The molecular basis of nicotine addition

Cigarette smoking and consciousness? To most people they don’t seem to go together. But one of the fun things about scientific research is the surprises it brings. This is the story about the chemical basis of nicotine addiction, which is also believed to be one of the important ingredients of the brain basis of human consciousness. Moreover, it allows us to explore a favorite question raised by philosophers — can high-level conscious experiences, involving activity in large parts of the brain, be understood in terms of molecules? Crudely stated, can conscious experiences be reduced to molecules?A recent study performed in the laboratories of Jean-Pierre Changeux and collaborators (see Maskos et al., 2005, Nature 436:103-107) provides some of the first convincing evidence that nicotine addiction, a complex behavioral state, can be attributed to a very specific molecule, a part of acetylcholine (a-SEE-til-koh-leen). Acetylcholine (abbreviated ACh) was one of the first chemicals discovered to be a neurotransmitter, a chemical messenger that allows a signal from one neuron to trigger activity in the next one. Most of the psychoactive drugs and chemicals we know seem to work by changing chemical transmission across the synapse, the tiny gap between two neurons. So neurotransmitters, and their close cousins, the neuromodulators, seem to be basic, even to questions like consciousness.

In this case, the crucial molecule is a subunit of the ACh receptor, called the ß2 subunit. The discovery that the ß2 subunit is involved in nicotine addiction is a remarkable accomplishment. It is reductionism at its best, showing how the cravings for a cigarette, can be controlled at the level of molecules. Nonetheless, one has to explain how this degree of reduction is possible for as complex a behavioral state as addiction. How do we connect a molecule operating at the level of Angstroms — 10 -10 meters, or about 1/10,000,000,000 of a yard — to the subjective feeling of wanting another smoke?

Secondly, is the same degree of reductionism possible for complex cognitive phenomena and global conscious states? Can we explain sensory consciousness, like your ability to see the words in front of you now, at the level of molecules?

Acetylcholine appears to play a role in both addiction and consciousness, so are both of those high-level phenomena reducible to molecules? Acetylcholine is a neuromodulator found widely This is reductionism at its best, showing how the cravings for a cigarette can be controlled at the level of molecules distributed throughout the CNS (see Fig. 1). Neuromodulators are sometimes called “spritzers” — because they look a little bit like a fountain, spraying a neurochemical widely throughout a large part of the brain. Their basic anatomy looks simple enough: a clump of neuronal cell bodies at the bottom of the brain sends long fibers to large parts of the brain, and secretes tiny squirts of neurochemical at synapses that are very widely dispersed. The ACh system participates in multiple functions, such as memory, selective attention, intellect and consciousness. This diversity of roles is enabled by the anatomy of the cholinergic system; it is a system that exerts a global influence over the whole of the cerebral cortex (Woolf, 1996).

Neuromodulators, such as acetylcholine, are so named because they adjust or modulate specific sensory information in thalamocortical pathways, like the visual and auditory nerves. Due to its enormous scope of projection, the cholinergic system modulates all of sensory processing, shifting the focus from one part of the cerebral cortex to another.

Much of what we know about the function of acetylcholine derives from animal studies in which cholinergic brain areas are damaged or cholinergic genes deleted. Behavioral deficits found for patients with Alzheimer’s disease or other dementias that compromise central cholinergic systems also attest to the roles played by acetylcholine.

Figure 1 Cholinergic pathways (black) and their interaction with dopaminergic pathways (red). Modified from Woolf, 1991. Abbreviations: ldt, laterodorsal tegmental area; N Acc, nucleus accumbens; PFC, prefrontal cortex; ppt, peduculopontine tegmental nucleus; VTA, ventral tegmental area.

The molecular mechanism of nicotine addiction described by Maskos et al. (2005) depends upon cholinergic receptors located on limbic dopamine neurons (see Fig. 1). In this sense, the anatomy of nicotine addiction is more restricted and more specific than the anatomy of global conscious states. After all, the conscious state relies on the totality of central cholinergic systems, as well as other cortical neurotransmitters. Depending on the qualitative state, acetylcholine modulates selective parts of the cerebral cortex through the activation of cholinergic pathways from the basal forebrain. The cholinergic basal forebrain is activated by the “pontine cholinergic nuclei” — the clumps of neurons located in the pons, a bulge that protrudes from the brainstem (the ppt and ldt shown in Fig 1). Those are the neurons that send their fibers secreting their chemicals all over cortex.

In addition to activating the basal forebrain, the pontine cholinergic nuclei activate the two major dopamine cell body regions in the brainstem, the substantia nigra (not shown) and the VTA (ventral tegmental area). Maskos et al. (2005) focused their study upon the VTA projections to the nucleus accumbens and prefrontal cortex (N Acc and PFC in Fig. 1). They also focused on nicotinic receptor mechanisms.

In the CNS, acetylcholine binds to two major classes of receptor, muscarinic and nicotinic receptors, each named after the drugs with which they bind besides acetylcholine. “Muscarinic” receptors are named after a mushroom chemical, and nicotinic receptors obviously provide the chemical docking stations for nicotine.

Muscarinic receptors predominate in the cerebral cortex; nicotinic receptors are present in far lesser concentrations. Centrally, nicotinic receptors are most abundant in the substantia nigra and the VTA. It was indeed an unexpected result when nicotinic receptors were first detected in the substantia nigra and the VTA (see Clarke et al., 1984). At that time it was widely believed that nicotinic receptors were exclusively peripheral and muscarinic receptors largely central. Soon after their discovery, addiction researchers began investigating nicotinic receptors on dopamine neurons. Since dopamine is closely associated with reward, it was logical to consider these CNS nicotinic receptors as a possible neural substrate of addiction.

Figure 2 Nicotinic receptors in the VTA contain α and ß subunits combined to form an ion channel that spans the neuronal membrane.

The nicotinic acetylcholine receptor is a pentamer, being comprised of 5 subunits, and various α and ß subunits combine differently to form functional receptors (see Fig. 2). There are regional differences in the subunits that make up nicotinic receptors. Nicotinic receptors in the VTA are composed of combinations of α2 – α10 and ß2 – ß4 subunits. The study by Maskos et al. (2005) focused very specifically on the ß2 subunit of the nicotinic receptor. It is this specificity, along with the way in which they did their study, which takes us one step closer to exactly pinpointing the mechanism underlying addiction.

First they showed the ß2 subunit of the nicotinic receptor was necessary for self-administration of nicotine, by measuring its absence in mice with the ß2 subunit deleted (ß2-null mice). While it was already known that the ß2 subunit was necessary for nicotine It is well known today that REM sleep is largely a cholinergic phenomenon addiction, it was essential to repeat this measure. Next, Maskos and colleagues re-expressed the ß2 subunit of the nicotinic receptor in behaviorally impaired ß2-null mice using a sophisticated lentiviral vector technique. When the ß2 subunit was reintroduced in this manner, ß2-null mice once again exhibited behavior patterns indicative of nicotine addiction. These results prove that the ß2 subunit of the nicotinic acetylcholine receptor is not only necessary, but also sufficient in producing addiction to nicotine. Moreover, the reintroduction of the ß2-subunit restored levels of slow exploration, a behavioral index related to cognition. Again, a global phenomenon like exploration was shown to be related to a molecular phenomenon.

This exciting result suggests genetic deletions of the ß2 subunit of the nicotinic acetylcholine receptor may be useful in carefully designed animal experiments testing well-developed theories of consciousness, such as Global Workspace and Higher-Order-Thought (HOT) (see Baars, 2005; Rosenthal, 1997). The involvement of the ß2 subunit of the nicotinic acetylcholine receptor seems likely in at least some forms of higher cognition, given the restoration of slow locomotor behavior after the reintroduction of the ß2 subunit.

Compared to the preferential involvement of nicotinic receptors in nicotine addiction, both nicotinic and muscarinic acetylcholine receptor subtypes are likely to contribute to cognition and global conscious states. This is indicated by correlations with sleep states, disease states and neuroreceptor mechanisms in anesthesia

It has long been known that rapid eye movement sleep (REM) is largely a cholinergic phenomenon, and that diseases that disrupt REM are associated with cholinergic deficits. In their 1999 review, Perry and co-authors note both nicotinic and muscarinic acetylcholine receptors underlie disrupted conscious phenomena (e.g., REM sleep abnormalities and hallucinations) in Alzheimer’s disease, Parkinson’s disease and Lewy body dementia. (look at Entrez PubMed to find research abstracts on these topics). It is also the case that both muscarinic and nicotinic receptor subtypes interact with volatile and injectable anesthetics, which obviously cause a loss of consciousness; however, inhibition of nicotinic receptors at clinically relevant doses may be associated with analgesia rather than loss of consciousness (Tassonyi et al., 2002). Nevertheless, analgesia involves a loss of conscious pain perception, so that even these results may be relevant to consciousness.

Among muscarinic receptors, the M1 muscarinic subtype is particularly implicated in conscious phenomena. Murasaki et al. (2003) found the injectable anesthetic propofol inhibits muscarinic receptor M1 by decoupling its activation from that of the G-protein, an intracellular component of chemical transmission. Similarly, Kamatchi et al. (2001) showed that volatile anesthetics decrease the ability of M1 receptors to modulate the L-type Ca2+ channel, another key mechanism of neuronal functioning. Despite a strong role for muscarinic receptors, multiple specific mechanisms have been identified in anesthesia — suggesting that consciousness is unlikely to rest wholly upon one receptor subtype or one mode of inhibition or decoupling. Quite the opposite: many receptors, in addition to muscarinic and nicotinic receptors, are affected by inhaled and injected anesthetics. Blocking GABA receptors, for example, appears to be critical far many types of anesthesia. Schofield and Harrison (2005) showed that the α3 subunit of GABAA is a pivotal site of action for the volatile anesthetic isoflurane. Last but not least, receptor mechanisms correlated with anesthesia imply, but do not necessarily prove, a role for any particular neuroreceptor in consciousness.

Why one nicotinic receptor subunit might be more critical than the others in addiction is a difficult question to answer, and it forces us to reevaluate our position of whether a reductionistic approach, at the level of molecules, will ultimately succeed or not.

Additional research suggests a reason that this kind of reductionism is possible for addiction. An essential feature of addiction is tolerance, so that more of a drug is needed to The real trick may be in reconciling reductionism with highly complex, yet unified cognitive and conscious states produce the same effect that a lower amount of the drug did initially. Tolerance is the reason why long-time addicted smokers may smoke several packs per day, and why chronic alcoholics may consume large volumes of alcohol per day. Given this basic logic, a possible molecular mechanism for addiction is a change in the working of a neurotransmitter receptor. McCallum et al. (2005) showed that the β2 subunit is responsible for the up-regulation of the other nicotinic receptor subunits. Thus, a very specific mechanism involving one subtype of nicotinic receptor can, at least in principle, cascade to have multiple divergent effects upon the other nicotinic subunits. This finding may go a long way towards enabling new treatments or even curing nicotine addiction.

This kind of reductionistic proof may be too narrowly focused for fully explaining global conscious states underlying many types of conscious contents. Combating diseases is by nature amenable to reductionism because of biochemical triggers and cascades; but when it comes to complex cognitive states, we may not be able to limit our perspective to one type of receptor subunit. Moreover, alterations in neuroreceptors may be merely correlative. Only more research will resolve these matters.

In the meantime, the molecular approach employed in the Maskos et al. (2005) is extraordinarily elegant, representing a major advance. It begins to elucidate the molecular mechanism of nicotine addiction, and shows how to use sophisticated molecular methods, like the lentiviral vectors to re-express deleted or dysfunctional proteins. These approaches stand on their own merits and open up far-reaching possibilities for treating age-relating disorders such as Parkinson’s disease or Alzheimer’s disease.

They may also tease out what receptor subunits are critical to various aspects of global conscious states, and they might be used to test theories such as Global Workspace and HOT. The real trick, however, may be in reconciling reductionism with highly complex, yet unified cognitive and conscious states.

Nancy J. Woolf
Behavioral Neuroscience
Department of Psychology
University of California Los Angeles
CA 90095-1563

Acknowledgment: Kind thanks to Uzi Awret for input.

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