An intriguing pitfall in chemical neuroanatomy: specific populations of unspecifically immunoreactive neurons in the brain of the mormyrid fish Gnathonemus petersii

Central connections of the trigeminal motor command system in the weakly electric Elephantnose fish (Gnathonemus petersii)

The highly mobile chin appendage of Gnathonemus petersii, the Schnauzenorgan, is used to actively probe the environment and is known to be a fovea of the electrosensory system. It receives an important innervation from both the trigeminal sensory and motor systems. However, little is known about the premotor control pathways that coordinate the movements of the Schnauzenorgan, or about central pathways originating from the trigeminal motor nucleus. The present study focuses on the central connections of the trigeminal motor system to elucidate premotor centers controlling Schnauzenorgan movements, with particular interest in the possible connections between the electrosensory and trigeminal systems. Neurotracer injections into the trigeminal motor nucleus revealed bilateral, reciprocal connections between the two trigeminal motor nuclei and between the trigeminal sensory and motor nuclei by bilateral labeling of cells and terminals. Prominent afferent input to the trigeminal motor nucleus originates from the nucleus lateralis valvulae, the nucleus dorsalis mesencephali, the cerebellar corpus C1, the reticular formation, and the Raphe nuclei. Retrogradely labeled cells were also observed in the central pretectal nucleus, the dorsal anterior pretectal nucleus, the tectum, the ventroposterior nucleus of the torus semicircularis, the gustatory sensory and motor nuclei, and in the hypothalamus. Labeled terminals, but not cell bodies, were observed in the nucleus lateralis valvulae and the reticular formation. No direct connections were found between the electrosensory system and the V motor nucleus but the central connections identified would provide several multisynaptic pathways linking these two systems, including possible efference copy and corollary discharge mechanisms.

Distribution of noradrenaline-immunoreactivity in the brain of the mormyrid teleost Gnathonemus petersii

The distribution of noradrenaline-immunoreactivity in the brain of the mormyrid fish Gnathonemus petersii was studied in order to evaluate the noradrenergic innervation of a number of specialized mormyrid brain regions, including electrosensory centers and a gigantocerebellum. Noradrenaline-immunoreactive (NAi) neurons occur in the hypothalamic paraventricular organ (PVO), the locus coeruleus, and the caudal rhombencephalon. In the PVO, NAi cerebrospinal fluid (CSF)-contacting neurons are located in the same regions where dopamine- and serotonin-containing CSF contacting neurons occur. The locus coeruleus consists, on each side, of at least 30 rather large NAi neurons with ventrolaterally directed dendrites and dorsolaterally coursing axons. In the caudal rhombencephalon, NAi neurons are located in the transition region between the ventromedial motor zone and the dorsolateral sensory zone. The density of NAi fibers is very high in the efferent tract of the locus coeruleus, the medial forebrain bundle, and two telencephalic, one preoptic, and one rhombencephalic subependymal axonal plexus. A marked NAi innervation is present in the dorsomedial and ventral telencephalon, the preoptic region, periventricular hypothalamic and thalamic regions, the midbrain tectum, cerebellar granular layers, the electrosensory lateral line lobe, the rhombencephalic transition region between the sensory and motor zones, and the area postrema. Other regions are more sparsely innervated by NAi fibers, but regions completely devoid of NAi fibers were not observed. Interestingly, NAi fibers form large club endings in some subdivisions of the precerebellar nucleus lateralis valvulae, and parallel fibers in the cerebellar granular layer. Comparison with the distribution of NAi or dopamine-beta-hydroxylase-immunoreactivity in other species shows that all teleosts studied to date have noradrenergic cells in the locus coeruleus and the caudal rhombencephalon. However, NAi CSF-contacting PVO cells have been described only in the teleost Gnathonemus petersii and the lizard Gekko gecko (Smeets and Steinbusch: J. Comp. Neurol. 285:453-466, '89). It is possible that they might pick up catecholamines as well as serotonin from the CSF, into which monoamines might be released by telencephalic and preoptic subependymal axonal plexuses.