Depression: a short textbook for GP's

2 Pathophysiology of Depression

2.3 Neurotransmitters and Mood

Neurotransmitters, especially noradrenaline and serotonin (also called 5-HT) are believed to be key in the control of mood and emotional behavior. The neurotransmitters we are concerned with - monoamines - all share a similar chemical structure and are also known as biogenic amines. These neurotransmitters include adrenaline, dopamine, serotonin and noradrenaline.

Noradrenaline and the Noradrenergic System

The noradrenaline system is referred to as the noradrenergic system, and receptors specific to noradrenaline or adrenaline are both referred to as adrenergic receptors.

Noradrenaline, which is synthesized from the dietary amino acid tyrosine, is believed to play a major role in the control of mood and emotional behavior.

The major concentration of noradrenergic neurons is in the locus ceruleus in the midbrain. The axons of these neurons project upward through the forebrain to the cerebral cortex, the limbic system, the thalamus, and the hippocampus.

the various parts of the brain along the noradrenergic pathway

Noradrenaline, a key neurotransmitter involved in the control of mood and emotional behavior, is believed to inhibit or stimulate a variety of emotional responses such as anxiety, aggression, stress, and sleep patterns.

Noradrenaline is released by a presynapse, and binds to receptor sites on a postsynaptic neuron. Residual noradrenaline (NE) is taken back up into the presynaptic neuron (reuptake) where it is recycled into storage vesicles or metabolized by the enzyme MAO (degradation).

Serotonin and the Serotonergic System

Like noradrenaline, serotonin (5-HT) is a key neurotransmitter involved in the control of mood. The axons of serotonergic neurons originate in the raphe nuclei of the brain stem and project to the cerebral cortex, the limbic system, cerebellum, and spinal cord.

the various parts of the brain along the serotonergic pathway

Serotonin is involved in the regulation of pain, pleasure, anxiety, panic, arousal, and sleep behavior (the sleep-wake cycle). Serotonin (5-HT) is synthesized from a dietary acid called tryptophan. Like noradrenaline, serotonin is a neurotransmitter. As such, serotonin is released by a presynaptic neuron, travels across a synapse, and binds to a receptor site in a postsynaptic neuron.

Residual serotonin may also be recycled by the presynaptic neuron through reuptake.

Neurotransmitter Receptor Sites

Just as there are different types of neurotransmitters, there are different receptor sites, each with an affinity for a specific neurotransmitter.

In addition to the receptor sites located on postsynaptic neurons in the brain, receptor sites exist on neurons in other parts of the body, such as the gut or salivary glands. (The side effect profile of antidepressant drugs varies with their affinity for these receptors.)

In recent years, many subtypes of these receptors have been discovered. Some adrenergic receptors, for example, are now known to be associated with inhibitory processes while others are stimulatory. While the science of mood disorders is still very incompletely understood, some very useful theories have emerged over the past two decades.

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