Richard W. Fitch - Research
Nicotinic Acetylcholine Receptors
The work in our laboratory centers around one primary biological target, the nicotinic acetylcholine receptor (nAChR). These receptors are key players in fast synaptic transmission in the central nervous system and play a number of roles in the periphery as well. Cholinergic neurotransmission within the brain is crucial in cognition, pain perception and a variety of neurological responses that require rapid signalling. Nicotinic receptors are implicated in a number of disease states, including Alzheimer's and Parkinson's diseases, certain epilepsies, and addictive behaviors. Cholinergic innervation of the mesolimbic dopamine system is considered one of the reasons for nicotine's addictive qualities. The nAChR is a pentameric protein that behaveas as a ligand gated ion channel, with the subunit proteins assembled like the staves of a barrel forming a pore through which ions, primarily Na+, Ca2+ and K+ can pass. Nicotinic receptors come in a number of subtypes distinguished by the number and type of subunits present, which in turn determines the pharmacology of the receptor and its conductance of ions. Peripheral neuromuscular nAChR, designated (a1)2b1gd is highly permeant to Na+, while the central neuronal types tend to conduct Ca2+ more easily. The major receptor subtype in the brain is composed of a4 and b2 subunits with the proposed stoichiometry (a4)2(b2)3 and is referred to as a4b2. Another key player whose dysfunction has been linked to schizophrenia is the (a7)5 homomer, referred to simply as a7.
The gating of a typical nicotinic receptor requires two binding sites to be occupied. These sites are typically allosterically coupled. That is the first binding event influences the second and in these receptors, enhances the affinity for the second ligand. When both are bound, a conformation change occurs that allows the flow of ions through the channel into and out of the cell.
This gating has two important biological consequences. First, upon activation, Na+ and Ca2+ flow in and K+ flows out, resulting in depolarization of the membrane. At the muscle endplate, this event causes the activation of voltage gated channels that enhance the depolarization, ultimately leading to muscle contraction. In neuronal cells, this leads to the development of the action potential, which is an important process in the propagation of nerve conduction. The second consequence is that if the receptor is permeable to calcium, the intracellular calcium level rises, triggering other events, such as enzyme activation, neurotransmitter release, or other events, often regulated by calmodulin binding.
The native ligand for these receptors is acetylcholine (below). However, a number of natural and synthetic ligands have been discovered and/or developed for this receptor, as it is important in analgesia, cognition, addiction, and a variety of other normal and pathological processes. Nicotinic receptors have been implicated in disease states ranging from Alzheimer’s dementia and Schizophrenia to Tourette’s syndrome, Parkinson’s disease, and certain epilepsies, such as autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). These receptors are thus important targets for the development of therapeutics.
We are interested in the discovery and development of selective ligands for nAChR and especially the molecular requirements for agonism at this receptor. This is a nontrivial matter, as designing molecules to bind to a receptor is a matter of identifying the pharmacophore, or mininmal structure for activity. For antagonists, one set of parameters is required for binding to the receptor in the correct location. Agonists take this one step further, in that they must activate the receptor, inducing the conformational changes required to cause the pore to open. If one considers Fisher's "lock and key" principle, the structural elements for binding are like the longs slots on your key that allow it to fit into a lock. An agonist must also have the right arrangement of teeth on the key to allow the lock to turn, which is a much more delicate matter. In chemical terms, it must have the proper spatial arrangement of attractive or groups to allow a movement of residues in the binding site that will translate to the gate in the pore of the receptor.
