The difficulty in studying neurotransmitter function in psychiatric patients is legion, despite the initial optimism that surrounded the studies of amine metabolites and neurotransmitter receptors in the brains of suicide victims. It is now generally accepted that factors such as the contribution of amine metabolites from the spinal cord, and the lack of diagnostic certainty in the case of suicide victims, raises serious difficulties when attempting to define the relationship between changes in neurotransmitter function and the symptoms of a psychiatric or neurological disease.
It is against this background that pharmacologists have become interested in the blood platelet as a peripheral model of the nerve terminal. The platelet has many important features in common with the nerve terminal. Thus (a) both the neuron and the platelet contain mitochondria and dense core vesicles in which transmitters are stored. (b) In both cases, the mitochondria provide energy for intracellular functions and also contain monoamine oxidase that catabolizes any free monoamines. (c) The vesicle acts as a storage site for serotonin in the platelet, and most transmitters in addition to serotonin in the neuron, and the release of the amines following activation of the platelet or neuronal membrane is facilitated by a calcium dependent excitation-excretion coupling mechanism. (d) Both platelets and neurons are of the same ectodermal origin and contain neuron specific enolases that, apart from platelets, are only found in nervous tissue. (e) Both platelets and neurons contain functional neurotransmitter and neuromodulator receptor sites on their outer membranes.
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