12.1 Essential ideas

12.1.5 Neuropharmacology (HL)

Neurotransmitters (NTs) are chemicals released at synapses by the pre-synaptic neuron and acting on the post-synaptic neuron

Acetylcholine

  • Found in synapses of the parasympathetic nervous system called cholinergic synapses.

Norephinephrine

  • Found in synapses of the sympathetic nervous system called andrenergic synapses.

Dopamine

  • Associated with the pleasure, motivation, and physical dependence/addiction.

Glutamate

  • An excitatory NT found in many parts of the brain.
  • Its action is inhibited by alcohol.

GABA (gamma-amino butyric acid)

  • An inhibitory NT found in many parts of the brain.
  • It regulates feelings of anxiety.

Serotonin

  • Contributes to mood, appetite, pain, sleep and body temperature control.
  • Low levels are associated with depression.

Excitatory vs inhibitory synapses

Summation is the cellular basis of neural decision-making

  • There are many pre-synaptic neurons connected to a single post-synaptic neuron, as shown in Figure 12.1.5a.
  • Information from one synapse is not enough to trigger an action potential in the post-synaptic neuron.
  • Instead, a nervous impulse is initiated or inhibited depending on the sum of all excitatory and inhibitory neurotransmitters in the post-synaptic neuron.
  • If the sum is greater than the threshold membrane potential, the result is an excitatory post-synaptic potential (EPSP).
  • If the sum is less than the threshold membrane potential, the result is an inhibitory post-synaptic potential (IPSP).

Figure 12.1.5a - Summation of excitatory and inhibitory neurotransmitters in a post-synaptic neuronFigure 12.1.5a - Summation of excitatory and inhibitory neurotransmitters in a post-synaptic neuron

Slow-acting neurotransmitters modulate fast synaptic transmission in the brain

  • When neurotransmitters bind directly to gated ion channels on the post-synaptic membrane, the result is a fast, and short-lived, synaptic transmission in the brain.
  • Sometimes neurotransmitters bind to membrane proteins that subsequently activate second messengers inside the post-synaptic membrane.
  • Second messengers modulate the speed and efficiency of fast synaptic transmission by altering the shape or state of an ion channel. 

Figure 12.1.5b - Slow neurotransmitters affect synaptic transmission through a second messenger.Figure 12.1.5b - Slow neurotransmitters affect synaptic transmission through a second messenger.

The role of slow-acting neurotransmitters in memory and learning

  • Secondary messengers are involved in metabolic cascades that result in the long-term modification of existing proteins or the synthesis of new proteins (by interacting with transcription factors).
  • Experimental evidence demonstrates that synapses are persistently strengthened by high-frequency transmissions. This phenomenon is called long-term potentiation (LTP).

Figure 12.1.5c - Long-term potentiation is a persistent strengthening of synapsesFigure 12.1.5c - Long-term potentiation is a persistent strengthening of synapses.

  • LTP is an example of the structural plasticity of the brain. It has been demonstrated in regions of the brain associated with memory and learning, most notably the hippocampus.

Figure 12.1.5d - Long-term potentiation results in modification of the structures associated with memoryFigure 12.1.5d - Long-term potentiation results in modification of the structures associated with memory 

  • Figure 12.1.5d shows a model for LTP involving the neurotransmitter glutamate and two membrane proteins in the hippocampus.
  • In this model, the increased number of receptors on the post-synaptic neuron explains the increased strength of the synaptic transmission.
  • Many neurobiologists think LTP could be a physiological mechanism for the encoding of memories.

Figure 12.1.5e - Woman with Alzheimer's diseaseFigure 12.1.5e – Woman with Alzheimer's disease
Glutamate is an excitatory neurotransmitter involved in learning and memory. Treatment of Alzheimer’s disease involves drugs that regulate its activity.

Concept help

Fast-acting does not necessarily mean excitatory. GABA is a fast-inhibiting neurotransmitter.

Nature of Science

Assessing risks associated with research: Patient advocates will often press for the speeding up of drug approval processes, encouraging more tolerance of risk.

International mindedness

Attitudes to drugs and the use of drugs differ globally. There are many cultures that use drugs to enhance rituals or religious experiences.

12.1.5f

Figure 12.1.5f – Priest wine

12.1.5gFigure 12.1.5g – Rastafari with ganja

Science and social responsibility (Aim 8)

What are the social consequences of psychoactive drug use and abuse … on the user? his or her family? the wider society?

Course link

In 6.2.5 you learned about cholinergic synapses. These are excitatory synapses.