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The development of serotonin-immunoreactive neurons in the central nervous system of Xenopus laevis larvae has been studied with special emphasis on the development of the raphe nuclei and raphespinal projections. The first serotonergic neurons were observed in the rostral part of the brain stem at stage 25, only 28 hr after fertilization. By stage 28 some 20 serotonin-immunoreactive neurons were found in the rostral part of the brain stem, bearing small protrusions on the ventromedial side of the soma. These initial axonal outgrowths reach the rostral part of the spinal cord at stage 32. By stage 35/36 the growth cones of the descending serotonergic axons in the spinal cord have reached the level of the anus (10th to 15th myotome). Up to stage 45 the majority of the descending serotonergic axons was found in the dorsolateral part of the marginal zone of the spinal cord. After stage 45 some serotonergic axons were also found scattered over other parts of the spinal marginal zone. Collateral branches were first observed in the caudal part of the brain stem at stage 35/36. Later they occurred also in the rostral (stage 43) and caudal (stage 50) spinal cord, usually on fibers in the ventral half of the spinal cord. The number of serotonergic neurons in the central nervous system (brain stem and hypothalamus) increased steadily throughout development until stage 45. After that the total number of serotonergic neurons in the central nervous system increased about two times faster than the number of serotonergic neurons in the raphe nuclei, due to a massive increase of serotonergic neurons in the hypothalamus. The present study shows that young, just differentiated raphe neurons already contain serotonin. The generation of these neurons appears to take place in the ventricular zone (matrix) of the brain stem between the caudal border of the mesencephalon and the entrance of the nervus octavus. From here these neurons seem to migrate to their final destination. The distribution of serotonin-immunoreactive neurons in the brain stem suggests that a superior (not described so far in Anura) and an inferior raphe nucleus can be distinguished in Xenopus. A rostrocaudal gradient seems to be present in the production of serotonergic neurons which project to the spinal cord. Spinal projections from the raphe nuclei are particularly extensive from the nucleus raphes inferior and gradually decrease rostralwards. In the rostral part of the nucleus raphes superior almost no neurons projecting to the spinal cord are found.

The distribution of serotonin-immunoreactive neurons and fibers was studied in the highly developed brain of the weakly electric fish Gnathonemus petersii with the aid of specific antibodies against serotonin. Serotoninergic cell bodies occur in three regions: the raphe region of the brainstem, the hypothalamus, and the transition zone between the dorsal thalamus and the pretectum. Serotoninergic raphe neurons are clustered in three groups: nucleus raphes superior, intermedius, and inferior. The latter has not been described in other teleosts and thus might be the source of the serotoninergic innervation of specific mormyrid electrosensory brain regions. Most hypothalamic serotoninergic neurons have cerebrospinal-fluid (CSF)-contacting processes and thus belong to the paraventricular organ (PVO), which in Gnathonemus is located around a number of small infundibular recesses. The distribution of serotonin in the PVO precisely matches the distribution of dopamine, as described previously. Serotoninergic cells in the thalamopretectal transition zone also have been described in other teleosts, but not in other vertebrate groups, and thus seem to represent a teleostean specialization. Serotoninergic fiber density is especially high in the medial forebrain bundle and surrounding preoptic and hypothalamic regions as well as in several telencephalic and preoptic subependymal plexus. Serotoninergic fibers appear to be almost completely absent in the large and differentiated corpus and valvula cerebelli. Comparison with the literature on teleostean serotoninergic innervation patterns reveals several mormyrid specializations, including the absence of serotonin in large parts of the mormyrid telencephalic lobes, a differentiated innervation pattern of distinct electrosensory and mechanosensory subnuclei of the torus semicircularis, a refined serotoninergic lamination pattern in the midbrain tectum, and a prominent innervation of the electrosensory lateral line lobe, the associated caudal cerebellar lobe, and the electromotor medullary relay nucleus. A distinct innervation of several types of (pre)motor neurons, such as the Mauthner cells and facial motor neurons, has not been reported previously for other teleosts. Consequently, the distribution of serotoninergic fibers as well as neurons in the mormyrid brain is substantially adapted to the high degree of differentiation of its electrosensory and telencephalic brain regions, but serotoninergic innervation is not involved in the circuitry of the most impressive part of the mormyrid brain; i.e., its large corpus and valvula cerebelli.

The distribution of dopamine-containing cell bodies and fibers was studied with aid of specific antibodies against dopamine in the highly developed brain of the weakly electric fish Gnathonemus petersii. In the telencephalon, dopamine-containing cell bodies were observed in a small area, i.e., area ventralis pars dorsalis and supracommissuralis. In the diencephalon, moderate numbers of dispersed dopamine-immunoreactive cells were present in the preoptic region, while large numbers of dopamine-containing neurons occurred in the hypothalamic paraventricular organ and neighbouring regions. The paraventricular organ, located around small (anterior, intermediate, and posterior) recesses contained many dopamine-immunoreactive cerebrospinal fluid-(CSF)-contacting neurons. Dopamine-containing cells were also observed in a magnocellular hypothalamic cell group, in the nucleus of the lateral recess, and in the nucleus posterior tuberis. In the mesencephalon only a few dopamine-containing cells were observed in a dorsal tegmental (possibly pretectal) area, whereas in ventral mesencephalic regions dopamine-containing cells were lacking. More caudally, dopamine-containing cells were observed in the presumed locus coeruleus, in the caudal region of the reticular formation, and in the presumed area postrema. Dopamine-immunoreactive fiber density was very high in the medioventral hypothalamus and in the preoptic region, where a dense subependymal plexus was observed along the preoptic recess. Such a plexus was also present in the caudal rhombencephalon, where it probably arises from the area postrema. Moderate numbers of dopamine-immunoreactive fibers were present in medioventral parts of the brain along its total rostrocaudal extent as well as in several subnuclei of the torus semicircularis, in the tectum mesencephali, and in the medial part of the dorsal telencephalic area. Other parts of the dorsal telencephalic area, as well as the large cerebellum and the electrosensory lateral line lobe of Gnathonemus, did not contain detectable amounts of dopamine. In spite of the high differentiation of the brain of Gnathonemus, the distribution of catecholamines as visualized with dopamine immunohistochemistry appears to be basically similar to that described in other teleostean and actinopterygian fishes on the basis of formaldehyde-induced fluorescence or tyrosine hydroxylase immunohistochemistry.(ABSTRACT TRUNCATED AT 400 WORDS)