High Throughput Pharmacological Screening for Lead Compounds at Molecular Targets of Relevance to Alcohol Dependence
J. M. Littleton. Kentucky Tobacco Research & Development Center, University of Kentucky, Lexington, KY, USA.
Effective medications for drug and alcohol dependence are greatly needed, but progress has been slow. One problem is that, although several molecular targets can be identified, many of these present specific difficulties. For example it is quite clear that nicotinic receptors for acetylcholine (nAChRs) are targets for smoking cessation medications, and the involvement of nAChRs in “reinforcing” pathways means that they are important for other types of dependence also. The difficulty here is the complexity of the subtypes of nAChRs, and their many roles in physiological function. Drugs that targeted different nAChR subtypes specifically might be valuable for different types of dependence, including alcoholism, and might have fewer of the side effects and abuse potential that reduce the value of agents like nicotine. Similarly, there is little doubt that the glutamatergic transmitter system in brain also plays an important role in reinforcement, and that glutamate receptors are therefore important molecular targets. In addition, the role of glutamate/NMDA receptors (NMDARs) in conditioning is well known, suggesting that this receptor is a target for medications aimed at reducing relapse caused by conditioned stimuli (‘cues’) previously associated with drug-taking (for example acamprosate in alcohol dependence). Once again, abuse potential and side effects related to the physiological role of this receptor (e.g. in learning and memory) make the NMDAR a difficult molecular therapeutic target. In this case the problem is less one of sub-type selectivity and more of targeting mechanisms that prevent NMDAR “overactivation” but which preserve normal function.
The first stage of drug discovery is to develop high throughput pharmacological screens (HTPS) for the molecular activity required. While it is easy to devise screens that measure “activity” at a receptor, it is more difficult to develop screens that identify compounds with more subtle selectivity, as is required here. We have attempted to do this using two or more HTPS sequentially, so that differences in activity at closely related binding sites are compared. Compounds with the selectivity we are seeking give a specific “signature” of activity in these screens. This differential/ sequential screening (DSS) has been applied to the identification of synthetic compounds and natural products that bind to nAChRs or to NMDARs with the kind of selectivity we require in potential medications for drug dependence. For NMDARs we have used radioligand binding of [3H]MK801 in the presence and absence of spermidine and have identified synthetic iminoguanidines that appear to inhibit the receptor only when it is co-activated by polyamines. For nAChRs we have used [3H]epibatidine, [3H]cytisine and [3H]methyl-lycaconitine sequentially and have identified several native plant extracts that contain compounds with receptor subtype selectivity that differs from that of known compounds. These may have been evolved as protection against insect predation, because some of them bind with very high affinity to insect nAChRs. If so, their affinity for different subtypes of mammalian nAChRs is “accidental” but potentially valuable. These synthetic compounds and plant extracts are now being tested in more complex screens and models of drug dependence to establish whether they are of any potential value as medications, lead compounds, or research tools.