Immunocytochemical localization of the AMPA glutamate receptor subtype GluR2/3 in the squid optic lobe.

As a major excitatory neurotransmitter in the cephalopod visual system, glutamate signaling is facilitated by ionotropic receptors, such as α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPAR). In cephalopods with large and well-developed brains, the optic lobes (OL) mainly process visual inputs and are involved in learning and memory. Although the presence of AMPAR in squid OL has been reported, the organization of specific AMPAR-containing neurons remains unknown. This study aimed to investigate the immunocytochemical localization of the AMPA glutamate receptor subtype 2/3-immunoreactive (GluR2/3-IR) neurons in the OL of Pacific flying squid (Tordarodes pacificus). Morphologically diverse GluR2/3-IR neurons were predominantly located in the tangential zone of the medulla. Medium-to-large GluR2/3-IR neurons were also detected. The distribution patterns and cell morphologies of calcium-binding protein (CBP)-IR neurons, specifically calbindin-D28K (CB)-, calretinin (CR)-, and parvalbumin (PV)-IR neurons, were similar to those of GluR2/3-IR neurons. However, two-color immunofluorescence revealed that GluR2/3-IR neurons did not colocalize with the CBP-IR neurons. Furthermore, the specific localizations and diverse types of GluR2/3-IR neurons that do not express CB, CR, or PV in squid OL were determined. These findings further contribute to the existing data on glutamatergic visual systems and provide new insights for understanding the visual processing mechanisms in cephalopods.