The mechanisms and pharmacology of neuronal nicotinic acetylcholine receptors in the central nervous system

Neuronal nicotinic acetylcholine receptors (nAChRs) are key players in both cognitive and autonomic processes. In the cognitive domains of the brain, destruction of cholinergic inputs or disruption of nAChR function result in cognitive deficits as observed in Alzheimer's disease, schizophrenia, brain trauma and aging. By contrast, moderate activation of nAChRs supports neuroprotection and improves cognitive functions. In addition, neuronal nAChRs are also expressed in important autonomic centers such as the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus (DMV) that support autonomic visceral reflexes and homeostasis. In this study, the underlying mechanisms of nAChR activation and its pharmacology were investigated in the hippocampus and the NTS, critical brain regions supporting cognitive and autonomic functions, respectively.;Specific Aim 1 of this study was to determine the capacity of physiological levels of choline to activate &agr;7 nAChRs in hippocampal CA1 pyramidal neurons and interneurons. A weak persistent activation of &agr;7 nAChRs can be neuroprotective. These levels of activation can be achieved by selective or non-selective &agr;7 nAChR agonists or inhibitors of ACh esterase (AChEI). However, nicotinic agonists desensitize &agr;7 nAChRs while AChEI produces side effects limiting their overall clinical and pre-clinical effectiveness. These limitations can be avoided by using a novel class of drugs; type-II positive allosteric modulators of &agr;7 nAChRs (&agr;7-PAMs) such as PNU-120596 (i.e., PNU). At physiological levels, choline alone is ineffective as an &agr;7 agonist because of its low concentration in the cerebrospinal fluid (∼10 microM) and low potency for &agr;7 activation (EC50 ∼1.5 mM). However, the results pertaining to Specific Aim 1 demonstrate that in the presence of PNU (1-5 microM) , 10 microM choline produces persistent &agr;7 activation expressed on CA1 pyramidal and interneurons which may be fine-tuned to achieve optimal neuroprotection and cognitive benefits. Specific Aim 2 was to test the novel concept that PNU mediated changes in &agr;7 receptor kinetics can alter the biophysical properties of &agr;7 channel-drug interactions and thereby increase the probability and the apparent affinity of open channel block. The results of this study suggest that the compounds (e.g., Bicuculline) that do not potently interact with &agr;7 ion channels in the absence of PNU begin to interact potently in its presence. These emergent properties of &agr;7 channel-drug interactions in the presence of PNU need to be recognized in drug development as they may lead to unanticipated side effects and serious misinterpretation of data. Specific Aim 3 investigated the pharmacology and mechanisms of action of pre-synaptic non-&agr;7 and &agr;7 nAChRs in the caudal NTS neurons. Although, activation of nAChRs is known to enhance pre-synaptic release of glutamate in subsets of caudal NTS neurons, its mechanism of action has been elusive. However, the results from this study demonstrated that nicotine-mediated enhancement of glutamate release requires Ca2+ influx via nAChRs but does not require any contribution from voltage-gated Ca2+ ion channels (VGCCs) and presynaptic Ca2+ stores. Moreover, both functional &agr;7 and non-&agr;7 nAChRs were found to contribute to the presynaptic effects of nicotine in subsets of NTS neurons. However, co-expression of &agr;7 and non-&agr;7 nAChRs on the same glutamatergic presynaptic terminals was not detected.;Collectively, these studies may help in developing new therapeutic strategies to selectively target nAChR-associated pathways that support cognitive and autonomic functions in health and disease.