Exogenous chemicals are substances that enter an organism from an external source and can interfere with the way neurons process signals from neurotransmitters

• Examples of exogenous chemicals include medications, pollutants, pesticides, food additives, narcotics and toxins in venoms
  

Neonicotinoids

• Neonicotinoids are pesticides that are structurally similar to acetylcholine and can bind irreversibly to cholinergic receptors

• Acetylcholine is a neurotransmitter released at neuromuscular junctions and triggers the contraction of muscles

• Neurons produce an enzyme (acetylcholinesterase) that breaks down acetylcholine in the synapse to prevent overstimulation 

• Neonicotinoids bind to the acetylcholine receptor but cannot be broken down by acetylcholinesterase

• This results in the continued depolarisation of post-synaptic neurons, leading to fatal convulsions and paralysis
  

• Insects have a much higher proportion of cholinergic receptors than mammals, making neonicotinoids an effective pesticide
  

Cocaine

• Cocaine is a stimulant drug that acts on the mesolimbic pathway of the brain to trigger a sensation of euphoria

• Cocaine binds to and blocks the dopamine reuptake pumps on pre-synaptic neurons in the dopamine reward pathway

• This causes dopamine to accumulate within the synapse and continue to stimulate the mesolimbic system