Session 8 ILOs - The Autonomic Nervous System (ANS)

Question 1

List the mechanisms by which drugs influence neurotransmission

Answer 1

In cholinergic synapses:

1. Degradation of the transmitter
2. Interaction with the post-synaptic receptors
3. Inactivation of the transmitter
4. Reuptake of the transmitter
5. Interaction with pre-synaptic receptors

In adrenergic synapses:

1. Metabolism enzymes (monamine oxidases and catechol-O-methyltransferase)
2. Post-junctional adrenoceptors

Question 2

Describe the major drug classes that exert all, or a part of their actions through altering ANS function

Answer 2

Cholinergic drugs:

• Acetylcholinesterase targeting drugs e.g. pyridostigmine for myasthenia graves or Donepezil to treat Alzheimer’s disease
• Muscarinic Ach receptor agonists e.g. Pilocarpine for glaucoma
• Muscarinic Ach receptor antagonists e.g. Ipratropium and tiotropium for some forms of asthma and COPD

Adrenergic drugs:
- Alpha1-adrenoceptor selective antagonists e.g. doxazosin in hypertension

• Beta1-adrenoceptor selective antagonists e.g. atenolol in hypertension
• Beta2-adrenoceptor selective agonists e.g. salbutamol in asthma

Question 3

Compare and contrast the following in cholingeric and adrenergic synapses:

• Neurotransmitter synthesis
• Neurotransmitter storage
• Neurotransmitter release
• Signal termination

Answer 3

Neurotransmitter synthesis:
- Cholingeric neurotransmitters are synthesised in the pre-synaptic portion, whereas adrenergic neurotransmitters are made in the nerve varicosities (adrenergic neurotransmitters can also be synthesised from other pre-cursors ‘on site’ from e.g. tyrosine)

Neurotransmitter storage:
- Both cholingeric and adrenergic neurotransmitters are stored in vesicles

Neurotransmitter release:
- Both cholingeric and adrenergic neurotransmitters are released upon the depolarisation of the neurone, however in cholingeric neurotransmitters, release is triggered by Ca2+, not the depolarisation itself

Signal termination:
- Cholingeric neurotransmission is terminated by the breakdown first (by acetylcholinesterase), then can be re-taken up by the nerve, whereas adrenergic neurotransmitter is terminated by the high affinity reuptake mechanism for NA direct reuptake