Distribution of dopamine immunoreactivity in the brain of the mormyrid teleost Gnathonemus petersii

Abstract

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. However, the absence of a mesencephalostriatal dopamine system and the presence of a very dominating and complex periinfundibular dopamine system indicate basic differences between actinopterygians and other anamniote (elasmobranch) as well as amniote groups (e.g., reptiles and mammals). Since many CSF-contacting neurons in the infundibular paraventricular organ seem to lack the dopaminesynthesizing enzyme tyrosine hydroxylase, we suggest that a substantial part of the teleostean paraventricular organ might receive dopamine from the intraventricular CSF, into which it might be released by the preoptic subependymal plexus.