Early visual experience of background contrast affects the expression of NMDA-like glutamate receptors in the optic lobe of cuttlefish, Sepia pharaonis

Visual experience is essential for the normal development of the visual system in vertebrates. Previous studies have demonstrated that N-methyl-d-aspartate (NMDA) receptors play significant roles in the activity-dependent plasticity of the developing visual system. In cephalopods, numerous findings indicate that cuttlefish behaviors show experience-dependent plasticity and are subject to environmental influences during development. Substantial evidence also suggests that glutamatergic currents and NMDA-like receptor subunits NR2A/2B are present in the optic lobe of cuttlefish. In light of these earlier findings, the present study was aimed to examine if early visual experience of background contrast may exert its influence on behaviors by affecting NMDA receptor expression in the optic lobe of cuttlefish (Sepia pharaonis). The expression of NMDA receptors in the optic lobe of S. pharaonis was first confirmed by the 1-amino-4-guanidobutane (AGB) functional assay. Newly hatched animals were then separated into two cohorts, one reared in a uniform-gray background (low-contrast, or L group) and the other raised in a black/white checkerboard background (high-contrast, or H group). At Weeks 4, 8, and 12, the optic lobes of cuttlefish were subjected to immunohistochemical and Western blot analyses using an antibody against the NR2A subunit of NMDA receptor. Although immunoreactivity of NR2A in the optic lobes of L and H group cuttlefish appeared similar, relative expression of NR2A protein in two groups of cuttlefish optic lobes changed during development, being lower at Week 4 and higher at Weeks 8 and 12 in the H group when compared to those in the L group. This result indicates that the expression of NMDA-like receptors in the optic lobe is dependent on rearing background contrast during cuttlefish development. This finding thus provides a good correlate between the expression of NMDA-like receptors and the behavioral plasticity in developing cuttlefish.