Amygdalic levels of dopamine and serotonin rise upon exposure to conditioned fear stress without elevation of glutamate.

Conditioned fear is an artificial stress, induced by a stimulus, such as a tone, that does not elicit fear in nature. This fear response is acquired by experimental animals when tone is combined with an unconditioned stimulus, such as electrical foot shock. The amygdala is considered to be the area involved in acquisition, consolidation and recall of fear. A series of previous pharmacological studies showed antagonists of dopamine D1 and D2, glutamate N-methyl-D-asparatate and (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors to prevent the acquisition of conditioned fear. However, little is known about the types of neurotransmitters released when conditioned fear is acquired and recalled. The present study was designed to continuously monitor changes in extracellular levels of glutamate, dopamine and serotonin in the amygdala, at the acquisition of conditioned fear on Day 1 and at fear recall in response to a tone as a conditioned stimulus on Day 2, using the in vivo microdialysis method. Glutamate was elevated only on Day 1, while dopamine and serotonin rose on both days. The periods of elevated dopamine and serotonin were longer on Day 1 than on Day 2. These results suggest that greater amounts of glutamate, dopamine and serotonin are necessary for acquisition than for recall of conditioned fear.