Neural receptors

Neurons are able to maintain membrane potential and synaptic integrity by an
intricate equilibrium of membrane transporter proteins and ion channels. Two
membrane proteins of particular importance in the vertebrate retina are the
excitatory amino acid transporters (EAATs) which are responsible for the reuptake
of glutamate into both glial and neuronal cells and the sodium potassium
chloride cotransporters (NKCCs) that are responsible for the uptake of chloride
ions into the cell. NKCCs are electro-neutral with the uptake of 2 Cl- coupled to
an exchange of a potassium and Na+ ion into the cells. Therefore, there is little
change of cell membrane potential in the action of NKCCs. In this study the
localization and function of EAATs in the distal retina is investigated. Whole cell
patch clamp recordings in lower vertebrate retina have demonstrated that EAAT2
is the main synaptic EAATs in rod photoreceptors and it is localized to the axon terminals. Furthermore, the action of the transporter seems to be modified by
intracellular calcium concentration. There is also evidence that EAAT2 might be
regulated by feedback from the neuron network by glycinergic and GABAergic
mechanisms. The second half of this study investigates expression of NKCCs in
the retina by western blot analysis and quantitative polymerase chain reaction.
There are two forms of NKCCs, NKCC1 and NKCC2. NKCC1 is mostly
expressed in the central nervous system and NKCC2 was thought to only be
expressed in the kidneys. NKCC1 is responsible for the majority of chloride
uptake into neuronal and epithelial cells and NKCC1 is expressed in the distal
retina where photoreceptors synapse on second order horizontal and bipolar
cells. This study found the expression of NKCC1 in the distal retina to be
regulated by temporal light and dark adaptation. Light adaptation increased
phosphorylated NKCC1 expression (the active form of the cotransporter). The
increase in NKCC1 expression during light adaptation was modulated by
dopamine. Specifically, a D1 receptor agonist increased phosphorylated NKCC1
expression. Dopamine is an essential chemical and receptor known for initiating
light adaptation in retina. Finally, an NKCC1 knockout mouse model was
examined and it revealed that both forms of NKCC are expressed in the
vertebrate retina.

Background: Light-adaptation is a multifaceted process in the retina that helps adjust the visual system to changing illumination levels. Many studies are focused on the photochemical mechanism of light-adaptation. Neural network adaptation mechanisms at the photoreceptor synapse are largely unknown. We find that large, spontaneous Excitatory Amino Acid Transporter (EAATs) activity in cone terminals may contribute to cone synaptic adaptation, specifically with respect to how these signals change in differing conditions of light. EAATs in neurons quickly transport glutamate from the synaptic cleft, and also elicit large thermodynamically uncoupled Cl- currents when activated. We recorded synaptic EAAT currents from cones to study glutamate-uptake events elicited by glutamate release from the local cone, and from adjacent photoreceptors. We find that cones are synaptically connected via EAATs in dark ; this synaptic connection is diminished in light-adapted cones. Methods: Whole-cell patch-clamp was performed on dark- and transiently light-adapted tiger salamander cones. Endogenous EAAT currents were recorded in cones with a short depolarization to -10mV/2ms, while spontaneous transporter currents from network cones were observed while a local cone holding at -70mV constantly. DHKA, a specific transporter inhibitor, was used to identify EAAT2 currents in the cone terminals, while TBOA identified other EAAT subtypes. GABAergic and glycinergic network inputs were always blocked with picrotoxin and strychnine. Results: Spontaneous EAAT currents were observed in cones held constantly at -70mV in dark, indicating that the cones received glutamate inputs from adjacent photoreceptors. These spontaneous EAAT currents disappeared in presence of a strong light, possibly because the light suppressed glutamate releases from the adjacent photoreceptors. The spontaneous EAAT currents were blocked with TBOA, but not DHKA, an inhibitor for EAAT2 subtype, suggesting that a

The amphibian retina is commonly used as a model system for studying function and mechanism of the visual system in electrophysiology, since the neural structure and synaptic mechanism of the amphibian retina are similar to higher vertebrate retinas. I determined the specific subtypes of receptors and channels that are involved in chemical and electrical synapses in the amphibian retina. My study indicates that glycine receptor subunits of GlyRº1, 3 and 4 and glutamate receptor subunit of GluR4 are present in bipolar and amacrine dendrites and axons to conduct chemical synapses in the retinal circuit. I also found that the gap junction channel, pannexin 1a (panx1a), is present in cone-dominated On-bipolar cells and rod-dominated amacrine processes possibly to connect rod-and cone-pathway in the inner retina. In addition, panx1a may form hemi-channels that pass ATP and Ca2+ signals. The findings of my study fill the gap of our knowledge about the subtypes of neurotransmitter receptors and gap junction channels conducting visual information in particular cell types and synaptic areas.