Voltage‐activated and calcium‐activated currents studied in solitary rod inner segments from the salamander retina

Abstract

.SUMMARY 1. Solitary rod inner segments were obtained by enzymatic dissociation of the tiger salamander (Ambystoma tigrinum) retina. Their membrane currents were studied with the single-pipette voltage-clamp technique. Individual currents were isolated with the aid of pharmacological agents.2. Extracellular caesium blocked a current activated by hyperpolarization from -30 mV. Changing external sodium and potassium concentrations altered the value of the reversal potential in a manner consistent with the current being carried equally by both ions.3. Extracellular tetraethylammonium (TEA) blocked a current activated by depolarization from -70 mV. In normal medium this current had a reversal potential of -72 mV. Changing the external potassium concentration altered the value of the reversal potential in a manner consistent with the current being carried predominantly by potassium.4. Extracellular cobalt blocked a current activated by depolarization that had an initial inward and a later outward component. 5. After EGTA was injected into an inner segment the outward component was suppressed. Cobalt then blocked an inward current. This current is believed to be carried predominantly by calcium. The conductance increased with depolarization from -45 mV and reached a maximum at approximately 0 mV. Following a step of depolarization the current activated rapidly (< 20 msec) and then remained constant for at least several seconds without evidence of inactivation.6. Injecting caesium into an inner segment eliminated a calcium-activated outward current believed to be carried by potassium ions.7. After the injection of caesium there remained another calcium-activated current with a reversal potential of -17 mV. Changing extracellular chloride concentration altered the value of the reversal potential in a manner consistent with chloride carrying at least 70 % of the current. Another anion may carry the balance.8. When the five currents mentioned in items 2, 3, 5, 6 and 7 were blocked, the C. R. BADER, D. BERTRAND AND E. A. SCHWARTZ membrane resistance between -90 and -25 mV was linear, time-independent, and had a high value (2-1 Ge).9. The five identified currents can all be activated in the physiological range of voltage in which salamander rods normally operate.

Synaptic transmission at retinal ribbon synapses

“…Moreover, single-channel recordings (Rodriguez-Contreras and Yamoah, 2003) of L-type Ca 2+ channels in hair cells reveal clearly that at any test voltage Ba 2+ results in much greater activation of the channel than Ca 2+ does, i.e., the channel has a much higher open probability in Ba 2+ . Barium also fails to activate significantly calcium-dependent channels, including a Ca 2+ -dependent potassium current (Bader et al, 1982;Moriondo et al, 2001) and a Ca 2+ -dependent chloride current (ICl Ca Maricq and Korenbrot, 1988;. Thus, the use of Ba is beneficial because it allows better isolation of I Ca , but it also obscures functionally important interactions between calcium and calcium-dependent channels (see Section 5.1).…”

“…Cl − flux can also alter membrane potential and a balance between these two effects may help to stabilize the tonic activation level of I Ca (Thoreson et al, 2003a). To better understand these simultaneous feedback interactions, we consider the case of rods where E Cl is positive to the dark membrane potential (Bader et al, 1982;Somlyo and Walz, 1985;Burkhardt et al, 1991;Thoreson et al, , 2003a. In this scenario, activation of I Ca stimulates I Cl(Ca) , producing a depolarization that leads to further activation of I Ca .…”

Activation of Ca2+‐activated Cl− channel ANO1 by localized Ca2+ signals

“…In the nervous system functional CaCCs are best characterized in neurons with various sensory functions, such as olfactory neurons (Kleene & Gesteland, 1991; Lowe & Gold, 1993), photosensitive rods and cones (Bader et al . 1982; Maricq & Korenbrot, 1988; Barnes & Hille, 1989), taste cells (Taylor & Roper, 1994) and somatosensory neurons (Liu et al . 2010; Cho et al .…”

Characterization of a voltage-gated K+ channel that accelerates the rod response to dim light

“…Large hyperpolarizations elicited by bright light activate a Cs + -sensitive inward current (Fain et al, 1978). This inward current, now called I h (Bader et al, 1982), produces, within a few hundred milliseconds, a reduction in the hyperpolarization, by up to 20 mV, that makes the photovoltage peak well in advance of the photocurrent (Bader et al, 1979;Baylor et al, 1984). However, a dim light response of only a few millivolts also peaks sooner than the corresponding photocurrent (Baylor et al, 1984).…”

“…Attwell and Wilson (1980) have suggested that one of two current components contributing to the net current elicited by hyperpolarization was a deactivating outward current, I x , which could be recorded in the presence of Cs + and reversed in the region of the K + equilibrium potential. Bader et al (1982) have described a voltage-gated K + current (I K ) and a Ca 2+ -activated K + current, but the role of these currents in the response to dim light has not been addressed. Using whole-cell, patch-clamp techniques applied to isolated rod photoreceptors for high-resolution recording, we characterize the activity of a population of K + channels that gate in the voltage range from -30 to -70 mV.…”