Changes in Neurotransmitter Extracellular Levels during Memory Formation

Excerpt

The current challenge of studying the mechanisms involving memory formation requires the possibility of capacity to analyze in the different brain regions that comprise the neuronal system and circuits underlying the different stages of memory. A timely detailed description of the dynamics of neurotransmitter release can and does provide information on the different areas of the brain, the chemical mechanisms involved, and their levels of participation during different physiological processes. Accordingly, the possibility of correlating behavior with changes in the extracellular level of neurotransmitters in CNS regions involved in information transmission and modulation is a great advantage in the study of memory formation. Also, the knowledge of the neurotransmitters released in different brain areas may result in the identification of important pharmacological targets.

In this regard, in vivo microdialysis is a well-established method for monitoring the extracellular levels of neurotransmitters in the CNS. This technique has been used extensively in neuroscience for almost 30 years. Microdialysis allows online estimates of neurotransmitters in living animals and is a suitable method for monitoring the extracellular levels of neurotransmitters during local administration of pharmacological agents. Different doses of a drug or a combination of agonists and antagonists can be administered in the same experiment without adding any fluid to extracellular spaces. Older alternative in vivo methods for the study of neurotransmitter release are the push–pull technique used in the brain, spinal cord, and intrathecal space.

Currently, measuring the changes in neurotransmitter extracellular levels in discrete brain areas is considered an important tool for identifying the neuronal systems involved in specific memory processes. Several neurotransmitters including acetylcholine (ACh), glutamate, γ-amino-butyric acid (GABA), and catecholamines have been investigated in a variety of memory models, with considerable evidence of extracellular level variations that correlated with changes in neuronal activity during memory formation.

This chapter summarizes and discusses the results obtained from investigating changes in ACh, glutamate, GABA, dopamine, and noradrenaline release during exposure to novel stimuli and performance of several kinds of long-term memory tasks such as operant and spatial memory tasks, and during taste recognition memory.