những phân tử này có thể có chức năng kép như cả hai chất dẫn truyền thần kinh và nội tiết tố: ví dụ, adrenaline và noradrenaline không phải là chất dẫn truyền thần kinh mà còn kích thích tố, phát hành từ tủy thượng thận, histamine là một nội tiết tố địa phương, | 20 Part I Drugs and Their Actions these molecules can have dual functions as both neurotransmitters and hormones for example adrenaline and noradrenaline are not only neurotransmitters but also hormones released from the adrenal medulla histamine is a local hormone a mediator of inflammation thyroliberin stimulates the release of thyroid-stimulating hormone from the anterior pituitary gland and vasopressin is a hormone released from the posterior pituitary gland. In addition the amino acids aspartate glutamate and glycine are found in proteins and neuropeptide transmitters . aspartate in cholecystokinin-8S glutamate in neurotensin and glycine in methionine-enkephalin . Up until the discovery of the neuropeptides scientists believed that a single neuron could only contain one neurotransmitter - Dale s law. However it became apparent that neuropeptides and amines could co-exist as co-transmitters in the same neuron with the neuropeptide modulating the release of the amine. Here the neuropeptides are referred to as neuromodulators and in most cells they inhibit the release of an amine for instance neurotensin inhibits the release of dopamine from certain forebrain neurons. Unlike many chemicals in the brain neurotransmitters are not homogeneously distributed but concentrated in certain regions. For example almost two-thirds of the dopamine in the brain is found in the bilateral nigrostriatal mesostriatal tract pathway where the neuronal cell bodies are located in the substantia nigra and the axons terminate in the corpus striatum. When over 85 of these dopaminergic neurons are lost the characteristic motor dysfunction of Parkinson s disease is seen. Neurotransmitters are synthesised at the point of use with the biosynthetic enzymes being located in the synaptic bouton. The enzymes can be identified when looking at neurotransmitter pathways by the fact that they usually end in the suffix -ase for example choline-O-acetyltransferase choline acetylase is necessary for
