Connectivity of the goldfish optic tectum with the mesencephalic and rhombencephalic reticular formation

Abstract

The optic tectum of goldfish, as in other vertebrates, plays a major role in the generation of orienting movements, including eye saccades. To perform these movements, the optic tectum sends a motor command through the mesencephalic and rhombencephalic reticular formation, to the extraocular motoneurons. Furthermore, the tectal command is adjusted by a feedback signal arising from the reticular targets. Since the features of the motor command change with respect to the tectal site, the present work was devoted to determining, quantitatively, the particular reciprocal connectivity between the reticular regions and tectal sites having different motor properties. With this aim, the bidirectional tracer, biotin dextran amine, was injected into anteromedial tectal sites, where eye movements with small horizontal and large vertical components were evoked, or into posteromedial tectal sites, where eye movements with large horizontal and small vertical components were evoked. Labeled boutons and somas were then located and counted in the reticular formation. Both were more numerous in the mesencephalon than in the rhombencephalon, and ipsilaterally than contralaterally, with respect to the injection site. Furthermore, the somas showed a tendency to be located in the area containing the most dense labeling of synaptic endings. In addition, labeled boutons were often observed in close association with retrogradely stained neurons, suggesting the presence of a tectoreticular feedback circuit. Following the injection in the anteromedial tectum, most of the boutons and labeled neurons were found in the reticular formation rostral to the oculomotor nucleus. Conversely, following the injection in the posteromedial tectum, most of the boutons and neurons were also located in the caudal mesencephalic reticular formation. Finally, boutons and neurons were found in the rhombencephalic reticular formation surrounding the abducens nucleus. They were more numerous following the injection in the posteromedial tectum. These results demonstrate characteristic patterns of reciprocal connectivity between physiologically different tectal sites and the mesencephalic and rhombencephalic reticular formation. These patterns are discussed in the framework of the neural substratum that underlies the codification of orienting movements in goldfish.

Development of Tectal Connectivity across Metamorphosis in the Bullfrog (Rana catesbeiana)

“…It is possible that this cell label reflects leakage of tracer from dap fibers into collaterals of abducens cells. On the other hand, cell body and terminal label in and around the abducens nucleus following injections of dextran amine into the OT have been observed in adult goldfish [Pérez-Pérez et al, 2003]. The absence of direct tectal efferents to the abducens nucleus in our data is consistent with previous results in adult R. temporaria [Cochran et al, 1984].…”

Visual Prey Capture in Larval Zebrafish Is Controlled by Identified Reticulospinal Neurons Downstream of the Tectum

“…MeLr and MeLc extend elaborate dendrites into the deeper layers of the tectum. Here, the cell bodies of tectofugal projection neurons are situated (Meek and Schellart, 1978;Nguyen et al, 1999), which, in other teleosts, have been reported to send a predominantly ipsilateral and excitatory projection to the nMLF (Bosch and Paul, 1993;Herrero et al, 1998a;Niida et al, 1998;Zompa and Dubuc 1998a,b;Isa and Sasaki, 2002;Torres et al, 2002;Perez-Perez et al, 2003). MeLr and MeLc extend axons into the spinal cord, arborizing predominantly in its rostral segments, in which their axon terminals colocalize with motor neurons and interneurons .…”

Genetic Single-Cell Mosaic Analysis Implicates ephrinB2 Reverse Signaling in Projections from the Posterior Tectum to the Hindbrain in Zebrafish

“…However, additional study is necessary to correlate the neuroanatomically identified projection patterns with the electrophysiologically characterized motor command map in the tectum. In goldfish, the injection sites of tracers in the tectum show a colinear anterior-posterior relationship with the sites containing the peak number of labeled synaptic boutons in the mesencephalic reticular formation, although such a relationship is not found in the rhombencephalic reticular formation (Perez-Perez et al, 2003). The discrepancy between these observations in goldfish and our results is likely attributable to the differences in labeling strategies (single-neuron labeling vs tracer injection) and in examined parameters for the tectum and the hindbrain (somatic locations and the final projection site vs injection sites and the number of synaptic boutons).…”