Cholinergics 

Acetylcholine 

Monoamines

Norepinephrine 

Dopamine 

Serotonin

Histamine 

Amino Acids

Gamma-aminobutyric acid (GABA) 

Glycine

Glutamate/Aspartate

Neuropeptides

Endorphins/Enkephalins

Substance P 

Somatostatin 

Acetylcholine

Functions: sleep, arousal, pain perception, movement, memory 

Increased levels: depression

Decreased levels: Alzheimer's, Huntington's, Parkinson's 

Norepinephrine

Functions: mood, cognition, perception, locomotion, cardiovascular functioning, sleep and arousal 

Increased levels: mania, anxiety, schizophrenia

Decreased levels: depression 

Dopamine

Functions: movement and coordination, emotions, voluntary judgment, release of prolactin (stimulates milk production after childbirth) 

Increased levels: mania and schizophrenia

Decreased levels: Parkinson's and depression 

Serotonin

Functions: sleep and arousal, libido, appetite, mood, aggression, pain perception, coordination, judgment

Increased levels: anxiety 

Decreased levels: depression 

Histamine 

Functions: wakefulness, pain sensation, inflammatory response 

Increased levels: - 

Decreased levels: depression 

Gamma-aminobutyric acid (GABA) 

Functions: slowdown of body activity 

Increased levels: - 

Decreased levels: Huntington's, anxiety, schizophrenia, and various forms of epilepsy 

Glycine

Functions: recurrent inhibition of motor neurons 

Toxic levels: "glycine encephalopathy", decreased levels are correlated with spastic motor movements 

Glutamate/Aspartate

Functions: relay of sensory information and in the regulation of various motor and spinal reflexes 

Increased levels: Huntington's, temporal lobe epilepsy, and spinal cerebellar degeneration

Decreased levels: - 

Endorphins/Enkephalins

Functions: modulation of pain and reduced peristalsis (enkephalins) 

Substance P 

Function: regulation of pain 

Increased levels: depression 

Decreased levels: Huntington's and Alzheimer's 

Somatostatin 

Function: can stimulate release of dopamine, serotonin, norepinephrine, and acetylcholine, and inhibits release of norepinephrine, histamine, and glutamate 

Increased levels: Huntington's

Decreased levels: Alzheimer's