**3.1 General mechanism of synaptic transmission in the brain**

Neurochemical studies have been performed to clarify the temporal and spatial distribution of substances (i.e., proteins, enzymes, etc.) in brain. In the brain, neuronal cells which are major components of nervous system including brain, spinal cord constitute synapses. When the neurons are electrically excitable, neurotransmitters are released from the terminal of neuronal synapses (i.e., pre-synaptic terminals). On the other hand, the other neurons which receive the neurotransmitters possess receptors (i.e., post-synaptic terminals). The receptors binding to neurotransmtters activate electrically post-synaptic neurons and thereby the neuronal information is processed in the brain (i.e., synaptic transmission). Furthermore, the synaptic transmission is modulated by various chemical mediators (i.e., carbon monooxide (CO), nitric oxide (NO) etc.). These mediators are also critical to induce synaptic plasticity (i.e., plastic changes of electrical strength or efficacy in synaptic transmission). Hence, the neurotransmitters such as glutamate, acethylcholine and monoamine are major neurochemical substances, furthermore, these chemical mediators are necessary for normal synaptic transmission and synaptic plasticity. Aberrant expressions of these neurochemicals impede on the synaptic functions.
