Brain-derived neurotrophic factor (BDNF) is the most widely distributed neurotrophin in the CNS, where it plays several pivotal roles in synaptic plasticity and neuronal survival. As a consequence, BDNF has become a key target in the physiopathology of several neurological and psychiatric diseases. Recent studies have consistently reported altered levels of BDNF in the circulation (i.e., serum or plasma) of patients with major depression, bipolar disorder, Alzheimer's disease, Huntington's disease and Parkinson's disease. Correlations between serum BDNF levels and affective, cognitive and motor symptoms have also been described. BDNF appears to be an unspecific biomarker of neuropsychiatric disorders characterized by neurodegenerative changes.

This review article gives an overview of a number of central neuro-transmitters, which are essential for integrating many functions in the central nervous system (CNS), such as learning, memory, sleep cycle, body movement, hormone regulation and many others. Neurons use neuro-transmitters to communicate, and a great variety of molecules are known to fit the criteria to be classified as such. A process shared by all neuro-transmitters is their release by excocytosis, and we give an outline of the molecular events and protein complexes involved in this mechanism. Synthesis, transport, inactivation, and cellular signaling can be very diverse when different neuro-transmitters are compared, and these processes are described separately for each neuro-transmitter system. Here we focus on the most well known neurotransmitters: acetyl-choline, catechol-amines (dopamine and nor-adrenalin), indole-amine (serotonin), glutamate, and γ- amino-butyric acid (GABA). Glutamate is the major excitatory neuro-transmitter in the brain and its actions are counterbalanced by GABA, which is the major inhibitory substance in the CNS. A balance of neuronal transmission between these two neuro-transmitters is essential to normal brain function. Acetyl-choline, serotonin and catechol-amines have a more modulatory function in the brain, being involved in many neuronal circuits. Apart from summarizing the current knowledge about the synthesis, release and receptor signaling of these transmitters, some disease states due to alteration of their normal neuro-transmission are also described.