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    • ABH Although our data failed to reproduce the robust enhance

    2023-12-08

    Although our data failed to reproduce the robust enhancement of LTP in the rat dentate with nicotine that was previously reported, our results with NS6740 suggest a more fundamental role for α7 receptor signaling in modulation of synaptic tone in the LTP pathway. The discovery of the cholinergic anti-inflammatory pathway (CAP) created an entirely new perspective on how α7 nAChR can function and be targeted for therapeutic effects [58]. ACh released by strong electrical stimulation of the vagus nerve was shown to stimulate an anti-inflammatory response mediated by α7 nAChR on macrophages and other ABH of the immune system [6], [47], [57]. Interestingly, GTS-21, a selective but relatively weak partial agonist for α7 nAChR [7] has been shown in many studies to be an effective activator of CAP [9], [11], [22], [25], [46], [52], [62], [63]. In contrast to being a weak activator of α7 ion channels, GTS-21 effectively induces prolonged desensitization of receptors that is reversible by the type II PAM PNU-120596 [35]. NS6740, like GTS-21, induces prolonged desensitization that is reversible by PNU-120596, while stimulating so little channel activation that it has been characterized as a “silent agonist” [12], [32], [33]. Both GTS-21 and NS6740 were shown to effectively suppress lipopolysaccharide (LPS) stimulated secretion of Tumor Necrosis Factor-alpha (TNF-α) by microglia cells, an in vitro model of CAP [52]. The effects of PNU-120596 in the oocyte experiments indicate that NS6740 may affect other receptor functions such as CAP (Cholinergic Anti-inflammatory Pathway), consistent with the activity of this agent as modulatory of neuropathic pain [32]. Prior to work of Thomsen and Mikkelsen that demonstrated the CAP activity of NS6740, the drug first appeared in the scientific literature as a putatively inactive α7 agonist that was able to block the cognitive effects of an efficacious α7 agonist [8]. NS6740 was also recently reported to similarly antagonize the effects of BMS-902483 in a novel object recognition test [44]. In the same work, BMS-902483 was shown to enhance hippocampal LTP, an activity presumably related to cognitive enhancement. In the present study, we show that NS6740 does not behave as a simple antagonist of α7 but, rather, alters the fundamental behavior of α7 nAChR in the hippocampus without having effects on other types of ion channel receptors. NS6740 then appears as a key to altering the modality of α7-mediated signaling. The activity of GTS-21, NS6740, other α7 silent agonists, and allosteric modulators [1], [15], [17], [32], [34], [56] in models of inflammatory disease and neuropathic pain appear to be independent of ion channel activation and, rather, depend on activation and modulation of intracellular signal transduction pathways [4], [14], [26], [49], [54]. In the case of the α7-mediated control of CAP, the cellular mediators of activity are themselves not even competent for generating nAChR channel currents, and so we must consider the complete receptor protein [50], multiple conformational states, and the complete receptor interactome [29], [42] as mediators of the activity. As we show, NS6740 does not behave like a simple receptor antagonist but, rather, is putting receptors into conformational states consistent with the modulation of intracellular signal transduction. There are a number of possible ways that α7-mediated signaling events might be able to alter basal synaptic function, for example, through changes in presynaptic calcium homeostasis or, alternatively, by post-synaptic changes in glutamate receptor trafficking. In any case, the present study challenges the naive assumption that all α7 ligands work by increasing intracellular calcium through ion channel activation. For most therapeutic/in vivo models this mode of action seems hardly even feasible since, in the absence of PAMs, α7 nAChR have extremely low probability of channel opening [61] and are especially ineffective at generating currents when agonist concentrations change slowly, as they would in the brain following a subcutaneous [44] or intraperitoneal injection [8]. It seems likely that the slice experiments NS6740 is down-regulating constitutive α7 ion channel receptor tone reducing tonic pre-synaptic facilitation [18].