Direct Participation of Starburst Amacrine Cells in Spontaneous Rhythmic Activities in the Developing Mammalian Retina

Membrane electric properties of displaced starburst (A, B, P0 retina) and ganglion cells (C, D, P3 retina). Currents shown here have been corrected for membrane leakage. A,Whole-cell currents from the starburst cell elicited by depolarizing voltage steps (13 steps in 10 mV increments) under voltage clamp from a V_h of −70 mV. The transient inward currents are carried mostly by Na⁺. B, Voltage responses from the same starburst cell to depolarizing current pulses of 200 pA increments under current clamp at approximately −70 mV. As in nearly all of the starburst cells tested, the action potentials in this cell contain a single spike and have an activation threshold between −30 and −35 mV. C, Whole-cell currents from a ganglion cell voltage-clamped at −70 mV in response to the same voltage protocol as in A. Compared with starburst cells, the ganglion cell has significantly larger inward Na⁺ currents, with the maximum amplitude similar to or larger than that of the outward K⁺ currents. D, Voltage responses of the ganglion cell in C to depolarizing current pulses of 80, 100, 140, and 160 pA in amplitude. The action potentials in the ganglion cell are larger and faster than those in the starburst cellB and have a threshold around −45, which is significantly lower than that in starburst cells. Data shown in this figure were filtered at 2 kHz and digitized at 10–40 kHz.

Rhythmic activities from the same starburst cell in Figure 2A,B. The cell was current-clamped at its resting membrane potential.A, Segment of continuous recording showing rhythmic bursts of membrane depolarization from the cell (upward deflections). The bursting activity in this cell occurred regularly at a rate of once every 1–1.5 min throughout the entire recording period of ∼2 hr. B, One of the bursts inA (arrow) shown on a faster time scale. The burst was ∼2 sec long and 5 mV in amplitude, and it consists of a slow depolarization with small peaks riding atop. Data shown in this figure were filtered at 200 Hz and digitized at 1.2 Hz.

The pattern of spontaneous bursts from a P1 displaced starburst cell under current and voltage clamp.A, Continuous patch-clamp recording first under current clamp (I = 0, top 5 traces) and then voltage clamp (−80 mV, bottom 2 traces). Periodic bursts of membrane depolarization are shown as upward deflections under current clamp, whereas rhythmic inward currents are seen as downward deflections under voltage clamp. The average interburst duration (∼2 min) is the same in either recording mode. To test the spiking capability of the cell, depolarizing current pulses were also injected into the cell during an early part of the recording (arrow). Data shown in A were filtered at 5 Hz and digitized at 15 Hz. B, One of the bursts of membrane depolarization (A, arrow) is shown on an expanded time scale. The burst consists of many superimposed fast events reminiscent of postsynaptic potentials. Data shown in this panel were filtered at 600 Hz and digitized at 1.67 kHz.C, Closer view of the voltage responses to depolarizing current pulses shown in A (first trace). The cell is obviously capable of generating somatic potentials, which have a characteristic single-spike profile and an activation threshold of approximately −35 mV. The cell was current-clamped at its resting membrane potential and was depolarized by current steps of 50 pA increments. Data were filtered at 2 kHz and digitized at 10 kHz. D, Closer view of a spontaneous burst of synaptic currents in A (last trace). The burst consists of numerous fast synaptic currents and lasts ∼2–3 sec. The inset shows a few of these synaptic currents in more detail. Data were filtered at 600 Hz and digitized at 1.67 kHz.

Spontaneous bursts of action potentials in a P0 ganglion cell. A, Current-clamp recording showing bursts of action potentials occurring at a frequency of once per 1–2 min. The cell was held near its resting membrane potential of −53 mV.B, Expanded view of the first burst shown inA. This burst consists of a train of four action potentials, each having a large spike amplitude and a distinctive plateau potential. Data were filtered at 10 Hz (sampled at 20 Hz) inA and at 200 Hz (sampled at 500 Hz) in B. Because of the low filter frequency, the spike amplitude inA appears much smaller.

Examples of different patterns of spontaneous activities seen in a small number of ganglion cells. A,Current-clamp recording from a P3 ganglion cell showing spontaneous bursting activities occurring in doublets. The overall interburst interval in this cell is once every 1–3 min, similar to that found in most ganglion cells. Data were filtered at 10 Hz and digitized at 20 Hz. B, Rhythmic activities from another ganglion cell current-clamped in the same P3 retina as in A. The bursts shown here have a much longer duration (20–50 sec) and consist of clusters of shorter bursts. The interburst interval is well within the range of 1–3 min. Data in this panel were filtered at 10 Hz and digitized at 20 Hz. The amplitude of the spikes shown here is much smaller than in D because of heavy filtering.C, Expanded view of a burst in B. Data were filtered at 100 Hz and digitized at 200 Hz. D, Two spontaneous action potentials in B shown on a faster time scale. Data were filtered at 2 kHz and digitized at 10 kHz. E,Voltage-clamp recording (V_h = −70 mV) from a ganglion cell in a P5 retina showing highly periodic bursts of synaptic currents at a frequency of approximately one per minute. The burst duration in this cell is of the order of 30 sec, which is much longer than in most other cells. Data were filtered at 50 Hz and sampled at 100 Hz.

Fluorescence photomicrograph of a pair of neighboring starburst (left) and ganglion (right) cells in a P0 rabbit retina immediately after simultaneous patch-clamp recording with pipettes filled with Lucifer yellow. Scale bar, 50 μm.