Tobacco Pharmacodynamics

Active vs. Inactive Ingredients: Nicotine is identified as the only primary active ingredient in tobacco. This means it's the main substance in tobacco that affects the body in a significant way. Other ingredients found in tobacco are considered generically toxic; they are reactive molecules that cause damage and "havoc" in the body without a specific pharmacodynamic mechanism associated with a particular receptor. These other ingredients can lead to various harmful effects but do not act through specific pathways like nicotine does.

Molecular Composition: The nicotine molecule contains two nitrogens that can be ionized independently of each other. This property affects how nicotine interacts with our body.

• Isomers: Nicotine exists in two isomeric forms: L-nicotine and D-nicotine.

  • L-nicotine: This is the predominant form found in tobacco and is responsible for most of the effects we associate with smoking or using tobacco.

  • D-nicotine: This form is likely a byproduct of a slightly inefficient synthesis process and does not serve a specific known purpose in the tobacco plant. This inefficiency shows how natural processes can lead to the production of substances that might not have a direct role in the plant’s survival.

Biological Purpose in Plants: Tobacco plants produce nicotine as a defense mechanism, specifically acting as an insecticide or insect repellent. This means that nicotine helps the plant to survive by deterring insects from eating it. Insects that attempt to eat the plant ingest the nicotine, which causes negative physiological effects or death. This highlights a fascinating example of how plants evolve to protect themselves from herbivores.

Physical State: Nicotine is a liquid at room temperature. This liquid state is important because it allows nicotine to be easily absorbed into the body when tobacco is smoked or chewed.

Ionization Characteristics: In the blood (physiological pH), nicotine is approximately $25\\%$ ionized.

• The molecule consists of a five-membered ring and a six-membered ring, each containing a nitrogen atom. This structure is crucial for how nicotine interacts with receptors in the body.

• The nitrogen on the five-membered ring ionizes more easily. At physiological pH, approximately $25\\%$ of these nitrogens are unionized. This means that a percentage of nicotine molecules carry a charge, which is important for their interaction with receptors.

• The nitrogen on the six-membered ring is essentially $0\\%$ ionized at physiological pH, indicating that it does not usually participate in receptor interactions at normal body pH levels.

Nicotinic Alkaloids: Several other molecules produced by different organisms act as agonists for the nicotinic acetylcholine receptor, often as defense mechanisms. This shows that nicotine's effect is mirrored in nature by other similar molecules.

• Cytosine: An alkaloid derived from another plant species.

• Anatoxin: Produced by a prokaryotic organism (bacteria). It is extremely toxic to animals that ingest the bacteria.

  • Alternative Name: Known as "Very Fast Death Factor." This highlights the level of danger that some natural substances can possess.

• Epibatidine: An alkaloid derived from a species of frog, which demonstrates how different species have evolved similar chemical defenses.

The Nicotinic Acetylcholine Receptor (nAChR):

• Terminology: The abbreviation nAChR is colloquially pronounced as "natcher" by some researchers, which might help students remember the term more easily.

• Identity and Evolution: Physiologically, it is an acetylcholine receptor. It did not evolve to respond specifically to nicotine, as nicotine is not an endogenous molecule. In other words, our body did not originally make nicotine; it was produced by plants. Nicotine was developed by plants to target and attack organisms that possess these acetylcholine receptors. This information might be helpful when thinking about biological interactions in ecosystems.

Classification: It is a ligand-gated ion channel. This means that certain molecules (ligands) must bind to it for it to open and allow ions to flow through.

• It is distinguished from the muscarinic acetylcholine receptor, which responds to the exogenous agonist muscarine but not to nicotine. This indicates different receptors can respond to different compounds.

Mechanism of Action: When a ligand (acetylcholine or nicotine) binds, the channel opens. This is how messages are sent across nerve cells.

• It functions primarily as a sodium channel ($Na^+$).

• Activation allows $Na^+$ to rush into the cell, resulting in a depolarization event. A depolarization event is a change in the electrical charge of the cell, essential for nerve signal transmission.

• Activation also allows potassium ($K^+$) to move out of the cell, helping balance the charge.

Pentameric Structure: Every nAChR is composed of five subunits arranged in a circular or pentagon shape around a central pore (ion channel). This unique structure allows for various functional properties.

• Subunit Varieties: There are different types: alpha ($\alpha$ ), beta ($\beta$), gamma ($\gamma$), delta ($\delta$), and epsilon ($\epsilon$).

  • Subtypes: Subunits are further categorized (e.g., $\alpha_2$ through $\alpha_9$ and $\beta_2$ through $\beta_4$), making it a complex family of receptors that have different roles in the body.

Single Subunit Features: Each individual subunit contains four transmembrane domains, anchoring the protein securely in the cell membrane. This helps ensure that the receptor remains properly positioned to interact with other molecules.

• Binding Pocket: Alpha-type subunits contain two specific cysteine residues at positions $192$ and $193$. These cysteines have sulfur groups that create the binding pocket for ligands. The binding pocket is essential for the receptor's ability to respond to nicotine and acetylcholine.

Regional Receptor Types:

• Central Nervous System (CNS): Predominantly features the $\alpha_4\beta_2$ receptor type (composed of two $\alpha_4$ and three $\beta_2$ subunits). Also includes the $\alpha_7$ receptor (homomeric, consisting of five $\alpha_7$ subunits).

• Peripheral Nervous System (PNS): Features forms such as $\alpha_3$, $\alpha_5$, and $\beta_4$, as well as variable $\beta$ subunits.

• Role in Disease: nAChRs are implicated in Alzheimer's disease, Parkinson's disease, Tourette's syndrome, and schizophrenia, illustrating the importance of these receptors to health.

• Addiction: The $\alpha_4\beta_2$ subtype is the specific receptor responsible for addiction, which might help explain why tobacco is so addictive.

Binding Site Specifics: Binding sites for nicotine and acetylcholine are generally present only on certain alpha subunits ($\alpha_3$, $\alpha_4$, $\alpha_7$). Notably, the $\alpha_5$ subunit does not possess a binding site. This specificity of binding sites shows how precise interactions at the molecular level can affect overall health outcomes.

Affinity Levels: Nicotine and acetylcholine demonstrate low nanomolar affinity, representing a very tight binding event. This high affinity is why nicotine can have strong effects even in very low concentrations.

Potency for Activation: To actually cause the channel to open, concentrations generally need to reach low micromolar levels.

Dynamic Conformational States:

• Resting State: The channel is closed but ready to open if a ligand binds. In this state, it is prepared and waiting for the right molecule to come along.

• Active State: The channel is open, allowing ion flow, demonstrating its crucial role in transmitting signals.

• Desensitized State: The receptor has a very high affinity for nicotine, but the channel is closed and unresponsive. Receptors often move from the active state to the desensitized state upon nicotine binding. There are various forms of desensitization; some can return to resting quickly, while others move toward an inactive form, showcasing the complex behavior of these receptors.

• Inactive Form: The receptor is "shut off" for a period. Even if nicotine leaves the binding site, the receptor remains in this inactive configuration. This means that even in the absence of nicotine, the receptor can remain non-functional for some time.

Physiological and Toxicological Effects:

• Catecholamine Release: Nicotine triggers the release of adrenaline and noradrenaline, initiating a fight-or-flight response. This causes physical responses like increased heart rate and energy levels.

• Cardiovascular Impacts: Increased blood pressure and heart rate can have long-term damaging effects on the heart and vessels.

• Gastrointestinal Effects: Increases gastric acid secretion, which elevates the risk of developing ulcers.

• CNS Stimulation: At lower doses, it can increase concentration and blunt certain emotions. However, overstimulation leads to tremors, indicating the fine line between stimulation and over-stimulation.

• Metabolism and Appetite: Nicotine increases the body's metabolic rate while simultaneously decreasing appetite and food intake, explaining why some use tobacco to manage weight.

• Hormonal Interference: It lowers estrogen levels, which can lead to early onset of menopause in women.

• Overdose and Poisoning:

  • Early Symptom: Vomiting is a hallmark sign of nicotine poisoning, showing how sensitive our bodies are to this chemical.

  • Cardiovascular Emergency: Extreme heart rate and blood pressure can lead to cardiac arrest or ruptured blood vessels, highlighting the severe risks of nicotine overdose.

  • System Shutdown: Neurons initially experience a rush of activity (depolarization), but eventually, the receptors enter desensitized or inactive states, causing the neurons to effectively shut off. This demonstrates how powerful and potentially harmful nicotine can be.

  • Lethality: Because nAChRs control muscle contractions, including those of the diaphragm, a severe overdose causes respiratory depression (stopping of breathing) and death. This exemplifies the critical role of proper receptor function in maintaining life.