Neurotransmitters can be classified into two groups according to its chemical structures: small molecular neurotransmitters and neuropeptides. The action and activities of neurotransmitters can be broadly defined as excitatory or inhibitory neurotransmitters. Small molecular neurotransmitters include groups of amino acids neurotransmitters such as glutamate, aspartate, glycine, D-serine, Gamma-aminobutyric acid (GABA). Another group of small molecular neurotransmitters are monoamines or other biogenic amines. Serotonin, norepinephrine, epinephrine, histamine, melatonin are among this group of neurotransmitters. Examples of neuropeptide neurotransmitters are: Beta-endorphin, Opioid peptides, Somatostatin, Calcitonin, Vasopressin, Oxytocin, Glucagon. Different neuropeptides are involved in a wide range of brain functions including learning and memory.

There are four major brain chemicals (neurotransmitters) that are important in cognitive function including memory (see post “brain chemical systems and memory“). Listed in this post are the neurotransmitters that are involved and implicated in the cognitive function:

• Acetylcholine: regulating the activities in certain areas of the brain, which are associated with attention, arousal, learning, and memory. (see post “Acetylcholine Cholinergic System For Cognitive Function“)
• Dopamine: Dopamine influences movement, learning, attention, and emotion.
• GABA: is the major inhibitory neurotransmitter that regulates and balances excitatory neurotransmission. GABA is the calming brain chemical.
• Serotonin: is the neurotransmitter that regulates mood. Because of the interaction between cognitive capacity and psychology. Deficiency in serotonin not only is associated with depression, but also is linked to cognitive function.
• Glutamate: Glutamate is an excitatory neurotransmitter. It is one of the most commonly found neurotransmitters in the central nervous system. Glutamate is mainly associated with cognitive functions (learning and memory). An excess of glutamate is however, toxic for the neurons. (see post “The Role of Glutamatergic Neurotransmission System In Aging Brain“)
• Aspartate: is an excitatory neurotransmitter that stimulates NMDA receptors (a type of glutamate receptor). Aspartate, as a neurotransmitter, is similar in function to glutamate. Although it is not as strongly as glutamate does.
• D-serine: is a physiological co-agonist of the N-methyl D-aspartate (NMDA) receptor ( a type of glutamate receptor). D-serine is a glia-derived neurotransmitter (or gliotransmitter).
• Norepinephrine (Noradrenaline): is an excitatory neurotransmitter which regulates mood, physical and mental alertness.
• Epinephrine (adrenaline): is an excitatory neurotransmitter that controls attention, arousal, cognition, and mental focus.
• Glycine: is the major inhibitory neurotransmitter in the brainstem and spinal cord, where it participates in a variety of motor and sensory functions. Glycine is also present in the forebrain, where it has recently been shown to function as a co-agonist at the N-methyl-D-aspartate (NMDA) type of glutamate receptor. In the latter context, glycine promotes the actions of glutamate, the major excitatory neurotransmitter
• Nitric oxide:  can increase the level of oxygen in the body, and improve memory, learning, alertness, and concentration by increasing the supply of oxygen in the brain.