UB-165: A Novel Nicotinic Agonist with Subtype Selectivity Implicates the α4β2* Subtype in the Modulation of Dopamine Release from Rat Striatal Synaptosomes

Abstract

Presynaptic nicotinic acetylcholine receptors (nAChRs) on striatal synaptosomes stimulate dopamine release. Partial inhibition by the alpha3beta2-selective alpha-conotoxin-MII indicates heterogeneity of presynaptic nAChRs on dopamine terminals. We have used this alpha-conotoxin and UB-165, a novel hybrid of epibatidine and anatoxin-a, to address the hypothesis that the alpha-conotoxin-MII-insensitive subtype is composed of alpha4 and beta2 subunits. UB-165 shows intermediate potency, compared with the parent molecules, at alpha4beta2* and alpha3-containing binding sites, and resembles epibatidine in its high discrimination of these sites over alpha7-type and muscle binding sites. (+/-)-Epibatidine, (+/-)-anatoxin-a, and (+/-)-UB-165 stimulated [(3)H]-dopamine release from striatal synaptosomes with EC(50) values of 2.4, 134, and 88 nM, and relative efficacies of 1:0.4:0.2, respectively. alpha-Conotoxin-MII inhibited release evoked by these agonists by 48, 56, and 88%, respectively, suggesting that (+/-)-UB-165 is a very poor agonist at the alpha-conotoxin-MII-insensitive nAChR subtype. In assays of (86)Rb(+) efflux from thalamic synaptosomes, a model of an alpha4beta2* nAChR response, (+/-)-UB-165 was a very weak partial agonist; the low efficacy of (+/-)-UB-165 at alpha4beta2 nAChR was confirmed in Xenopus oocytes expressing various combinations of human nAChR subunits. In contrast, (+/-)-UB-165 and (+/-)-anatoxin-a were similarly efficacious and similarly sensitive to alpha-conotoxin-MII in increasing intracellular Ca(2+) in SH-SY5Y cells, a functional assay for native alpha3-containing nAChR. These data support the involvement of alpha4beta2* nAChR in the presynaptic modulation of striatal dopamine release and illustrate the utility of exploiting a novel partial agonist, together with a selective antagonist, to dissect the functional roles of nAChR subtypes in the brain.

Dose‐related neuroprotective effects of chronic nicotine in 6‐hydroxydopamine treated rats, and loss of neuroprotection in α4 nicotinic receptor subunit knockout mice

“…The nigrostriatal system however, which is speci®cally damaged in PD, expresses a diverse array of nAChR subunits (Elliott et al, 1998) including, but not restricted to, a4/b2 and a7 nAChR subtypes. Further, nicotinic modulation of striatal dopamine release is consistent with a heterogeneous nAChR population on dopaminergic nigrostriatal neurons, although the nAChR subtypes present have not been identi®ed with certainty (see Reuben et al, 2000;Sharples et al, 2000).…”

“…The current results do not rule out the involvement of additional nAChR subtypes in neuroprotection in vivo: although failing to reach statistical signi®cance, a trend towards increased dopaminergic neuron survival following nicotine treatment was observed in a4 knockout mice. This eect may be due to the involvement of additional nAChR subtype(s) in vivo: the expression of multiple nAChR subunits and modulation of dopaminergic function by a heterogeneous nAChR population within nigrostriatal neurons (see Reuben et al, 2000;Sharples et al, 2000) adds strength to this possibility. Further, the wide distribution of nAChRs, particularly a4 subunit-containing nAChRs, implies that the eect of systemically administered nicotine need not be restricted to direct actions upon the nigrostriatal system, but may involve nAChRs at several sites within the brain.…”

The subtypes of nicotinic acetylcholine receptors on dopaminergic terminals of mouse striatum

“…Since that time, others have established that nicotine, and other nicotinic agonists, will elicit Ca ++ -dependent release of dopamine from striatal tissue slices (see, for examples [98,99]) and synaptosomes [39]. We have used a variant of the synaptosomal dopamine release assay that was originally developed in the Wonnacott laboratory [39] in a series of studies that characterized the pharmacological properties of dopamine release from striatum [40,89,90,100,101,102], as well as in other brain regions such as the nucleus accumbens, olfactory tubercles and frontal cortex [103]. Dose-response curves for agonist-stimulated dopamine release were obtained with many agonists, and, without exception, the results suggested that dopamine release is modulated by a single receptor subtype.…”

Cyanobacterial Toxins of the Laurentian Great Lakes, their Toxicological Effects, and Numerical Limits in Drinking Water

“…Structurally, anatoxin-a is a bicyclic amine alkaloid, similar to that of epibatidine, but lacking the peperidine motif [212]. Upon binding irreversibly to acetylcholine receptors, anatoxin-a causes depolarization of postsynaptic neuronal cells, or efflux of Ca + and Na + ions, generating an action potential.…”