Two pharmacological classes of quisqualate-induced electrical responses in rat retinal ganglion cells in vitro

Light triggers expression of philanthotoxin-insensitive Ca2+-permeable AMPA receptors in the developing rat retina

Ca2+-permeable AMPA receptors (AMPARs) are expressed throughout the adult CNS but yet their role in development is poorly understood. In the developing retina, most investigations have focused on Ca2+ influx through NMDARs in promoting synapse maturation and not on AMPARs. However, NMDARs are absent from many retinal cells suggesting that other Ca2+-permeable glutamate receptors may be important to consider. Here we show that inhibitory horizontal and AII amacrine cells lack NMDARs but express Ca2+-permeable AMPARs. Before eye-opening, AMPARs were fully blocked by philanthotoxin (PhTX), a selective antagonist of Ca2+-permeable AMPARs. After eye-opening, however, a subpopulation of Ca2+-permeable AMPARs were unexpectedly PhTX resistant. Furthermore, Joro spider toxin (JSTX) and IEM-1460 also failed to antagonize, demonstrating that this novel pharmacology is shared by several AMPAR channel blockers. Interestingly, PhTX-insensitive AMPARs failed to express in retinae from dark-reared animals demonstrating that light entering the eye triggers their expression. Eye-opening coincides with the consolidation of inhibitory cell connections suggesting that the developmental switch to a Ca2+-permeable AMPAR with novel pharmacology may be critical to synapse maturation in the mammalian retina.

The rat retinal ganglion cell in culture: An accessible CNS neurone

Retinal ganglion cells are vital for vision, some have intrinsic light sensing properties and in retinal networks display complex computational abilities. Furthermore they are implicated in a very common form of blindness, glaucoma as well some the symptoms of AIDS. Retinal ganglion cells, unlike many neurones of the central nervous system, have a clearly defined physiological role and can be identified in primary cultures with ease. Here we detail the cell culture and electrophysiological methods required to obtain recordings on the voltage-gated and ligand-gated ion currents and channels expressed by these neurones. Information is given on the range of non-ionotropic receptors that are thought to be present on these cells and what role they may have as model systems in the pharmacological and pharmaceutical research environment.

Co-expression of AMPA/kainate receptor-operated channels with high and low Ca2+ permeability in single rat retinal ganglion cells

The patch-clamp technique was used to record whole-cell currents induced by alpha-amino-3-hydroxyl-5-methyl-isoxazol-4-propionic acid (AMPA) or kainate in solitary rat retinal ganglion cells (n = 125) in vitro. Two groups of retinal ganglion cells could be distinguished according to their responses to kainate or AMPA in extracellular solutions with Ca2+ as the only permeant cation. The ratio of the steady-state currents evoked by a given concentration of AMPA compared to kainate was low (0.08) in the first group and high (0.61) in the second group of retinal ganglion cells. The Ca2+ permeability through AMPA/kainate receptor-operated channels was low (PCa2+/PCs+ < 0.1) in the first group (n = 74, 59%) and moderate (PCa2+/PCs+ = 0.53) in the second group (n = 51, 41%) of retinal ganglion cells. The fraction of the total current induced by stimulation of non-N-methyl-D-aspartate receptors that is flowing through Ca2+ permeable AMPA/kainate channels in single cells with high Ca2+ permeability was estimated by comparing the current-voltage relationship in extracellular solutions with either Ca2+ or Na+ as the sole charge carrier. The contribution of Ca(2+)-permeable channels to the non-N-methyl-D-aspartate receptor induced whole-cell current in single Ca2+ permeable cells (n = 12) ranged from 40 to 70%, correlating with the intermediate level of Ca2+ permeability (PCa2+/PCs+ = 0.22-0.80) measured by an independent method in these cells. Thus, single Ca(2+)-permeable cells appear to express at least two types of AMPA/kainate receptor-operated channels with high or low Ca2+ permeability. Using the polymerase chain reaction, transcripts for the glutamate receptor subunits 1-4, including their "flip" and "flop" versions, were identified in retinal ganglion cells. Together, these findings suggest that among rat retinal ganglion cells there are differences in the pattern of expression of AMPA/kainate receptor-operated channels. Moreover, individual cells co-express multiple heterologous non-N-methyl-D-aspartate receptors with distinct functional properties. The functional diversity of these receptors may play an important role in controlling Ca2+ entry into neurons. We speculate that the low Ca2+ permeability and the preference for kainate in one group of retinal ganglion cells may be due to the predominant expression of non-N-methyl-D-aspartate receptors containing the edited form of the glutamate receptor subunit 2 flop splice variant.

Expression of NMDA and high-affinity kainate receptor subunit mRNAs in the adult rat retina

The expression patterns of nine genes encoding the N-methyl-D-aspartate (NMDA) receptor subunits NR1 and NR2A, NR2B, NR2C and NR2D, and the high-affinity kainate receptor subunits KA1, KA2, GluR6 and GluR7, were studied in the adult rat retina by in situ hybridization. Hybridization with [35S]dATP-labelled oligonucleotide probes revealed the expression of four of the NMDA receptor subunits (NR1, NR2A, NR2B and NR2C) and three of the high-affinity kainate receptor subunits (KA2, GluR6 and GluR7) in the retina. The NMDA receptor subunit NR2D and the high-affinity kainate receptor subunit KA1 could not be detected. In the ganglion cell layer, virtually every ganglion cell or displaced amacrine cell expressed the receptor subunits NR1, NR2A, NR2B, NR2C, KA2 and GluR7. The GluR6 subunit was expressed in a more restricted manner in the ganglion cell layer. In the inner nuclear layer, the receptor subunits NR1 and KA2 were homogeneously distributed, and therefore are most likely expressed by all cell types in this layer. The GluR6, NR2A, NR2B and NR2C subunits were expressed by subsets of amacrine cells. Labelling for NR2C was also found above the middle of the inner nuclear layer, corresponding to the location of bipolar cell somata. The GluR7 subunit was expressed by most amacrine and bipolar cells. These findings suggest that NMDA and high-affinity kainate receptor subunits could be present at a majority of glutamatergic retinal synapses.

Species-dependent functional properties of non-NMDA receptors expressed in Xenopus laevis oocytes injected with mammalian and avian brain mRNA

1. Species-dependent variation in the functional properties of non-NMDA receptors was investigated by intracellular recording in Xenopus laevis oocytes injected with rat, chick and calf brain mRNA. 2. In all mRNA-injected oocytes, kainic acid (KA), domoic acid (Dom) and 5-bromowillardiine (BrW) evoked large, maintained membrane currents, in contrast to the smaller, desensitizing responses elicited by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), quisqualic acid (QA) and L-glutamic acid (L-Glu). Dose-response curves for KA in oocytes injected with calf (EC50 = 96.4 +/- 12.3 microM; mean +/- s.e. mean), chick (87.0 +/- 8.9 microM) or rat (88.7 +/- 4.3 microM) brain mRNA were similar. 3. Current-voltage (I-V) relationships determined with KA inwardly rectified in oocytes injected with calf or chick mRNA; whereas, outward rectification was observed in oocytes injected with rat brain mRNA. 4. In oocytes injected with rat brain mRNA, AMPA antagonized responses evoked by KA in a competitive manner. The absolute amplitudes of KA and AMPA responses in the same oocytes were significantly correlated, which is consistent with both agonists acting on the same receptor-ionophore complex. 5. In contrast, in oocytes injected with calf or chick brain mRNA, AMPA (QA and L-Glu) antagonized the response evoked by KA in a non-competitive manner. The response amplitudes of KA compared to AMPA, QA or L-Glu in the same oocytes were not correlated suggesting discrete receptor-ionophores. 6. This study favours the existence of distinct non-NMDA receptor subtypes that are equi-sensitive to KA. The expressed receptors from different species of mRNA may be distinguished by their voltage sensitivities and the type of antagonism exerted by AMPA on KA-activated responses. Our observations may reflect further heterogeneity of non-NMDA receptors in the central nervous system of different vertebrate species.

Preferential activation of [3H]phorbol-12,13-dibutyrate binding by AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) in neonatal striatal cell cultures

Activation of excitatory amino acid receptors increased [3H]phorbol-12,13-dibutyrate ([3H]PdBu) binding in four week cultures of striatal cells from postnatal day 7 rat pups (PN7), and in PN7 cells co-cultured the fourth week with striatal cells from postnatal day 1 rat pups. Kainate (KA), trans-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD), and N-methyl-D-aspartate (NMDA) increased [3H]PdBu binding equally in both types of cultures, but alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) increased binding by 3-fold in the co-cultures. Thus, [3H]PdBu binding in these two types of striatal cultures offers a simple model system for studying the regulation of AMPA/KA receptor responses.

Differential expression of glutamate receptor genes (GluR1-5) in the rat retina

The recent isolation of at least five different cDNAs encoding functional subunits of glutamate receptors (GluR1 to GluR5) has revealed a diversity whose function is not understood. To learn more about how these different receptor subunits are used in the brain, we undertook an in situ hybridization study of the retina to define how the different glutamate receptor genes are expressed. We chose the retina because the glutamate sensitivities of its different cell types have been characterized, and these different neurons reside in different laminae. Hybridization of [35S]UTP-labeled cRNA probes with transverse sections and freshly dissociated cells reveals that all five receptor subunits are expressed in the retina. Hybridization signal is detected in different, but overlapping, sets of cells in the retina. GluR1, GluR2, and GluR5 are expressed by many somata, and GluR4 by a few, in the outer third of the inner nuclear layer, where the horizontal cells reside. Transcripts for GluR1, GluR2, and GluR5 are found in the somata within the middle third of the inner nuclear layer, which is where the bipolar cell somata are located, and GluR2 probes label freshly dissociated rod bipolar cells. All of the probes produce labeling over the cells at the inner edge of the inner nuclear layer, which are probably amacrine cells, as well as over the cell bodies in the ganglion cell layer.

Quisqualate agonists occlude kainate-induced current in cultured striatal neurons

We employed the whole cell patch-clamp technique to examine the ionic currents induced via activation of kainate/quisqualate receptors on striatal neurons in primary culture when N-methyl-D-aspartate receptors were blocked by selective antagonists. Bath perfusion of 10 microM-1 mM each of quisqualate, glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (a selective quisqualate agonist) or kainate, induced only a sustained current, but more rapid application by pressure ejection of each of the first three agonists (but not kainate) also activated a rapidly desensitizing current. The current induced by a near-saturating concentration of kainate (1 mM) was, on average, 16-fold larger than the maximum sustained current induced by quisqualate (10 microM), or 7.5-fold larger than that induced by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (100 microM) or glutamate (100 microM). When kainate (100 microM-10 mM) was co-applied with each of the agonists (1 microM-1 mM), the sustained current was not the algebraic sum of the currents activated by kainate or the other agonist alone; rather, the kainate-induced current was increasingly occluded by co-application with increasing concentrations of another agonist. The potency to occlude kainate-induced current had a rank order of quisqualate greater than alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate approximately glutamate; although at sufficiently high concentrations all three agonists could occlude the kainate-induced current completely. When kainate and quisqualate were co-applied during the continued presence of quisqualate, the onset of the kainate-induced sustained current was dramatically slowed. However, the steady-state occlusion by quisqualate could be abolished when the ratio kainate to quisqualate was raised to 100:1; therefore, the occlusion appears to involve a competition between kainate and quisqualate at some shared receptor binding sites which have a higher affinity for quisqualate than kainate.