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Agonists of nicotinic acetylcholine receptors (nAChRs) produce long-lasting cognitive efyfects in animal models and humans. The duration of these cognitive effects can outlast the presence of the agonists in the system, and the persistence of cognitive enhancement is increased further by repeated exposure. The basis for this discrepancy appears be the cellular and systemic mechanisms of learning and memory. Agonists of nAChRs induce long-term potentiation (LTP), which is a strengthening of synaptic connections that is associated with learning and memory formation. Some of the cellular effects of nAChR agonists overlap with the known cellular mechanisms of LTP, including long-lasting increases in intracellular concentrations of [Ca.sup.2+], activation of second-messenger systems and transcription factors, elevated levels of gene products and enhanced neurotransmitter release. A better understanding of this phenomenon might shed new light on the role of nAChR systems in memory formation and retrieval.

ABSTRACT The development of selective ligands targeting neuronal nicotinic acetylcholine receptors to alleviate symptoms associated with neurodegenerative diseases presents the advantage of affecting multiple deficits that are the hallmarks of these pathologies. TC-1734 is an orally active novel neuronal nicotinic agonist with high selectivity for neuronal nicotinic receptors. Microdialysis studies indicate that TC-1734 enhances the release of acetylcholine from the cortex. TC-1734, by either acute or repeated administration, exhibits memory enhancing properties in rats and mice and is neuroprotective following excitotoxic insult in fetal rat brain in cultures and against alterations of synaptic transmission induced by deprivation of glucose and oxygen in hippocampal slices. At submaximal doses, TC-1734 produced additive cognitive effects when used in combination with tacrine or donepezil. Unlike (-)-nicotine, behavioral sensitization does not develop following repeated administration of TC-1734. Its pharmacokinetic (PK) profile (half-life of 2 h) contrasts with the long lasting improvement in working memory (18 h) demonstrating that cognitive improvement extends beyond the lifetime of the compound. The very low acute toxicity of TC-1734 and its receptor activity profile provides additional mechanistic basis for its suggested potential as a clinical candidate. TC-1734 was very well tolerated in acute and chronic oral toxicity studies in mice, rats and dogs. Phase I clinical trials demonstrated TC-1734's favorable pharmacokinetic and safety profile by acute oral administration at doses ranging from 2 to 320 mg. The bioavailability, pharmacological, pharmacokinetic, and safety profile of TC-1734 provides an example of a safe, potent and efficacious neuronal nicotinic modulator that holds promise for the management of the hallmark symptomatologies observed in dementia. [ABSTRACT FROM AUTHOR]

Abstract: Nicotinic α4β2* agonists are known to be effective in a variety of preclinical pain models, but the underlying mechanisms of analgesic action are not well-understood. In the present study, we characterized activation and desensitization properties for a set of seventeen novel α4β2*-selective agonists that display druggable physical and pharmacokinetic attributes, and correlated the in vitro pharmacology results to efficacies observed in a mouse formalin model of analgesia. ABT-894 and Sazetidine-A, two compounds known to be effective in the formalin assay, were included for comparison. The set of compounds displayed a range of activities at human (α4β2)2β2 (HS-α4β2), (α4β2)2α5 (α4β2α5) and (α4β2)2α4 (LS-α4β2) receptors. We report the novel finding that desensitization of α4β2* receptors may drive part of the antinociceptive outcome. Our molecular modeling approaches revealed that when receptor desensitization rather than activation activitiesat α4β2* receptors are considered, there is a better correlation between analgesia scores and combined in vitro properties. Our results suggest that although all three α4β2 subtypes assessed are involved, it is desensitization of α4β2α5 receptors that plays a more prominent role in the antinociceptive action of nicotinic compounds. For modulation of Phase I responses, correlations are significantly improved from an r 2 value of 0.53 to 0.67 and 0.66 when HS- and LS-α4β2 DC50 values are considered, respectively. More profoundly, considering the DC50 at α4β2α5 takes the r 2 from 0.53 to 0.70. For Phase II analgesia scores, adding HS- or LS-α4β2 desensitization potencies did not improve the correlations significantly. Considering the α4β2α5 DC50 value significantly increased the r 2 from 0.70 to 0.79 for Phase II, and strongly suggested a more prominent role for α4β2α5 nAChRs in the modulation of pain in the formalin assay. The present studies demonstrate that compounds which are more potent at desensitization of α4β2* receptors display better analgesia scores in the formalin test. Consideration of desensitization propertiesat α4β2* receptors, especially at α4β2α5, in multiple linear regression analyses significantly improves correlations with efficacies of analgesia. Thus, α4β2* nicotinic acetylcholine receptor desensitization may contribute to efficacy in the mediation of pain, and represent a mechanism for analgesic effects mediated by nicotinic agonists. [Copyright &y& Elsevier]

In recent years, there has been considerable effort to design and synthesize radiotracers suitable for use in Positron Emission Tomography (PET) imaging of the α4β2 neuronal nicotinic acetylcholine receptor (nAChR) subtype. A new fluoropyridinyl derivative of (−)-cytisine (1), namely (−)-9-(2-fluoropyridinyl)cytisine (3, Ki values of 24 and 3462 nM for the α4β2 and α7 nAChRs subtypes, respectively) has been synthesized in four chemical steps from (−)-cytisine and labelled with fluorine-18 (T1/2: 119.8 min) using an efficient two-step radiochemical process [(a) nucleophilic heteroaromatic ortho-radiofluorination using the corresponding N-Boc-protected nitro-derivative, (b) TFA removal of the Boc protective group]. Typically, 20–45 mCi (0.74–1.67 GBq) of (−)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3, 2–3 Ci/μmol or 74–111 GBq/μmol) were easily obtained in 70–75 min starting from a 100 mCi (3.7 GBq) aliquot of a cyclotron-produced [18F]fluoride production batch (20–45% non decay-corrected yield based on the starting [18F]fluoride). The in vivo pharmacological profile of (−)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3) was evaluated in rats with biodistribution studies and brain radioactivity monitoring using intracerebral radiosensitive β-microprobes. The observed in vivo distribution of the radiotracer in brain was rather uniform, and did not match with the known regional densities of nAChRs. It was also significantly different from that of the parent compound (−)-[3H]cytisine. Moreover, competition studies with (−)-nicotine (5 mg/kg, 5 min before the radiotracer injection) did not reduce brain uptake of the radiotracer. These experiments clearly indicate that (−)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3) does not have the required properties for imaging nAChRs using PET. [Copyright &y& Elsevier]

To date, the primary treatments for Alzheimer's disease with proven efficacy have been acetylcholinesterase inhibitors that prevent the hydrolysis of acetylcholine (ACh) in the synaptic cleft, thereby prolonging its activity. Although these agents have some benefit in alleviating cognitive impairment, they have limited clinical utility because of insufficient efficacy and marginal tolerability. Within the last decade, there has been much experimental support for the use of therapeutics that directly target nicotinic ACh receptors (nAChRs) to improve cognitive function and slow neurodegenerative disease progression. These findings have spurred considerable research efforts to develop ligands selective for nAChRs, such as ABT-418 (Arneric et al., 1995), SIB-1553 (Bontempi et al., 2001), TC-2403 (Lippiello et al., 1996), and TC-2559 (Bencherif et al., 2000). There is abundant evidence that nAChR modulators have the potential to alleviate cognitive impairment in demented states. In addition to improving cognitive function, a large body of research implicates a role for nAChRs in neuroprotection, suggesting potential for disease modification. An impact of nAChR agonists on disease progression would provide an advantage over currently available treatments for memory loss. The profile of previous nAChR-targeted clinical candidates has not been adequate to warrant further development owing to poor oral bioavailability, side effects, and/or lack of efficacy. Thus, a challenge in nAChR drug design and development has been the reduction of undesirable effects that result from activity at specific nAChRs in the CNS and PNS, including cardiovascular toxicity, emesis, seizures, and hypothermia.

Heterologous expression and lesioning studies were conducted to identify possible subunit assembly partners in nicotinic acetylcholine receptors (nAChR) containing alpha6 subunits (alpha6(*) nAChR). SH-EP1 human epithelial cells were transfected with the requisite subunits to achieve stable expression of human alpha6beta2, alpha6beta4, alpha6beta2beta3, alpha6beta4beta3, or alpha6beta4beta3alpha5 nAChR. Cells expressing subunits needed to form alpha6beta4beta3alpha5 nAChR exhibited saturable [(3)H]epibatidine binding (K(d) = 95.9 +/- 8.3 pM and B(max) = 84.5 +/- 1.6 fmol/mg of protein). The rank order of binding competition potency (K(i)) for prototypical nicotinic compounds was alpha-conotoxin MII (6 nM) > nicotine (156 nM) approximately methyllycaconitine (200 nM) > alpha-bungarotoxin (>10 microM), similar to that for nAChR in dopamine neurons displaying a distinctive pharmacology. 6-Hydroxydopamine lesioning studies indicated that beta3 and alpha5 subunits are likely partners of the alpha6 subunits in nAChR expressed in dopaminergic cell bodies. Similar to findings in rodents, quantitative real-time reverse transcription-polymerase chain reactions of human brain indicated that alpha6 subunit mRNA expression was 13-fold higher in the substantia nigra than in the cortex or the rest of the brain. Thus, heterologous expression studies suggest that the human alpha5 subunit makes a critical contribution to alpha6beta4beta3alpha5 nAChR assembly into a ligand-binding form with native alpha6(*)-nAChR-like pharmacology and of potential physiological and pathophysiological relevance.