Tetrahydroaminoacridine induces opposite changes in muscarinic and nicotinic receptors in rat brain

Mechanism of action of cholinesterase inhibitors in Alzheimer's disease

Cholinesterase inhibitors, such as physostigmine and tacrine, have lately gained interest as potential drugs in the treatment of Alzheimer's disease. Already in the 1950s, it was discovered that physostigmine and tacrine were potent inhibitors of acetylcholinesterase and butyrylcholinesterase. However, later studies have shown that cholinesterase inhibitors also interact with cholinergic receptors, with sodium and potassium ion channels and effect the uptake, synthesis and release of neurotransmitters. In summary, cholinesterase inhibitors are drugs with many modes of action, which may be of advantage in the treatment of a complex disorder such as Alzheimer's disease.

Effects of cholinesterase inhibitors on rat nicotinic receptor levels in vivo and in vitro

Cholinesterase inhibitors (ChEIs) are the mainstay of treatment for AD but differ by secondary mechanisms of action. We determine the effects of sub-chronic dosing of ChEIs on alpha7 and non-alpha7 nAChRs and determine if differences can be observed between them. Sprague-Dawley rats were administered donepezil, galantamine; rivastigmine at two doses each, in saline SQ twice daily or with nicotine (0.4 mg/kg) as a positive control. After 14 days the animals were sacrificed, and the levels of nAChRs were measured using [3H]-EPI to measure non-alpha7 nAChRs and [3H]-MLA to measure alpha7 nAChRs. In the cortex, all compounds tested at the higher doses significantly increased the levels of both [3H]-EPI and [3H]-MLA. In the hippocampus all compounds significantly increased [3H]-EPI but had no effect on [3H]-MLA binding. No effects were observed in the striatum with treatment. There were no differences observed among the ChEIs. In cell cultures, none of the ChEIs increased non-alpha7 or alpha7 receptor binding. Treatment with ChEIs result in similar increases in receptor levels which suggest that the increases in nAChRs may be due simply to the increases in synaptic levels of acetylcholine.

Role of the cholinergic system in regulating survival of newborn neurons in the adult mouse dentate gyrus and olfactory bulb

Neurogenesis in the subgranular zone of the hippocampal dentate gyrus and olfactory bulbs continues into adulthood and has been implicated in the cognitive function of the adult brain. The basal forebrain cholinergic system has been suggested to play a role in regulating neurogenesis as well as learning and memory in these regions. Herein, we report that highly polysialylated neural cell adhesion molecule (PSA-NCAM)-positive immature cells as well as neuronal nuclei (NeuN)-positive mature neurons in the dentate gyrus and olfactory bulb express multiple acetylcholine receptor subunits and make contact with cholinergic fibers. To examine the function of acetylcholine in neurogenesis, we used donepezil (Aricept), a potent and selective acetylcholinesterase inhibitor that improves cognitive impairment in Alzheimer's disease. Intraperitoneal administrations of donepezil significantly enhanced the survival of newborn neurons, but not proliferation of neural progenitor cells in the subgranular zone or the subventricular zone of normal mice. Moreover, donepezil treatment reversed the chronic stress-induced decrease in neurogenesis. Taken together, these results suggest that activation of the cholinergic system promotes survival of newborn neurons in the adult dentate gyrus and olfactory bulb under both normal and stressed conditions.

Effect of subchronic galantamine treatment on neuronal nicotinic and muscarinic receptor subtypes in transgenic mice overexpressing human acetylcholinesterase

Overexpression of acetylcholinesterase (AChE) in mice causes cholinergic deficits with memory impairment. In this study, AChE overexpressing (hAChE-Tg) and control (FVB/N) mice were treated with the AChE inhibitor (AChEI) galantamine (4 mg/kg/day) for 10 days. The concentration of galantamine in plasma was 75-80 ng/ml. The inhibition of AChE was 20% in red blood cells (RBC) and 30% in brain cortical tissue. A significant increase in [(3)H]cytisine (alpha4 nicotinic receptor) binding was measured in the CA1 and CA3 area of the hippocampus of FVB/N mice following galantamine treatment. Similarly, a significant increase in [(125)I]alphabungarotoxin (alpha7 nicotinic receptor) binding was found in the frontal cortex, retrosplenial gr. cortex, motor cortex and thalamus in galantamine treated FVB/N compared to saline treated mice. No significant changes in nicotinic receptor binding sites were observed in galantamine treated hAChE-Tg mice. Significant decreases in the muscarinic receptors measured by [(3)H]AF-DX-384 (M2 muscarinic receptor) and [(3)H]pirenzepine (M1 muscarinic receptor) were observed in several brain regions of galantamine treated FVB/N and hAChE-Tg mice. This study shows regional and receptor subtype specific changes in the nicotinic receptor subtypes compared to the muscarinic receptors following galantamine treatment in FVB/N and hAChE-Tg mice.

Tacrine attenuates hydrogen peroxide-induced apoptosis by regulating expression of apoptosis-related genes in rat PC12 cells

The present studies investigated the effects of tacrine, a selective acetylcholinesterase (AChE) inhibitor and promising anti-dementia agent, on hydrogen peroxide (H(2)O(2))-induced apoptosis and the expression of apoptosis-related genes in rat pheochromocytoma line PC12 cells. Transient exposure of the cells to H(2)O(2) (100 microM) triggered typical apoptosis as evidenced by chromatin condensation, nuclei fragmentation and DNA laddering. RT-PCR studies showed upregulated p53 and bax mRNA levels with H(2)O(2) treatment. The results were further confirmed at protein levels by immunocytochemistry with specific antibodies. Preincubation with tacrine significantly attenuated H(2)O(2)-induced injury, prevented the cells from apoptosis and attenuated H(2)O(2)-induced overexpression of bax and p53. The present findings suggest that tacrine exert significant protection against H(2)O(2)-induced apoptosis possibly through inhibiting expression of pro-apoptosis genes.

Increased levels of tau protein in SH-SY5Y cells after treatment with cholinesterase inhibitors and nicotinic agonists

Several cholinesterase inhibitors used in the treatment of Alzheimer's disease (AD) have been shown to interact with an allosteric site on the nicotinic acetylcholine receptor (nAChR). A possible linkage between the phosphorylation state of tau, the major component of paired helical filaments found in AD brain, and stimulation of nAChRs by cholinesterase inhibitors and nicotinic agonists was investigated. Western blot analysis showed that treatment of SH-SY5Y cells for 72 h with the cholinesterase inhibitors tacrine (10(-5) M), donepezil (10(-5) M), and galanthamine (10(-5) M), nicotine (10(-5) M), and epibatidine (10(-7) M) increased tau levels as detected with Tau-1, AT 8, and AT 270 monoclonal antibodies and binding of [3H]epibatidine. The increase in tau immunoreactivity induced by nicotine, epibatidine, and tacrine, but not the up-regulation of nAChRs, was prevented by the antagonists d-tubocurarine and mecamylamine. Both antagonists were synergistic with the nicotinic agonists in causing up-regulation, but only d-tubocurarine showed a synergistic effect with tacrine. The increased tau immunoreactivity induced by tacrine was not prevented by atropine, indicating that in terms of cholinergic receptors, tacrine modulates tau levels mainly through interactions with nAChRs and not with muscarinic receptors. Additional work is needed to determine the exact mechanism by which cholinesterase inhibitors and nicotinic agonists modulate phosphorylation and levels of tau protein.

Effect of subchronic metrifonate treatment on cerebral glucose metabolism in young and aged rats

The effects of subchronic administration of metrifonate, a long-lasting cholinesterase inhibitor, on local cerebral glucose utilization were assessed in 3- and 27-month old Sprague-Dawley rats, using the autoradiographic [14C]2-deoxyglucose technique. Rats were treated twice daily with metrifonate (80 or 120 mg/kg) for 3 weeks. The [14C]2-deoxyglucose experiment was performed 18 h after the last metrifonate administration. In 3-month old rats, metrifonate 80 mg/kg increased the average hemispheric cerebral glucose utilization by 12% (P > 0.001). Significant effects were observed in 19 of the 54 regions studied, including cortical and limbic regions. The higher dose induced a larger effect (average increase 17%, 24 of the 54 regions affected). In 27-month old rats, very similar effects were obtained. These results show that repeated administration of metrifonate leads to a sustained metabolic activation in rat brain, at a level comparable to the activation observed previously after a single administration of the drug.

Is there a rationale for the use of acetylcholinesterase inhibitors in the therapy of Alzheimer's disease?

Since the 1980s, the cholinergic hypothesis of the pathogenesis of Alzheimer's disease has proven to be a strong stimulus to pharmacological strategies aimed at correcting the cognitive deficit by manipulating cholinergic neurotransmission. Among these strategies, the one based on acetylcholinesterase inhibition is currently the most extensively developed for the therapy of Alzheimer's disease. The inhibitors' mechanisms of action are complex, including changes in the release of acetylcholine, and modulation of acetylcholine receptors. Various clinical trials of various inhibitors have shown that, on the whole, their effects were modest and, in the case of some drugs, were associated with frequent adverse reactions. Among the conceivable reasons for the limited efficacy of these drugs, those related to the pharmacological target deserve particular attention. This review, therefore, focuses on the complex nature of the acetylcholine system, the alterations of acetylcholinesterase and muscarinic receptor signal transduction in Alzheimer's disease, and the involvement of other neurotransmitters.

Effect of tacrine on intracellular calcium in cholinergic SN56 neuronal cells

We have found earlier that the depolarization-induced release of acetylcholine from the brain could be inhibited by tacrine (tetrahydroaminoacridine) but the mechanism of this action of tacrine was not clarified (S. Tucek, V. Dolezal, J. Neurochem. 56 (1991) 1216). We have now investigated whether tacrine has an effect on the changes in the intracellular concentration of calcium ions ([Ca2+]i) induced by depolarization. Experiments were performed on the cholinergic SN56 neuronal cell line with Fura-2 fluorescence technique of calcium imaging. The depolarization by 71 mmol/l K+ evoked minimum increases of [Ca2+]i up to day 5 in culture. Then the response gradually increased and reached a plateau after 7 days in culture. A similar time course was observed for acetylcholinesterase activity. The effect of K+ ions was concentration-dependent and the concentration of 71 mmol/l K+ evoked maximum [Ca2+]i responses. The increases of [Ca2+]i did not occur in the absence of extracellular calcium. They were mediated by high voltage-activated calcium channels of the L-type and the N-type. Nifedipine (2 micromol/l; L-type calcium channel blocker) and omega-conotoxin GVIA (100 nmol/l; N-type calcium channel blocker) diminished the response to 71 mmol/l K+ by 53% and 39%, respectively, and their effects were additive (decrease to 8% of controls). Non-selective inorganic blocker of voltage-activated calcium channels LaCl3 (0.1 mmol/l) decreased the response by 83%. Tacrine attenuated the [Ca2+]i response in a concentration-dependent manner. At a concentration of 10 micromol/l it inhibited the [Ca2+]i response by 55% and its inhibitory effect was additive with that of omega-conotoxin GVIA but not with that of nifedipine. An equimolar concentration of paraoxon, an irreversible inhibitor of cholinesterases, had no influence on [Ca2+]i response. Tacrine exhibited the same inhibitory effect when paraoxon was present. In conclusion, our data indicate that high-voltage-activated calcium channels of the L-type and the N-type are both present in the SN56 cells but that they are fully expressed only after 6-7 days in culture. Tacrine attenuates the influx of calcium by inhibiting the L-type calcium channels. This inhibitory effect is not a consequence of the anticholinesterase activity of tacrine. The finding that low micromolar concentrations of tacrine may interfere with calcium-dependent events is likely to be of importance for the evaluation of the therapeutic potential of the drug.

Tacrine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy in Alzheimer's disease

Tacrine is a centrally acting cholinesterase inhibitor with additional pharmacological activity on monoamine levels and ion channels. It has been postulated that some or all of these additional properties may also be relevant to the mode of action of the drug. There are wide interindividual variations in pharmacological and clinical response to tacrine, possibly related to interindividual variation in bioavailability. Tacrine appears to improve cognitive function and behavioural deficits in a proportion of patients with Alzheimer's disease, at dosages of 80 to 160 mg/day. In the best designed trials, 30 to 51% of evaluable patients showed an improvement of at least 4 points on the cognitive subscale of the Alzheimer's Disease Assessment Scale, versus 16 to 25% of placebo recipients. A similar proportion of tacrine recipients were judged to have improved when global assessment scales were used. There was a significant dose-response relationship up to 160 mg/day. However, large numbers of patients were withdrawn during the trials, many because of tacrine-associated increases in transaminase levels. Elevated liver enzyme levels occurred in about 50% of tacrine recipients (reaching clinical significance in about 25%). Cholinergic symptoms also occurred more often in tacrine recipients than in those receiving placebo. A gradual increase in tacrine dosage, at 6-week intervals, is recommended when initiating therapy, and weekly serum transaminase monitoring is required for 6 weeks after each dosage increase. Despite the limitations implied by the low proportion of responders and high incidence of hepatic adverse effects associated with therapy, tacrine appears to make a measurable difference in both cognitive and behavioural function in a proportion of patients with Alzheimer's disease--a welcome advance in an area previously devoid of acceptable treatment options.

Tetrahydroaminoacridine and physostigmine increase cerebral glucose utilization in specific cortical and subcortical regions in the rat

The effects of the anticholinesterases tetrahydroaminoacridine (THA) and physostigmine on local cerebral glucose utilization (LCGU) were studied in the conscious rat, using the autoradiographic [14C]deoxyglucose technique. THA (5 mg/kg i.p.) increased LCGU significantly in 8 of the 43 regions studied. A higher dose of THA (10 mg/kg) produced a metabolic activation in 19 of the 43 regions. LCGU increased in cortical areas (including parietal and temporal cortices), the septohippocampal system, the thalamus, the lateral habenula, the basolateral amygdala, the superior colliculus, and the substantia nigra. Scopolamine (4 mg/kg i.p.) reversed the THA-induced LCGU increase. Physostigmine (0.2 and 0.5 mg/kg) increased LCGU in 15 and 22 regions, respectively. The average magnitude of the change induced by 0.5 mg/kg of physostigmine was similar to that observed after THA at 10 mg/kg, but the topography of the effects was somewhat different. Physostigmine increased LCGU in the preoptic magnocellular area, the brainstem, and the cerebellum but not in the parietal cortex. The effects in the septohippocampal system were smaller than those induced by THA. The regional topography of the LCGU increase overlapped the distribution of the M2 muscarinic receptors and that of acetylcholinesterase activity. These data suggest that the major effects of THA and physostigmine on LCGU result from their anticholinesterase action.

Chronic treatments with cholinoceptor drugs influence spatial learning in rats

Nicotine, scopolamine, oxotremorine, diisopropyl-fluorophosphate (DFP) and tetrahydroaminoacridine (THA) were administered chronically to different groups of rats in doses reported to alter central muscarinic and/or nicotinic receptor numbers. Beginning 24 h after final drug injection, the groups were compared to a vehicle control group on acquisition of a hidden platform position in the Morris water maze over 20 trials with a 30-min inter-trial interval. Chronic treatment with either nicotine or scopolamine significantly improved the rate of learning, but oxotremorine and DFP retarded learning and THA had no effect on learning. The chronic drug effects on behaviour were consistent with known effects of the injected drugs on muscarinic and nicotinic binding in the forebrain and on the sensitivity of frontal cortex neurones to iontophoretically applied cholinoceptor agonists. However, alternative explanations for the observed changes cannot be ruled out, since the drugs used are known to have a wide range of effects on other neurotransmitters.

Tacrine restores cholinergic nicotinic receptors and glucose metabolism in Alzheimer patients as visualized by positron emission tomography

Three patients with Alzheimer's disease, a 68-year-old woman with mild dementia and 2 men (aged 64 and 72 years) with moderate dementia were treated orally with the cholinesterase inhibitor tacrine (tetrahydroaminoacridine), 80 mg daily, for several months. The patients were investigated using positron emission tomography (PET) prior to, and after 3 weeks and 3 months of treatment. The PET studies involved a multi-tracer system consisting of [18F]-fluoro-deoxy-glucose (18F-FDG) (tracer for glucose metabolism); 11C-butanol (cerebral blood flow) and (S)(-)- and (R)(+)-[N-11C-methyl]-nicotine (nicotinic receptors; cholinergic neural activity). Tacrine treatment increased the uptake of 11C-nicotine to the brain. Significant reduced difference in uptake between the two enantiomers (S)(-)- and (R)(+)11C-nicotine was observed in the frontal and temporal cortices after tacrine treatment in all three patients. The kinetic analysis indicated increased binding of (S)(-)11C-nicotine in brain compatible with a restoration of nicotinic cholinergic receptors. The most pronounced effect was observed after 3 weeks and 3 months treatment in the patient with mild dementia. An increase in cerebral glucose utilization was found in the 68-year-old patient with mild dementia but also slightly in the 64-year-old man with moderate dementia when treated with tacrine for 3 months. Tacrine administration did not affect cerebral blood flow. The PET data obtained after 3 weeks of tacrine treatment was paralleled by improvement in neuropsychological performance. This study shows in vivo by PET neurochemical effects induced in brain by treatment with tacrine to Alzheimer patients. Intervention with tacrine in the early course of the disease might be necessary for clinical improvement.