Expression of AMPA‐type glutamate receptor subunit (GluR2) in ON‐bipolar neurons in the rat retina

Abstract

The role of glutamate receptors (GluR) in the signal pathways of the retina is widely recognized. Photoreceptors make synaptic contact with functionally different classes of bipolar cells. The OFF-type bipolar cells mediate light offset-evoked responses and use ionotropic alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA)- or kainate-type GluRs, whereas bipolars involved in the ON-pathway use the metabotropic GluR6. This dichotomy predicts a defined expression pattern of AMPA-type GluRs and mGluR6 in bipolar cell classes. This hypothesis was tested by performing immunocytochemical double labeling studies combining GluR-specific antibodies with markers specific for the diverse bipolar cell populations in the rat retina. AMPA-type receptors are composed of combinations of four types of subunits, GluR1-4. GluR1 is expressed by a few somata in the outer part of the inner nuclear layer (INL). Sparse colocalization with any of the bipolar markers used could be established. In contrast, GluR2 is expressed by many of the somata in the outer zone of the INL. At the transcript level, in situ hybridizations demonstrated abundant GluR2 expression over the complete width of the INL. In contrast to our expectations, approximately 70% of the somata labeled by the rod ON-bipolar markers protein kinase C (PKC) or Goalpha, colocalized with GluR2. Approximately 90% of the OFF-type bipolar cells, identified as recoverin-positive, showed GluR2 immunoreactivity. At least 40% of the somata that were mGluR6-immunoreactive, a both rod and cone ON-type bipolar marker, were GluR2-immunopositive. Ultrastructurally, examples were observed of GluR2 localization in bipolar processes with labeling outside the actual compartment associated with the synaptic complex of the rod terminal. No specific antibody was available against GluR3, but 74% of the PKC-positive cells were GluR2/3-positive. GluR4 did not show a somatic localization making double labeling impossible. On the basis of these results, we conclude that ionotropic GluRs are expressed by rod ON-type bipolar cells (PKC- or Goalpha-immunoreactive), and by cone ON- and OFF-type bipolars based on a colocalization with nearly all of the present recoverin-positive somata. Our observations show that the functional dichotomy in ON- and OFF-type bipolars is not reflected in a matching expression pattern of ionotropic and metabotropic GluRs. This finding raises the intriguing possibility that the AMPA-type GluRs are, in an as yet unclear manner, involved in the ON signaling pathways of rods and cones.

P2X receptor signaling inhibits BDNF-mediated spiral ganglion neuron development in the neonatal rat cochlea

“…The single-cell transcript levels were comparable to those previously described for other ion channel subunits (Sucher et al, 2000) and for transcription factors and housekeeping genes (Wagatsuma et al, 2005;Warrington et al, 2000). Our application of TaqMan-based real-time PCR in single cells is also supported by recent investigations of dopaminergic receptor mRNA transcript levels (Liss et al, 2001), mGluR in rod photoreceptors (Kamphuis et al, 2003) and 5HT receptors in hypoglossal motoneurons (Zhan et al, 2002). These studies, alongside critical reviews of quantitative RT-PCR technology (Bustin, 2002;Fink et al, 1998;Ginzinger, 2002;Pfaffl et al, 2002;Wagatsuma et al, 2005), establish criteria for determining the validity of quantifying transcripts at low copy number.…”

Photons Do Collapse In the Retina Not in the Brain Cortex: Evidence from Visual Illusions

“…Beyond a certain size, further increases in the spot size have no additional effect on ganglion cell output (cf. Kingsley, 1996) (Vardi et al, 2000;Duvoisin et al, 2005;Tian and Kammermeier, 2006 Kamphuis et al, 2003), and this makes perfect sense in view of the sign-reversed transmission performed by rods and cones. A reader who is acquainted with basic neurobiology can easily verify that due to expression of AMPA and kainate receptors the OFF-center bipolar cell and the corresponding OFF-center ganglion cell will be excited when the central photoreceptor is at dark.…”

The expression and localization of cytoplasmic polyadenylation element binding proteins in the retina.

“…If similar machinery is harnessed for synaptogenesis, then one may relate such key molecules to CPEB. The expression pattern for CPEB3 presented here is similar to the pattern for retinal GluR2 (Dijk and Kamphuis 2004;Grunder et al 2000a;Hack et al 2001;Hamassaki-Britto et al 1993;Kamphuis et al 2003). Both are 176 enhanced expression of CPEBs.…”

“…Such mRNAs include: CaMKIl (Liu et al 2000), NR1, NR2A-D (Grunder et al 2000b;Araki and Hamassaki-Britto 2000), GluR1, and GluR2 (Grunder et al 2000a;Hack et al 2001;Kamphuis et al 2003;Dijk and Kamphuis 2004).…”