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Noradrenaline

Location, synthesis and release

Noradrenaline is synthesised from tyrosine in several different areas of the brain. It is released from synaptic vesicles into the synaptic cleft  where it can bind to specific noradrenaline receptors on both the pre- and post-synaptic membrane.

Adrenoreceptors and post-synaptic ß1 receptors

There are two main types of noradrenaline receptor  found on the post-synaptic membrane - α₁ and ß₁. The most important is the ß₁ subtype, which has a stimulatory effect on the post-synaptic neuron .

Pre-synaptic α 2 receptors

In contrast to the post-synaptic membrane, the main adrenoreceptor sub-type found on the pre-synaptic membrane is the α₂ receptor. Activation of this receptor has a negative affect, inhibiting the release of noradrenaline from the pre-synaptic neuron.

Pre-synaptic ß2 receptors

Pre-synaptic ß₂ receptor activation results in an increased release of noradrenaline from the pre-synaptic neuron. Low levels of noradrenaline in the synaptic cleft cause its activation.

Noradrenaline degradation

Noradrenaline is either degraded by monoamine oxidase in the pre-synaptic neuron, or by COMT, which is found on the post-synaptic membrane.

Serotonin

Location, synthesis and release

Serotonin (5HT) is synthesised in the pre-synaptic neuron from tryptophan and serotonin nerve fibres innervate most of the brain. As noradrenaline, serotonin is released from synaptic vesicles into the synaptic cleft where it can bind to its specific receptors.

Post-synaptic 5HT1A receptors

The 5HT_1A receptor has a negative effect on the propagation of the nerve impulse. The binding of serotonin to this receptor causes the opening of ion channels and activation of other proteins, resulting in the inhibition of the nerve impulse.

Post-synaptic 5HT2A receptors

As opposed to the 5HT_1A receptor_, activation of the 2A receptor has a stimulatory effect on nerve impulse transmission, but it is also mediated via ion channels and other proteins.

Serotonin degradation

In contrast to noradrenaline, serotonin is degraded by monoamine oxidase in the pre-synaptic neuron only and cannot be broken down by COMT.

Serotonin and mood disorders

Many mood disorders are associated with reduced levels of serotonin. Therapy preventing the degradation of serotonin is often used for the treatment of these affective disorders.