strychnine and (1-2-5-6-tetrahydropyridin-4-yl)methylphosphinic-acid

In hypoglossal motoneurons, a sustained anionic current, sensitive to a blocker of ρ-containing GABA receptors, (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) and insensitive to bicuculline, was previously shown to be activated by gabazine. In order to better characterize the receptors involved, the sensitivity of this atypical response to pentobarbital (30μM), allopregnanolone (0.3μM) and midazolam (0.5μM) was first investigated. Pentobarbital potentiated the response, whereas the steroid and the benzodiazepine were ineffective. The results indicate the involvement of hybrid heteromeric receptors, including at least a GABA receptor ρ subunit and a γ subunit, accounting for the pentobarbital-sensitivity. The effects of the endogenous β amino acids, taurine and β-alanine, which are released under various pathological conditions and show neuroprotective properties, were then studied. In the presence of the glycine receptor blocker strychnine (1μM), both taurine (0.3-1mM) and β-alanine (0.3mM) activated sustained anionic currents, which were partly blocked by TPMPA (100μM). Thus, both β amino acids activated ρ-containing GABA receptors in hypoglossal motoneurons. Bicuculline (20μM) reduced responses to taurine and β-alanine, but small sustained responses persisted in the presence of both strychnine and bicuculline. Responses to β-alanine were slightly increased by allopregnanolone, indicating a contribution of the bicuculline- and neurosteroid-sensitive GABAA receptors underlying tonic inhibition in these motoneurons. Since sustained activation of anionic channels inhibits most mature principal neurons, the ρ-containing GABA receptors permanently activated by taurine and β-alanine might contribute to some of their neuroprotective properties under damaging overexcitatory situations.

Topics: Animals; beta-Alanine; Bicuculline; Brain Stem; Central Nervous System Agents; Chlorine; GABA Agents; Hypoglossal Nerve; Midazolam; Motor Neurons; Neuroprotective Agents; Patch-Clamp Techniques; Pentobarbital; Phosphinic Acids; Pregnanolone; Pyridazines; Pyridines; Rats, Sprague-Dawley; Receptors, GABA; Receptors, GABA-A; Strychnine; Taurine; Tissue Culture Techniques

Direction-selective ganglion cells (DSGCs) are tuned to motion in one direction. Starburst amacrine cells (SACs) are thought to mediate this direction selectivity through precise anatomical wiring to DSGCs. Nevertheless, we previously found that visual adaptation can reverse DSGCs's directional tuning, overcoming the circuit anatomy. Here we explore the role of SACs in the generation and adaptation of direction selectivity. First, using pharmacogenetics and two-photon calcium imaging, we validate that SACs are necessary for direction selectivity. Next, we demonstrate that exposure to an adaptive stimulus dramatically alters SACs' synaptic inputs. Specifically, after visual adaptation, On-SACs lose their excitatory input during light onset but gain an excitatory input during light offset. Our data suggest that visual stimulation alters the interactions between rod- and cone-mediated inputs that converge on the terminals of On-cone BCs. These results demonstrate how the sensory environment can modify computations performed by anatomically defined neuronal circuits.

Topics: Adaptation, Physiological; Amacrine Cells; Animals; Cell Polarity; Connexins; GABA Antagonists; Gap Junction delta-2 Protein; Glycine Agents; Membrane Potentials; Membrane Transport Proteins; Mice; Mice, Inbred C57BL; Mice, Transgenic; Neural Inhibition; Phosphinic Acids; Photic Stimulation; Propionates; Pyridazines; Pyridines; Receptors, Glycine; Retina; Strychnine; Visual Pathways

Nonspiking cells of several sensory systems respond to stimuli with graded changes in neurotransmitter release and possess specialized synaptic ribbons. Here, we show that manipulations to synaptic ribbons caused dramatic effects on mEPSC-like (mlEPSC) amplitude and frequency. Damage to rod-bipolar cell ribbons using fluorophore-assisted light inactivation resulted in the immediate reduction of mlEPSC amplitude and frequency, whereas the first evoked response after damage remained largely intact. The reduction in amplitude could not be recovered by increasing release frequency after ribbon damage. In parallel experiments, we looked at mlEPSCs from cones of hibernating ground squirrels, which exhibit dramatically smaller ribbons than awake animals. Fewer and smaller mlEPSCs were observed postsynaptic to cones from hibernating animals, although depolarized cones were able to generate larger mlEPSCs. Our results indicate that ribbon size may influence mlEPSC frequency and support a role for ribbons in coordinating multivesicular release.

Topics: Alcohol Oxidoreductases; Aminobutyrates; Animals; Biophysics; Co-Repressor Proteins; DNA-Binding Proteins; Dose-Response Relationship, Drug; Electric Stimulation; Excitatory Amino Acid Agonists; Excitatory Postsynaptic Potentials; GABA Antagonists; Glycine Agents; Hibernation; In Vitro Techniques; Mice; Mice, Inbred C57BL; Patch-Clamp Techniques; Phosphinic Acids; Phosphoproteins; Picrotoxin; Pyridines; Receptors, AMPA; Retina; Sciuridae; Strychnine; Synapses; Visual Pathways; Wakefulness

NiCl(2) (15 microM) stimulates the electroretinogram (ERG) b-wave amplitude of vertebrate retina up to 1.5-fold through its blocking of E/R-type voltage-gated Ca(2+) channels. Assuming that such an increase is mediated by blocking the release of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) via ionotropic GABA receptors, we tested the effect of both GABA itself and GABA-receptor antagonists such as (-)bicuculline (1.51-fold increase) and (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA; 1.46-fold increase) on the b-wave amplitude.. Recording of the transretinal potentials from the isolated bovine retina.. GABA (100 microM) reduced the b-wave amplitude only when NiCl(2) (15 microM) was applied first. Each antagonist applied on its own stimulated the b-wave amplitude only partially: subsequent NiCl(2) superfusion caused a small but additional increase, leading to a 1.69- and a 1.88-fold total increase of the amplitude by Ni(2+) plus (-)bicuculline or Ni(2+) plus TPMPA, respectively. Only the application of both antagonists in combination, before superfusing low NiCl(2) (15 microM), completely prevented subsequent stimulation by NiCl(2) with a similar 1.90-fold total increase of b-wave amplitude. Those retina segments that did not respond to NiCl(2) could not be stimulated by (-)bicuculline and vice versa.. The stimulatory effect of NiCl(2) on the ERG b-wave amplitude is mainly, but not only, mediated by a NiCl(2)-sensitive, Ca(v)2.3-triggered GABA release acting through ionotropic GABA-A and GABA-C receptors.

Topics: Animals; Bicuculline; Calcium Channels; Calcium Channels, R-Type; Cation Transport Proteins; Cattle; Dose-Response Relationship, Drug; Drug Combinations; Electroretinography; GABA Antagonists; gamma-Aminobutyric Acid; Glycine Agents; In Vitro Techniques; Nickel; Night Vision; Phosphinic Acids; Photoreceptor Cells, Vertebrate; Pyridines; Receptors, Glycine; Retina; Signal Transduction; Strychnine

Synaptic integration is modulated by inhibition onto the dendrites of postsynaptic cells. However, presynaptic inhibition at axonal terminals also plays a critical role in the regulation of neurotransmission. In contrast to the development of inhibitory synapses onto dendrites, GABAergic/glycinergic synaptogenesis onto axon terminals has not been widely studied. Because retinal bipolar cells receive subclass-specific patterns of GABAergic and glycinergic presynaptic inhibition, they are a good model for studying the development of inhibition at axon terminals. Here, using whole cell recording methods and transgenic mice in which subclasses of retinal bipolar cells are labeled, we determined the temporal sequence and patterning of functional GABAergic and glycinergic input onto the major subclasses of bipolar cells. We found that the maturation of GABAergic and glycinergic synapses onto the axons of rod bipolar cells (RBCs), on-cone bipolar cells (ON-CBCs) and off-cone bipolar cells (OFF-CBCs) were temporally distinct: spontaneous chloride-mediated currents are present in RBCs earlier in development compared with ON- and OFF-CBC, and RBCs receive GABAergic and glycinergic input simultaneously, whereas in OFF-CBCs, glycinergic transmission emerges before GABAergic transmission. Because on-CBCs show little inhibitory activity, GABAergic and glycinergic events could not be pharmacologically distinguished for these bipolar cells. The balance of GABAergic and glycinergic input that is unique to RBCs and OFF-CBCs is established shortly after the onset of synapse formation and precedes visual experience. Our data suggest that presynaptic modulation of glutamate transmission from bipolar cells matures rapidly and is differentially coordinated for GABAergic and glycinergic synapses onto distinct bipolar cell subclasses.

Topics: Age Factors; Animals; Animals, Newborn; Axons; Bicuculline; Drug Interactions; GABA Antagonists; Glycine Agents; Green Fluorescent Proteins; In Vitro Techniques; Inhibitory Postsynaptic Potentials; Luminescent Proteins; Mice; Mice, Transgenic; Neural Inhibition; Patch-Clamp Techniques; Phosphinic Acids; Potassium Channels, Inwardly Rectifying; Presynaptic Terminals; Pyridazines; Pyridines; Receptors, Metabotropic Glutamate; Retina; Retinal Bipolar Cells; Strychnine

Retinal bipolar cells can be divided into on and off types based on the polarity of their response to light. Bipolar activity is further shaped by inhibitory inputs, characterized here by the events that occur immediately after the onset of a light step: 1) in most off bipolar cells, excitatory current decreased, whereas inhibitory current increased. These currents reinforced each other, enhancing the light response. 2) In about half of the on cone bipolar cells, the excitatory current increased, whereas inhibitory current decreased, also reinforcing the light response. Both of these reinforcing interactions were mediated by glycinergic inhibition. 3) In the remaining on cone bipolar cells, excitation and inhibition both increased, but inhibition was delayed so that these cells responded transiently. 4) Finally, in rod bipolar cells, excitation and inhibition both increased so that inhibition suppressed excitation, reducing the light response at all time scales. The suppressive inhibition seen in on cone and rod bipolar cells was mediated by GABA. Thus morphologically diverse bipolar cells receive only four main types of inhibitory input, and the majority of "inhibitory" inputs actually serve to enhance excitation.

Topics: Aminobutyrates; Animals; Axons; Electrophysiology; Feedback; GABA Antagonists; gamma-Aminobutyric Acid; Glycine; Glycine Agents; Image Processing, Computer-Assisted; In Vitro Techniques; Patch-Clamp Techniques; Phosphinic Acids; Photic Stimulation; Pyridines; Rabbits; Receptors, GABA; Retina; Retinal Bipolar Cells; Retinal Cone Photoreceptor Cells; Retinal Rod Photoreceptor Cells; Strychnine

Glycine and gamma-aminobutyric acid (GABA) are the major inhibitory transmitters of the mammalian retina, and bipolar cells receive GABAergic and glycinergic inhibition from multiple amacrine cell types. Here we evaluated the functional properties and subunit composition of glycine receptors (GlyRs) in bipolar cells. Patch-clamp recordings were performed from retinal slices of wild-type, GlyRalpha1-deficient (Glra1(spd-ot)) and GlyRalpha3-deficient (Glra3(-/-)) mice. Whole-cell currents following glycine application and spontaneous inhibitory postsynaptic currents (IPSCs) were analysed. During the recordings the cells were filled with Alexa 488 and, thus, unequivocally identified. Glycine-induced currents of bipolar cells were picrotoxinin-insensitive and thus represent heteromeric channels composed of alpha and beta subunits. Glycine-induced currents and IPSCs were absent from all bipolar cells of Glra1(spd-ot) mice, indicating that GlyRalpha1 is an essential subunit of bipolar cell GlyRs. By comparing IPSCs of bipolar cells in wild-type and Glra3(-/-) mice, no statistically significant differences were found. OFF-cone bipolar (CB) cells receive a strong glycinergic input from AII amacrine cells, that is preferentially based on the fast alpha1beta-containing channels (mean decay time constant tau = 5.9 +/- 1.4 ms). We did not observe glycinergic IPSCs in ON-CB cells and could elicit only small, if any, glycinergic currents. Rod bipolar cells receive a prominent glycinergic input that is mainly mediated by alpha1beta-containing channels (tau = 5.5 +/- 1.6 ms). Slow IPSCs, the characteristic of GlyRs containing the alpha2 subunit, were not observed in bipolar cells. Thus, different bipolar cell types receive kinetically fast glycinergic inputs, preferentially mediated by GlyRs composed of alpha1 and beta subunits.

Topics: Animals; Dose-Response Relationship, Radiation; Drug Interactions; Electric Stimulation; GABA Antagonists; gamma-Aminobutyric Acid; Glycine; Glycine Agents; Green Fluorescent Proteins; Immunohistochemistry; In Vitro Techniques; Lewis X Antigen; Membrane Potentials; Mice; Mice, Inbred C57BL; Mice, Transgenic; Neural Inhibition; Neurons; Patch-Clamp Techniques; Phosphinic Acids; Protein Kinase C-alpha; Pyridazines; Pyridines; Receptors, Glycine; Retina; Strychnine; Synaptic Transmission; Time Factors; Transducin