the response. In both differentiated and the control cells, the response originated in the periphery and gradually extended into the center of the cell. However, the elevation of cytosolic calcium concentration ([Ca(2+)](i)) was particularly evident within the processes and adjacent to the inner cell membrane of the differentiated astrocytes. In addition, differentiation significantly prolonged the duration of the [Ca(2+)](i) elevation. Potentiation of the calcium transients was mimicked by forskolin-induced differentiation and abolished by a specific protein kinase-A blocker. Conversely, the enhancement of the calcium transients was not mimicked by brief exposure to cAMP not causing morphological differentiation, and in PC12 cells that did not undergo morphological changes after 100 min of cAMP treatment. Impairing cAMP-induced cytoskeleton re-organization, by means of cytochalasin D and nocodazole, prevented the potentiation of the calcium transients in cAMP-treated astrocytes. Phospholipase C activity and sensitivity to inositol (1,4,5)-trisphosphate were not involved in the enhancement of the calcium responses. Also, potentiation of the calcium transients was dependent on extracellular calcium. Calcium storage and thapsigargin-depletable intracellular calcium reservoirs were analogously not increased in differentiated astrocytes. Rearrangement of the cell shape also caused a condensation of the endoplasmic reticulum and altered the spatial relationship between the endoplasmic reticulum and the cell membrane. In conclusion, morphological rearrangements of type I astrocytes increase the magnitude and the duration of agonist-induced calcium transients via enhancement of capacitative calcium entry and is associated with a spatial reorganization of the relationship between cell membrane and the endoplasmic reticulum structures.

Calexcitin interaction with neuronal ryanodine receptors.

Calexcitin (CE), a Ca2+- and GTP-binding protein, which is phosphorylated during memory consolidation, is shown here to co-purify with ryanodine receptors (RyRs) and bind to RyRs in a calcium-dependent manner. Nanomolar concentrations of CE released up to 46% of the 45Ca label from microsomes preloaded with 45CaCl2. This release was Ca2+-dependent and was blocked by antibodies against the RyR or CE, by the RyR inhibitor dantrolene, and by a seven-amino-acid peptide fragment corresponding to positions 4689-4697 of the RyR, but not by heparin, an Ins(1,4,5)P3-receptor antagonist. Anti-CE antibodies, in the absence of added CE, also blocked Ca2+ release elicited by ryanodine, suggesting that the CE and ryanodine binding sites were in relative proximity. Calcium imaging with bis-fura-2 after loading CE into hippocampal CA1 pyramidal cells in hippocampal slices revealed slow, local calcium transients independent of membrane depolarization. Calexcitin also released Ca2+ from liposomes into which purified RyR had been incorporated, indicating that CE binding can be a proximate cause of Ca2+ release. These results indicated that CE bound to RyRs and suggest that CE may be an endogenous modulator of the neuronal RyR.

Evolution of adaptive neural networks: the role of voltage-dependent K+ channels.

The vestibular pathway of the mollusk Hermissenda crassicornis mediates a reflexive, unconditioned response to disorientation, clinging, that has been conserved during evolution even to the emergence of our own species. This response becomes associated with a visual stimulus (mediated by a precisely ordered visual-vestibular synaptic network) according to principles of Pavlovian conditioning that are also followed in human learning. It is not entirely surprising therefore that molecular and biophysical cascades responsible for this associative learning appear to function in both mollusks and mammals. In brief, combinational elevation of (Ca2+)i, diacylglycerol, and arachidonic acid activates protein kinase C to phosphorylate the Ca2+ and guanosine triphosphate-binding protein, cp20 (now called calexcitin (Nelson T, et al. Proc Natl Acad Sci USA 1996;93:13808-13)), which potently inactivates postsynaptic voltage-dependent K+ currents and thereby increases synaptic weight. Longer term changes included rearrangement of synaptic terminals and modified protein synthesis. This cascade has also been implicated in other associative-learning paradigms (e.g., spatial maze, olfactory discrimination) and as a pathophysiologic target in early Alzheimer's disease. Recent molecular biologic experiments also demonstrate the dependence of associative memory (but not long-term potentiation) on voltage-dependent K+ currents. Theoretic learning models based on these findings focus on dendritic spine clusters and yield computer implementations with powerful pattern-recognition capabilities.

Alzheimer's-specific effects of soluble beta-amyloid on protein kinase C-alpha and -gamma degradation in human fibroblasts.

Alzheimer's disease (AD) is a multifactorial disease in which beta-amyloid peptide (betaAP) plays a critical role. We report here that the soluble fraction 1-40 of betaAP differentially degrades protein kinase C-alpha and -gamma (PKCalpha and PKCgamma) isoenzymes in normal (age-matched controls, AC) and AD fibroblasts most likely through proteolytic cascades. Treatment with nanomolar concentrations of betaAP(1-40) induced a 75% decrease in PKCalpha, but not PKCgamma, immunoreactivity in AC fibroblasts. In the AD fibroblasts, a 70% reduction of the PKCgamma, but not PKCalpha, immunoreactivity was observed after betaAP treatment. Preincubation of AC or AD fibroblasts with 50 microM lactacystine, a selective proteasome inhibitor, prevented beta-AP(1-40)-mediated degradation of PKCalpha in the AC cells, and PKCgamma in the AD fibroblasts. The effects of betaAP(1-40) on PKCalpha in AC fibroblasts were prevented by inhibition of protein synthesis and reversed by PKC activation. A 3-hr treatment with 100 nM phorbol 12-myristate 13-acetate restored the PKCalpha signal in treated AC cells but it did not reverse the effects of betaAP(1-40) on PKCgamma in the AD fibroblasts. Pretreatment with the protein synthesis inhibitor, cycloheximide (CHX, 100 microM), inhibited the effects of betaAP(1-40) on PKCalpha and blocked the rescue effect of phorbol 12-myristate 13-acetate in AC fibroblasts but did not modify PKCgamma immunoreactivity in AD cells. These results suggest that betaAP(1-40) differentially affects PKC regulation in AC and AD cells via proteolytic degradation and that PKC activation exerts a protective role via de novo protein synthesis in normal but not AD cells.

Calexcitin: a signaling protein that binds calcium and GTP, inhibits potassium channels, and enhances membrane excitability.

A previously uncharacterized 22-kDa Ca(2+)-binding protein that also binds guanosine nucleotides was characterized, cloned, and analyzed by electrophysiological techniques. The cloned protein, calexcitin, contains two EF-hands and also has homology with GTP-binding proteins in the ADP ribosylation factor family. In addition to binding two molecules of Ca2+, calexcitin bound GTP and possessed GTPase activity. Calexictin is also a high affinity substrate for protein kinase C. Application of calexcitin to the inner surface of inside-out patches of human fibroblast membranes, in the presence of Ca2+ and the absence of endogenous Ca2+/calmodulin kinase type II or protein kinase C activity, reduced the mean open time and mean open probability of 115 +/- 6 pS K+ channels. Calexcitin thus appears to directly regulate K+ channels. When microinjected into molluscan neurons or rabbit cerebellar Purkinje cell dendrites, calexcitin was highly effective in enhancing membrane excitability. Because calexcitin translocates to the cell membrane after phosphorylation, calexcitin could serve as a Ca(2+)-activated signaling molecule that increases cellular excitability, which would in turn increase Ca2+ influx through the membrane. This is also the first known instance of a GTP-binding protein that binds Ca2+.

Imaging protein kinase C activation in living sea urchin eggs after fertilization.

The fluorescent dye NBD-phorbol acetate was used to visualize the activation of protein kinase C (PKC) in living Lytechinus pictus eggs during fertilization. The dye interacts directly with PKC as determined using a competitive binding assay. Quantitative image analysis of sequential images from laser-scanning confocal microscopy showed a significant reorganization of the signal in the vicinity of the cortical granules and the plasma membrane that began immediately following fertilization and persisted up to 1 hr (P<0.0001). At the concentrations employed, the NBD-phorbol dye was not capable of inducing a significant translocation of the fluorescent signal to the membrane, nor did it appear to interfere with the cell cycle. It therefore seems likely that the present in vivo results reflect the previously reported in vitro activation of protein kinase C immediately subsequent to fertilization. Such an interpretation is parsimonious with the results of parallel subcellular fractionation experiments using an N-terminal polyclonal antibody to sea urchin PKC which showed a significant (P<0.037) translocation of the enzyme from the cytosolic fraction to the membrane fraction 40 min subsequent to fertilization. This study supports and extends previous in vitro data suggesting that PKC activation subsequent to fertilization occurs at or near the egg plasma membrane, perhaps in association with arachadonic acid-rich cortical granules.

Pituitary adenylate cyclase-activating polypeptide activates different signal transducing mechanisms in cultured cerebellar granule cells.

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a novel 38-residue neuropeptide which stimulates adenylate cyclase activity in rat pituitary cells as well as in other neuronal and non-neuronal tissues. In this study we have investigated whether PACAP27 and PACAP38 may stimulate either cyclic AMP accumulation or phosphoinositide formation in cultured cerebellar granule cells. In cultures at 8 days of maturation in vitro (DIV), a 15-min exposure to PACAP27 or PACAP38 equally promoted a concentration-dependent increase in intracellular cAMP content: the effect was significant at 1-5 nM and maximal between 10 and 100 nM, while VIP was 1,000-fold less potent in elevating cAMP levels. In the presence of 3-isobutyl-1-methylxanthine (200 microM), stimulation by PACAP was present already at 0.1 nM and was maximal (6-fold increase) at 1 nM. A rapid elevation in intracellular cAMP (about 80%) was observed within a 30-second exposure to 10 microM PACAP38 or PACAP27; the maximal activity of PACAP was present between 15 and 30 min and progressively declined at 60 min without reaching basal values. PACAP27 and PACAP38, but not VIP, were also able to stimulate inositol phospholipid hydrolysis: PACAP38 (EC50: 0.16 nM) was 10-fold more potent than PACAP27 (EC50: 2.1 nM) in stimulating [3H]inositol phosphate formation. The effect of PACAP was rapid: fractionation of [3H]inositol phosphates revealed that inositol trisphosphate and inositol bisphosphate increased earlier (within 20 s) than inositol monophosphate (within 60 s).(ABSTRACT TRUNCATED AT 250 WORDS)

The inhibition of peroxide formation as a possible substrate for the neuroprotective action of dihydroergocryptine.

Dihydroergocryptine is an ergot alkaloid endowed with pharmacological actions mainly related to its dopaminomimetic activity. Free radical formation and subsequent lipid peroxidation had been postulated to participate broadly to the pathogenesis of tissue injury, including the brain injury induced by hypoxia, ischemia or trauma, as well as in the physiopathology of chronic neurodegenerative diseases, such as Parkinson's disease. Here we report that dihydroergocryptine protects cultured rat cerebellar granule cells against age-dependent and glutamate-induced neurotoxicity. Dihydroergocryptine antagonizes in fact both the neuronal death produced by acute exposure to a toxic glutamate concentration as well as the normal age-dependent degeneration in culture, presumably by exerting a scavenger action. This effect does not seem mediated entirely by interactions with the dopamine D2 receptors. The neuroprotective action of dihydroergocryptine suggests a potential usefulness in halting the acute and chronic neurodegenerative diseases related to excitotoxic damage and free radical formation, including Parkinson's disease.

Differential expression of gamma-aminobutyric acid receptors in immortalized luteinizing hormone-releasing hormone neurons.

gamma-Aminobutyric acid (GABA) has been shown both to stimulate and inhibit LH secretion in vivo. GABA apparently exerts these effects at the hypothalamic level by regulating the release of LHRH. In this study, we have investigated the effect of GABAergic agents on LHRH secretion from an immortalized hypothalamic neuronal cell line (GT1-7). LHRH secretion was stimulated in a dose-dependent manner with increasing concentrations of GABA. This effect was mimicked by the GABAA receptor agonist, muscimol, and was blocked by the selective antagonist, bicuculline. The stimulatory effect of muscimol on LHRH secretion was synergistic with low concentrations of [K+]. By comparison, neither activation of the GABAB receptors with baclofen nor blockade with phaclofen influenced basal LHRH secretion. Baclofen, however, did depress [K+]-induced LHRH release. Binding studies confirmed the presence of GABAA and GABAB receptors on GT1-7 cells. In addition, Northern blots with probes to the GABAA receptor alpha 1, beta 3, and gamma 2L subunits revealed that only the beta 3 messenger RNA (mRNA) was expressed in the GT1-7 cells. These data provide the first demonstration that immortalized LHRH neurons are directly responsive to GABAergic agents. To the extent that these immortalized neurons may resemble those in vivo, our results suggest that GABAergic agents may play a dual role in reproductive physiology by exerting both stimulatory and inhibitory control over LHRH release.

Protection by dihydroergocryptine of glutamate-induced neurotoxicity.

Dihydroergocryptine is a hydrogenated ergot derivative with pharmacological actions mainly related to its dopaminomimetic activity. Here we report that dihydroergocryptine can protect cultured rat cerebellar granule cells against glutamate-induced neurotoxicity, assessing cell viability with the fluorescein diacetate-propidium iodide technique. Dihydroergocryptine antagonized both the neuronal death produced by acute exposure to a toxic glutamate concentration as well as the normal age-dependent degeneration in culture. The effect of dihydroergocryptine might be mediated by a scavenger action as suggested by the fact that the compound in a concentration-dependent manner reduced the formation of intracellular peroxides produced in cerebellar granule cells by exposure to 100 microM glutamate. This action is apparently not mediated entirely by interactions with the dopamine D2 receptors. The neuroprotective action suggests that dihydroergocryptine might be a potential useful drug in the therapy and/or prophylaxis of acute and chronic neurodegenerative diseases related to excitotoxic damage.

Dihydroergocryptine protects from acute experimental allergic encephalomyelitis in the rat.

The effect of dihydroergocryptine, a natural alkaloid derivative which exhibits D2 dopaminomimetic properties, has been studied in Lewis female rats with experimentally induced allergic encephalomyelitis. A chronic treatment with dihydroergocryptine started two days before immunization, induced a dramatic reduction of prolactin levels accompanied by a marked amelioration of neurological signs. In addition, the proliferative activity of splenic lymphocytes induced by the mitogen Concanavalin-A (Con-A) was reduced in dihydroergocryptine-treated animals. It is suggested that this effect is related to the ability of dihydroergocryptine to lower prolactin concentrations or also, partially, to a neuroprotective action of this drug.

Ubiquinone protects cultured neurons against spontaneous and excitotoxin-induced degeneration.

Ubiquinone is an endogenous quinone with pharmacological actions mainly related to its antioxidant properties. Here we report that ubiquinone protects cultured cerebellar granule cells against glutamate-induced neurotoxicity. In control cultures at 9 days of maturation in vitro (DIV), a 30-min exposure to 100 microM glutamate induced neuronal degeneration, as reflected by the great percentage (greater than 90%) of cells labeled with propidium iodide 24 h after the exposure. Glutamate-induced neuronal death was dramatically reduced in cultures treated daily with ubiquinone since the second DIV. In these cultures, glutamate failed to induce a "delayed" increase in the influx of 45Ca2+, an established parameter of excitotoxicity. Similarly, repeated addition of ubiquinone attenuated in a concentration-dependent manner the age-dependent degeneration of granule cells that is due to the toxic action of the endogenous glutamate progressively released into the medium. These results suggest that ubiquinone may be a useful drug in the therapy of acute and chronic neurodegenerative diseases related to hyperactivity of excitatory amino acid neurotransmission.

Trophic action of acetyl-L-carnitine in neuronal cultures.

Daily addition of acetyl-L-carnitine (100 microM) to cultured cerebellar granule cells since the first day of maturation led to an increased rate of expression of D-[3H]aspartate uptake (an established marker of maturation of glutamatergic neurons) and of N-methyl-D-aspartate (NMDA) receptors linked to large conductance ion channels permeable to Ca2+. Acetyl-L-carnitine treatment also increased neuronal survival, as reflected by a greater percentage of cultures retaining functional NMDA receptors after 15 days of maturation. These results support the view that acetyl-L-carnitine exerts neuronotrophic activity and prevents age-dependent neuronal degeneration.

Estrogen modulates stimulation of inositol phospholipid hydrolysis by norepinephrine in rat brain slices.

The influence of estrogen on stimulation of inositol phospholipid hydrolysis by norepinephrine and carbamylcholine has been studied by measuring the accumulation of [3H]inositol-monophosphate ([3H]InsP) in cortical, hippocampal and striatal slices from ovariectomized rats. Repeated (but not a single) subcutaneous injections of estradiol benzoate (EB) (2 micrograms/animal once every 2 days for 10 days) markedly reduced stimulation of inositol phospholipid hydrolysis by norepinephrine in hippocampus and corpus striatum. Conversely, the efficacy of norepinephrine was increased in cortical slices. Estrogen treatment did not affect basal or carbamylcholine-stimulated [3H]InsP formation. In vitro addition of 17 beta-estradiol (1-100 nM) failed to modify norepinephrine- or carbamylcholine-induced [3H]InsP production in all regions examined. An increased density of alpha 1-adrenergic binding sites in cortical membranes paralleled the enhanced responsiveness of inositol phospholipid hydrolysis to norepinephrine induced by EB treatment in this area, whereas no significant changes in [3H]prazosin binding were found in membranes from hippocampus and corpus striatum. These results indicate that estrogen may affect inositol phospholipid hydrolysis in discrete brain areas, suggesting a complex role for estradiol in modulating noradrenergic receptor activity in the central nervous system.

Repeated injections of piracetam improve spatial learning and increase the stimulation of inositol phospholipid hydrolysis by excitatory amino acids in aged rats.

Repeated injections of piracetam (400 mg/kg, i.p. once a day for 15 days) to 16-month old rats led to an improved performance on an 8-arm radial maze, used as a test for spatial learning. This effect was accompanied by a greater ability of excitatory amino acids (ibotenate and glutamate) to stimulate [3H]inositol-monophosphate formation in hippocampal slices incubated in the presence of 10 mM Li+. Repeated administration of piracetam did not induce changes in excitatory amino acid-stimulated polyphosphoinositide hydrolysis in hippocampal slices prepared from 2-month old animals.

Receptor-mediated stimulation of inositol phospholipid hydrolysis in human brain.

Stimulation of inositol phospholipid hydrolysis by transmitter receptor agonists was studied in slices from the occipital, temporal and cerebellar cortex that were surgically removed from three patients during ablation of brain tumors. Norepinephrine and the muscarinic acetylcholine agonist carbamylcholine increased the content of [3H]inositol monophosphate (InsP) in all the regions examined, whereas the glutamate receptor agonist quisqualate was effective only in the cerebellar slices. These results provide evidence that neurotransmitter receptors are coupled to inositol phospholipid hydrolysis in human brain tissue.

Repeated calcitonin treatment reduces the stimulation of inositol phospholipid by norepinephrine and serotonin in rat hippocampus and cerebral cortex.

Stimulation of inositol phospholipid hydrolysis by norepinephrine or 5-hydroxytryptamine was reduced in hippocampal or cortical slices from rats repeatedly injected with (Asu1.7)eel-calcitonin (2.5 IU/kg i.p.). This effect was specific, as the basal or carbamylcholine-stimulated inositol phospholipid hydrolysis was unchanged in slices from calcitonin-injected animals. The reduced responsiveness to norepinephrine did not reflect a decreased number or affinity of alpha 1-adrenergic recognition sites, suggesting that calcitonin treatment leads to a reduced coupling between alpha 1-adrenoceptors and phospholipase C.