Effect of muscarinic ligands on the electrical activity recorded from the hippocampus: a quantitative approach

Design of [R-(Z)]-(+)-alpha-(methoxyimino)-1-azabicyclo[2.2.2]octane-3-acetonitri le (SB 202026), a functionally selective azabicyclic muscarinic M1 agonist incorporating the N-methoxy imidoyl nitrile group as a novel ester bioisostere

Loss of cholinergic function is believed to be implicated in the cognitive decline associated with senile dementia of the Alzheimer type (SDAT). The disease is characterized by progressive loss of muscarinic receptors located on nerve terminals while postsynaptic muscarinic M1 receptors appear to remain largely intact. Muscarinic agonists acting directly on postsynaptic receptors offer the prospect of countering the cholinergic deficit in SDAT. This study describes a novel series of azabicyclic muscarinic agonists, which incorporate an oxime ether or modified oxime ether group as an ester bioisostere. Modification of the oxime ether function by the introduction of electron withdrawing groups led to the finding that the (Z)-N-methoxy imidoyl nitrile group serves as a stable methyl ester bioisostere. This culminated in the discovery of the quinuclidinyl N-methoxy imidoyl nitrile R-(+)-(Z)-5g which is a functionally selective muscarinic M1 partial agonist currently in phase III clinical trials for the treatment of SDAT. The selective profile of R-(+)-(Z)-5g can be rationalized in terms of the relative affinity of the compound at muscarinic receptor subtypes, the degree of agonist efficacy, and brain penetrancy.

Possible role of the cholinergic system and disease models

Memory impairment associated with the loss of cortical cholinergic neurons in AD has stimulated the development of animal models based on blockade or destruction of these systems. Strategies include mechanical lesions, local injection of excitotoxic amino acids or ethylcholine aziridinium (AF 64A), which disrupt reference and working memory in rats, but lack specificity for cholinergic systems. Other models involving, reduction in cerebral blood flow and interference with oxidative metabolism of glucose, mimic those found in AD, and also interfere with working and long-term memory in the rat. Memory impairments can be reversed by acetylcholinesterase inhibitors and cholinergic agonists but beneficial effects of these agents in AD patients are small and inconsistent. This may be partly due to unfavorable pharmacokinetics and dose-limiting side effects of existing drugs. Newer, brain specific acetylcholinesterase inhibitors and M1 muscarinic agonists with a lower incidence of unwanted effects are currently being evaluated.

SDZ ENS 163 a novel pilocarpine like drug: pharmacological in vitro and in vivo profile

The thiolactone analogue of pilocarpine, SDZ ENS 163, acts in vitro and in vivo as a partial agonist at M1/M3 and as an antagonist at M2 muscarinic receptors. In vitro, the properties of SDZ ENS 163 have been investigated in several functional models for muscarinic receptors: it is a full agonist at M1 (rat superior cervical ganglion, carbachol = 100%) and a partial agonist at M3 receptors (guinea pig ileum). However, the drug shows antagonistic properties at M2 receptors (rat atria). Radioligand binding studies with 3H-N-methylscopolamine (3H-NMS) using CHO cells expressing m1 or m3 receptors indicate that SDZ ENS 163 does not discriminate between m1 and m3 receptors (Ki 1.5 and 2.4 microM respectively). Regarding phosphoinositide (PI) turnover in A9L cells, SDZ ENS 163 is a partial agonist at m1 receptors. In ex vivo neurochemical studies in rats SDZ ENS 163 displays effects characteristic of muscarinic antagonists regarding the turnover of ACh which is increased in the brain. At a similar dose-range SDZ ENS 163 accelerates PI metabolism in the rat brain in vivo and increases the energy of the low frequency band (2-5 Hz) in the rat hippocampal EEG. These effects observed in vivo are consistent with postsynaptic M1 agonistic and presynaptic M2 antagonistic activities. Since SDZ ENS 163 at centrally active doses exerts no peripheral cholinergic effects, it may be useful for the symptomatic treatment of Alzheimer's disease.

Brain selective inhibition of acetylcholinesterase: a novel approach to therapy for Alzheimer's disease

It could be argued that clinical experience with cholinergic drugs in the therapy of AD has not yet shown relevant symptomatic improvements. The main reasons for this might be attributed to peripheral cholinergic effects and the liver toxicity of some of these drugs, which limit their use and prevent confirmation of the cholinergic hypothesis (Gray et al., 1989). The main disadvantages of the cholinesterase inhibitors used in clinical trials are the short duration of action in the case of physostigmine and the potential for liver toxicity seen with the aminoacridine derivatives. The results presented with SDZ ENA 713 indicate that the disadvantages of AChE inhibitors might be overcome by improving CNS selectivity and thereby decreasing the peripheral cholinergic effects and toxicity. Clinico-pharmacological studies with SDZ ENA 713 have been performed in healthy volunteers; while central activity was clearly demonstrated in an EEG-sleep study (Holsboer et al., 1992), no prohibitive peripheral side effects were seen, confirming in humans the results obtained in experimental animals (Enz et al., 1991). A multicentre clinical investigation in AD patients has been performed in Europe and is currently being evaluated.

Central effects of muscarinic agonists and antagonists on hippocampal theta rhythm and blood pressure in the anaesthetised rat

The in vivo central effects of a range of full and partial muscarinic receptor agonists have been investigated on hippocampal theta rhythm and blood pressure. In the isoflurane-anaesthetised rat, pretreated with N-methylscopolamine, i.v. administration of arecoline, oxotremorine, arecaidine propargyl ester, aceclidine and pilocarpine produced dose-dependent increases in the frequency of hippocampal theta rhythm and blood pressure, with an order of potency of arecoline = oxotremorine = arecaidine propargyl ester greater than aceclidine greater than or equal to pilocarpine. To increase theta wave frequency, pilocarpine showed a low maximum response and possessed antagonist activity against arecoline, indicating that pilocarpine was acting as a partial agonist. AF102B failed to alter blood pressure or theta rhythm. Intraventricular injections of scopolamine and the M1 receptor-selective antagonist, pirenzepine, produced dose-dependent antagonism of the enhanced theta wave frequency and hypertensive response produced by arecoline. The differences in antagonist potency for the two responses was less than 6-fold, which indicated that both the increase in hippocampal theta wave activity and increase in blood pressure may have been mediated through muscarinic receptors of the M1 subtype. Further studies using a wider range of antagonists will be required to confirm this conclusion.

Effects of muscarinic receptor agonists and antagonists on swim-stress-induced 'behavioural despair' in rats

The effects of some selective agonists and antagonists of cholinergic muscarinic receptor subtypes on swim-stress-induced immobility (SI) was investigated in rats. This paradigm has been proposed to assess 'behavioural despair' in rodents as a laboratory model for clinical depression. All the rats were pre-treated with atropine ethoiodide i.p. to obviate any peripheral cholinergic influence. SI was only marginally less in intracerebroventricular (i.c.v.) cannulated rats administered artificial cerebrospinal fluid than in uncan nulated rats administered normal saline i.p. The drugs which do not have access across the blood-brain barrier were administered i.c.v. The muscarinic M(1) receptor agonists, arecholine and McN-A-343, and the non-specific muscarinic receptor agonist, oxotremorine, induced dose-related increases in SI, whereas the muscarinic M(1) receptor antagonists, scopolamine and pirenzepine, and the non-specific antagonist, atropine, produced significant decreases in SI. Carbachol, an M(2) receptor agonist, and physostigmine induced a dose- dependent dual effect, with lower doses attenuating and higher doses augmenting SI. The M(2) receptor antagonists, gallamine and AF-DX 116, increased SI. The data may be interpreted to suggest that muscarinic M(1) receptors may function to accentuate depression whereas muscarinic M(2) receptors may exert an inhibitory modulatory effect.

The pharmacological assessment of RS 86 (2-ethyl-8-methyl-2,8-diazaspiro-[4,5]-decan-1,3-dion hydrobromide). A potent, specific muscarinic acetylcholine receptor agonist

The effects of the compound RS 86 (2-ethyl-8-methyl-2,8-diazaspiro-[4,5]-decan-1,3-dion hydrobromide) in a number of in vitro and in vivo test systems for muscarinic cholinergic activity were analyzed and compared to those of classical muscarinic receptor agonists. In radioligand binding assays RS 86 presented high nanomolar apparent affinity only for sites labeled by 3H-muscarinic receptor agonists while its apparent affinity for sites labeled by 3H-muscarinic receptor antagonists including [3H]QNB, [3H]NMS and [3H]pirenzepine was in the micromolar range. RS 86 had no or only low affinity (IC50 greater than 10 microM) for other neurotransmitter or drug receptor sites. The compound induced scopolamine-sensitive contractions of the isolated guinea-pig ileum showing a pD2 of 6 in this model. In the isolated rat superior cervical ganglion RS 86 was also an agonist with a pD2 of 6.7. When given to mice or rats by different routes RS 86 induced central and peripheral effects typical of a muscarinic receptor agonist, such as hypothermia, tremor, mydriasis, salivation, lacrimation, diarrhoea and modification of behavior as observed in an open field. In several of these tests RS 86 was about 10 times less potent than oxotremorine but more potent than arecoline, pilocarpine, aceclidine or the compound (cis) AF-30. The ED50 values for some central effects, including the induction of hypothermia and alert non-mobile behavior were lower than those for tremor and peripheral effects. Some of the effects lasted for up to 6 h, depending on the dose. Finally, RS 86 administration resulted in modifications of brain acetylcholine turnover and high affinity choline uptake typical of a central muscarinic receptor agonist. Taken together these results demonstrate clearly that RS 86 is a potent, centrally acting, selective muscarinic receptor agonist. RS 86 appears to be an adequate tool for the clinical examination of the cholinergic hypothesis of Alzheimer's disease.