Dissociation between duration of action in the forced swim test in mice and nicotinic acetylcholine receptor occupancy with sazetidine, varenicline, and 5-I-A85380.

RATIONALE: Nicotinic acetylcholine receptor (nAChR) agonists, partial agonists, and antagonists have antidepressant-like effects in rodents and reduce symptoms of depression in humans. OBJECTIVES: The study determined whether the antidepressant-like effect of the nAChR ß2* partial agonist sazetidine-A (sazetidine) in the forced swim test was due to activation or desensitization of ß2* nAChRs. The study also determined if sazetidine's behavioral responses in the forced swim test corresponded to ß2* nAChRs receptor occupancy and drug bioavailability. RESULTS: Acute antidepressant-like effects in the forced swim test were seen with sazetidine and the full ß2* agonist 5-I-A8350 (BALB/cJ mice) and the less selective ß2* partial agonist varenicline in C57BL/6J but not BALB/cJ mice. The role of ß2* nAChRs was confirmed by results showing: (1) reversal of sazetidine's antidepressant-like effects in the forced swim test by nAChR antagonists mecamylamine and dihydro-ß-erythroidine; (2) absence of sazetidine's effect in mice lacking the ß2 subunit of the nAChR; and (3) a high correspondence between behaviorally active doses of sazetidine and ß2* receptor occupancy. ß2* receptor occupancy following acute sazetidine, varenicline, and 5-I-A8350 lasted beyond the duration of action in the forced swim test. Sazetidine's long lasting receptor occupancy did not diminish behavioral efficacy in the forced swim test following repeated dosing. CONCLUSIONS: Results demonstrate that activation of a small population of ß2* nAChRs (10-40%) is sufficient to elicit sazetidine's antidepressant-like actions without producing tolerance and suggest that ligands that activate ß2* nAChRs would be promising targets for the development of a new class of antidepressant.

Chemistry and pharmacology of nicotinic ligands based on 6-[5-(azetidin-2-ylmethoxy)pyridin-3-yl]hex-5-yn-1-ol (AMOP-H-OH) for possible use in depression.

AMOP-H-OH (sazetidine-A; 6-[5-(azetidin-2-ylmethoxy)pyridin-3-yl]hex-5-yn-1-ol) and some sulfur-bearing analogues were tested for their activities in vitro against human alpha4beta2-, alpha4beta4-, alpha3beta4*- and alpha1*-nicotinic acetylcholine receptors (nAChRs). AMOP-H-OH was also assessed in an antidepressant efficacy model. AMOP-H-OH and some of its analogues have high potency and selectivity for alpha4beta2-nAChRs over other nAChR subtypes. Effects are manifested as partial agonism, perhaps reflecting selectivity for high sensitivity (alpha4)(3)(beta2)(2)-nAChRs. More prolonged exposure to AMOP-H-OH and its analogues produces inhibition of subsequent responses to acute challenges with full nicotinic agonists, again selectively for alpha4beta2-nAChRs over other nAChR subtypes. The inhibition is mediated either via antagonism or desensitization of nAChR function, but the degree of inhibition of alpha4beta2-nAChRs is limited by the partial agonist activity of the drugs. Certain aspects of the in vitro pharmacology suggest that AMOP-H-OH and some of its analogues have a set of binding sites on alpha4beta2-nAChRs that are distinct from those for full agonists. The in vitro pharmacological profile suggests that peripheral side effects of AMOP-H-OH or its analogues would be minimal and that their behavioral effects would be dominated by central nAChR actions. AMOP-H-OH also has profound and high potency antidepressant-like effects in the forced swim test. The net action of prolonged exposure to AMOP-H-OH or its analogues, as for nicotine, seems to be a selective decrease in alpha4beta2-nAChR function. Inactivation of nAChRs may be a common neurochemical endpoint for nicotine dependence, its treatment, and some of its manifestations, including relief from depression.

Sazetidine-A, a novel ligand that desensitizes alpha4beta2 nicotinic acetylcholine receptors without activating them.

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels found throughout the central and peripheral nervous systems. They are crucial to normal physiology and have been clearly implicated in nicotine addiction. In addition, they are possible therapeutic targets in a wide range of pathological conditions, including cognitive disorders, Parkinson's disease, and neuropathic pain. Nicotinic ligands are usually classified as agonists (or partial agonists), competitive antagonists, or noncompetitive antagonists. Sazetidine-A is a new nicotinic ligand that shows a different pharmacological profile from any of these known classes of ligands. Sazetidine-A competes with very high binding affinity (Ki approximately 0.5 nM) and selectivity for the alpha4beta2 nAChR subtype (Ki ratio alpha3beta4/alpha4beta2 approximately 24,000). Despite its high affinity, sazetidine-A neither activates nAChR channel function nor prevents channel activation when it is applied simultaneously with nicotine. However, when it is pre-incubated for 10 min with the receptors, it potently blocks nicotine-stimulated alpha4beta2 nAChR function (IC50 approximately 30 nM). The action of sazetidine-A may be explained by its very low affinity for the resting conformation of the alpha4beta2 nAChRs, and its very high affinity for the desensitized state of the receptor. We propose that sazetidine-A is a "silent desensitizer" of nAChRs, meaning that it desensitizes the receptor without first activating it. Furthermore, comparison of the effects of sazetidine-A and nicotine at alpha4beta2 nAChRs suggests that the predominant effects of nicotine and other nicotinic agonists are related to desensitization of the receptors and that sazetidine-A potently mimics these effects.

Synthesis and pharmacological evaluation of novel 9- and 10-substituted cytisine derivatives. Nicotinic ligands of enhanced subtype selectivity.

We report the synthesis and pharmacological properties of several cytisine derivatives. Among them, two 10-substituted derivatives showed much higher selectivities for the alpha4beta2 nAChR subtype in binding assays than cytisine. The 9-vinyl derivative was found to have a very similar agonist activity profile to that of cytisine.