GluR2/3 label expression of the AMPA-type glutamate receptor in the hippocampal formation of the homing pigeon stabilizes just after birth

Development of cone photoreceptors and their synapses in the human and monkey fovea

During retinal development, ribbon synapse assembly in the photoreceptors is a crucial step involving numerous molecules. While the developmental sequence of plexiform layers in human retina has been characterized, the molecular steps of synaptogenesis remain largely unknown. In the present study, we focused on the central rod-free region of primate retina, the fovea, to specifically investigate the development of cone photoreceptor ribbon synapses. Immunocytochemistry and electron microscopy were utilized to track the expression of photoreceptor transduction proteins and ribbon and synaptic markers in fetal human and Macaca retina. Although the inner plexiform layer appears earlier than the outer plexiform layer, synaptic proteins, and ribbons are first reliably recognized in cone pedicles. Markers first appear at fetal week 9. Both short (S) and medium/long (M/L) wavelength-selective cones express synaptic markers in the same temporal sequence; this is independent of opsin expression which takes place in S cones a month before M/L cones. The majority of ribbon markers, presynaptic vesicular release and postsynaptic neurotransduction-related machinery is present in both plexiform layers by fetal week 13. By contrast, two crucial components for cone to bipolar cell glutamatergic transmission, the metabotropic glutamate receptor 6 and voltage-dependent calcium channel α1.4, are not detected until fetal week 22 when bipolar cell invagination is present in the cone pedicle. These results suggest an intrinsically programmed but nonsynchronous expression of molecules in cone synaptic development. Moreover, functional ribbon synapses and active neurotransmission at foveal cone pedicles are possibly present as early as mid-gestation in human retina.

Immunohistochemical localization of AMPA-type glutamate receptor subunits in the nucleus of the Edinger-Westphal in embryonic chick

The Edinger-Westphal nucleus (EW) in birds is responsible for the control of pupil constriction, accommodation, and choroidal blood flow. The activation of EW neurons is mediated by the neurotransmitter glutamate, in large part through AMPA-type glutamate receptors (GluRs), whose behavior varies according to the subunit composition. We investigated the developmental expression of the GluR subunits in EW of the chick (Gallus gallus) using immunohistochemistry on tissue from embryonic days 10 through 20 (E10-E20). Of the three antibodies used, one recognized the GluR1 subunit, another the GluR4 subunit, and the third recognized a sequence common to GluR2 and GluR3 subunits. No immunolabeling of EW neurons for any GluR subunits was observed prior to E12, although immunolabeling was seen in somatic oculomotor prior to E12. At E12, immunoreactivity for each of the three antibodies was in only approximately 2% of EW neurons. By E14, the abundance of GluR1+ perikarya in EW had increased to 13%, and for GluR2/3 had increased to 48%. The perikaryal abundance of the immunoreactivity for GluR1 and GluR2/3 declined to 3% and 23%, respectively, by E16. At E14, 33% of EW neurons immunolabeled for GluR4, and their frequency increased to 43% by E16, and remained at that approximate percentage through hatching. The increased expression of GluR1 and GluR4 in EW at E14 coincides with the reported onset of the expression of the calcium-binding protein parvalbumin, and the calcium currents associated with AMPA receptors formed by these two subunits may play a role in the occurrence of parvalbumin expression.