Two metabotropic gamma-aminobutyric acid receptors differentially modulate calcium currents in retinal ganglion cells

GABAB receptors enhance excitatory responses in isolated rat retinal ganglion cells

KEY POINTS

GABA is an inhibitory transmitter but can sometimes produce paradoxical excitatory effects through synaptic networks. We found a novel GABA-mediated excitation within a single retinal cell. It involves a chain of events from receptor stimulation to the sequential modulation of two associated channels, resulting in enhanced neuroexcitability. GABAB receptor activation selectively suppresses N-type calcium channels. The BK-type potassium channels are exclusively linked to the N-type calcium channel. Thus, stimulation of GABAB receptors suppresses an outward current, increasing the excitatory range of single neurons.

ABSTRACT

GABAB receptors (GABAB Rs) suppress voltage-gated calcium channels and activate G-protein coupled potassium channels (GIRK and TREK channels), both mechanisms serving to inhibit neurons. In isolated rat retinal spiking neurons, GABAB Rs produce both actions but the net effect is to enhance excitatory signals. This is because GABAB Rs selectively suppress N-type calcium channels, which in turn are specifically linked to BK channels. Consequently, when GABAB Rs are stimulated there is a reduction in outward current, allowing neurons to extend their level of depolarization. Whereas many retinal neurons use L-type channels to stimulate vesicle fusion, the suppression of N-type channels augments dynamic range without affecting transmitter release.

GABAergic neurotransmission and retinal ganglion cell function

Ganglion cells are the output retinal neurons that convey visual information to the brain. There are ~20 different types of ganglion cells, each encoding a specific aspect of the visual scene as spatial and temporal contrast, orientation, direction of movement, presence of looming stimuli; etc. Ganglion cell functioning depends on the intrinsic properties of ganglion cell's membrane as well as on the excitatory and inhibitory inputs that these cells receive from other retinal neurons. GABA is one of the most abundant inhibitory neurotransmitters in the retina. How it modulates the activity of different types of ganglion cells and what is its significance in extracting the basic features from visual scene are questions with fundamental importance in visual neuroscience. The present review summarizes current data concerning the types of membrane receptors that mediate GABA action in proximal retina; the effects of GABA and its antagonists on the ganglion cell light-evoked postsynaptic potentials and spike discharges; the action of GABAergic agents on centre-surround organization of the receptive fields and feature related ganglion cell activity. Special emphasis is put on the GABA action regarding the ON-OFF and sustained-transient ganglion cell dichotomy in both nonmammalian and mammalian retina.

GABAB1 and GABAB2 receptor subunits co-expressed in cultured human RPE cells regulate intracellular Ca2+ via Gi/o-protein and phospholipase C pathways

GABAB receptors associate with Gi/o-proteins that regulate voltage-gated Ca(2+) channels and thus the intracellular Ca(2+) concentration ([Ca(2+)]i), there is also reported cross-regulation of phospholipase C. These associations have been studied extensively in the brain and also shown to occur in non-neural cells (e.g. human airway smooth muscle). More recently GABAB receptors have been observed in chick retinal pigment epithelium (RPE). The aims were to investigate whether the GABAB receptor subunits, GABAB1 and GABAB2, are co-expressed in cultured human RPE cells, and then determine if the GABAB receptor similarly regulates the [Ca(2+)]i of RPE cells and if phospholipase C is involved. Human RPE cells were cultured from five donor eye cups. Evidence for GABAB1 and GABAB2 mRNAs and proteins in the RPE cell cultures was investigated using real time polymerase chain reaction, western blots and immunofluorescence. The effects of the GABAB receptor agonist baclofen, antagonist CGP46381, a Gi/o-protein inhibitor pertussis toxin, and the phospholipase C inhibitor U73122 on [Ca(2+)]i in cultured human RPE were demonstrated using Fluo-3. Both GABAB1 and GABAB2 mRNA and protein were identified in cell cultures of human RPE; antibody staining was co-localized to the cell membrane and cytoplasm. One-hundred micromolars of baclofen caused a transient increase in the [Ca(2+)]i of RPE cells regardless of whether Ca(2+) was added to the buffer. Baclofen-induced increases in the [Ca(2+)]i were attenuated by pre-treatment with CGP46381, pertussis toxin, and U73122. GABAB1 and GABAB2 are co-expressed in cell cultures of human RPE. GABAB receptors in RPE regulate the [Ca(2+)]i via a Gi/o-protein and phospholipase C pathway.

R-Isovaline: a subtype-specific agonist at GABA(B)-receptors?

The R-enantiomer of isovaline, an analgesic amino acid, has a chemical structure similar to glycine and GABA. Although its actions on thalamic neurons are strychnine-resistant and independent of the Cl(-) gradient, R-isovaline increases membrane conductance for K(+). The purpose of this study was to determine if R-isovaline activated metabotropic GABA(B) receptors. We used whole-cell voltage-clamp recordings to characterize the effects of R-isovaline applied by bath perfusion and local ejection from a micropipette to thalamic neurons in 250 μm thick slices of rat brain. The immunocytochemical methods that we employed to visualize GABA(B1) and GABA(B2) receptor subunits showed extensive staining for both subunits in ventrobasal nuclei, which were the recording sites. Bath or local application of R-isovaline caused a slowly developing increase in conductance and outward rectification in 70% (54/77) of neurons, both effects reversing near the K(+) Nernst potential. As with the GABA(B) agonist baclofen, G proteins likely mediated the R-isovaline effects because they were susceptible to blockade by non-hydrolyzable substrates of guanosine triphosphate. The GABA(B) antagonists CGP35348 and CGP52432 prevented the conductance increase induced by R-isovaline, applied by bath or local ejection. The GABA(B) allosteric modulator CGP7930 enhanced the R-isovaline induced increase in conductance. At high doses, antagonists of GABA(A), GABA(C), glycine(A), μ-opioid, and nicotinic receptors did not block R-isovaline responses. The observations establish that R-isovaline increases the conductance of K(+) channels coupled to metabotropic GABA(B) receptors. Remarkably, not all neurons that were responsive to baclofen responded to R-isovaline. The R-isovaline-induced currents outlasted the fast baclofen responses and persisted for a 1-2-h period. Despite some similar actions, R-isovaline and baclofen do not act at identical GABA(B) receptor sites. The binding of R-isovaline and baclofen to the GABA(B) receptor may not induce the same conformational changes in receptor proteins or components of the intracellular signaling pathways.

GABA(B) receptor feedback regulation of bipolar cell transmitter release

GABAergic amacrine cell feedback to bipolar cells in retina has been described, activating both GABA(A) and GABA(C) receptors. We explored whether metabotropic GABA(B) receptors also participate in this feedback pathway. CGP55845, a potent GABA(B) receptor antagonist, was employed to determine the endogenous role of these receptors. Ganglion cell EPSCs and IPSCs were monitored to measure the output of bipolar and amacrine cells. Using the tiger salamander slice preparation, we found that GABA(B) receptor pathways regulate bipolar cell release directly and indirectly. In the direct pathway, the GABA(B) receptor antagonist reduces EPSC amplitude, indicating that GABA(B) receptors cause enhanced glutamate release from bipolar cells to one set of ganglion cells. In the indirect pathway, the GABA(B) receptor antagonist reduces EPSC amplitude in another set of ganglion cells. The indirect pathway is only evident when GABA(A) receptors are inhibited, and is blocked by a glycine receptor antagonist. Thus, this second feedback pathway involves direct glycine feedback to the bipolar cell and this glycinergic amacrine cell is suppressed by GABAergic amacrine cells, through both GABA(A) and GABA(B) but not GABA(C) receptors. Overall, GABA(B) receptors do contribute to feedback regulation of bipolar cell transmitter release. However, unlike the ionotropic GABA receptor pathways, the metabotropic GABA receptor pathways act to enhance bipolar cell transmitter release. Furthermore, there are three discrete subsets of bipolar cell output regulated by GABA(B) receptor feedback (direct, indirect and null), implying three distinct, non-overlapping bipolar cell to ganglion cell circuits.

Synaptic inhibition by glycine acting at a metabotropic receptor in tiger salamander retina

Glycine is the lone fast neurotransmitter for which a metabotropic pathway has not been identified. In retina, we found a strychnine-insensitive glycine response in bipolar and ganglion cells. This glycine response reduced high voltage-activated calcium current. It was G-protein mediated and protein kinase A dependent. The EC(50) of the metabotropic glycine response is 3 mum, an order of magnitude lower than the ionotropic glycine receptor in the same retina. The bipolar cell glutamatergic input to ganglion cells was suppressed by metabotropic glycine action. The synaptic output of about two-thirds of bipolar cells and calcium current in two-thirds of ganglion cells are sensitive to the action of glycine at metabotropic receptors, suggesting this signal regulates specific synaptic pathways in proximal retina. This study resolves the curious absence of a metabotropic glycine pathway in the nervous system and reveals that the major fast inhibitory neurotransmitters, GABA and glycine, both activate G-protein-coupled pathways as well.

Characterization of the binding of [3H]CGP54626 to GABAB receptors in the male bullfrog (Rana catesbeiana)

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the vertebrate brain. GABA activates both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors in mammals. Whether non-mammalian vertebrates possess receptors with similar characteristics is not well understood. We used a mammalian GABA(B)-specific antagonist to determine the pharmacology of putative receptors in the brain of an anuran amphibian, the male bullfrog (Rana catesbeiana). Receptor binding assays with the antagonist [(3)H]CGP54626 revealed a single class of high affinity binding sites (with a K(D) of 2.97 nM and a B(max) of 2619 fmol/mg protein). Binding was time- and temperature-dependent, saturable and specific. Specific binding of [(3)H]CGP54626 was inhibited by several mammalian GABA(B) receptor agonists and antagonists. The rank order potency of agonists was: GABA = SKF97541 > (R)-Baclofen > 3-APPA. The rank order for antagonists was: CGP54626 = CGP55845 > CGP52432 > CGP35348. The GABA(A) receptor ligands muscimol and SR95531 had very low affinity for [(3)H]CGP54626 binding sites, while bicuculline compounds had no affinity. Binding of GABA was positively modulated by CGP7930. Taurine did not allosterically modulate GABA binding but did inhibit [(3)H]CGP54626 binding in a linear fashion. Bullfrog brain thus possesses binding sites with significant similarity to mammalian GABA(B) receptors. These receptors differ from mammalian receptors, however, in dissociation kinetics, ligand specificity and allosteric modulation.

Compartmental localization of gamma-aminobutyric acid type B receptors in the cholinergic circuitry of the rabbit retina

Although many effects of gamma-aminobutyric acid (GABA) on retinal function have been attributed to GABA(A) and GABA(C) receptors, specific retinal functions have also been shown to be mediated by GABA(B) receptors, including facilitation of light-evoked acetylcholine release from the rabbit retina (Neal and Cunningham [1995] J. Physiol. 482:363-372). To explain the results of a rich set of experiments, Neal and Cunningham proposed a model for this facilitation. In this model, GABA(B) receptor-mediated inhibition of glycinergic cells would reduce their own inhibition of cholinergic cells. In turn, muscarinic input from the latter to the glycinergic cells would complete a negative-feedback circuitry. In this study, we have used immunohistochemical techniques to test elements of this model. We report that glycinergic amacrine cells are GABA(B) receptor negative. In contrast, our data reveal the localization of GABA(B) receptors on cholinergic/GABAergic starburst amacrine cells. High-resolution localization of GABA(B) receptors on starburst amacrine cells shows that they are discretely localized to a limited population of its varicosities, the majority of likely synaptic-release sites being devoid of detectable levels of GABA(B) receptors. Finally, we identify a glycinergic cell that is a potential muscarinic receptor-bearing target of GABA(B)-modulated acetylcholine release. This target is the DAPI-3 cell. We propose, based on these data, a modification of the Neal and Cunningham model in which GABA(B) receptors are on starburst, not glycinergic amacrine cells.

Functional GABA(B) receptors are expressed at the cone photoreceptor terminals in bullfrog retina

GABA(B) receptors at the cone terminals in bullfrog retina were characterized by immunocytochemical and whole-cell patch clamp techniques in retinal slice preparations. Somata, axons and synaptic terminals (pedicles) of cones were both GABA(B) receptor (GABA(B)R) 1 and GABA(B)R2 immunoreactive. Physiologically, barium/calcium currents of cones to voltage steps were significantly reduced in size when GABA was puffed to cone terminals in the presence of picrotoxin that is supposed to block both GABA(A) and GABA(C) receptors. Similar reduction in barium currents was obtained with puff application of baclofen to cone terminals. These results suggest the presence of functional GABA(B) receptors at the bullfrog cone terminals. Suppression of barium currents of cones by baclofen was dose-dependent. Moreover, barium currents of cones were potentiated by background illumination, as compared with those recorded in the dark. 6,7-Dinitroquinoxaline-2,3-dione, an antagonist of non-NMDA receptors that hyperpolarizes horizontal cells and reduces GABA release from these cells, and saclofen, a GABA(B) receptor antagonist, both potentiated barium currents of cones in the dark, thereby mimicking the effects of background illumination. It is suggested that changes in calcium influx into the cone synaptic terminals due to activation of GABA(B) receptors may provide a negative feedback mechanism for regulating signal transmission between cones and second-order neurons in the retina by modifying the amount of glutamate released from the cones.

Ethanol modulates the expression of GABAB receptor mRNAs in the prenatal rat brain in an age and area dependent manner

Prenatal ethanol exposure has various deleterious effects on neuronal development. As GABA(B) receptor is known to play an important role during the development of the CNS, we now focused on its mRNA expression pattern in the rat brain during the late gestational days (GD) from 15.5 to GD 21.5. Ethanol's effect was also observed from GD 11.5 to GD 21.5. GABA(B1) receptor mRNA showed a high expression level in GD 15.5 and 19.5, while GABA(B2) receptor mRNA did in GD 15.5 and 21.5. The mRNAs levels depended on age and area during development. Ethanol exposure decreased GABA(B1) receptor from GD 11.5 to GD 19.5 with slight increases in GD 21.5. The decreasing effects were area dependent, with the highest effects in the forebrain including cortex, whereas slight effects were observed in the midbrain and hindbrain. The present results suggest an important role of GABA(B) receptor in the effects of ethanol on prenatal brain developmental processes.

Modulation of voltage-dependent calcium channels by neurotransmitters and neuropeptides in parasympathetic submandibular ganglion neurons

The control of saliva secretion is mainly under parasympathetic control, although there also could be a sympathetic component. Sympathetic nerves are held to have a limited action in secretion in submandibular glands because, on electrical stimulation, only a very small increase to the normal background, basal secretion occurs. Parasympathetic stimulation, on the other hand, caused a good flow of saliva with moderate secretion of acinar mucin, plus an extensive secretion of granules from the granular tubules. The submandibular ganglion (SMG) is a parasympathetic ganglion which receives inputs from preganglionic cholinergic neurons, and innervates the submandibular salivary gland to control saliva secretion. Neurotransmitters and neuropeptides acting via G-protein coupled receptors (GPCRs) change the electrical excitability of neurons. In these neurons, many neurotransmitters and neuropeptides modulate voltage-dependent calcium channels (VDCCs). The modulation is mediated by a family of GPCRs acting either directly through the membrane delimited G-proteins or through second messengers. However, the mechanism of modulation and the signal transduction pathway linked to an individual GPCRs depend on the animal species. This review reports how neurotransmitters and neuropeptides modulate VDCCs and how these modulatory actions are integrated in SMG systems. The action of neurotransmitters and neuropeptides on VDCCs may provide a mechanism for regulating SMG excitability and also provide a cellular mechanism of a variety of neuronal Ca(2+)-dependent processes.

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.

Long-term depression of synaptic inhibition is expressed postsynaptically in the developing auditory system

Inhibitory transmission is critically involved in the functional maturation of neural circuits within the brain. However, the mechanisms involved in its plasticity and development remain poorly understood. At an inhibitory synapse of the developing auditory brain stem, we used whole cell recordings to determine the site of induction and expression of long-term depression (LTD), a robust activity-dependent phenomenon that decreases inhibitory synaptic gain and is postulated to underlie synapse elimination. Recordings were obtained from lateral superior olivary (LSO) neurons, and hyperpolarizing inhibitory potentials were evoked by stimulation of the medial nucleus of the trapezoid body (MNTB). Both postsynaptic glycine and GABAA receptors could independently display LTD when isolated pharmacologically. Focal application of GABA, but not glycine, on the postsynaptic LSO neuron was sufficient to induce depression of the amino acid-evoked response, or MNTB-evoked inhibitory postsynaptic potentials. This GABA-mediated depression, in the absence of MNTB stimulation, was blocked by a GABAB receptor antagonist. To assess whether a change in neurotransmitter release is associated with the LTD, the polyvalent cation, ruthenium red, was used to increase the frequency of miniature inhibitory synaptic events. Consistent with a postsynaptic locus of expression, we found that the mean amplitude of miniature events decreased after LTD with no change in their frequency of occurrence. Furthermore, there was no change in the paired-pulse ratio or release kinetics of evoked inhibitory responses. Together, these results provide direct evidence that activity-dependent LTD of inhibition has a postsynaptic locus of induction and alteration, and that GABA but not glycine plays a pivotal role.

Cyclic AMP-dependent protein kinase phosphorylation facilitates GABA(B) receptor-effector coupling

GABA (gamma-aminobutyric acid)(B) receptors are heterodimeric G protein-coupled receptors that mediate slow synaptic inhibition in the central nervous system. Here we show that the functional coupling of GABA(B)R1/GABA(B)R2 receptors to inwardly rectifying K(+) channels rapidly desensitizes. This effect is alleviated after direct phosphorylation of a single serine residue (Ser892) in the cytoplasmic tail of GABA(B)R2 by cyclic AMP (cAMP)-dependent protein kinase (PKA). Basal phosphorylation of this residue is evident in rat brain membranes and in cultured neurons. Phosphorylation of Ser892 is modulated positively by pathways that elevate cAMP concentration, such as those involving forskolin and beta-adrenergic receptors. GABA(B) receptor agonists reduce receptor phosphorylation, which is consistent with PKA functioning in the control of GABA(B)-activated currents. Mechanistically, phosphorylation of Ser892 specifically enhances the membrane stability of GABA(B) receptors. We conclude that signaling pathways that activate PKA may have profound effects on GABA(B) receptor-mediated synaptic inhibition. These results also challenge the accepted view that phosphorylation is a universal negative modulator of G protein-coupled receptors.

Adenosine inhibits calcium channel currents via A1 receptors on salamander retinal ganglion cells in a mini-slice preparation

The effects of adenosine on high-voltage-activated calcium channel currents in tiger salamander retinal ganglion cells were investigated in a mini-slice preparation. Adenosine produced a concentration-dependent decrease in the amplitude of calcium channel current with a maximum inhibition of 26%. The effects of adenosine on calcium channel current were both time- and voltage-dependent. In cells dialyzed with GTP-gamma-s, adenosine caused a sustained and irreversible inhibition of calcium channel current, suggesting involvement of a GTP-binding protein. The inhibitory effect of adenosine on calcium channel current was blocked by the A1 antagonist 8-cyclopentyltheophylline (DPCPX, 1-10 microm), but not by the A2 antagonist 3-7-dimethyl-1-propargylxanthine (DMPX, 10 microm), and was mimicked by the A1 agonist N6-cyclohexyladenosine (CHA, 1 microm) but not by the A2 agonist 5'-(N-cyclopropyl) carbox-amidoadenosine (CPCA, 1 microm). Adenosine's inhibition of calcium channel current was not affected by the L-type calcium channel blocker nifedipine (5 microm). However, adenosine's inhibition of calcium channel current was reduced to approximately 10% after application of omega-conotoxin GVIA (1 microm), suggesting that adenosine inhibits N-type calcium channels. These results show that adenosine acts on an A1 adenosine receptor subtype via a G protein-coupled pathway to inhibit the component of calcium channel current carried in N-type calcium channels.

Calcium and retinal function

We survey the primary roles of calcium in retinal function, including photoreceptor transduction, transmitter release by different classes of retinal neuron, calcium-mediated regulation of gap-junctional conductance, activation of certain voltage-gated channels for K+ and Cl-, and modulation of postsynaptic potentials in retinal ganglion cells. We discuss three mechanisms for changing [Ca2+]i, which include flux through voltage-gated calcium channels, through ligand-gated channels, and by release from stores. The neuromodulatory pathways affecting each of these routes of entry are considered. The many neuromodulatory mechanisms in which calcium is a player are described and their effects upon retinal function discussed.

A dopamine- and protein kinase A-dependent mechanism for network adaptation in retinal ganglion cells

Vertebrates can detect light intensity changes in vastly different photic environments, in part, because postreceptoral neurons undergo "network adaptation." Previous data implicated dopaminergic, cAMP-dependent inhibition of retinal ganglion cells in this process yet left unclear how this occurs and whether this occurs in darkness versus light. To test for light- and dopamine-dependent changes in ganglion cell cAMP levels in situ, we immunostained dark- and light-adapted retinas with anti-cAMP antisera in the presence and absence of various dopamine receptor ligands. To test for direct effects of dopamine receptor ligands and membrane-permeable protein kinase ligands on ganglion cell excitability, we recorded spikes from isolated ganglion cells in perforated-patch whole-cell mode before and during application of these agents by microperfusion. Our immunostainings show that light, endogenous dopamine, and exogenous dopamine elevate ganglion cell cAMP levels in situ by activating D1-type dopamine receptors. Our spike recordings show that D1-type agonists and 8-bromo cAMP reduce spike frequency and curtail sustained spike firing and that these effects entail protein kinase A activation. These effects resemble those of background light on ganglion cell responses to light flashes. Network adaptation could thus be produced, to some extent, by dopaminergic modulation of ganglion cell spike generation, a mechanism distinct from modulation of transmitter release onto ganglion cells or of transmitter-gated currents in ganglion cells. Combining these observations with results obtained in studies of photoreceptor, bipolar, and horizontal cells indicates that all three layers of neurons in the retina are equipped with mechanisms for adaptation to ambient light intensity.

A dopamine- and protein kinase A-dependent mechanism for network adaptation in retinal ganglion cells

Vertebrates can detect light intensity changes in vastly different photic environments, in part, because postreceptoral neurons undergo "network adaptation." Previous data implicated dopaminergic, cAMP-dependent inhibition of retinal ganglion cells in this process yet left unclear how this occurs and whether this occurs in darkness versus light. To test for light- and dopamine-dependent changes in ganglion cell cAMP levels in situ, we immunostained dark- and light-adapted retinas with anti-cAMP antisera in the presence and absence of various dopamine receptor ligands. To test for direct effects of dopamine receptor ligands and membrane-permeable protein kinase ligands on ganglion cell excitability, we recorded spikes from isolated ganglion cells in perforated-patch whole-cell mode before and during application of these agents by microperfusion. Our immunostainings show that light, endogenous dopamine, and exogenous dopamine elevate ganglion cell cAMP levels in situ by activating D1-type dopamine receptors. Our spike recordings show that D1-type agonists and 8-bromo cAMP reduce spike frequency and curtail sustained spike firing and that these effects entail protein kinase A activation. These effects resemble those of background light on ganglion cell responses to light flashes. Network adaptation could thus be produced, to some extent, by dopaminergic modulation of ganglion cell spike generation, a mechanism distinct from modulation of transmitter release onto ganglion cells or of transmitter-gated currents in ganglion cells. Combining these observations with results obtained in studies of photoreceptor, bipolar, and horizontal cells indicates that all three layers of neurons in the retina are equipped with mechanisms for adaptation to ambient light intensity.

Comparative immunohistochemical localisation of GABA(B1a), GABA(B1b) and GABA(B2) subunits in rat brain, spinal cord and dorsal root ganglion

GABA(B) receptors are G-protein-coupled receptors mediating the slow onset and prolonged synaptic actions of GABA in the CNS. The recent cloning of two genes, GABA(B1) and GABA(B2), has revealed a novel requirement for GABA(B) receptor signalling. Studies have demonstrated that the two receptor subunits associate as a GABA(B1)/GABA(B2) heterodimer to form a functional GABA(B) receptor. In this study we have developed polyclonal antisera specific to two splice variants of the GABA(B1) subunit, GABA(B1a) and GABA(B1b), as well as an antiserum to the GABA(B2) subunit. Using affinity-purified antibodies derived from these antisera we have mapped out the distribution profile of each subunit in rat brain, spinal cord and dorsal root ganglion. In brain the highest areas of GABA(B1a), GABA(B1b) and GABA(B2) subunit expression were found in neocortex, hippocampus, thalamus, cerebellum and habenula. In spinal cord, GABA(B1) and GABA(B2) subunits were expressed in the superficial layers of the dorsal horn, as well as in motor neurones in the deeper layers of the ventral horn. GABA(B) receptor subunit immunoreactivity in dorsal root ganglion suggested that expression of GABA(B1b) was restricted to the large diameter neurones, in contrast to GABA(B1a) and GABA(B2) subunits which were expressed in both large and small diameter neurones. Although expression levels of GABA(B1) and GABA(B2) subunits varied we found no areas in which GABA(B1) was expressed in the absence of GABA(B2). This suggests that most, if not all, GABA(B1) immunoreactivity may represent functional GABA(B) receptors. Although our data are in general agreement with functional studies, some discrepancies in GABA(B1) subunit expression occurred with respect to other immunohistochemical studies. Overall our data suggest that GABA(B) receptors are widely expressed throughout the brain and spinal cord, and that GABA(B1a) and GABA(B1b) subunits can associate with GABA(B2) to form both pre- and post-synaptic receptors.

Multireceptor GABAergic regulation of synaptic communication in amphibian retina

The synaptic output of retinal bipolar cells was monitored by recording light-evoked EPSCs in ganglion cells. Application of (RS)-2-amino-3-(3-hydroxy-5-tert-butyl-4-isoxazolyl (ATPA), a selective agonist at kainate receptors, depolarized amacrine cells and reduced the light-evoked excitatory current (L-EPSC) in ganglion cells. ATPA had only a slight effect on the light responses of bipolar cells. Therefore, ATPA suppresses bipolar cell synaptic output to ganglion cells. ATPA reduced the transient L-EPSC, but had comparatively little effect on sustained L-EPSC, of ganglion cells. The transient ON L-EPSC was more suppressed than the transient OFF L-EPSC. Thus, ATPA preferentially suppressed transient output from bipolar cells.GABA receptor antagonists blocked the effect of ATPA. This indicates that ATPA stimulated an endogenous inhibitory feedback pathway that suppressed bipolar cell output.CGP55845 and CGP35348 reduced the ATPA-induced suppression of L-EPSCs in ganglion cells, signifying that part of the feedback pathway is mediated by metabotropic GABA receptors.(1,2,5,6-Tetrahydropyridine-4-yl)-methylphosphinic acid (TPMPA) and picrotoxin, GABAC receptor antagonists, reduced the ATPA effect. Picrotoxin was more effective than ATPA. However, picrotoxin blocked only a part of this GABAC effect, while imidazole-4-acetic acid (I4AA) blocked another segment of the effect. This indicates that two pharmacologically distinct GABAC receptors mediate feedback to bipolar cells. SR95531 produced a very small suppression of the ATPA effect. Thus, GABAA receptors provide a negligible component to this feedback pathway. The experiments indicate that endogenous GABAergic feedback to bipolar cells suppresses their output, and that this feedback is mediated by at least three types of GABA receptor, both metabotropic and ionotropic.In conjunction with previous studies, the results indicate that feedback inhibition is the predominant factor regulating transient signalling in ganglion cells, while feedforward inhibition is the primary regulator of tonic ganglion cell signals.

Deactivation, recovery from inactivation, and modulation of extra-synaptic ion currents in fish retinal ganglion cells

As is shown magnificently by Heron Island's reef, the visual environment of many fishes includes various light intensities, hues and shapes that can change on large and small scales in space and time. Several articles in this issue address why fishes are sensitive to some of these properties, and how fishes and other aquatic species have acquired or fostered these sensitivities. This article discusses contributions of extrasynaptic ion currents, in a specific population of neurons, to the detection of ambient light levels, the appearance of certain visual stimuli and the disappearance of others.

Neuromodulation of ligand- and voltage-gated channels in the amphibian retina

To understand information processing in the retina, it is important to identify and characterize the types of synaptic receptors and intrinsic ion channels in retinal neurons. In order to achieve a high degree of adaptability, retinal synapses have evolved multiple neuromodulatory mechanisms. Light or modulatory agents can alter the efficacies of both electrical and chemical synaptic transmission in the retina. Recent studies indicate that interaction of voltage-gated channels with those activated by neurotransmitters plays a significant role in shaping the light-evoked postsynaptic responses of retinal neurons. The fact that both types of channels are subject to modulation by multiple second messenger-mediated intracellular processes is a clear indicator of the importance of neuromodulation in retinal function. The whole-cell patch clamp technique provides a means to study mechanisms of regulation of ion channels by controlling intracellular as well as the extracellular environment. This review describes the experimental evidence, mostly obtained in our laboratory, which indicates the important role of Ca-dependent neuromodulatory processes in the regulation of signal transmission in the vertical pathway of the amphibian retina.

Suppressive actions of betaxolol on ionic currents in retinal ganglion cells may explain its neuroprotective effects

Betaxolol, a beta 1-selective adrenoceptor antagonist, is widely used in the treatment of glaucoma. In addition to its ocular hypotensive effects, betaxolol has been suggested to act as a retinal neuroprotective agent (Osborne et al., 1997). To investigate possible mechanisms underlying the neuroprotective effects, we tested the actions of betaxolol on ion channels and calcium signaling in isolated retinal ganglion cells. Betaxolol (50 microM) reduced by about 20% the high-voltage-activated (HVA) Ca channel currents in ganglion cells isolated from tiger salamander retina. In contrast, the beta 1-adrenoceptor antagonists propranolol (10 microM) and timolol (50 microM) had no inhibitory actions on HVA Ca channel currents. The L-type Ca channel antagonist, nisoldipine, blocked the HVA Ca channel current partially and the remaining current was not inhibited by betaxolol. Outward current was inhibited in the presence of betaxolol. Both iberiotoxin (IBTX; 10 nM), a selective inhibitor of large-conductance Ca-activated K channels, and Cd2+ (100 microM), which suppresses Ca-activated K channels subsequent to its block of Ca channels, reduced outward current and the remaining current was not blocked significantly with betaxolol. In the presence of betaxolol, Na channel currents were reduced by about 20%, as were currents evoked by glutamate (10 mM) and GABA (1 mM). Current clamp recordings from isolated ganglion cells showed that betaxolol had several effects on excitability: spike height decreased, repetitive spike activity was suppressed, spike width increased and hyperpolarization following spikes was reduced. Calcium imaging in isolated rat retinal ganglion cells revealed that betaxolol inhibited glutamate-induced increases in [Ca2+]i. These results suggest that betaxolol has a diversity of suppressive actions on ganglion cell ion channels and that, as a consequence, one of the net actions of the drug is to reduce Ca2+ influx. The subsequent reduction in [Ca2+]i may contribute to the apparent neuroprotective actions of betaxolol in promoting ganglion cell survival following ischemic insult, as may occur in glaucoma and retinal disease.

Differential expression of high- and two types of low-voltage-activated calcium currents in rod and cone bipolar cells of the rat retina

Whole cell voltage-clamp recordings were performed to investigate voltage-activated Ca(2+) currents in acutely isolated retinal bipolar cells of rats. Two groups of morphologically different bipolar cells were observed. Bipolar cells of the first group, which represent the majority of isolated bipolar cells, were immunoreactive to protein kinase C (PKC) and, therefore likely to be rod bipolar cells. Bipolar cells of the second group, which represent only a small population of isolated bipolar cells, did not show PKC immunoreactivity and were likely to be cone bipolar cells. The validity of morphological identification of bipolar cells was further confirmed by the presence of GABA(C) responses in these cells. Bipolar cells of both groups displayed low-voltage-activated (LVA) Ca(2+) currents with similar voltage dependence of activation and steady-state inactivation. However, the activation, inactivation, and deactivation kinetics of the LVA Ca(2+) currents between rod and cone bipolar cells differed. Particularly, the LVA Ca(2+) currents of rod bipolar cells displayed both transient and sustained components. In contrast, the LVA Ca(2+) currents of cone bipolar cells were mainly transient. In addition, the LVA Ca(2+) channels of rod bipolar cells were more permeable to Ba(2+) than to Ca(2+), whereas those of cone bipolar cells were equally or less permeable to Ba(2+) than to Ca(2+). The LVA Ca(2+) currents of both rod and cone bipolar cells were antagonized by high concentrations of nimodipine with IC(50) of 17 and 23 microM, respectively, but largely resistant to Cd(2+) and Ni(2+). Bipolar cells of both groups also displayed high-voltage-activated (HVA) Ca(2+) currents. The HVA Ca(2+) currents were, at least in part, to be L-type that were potentiated by BayK-8644 (1 microM) and largely antagonized by low concentrations of nimodipine (5 microM). The L-type Ca(2+) channels were almost exclusively located at the axon terminals of rod bipolar cells but expressed at least in the cell soma of cone bipolar cells. Results of this study indicate that rod and cone bipolar cells of the mammalian retina differentially express at least two types of LVA Ca(2+) channels. Rod and cone bipolar cells also show different spatial distribution of L-type Ca(2+) channels.

Calcium channel activation facilitated by nitric oxide in retinal ganglion cells

We investigated the modulation of voltage-gated Ca channels by nitric oxide (NO) in isolated salamander retinal ganglion cells with the goals of determining the type of Ca channel affected and the signaling pathway by which modulation might occur. The NO donors, S-nitroso-N-acetyl-penicillamine (SNAP, 1 mM) and S-nitroso-cysteine (1 mM) induced modest increases in the amplitude of Ca channel currents recorded with ruptured- and permeabilized-patch techniques by causing a subpopulation of the Ca channels to activate at more negative potentials. The Ca channel antagonists omega-conotoxin GVIA and nisoldipine each reduced the Ca channel current partially, but only omega-conotoxin GVIA blocked the enhancement by SNAP. The SNAP-induced increase was blocked by oxadiazolo-quinoxaline (50 microM), suggesting that the NO generated by SNAP acts via a soluble guanylyl cyclase to raise levels of cGMP. The membrane-permeant cGMP analog 8-(4-chlorophenylthio) guanosine cyclic monophosphate also enhanced Ca channel currents and 8-bromo guanosine cyclic monophosphate (1 mM) occluded enhancement by SNAP. Consistent with these results, isobutyl-methyl-xanthine (IBMX, 10 microM), which can raise cGMP levels by inhibiting phosphodiesterase activity, increased Ca channel current by the same amount as SNAP and occluded subsequent enhancement by SNAP. Neither IBMX, the cGMP analogs, nor SNAP itself, led to activation of cGMP-gated channels. N-[2-(methylamino)ethyl]-5-isoquinoline-sulfonamide (2 microM), a broad spectrum inhibitor of protein kinase activity, KT5823 (1 microM), a specific protein kinase G (PKG) inhibitor, and a peptide inhibitor of PKG (200 microM) blocked SNAP enhancement, as did 5'-adenylylimidophosphate (1.5 mM), a nonhydrolyzable ATP analog that prevents protein phosphorylation. A peptide inhibitor of protein kinase A (10 nM) did not block the facilitory effects of SNAP. Okadaic acid (1 microM), a phosphatase inhibitor, had no effect by itself but increased the enhancement of Ca channel current by SNAP. These results suggest that NO modulates retinal ganglion cell N-type Ca channels by facilitating their voltage-dependent activation via a mechanism involving guanylyl cyclase/PKG-dependent phosphorylation. This effect could fine-tune neural integration in ganglion cells or play a role in ganglion cell disease by modulating intracellular calcium signaling.

Internal calcium modulates apparent affinity of metabotropic GABA receptors

The metabotropic GABA receptor (GABA(B)R) regulates calcium influx in neurons. Whole cell voltage-clamp techniques were employed to determine the effects of internal calcium on the activity of GABA(B)Rs. GABA(B)R receptor apparent affinity was maximal when bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) maintained internal calcium below 70 nM. Apparent affinity was reduced as internal calcium increased. EGTA did not produce similar effects, suggesting that localized increases in calcium influenced GABA(B)R apparent affinity. Confocal imaging disclosed relatively high internal calcium just below the plasma membrane of isolated neurons. BAPTA, but not EGTA, reduced this ring of high calcium. Heparin, dantrolene, and ryanodine increased GABA(B)R apparent affinity, effects similar to that of BAPTA. Calmodulin inhibitors also increased receptor apparent affinity. These results suggest that internally released calcium activates calmodulin, which reduces GABA(B)R apparent affinity. This identifies a reciprocal system in which the metabotropic GABA receptor can reduce calcium influx, but internal calcium can suppress this receptor pathway. Metabotropic glutamate receptors linked to inositol 1,4,5 trisphosphate (InsP(3)) raised internal calcium and suppressed the action of GABA(B)Rs. Thus negative feedback systems control the balance between excitatory and inhibitory metabotropic receptor pathways in retinal neurons.

GABA(B) receptors in Müller cells of the bullfrog retina

GABA is a major inhibitory neurotransmitter in the vertebrate retina. Using GABA(B) receptor-specific antibody combined with glial fibrillary acidic protein (GFAP) antiserum, we demonstrated that primary processes, vitreal endfeet and somata of virtually all Müller glial cells in the bullfrog retina prominently showed GABA(B) receptor immunoreactivity by light and electron microscopy. This study provides the first evidence that glial elements in the vertebrate retina express GABA(B) receptors. Such receptors on Müller cells may play an important role in retinal information processing through regulating the conductance of the inward-rectifying K+ channels.

Metabotropic GABA receptors facilitate L-type and inhibit N-type calcium channels in single salamander retinal neurons

1. Whole-cell voltage clamp experiments were performed on isolated spiking retinal neurons from the salamander retina. Calcium channel currents were studied using barium as the charge carrier while potassium and sodium currents were suppressed with TEA and TTX, respectively. 2. Baclofen, a metabotropic GABA receptor agonist, both enhanced and suppressed high-voltage-activated calcium channel current. Baclofen facilitated an L-type channel current, and this effect was not voltage dependent. As reported previously, baclofen inhibited an N-type channel current and this action was voltage dependent. 3. While the suppressive effect was mediated by a fast-acting, direct G-protein action, the facilitatory effect was slower and was blocked by inhibitors of protein kinase C (PKC), either GF-109203x or the PKC (19-36) sequence fragment. 4. The pharmacology of the inhibitory and facilitatory responses differed. Commonly used antagonists of metabotropic GABA receptors, CGP35348 and CGP55845, were more potent antagonists of the inhibitory response. Similarly, a selective agonist at the metabotropic GABA receptor, APMPA, was also more effective in eliciting the inhibitory response. 5. These observations indicate that there may be two baclofen-sensitive metabotropic GABA receptors with opposing effects on calcium channel current. This is the first description of a facilitatory action of GABAB receptors and indicates that GABA may not function exclusively as an inhibitory transmitter.

AMPA receptor activates a G-protein that suppresses a cGMP-gated current

The AMPA receptor, ubiquitous in brain, is termed "ionotropic" because it gates an ion channel directly. We found that an AMPA receptor can also modulate a G-protein to gate an ion channel indirectly. Glutamate applied to a retinal ganglion cell briefly suppresses the inward current through a cGMP-gated channel. AMPA and kainate also suppress the current, an effect that is blocked both by their general antagonist CNQX and also by the relatively specific AMPA receptor antagonist GYKI-52466. Neither NMDA nor agonists of metabotropic glutamate receptors are effective. The AMPA-induced suppression of the cGMP-gated current is blocked when the patch pipette includes GDP-beta-S, whereas the suppression is irreversible when the pipette contains GTP-gamma-S. This suggests a G-protein mediator, and, consistent with this, pertussis toxin blocks the current suppression. Nitric oxide (NO) donors induce the current suppressed by AMPA, and phosphodiesterase inhibitors prevent the suppression. Apparently, the AMPA receptor can exhibit a "metabotropic" activity that allows it to antagonize excitation evoked by NO.

AMPA receptor activates a G-protein that suppresses a cGMP-gated current

The AMPA receptor, ubiquitous in brain, is termed "ionotropic" because it gates an ion channel directly. We found that an AMPA receptor can also modulate a G-protein to gate an ion channel indirectly. Glutamate applied to a retinal ganglion cell briefly suppresses the inward current through a cGMP-gated channel. AMPA and kainate also suppress the current, an effect that is blocked both by their general antagonist CNQX and also by the relatively specific AMPA receptor antagonist GYKI-52466. Neither NMDA nor agonists of metabotropic glutamate receptors are effective. The AMPA-induced suppression of the cGMP-gated current is blocked when the patch pipette includes GDP-beta-S, whereas the suppression is irreversible when the pipette contains GTP-gamma-S. This suggests a G-protein mediator, and, consistent with this, pertussis toxin blocks the current suppression. Nitric oxide (NO) donors induce the current suppressed by AMPA, and phosphodiesterase inhibitors prevent the suppression. Apparently, the AMPA receptor can exhibit a "metabotropic" activity that allows it to antagonize excitation evoked by NO.

Molecular identification of the human GABABR2: cell surface expression and coupling to adenylyl cyclase in the absence of GABABR1

We have identified a gene encoding a GABAB receptor, the human GABABR2, located on chromosome 9q22.1, that is distinct from the recently reported rat GABABR1. GABABR2 structurally resembles GABABR1 (35% identity), having seven transmembrane domains and a large extracellular region, but differs in having a longer carboxy-terminal tail. GABABR2 is localized to the cell surface in transfected COS cells, and negatively couples to adenylyl cyclase in response to GABA, baclofen, and 3-aminopropyl(methyl)phosphinic acid in CHO cells lacking GABABR1. Baclofen action is inhibited by the GABABR antagonist, 2-hydroxysaclofen. The human GABABR2 and GABABR1 genes are differentially expressed in the nervous system, with the greatest difference being detected in the striatum in which GABABR1 but not GABABR2 mRNA transcripts are detected. GABABR2 and GABABR1 mRNAs are also coexpressed in various brain regions such as the Purkinje cell layer of the cerebellum. Identification of a functional homomeric GABABR2 coupled to adenylyl cyclase suggests that the complexity of GABAB pharmacological data is at least in part due to the presence of more than one receptor and opens avenues for future research leading to an understanding of metabotropic GABA receptor signal transduction mechanisms.

Neuronal discriminator formed by metabotropic gamma-aminobutyric acid receptors

Neuronal discriminator formed by metabotropic gamma-aminobutyric acid receptors. J. Neurophysiol. 80: 3365-3368, 1998. Neurotransmitters function in one of two modes, promoting either inhibition or excitation. However, the metabotropic gamma-aminobutyric acid receptor (GABABR) system can switch between these modes. In the presence of a small excitatory stimulus, the GABABR mediates a shunting inhibition that suppresses excitation. However, in the presence of a strong excitatory stimulus, the GABABR potentiates the response. This bipartite action is accomplished by linking the GABABR to two electrogenic mechanisms; one activates an outward current and another reduces an outward current. As a consequence, the GABABR serves as a discriminator that reduces the influence of weak signals while augmenting responses to strong signals. In retinal ganglion cells, this mechanism acts to promote the communication of phasic information.

Differential localization of GABA(B) receptors in the mouse retina

The mRNA distribution of the two cloned GABA(B) receptor variants, GABA(B)R1a and -R1b, was analysed in the retina by non-radioactive in situ hybridization. GABA(B)R1a transcripts were found in the inner nuclear and ganglion cell layers, probably in horizontal, amacrine and ganglion cells, whereas GABA(B)R1b transcripts were detected in the ganglion cell layer only. Together with a recent immunohistochemical localization of GABA(B)R1 in the retina, this indicates a differential targeting of the receptor variants to pre- and postsynaptic sites with GABA(B)R1a and -R1b localized to axonal and dendritic compartments, respectively. In this way, inhibition of neurotransmitter release and slow postsynaptic inhibition could be provided by receptor variants derived from the same gene.

Characterization of spontaneous inhibitory synaptic currents in salamander retinal ganglion cells

Spontaneous and light-evoked postsynaptic currents (sPSCs and lePSCs, respectively) in retinal ganglion cells of the larval tiger salamander were recorded under voltage-clamp conditions from living retinal slices. The focus of this study is to characterize the spontaneous inhibitory PSCs (sIPSCs) and their contribution to the light-evoked inhibitory PSCs (leIPSCs) in ON-OFF ganglion cells. sIPSCs were isolated from spontaneous excitatory PSCs (sEPSCs) by application of 10 microM 6,7-dinitroquinoxaline-2,3-dione (DNQX) + 50 microM 2-amino-5-phosphonopentanoic acid (AP5). In approximately 70% of ON-OFF ganglion cells, bicuculline (or picrotoxin) completely blocks sIPSCs, suggesting all sIPSCs in these cells are mediated by GABAergic synaptic vesicles and gamma-aminobutyric acid-A (GABAA) receptors (GABAergic sIPSCs, or GABAsIPSCs). In the remaining 30% of - ganglion cells, bicuculline (or picrotoxin) blocks 70-98% of the sIPSCs, and the remaining 2-30% are blocked by strychnine (glycinergic sIPSCs, or GLYsIPSCs). GABAsIPSCs occur randomly with an exponentially distributed interval probability density function, and they persist without noticeable rundown over time. The GABAsIPSC frequency is greatly reduced by cobalt, consistent with the idea that they are largely mediated by calcium-dependent vesicular release. GABAsIPSCs in DNQX + AP5 are tetrodotoxin (TTX) insensitive, suggesting that amacrine cells that release GABA under these conditions do not generate spontaneous action potentials. The average GABAsIPSCs exhibited linear current-voltage relation with a reversal potential near the chloride equilibrium potential, and an average peak conductance of 319.67 +/- 252.83 (SD) pS. For GLYsIPSCs, the average peak conductance increase is 301.68 +/- 94.34 pS. These parameters are of the same order of magnitude as those measured in inhibitory miniature postsynaptic currents (mIPSCs) associated with single synaptic vesicles in the CNS. The amplitude histograms of GABAsIPSCs did not exhibit multiple peaks, suggesting that the larger events are not discrete multiples of elementary events (or quanta). We propose that each GABAsIPSC or GLYsIPSC in retinal ganglion cells is mediated by a single or synchronized multiple of synaptic vesicles with variable neurotransmitter contents. In a sample of 16 ON-OFF ganglion cells, the average peak leIPSC (held at 0 mV) at the light onset is 509.0 +/- 233.85 pA and that at the light offset is 529.0 +/- 339.88 pA. The approximate number of GABAsIPSCs and GLYsIPSCs required to generate the average light responses, calculated by the ratio of the charge (area under current traces) of the leIPSCs to that of the average single sIPSCs, is 118 +/- 52 for the light onset, and 132 +/- 76 for the light offset.

Calcium released from intracellular stores inhibits GABAA-mediated currents in ganglion cells of the turtle retina

We studied spiking neurons isolated from turtle retina by the whole cell version of the patch clamp. The studied cells had perikaryal diameters > 15 microns and fired multiple spikes in response to depolarizing current steps, indicating they were ganglion cells. In symmetrical [Cl-], currents elicited by puffs of 100 microM gamma-aminobutyric acid (GABA) were inward at a holding potential of -80 mV. All of the GABA-evoked current was blocked by SR95331 (20 microM), indicating that it was mediated by a GABAA receptor. The GABA-evoked currents were unaltered by eliciting a transmembrane calcium current either just before or during the response to GABA. On the other hand caffeine (10 mM), which induces Ca2+ release from intracellular stores, inhibited the GABA-evoked current on average by 30%. The caffeine effect was blocked by introducing the calcium buffer bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) into the cell but was unaffected by replacing [Ca2+]o with equimolar cobalt. Thapsigargin (10 microM), an inhibitor of intracellular calcium pumps, and ryanodine (20 microM), which depletes intracellular calcium stores, both markedly reduced a caffeine-induced inhibition of the GABA-evoked current. Another activator of intracellular calcium release, inositol trisphosphate (IP3; 50 microM), also progressively reduced the GABA-induced current when introduced into the cell. Dibutyryl adenosine 3'5'-cyclic monophosphate (cAMP; 0.5 mM), a membrane-permeable analogue of cAMP, did not reduce GABA-evoked currents, suggesting that cAMP-dependent kinases are not involved in suppressing GABAA currents, whereas calmidazolium (30 microM) and cyclosporin A (20 microM), which inhibit Ca/calmodulin-dependent phosphatases, did reduce the caffeine-induced inhibition of the GABA-evoked current. Alkaline phosphatase (150 micrograms/ml) and calcineurin (300 micrograms/ml) had a similar action to caffeine or IP3. Antibodies directed against the ryanodine receptor or the IP3 receptor reacted with the great majority of neurons in the ganglion cell layer. We found that these two antibodies colocalized in large ganglion cells. In summary, intracellular calcium plays a role in reducing the currents elicited by GABA, acting through GABAA receptors. The modulatory action of calcium on GABA responses appears to work through one or more Ca-dependent phosphatases.

Developmental changes of agonist affinity at GABABR1 receptor variants in rat brain

Recently, two N-terminal splice variants of the metabotropic receptor for GABA (gamma-amino-butyric acid) were cloned. Here, we describe an antiserum that recognizes the two receptor variants. We demonstrate that these proteins are identical with GABAB receptors that are photoaffinity labeled with [125I]CGP71872 in rat brain. The C-terminal epitopes recognized by the antiserum are conserved in several vertebrate species but not in chicken. No hints for the existence of additional closely related receptor subtypes or variants are found in double-labeling experiments with antibody and photoaffinity ligand. Western blot analysis reveals widespread expression of the GABABR1 receptor proteins in rat brain with the highest level of expression at early postnatal stages. The binding affinity of the GABAB receptor agonist L-baclofen at native R1a and R1b variants is similar. In early postnatal development the affinity at R1a and R1b is 10-fold lower than in adult brain and gradually increases with aging.

Metabotropic and ionotropic glutamate receptors regulate calcium channel currents in salamander retinal ganglion cells

1. Glutamate suppressed high-voltage-activated barium currents (IBa, HVA) in tiger salamander retinal ganglion cells. Both ionotropic (iGluR) and metabotropic (mGluR) receptors contributed to this calcium channel inhibition. 2. Trans-ACPD (1-aminocyclopentane-trans-1S,3R-dicarboxylic acid), a broad-spectrum metabotropic glutamate receptor agonist, suppressed a dihydropyridine-sensitive barium current. Kainate, an ionotropic glutamate receptor agonist, reduced an omega-conotoxin GVIA-sensitive current. 3. The relative effectiveness of selective agonists indicated that the predominant metabotropic receptor was the L-2-amino-4-phosphonobutyrate (L-AP4)-sensitive, group III receptor. This receptor reversed the action of forskolin, but this was not responsible for calcium channel suppression. l-AP4 raised internal calcium concentration. Antagonists of phospholipase C, inositol trisphosphate (IP3) receptors and ryanodine receptors inhibited the action of metabotropic agonists, indicating that group III receptor transduction was linked to this pathway. 4. The action of kainate was partially suppressed by BAPTA, by calmodulin antagonists and by blockers of calmodulin-dependent phosphatase. Suppression by kainate of the calcium channel current was more rapid when calcium was the charge carrier, instead of barium. The results indicate that calcium influx through kainate-sensitive glutamate receptors can activate calmodulin, which stimulates phosphatases that may directly suppress voltage-sensitive calcium channels. 5. Thus, ionotropic and metabotropic glutamate receptors inhibit distinct calcium channels. They could act synergistically, since both increase internal calcium. These pathways provide negative feedback that can reduce calcium influx when ganglion cells are depolarized.

A developmental shift from GABAergic to glycinergic transmission in the central auditory system

GABAergic and glycinergic circuits are found throughout the auditory brainstem, and it is generally assumed that transmitter phenotype is established early in development. The present study documents a profound transition from GABAergic to glycinergic transmission in the gerbil lateral superior olive (LSO) during the first 2 postnatal weeks. Whole-cell voltage-clamp recordings were obtained from LSO neurons in a brain slice preparation, and IPSCs were evoked by electrical stimulation of the medial nucleus of the trapezoid body (MNTB), a known glycinergic projection in adult animals. GABAergic and glycinergic components were identified by blocking transmission with bicuculline and strychnine (SN), respectively. In the medial limb of LSO, there was a dramatic change in the GABAergic IPSC component, decreasing from 78% at postnatal day 3 (P3)-P5 to 12% at P12-P16. There was an equal and opposite increase in the glycinergic component during this same period. Direct application of GABA also elicited significantly larger amplitude and longer duration responses in P3-P5 neurons compared with glycine-evoked responses. In contrast, MNTB-evoked IPSCs in lateral limb neurons were more sensitive to SN throughout development. Consistent with the electrophysiological observations, there was a reduction in staining for the beta2,3-GABAA receptor subunit from P4 to P14, whereas staining for the glycine receptor-associated protein gephyrin increased. Brief exposure to baclofen depressed transmission at excitatory and inhibitory synapses for approximately 15 min, suggesting a GABAB-mediated metabotropic signal. Collectively, these data demonstrate a striking switch from GABAergic to glycinergic transmission during postnatal development. Although GABA and glycine elicit similar postsynaptic ionotropic responses, our results raise the possibility that GABAergic transmission in neonates may play a developmental role distinct from that of glycine.

A developmental shift from GABAergic to glycinergic transmission in the central auditory system

GABAergic and glycinergic circuits are found throughout the auditory brainstem, and it is generally assumed that transmitter phenotype is established early in development. The present study documents a profound transition from GABAergic to glycinergic transmission in the gerbil lateral superior olive (LSO) during the first 2 postnatal weeks. Whole-cell voltage-clamp recordings were obtained from LSO neurons in a brain slice preparation, and IPSCs were evoked by electrical stimulation of the medial nucleus of the trapezoid body (MNTB), a known glycinergic projection in adult animals. GABAergic and glycinergic components were identified by blocking transmission with bicuculline and strychnine (SN), respectively. In the medial limb of LSO, there was a dramatic change in the GABAergic IPSC component, decreasing from 78% at postnatal day 3 (P3)-P5 to 12% at P12-P16. There was an equal and opposite increase in the glycinergic component during this same period. Direct application of GABA also elicited significantly larger amplitude and longer duration responses in P3-P5 neurons compared with glycine-evoked responses. In contrast, MNTB-evoked IPSCs in lateral limb neurons were more sensitive to SN throughout development. Consistent with the electrophysiological observations, there was a reduction in staining for the beta2,3-GABAA receptor subunit from P4 to P14, whereas staining for the glycine receptor-associated protein gephyrin increased. Brief exposure to baclofen depressed transmission at excitatory and inhibitory synapses for approximately 15 min, suggesting a GABAB-mediated metabotropic signal. Collectively, these data demonstrate a striking switch from GABAergic to glycinergic transmission during postnatal development. Although GABA and glycine elicit similar postsynaptic ionotropic responses, our results raise the possibility that GABAergic transmission in neonates may play a developmental role distinct from that of glycine.

GABAB receptors: drugs meet clones

Recently, the long-awaited cloning of the GABAB receptors, the last of the major known neurotransmitter receptors to be identified, has been reported. In addition to an emerging molecular understanding, there have been advances in discerning the specific coupling partners of GABAB receptors in the brain.