12.2 Applications and skills

12.2.5 Drugs and addiction (HL)

Figure 12.2.5a – Psychoactive drugs affect post-synaptic transmission.Figure 12.2.5a – Psychoactive drugs affect post-synaptic transmission.

Psychoactive drugs affect the brain by either increasing or decreasing post-synaptic transmission. This is achieved by:

  • affecting the concentration of neurotransmitter in the synaptic cleft (e.g. amphetamines)
  • blocking re-uptake of neurotransmitter at the pre-synaptic neuron (e.g. cocaine)
  • mimicking neurotransmitters or binding to receptors on the post-synaptic neuron (e.g. benzodiazepines)
  • affecting release of neurotransmitters from the pre-synaptic neuron (e.g. THC).

Application: Effect of stimulants and sedatives on the nervous system

Match the drug from the left column with its mode of action and effect on the nervous system from the right column. Check your answers here1F, 2C, 3A, 4E, 5B, 6D.

1 Cocaine

A Stimulant

  • Binds to acetylcholine receptors of the post-synaptic membrane
  • Increases post-synaptic transmission for a short time

2 Metham-
phetamine

B Sedative

  • Binds to cannabinoid receptors in pre-synaptic membrane
  • Prevents release of inhibitory neurotransmitters that normally prevent dopamine from entering the synaptic cleft

3 Nicotine

C Stimulant

  • Mimics dopamine and is taken into the pre-synaptic neuron through transporter proteins
  • Causes increased dopamine release into the synaptic cleft and post-synaptic excitation

4 Benzo-
diazepenes,
e.g. valium

Sedative

  • Decreases action of glutamate (an excitatory neurotransmitter)
  • Increases the action of GABA (an inhibitory neurotransmitter)

5 Tetra-
hydro-
cannibinol
(THC)

E Sedative

6 Alcohol

F Stimulant

  • Blocks transporter proteins (involved in the re-uptake of dopamine) on the pre-synaptic neuron
  • Continuous excitation of the post-synaptic neuron results from the accumulation of dopamine in the synaptic cleft
  • Addiction can be affected by genetic predisposition, social environment and dopamine secretion.

Application: Effect of anesthetics on awareness

  • Anesthetics act by interfering with neural transmission between areas of sensory perception and the CNS.
  • Local anesthetics work by stabilising the membrane proteins to prevent Na+ ions from entering the neuron and causing depolarisation.

Figure 12.2.5b – Anesthetics interfere with neural transmission to the CNS.Figure 12.2.5b – Anesthetics interfere with neural transmission to the CNS.

  • General anesthetics are used with the aim of inducing sleep and amnesia so that a medical intervention can be performed on a patient.
  • Precise doses of different drugs are used to induce the level of consciousness appropriate for each procedure.

Application: Endorphins act as painkillers

  • Endorphins are a class of neuropeptides that function in neural pathways associated with pain and emotions.
  • Endorphins are released by the pituitary gland during exercise, excitement or when experiencing pain. They are known to have analgesic effects.

Skill: The impact of MDMA on serotonin and dopamine metabolism

Researchers assayed the extracellular fluid from the nucleus accumbens of living rats that had been injected with one of three psychoactive drugs. The results showing concentrations of neurotransmitter at different times are set out in Figure 12.2.5c. 

Figure 12.2.5c

Figure 12.2.5c – Effect of psychoactive drugs
Effect of psychoactive drugs on serotonin and dopamine metabolism (source: NIDA accessed at: drugabuse.gov

  1. Explain the trend in neurotransmitter concentration after injection with psychoactive drugs.
  2. Compare the effect of MDMA on serotonin levels to its effect on dopamine levels.
  3. Calculate the increase in serotonin concentration in rats 20 minutes after injection with 1.0mg mephedrone/kg body mass.
  4. Evaluate the hypothesis that the three drugs cause similar psychoactive effects.

Addiction can be affected by dopamine secretion, social environment and genetic predisposition

  • Dopamine secretion: Most drugs that cause addiction reinforce the reward pathway involving dopamine. As dopamine receptors are repeatedly stimulated, the user needs more of the drug to get the same pleasurable effect.
  • Social environment: Addiction is more prevalent in some social environments, based on social or economic factors. Certain drugs may be more socially acceptable in certain cultures or environments. Peer pressure is also a factor.
  • Genetic predisposition: There is correlative evidence for a genetic predisposition to addiction: family members of addicts are more likely to become addicts. It is possible that any polymorphism in genes of the dopamine reward pathway may be involved in predisposing some people to addiction – but no clear links have been found so far.

12.2.5d – Fight or flightFigure 12.2.5d – Fight or flight
Stimulant drugs mimic the effect of the sympathetic nervous system. Aggression, fear and anxiety are common physiological effects of the sympathetic ‘fight or flight’ response.

Figure 12.2.3e – Epidural

Figure 12.2.5e – Epidural
An epidural is an anesthetic injection given to women to dull the pain of childbirth. The needle must be administered by a trained professional.

12.2.5fFigure 12.2.5f – 'Bath salts'
MDMA is also known as ecstasy. Mephedrone (shown above) and methylone are sometimes called ‘bath salts’ because of their similarity to magnesium sulphate bath salts.

Concept help

The nucleus accumbens is the part of the brain associated with reward and pleasure. See 12.2.2.