FIGURE 11.2 Structure of nicotine (2). ©2000 by CRC Press LLC
E,J, orG).19 They are also found in high concentrations throughout the brain and spinal
cord. Neuronal nAChRs are believed to be pentameric structures composed of D and E sub- units with a vast possibility of subunit combinations.20-22 To date, 11 neuronal subunit genes
have been identified in rat and chick, 8 which code for the D subunit (D2 to D9)22-24 and
3 which code for non-D (n D) or E subunits (E2 to E4 in rat and n D1 to n D3 in chick).22 Human
genes for D2 to D5, D7, E2 to E4 have been cloned.25-29 These receptors have been found to exist
as homo-oligomeric combinations of D7, D8, and D9 subunits alone. They are also known to
exist as hetero-oligomeric complexes containing D2, D3, and D4 subunits separately coex-
pressed with E2 or E4 subunits. This large number of subtypes suggests a multitude of
potential combinations which could give rise to many functional subtypes of the neuronal nAChR. However, in spite of the apparent potential for a high degree of neuronal nAChR diversity, only three major neuronal nAChR subclasses have been identified using radio- ligand binding techniques. The three classes are those with high affinity for (–)-nicotine which are labeled by [3H]-acetylcholine,30 [3H]-(–)-nicotine,31 [3H]-(–)-cytisine,32 and [3H]-
methylcarbamylcholine,32 which correlate with the D
4E2 subtype nAChR; those which are
D-bungarotoxin sensitive, which correlate with the D7 distribution33,34; and a group of recep-
tors that are selective for neuronal bungarotoxin which represent the D3 subunit.35 Immu-
noprecipitation studies have found that 90% of the nAChRs which bind (–)-nicotine (2) with high affinity are of the D4E2 subtype.36 The composition, location, and function of the
remaining 10% of these high-affinity nicotinic sites are not yet understood. In order to char- acterize fully these minor subtypes as well as the major D4E2 subtype, selective agonists or
antagonists for these various subtypes are needed.
11.4 Beneficial Effects and Liabilities Associated with nAChRs
The beneficial effects of ChCMs are many. Nicotine has been reported to enhance cognition, increase attention, reduce anxiety, decrease nociceptive perceptions, and act as a neuropro- tectant.37 However, the liabilities associated are significant as nicotine causes decreased
body temperature, reduction of locomotor activity, and induction of seizures.37 Nicotine
also possesses a drug discrimination or cue upon administration.37 These benefits and lia-
bilities show the diversity of nAChRs and the potential for defining each pharmacological effect with selective agents.
11.5 Pharmacological Profile of Lobeline
Compared with nicotine, lobeline acts similarly in many behavioral and pharmacological experiments. Both nicotine and lobeline bind to central nicotinic receptors with high affinity1 and stimulate autonomic ganglia.38 Furthermore, they possess anxiolytic proper-
ties,39 reduce locomotor activity,40 and induce seizures when high doses are injected into the
brain.41 They also enhance basal forebrain stimulation-induced changes in cerebral blood
flow.42 In addition, both compounds reduce body temperature and rearing in an open-field
test; however, the effects of lobeline were not modified by the long-acting nicotinic antag- onist, chlorisondamine, as were the effects of nicotine.43 Lobeline has been found to be
10-fold less potent than nicotine in all aspects with the exception of the anxiolytic-like effects where it is equipotent.43 Lobeline also has been reported to cause tachycardia and ©2000 by CRC Press LLC
hypertension.44 Conversely, in urethane- and pentobarbital-anesthetized rats it is reported
to cause bradycardia and hypotension.45 Lobeline has been used as a smoking cessation
agent and is currently in Phase III clinical trials as a sublingual tablet.46 However, nicotine
and lobeline differ in several important aspects. Lobeline does not produce the nicotine cue in drug discrimination experiments, and has been reported not to stimulate the release of catecholamines.47-50 More recently, lobeline has been reported to cause the release of
dopamine.2However, it acts as a potent inhibitor of dopamine uptake into synaptic vesi-
cles, and subsequently alters presynaptic dopamine storage.2 Lobeline does not cause an
increase in the number of nicotinic receptors in many regions of the brain with chronic administration.23In contrast, nicotine has shown the ability to increase the receptor popu-
lation as seen in postmortem human brain tissue obtained from smokers.51 These differ-
ences suggest that lobeline is interacting with a different subtype of central nicotinic receptors or working through non-nicotinic mechanisms. Most recently, lobeline has been reported to bind with high affinity to neuronal nicotinic receptors, but there is strong evi- dence to support it does not activate the D4E2 subtype.2 This is another indication that
lobeline is either working through minor subset populations of nicotinic receptors, non- nicotinic mechanisms, or is causing an allosteric effect at the nicotinic receptor facilitating the major subunits ability to bind endogenous compounds.
11.6 Nicotinic Pharmacophore Models
Several attempts to describe a nicotinic pharmacophore have been reported. The only accepted pharmacophore is that of the peripheral neuromuscular nicotinic receptor. Since the discovery of the multitude of receptor subtype possibilities in the central nervous sys- tem, there has not been an attempt to describe a true central nicotinic pharmacophore. Models proposed by Beers and Reich53 (Figure 11.3) through a conformational analysis,
suggest a key element in the binding of ligands to nAChRs is a hydrogen bond formed between a receptor hydrogen donor and an acceptor group in the ligand. The distance is believed to be 5.9 Å from the positively charged nitrogen atom.
However, it is known that acetylcholine (4), the endogenous ligand of the receptor, assumes a conformation giving to a 4.4 Å distance at the muscarinic receptor between the hydrogen acceptor and the positively charged nitrogen. A more exhaustive model was pro- posed by Sheridan and co-workers54 (Figure 11.4), using a distance geometry approach,
gave a distance of 4.8 Å between these two sites.
Epibatidine (5,Figure 11.5), a natural alkaloid isolated by Daly et al.55 from the Ecuadorian
poison dart frog, Epipedobates tricolor, has recently proved that the proposed pharmacophores are not complete. Epibatidine is the most potent central nicotinic receptor ligand reported to
FIGURE 11.3
Beers and Reich53 pharmacophore.
©2000 by CRC Press LLC
date and its intramolecular distance of 5.51 Å between the hydrogen acceptor site and the positively charged nitrogen is greater than any previously theorized to be essential.
Currently, Glennon and Dukat56 have been working toward developing an understand-
ing of binding affinities for various nicotinic ligands and have published an exhaustive review of nAChR pharmacophores. As previously mentioned, these models were devel- oped with the understanding that there was one receptor. This is now known to be untrue, and more detailed models should be developed as more ligands are produced.
11.7 Geometric Comparisons of Lobeline and Nicotine
Barlow and Johnson57 have published x-ray crystallography data comparing nicotine and
several nicotinic ligands including lobeline. Their conclusions suggested that lobeline, a nearly symmetrical molecule, had an agonist portion and an antagonist portion. They believed that the moiety containing the ketone portion of the molecule acted at the receptor as the agonist based on its close overlap with nicotine and that the moiety containing the alcohol acted either antagonistically or was not involved in the activity at all. Terry and co- workers58 recently reported that the entire molecule of lobeline is believed to be needed to
exert its high-affinity binding and pharmacological effects. In this report, the “two halves” of lobeline (shown in Figure 11.6), CRM1-32-1 (6) and CRM1-13-1 (7), were synthesized and subjected to rigorous receptor binding and rubidium efflux assays.
Interestingly, the two halves bind with much less affinity but cause an efflux of rubidium, suggesting that both act as agonists. Furthermore, acting in a similar fashion to lobeline, the effects of the compounds could not be reversed by mecamylamine or other nicotinic antag- onists. These compounds also exhibited a similar pharmacological profile to lobeline in
FIGURE 11.4
Sheridan et al.54 pharmacophore. Atom labeled D is a dummy atom along the bond angle bisector and 1.2 Å from the atom to which it is attached. Three essential groups are needed: the cationic center (A), an electronegative atom (B), and an atom (C) that forms a dipole with B.
FIGURE 11.5