Drug Tolerance and Psychological Dependence

Drug tolerance is when the same dosage of a drug has a reduced effect over time, requiring a higher dose to achieve the original effect.
This is represented formally as a rightward shift in the dose-response curve.
Dose-response curve: Plots the drug dose against its effect. Typically, increasing the dose increases the effect, though with diminishing returns at higher doses.
Drug tolerance shifts the entire dose-response curve to the right, meaning a higher dose is needed for the same effect.

Drug tolerance can be seen as a form of negative feedback, where the body tries to maintain homeostasis.
Homeostasis: Maintaining a stable internal environment with parameters within acceptable limits (e.g., body temperature).
When a drug disrupts homeostasis (e.g., increasing blood pressure), the body employs mechanisms to counter the drug's effects and restore balance.
These mechanisms can include:
- Enhanced drug elimination.
- Changes in receptor numbers (reducing or increasing them).
- Receptor modification (replacing them with less sensitive versions).
- Changes in intracellular processes after receptor binding.
Tolerance is an active response of the body to the drug and develops over time.
Example: Regular ibuprofen use may lead to reduced effectiveness for headaches over time.

When drug use stops, tolerance mechanisms persist, pushing the body in the opposite direction of the drug's effects.
This results in withdrawal effects, which are the opposite of the drug's effects.
- Example: Discontinuing sleeping pills can cause insomnia; stopping pain medication can increase pain; discontinuing antidepressants can cause stronger depressive symptoms.
Withdrawal effects diminish over time as the body readjusts to homeostasis.
Experiencing withdrawal symptoms indicates physical dependence on a drug.
To mitigate withdrawal, drug cessation should be gradual and supervised.
- Example: Using nicotine patches or gum to slowly reduce nicotine intake when quitting smoking.

Tolerance can be classically conditioned to the context in which the drug is taken.
Heroin addicts often associate drug-taking with specific environments or people.
The context becomes a conditioned stimulus that triggers the tolerance response, which are compensatory reactions opposing the drug's effects.
This has implications:
- Overdose: A dose safe in a familiar environment can be lethal in a novel environment where the tolerance response is not triggered.
- Relapse: Entering a familiar drug-taking environment can trigger withdrawal symptoms, leading to relapse even after detoxification.
In summary, tolerance is drug-specific, develops over time, and can be conditioned to the context.

Psychological dependence is rooted in neurobiology.
It relates to operant conditioning, where a behavior is reinforced by a reward, leading to its repetition.
Operant conditioning: A stimulus triggers a behavior, which leads to a reward, reinforcing the behavior.
Evolutionarily, behaviors that lead to essential rewards (food, sex) are reinforced.

Olds and Milner discovered a brain region where electrical stimulation reinforced behavior.
Intracranial self-stimulation: Rats press a lever to receive electrical stimulation in specific brain areas.
Mapping the brain revealed numerous reward locations, but the nucleus accumbens and ventral tegmental area (VTA) are the most crucial.
These areas are part of the mesolimbic dopamine system, which connects the midbrain (mesencephalon) to the forebrain (telencephalon) using dopamine as a neurotransmitter.
The reward system is a subset of this broader dopamine system.
The medial forebrain bundle consists of dopaminergic axons running from the mesencephalon to the telencephalon.
Stimulating this bundle releases dopamine.

Brain development starts with three primary vesicles: forebrain, midbrain, and hindbrain.
The forebrain divides into the telencephalon (cortex and basal ganglia) and diencephalon (thalamus and hypothalamus).
The midbrain remains the mesencephalon.
The hindbrain divides into the metencephalon (pons and cerebellum) and myelencephalon (medulla oblongata).
The adult human brain can be thought of as five major subdivisions from back to front: medulla, pons/cerebellum, midbrain, diencephalon, and cortex/basal ganglia.

Dopamine release in the nucleus accumbens from the VTA correlates with reward.
Experiment: Blocking dopamine receptors in the nucleus accumbens prevents rats from learning to press the lever for stimulation.
Conclusion: Dopamine release from the VTA to the nucleus accumbens is crucial for reinforcement.
Dopamine is not just about pleasure. It might be more strongly associated with wanting and seeking rather than liking.
Dopamine is released during both reward and punishment, suggesting its role in compulsion and information gathering.
Experiment: Dopamine levels increase in male rats in novel environments and during interactions with females.
Dopamine drives exploration (seeking new information) and compels repetition of behaviors, even if they are no longer pleasurable.

Psychological dependence: Drug addicts crave drugs even when they no longer enjoy their effects.
Evolution favors behaviors that increase fitness, regardless of pleasure.
Cocaine and amphetamines increase dopamine levels and are psychologically addictive.
Experiment: Rats self-inject cocaine or amphetamines, leading to increased dopamine levels.
Infusing cocaine directly into the nucleus accumbens reinforces self-administration, indicating a direct effect on the mesolimbic dopamine system.
Cocaine and amphetamines bypass normal reward assessment systems and directly activate dopamine release, creating compulsion.

The nucleus accumbens connection is the key reinforcement system, with dopamine as a crucial underlying neurotransmitter.
This system drives exploration and reinforces behaviors.
Drugs that bypass normal reward pathways and directly activate dopamine release in the nucleus accumbens are highly addictive.
Physical Drug dependence is due to drug tolerance, Drug tolerance is the reason that withdrawal symptoms appear. Easiest response which is to take the drug again to get rid of the withdrawal symptoms.
Psychological dependence, Activation of this ventral tegmental area and nucleus accumbens connection that are involved in this rewarding or seeking system.