[125I]‐Epibatidine binds to multiple nicotinic acetylcholine receptor (nAChR) subtypes with high affinity. In this study, [125I]‐epibatidine was used to label and characterize a novel nAChR subtype found in mouse brain inferior colliculus, interpeduncular nucleus, and olfactory bulb homogenates.
Binding of [125I]‐epibatidine was saturable and apparently monophasic in each brain region (KD=71±12 pM mean±s.e.mean across regions) but inhibition of [125I]‐epibatidine binding (200 pM) by A85380, cytisine and (−)‐nicotine was biphasic, indicating the presence of multiple binding sites.
The sites with lower agonist affinity comprised 30.0±2.2, 58.6±0.1 and 48.7±3.3% of specific [125I]‐epibatidine (200 pM) binding in inferior colliculus, interpeduncular nucleus, and olfactory bulb homogenates, respectively.
The affinity difference between A85380‐sensitive and ‐resistant binding sites was particularly marked (approximately 1000 fold). Thus A85380 was used to differentiate agonist‐sensitive and ‐resistant sites.
The pharmacological profiles of the A85380‐resistant sites in each region were assessed with inhibition binding experiments, using 14 agonists and five antagonists. The profiles were indistinguishable across regions, implying that A85380‐resistant [125I]‐epibatidine binding sites in inferior colliculus, interpeduncular nucleus, and olfactory bulb represent a single nAChR subtype.
The pharmacological profile of the A85380‐resistant sites is very different from that previously reported for high affinity (−)‐[3H]‐nicotine‐, [125I]‐α‐bungarotoxin‐, or [125I]‐α‐conotoxin MII‐binding sites, suggesting that they represent a novel nAChR population in mouse brain.
British Journal of Pharmacology (2000) 131, 729–739; doi:
In a recent study, we reported that a restriction fragment length polymorphism associated with the alpha4 nicotinic receptor gene (Chrna4) may play a role in regulating differential sensitivity of LS and SS mouse lines to the seizure-inducing effects of nicotine. Since the alpha4 subunit (CHRNA4) is often found as a heteromer with the beta2 subunit (CHRNB2), alpha4 and beta2 cDNAs from the LS and SS mice were cloned and sequenced. A polymorphism in the coding portion of the alpha4 gene was found (1587A to G) which should result in a threonine/alanine substitution at position 529 (T529A). The LS and SS beta2 nicotinic receptor subunit cDNAs were identical. The potential consequences of the alpha4 polymorphism were evaluated using an ion (86Rb+) flux assay that likely measures the function of alpha4beta2-type receptors. LS-SS differences in maximal nicotine-stimulated ion flux were seen when bovine serum albumin (BSA) was not included but this difference was not seen when BSA was included in the perfusion buffer. Current evidence suggests that BSA may alter the ratio of nicotinic receptors that are in the ground state and desensitized forms. Thus, it may be that the Chrna4 T529A substitution leads to a difference in the ratio of the two receptor forms which then promotes differences in receptor function, as well as differential behavioural sensitivity to nicotine.
In patients with PBC refractory to UDCA, obeticholic acid or a fibrate is a reasonable choice as an adjunctive treatment to UDCA. Further investigation with randomized controlled trials is needed to provide high quality evidence to formulate standardized therapies in this difficult to treat population.
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