Galantamine is an allosterically potentiating ligand of the human alpha4/beta2 nAChR

Galantamine (Reminyl) is a novel drug treatment for mild to moderate Alzheimer's disease (AD). Originally established as a reversible inhibitor of the acetylcholine-degrading enzyme acetylcholinesterase (AChE), galantamine also acts as an allosterically potentiating ligand (APL) on nicotinic acetylcholine receptors (nAChR). Having previously established this second mode of action on nAChRs from murine brain, we demonstrate here the same action of galantamine on the most abundant nAChR in the human brain, the alpha4/beta2 subtype. This nAChR-sensitizing action is not a common property of all, or most, AChE inhibitors, as is shown by the absence of this effect for other therapeutically applied AChE inhibitors including tacrine, metrifonate, rivastigmine and donepezil. The possible benefits for therapy of AD of an APL action on nicotinic receptors is discussed.

Allosteric sensitization of nicotinic receptors by galantamine, a new treatment strategy for Alzheimer's disease

Cholinesterase inhibitors are the only approved drug treatment for patients with mild to moderately severe Alzheimer's disease. Interestingly, the clinical potency of these drugs does not correlate well with their activity as cholinesterase inhibitors, nor is their action as short lived as would be expected from purely symptomatic treatment. A few cholinesterase inhibitors, including galantamine, produce beneficial effects even after drug treatment has been terminated. These effects assume modes of action other than mere esterase inhibition and are capable of inducing systemic changes. We have recently discovered a mechanism that could account, at least in part, for the above-mentioned unexpected properties of some cholinesterase inhibitors. We have found that a subgroup of cholinesterase inhibitors, including galantamine but excluding tacrine, directly interacts with nicotinic acetylcholine receptors. These compounds, named allosterically potentiating ligands, sensitize nicotinic receptors by increasing the probability of channel opening induced by acetylcholine and nicotinic agonists and by slowing down receptor desensitization. The allosterically potentiating ligand action, which is not necessarily associated with cholinesterase inhibition, has been demonstrated by whole-cell patch-clamp recordings to occur in natural murine and human neurons and in murine and human cell lines expressing various subtypes of neuronal nicotinic acetylcholine receptors.

Allosterically potentiating ligands of nicotinic receptors as a treatment strategy for Alzheimer's disease

One of the most prominent cholinergic deficit in Alzheimer's disease (AD) is the reduced number of nicotinic acetylcholine receptors (nAChR) in the hippocampus and cortex of AD patients, as compared to age-matched controls. This deficit results in reduced nicotinic cholinergic excitation which may not only impair postsynaptic depolarization but also presynaptic neurotransmitter release and Ca2+-dependent intracellular signaling, including transcriptional activity. Presently, the most common approach to correct the nicotinic cholinergic deficit in AD is the application of cholinesterase inhibitors. Due to the resulting increase in synaptic acetylcholine levels, both in concentration and time, additional nAChR molecules, e.g. those more distant from the ACh release sites, could be activated. As an obvious disadvantage, this approach affects cholinergic neurotransmission as a whole, including muscarinic neurotransmission. As a novel and alternative approach, a treatment strategy which exclusively targets nicotinic receptors is suggested. The strategy is based on a group of modulating ligands of nicotinic receptors, named allosterically potentiating ligands (APL), which increase the probability of channel opening induced by ACh and nicotinic agonists, and in addition decrease receptor desensitization. The action of APL on nicotinic receptors is reminiscent of that of benzodiazepines on GABA(A) receptors and of that of glycine on the NMDA-subtype of glutamate receptor. Representative nicotinic APL are the plant alkaloids physostigmine, galanthamine and codeine, and the neurotransmitter serotonin (5HT). The potentiating effect of APL on nicotinic neurotransmission has been shown by whole-cell patch-clamp studies in natural murine and human neurons, and in murine and human cell lines expressing various subtypes of neuronal nAChR.

Aging and beta-amyloid peptides decrease cholinergic receptor-mediated calcium increase in brain cortex synaptoneurosomes

In this study, the muscarinic cholinergic receptor (MAChR)-evoked inositol 1,4,5-trisphosphate (IP3)-mediated increase of cytosolic calcium concentration ([Ca]i) in synaptoneurosomes from brain cortex of adult and aged rats was investigated. In addition, the effect of two beta-amyloid (A beta) peptides, 1-28 and 25-35, on the resting and MAChR-induced increase of [Ca]i in brain cortex synaptoneurosomes of adult rats was evaluated. Release of IP3 was measured after prelabeling of synaptoneurosomal phosphoinositides with myo-[2-3H]inositol. Changes in [Ca]i were monitored by using fura-2 indicator. The effect of A beta peptides was evaluated following their preincubation with synaptoneurosomal protein for 1, 5, 30 and 60 min. It was observed that in brain cortex synaptoneurosomes from aged rats, Ca(2+)-dependent and MAChR-mediated IP3 production was not changed in comparison with that estimated in adult brain, over 60 min of incubation. Activation of MAChR in synaptoneurosomes from brain cortex of adult rats for 10 min increased [Ca]i by about 60% over its resting level (240 nM). This increase was completely blocked by muscarinic antagonists, atropine and pirenzepine, as well as by the antagonist of IP3 receptor, 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8). In aged brain, there was no detectable change in resting [Ca]i (165 nM) due to MAChR stimulation. The 25-35 A beta peptide caused a time-dependent significant increase of resting [Ca]i in synaptoneurosomes from brain cortex of adult rats, which was almost five-fold after 60 min. In the same conditions, the action of 1-28 A beta peptide was statistically insignificant up to 30 min, then a rapid increase of resting [Ca]i by two-fold was observed up to 60 min. Both A beta peptides decreased markedly the MAChR-dependent elevation of [Ca]i in respect to control (resting [Ca]i) in synaptoneurosomes from brain cortex of adult rats. These results indicate that beta-amyloid 1-28 and 25-35 peptides may be involved in alteration of muscarinic receptor-mediated signal transduction during brain aging.

Nitric oxide responsible for NMDA receptor-evoked inhibition of arachidonic acid incorporation into lipids of brain membrane

The activation of the glutamatergic NMDA receptor has no effect on arachidonic acid release from cortical synaptoneurosomal lipids prelabeled with [1-14C]arachidonic acid ([14C]AA). However, activation of NMDA receptor leads to the reduction of AA incorporation into rat brain cortex synaptoneurosomal membrane phosphatidylinositol (PI). The competitive NMDA receptor antagonist, 2-amino-5-phosphovaleric acid (APV), completely eliminates the effect of NMDA on this process. More precise analysis of the sequence of events leading to NMDA-induced decrease of AA incorporation indicates that this process is significantly blocked by voltage-gated sodium and calcium channels inhibitors, such as tetrodotoxin (TTX) and omega-conotoxin (CTX), respectively. Then the antagonist of inositol trisphosphate receptor, TMB-8, totally abolishes the effect of NMDA on AA incorporation into PI. The lowering of AA incorporation evoked by NMDA is significantly diminished by nitric oxide (NO) synthase inhibitor, NG-nitro-L- arginine (NNLA). Further studies were carried out with NO donor(s) to explain the mechanism of NO action in the inhibition of AA incorporation into PI. Our results suggest the following sequence of events: opening of voltage-dependent sodium and calcium channels, subsequent activation of PI-4,5-bisphosphate-specific phospholipase C (PLC), elevation of inositol trisphosphate (IP3)-sensitive calcium ions, stimulation of NO production and NO-mediated S-nitrosylation, or free radical effect on enzymes involved in AA incorporation. Our data suggest that NO-mediated events may be responsible for NMDA-evoked inhibition of AA incorporation into PI of synaptoneurosomal membrane.

Activation of serotonergic 5-HT1A receptor reduces Ca(2+)- and glutamatergic receptor-evoked arachidonic acid and No/cGMP release in adult hippocampus

Stimulation of glutamatergic NMDA receptor in adult rat hippocampal synaptoneurosomes induces statistically significant Ca(2+)-dependent liberation of arachidonic acid (AA) and nitric oxide (NO)-activated cGMP synthesis. NMDA acting for 5 min at 100 microM markedly increases, by approx. 25%, Ca(2+)-mediated AA release from phospholipids of hippocampal synaptoneurosomes. Prolonged stimulation of NMDA receptor up to 10 min has smaller stimulatory effect and enhances AA release by about 6%. Moreover, NMDA activates NO-dependent cGMP production by approx. 5 times more than the Ca2+ itself. Release of both these second messengers is completely blocked by the competitive NMDA antagonist, APV (100 microM). The NMDA-mediated cGMP elevation completely depends on NO action, and is abolished by the specific inhibitor of NO synthase, NG-nitro-L-arginine. Moreover, serotonin at 10 microM in the presence of 10 microM pargyline, potently decreases both Ca(2+)- and NMDA receptor-mediated AA and cGMP release in hippocampal synaptoneurosomes. The agonist of 5-HT1A receptor, buspirone, in a way similar to serotonin itself, counteracts the Ca(2+)- and also NMDA receptor-evoked AA release and cGMP accumulation. An antagonist of 5-HT1A receptor, NAN-190, eliminates the effect of serotonin and buspirone on AA and NO/cGMP liberation. An antagonist of serotonergic 5-HT2 receptor, ketanserin, has no effect on the Ca2+ and serotonin action. These results indicate that serotonin, through 5-HT1A receptor, potently antagonizes the action of excitatory amino acid for AA release and NO/cGMP synthesis in the adult rat hippocampus. In conclusion, the interaction of serotonin with the glutamatergic system in the hippocampus may play an important role in the modulation of a signal transduction pathway, and by this molecular mechanism serotonin may exert a neuroprotective effect on hippocampal neurons.

The negative coupling between serotonin and muscarinic receptor(s) for arachidonic acid and inositol phosphates release in brain cortex synaptoneurosomes. Effect of aging

In the present study we have investigated the relationship between serotonergic receptor(s) (5-HTR) and muscarinic cholinergic receptor (mAChR) activation, leading to the arachidonic acid (AA) release and inositol phosphates (IPs) formation in adult and aged brain cortex synaptoneurosomes. It was observed that serotonin (5-HT) almost completely inhibits carbachol-stimulated AA release in adult brain. This negative coupling between 5-HTR and mAChR probably depends on the direct stimulation of the acylation reaction coupled with 5-HT1AR. In the aged brain this type of interaction does not occur because aging eliminates carbachol and 5-HT effect. On the other hand, in aged brain, both 5-HT and carbachol more actively stimulate IPs accumulation than in adult brain. Serotonin inhibits carbachol-stimulated IPs release to the level observed during 5-HT2R activation. Our study indicates for the first time, the negative coupling between 5-HTR and mAChR for AA liberation in adult brain and a lack of this kind of receptors interaction in senescent brain. In adult brain, 5-HT1AR-stimulated AA incorporation is responsible for this coupling. Moreover, serotonin through the stimulation of 5-HT2R, suppresses mAChR-dependent IPs liberation in adult and aged brain similarly. The interactions between these two neurotransmitter receptors lead to the modification of lipid mediators formation and may have important implications in alteration of signal transduction in adult and in aged brain.

Age-related changes of GABA-activated chloride channel properties in brain cortex synaptic plasma membrane: evidence for phospholipase involvement

Brain aging decreases binding of tert-butylbicyclophosphorothionate (TBPS), a specific ligand for Cl- channels, but has no effect on Cl- influx. Detailed studies on the kinetics of TBPS dissociation allowed the characterization of Cl- channel properties. Aging lowers, exclusively in the presence of GABA agonist, muscimol, the half-life of the fast phase of TBPS dissociation, indicating an opening time of receptor-dependent Cl- channels shorter than that in adult brain. The half-life of the slow phase of TBPS dissociation is significantly lower in aged brain in the presence and absence of muscimol. These results suggest a sustained Cl- current, including also the other channel(s) not connected with GABAA receptor activation. The analysis of biphasic TBPS dissociation demonstrates a lowered number of binding sites resulted in the reduction of the number of Cl- channels in the "open" state. This may explain an observed decrease of TBPS binding in aged brain. One of the possible factors involved in modification of GABAA receptor behavior during aging may be arachidonic acid or diacylglycerol, known to be accumulated in aged brain. The action of these compounds on the Cl- channel, observed in this study, correlates well with the effect of aging.

Serotonin, a potent modulator of arachidonic acid turnover, interaction with glutamatergic receptor in brain cortex

Brain cortex synaptoneurosomes actively incorporated [14C]arachidonic acid (AA) into lipids. Serotonin (5-HT), at a concentration range of 10 microM-1 mM, significantly stimulates the incorporation of AA mainly into phosphatidylinositol (PI) of brain cortex synaptoneurosomes. The stimulation rate of AA incorporation by 5-HT was the same in the presence and absence of lysophosphatidylinositol (LPI). However, in the absence of LPI some stimulation of AA uptake was also observed into phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid. Buspirone, an agonist of 5-HT1A receptor, has a similar effect on AA incorporation into membrane lipids as serotonin itself. Moreover, ketanserin, an antagonist of 5-HT2 receptor, also induces activation of AA incorporation into membrane lipids. On the other hand, glutamate, in a concentration dependent manner, significantly inhibits AA uptake into PI and also has some inhibitory action on AA uptake into the other lipids. Serotonin itself and the agonist of 5-HT1A receptor through the activation of AA turnover counteract glutamate-induced inhibition of AA uptake into lipids of brain cortex. Our results indicated that serotonin directly, through the specific receptors, or indirectly, through the interaction with glutamatergic receptors, modulates turnover and the level of arachidonic acid in the brain.

Aging diminishes serotonin-stimulated arachidonic acid uptake and cholinergic receptor-activated arachidonic acid release in rat brain cortex membrane

Synaptoneurosomal and synaptosomal fractions from the brain cortex of adult (4-month-old) and aged (27-month-old) rats were used for studies on the uptake and subsequent release of [14C]arachidonic acid ([14C]AA) from brain lipids. The incorporation of AA and the pattern of its uptake into lipids of the aged brain cortex synaptoneurosomes and synaptosomes were not significantly different when compared with those in the adult brain cortex fractions. Serotonin (5-HT), at 10 microM to 1 mM in the presence of pargyline and the agonist of the 5-HT1A receptor, buspirone, stimulated AA uptake into membrane lipids, mainly into phosphatidylinositol, by about 40% exclusively in adult brain synaptoneurosomes. Aging significantly diminished the effect of 5-HT on AA uptake. Synaptoneurosomal and synaptosomal fractions prelabeled with [14C]AA were used subsequently for investigation of voltage-dependent, muscarinic and 5-HT receptor-mediated AA release. Aging diminished markedly carbachol-stimulated Ca(2+)-dependent AA liberation from membrane lipids of synaptoneurosomes and synaptosomes. Moreover, aging decreased voltage-dependent and 5-HT2 receptor-mediated AA release. These results show that aging affects receptor-dependent AA uptake and pre- and postsynaptic receptor-mediated AA release. These modulations of AA incorporation and release in aged brain may be of pathophysiological significance, in view of the importance of these processes for signal transmission in the brain. The changes of receptor-dependent processes of deacylation and reacylation may be responsible for alteration in the function of neuronal cells and may affect learning and memory ability and brain plasticity during aging.

Aging modulates calcium-dependent phosphatidylinositol degradation by cerebral cortex synaptic plasma membrane phospholipases

The synaptic plasma membrane (SPM) and cytosol fractions from cerebral cortex of adult (4-mo-old) and aged (27-mo-old) rats were used as a source of phospholipase A2 (PLA2) and phospholipase C (PLC). The activity of PLC acting on [3H-inositol]phosphatidylinositol ([3H]PtdIns) was investigated in the presence of endogenous and 2 mM Ca2+. Arachidonic acid (AA) release was studied in the same conditions, using 1-stearoyl-[2-14C]arachidonyl-sn-glycerophosphoinositol ([14C]PtdIns) as a substrate. In the presence of endogenous Ca2+ (i.e., no added Ca2+) SPM-bound PLC and PLA2 or diacylglycerol (DAG) lipase of aged brain exert significantly higher activity in degradation of PtdIns as compared to their activities in adult brain. Moreover, these enzymes of aged brain are less or not further activated by 2 mM Ca2+, contrary to the enzymes isolated from adult brain. The activity of cytosolic enzymes involved in degradation [3H]PtdIns and [14C]PtdIns and their regulation by Ca2+ ions are not significantly changed in senescent cerebral cortex as compared to the adult. The intracellular calcium concentration ([Ca2+]i), measured with fura-2, is lower in aged brain compared to adult brain, which may suggest the modification in Ca2+ ion redistribution in aged brain and probably its higher concentration in membranes. These results indicate that aging modifies significantly the activity of membrane-bound, Ca(2+)-dependent phospholipase(s) degrading PtdIns, which may be connected with alteration of Ca2+ ion redistribution and may influence the formation and accumulation of very potent lipid messengers as diacylglycerol, lysophospholipid, and arachidonic acid, known to be involved in neurotransmission processes.

Modulatory action of arachidonic acid on GABAA/chloride channel receptor function in adult and aged brain cortex membranes

Effect of arachidonic acid on binding parameters of two binding sites on the GABAA receptor and on GABA activated Cl- uptake was investigated in synaptic plasma membrane and in synaptoneurosomes from brain cortex of adult (4-months old) and aged (27-months old) rats. The ligands used were [3H]muscimol, a GABA agonist and [35S]-t-butylbicyclophosphorothionate ([35S]TBPS), a convulsant that binds to the site near the chloride channel. Arachidonic acid increases significantly GABAA agonist binding and concomitantly decreases [35S]TBPS binding in a concentration dependent manner. The analysis of binding parameters in adult brain showed a significant decrease by AA of KD value for low but not for high affinity of [3H]muscimol binding. Concomitantly, AA enhances Bmax values for high affinity binding and has no effect on Bmax value for low affinity binding in synaptic plasma membrane (SPM) from adult brain. In synaptic plasma membrane from aged brain AA increases low and high affinity binding of agonist to GABAA receptor, modified significantly KD value by about 30 and 66%, respectively. On the other hand, AA significantly decreases of [35S]TBPS binding to chloride channel recognition site. Scatchard's analysis indicates that this inhibition results from a decrease of total number of binding sites. Moreover, the affinity of [35S]TBPS binding was increased (KD = 37.0 nM for AA-treated membrane, as compared to 69.3 nM KD in control membrane). GABA-dependent chloride uptake into synaptoneurosomes is also inhibited by AA in a concentration dependent manner in adult brain. In aged brain synaptoneurosomes AA has similar inhibitory effect on GABA-activated chloride uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbachol-stimulated release of arachidonic acid and eicosanoids from brain cortex synaptoneurosome lipids of adult and aged rats

Synaptoneurosomes from the brain cortex of adult rats (4 months old) and aged rats (27 months old), prelabeled with [14C]arachidonic acid (AA), were used as the source of enzyme(s) and substrates to study the effect of a cholinergic agonist on the release of AA and eicosanoids. In synaptoneurosomes from adult brains, carbachol, the nonhydrolyzable analog of acetylcholine, increased AA release by 16% in the presence of 2 mM calcium. This agonist-mediated AA release occurred specifically from phosphatidylinositol (PI). Concomitantly, carbachol in the presence of 2 mM Ca2+ significantly activated the formation of 15-HETE and PGF2 alpha. This effect of carbachol on the level of eicosanoids was also observed in the presence of endogenous calcium. In synaptoneurosomes from aged brains, carbachol had no effect on the release of AA and eicosanoids. The results of studies involving inhibitors of phospholipase A2 (PLA2) and phospholipase C (PLC) suggested that PLA2 is almost completely responsible for the Ca(2+)-dependent, carbachol-mediated AA liberation. The distribution of labeled AA in the lipids after incubation of synaptoneurosomes in the presence of 2 Mm Ca2+ and carbachol indicated that in aged synaptoneurosomes, the muscarinic receptor-mediated degradation of phosphoinositides through phospholipase C is preserved, but the turnover of the phosphoinositide cycle is probably suppressed. These results indicate that aging significantly affects the population of cholinergic-muscarinic receptors coupled to PLA2.

Ca(2+)-independent, Ca(2+)-dependent, and carbachol-mediated arachidonic acid release from rat brain cortex membrane

Synaptoneurosomes obtained from the cortex of rat brain prelabeled with [14C]arachidonic acid [( 14C]AA) were used as a source of substrate and enzyme in studies on the regulation of AA release. A significant amount of AA is liberated in the presence of 2 mM EGTA, independently of Ca2+, primarily from phosphatidic acid and polyphosphoinositides (poly-PI). Quinacrine, an inhibitor of phospholipase A2 (PLA2), suppressed AA release by about 60% and neomycin, a putative inhibitor of phospholipase C (PLC), reduced AA release by about 30%. An additive effect was exhibited when both inhibitors were given together. Ca2+ activated AA release. The level of Ca2+ present in the synaptoneurosomal preparation (endogenous level) and 5 microM CaCl2 enhance AA liberation by approximately 25%, whereas 2 mM CaCl2 resulted in a 50% increase in AA release relative to EGTA. The source for Ca(2+)-dependent AA release is predominantly phosphatidylinositol (PI); however, a small pool may also be liberated from neutral lipids. Carbachol, an agonist of the cholinergic receptor, stimulated Ca(2+)-dependent AA release by about 17%. Bradykinin enhanced the effect of carbachol by about 10-15%. This agonist-mediated AA release occurs specifically from phosphoinositides (PI + poly-PI). Quinacrine almost completely suppresses calcium-and carbachol-mediated AA release. Neomycin inhibits this process by about 30% and totally suppresses the effect of bradykinin. Our results indicate that both phospholipases PLA2 and PLC with subsequent action of DAG lipase are responsible for Ca(2+)-independent AA release. Ca(2+)-dependent and carbachol-mediated AA liberation occurs mainly as the result of PLA2 action. A small pool of AA is probably also released by PLC, which seems to be exclusively responsible for the effect of bradykinin.

Dolichol alters GABA uptake and high affinity binding of agonist to rat brain synaptic plasma membranes

The effects of dolichol on high affinity [3H]muscimol binding to synaptic plasma membrane (SPM) and [3H]GABA uptake into synaptosomes from rat brain were analyzed. Membranes were enriched with dolichol, by preincubation, in the presence of bovine serum albumin (BSA) as a vehicle (40-100 micrograms of dolichol/mg protein + 5% BSA). The rate of dolichol incorporation into the membrane was determined using [1-3H]dolichol C95, and it was in the range of 5-7 nmol/mg protein/h for synaptosomes and SPM, respectively. The uptake of [3H]GABA into synaptosomes enriched with dolichol decreased significantly by about 30%. Dolichol alone added into the incubation medium produced only a negligible effect. Specific binding of [3H]muscimol, which was higher than 90% of total binding, was significantly reduced to SPM enriched with dolichol as compared to controls. The Kd of the high affinity sites was significantly elevated by about 30% in SPM enriched with dolichol (10.8 +/- 0.3 nM vs 7.3 +/- 0.2 nM in control). This difference was more pronounced for SPM isolated from cerebellum (Kd increased by about 50%). The Bmax value was not changed. Dolichol alone did not alter the agonist binding. These results indicate for the first time that the higher level of dolichol in SPM might influence the GABAergic transmission system. An increase in dolichols in membranes may be an important factor in the decline of brain function during aging.