Neurotransmitters in the human brain Miklós Palkovits Research Group of Neuromorphology, Hungarian Academy of Sciences - Semmelweis University and National Institute of Mental Health, Bethesda

During the past decades, a great variety of neurotransmitters and receptors has been organized, isolated, characterized and localized in the human central nervous system. Instead of the neuroanatomical or neurochemical description of individual neurotransmitters, some of their common features, their characteristics and their possible functional role are summarized here:

1) On the base of their chemical nature, neurotransmitters can be classified as biogenic amines, transmitter amino acids, acetylcholine and neuropeptides. 2) Neurotransmitters are present in all brain areas but there are high variations regarding their concentrations and cell/synaptic densities. 3) The degree of the convergence of nerve fibers with various neurotransmitters in brain regions is very high. None of the cell groups in the brain receives single input, various fibers of different origins from terminal networks there. A similar degree of neuronal divergence has been revealed by immunohistochemistry: a single neuropeptidergic neurons can innervate hundreds, on aminergic neuron thousands, of other neurons. 4) During the past decades, it has become evident that we can not find any homogeneous cell groups in the brain that contain only a single neurotransmitters: neurotransmitters are co-localized in brain nuclei. It is hard to find any neuronal cell that may express only one neurotransmitter: neurotransmitter is synthesized in the same neuron (co-expression). 5) Depending on the site of action, neuropeptides can act as neurotransmitters, neurohormones or both.

Some of the neurotransmitters act at presynaptic levels, stimulating or inhibiting the release of other neurotransmitters, they are considered to have neuromodulator activity.

The increasing arsenal of powerful new techniques, such as subtractive hybridization approach, orphan receptor strategy, combinatorial chemistry and molecular modeling, helps researcher to identify new neurotransmitters.