Cholinergic receptors normally bind the endogenous neurotransmitter acetylcholine (ACh), thereby triggering the opening of ion channels. ACh receptors in the mammalian central nervous system can be divided into muscarinic (mAChR) and nicotinic (nAChR) subtypes based on the agonist activities of muscarine and nicotine, respectively. The nicotinic acetylcholine receptors are ligand-gated ion-channels containing five subunits. Members of the nAChR subunit gene family have been divided into two groups based on their amino acid sequences; one group containing so-called (3 subunits, and a second group containing a subunits. Three kinds of a subunits, α7, α8 and α9, have been shown to form functional receptors when expressed alone and thus are presumed to form homooligomeric pentameric receptors.
An allosteric transition state model of the nAChR has been developed that involves at least a resting state, an activated state and a “desensitized” closed channel state, a process by which receptors become insensitive to the agonist. Different nAChR ligands can stabilize the conformational state of a receptor to which they preferentially bind. For example, the agonists ACh and (−)-nicotine respectively stabilize the active and desensitized states.
Changes of the activity of nicotinic receptors has been implicated in a number of diseases. Some of these, for example myasthenia gravis and ADNFLE (autosomal dominant nocturnal front lobe epilepsy) are associated with reductions in the activity of nicotinic transmission either because of a decrease in receptor number or increased desensitization.
Reductions in nicotinic receptors have also been hypothesized to mediate cognitive deficits seen in diseases such as Alzheimer's disease and schizophrenia.
The effects of nicotine from tobacco are also mediated by nicotinic receptors. and since the effect of nicotine is to stabilize receptors in a desensitized state, an increased activity of nicotinic receptors may reduce the desire to smoke.
Compounds which bind nAChRs have been suggested for the treatment of a range of clinical psychiatric and neurological disorders involving cognitive deficits in attention, alertness, and memory. These may include those that may benefit from selective enhancement in cholinergic transmission such as attention deficit, chronic psychotic disorders, jetlag, selected pain syndromes, and smoking cessation; and those thought to involve reduced cholinergic function such as, neurodegenerative disorders, central inflammatory or autoimmune diseases, brain trauma and cerebral vascular disease. Modulation of α7 nicotinic receptor activity is expected to be beneficial in a number of diseases including Alzheimer's disease, Mild Cognitive Impairment (MCI) and related syndromes, Lewy Body Dementia, vascular dementia, Multiple Sclerosis, post-encephalitic dementias, Parkinson's disease, Huntington's disease, Attention Deficit Hyperactivity Disorder (ADHD), schizophrenia, bipolar mood disorder, schizoaffective disorder, Tourette's syndrome, and brain trauma.
However, treatment with nicotinic receptor agonists which act at the same site as ACh is problematic because ACh not only activates, but also blocks receptor activity through processes which include desensitization and uncompetitive blockade. Furthermore, prolonged activation appears to induce a long-lasting inactivation. Therefore, agonists of ACh can be expected to reduce activity as well as enhance it.
At nicotinic receptors in general, and of particular note at the α7-nicotinic receptor, desensitization limits the duration of action of an applied agonist.