Functional Anatomy and Evolution of Monoaminergic Systems

1. The organization of brain neuronal systems utilizing monoamines (primary techolamines and serotonin) as neurotransmitters and/or neuromodulators, has been studied in representatives of each major vertebrate class using fluorescence histochemical methods. This long-term investigation has led to the following conclusions: 2. (1) The monoaminergic neurons seem to have occurred early in phylogeny. These neurons, particularly the serotoninergic systems, are already well-developed in the brains of cyclostomes (lampreys) and primitive bony fishes. 3. (2) The monoaminergic cell bodies are especially abundant within the hypothalamus in anamniotes whereas they predominate at brainstem levels in amniotes. However, a similar pattern of distribution of monoaminergic cell bodies has been found in the brainstem of all vertebrates. 4. (3) The monoaminergic neurons may act as an important informative link between the cerebrospinal fluid and the neural tissue in all vertebrates through processes contacting the ventricular space. 5. (4) The monoamingeric neuronal systems are widespread and highly collateralized inall vertebrates. A single brainstem monoaminergic neuron may send divergent axon collaterals to various remote forebrain areas and, thus, could strongly influence vast neuronalassemblies. 6. (5) The terminal arborization of monoaminergic systems, particularly that of the serotoninergicneurons, displays patterns that are strikingly constant from one vertebrate class to the other. Such a constancy strongly suggests that these chemospecific neuronal systems are indeedphylogenetically ancient and subserve similar fundamental brain functions, such as the controlof sleep-waking cycle and of feeding and reproductive behaviors, in all vertebrates.