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

Abstract

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 reminescent 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.

Introduction

While it is well established that cholinergic neurotransmission plays a crucial role in learning and memory, the cellular and molecular basis for this role is not yet well understood. Initially based on the observation that the muscarinic antagonists scopolamine produces deficits in short-term memory, it has been proposed that the cholinergic deficit in Alzheimer’s disease (AD) is predominantly of muscarinic nature [13], [26]. This view, however, is challenged by a large body of evidence, including autoradiographic and histochemical studies of autopsy brain tissue [36], [42], [52], [61], and brain imaging studies of patients [37], which points to a specific loss in AD of nicotinic rather than muscarinic acetylcholine receptors. In contrast, these data consistently show that muscarinic receptors, including M2 receptors, are much less, if at all, reduced in AD. In the upper cortical layers of the frontal cortex, and in the temporal cortex, the loss of nicotinic acetylcholine receptors (nAChR) appears to concern predominantly an α4 subunit-bearing subtype, rather than the α7 nAChR, as has been established by histochemical studies [30], [60] and radioligand binding [44], [59].

Of the many nAChR subtypes that are expressed in the mammalian brain, the α4β2 and the α7 subtype are the most prominent ones. They are both found in postsynaptic as well as in presynaptic and perisynaptic locations [2], [6]. The α7 nAChR displays functional properties quite different from those of the α4β2 nAChR, among which are a much higher Ca²⁺ permeability, very fast desensitization and different pharmacology, including activation by choline and blockade by α-bungarotoxin (αBTX) [6], [11]. Due to its sensitivity to choline, the α7 nAChR can be chemically excited even after the natural transmitter has been enzymatically cleaved. α7 nAChR therefore can respond not only to synaptic events of ACh release but also to volume changes in ACh/choline concentration. (Rapid desensitization of α7 nAChR and a significant refractory period may be prerequisites for the latter response mode.) Due to its Ca²⁺ permeability, α7 nAChR activation can produce metabotropic responses in the excited cell, including Ca²⁺-controlled transmitter release and stimulation of gene transcription and protein biosynthesis. Very recently, the first electrophysiological studies of human cerebral cortical interneurons have been reported [5]. These studies established that both α4β2 and α7 nAChR are located on the somatodendritic regions of human interneurons and, as demonstrated by their ability to modulate GABA release, could be involved in inhibitory and disinhibitory mechanisms in the human cortex. The inhibitory action could enhance the signal-to-noise ratio of neuronal circuitry whereas the disinhibitory action could lead to synaptic strengthening which is an essential element of the learning paradigm long-term potentiation [5]. So far, such properties of ACh action in the human cortex have all been ascribed to muscarinic receptors [45], [47]. Together with the receptor expression data discussed above, and the fact that clinical studies with muscarinic agonists have not been able to demonstrate significant beneficial effects in AD, it therefore seems reasonable to focus the original ‘cholinergic hypothesis’ of Bartus et al. [7] on nicotinic cholinergic neurotransmission.

There is a large body of evidence to indicate that nicotinic drugs indeed affect learning and memory. Nicotine and other nicotinic agonists can improve cognitive and psychomotor function [62], [63], whereas nicotinic antagonists lead to cognitive impairment [33], [34]. Moreover, the incidence of AD in smokers is lower than that in non-smokers (Nitta et al., 1994) which may relate to the increased nAChR expression levels observed in the brains of smokers [37], [43]. Thus, nicotinic drugs may have both acute and chronic effects on cognitive function, the chronic effects possibly including a neuroprotective effect.

Three major strategies have so far been applied to balance nicotinic cholinergic deficits, stimulation of ACh synthesis, inhibition of ACh degradation and administration of nicotinic agonists. Practically no therapeutic effects have been achieved by the administration of ACh precursors [15]. Administration of choline esterase inhibitors (AChE-I) presently is the most commonly applied therapeutic approach. AChE-I have proven albeit limited therapeutic value [35], and most of them do not prevent progression of the disease to any significant extend [16], [46]. A number of nicotinic agonists are presently in preclinical and clinical testing [9], [17], [32], even though they are difficult to dose, as higher levels may cause desensitization rather than increased activation of nicotinic receptors [28]. Other unsolved problems are drug transport to the targeted nAChR(s) in the brain, and target selectivity (nAChR subtype).

A novel approach to drug treatment in AD is the application of allosteric modulators of nicotinic receptors [28], [29]. Allosteric modulators are compounds that interact with the receptor via binding sites that are distinct from those for ACh and nicotinic agonists and antagonists. Consequently, modulators are not directly involved in the neurotransmission process they affect and hence usually do not induce compensatory processes, as agonists and antagonists may do (e.g. receptor desensitization, down-regulation of expression). Because AD is associated with a deficit in nicotinic neurotransmission, allosteric modulators are needed that up-modulate (potentiate) the channel activity of nicotinic receptors in response to ACh. Such properties are displayed by a novel class of nAChR ligands, named ‘allosterically potentiating ligands (APL)’ [28], [50].

Allosteric modulation of receptor activity is a quite common mechanism in neurotransmission. Arguably the most prominent example is the benzodiazepines which positively modulate (potentiate) the activity of the GABA_A receptor by facilitating opening of the receptor-integral Cl⁻ channel (increase in the probability of channel opening at given concentrations of GABA). This effect is the underlying principle of the anxiolytic action of benzodiazepines [31].