Acetylcholine, released by cholinergic neurons, activates two groups of acetylcholine receptors (AChRs); muscarinic AChRs (mAChRs) which belong to the superfamily of G-protein coupled receptors (GPCRs) and nicotinic AChRs (nAChRs) which belong to the ligand-gated ion channel superfamily. nAChRs also respond to nicotine, hence their name¹. These channel receptors are usually non-selective cation channels activated upon ligand binding which ultimately leads to the depolarization of postsynaptic cell membranes.

To date, 17 different but related subunits of nAChRs have been identified and cloned. They consist of a subunits (a1-10), which is responsible for the binding of ligands. In fact, this subunit includes a Cys-loop in the first extracellular domain that is required for agonist binding². The other subunits responsible for making up the active receptor are the b (b1-4), g, d and e subunits³. Structurally, all subunits have the following: a conserved large extracellular N-terminal domain, 3 conserved transmembrane domains, a variable cytoplasmic loop and a fourth transmembrane domain with a short extracellular C-terminal domain.  An active nAChR is generally a heteropentamer (homopentamers also exist) of these various subunits organized around a central pore¹.

All a subunits are expressed in neuronal cells except for the a1 subunit which is specifically expressed in skeletal muscle. They are also expressed in non-neuronal cells such as bronchial epithelial cells⁴, as well lymphocytes⁵. The diversity of these receptors and their functional organization gives rise to unique properties and functions. The a4b2 receptor composition makes up a high affinity nicotinic receptor. In fact, its upregulation (mainly expressed by the increase of functional receptors at the membrane and not expression per se) is responsible for the increased appearance of binding sites following nicotine administration^1,3.

Animal studies have shown that nAChR-related mechanisms are involved in attention function⁶. Indeed a4b2 nAChR seems to also be involved in attention-deficit hyperactivity disorder (ADHD), a disease distinguished by a lack of attention, distractibility and hyperactivity³. The a4b2 and a7 nAChRs appear to be critical in rats for attention and working memory. Also, a a4b2 specific agonist was shown to reduce impulsivity, hyperactivity and attention deficits in adults with ADHD⁷. This same receptor subtype may also be involved in Parkinson’s disease (PD) as smoking and a4b2 nAChR agonists show beneficial effects in PD^8,9.