Voltage-clamp analysis and computational model of dopaminergic neurons from mouse retina

Abstract

Isolated dopaminergic amacrine (DA) cells in mouse retina fire rhythmic, spontaneous action potentials and respond to depolarizing current with trains of low-frequency action potentials. To investigate the roles of voltage-gated ion channels in these processes, the transient A-type K+ current (I(K,A)) and Ca2+ current (I(Ca)) in isolated mouse DA cells were analyzed by voltage clamp. The I(K,A) activated at -60 mV and inactivated rapidly. I(Ca) activated at around -30 mV and reached a peak at 10 mV without apparent inactivation. We also extended our previous computational model of the mouse DA cell to include the new electrophysiological data. The model consisted of a membrane capacitance in parallel with eight currents: Na+ transient (I(Na,T)), Na+ persistent (I(Na,P)), delayed rectifier potassium (I(Kdr)), I(K,A), calcium-dependent potassium (I(K,Ca)), L-type Ca2+ I(Ca), hyperpolarization-activated cation current (I(h)), and a leak current (I(L)). Hodgkin-Huxley type equations were used to define the voltage- and time-dependent activation and inactivation. The simulations were implemented using the neurosimulator SNNAP. The model DA cell was spontaneously active from a wide range of initial membrane potentials. The spontaneous action potentials reached 35 mV at the peak and hyperpolarized to -76 mV between spikes. The spontaneous firing frequency in the model was 6 Hz. The model DA cell responded to prolonged depolarizing current injection by increasing its spiking frequency and eventually reaching a depolarization block at membrane potentials greater than -10 mV. The most important current for determining the firing rate was I(K,A). When the amplitude of I(K,A) was decreased, the firing rate increased. I(Ca) and I(K,Ca) also affected the width of action potentials but had only minor effects on the firing rate. Ih affected the firing rate slightly but did not change the waveform of the action potentials.

Dynamic, Nonlinear Feedback Regulation of Slow Pacemaking by A-Type Potassium Current in Ventral Tegmental Area Neurons

“…These models have used Hodgkin-Huxley-like equations based on independent activation and inactivation processes (Connor and Stevens, 1971b; Gerber and Jakobsson, 1993; Rush and Rinzel, 1995; Xiao et al, 2004) and have had overlapping steady-state activation and inactivation curves, thus predicting substantial steady-state I A “window” current. For example, the model of Connor and Stevens (1971b) for I A predicts steady-state I A near −60 mV amounting to 4% of maximally activated current.…”

“…Thus, it is reasonable to suppose that I A may play an important role in regulating firing of VTA neurons. However, the mechanism by which I A influences firing rate (i.e., the time dependence and magnitude of I A flowing between spikes) in VTA or other neurons is unclear and has been approached mainly by computer modeling (Connor and Stevens, 1971b; Gerber and Jakobsson, 1993; Rush and Rinzel, 1995; Xiao et al, 2004), with a lack of experimental data attempting to directly quantify flow of I A during natural pacemaking.…”

Heterogeneous transgene expression in the retinas of the TH-RFP, TH-Cre, TH-BAC-Cre and DAT-Cre mouse lines

“…For example, transgenic labeling of type 1 DA amacrine cells have aided in defining the connectivity between these cells and AII and GABA-containing amacrine cells, ON-cone bipolar cells, and melanopsin-containing ganglion cells (Gustincich et al, 1997, Feigenspan et al, 2000, Zhang et al, 2007, Contini et al, 2010, Van Hook et al, 2012, Zhang et al, 2012, Newkirk et al, 2013). Other studies that used isolated type 1 DA amacrine cells, labeled by human placental alkaline phosphatase (hPLAP), report these cells have transient A-type K + , Ca 2+ and TTX-sensitive Na + currents, and rhythmic spontaneous action potentials (Feigenspan et al, 1998, Xiao et al, 2004). More recently, type 1 DA amacrine cells identified by green fluorescent protein (GFP) in retinal whole mounts of a dopamine receptor 2 transgenic mouse line were used to characterize their resting spontaneous spike properties and light responses (Newkirk et al, 2013).…”

Ionic currents influencing spontaneous firing and pacemaker frequency in dopamine neurons of the ventrolateral periaqueductal gray and dorsal raphe nucleus (vlPAG/DRN): A voltage-clamp and computational modelling study

“…For the operation of the model and in line with Hodgkin and Huxley (1952) we have used three activation gates for sodium currents and four activation gates for the delayed rectifier potassium current. Only one activation gate was used for calcium currents, I A current, I M and I H current as described previously (Xiao et al 2004) and in line with our experimental fitting. Barium currents were shifted 10 mV more positive to mirror the V 50 values for calcium currents.…”