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.
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 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.
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 2015) demonstrated 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.