Konnektivität und cytochemische Organisation des tecto-reticulären Systems im Gehirn von Amphibien

Abstract

In the tecto-reticular system of plethodontid salamanders, ?-aminobutyric acid (GABA)-, glycine (gly)-, and glutamate (glu)-like immunoreactivity (ir) of neurons was studied. Combined tracing and immunohistochemical experiments were performed to analyze the transmitter content of medullary nuclei with reciprocal connections with the tectum mesencephali as well as tectal neurons with descending projections to the medulla oblongata. Immunogold double-labelling experiments for biocytin and GABA, glycine or 5-HT were carried out to study synaptic contacts between medullary afferents and tecto-bulbar projection neurons in the tectum mesencephali. In the medulla oblongata, the distribution of transmitters differed significantly between the rostral and caudal part of the medulla; dual or triple localization of transmitters was present in somata throughout the rostrocaudal extent of the medulla. Regarding the rostral medulla, the largest number of GABA-, gly-, and glu-ir neurons was found in the medial zone. Neurons of the nucleus reticularis medius (NRM) retrogradely labeled by tracer application into the tectum revealed predominantly gly-ir, often colocalized with glu-ir. The NRM appears to be homologous to the mammalian gigantocellular reticular nucleus, and its glycinergic projection is mostlikely part of a negative feedback loop between the medulla and tectum. Neurons of the dorsal and vestibular nucleus projecting to the tectum were glu-ir, and neurons of the vestibular nucleus often revealed GABA- and/or gly-ir in addition. Regarding the caudal medulla, the highest densitiy of GABA-, gly- and glu-ir cells was found in the lateral zone. Differences in the neurochemistry of the rostral versus the caudal medulla correlate with the different projection patterns of these regions: Only nuclei of the rostral medulla are involved in the tecto-reticular system.In the tectum mesencephali, the distribution of transmitters differed significantly between the rostral/central and caudal part of the tectum; dual or triple localization of transmitters was present in somata throughout the rostrocaudal extent of the tectum. Regarding the rostral and central tectum, the largest number of GABA-, gly-, and glu-ir cells was found in the deep cellular layer 8 of the tectum mesencephali, which is characterized by a large amount of GABAergic interneurons. In contrast, in the caudal tectumthe distribution of single, dual or triple localization of transmitters is predominantly found in the superficial cellular layer 6 of the tectum. Differences in the neurochemistry of the rostral or central versus the caudal tectum appear to result from different functional processes by the representation of the visual field. The rostral and central tectum is mainly involved in the processes of object recognition and localization in the binocular field, whereas the caudal tectum processes visual information of the lateral monocular field. Neurons with descending projections to the ipsi- or contralateral medulla were predominantly localized in the cellular layer 6 of the tectum, and were mostly glu-ir. About 50% of these neurons revealed additional GABA-ir. Type TO2 and TO4 projection neurons, the axons of which descend in the ventrolateral tract, were mainly glutamatergic, whereas many neurons of the type TO1 and TO3, the axons of which descend in the ventromedial tract, were immunoreactive for glutamate and GABA. The tectal inhibitory system consists of GABAergic tecto-bulbar projection neurons and a great amount of GABAergic interneurons. These GABAergic cells affect neighboring tectal projection neurons or interneurons via dendro-dendritic or axo-dendritic contacts and thus, are able to modulate intratectal processing of visual and multimodal input. These intratectal processes are influenced by glycinergic projections from the NRM or serotonergic afferents of the nuclei raphes (NR), which are involved in the control of vigilance and attention. Both nuclei most likely inhibit intratectal inhibitory interactions. This disinhibition results in a selective activation of tectal neurons involved in the processing of relevant objects (up-regulation). In addition, serotonergic afferents of the NR influence the tecto-bulbar transmission via direct inhibition of projection neurons (down-regulation). The results likewise underline the role of the NRM and NR in visual object selection and orientation, as suggested by behavioral studies and electrophysiological recordings from tectal neurons.