Fiber Connections of the Nucleus isthmi in the Carp <i>(Cyprinus carpio)</i> and Tilapia <i>(Oreochromis niloticus)</i>

Abstract

The nucleus pretectalis (NP) is a prominent nucleus in the percomorph pretectum and has been shown to project to the nucleus isthmi in the filefish by an HRP tract-tracing method [Ito et al., 1981], but a homologous nucleus to the NP is apparently lacking in ostariophysans. The present study examined fiber connections of the nucleus isthmi in an ostariophysan teleost, the carp (Cyprinidae, Cyprinus carpio), to identify a nucleus homologous to the percomorph nucleus pretectalis. Identical studies in a percomorph tilapia (Cichlidae, Oreochromis niloticus) were also performed. Injections of biotinylated dextran amine (BDA) or biocytin to the carp nucleus isthmi labeled cells in the ipsilateral optic tectum and nucleus ruber of Goldstein [1905]. Labeled tectal neurons were located in the stratum periventriculare (SPV) and the stratum fibrosum et griseum superficiale (SFGS). The somata in the SPV were pyriform and those in the SFGS were fusiform. No labeled cells were found in the pretectum. Labeled terminals were seen in the ipsilateral nucleus pretectalis superficialis pars parvocellularis (PSp), optic tectum, and bilateral nucleus ruber. Terminals in the nucleus ruber appear to come from tectal neurons in the SFGS labeled by isthmic injections. Thus the nucleus isthmi has reciprocal fiber connections with the ipsilateral optic tectum, receives projections from the ipsilateral nucleus ruber, and projects to the ipsilateral PSp. The nucleus pretectalis homologue is apparently absent in the carp. Studies in tilapia showed that the nucleus isthmi receives bilateral projections from the NP and optic tectum. In addition, the present study revealed a previously unknown afferent from the nucleus ruber to the percomorph nucleus isthmi. The tilapia nucleus isthmi projects to the same targets as in the carp. Isthmic projection neurons in the tilapia optic tectum were located in the SPV and pyriform with a similar shape to the carp SPV neurons that project to the nucleus isthmi. No labeled cells were found in the SFGS of tilapia optic tectum. The fusiform neurons in the SFGS of the carp optic tectum possess various hodological similarities with the NP and may correspond to the NP neurons of percomorphs.

Influencing and Interpreting Visual Input: The Role of a Visual Feedback System

“…As described above, the OT and NI are reciprocally connected in topographic manner (Sakamoto et al, 1981;Dunn-Meynell and Sharma, 1984;Xue et al, 2001). NI generates bursts of graded amplitude spikes evoked by visual and mechanoreceptor stimulation.…”

“…Its hair-trigger response to novelty and motion broadcast over the OT suggests an alerting or attentional function, an idea supported by the finding that NI can be "distracted" by a salient ipsilateral stimulus (Northmore, 1991). The crossing pathway mediating this effect could involve the inhibitory prectecto-isthmal fibers (Sakamoto et al, 1981;DunnMeynell and Sharma, 1984;Xue et al, 2001). The most effective visual stimulus for NI is an object advancing toward the contralateral eye (Gallagher and Northmore, 2006), evoking a linear ramp-up in firing.…”

Anatomical and functional analysis of non-retinal tectal afferents in larval zebrafish

“…Across species, tectal afferents include the ipsilateral and contralateral telencephalon (Fiebig et al, 1983), the pretectum, and both the anterior and ventro-medial thalamic nuclei (Northcutt, 1982;Fiebig et al, 1983). In piranhas, tectal afferents also include the dorsal hypothalamus, whose connections are theorized to be the equivalent of the tectal afferents originating in the mesencephalic tegmentum in carp (Fiebig et al, 1983;Xue et al, 2001). Additional tectal afferents include the torus longitudinalis, torus semicircularis, and the nucleus isthmi (Northcutt, 1982;Xue et al, 2001;Fame et al, 2006;Folgueira et al, 2007).…”

“…In piranhas, tectal afferents also include the dorsal hypothalamus, whose connections are theorized to be the equivalent of the tectal afferents originating in the mesencephalic tegmentum in carp (Fiebig et al, 1983;Xue et al, 2001). Additional tectal afferents include the torus longitudinalis, torus semicircularis, and the nucleus isthmi (Northcutt, 1982;Xue et al, 2001;Fame et al, 2006;Folgueira et al, 2007). In the hindbrain, structures including the reticular formation and dorsal funicular nucleus (Northmore, 1991), the cerebellum, and the trigeminal nuclei (Northcutt, 1982;Heap et al, 2013) also project to the optic tectum.…”

“…In addition to pyramidal cells, the SFGS houses small bipolar neurons, which extend processes within the SFGS, as well as horizontal cells which populate the plexiform layers within the structure (Vanegas et al, 2004). In carp, isthmic projections to the tectum localise in the SFGS, where they specifically target fusiform cells (Sas and Maler, 1986;Xue et al, 2001). Cells within the SGC of the teleost tectal neuropil are not confined to this layer; fusiform neurons extend vertically into the SO, SFGC and SAC, whereas bipolar neurons project dorsally into the SFGS and fusiform neurons project into the SO and within the SGC (Meek, 1983;Sas and Maler, 1986;Vanegas et al, 2004).…”