The organization of monoamine‐containing neurons in the brain of the salamander, Necturus maculosus

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Abstract
The organization of monoamine (MA)‐containing neuronal systems in the brain of the salamander (Necturus maculosus) was studied by means of paraformaldehyde and glyoxylic acid histofluorescence methods. The MA neurons were further characterized morphologically andhistochemically in material prepared for the demonstration of cholinesterase activity.In the brain ofNecturus, the most caudally located catecholamine (CA)‐containing cells occur in the nucleus of the solitary tract. A large population of serotonin (5‐HT)‐containing cells is also found along the entire rostrocaudal extent of the raphe region of the brainstem. At isthmal levels, a few scattered CA cells lie dorsolaterally in the so‐called acousticolateral area. These neurons can be seen to send axonal projections to the spinal cord and the cerebellum, but not to the telencephalon, and thus can not be considered as fully homologous to the locus coeruleus of reptiles, birds, and mammals. Within the midbrain, some cells displaying a strong CA fluorescence and staining intensely for cholinesterases are present near the oculomotor nerve root fibers. They give rise to a long ascending fiber system arborizing in the striatum, and this system appears homologous to the mesostriatal CA pathway of amniotic vertebrates. At diencephalic levels a multitude of CSF‐contacting cells of both CA and 5‐HT types occurs. They abound particularly in the ventral thalamus, along the infudibular recess, in the dorsal infundibular nucleus, and in the ventral thalamus. As a whole, this population of CSF‐contacting MA cells is considered homologous to the paraventricular organ present in all nonmammalian vertebrates. Some CA cells also occur along the preoptic recess, around the hippocampal commissure, and even within the caudalmost aspect of the medial pallium. The presence of CA cell bodies among telencephalic cell groups has not been demonstrated in other vertebrates. The heaviest CA innervation in the telencephalon is found in the neuropil of the corpus striatum and in the “prominentia ventralis” (nucleus accumbens septi), both of which also stain significantly for cholinesterases. Finally, numerous CA cells are scattered throughout the magnocellular, molecular, and granular cell layers of the olfactory bulb. These cells provide a strong CA innervation to the glomerular layer of the bulb which also exhibits a high cholinesterase activity seemingly derived from the processes ofperiglomerular cells. Thus, the pattern of organization of MA systems in Necturus appears unique in many aspects, but it also displays striking similarities to the arrangement of brain MA neurons disclosed in other vertebrates.