Effects of neuronal nicotinic acetylcholine receptor allosteric modulators in animal behavior studies

Abstract

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation-conducting transmembrane channels from the cys-loop receptor superfamily. The neuronal subtypes of these receptors (e.g. the α7 and α4β2 subtypes) are involved in neurobehavioral processes such as anxiety, the central processing of pain, food intake, nicotine seeking behavior, and a number of cognitive functions like learning and memory. Neuronal nAChR dysfunction is involved in the pathophysiology of many neurological disorders, and behavioral studies in animals are useful models to assess the effects of compounds that act on these receptors. Allosteric modulators are ligands that bind to the receptors at sites other than the orthosteric site where acetylcholine, the endogenous agonist for the nAChRs, binds. While conventional ligands for the neuronal nAChRs have been studied for their behavioral effects in animals, allosteric modulators for these receptors have only recently gained attention, and research on their behavioral effects is growing rapidly. Here we will discuss the behavioral effects of allosteric modulators of the neuronal nAChRs.

Introduction

Nicotinic acetylcholine receptors (nAChRs) are cation-conducting pentameric transmembrane proteins that are activated by the endogenous neurotransmitter acetylcholine (ACh), and are located at the neuromuscular junction and in the central and peripheral nervous systems [1], [2], [3]. For the neuronal nAChRs, 12 genes have been identified thus far (nine α (α2–α10) and three β (β2–β4) subunits) that are widely expressed in the nervous system, on both neuronal and non-neuronal cells [1], [3], [4], [5], [6], [7], [8], [9].

The different neuronal nAChR subunits combine in various permutations to form functional receptors [1], [3]. Of the many possible subtypes of nAChRs that have previously been described, the α7 and the α4β2 receptors are the two main subtypes widely expressed in the brain, in particular in the hippocampus [10], [11], [12], [13], [14], [15], [16]. While the α7 subunits can form functional homopentameric receptors [1], [3], there is evidence that in both heterologous expression systems and in the brain, the α7 subunit can also form functional receptors in combination with the β2 subunit [16], [17], [18]. The α7 receptors are characterized by relatively high calcium permeability and rapid desensitization upon exposure to agonists [19], [20], [21]. The heteromeric α4β2 receptors (composed of both the α4 and β2 subunits) are expressed in at least two different stoichiometries; the (α4)₂(β2)₃ stoichiometry which binds to ACh and nicotine with higher affinity, and the (α4)₃(β2)₂ stoichiometry which binds with lower affinity [22], [23], [24], [25], [26]. The α7 and α4β2 nAChRs are found at pre- and postsynaptic sites, as well as non-synaptic sites in various regions of the brain [27], [28], [29], [30], [31], [32], [33]. Due to the widespread distribution of nAChRs in the central nervous system, it is not surprising that they are involved in the pathophysiology of many neurological diseases and conditions [1], [3], [34], including (but not limited to) nicotine addiction [35], [36], [37], Alzheimer's disease [33], [38], [39], human epilepsy syndrome [40], [41], [42], [43], schizophrenia [44], [45], [46] and autism [47], [48], [49]. Experimental evidence also suggests that nAChRs are involved in cognitive processes like attention [50], [51], [52], [53] and learning and memory [54], [55], in the central processing of pain [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], and in psychological behaviors such as anxiety [54], [66], [67], [68], [69] and depression [68], [70], [71].

The neuronal nAChRs behave as allosteric proteins with multiple, inter-convertible conformations [72], [73], [74], [75], [76], [77]. Many different pharmacological ligands preferentially bind to and stabilize the nAChR in a specific confirmation [12], [77], [78]. While agonists bind to the orthosteric site, there are many ligands which are known to bind to the receptor at sites separate and distinct from the orthosteric site; these are referred to as allosteric sites [12], [77], [78], [79], [80], [81], [82], [83], [84], and the ligands that bind to the nAChRs at these sites are referred to as allosteric modulators [12], [78], [85]. These allosteric modulators can be classified as positive (PAMs) or negative allosteric modulators (NAMs) [12], [78], [85], [86]; PAMs are compounds that increase the receptor response induced by an agonist [12], [78], while NAMs on the other hand are compounds which decrease the receptor response [12], [83]. PAMs do not have any agonist activity on their own, but they change the ability of the orthosteric ligand (agonist) to affect channel opening [12], [78], [85]. Therefore, allosteric modulators provide an alternative approach to manipulate nAChR function [39], [87], [88], [89]. For example, PAMs can potentially increase the effectiveness of endogenous ACh that is released into the synapse, and strengthen the cholinergic tone without directly activating nAChRs [12], [85]. It is known that partial and full agonists cause desensitization of nAChRs and an upregulation of receptor expression levels [90], [91], [92], [93]. Since the PAMs do not activate the nAChRs directly, they do not induce either desensitization or upregulation. In the presence of PAMs, the nAChR-driven cholinergic synapse will remain under the control of the released endogenous ACh, and only the magnitude of the nAChR response will increase due to the PAM. Thus the nAChR PAMs provide a distinct advantage over agonists. Since the α7 and α4β2 nAChRs are widely distributed in the brain and are involved in the pathophysiology of a variety of neurological diseases and conditions, allosteric modulators for these receptors hold immense therapeutic potential.

Agonists for nAChRs have been under investigation for a variety of neurological disorders for some time [94], [95], [96]. Animal behavioral studies, including animal models with nAChR gene knockouts, provide a suitable and useful tool to assess the effects of pharmacological agents that target these receptors [96], [97], [98], [99]. Behavioral studies with nAChR agonists have primarily focused on animal models of cognition, depression and neuropathic pain [50], [51], [52], [53], [54], [55], [59], [60], [61], [68], [70], [71]. However to date, there are few studies that identify the effects of nAChR PAMs in different paradigms of animal behavior, with little to no published clinical data. Here we will discuss what has been reported thus far on the behavioral studies using different allosteric modulators for the neuronal nAChRs.