No response to high-dose muscarinic agonist therapy in Alzheimer's disease

Effects of acute infusion of the muscarinic cholinergic agonist arecoline on verbal memory and visuo-spatial function in dementia of the Alzheimer type

1. Treatment of patients with dementia of the Alzheimer type (DAT) with arecoline, a muscarinic cholinergic receptor agonist, reportedly improves performance on a picture recognition memory task, but not on other memory measures. To examine further possible performance improvements following arecoline treatment, patients with DAT were treated with a 30 min intravenous infusion of arecoline (5 mg). 2. Psychometric testing was done at five time points (two before and three following the infusion). Patients were tested on a memory task (Buschke selective reminding) and a test of visuo-spatial performance (figure copying). 3. No net change from baseline was seen in mean scores following arecoline infusion. However, the changes in performance on the two tasks were correlated (p less than 0.02) over subjects at 10 min but not at 1.5 or 5.5 hr following the infusion. 4. This result suggests that although individual patients vary in their response to a given dose of arecoline, their responses are consistent across types of tasks. Thus the lack of a mean drug effect may be due to individual differences in response rather than to a lack of response.

A novel series of non-quaternary oxadiazoles acting as full agonists at muscarinic receptors

1 A novel series of non-quaternary oxadiazole-based muscarinic agonists demonstrated high affinity for muscarinic receptors. 2. These agonists possessed high efficacy in the nanomolar range at muscarinic receptors in the superior cervical ganglion, atrium and ileum but did not show selectivity across the tissue preparations. 3. Two amino oxadiazoles, one from a quinuclidine series (L-660,863) and one from a 1-azanorbornane series (L-670,207) possessed a high ratio of potency for displacing the binding of [3H]-N-methyl-scopolamine ([3H]-NMS) to potency for displacing the agonist [3H]-oxotremorine-M cortex. 4. The two azanorbornane derivatives L-670,548 and L-670,207 stimulated the turnover of phosphatidylinositol in the cortex with a potency higher than that obtained with any other known muscarinic agonist (ED50 0.26 and 0.18 microM respectively). 5. The maximum response obtained with L-670,207 was greater than that observed for carbachol but was comparable to that of the natural ligand acetylcholine. 6. These oxadiazole muscarinic agonists are among the most potent and efficacious non-quaternary muscarinic agonists ever described.

Direct comparison of cognitive facilitation by physostigmine and tetrahydroaminoacridine in two primate models

Cognitive facilitation by physostigmine and tetrahydroaminoacridine (THA) was compared in two primate models. Disruption of spatial delayed response performance by scopolamine (0.03 mg/kg) was fully reversed by coadministration of 5 doses of physostigmine in the range 0.03-0.08 mg/kg, but by only one dose (4.0 mg/kg) of THA; partial reversal of some effects of scopolamine was observed at 1 and 3 mg/kg of THA. Visual recognition memory was enhanced following treatment with 4 doses of physostigmine in the range 0.001-0.03 mg/kg. The effect of THA across the group of animals was not significant but performance tended to improve using a dose of 0.8 mg/kg. Our findings indicate that THA does not have a superior profile to physostigmine as a cognitive enhancer in primates.

In vivo characterisation of novel efficacious muscarinic receptor agonists

Although a number of muscarinic agonists have been used in clinical trials for Alzheimer's Disease, many of these compounds are low in potency and have only limited intrinsic efficacy. The present study describes four non-quaternary oxadiazole based muscarinic agonists from a quinuclidine and a 1-azanorbornane series. These displayed up to 1000 fold higher affinity than arecoline and were efficacious muscarinic agonists at cortical receptors. All four compounds produced peripherally mediated salivation and centrally mediated hypothermia at doses 50-50,000 fold lower than arecoline. The most potent was L-670,548, the methyl oxadiazole in the 1-azanorbornane series, which had an ED50 of 0.0016 mg/kg on the hypothermia model. This derivative was also the most potent compound in ex vivo binding studies (ED50 0.0069 mg/kg) and showed excellent brain penetration (3.8% of the administered dose). These derivatives are the first non quaternary efficacious agonists which show good penetration into the CNS (central nervous system), and will prove useful tools in understanding the role of muscarinic receptors in CNS function.

Potentiation of cholinergic activity with pyridino[1,2-a]imidazo[5,4-b]indole: in vitro studies

1. Effects of a novel imidazoindole derivative on cholinergic function were studied in isolated tissue preparations. 2. The compound demonstrated a dose-dependent (10(-11)-10(-9) potentiation (20-60%) of acetylcholine induced tension in guinea pig ileal tissue. 3. Increases in the size of end-plate potentials and nerve evoked muscle twitches were observed in frog nerve-skeletal muscle preparations. 4. Cholinesterase activity was not inhibited. 5. The results suggest that the compound has actions at the post-synaptic muscarinic receptor complex in smooth muscle and causes pre-synaptic increases in ACh release at the neuromuscular junction.

Therapeutic effect of THA on hemicholinium-3-induced learning impairment is independent of serotonergic and noradrenergic systems

Tetrahydroaminoacridine (THA: Tacrine) has previously been shown to reverse deficits in spatial discrimination learning induced by hemicholinium-3 (HC-3). In the present experiments the effects of prior depletion of serotonin (5-HT) or noradrenaline (NA) on this reversal were examined. In the first experiment 5-HT lesions were made by injecting 5,7-DHT (2 x 50 micrograms/5 microliters) into the lateral ventricles of rats pretreated with desmethylimipramine (DMI 25 mg/kg IP). A permanently indwelling guide tube was then implanted over the right lateral ventricle. Subsequent testing under drug-free conditions, revealed no effect of the lesion on the number of trials needed to attain criterion (nine consecutive correct choices) in two-platform spatial discrimination learning in a watermaze. Using a latin square design rats were then tested for the effects of HC-3 and THA. HC-3 (5 micrograms/5 microliters ICV) or placebo (CSF) were injected 60 min before the start of a 30-trial training session. THA (4.6, 10 mg/kg SC) or placebo were then injected 15 min before training. Choice accuracy but not choice latency was significantly impaired by HC-3 and the effect was reversed by THA in both sham operated and 5-HT lesioned rats. In the second experiment two injections of DSP-4 (50 mg/kg IP) were given, following cannulation, to deplete forebrain NA. The lesion had no effect on spatial learning under drug-free conditions and failed to block the THA-induced reversal of spatial discrimination learning deficits following HC-3. These results confirm that forebrain Ach depletion by HC-3 impairs spatial discrimination learning and that the deficit is reversed by THA.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of oral pyridostigmine on memory and cognitive function in SDAT

This study reports the effects of the anticholinesterase pyridostigmine on cognitive function in 15 elderly patients with senile dementia of the Alzheimer type (SDAT). Each patient was given placebo and pyridostigime 60 mg orally four times over 26 hours, with a seven day washout between, using a randomized double-blind cross-over design. There was no significant improvement in cognitive function following placebo or pyridostigmine. We did not perform any preliminary dose-finding trials to find the optimum dose of pyridostigmine, but gave the drug in a predetermined fixed dosage to each patient. We conclude that with this dosage, pyridostigmine did not significantly improve cognitive function more than placebo in this group of elderly patients.

[Pharmacology of central cholinergic synapse]

Because it intervenes in normal and pathological cerebral ageing, plays a role in late dyskinesias of Parkinson's syndrome and is involved in the memorization processes, central cholinergic neurotransmission deserves to be known with precision. Its physiological approach deals with the anatomical organization of the central cholinergic system and with the mechanisms of synthesis, release and degradation of acetylcholine and its interaction with its receptors. The pharmacological changes (inhibition or stimulation) conceivable at different levels lead to therapeutic applications.