Retinotectal transmission in the optic tectum of rainbow trout

Retinotectal transmission has not yet been well characterized at the cellular level in the optic tectum. To address this issue, we used a teleost, the rainbow trout, and characterized periventricular neurons as postsynaptic cells expected to receive the retinotectal inputs to the optic tectum. The somata of periventricular neurons are localized in the upper zone of the stratum periventriculare (SPV), whereas the lower zone of the SPV comprises the cell body layer of radial glial cells. Ca²⁺ imaging identified functional ionotropic glutamate receptors in periventricular neurons. We also cloned cDNAs encoding the NR1 subunit of N-methyl-D-aspartic acid (NMDA) receptors and the GluR2 subunit of (±)-α-amino-3-hydroxy-5-methyl-isoxazole-4- propionic acid (AMPA) receptors, and detected their mRNAs in periventricular neurons by in situ hybridization. The presence of the receptor subunit proteins was also confirmed in the dendrites of periventricular neurons by immunoblotting and immunohistochemistry. On the other hand, radial glial cells in the lower zone of the SPV did not respond to glutamate applications, and mRNA and immunoreactivities of ionotropic glutamate receptors were not detected in glial cells. The present findings suggest that glutamatergic transmission at synapses between retinotectal afferents and periventricular neurons is mediated by the functional NMDA and AMPA receptors.