Localisation of the GABA(C) receptors at the axon terminal of the rod bipolar cells of the mouse retina

Synaptic or Non-synaptic? Different Intercellular Interactions with Retinal Ganglion Cells in Optic Nerve Regeneration

Axons of adult neurons in the mammalian central nervous system generally fail to regenerate by themselves, and few if any therapeutic options exist to reverse this situation. Due to a weak intrinsic potential for axon growth and the presence of strong extrinsic inhibitors, retinal ganglion cells (RGCs) cannot regenerate their axons spontaneously after optic nerve injury and eventually undergo apoptosis, resulting in permanent visual dysfunction. Regarding the extracellular environment, research to date has generally focused on glial cells and inflammatory cells, while few studies have discussed the potentially significant role of interneurons that make direct connections with RGCs as part of the complex retinal circuitry. In this study, we provide a novel angle to summarize these extracellular influences following optic nerve injury as "intercellular interactions" with RGCs and classify these interactions as synaptic and non-synaptic. By discussing current knowledge of non-synaptic (glial cells and inflammatory cells) and synaptic (mostly amacrine cells and bipolar cells) interactions, we hope to accentuate the previously neglected but significant effects of pre-synaptic interneurons and bring unique insights into future pursuit of optic nerve regeneration and visual function recovery.

Elevating Growth Factor Responsiveness and Axon Regeneration by Modulating Presynaptic Inputs

Despite robust effects on immature neurons, growth factors minimally promote axon regeneration in the adult central nervous system (CNS). Attempting to improve growth-factor responsiveness in mature neurons by dedifferentiation, we overexpressed Lin28 in the retina. Lin28-treated retinas responded to insulin-like growth factor-1 (IGF1) by initiating retinal ganglion cell (RGC) axon regeneration after axotomy. Surprisingly, this effect was cell non-autonomous. Lin28 expression was required only in amacrine cells, inhibitory neurons that innervate RGCs. Ultimately, we found that optic-nerve crush pathologically upregulated activity in amacrine cells, which reduced RGC electrical activity and suppressed growth-factor signaling. Silencing amacrine cells or pharmacologically blocking inhibitory neurotransmission also induced IGF1 competence. Remarkably, RGCs regenerating across these manipulations localized IGF1 receptor to their primary cilia, which maintained their signaling competence and regenerative ability. Thus, our results reveal a circuit-based mechanism that regulates CNS axon regeneration and implicate primary cilia as a regenerative signaling hub.

Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAA and GABAC receptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

An Update on GABAρ Receptors

The present review discusses the functional and molecular diversity of GABAρ receptors. These receptors were originally described in the mammalian retina, and their functional role in the visual pathway has been recently elucidated; however new studies on their distribution in the brain and spinal cord have revealed that they are more spread than originally thought, and thus it will be important to determine their physiological contribution to the GABAergic transmission in other areas of the central nervous system. In addition, molecular modeling has revealed peculiar traits of these receptors that have impacted on the interpretations of the latest pharmacolgical and biophysical findings. Finally, sequencing of several vertebrate genomes has permitted a comparative analysis of the organization of the GABAρ genes.

GABA(C) Receptors in the Lateral Amygdala: A Possible Novel Target for the Treatment of Fear and Anxiety Disorders?

Activation of GABA(A)Rs in the lateral nucleus of the amygdala (LA), a key site of plasticity underlying fear learning, impairs fear learning. The role of GABA(C)Rs in the LA and other brain areas is poorly understood. GABA(C)Rs could be an important novel target for pharmacological treatments of anxiety-related disorders since, unlike GABA(A)Rs, GABA(C)Rs do not desensitize. To detect functional GABA(C)Rs in the LA we performed whole cell patch clamp recordings in vitro. We found that GABA(A)Rs and GABA(B)Rs blockade lead to a reduction of evoked inhibition and an increase increment of excitation, but activation of GABA(C)Rs caused elevations of evoked excitation, while blocking GABA(C)Rs reduced evoked excitation. Based on this evidence we tested whether GABA(C)Rs in LA contribute to fear learning in vivo. It is established that activation of GABA(A)Rs leads to blockage of fear learning. Application of GABA(C) drugs had a very different effect; fear learning was enhanced by activating and attenuated by blocking GABA(C)Rs in the LA. Our results suggest that GABA(C) and GABA(A)Rs play opposing roles in modulation of associative plasticity in LA neurons of rats. This novel role of GABA(C)Rs furthers our understanding of GABA receptors in fear memory acquisition and storage and suggests a possible novel target for the treatment of fear and anxiety disorders.

Transporter-mediated GABA responses in horizontal and bipolar cells of zebrafish retina

GABA-mediated interactions between horizontal cells (HCs) and bipolar cells (BCs) transform signals within the image-processing circuitry of distal retina. To further understand this process, we have studied the GABA-driven membrane responses from isolated retinal neurons. Papain-dissociated retinal cells from adult zebrafish were exposed to GABAergic ligands while transmembrane potentials were monitored with a fluorescent voltage-sensitive dye (oxonol, DiBaC4(5)). In HCs hyperpolarizing, ionotropic GABA responses were almost never seen, nor were responses to baclofen or glycine. A GABA-induced depolarization followed by after hyperpolarization (dep/AHP) occurred in 38% of HCs. The median fluorescence increase (dep component) was 0.17 log units, about 22 mV. HC dep/AHP was not blocked by bicuculline or picrotoxin. Muscimol rarely evoked dep/AHP responses. In BCs picrotoxin sensitive, hyperpolarizing, ionotropic GABA and muscimol responses occurred in most cells. A picrotoxin insensitive dep/AHP response was seen in about 5% of BCs. The median fluorescence increase (dep component) was 0.18 log units, about 23 mV. Some BCs expressed both muscimol-induced hyperpolarizations and GABA-induced dep/AHP responses. For all cells, the pooled Hill fit to median dep amplitudes, in response to treatments with a GABA concentration series, gave an apparent k of 0.61 muM and an n of 1.1. The dep/AHP responses of all cells required both extracellular Na+ and Cl(-), as dep/AHP was blocked reversibly by Li+ substituted for Na+ and irreversibly by isethionate substituted for Cl(-). All cells with dep/AHP responses in zebrafish have the membrane physiology of neurons expressing GABA transporters. These cells likely accumulate GABA, a characteristic of GABAergic neurons. We suggest Na+ drives GABA into these cells, depolarizing the plasma membrane and triggering Na+, K+-dependent ATPase. The ATPase activity generates AHP. In addition to a GABA clearance function, these large-amplitude transporter responses may provide an outer plexiform layer GABA sensor mechanism.

Electrophysiological evidence of GABAA and GABAC receptors on zebrafish retinal bipolar cells

To refine inhibitory circuitry models for ON and OFF pathways in zebrafish retina, GABAergic properties of zebrafish bipolar cells were studied with two techniques: whole cell patch responses to GABA puffs in retinal slice, and voltage probe responses in isolated cells. Retinal slices documented predominantly axon terminal responses; isolated cells revealed mainly soma-dendritic responses. In the slice, GABA elicited a conductance increase, GABA responses were more robust at axon terminals than dendrites, and Erev varied with [Cl(-)]in. Axon terminals of ON- and OFF-type cells were similarly sensitive to GABA (30-40 pA peak current); axotomized cells were unresponsive. Bicuculline-sensitive, picrotoxin-sensitive, and picrotoxin-insensitive components were identified. Muscimol was as effective as GABA; baclofen was ineffective. Isolated bipolar cells were either intact or axotomized. Even in cells without an axon, GABA or muscimol (but not baclofen) hyperpolarized dendritic and somatic regions, suggesting significant distal expression. Median fluorescence change for GABA was -0.22 log units (approximately -16 mV); median half-amplitude dose was 0.4 microM. Reduced [Cl(-)]out blocked GABA responses. GABA hyperpolarized isolated ON-bipolar cells; OFF-cells were either unresponsive or depolarized. Hyperpolarizing GABA responses in isolated cells were bicuculline and TPMPA insensitive, but blocked or partially blocked by picrotoxin or zinc. In summary, axon terminals contain bicuculline-sensitive GABAA receptors and both picrotoxin-sensitive and insensitive GABAC receptors. Dendritic processes express zinc- and picrotoxin-sensitive GABAC receptors.

Physiology and pathology of somatostatin in the mammalian retina: a current view

In the retina, peptidergic signalling participates in multiple circuits of visual information processing. The neuropeptide somatostatin (SRIF) is localised to amacrine cells and, in some instances, in a subset of ganglion cells. The variegated expression patterns of SRIF receptors (sst(1)-sst(5)) and the variety of signalling mechanisms activated by retinal SRIF suggest that this peptide may exert multiple actions on retinal neurons and on retinal physiology, although our current understanding reflects a rather complicated picture. SRIF, mostly through sst(2), may act as a positive factor in the retina by regulating retinal homeostasis and protecting neurons against damage. In this respect, SRIF analogues seem to constitute a promising therapeutic arsenal to cure different retinal diseases, as for instance, ischemic and diabetic retinopathies. However, further investigations are needed not only to fully understand the functional role of the SRIF system in the retina but also to exploit new chemical space for drug-like molecules.

GABA(C) receptors in retina and brain

The expression of GABA(C) receptors has long been regarded as a specific property of bipolar cells in the inner retina where they control the information transfer from bipolar to retinal ganglion cells. A number of recent anatomical and physiological studies, however, have provided evidence that GABA(C) receptors are also expressed in many brain structures apart from the retina. The presence of GABA(C) receptors in many GABAergic neurons suggests that this receptor type may be involved in the regulation of local inhibition. This chapter focuses on the distribution of GABA(C) receptors and their possible function in various brain areas.

Patch-clamp investigations and compartmental modeling of rod bipolar axon terminals in an in vitro thin-slice preparation of the mammalian retina

To extend the usefulness of rod bipolar cells for studies of chemical synaptic transmission, we have performed electrophysiological recordings from rod bipolar axon terminals in an in vitro slice preparation of the rat retina. Whole cell recordings from axon terminals and cell bodies were used to investigate the passive membrane properties of rod bipolar cells and analyzed with a two-compartment equivalent electrical circuit model developed by Mennerick et al. For both terminal- and soma-end recordings, capacitive current decays were well fitted by biexponential functions. Computer simulations of simplified models of rod bipolar cells demonstrated that estimates of the capacitance of the axon terminal compartment can depend critically on the recording location, with terminal-end recordings giving the best estimates. Computer simulations and whole cell recordings demonstrated that terminal-end recordings can yield more accurate estimates of the peak amplitude and kinetic properties of postsynaptic currents generated at the axon terminals due to increased electrotonic filtering of these currents when recorded at the soma. Finally, we present whole cell and outside-out patch recordings from axon terminals with responses evoked by GABA and glycine, spontaneous inhibitory postsynaptic currents, voltage-gated Ca(2+) currents, and depolarization-evoked reciprocal synaptic responses, verifying that the recorded axon terminals are involved in normal pre- and postsynaptic relationships. These results demonstrate that axon terminals of rod bipolar cells are directly accessible to whole cell and outside-out patch recordings, extending the usefulness of this preparation for detailed studies of pre- and postsynaptic mechanisms of synaptic transmission in the CNS.

Capacitance measurements in the mouse rod bipolar cell identify a pool of releasable synaptic vesicles

The mouse is an important model system for understanding the molecular basis of neuronal signaling and diseases of synaptic communication. However, the best-characterized retinal ribbon-style synapses are those of nonmammalian vertebrates. To remedy this situation, we asked whether it would be feasible to track synaptic vesicle dynamics in the isolated mouse rod bipolar cell using time-resolved capacitance measurements. The results demonstrate that membrane depolarization triggered an increase in membrane capacitance that was Ca(2+) dependent and restricted to the synaptic compartment, consistent with exocytosis. The amplitude of the capacitance response recorded from the easily accessible soma of an intact mouse rod bipolar cell was identical to that recorded directly from the small synaptic terminal, suggesting that in the carefully selected cohort of cells presented here, axonal resistance was not a significant barrier to current flow. This supposition was supported by the analysis of passive membrane properties and a comparison of membrane capacitance measurements in cells with and without synaptic terminals and reinforced by the lack of an effect of sine-wave frequency (200-1,600 Hz) on the measured capacitance increase. The magnitude of the capacitance response increased with Ca(2+) entry until a plateau was reached at a spatially averaged intraterminal calcium of about 600 nM. We interpret this plateau, nominally 30 fF, as corresponding to a releasable pool of synaptic vesicles. The robustness of this measure suggests that capacitance measurements may be used in the mouse rod bipolar cell to compare pool size across treatment conditions.

Modulation by brain natriuretic peptide of GABA receptors on rat retinal ON-type bipolar cells

Natriuretic peptides (NPs) may work as neuromodulators through their associated receptors [NP receptors (NPRs)]. By immunocytochemistry, we showed that NPR-A and NPR-B were expressed abundantly on both ON-type and OFF-type bipolar cells (BCs) in rat retina, including the dendrites, somata, and axon terminals. Whole-cell recordings made from isolated ON-type BCs further showed that brain natriuretic peptide (BNP) suppressed GABAA receptor-, but not GABAC receptor-, mediated currents of the BCs, which was blocked by the NPR-A antagonist anantin. The NPR-C agonist c-ANF [des(Gln18, Ser19, Gln20, Leu21, Gly22)ANF(4-23)-NH2] did not suppress GABAA currents. The BNP effect on GABAA currents was abolished with preincubation with the pGC-A/B antagonist HS-142-1 but mimicked by application of 8-bromoguanosine-3',5'-cyclomonophosphate. These results suggest that elevated levels of intracellular cGMP caused by activation of NPR-A may mediate the BNP effect. Internal infusion of the cGMP-dependent protein kinase G (PKG) inhibitor KT5823 essentially blocked the BNP-induced reduction of GABAA currents. Moreover, calcium imaging showed that BNP caused a significant elevation of intracellular calcium that could be caused by increased calcium release from intracellular stores by PKG. The BNP effect was blocked by the ryanodine receptor modulators caffeine, ryanodine, and ruthenium red but not by the IP3 receptor antagonists heparin and xestospongin-C. Furthermore, the BNP effect was abolished after application of the blocker of endoplasmic reticulum Ca2+-ATPase thapsigargin and greatly reduced by the calmodulin inhibitors W-7 and calmidazolium. We therefore conclude that the increased calcium release from ryanodine-sensitive calcium stores by BNP may be responsible for the BNP-caused GABAA response suppression in ON-type BCs through stimulating calmodulin.

Depolarizing effect of GABA in rod bipolar cells of the mouse retina

Gamma-amino butyric acid (GABA) has been characterized as inhibitory neurotransmitter through chloride mediated channels in the adult nervous system. However, using gramicidin perforated patch-clamp recordings from rod bipolar cells dissociated from retinas of adult mice, we find that GABA is capable of inducing cell depolarization. Currents mediated by GABA(A) and GABA(C) receptors were further isolated by the use of GABA receptor specific blockers. In rod bipolar cells dissociated from the mouse retina, activation of GABA(A) receptors located at the cell dendrites induces ionic currents which show a reversal potential of -33 mV. However, local activation of GABA(C) receptors located at the axon terminal induces ionic currents with a reversal potential of -60 mV. According to Nernst equation, the dendrites of rod bipolar cells of the mouse retina would have a high intracellular chloride concentration ([Cl(-)](i)) and there must be an intracellular gradient in [Cl(-)](i), being the [Cl(-)](i) more elevated in the dendrites than in the axon terminal. The depolarizing effect of GABA at the dendrites of rod bipolar cells may contribute to the lateral interaction in the mammalian retina, thereby enhancing visual discrimination of stimuli input.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

Temporal modulation of scotopic visual signals by A17 amacrine cells in mammalian retina in vivo

We examined function of the feedback pathway from A17 GABAergic amacrine cells to rod bipolar cells (A17 feedback), a critically located inhibitory circuit in the classic rod pathway of the mammalian retina whose role in processing of scotopic visual information is still poorly understood. We show evidence that this A17 feedback has a profound influence on the temporal properties of rod-driven postphotoreceptoral responses (assessed with the scotopic electroretinogram b-wave). Application of a GABA(c) antagonist prolonged preferentially the decay of the scotopic b-wave. The degree of prolongation increased as the light intensity decreased. Application of selective GABA(a) antagonists accelerated the kinetics of the scotopic b-wave. This effect was abolished when the GABA(c) antagonist was coapplied. Selective ablation of A17 cells mimicked the action of the GABA(c) antagonist. In A17 cell-ablated retinas, the GABA(c) antagonist was no longer very effective to slow the decay of the scotopic b-wave. Thus the A17 feedback, activated by light stimulation and mediated mainly by the GABA(c) receptors, makes the scotopic b-wave more transient by accelerating preferentially its decay. The strength of the feedback can be modulated by GABA(a) receptor-mediated inhibition and by light intensity. Our results also suggest that in the mammalian retina the feedback may be a novel mechanism that contributes postphotoreceptorally to the termination of rod signals, especially those elicited by very dim light stimuli.

Functional modifications in rod bipolar cells in a mouse model of retinitis pigmentosa

The rd mouse has been widely used as an animal model of retinitis pigmentosa. In this model, a mutation of rod-specific phosphodiesterase leads to a loss of rods during the early period of postnatal life. Morphological modifications at the level of the outer plexiform layer have been shown (Proc. Nat. Acad. Sci. USA 97 (2000) 11020) in bipolar and horizontal cells. However, very little is known about the functional changes suffered by these cells postsynaptic to the degenerated rods. In the present work we have studied the neurotransmitter-induced currents in rod bipolar cells from the rd mouse retina. Currents induced by glutamate and GABA were studied by the patch clamp-whole cell technique, on rod bipolar cells enzymatically dissociated from the rd mouse retina. Data from rd animals were compared with non-dystrophic NMRI mice. GABA (30-100 micro M) and glutamate (100 micro M) were applied from a puff pipette in the near proximity of rod bipolar cell dendrites, clamped at physiological membrane potentials, and their evoked currents were studied. In rod bipolar cells from non-dystrophic mouse, puff application of glutamate induced an outward current. This current was increased twofold in absence of extracellular calcium (nominally 0 calcium). In rod bipolar cells from adult rd mouse, currents induced by glutamate were absent. Two types of GABA mediated currents were isolated in rod bipolar cells both in control and rd mouse retinas. The currents mediated by GABA(C) receptors were observed exclusively at the axon terminal, while the currents mediated by the GABA(A) receptors were observed upon GABA application to the bipolar cell dendrites. The currents mediated by GABA(A) receptors in rod bipolar cells from rd mouse were larger than those from control animals. We conclude that after the degeneration of rod photoreceptors in rd mouse, rod bipolar cells lost their glutamate (rod-neurotransmitter) input while they increase their response to GABA (horizontal cell-neurotransmitter). In our opinion, this work describes for the first time the changes in neurotransmitter sensitivity that affect rod bipolar cells after photoreceptor degeneration of the mouse retina.

Somatostatin (SRIF) and SRIF receptors in the mouse retina

In the retina, somatostatin (SRIF) acts as a neuromodulator by interacting with specific SRIF subtype (sst) receptors. The aim of this study was to detect mRNAs for sst(1-5) receptors by semiquantitative RT-PCR and to determine the cellular localization of either SRIF or individual SRIF receptor immunoreactivities. Size, density and absolute number of immunolabeled somata were measured using computer-assisted image analysis. With RT-PCR we found that all five sst receptor mRNAs were expressed, with highest levels of sst(2) and sst(4) receptors. SRIF immunolabeling was localized to sparse-occurring amacrine cells in the inner nuclear layer (INL) and to displaced amacrine cells in the ganglion cell layer (GCL). sst(2A) receptors were localized to protein kinase- (PKC) immunoreactive (IR) rod bipolar cells, calbindin- (CaBP-) IR horizontal cells, tyrosine hydroxylase- (TH-) IR amacrine cells and glycinergic amacrine cells. None of the sst(2A)-IR amacrine cells were found to express parvalbumin (PV) immunoreactivity. sst(4) receptor immunolabeling was localized to CaBP-IR and CaBP-non-IR cells in the GCL that originated long process bundles in the GC axon layer. These cells were not observed after optic nerve transection and they were therefore interpreted as ganglion cells. Quantitative analysis showed that all of the PKC-IR rod bipolar cells, CaBP-IR horizontal cells, and TH-IR amacrine cells and 5% of the glycinergic amacrine cells expressed sst(2A) receptors. In addition, 4-6% of the putative ganglion cells expressed sst(4) receptors. The localization of SRIF to sparse-occurring retinal neurons, together with the widespread expression of sst(2A) and sst(4) receptors suggests that SRIF acts at multiple levels of retinal circuitry. These results provide a database for investigations of the functional retinal networks in mice with genetic alterations of somatostatinergic transmission.

The effects of vigabatrin on electrophysiology and visual fields in epileptics: a controlled study with a discussion of possible mechanisms

PURPOSE

To compare the visual electrophysiology and visual fields of patients taking vigabatrin to those of a control group of epileptics on other anti-epileptic drugs (AEDs).

METHODS

Fourteen epileptics treated with vigabatrin and 10 control patients treated with other AEDs underwent ERG and EOG. Goldmann visual fields were performed and analysed using standard software to measure areas contained within I4e isopters.

RESULTS

The cone and rod b-waves of the ERG, the oscillatory potential amplitudes and Arden indices were reduced in vigabatrin-treated subjects and the oscillatory potentials delayed. The Arden indices were reduced due to an increased dark trough. The areas contained within the I4e isopter of vigabatrin treated subjects were reduced compared to the control group and these areas correlated well with oscillatory potential amplitudes and b-wave amplitudes in the vigabatrin group only.

CONCLUSIONS

The use of vigabatrin is associated with a reduction of the ERG cone b-wave amplitude and oscillatory potentials which correlates with visual field loss. The Arden ratio is reduced in subjects taking vigabatrin but may recover after cessation. However, visual loss may persist in the presence of a recovered EOG. These findings suggest further effects of the drug than those mediated by GABA receptors, and support the contention that the cause of the field loss may be at least in part due to retinal effects. Possible mechanisms are discussed.

Developmentally regulated expression of GABA receptor rho1 and rho2 subunits, L7 and cone-rod homeobox (CRX) genes in mouse retina

In this study, we compared the temporal expression pattern of four retinal genes; rho1 and rho2 that encode subunits of GABA(c) receptors, L7 that encodes Purkinje cell protein and CRX that encodes the cone-rod homeobox transcription factor. A reverse-transcription-polymerase chain reaction (RT-PCR) strategy that generated a linear correlation between the amount of retinal RNA and the amount of amplified product was used to quantify transcripts from each gene. Results with this method showed that the rho1 and L7 have similar developmental patterns. Both exhibit basal level expression before P7. From P7 to P20, the RNA levels for both genes were increased about 12-fold. After P20, the RNA levels remained unchanged. Compared to rho1 and L7, expression of rho2 began later, since the rho2 RNA could not be detected until P10. At P10, the rho2 RNA level was about 10% of its level at P35. Expression of rho2 reached its peak at a later developmental stage compared to that of rho1 and L7. The different temporal patterns were confirmed by co-amplification of rho1, rho2, and L7 in a single PCR tube. CRX RNA was detected at embryonic day 15 (E15) and increased progressively, in agreement with a prior study using in situ hybridization. These data, combined with evidence that the tissue distribution of rho1 and L7 RNA in the CNS are similar, indicates that rho1 and L7 may share common transcriptional regulatory elements. Furthermore, the difference in the timing of rho subunit expression suggests that the subunit composition of GABA(c) receptors vary during retinal development.