Drug addiction

Drugs

• chemical substances which interact with the biochemistry of the body

• Inhibit or reinforce enzyme activity

• Block or activate receptors

• Interact with neurotransmitters/ hormones in other ways

• Attack ‘invaders’ e.g. antibiotics

Psycho-active drugs

• chemicals that influence the way we feel/ act

• Usually interact with the nervous/ endocrine system

• Mostly act at synapses

Agonist vs antagonist drugs

• agonist drugs mimics the action of neurotransmitters either at a synaptic level (effecting the post-synaptic cell) or effect receptors - this can cause the postsynaptic neuron to signal to stop sending neurotransmitters even though none were working causing an antagonistic effect on the synapse

• Antagonistic drugs effect the same areas but block the action of neurotransmitters by binding to receptors so other neurotransmitters can’t bind or change the structure of the receptor to prevent neurotransmitters binding. - acting as an agonist in the synapse as blockage of neurotransmitters will cause signals to naturally raise the release of neurotransmitters.

intake of drugs

• Digestive tract e.g. pill (slowest route)

• Respiratory tract e.g. inhalation/ smoked

• Skin e.g. patch

• Mucous membranes e.g. cocaine (through nose)

• Intravenous Injection (directly into the blood) - fastest route

• Intramuscular injection (into muscles) e.g. vaccine

• Subcutaneous injection (under the skin)

• Routes chosen will depend on the drug and how quickly you want to feel the effects - the aim is to get into the bloodstream so it can reach the brain in order to distribute itself

Distribution of drugs

• through blood as it travels all around the body

• Water soluble molecules can be directly dissolved in the blood but doesn’t pass through cell membranes

• Lipid (fat) soluble molecules need carriers to transport them through blood but can pass directly through cell membranes

Blood brain barrier

• capillaries in the brain have no gaps like they do in the rest of the body so substances (water soluble drugs) cannot freely pass in and out of the bloodstream when in the brain but lipid soluble drugs can

• Most psychoactive drugs are fat soluble as this affects the brain

Elimination of drugs

• all drugs are eventually eliminated from the body (easier for water soluble drugs) either by chemical breakdown (enzymes) or excretion (urine)

• Lipid soluble drugs are stored in the body for a much longer time (within fat tissue)

• Biological half life (time from peak concentration to half peak) can vary from minutes to weeks - shorter half life = quicker elimination (lipohilic drugs have a longer half life

Psychical dependence: drug tolerance

• exists with every drug but in different forms

• Drug tolerance is a shift in the dose-response curve meaning intolerant subjects wont have the same effect with the same dose so a greater dose would be required to have the same effect

• Impacts metabolic tolerance (become better at eliminating the drug) making the drugs have less of an effect

• Over time, impacts functional tolerance: causing changes in receptor numbers, sensitivity and intracellular cascades

Withdrawal

• adaptive neural changes that produce tolerance by counteracting the drug effect

• If there is no drug intake to counteract this, the neural adaptations produce withdrawal effects opposite to the effects of the drug

• The easiest way to prevent withdrawal effects is to keep taking the drug → dependency/ addiction

Classical conditioning and drug tolerance

• sight of syringe = neutral stimuli

• Primary effects of drug on neurons in brain = unconditioned stimulus

• Compensatory reactions that oppose the primary effects of drug = unconditioned response

• Once pairing the neutral stimulus (sight of drug) with the effects of drugs it becomes a conditioned stimulus and automatically produces the compensatory/ withdrawal response (conditioned response) without needing to actually take the drug

• Overdosing is easier in novel surroundings

• Withdrawal symptoms can occur in familiar settings through association → relapse

Operant conditioning and drug addiction

• Stimulus (sight of needle) triggers out perceptual system which triggers our motor system which leads to the behaviour

• Behaviour produces a reinforcing stimulus (taking drug leads to good feeling) which creates a reinforcement system that strengthens the connection between the perceptual and motor systems

• Continued reinforcement/ taking of the drug will lead to tolerance which leads to withdrawal

Intra-cranial self stimulation

• we can map out the brain by using electrodes to identify what areas are active during stimulation

• If stimuli is rewarding we can see where this happens in the brain and use the electrodes to stimulate those parts of the brain to teach reinforcement to rats

• Once sites are stimulated → rewarding feeling

Brain areas involved in self-stimulation/ reward in rats

• telancephalon: contains nucleus accumbens (requires little stimulation to trigger reward)

• Mesencephalon: contains ventral tegmental area (requires little stimulation to achieve reward)

Dopamine and reward

• Allyn and bacon found when rats pressed the lever to cause electrodes to stimulate the nucleus accumbens → increased dopamine levels - suggesting a correlation with dopamine and reward

• Stellar et al wanted to test if this correlation can be changed to cause and effect by stimulating the dopaminergic neuron in the ventral tegmental area but used a dopamine receptor blocker in the nucleus accumbens and found a decreased effect of reinforcing brain stimulation

• However dopamine isn’t necessarily linked to pleasure as research has found it is also released during punishing stimuli as well, overtrained rats don’t release dopamine upon reward and dopamine blockers made rats work less hard for food but they still enjoyed it

Dopamine and seeking/ wanting - an alternative hypothesis (pfaus)

• Found when placing male rats alone in a new environment their dopamine levels started to rise when there wasn’t anything rewarding present - suggests the desire to explore increases dopamine

• Dopamine levels peaked when a female rat was introduced into the environment due to the male wanting her

• Suggests dopamine is more related to wanting/ exploration rather than the reward itself

Psychological dependence

• when drugs directly interact with the brains reward system

• Addicts will crave the drugs even whilst disliking their effects - suggests drugs aren’t necessarily a rewarding/ reinforcing stimulus to us but because they directly interact with or brains reward system we still crave it

Conclusion

• different drugs have different pharmacokinetics

• Physical drugs dependence is due to drug tolerance

• The ventral tegmental area and nucleus accumbens are involved in the reward/ seeking system in the brain where the reinforcement system is maintained through dopamine

• Psychological dependence works through this mesotelencephalic dopamine pathway