Pharmacodynamics of Tobacco and Nicotine

The primary focus is on nicotine, which is the only active ingredient in tobacco, while other components are largely toxic and lack significant pharmacodynamic mechanisms.

Chemical Structure: Nicotine contains two nitrogen atoms that can be ionized independently.
Isomers:
- L-Nicotine: The dominant isomer found in tobacco, likely used by the plant as an insecticide or insect repellent to deter herbivores.
- D-Nicotine: Present possibly as a byproduct of an inefficient synthesis, with no clear purpose in tobacco.
Physical State: Nicotine is a liquid at room temperature and is approximately 25% ionized in blood.
- Ionization of components is influenced by drying processes and additives in tobacco products.

At physiological pH, about 25% of the nitrogen in the five-membered ring is ionized, while virtually 0% of the nitrogen in the six-membered ring is ionized.

Other alkaloids can also act as agonists for the nicotinic acetylcholine receptor (nAChR):
- Cytosine: Derived from a plant.
- Anatoxin: A toxin from a prokaryotic organism, known as very fast death factor, dangerous to animals ingesting it.
- Epobetidine: Derived from a species of frog.

Abbreviated as nAChR or read as natur by some researchers.
Functional Characteristics:
- Type of Receptor: Ligand-gated ion channel.
- Physiological Role: Responds primarily to acetylcholine but also to nicotine.
Comparison with Muscarinic Receptors:
- Muscarinic acetylcholine receptors respond to the endogenous ligand muscarine, not nicotine.

The nAChR is composed of five subunits, arranged in a circular or pentagonal shape:
- Subunit types include alpha, beta, gamma, delta, and epsilon.
- Subtypes of alpha and beta exist (Alpha 2-9, Beta 2-4).
CNS vs. PNS Variations:
- CNS: Primarily composed of alpha4 and beta2 subunits (2 alpha4, 3 beta2) and alpha7 receptors.
- PNS: Includes additional forms such as alpha3 and beta4.

Involved in autonomic nervous system transmission and muscle contractions at the neuromuscular junction.
Associated Diseases: Linked to various disorders like Alzheimer's, Parkinson's, Tourette syndrome, and schizophrenia.
The alpha4 beta2 receptor is particularly associated with nicotine addiction.

Each subunit has four transmembrane domains, conferring a well-buried ion channel within the membrane.
Specific regions contain cysteine residues which create binding pockets for nicotine and acetylcholine.

Activation process involves binding leading to conformational changes, opening the ion channel and allowing sodium to flow in and potassium to move out.
Affinity and Potency of different agonists:
- Affinities: Low nanomolar affinity for nicotine and acetylcholine.
- To open the nAChR, low micromolar concentrations of agonists are typically required.
Desensitization states exist alongside resting states, with nicotine primarily promoting desensitization after activation.

Sympathetic Nervous System Activation: Nicotine leads to the release of catecholamines (adrenaline, noradrenaline), stimulating the fight or flight response, which results in:
- Increased blood pressure.
- Increased heart rate.
Stomach Effects: Increases gastric acid secretion, raising the risk of ulcers.
CNS Stimulation: Causing effects such as:
- Enhanced concentration and potential intellectual benefits.
- Tremors and emotional blunting.
Respiratory Effects: Increased respiration as part of the fight or flight response.
Overdose Symptoms:
- Initial stimulation followed by respiratory depression, vomiting, and potential cardiovascular emergencies (e.g., cardiac arrest, ruptured blood vessels).
Hormonal Impact: Decrease in estrogen levels has been noted, potentially leading to early menopause in women.

In summary, nicotine has significant pharmacodynamic effects impacting several physiological systems, leading to both stimulatory and potentially harmful effects, including addiction and various health complications.
Future discussions will delve into pharmacokinetics and further exploration of tobacco effects on health.