Positive and negative effects of tacrine (tetrahydroaminoacridine) and methoxytacrine on the metabolism of acetylcholine in brain cortical prisms incubated under "resting" conditions

Phytoceramide shows neuroprotection and ameliorates scopolamine-induced memory impairment

The function and the role phytoceramide (PCER) and phytosphingosine (PSO) in the central nervous system has not been well studied. This study was aimed at investigating the possible roles of PCER and PSO in glutamate-induced neurotoxicity in cultured neuronal cells and memory function in mice. Phytoceramide showed neuro-protective activity in the glutamate-induced toxicity in cultured cortical neuronal cells. Neither phytosphingosine nor tetraacetylphytosphingosine (TAPS) showed neuroproective effects in neuronal cells. PCER (50 mg/kg, p.o.) recovered the scopolamine-induced reduction in step-through latency in the passive avoidance test; however, PSO did not modulate memory function on this task. The ameliorating effects of PCER on spatial memory were confirmed by the Morris water maze test. In conclusion, through behavioral and neurochemical experimental results, it was demonstrated that central administration of PCER produces amelioration of memory impairment. These results suggest that PCER plays an important role in neuroprotection and memory enhancement and PCER could be a potential new therapeutic agent for the treatment of neurodegenerative diseases such as Alzheimer's disease.

Functional cholinergic damage develops with amyloid accumulation in young adult APPswe/PS1dE9 transgenic mice

We investigated the functional characteristics of pre- and postsynaptic cholinergic transmission in APPswe/PS1dE9 double transgenic mice at a young age (7-10 weeks) before the onset of amyloid plaque formation and at adult age (5-6 months) at its onset. We compared brain slices from cerebral cortex and hippocampus with amyloid deposits to slices from striatum with no amyloid plaques by 6 months of age. In young transgenic mice we found no impairments of preformed and newly synthesized [(3)H]-ACh release, indicating intact releasing machinery and release turnover, respectively. Adult transgenic mice displayed a significant increase in preformed [(3)H]-ACh release in cortex but a decrease in hippocampus and striatum. The extent of presynaptic muscarinic autoregulation was unchanged. Evoked release of newly synthesized [(3)H]-ACh was significantly reduced in the cortex and hippocampus but unchanged in the striatum. Carbachol-induced G-protein activation in cortical membranes displayed decreased potency but normal efficacy in adult animals and no changes in young animals. These results indicate that functional pre- and postsynaptic cholinergic deficits are not present in APPswe/PS1dE9 transgenic mice before 10 weeks of age, but develop along with beta-amyloid accumulation in the brain.

Novel cognitive improving and neuroprotective activities of Polygala tenuifolia Willdenow extract, BT-11

We carried out this study to search a new active constituent that had cognitive enhancing activity and low side effects from natural source. We found that the extract of dried root of Polygala tenuifolia Willdenow (BT-11, 10 mg/kg, i.p.) could significantly reverse scopolamine-induced cognitive impairments in rat, using a passive avoidance and a water maze test. We also investigated the effects of BT-11 on neurotoxicity induced by glutamate (Glu) and toxic metabolites of amyloid precursor protein (APP) such as amyloid beta protein (A beta) and C-terminal fragment of APP (CT) in primary cultured neurons of rat. The pretreatment of BT-11 (0.5, 3, and 5 micro g/ml) significantly reduced cell death induced by Glu (1 mM), A beta (10 micro M) and CT105 (10 micro M) in a dose-dependent manner. In addition, BT-11 inhibited acetylcholinesterase (AChE) activity in a dose-dependent and non-competitive manner (IC(50) value; 263.7 micro g/ml). Our novel findings suggest the possibility that this extract may have some protective effects against neuronal death and cognitive impairments in Alzheimer's disease (AD), or other neurodegenerative diseases related to excitotoxicity and central cholinergic dysfunction.

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.

Influence of retinoic acid and of cyclic AMP on the expression of choline acetyltransferase and of vesicular acetylcholine transporter in NG108-15 cells

Treatment of the cholinergic cell line NG108-15 with retinoic acid or cAMP results in an increase of choline acetyltransferase activity (ChAT) whereas none of these agents influences the amount of the vesicular acetylcholine transporter (VAChT) as judged from vesamicol binding and immunoblot studies. We suggest that immaturity of posttranslational events controlling the expression of VAChT protein is responsible for the apparent absence of coregulation of ChAT and VAChT protein expression.

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 increases stimulation-evoked acetylcholine release from rat hippocampal slices

We examined the effects of tacrine (9-amino-1,2,3,4-tetrahydroacridine) on endogenous acetylcholine (ACh) release from rat hippocampal slices. Tacrine (more than 1 microM) increased the measurable amount of basal ACh release. On the other hand, in the presence of physostigmine (50 microM; under this condition, cholinesterase activity was inhibited), tacrine did not enhance the basal ACh release. Tacrine at more than 100 microM increased the submaximal electrical stimulation-evoked release of ACh in both the absence and presence of physostigmine (50 microM). This effect of tacrine was abolished by a combination of atropine (100 mM) and physostigmine. These results indicate that a high-dose of tacrine increases cholinergic neurotransmission not only by inhibition of cholinesterase but also by increasing ACh release through an atropine-like effect, perhaps by blockade of part of the process of muscarinic autoinhibition.

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.

Investigation of the mechanism of the effect of tacrine (tetrahydroaminoacridine) on the metabolism of acetylcholine and choline in brain cortical prisms

The mechanism by which tacrine increases the content and synthesis of acetylcholine (ACh) in cerebrocortical prisms exposed to an irreversible inhibitor of cholinesterases and incubated under resting conditions (Dolezal and Tucek, 1991) is not known. As found in the present experiments, this effect of tacrine is only apparent if its application had been preceded by a period of preincubation, but the preincubation is ineffective if it occurs in the presence of hemicholinium-3. Apparently, choline or a choline-containing compound accumulates in the slices during the preincubation and is then utilized for the enhanced synthesis of ACh in the presence of tacrine. Tacrine did not induce a decrease in the amount of radiolabel that had been incorporated from choline into acid-insoluble compounds, which suggests that the choline which is used for the synthesis of additional ACh does not originate from choline lipids. However, tacrine was found to diminish the efflux of choline from prisms which had been preincubated with an increased concentration of choline in the medium, and from prisms incubated in the presence of hemicholinium-3. It also diminished the efflux of radioactive choline that had accumulated in the prisms during preincubation with a very low concentration of tacrine, when the prisms were subsequently incubated with 4-aminopyridine. It is proposed that the potency of tacrine to increase the content and synthesis of ACh in cerebrocortical prisms whose cholinesterases had been inhibited is due to its ability to diminish the efflux of endogenous choline from the nerve terminals.

Dementia: the neurochemical basis of putative transmitter orientated therapy

The neurotransmitter deficits of dementias, including Alzheimer's dementia, Lewy body dementia and Parkinson's disease are discussed in relation to cognitive and behavioural impairments together with neuropathological changes and available data on the status of receptor transmembrane signalling. Potential therapeutic strategies for dementia are outlined based on the following systems: excitatory amino acids, gamma-amino butyric acid, acetylcholine (muscarinic and nicotinic), noradrenaline, serotonin and peptides. These include the attenuation of transmitter deficits by agonists and agents inhibiting transmitter breakdown and support for surviving neurons by suppression of inhibitory inputs, trophic factors and neural implantation.

Negative effects of tacrine (tetrahydroaminoacridine) and methoxytacrine on the metabolism of acetylcholine in brain slices incubated under conditions stimulating neurotransmitter release

The effects of tacrine (1,2,3,4-tetrahydro-9-aminoacridine) and 7-methoxytacrine on the metabolism of brain acetylcholine were investigated in experiments in which acetylcholine turnover was stimulated by tissue depolarization or by 4-aminopyridine. Rat cerebrocortical prisms were preincubated under "resting" conditions (Krebs-Ringer buffer with 3 mmol/L K+ and with paraoxon to inhibit cholinesterases) and then incubated in the presence of tacrine or methoxytacrine and of 50 mmol/L K+. Both drugs diminished the amount of acetylcholine released by depolarization and the amount of acetylcholine synthesized during incubation; in experiments in which [14C]choline was present in the incubation medium simultaneously with tacrine or methoxytacrine, the drugs diminished the uptake of [14C]choline by the tissue and the amount of [14C]-acetylcholine synthesized and released into the medium. In these experiments, it was not possible to distinguish whether the inhibitory effects of tacrine and methoxytacrine were primarily on the process of acetylcholine synthesis (particularly on the uptake of choline), or whether the drugs also acted directly on the process of neurotransmitter release. In subsequent experiments the prisms were preincubated with [14C]choline and only then subjected to a short depolarization in the presence of hemicholinium-3 and tacrine or methoxytacrine. Both drugs severely inhibited the release of preformed [14C]acetylcholine and prevented the diminution of tissue [14C]acetylcholine stores. Methoxytacrine was also found to diminish the release of acetylcholine induced by 4-aminopyridine while increasing the content of acetylcholine in the tissue. Tacrine and methoxytacrine had no effect on the activity of choline acetyltransferase (EC 2.3.1.6).(ABSTRACT TRUNCATED AT 250 WORDS)