Molecular Basis of Nicotine Toxicity

Molecular basis of nicotine toxicity

Nicotine toxicity predominantly occurs through a well-understood pathway involving acetylcholine receptors and subsequently downstream mediated modification of ion channels and release of specific neurotransmitters. In order to fully characterize the condition, it is important to wholly recognize and understand each of the steps involved within the pathway, and how they go awry in relation to usual operating behaviors.

Figure 1: Graphic representation of steps occuring at the nAChR site prior to and following nicotine binding. (Grando 2014)
Figure 1: Graphic representation of steps occuring at the nAChR site prior to and following nicotine binding. (Grando 2014)

Nicotine and Acetylcholine

Acetylcholine is a naturally occurring neurotransmitter found within the human body which mediates ligand-gated movement of ions across the cell membrane of neurons and neuromuscular junctions. Acetylcholine mediated channels fall into two large categories: nicotine affected (Nicotinic) and muscarine affected (Muscarinic), corresponding to which natural product acts as an alternative agonist upon the channel.  The channel complex itself exists in two discrete states: open and closed. Open nAChR complexes allow for the free movement of cations through the cell membrane, with particular note given to the K+, Na+, and Ca2+ ions within the system. Conversely, closed channel systems without bound Acetylcholine or Nicotinic ligands do not allow for the same movement of charge.

Upon the free movement of K+ and Na+ ions across the cell membrane into the cell in question, the charge gradient between the interior and exterior facilitates the occurrence of an excited post-synaptic potential (EPSP), which in turn promotes the firing of an action potential by further depolarizing the cell. The action potential itself allows for proper intracellular communication, with the location of the original neuron dictating the specific message and role conveyed. In neuromuscular junctions, where nAChR complexes are often found, the action potential allows for communication and flexion of muscles within the body. Alternatively, nAChR complexes in the brain facilitate the neural stimulation effects of nicotine with a corresponding increase in other membranes incidences of EPSP by way of utilizing their action potential to depolarize the membranes of neighboring neurons and affect the charge gradient (Purves 2001). By such mechanisms, neural tissue can communicate with each other and muscle contractions which they are paired with controlled.

While nicotine toxicity itself does not have a genetic component of inheritance, as it exists as a toxic natural product removed from human biology, there are avenues of focus where genetic information proves useful. The role of genetics in moderating nicotine dependency is a highly relevant field, as due to nicotine’s well understood mechanism of addiction, overall patterns of addiction in relation to genetic factors may be further discerned. Prior to the sequencing of the human genome, it was known that likelihood for alcoholism and nicotine addiction persisted across twins. In a 2006 paper, oxford researchers analyzed over 3700 single nucleotide polymorphisms (SNP’s) against a large gene screening in a population of heavy smokers. From this data, it was determined that SNP’s within genes responsible for nAChR complex formation were the most likely to have relevant variance, with the non-synonymous SNP’s in the CHRNA5 gene correlating with both a recessive mode of inheritance and a 200% increase in risk of developing nicotine addiction once exposed to cigarettes (Saccone 2007).  The CHRNA5 gene is responsible for the development and proper function of ligand gated ion channels, specifically those belonging to the nicotinic group. It may be conjectured that the non-synonymous SNP serves to increase the binding affinity of nicotine to the receptors in some fashion, or otherwise facilitates the free passage of cations through the cell membrane. Additionally, KCNJ6 and GABRA4 both had statistically significant changes in signaling between the addict population and the baseline population.

KCNJ6 is responsible for the proper movement of K+ ions across the cell membrane, and as such is tied into the depolarization event mediated by the nAChR complex binding to a nicotine molecule. GABRA4 itself is linked to the function of GABA receptors in the brain, specifically within an inhibitory role. In this role, the activation of a GABA receptor within the brain leads to a hyper-polarization of the cell, with CL ions entering and preventing a depolarization event. By this mechanism, action potentials and consequent neural activity can be suppressed. In the event that GABRA4 functioned incorrectly, and there were less inhibitory hyperpolarization events, it would be consequently more likely for the triggered action potential by way of nAChR’s to propagate further, and the bodies neural network to become dependent upon such rapid propagations facilitated by the presence of nicotine.

Supporting this theory of nicotine-advanced nicotine addiction is the research finding that consistent use of nicotine containing products over time leads to increased propagation of nAChR’s within the body, as compared to that of a non-smoker. A reasonable explanation for this is predicated on the systematic desensitization of nAChR’s existing within the body, and the necessity for higher levels of nAChR’s in order to attain the original degree of signaling efficacy. (Purves 2001)

Proteins

Proteins also play a direct role in response to toxic quantities of nicotine, with significant research having already been done examining the role of substrate-mediated binding patterns within the nAChR’s in various species, and the resulting systemic effects upon the organism. The role of nicotine-containing compounds (neonicotinoids) within the broader context of pesticide use provides avenues to discern the implications of different conformations and structural features upon binding activity and downstream effects. Nicotine derived pesticides provide a more immersive avenue than commercially available nicotine products for consumption, as the pesticides possess a wider range of stereoconformations and the capability to examine them in a mammalian or insect test population is more relevant during the initial development phase. Research done in insect populations points to the difference between ionic neonicotinoids and neonicotinoids possessing a negative charge at the active binding site, either by way of a cyano or nitro group (Tomizawa 2003). The authors demonstrated therein that the negatively charged endgroups were less functional against mammalian targets, as compared to unmodified nicotine. The rationale here was that the presence of anionic lipid-based anionic subsites within the mammalian nAChR  complex interacts more effectively with the anionic nicotine than with the insect-targeted negatively charged neonicotinoids. (Tonstad 2014)

This is particularly relevant due to the propensity for nicotine toxicity via alternate methods then direct consumption of tobacco, such as working with or unknowingly ingesting neonicotinoid pesticides.

Figure 2: The mechanism by which neonicotinoids engage with nAChR sites, as seen within pesticies (Motohiro 2003)
Figure 2: The mechanism by which neonicotinoids engage with nAChR sites, as seen within pesticides (Motohiro 2003)

As neonicatinoid based pesticides are the most widely used commercial family of pesticides, in recent years there has been examinations to identify possible nicotine-like harmful effects upon mammals in chronic application conditions, so as to determine the risk of public health in application of pesticides.A 2015 paper (Ding 2015demonstrated the neonicotinoid Imidacloprid, the most commonly used insecticide in the world, breaks down within mammalian systems to produce highly hydrophilic metabolites. As hydrophilic molecules are more susceptible to bonding with polar elements of globular proteins, such as found within nAChR’s, the binding affinity and resulting chances of toxic interactions merits concern from these findings. The role of polar metabolites arising from the otherwise ionic neonicotinoids presents an additional and intriguing area of study, as biological reactions may turn these insect-targeted compounds into toxins with a high mammalian binding affinities, subsequently leading to accidental neural damage to receptor sites via the nAChR mediated pathway.

4 Replies to “Molecular Basis of Nicotine Toxicity”

  1. Hey Anthony! Well written and well organized. I have a few questions. 1. I noticed that you mentioned the two functions of the acetylcholine mediated channels, one of them being nicotine related and the other being muscarine affected. How similar are the effects muscarine affect channels and nictotine affected channels? Do the receptors differ in structure at all? Are the any potential reasons to focus on muscarine as a field of study?
    2. Have there been in depth studies on the nature of the common genetic mutations you mentioned that effect nicotine addiction?
    3. What do you mean when you say “protein-mediated binding patterns”? Is this a protein mediated binding of nicotine to sense nicotine levels? I think a specific specific example here would help get your point across, that is, if there is research is available to make that claim. A figure in this section would also be helpful.
    Thanks for a great read!

    1. Hey Matt,

      Thank you for pointing out the lack of accessible figures within the paper, I hope the directly embedded figures allow for more understanding of the concepts at question.

      Muscarine affected channels operate in much the same way as nAChR’s: they allow for release of Ach in the event that a muscarine substrate binds to the pocket and allows hyperpolarization of the membrane to occur. The primary difference occurs in the structure used to do so: While nAChR’s utilize a rapid ligand-gated ion channel, mAChR’s use G-coupled protein receptors to manipulate the charge state of the membrane. While Muscarine is an alternative field of study, it is mostly found in mushrooms and is less produced as compared to nicotine-derived compounds. Hence, it is less of a pressing epidemiological concern.

      As of yet, the genetic studies indicate SNP alterations within the nAChR’s themselves, leading to increased hyperpolarization and consequent downstream increases in neurotransmitters such as dopamine and serotonin. These neurotransmitter modifications are what primarily effect the
      addictive states.

      Protein mediated binding patterns is a typo on my part: I instead meant to state substrate mediated binding patterns, as different neonicotinoids possess different binding affinities and consequent activity upon the nAChR complex. Sorry!

  2. Hi Anthony. After reading your page, I have a few questions. 1) I was wondering if the mechanisms by which the body metabolizes alcohol and nicotine overlap. I have observed that consumption of alcohol makes people want to smoke more. Do you have any thoughts about this? 2) Regarding the genetic component of nicotine toxicity, I know that there are genetic predispositions to developing lung cancer. Do you know if there are any correlations between a propensity to developing lung cancer and a propensity to develop a smoking habit? Thanks!

    1. Hi Zach,
      Great question!

      As far as I have been able to discern, there are related pharmacogenetic factors in nicotine and alcohol use, specifically CYP2E1. In a more mechanistic sense, the stimulatory effects of nicotine are oft described as pleasurable in concert with the depressive effects of alcohol: taking a mixture of “uppers” and “downers” if you will.

      In regards to lung cancer, the CHRNA5 gene has been visualized as an oncogene in that regard, and I can hypothesize a role of nicotine in lung cancer by way of oxidative damage occasioning from modification of acetylcholine-regulated actions within respiratory muscles.

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