High-frequency, short-latency disinhibition bursting of midbrain dopaminergic neurons

Abstract

During reinforcement and sequence learning, dopaminergic neurons fire bursts of action potentials. Dopaminergic neurons in vivo receive strong background excitatory and inhibitory inputs, suggesting that one mechanism by which bursts may be produced is disinhibition. Unfortunately, these inputs are lost during slice preparation and are not precisely controlled during in vivo experiments. In the present study we show that dopaminergic neurons can be shifted into a balanced state in which constant synaptic N-methyl-d-aspartate (NMDA) and GABA(A) conductances are mimicked either pharmacologically or using dynamic clamp. From this state, a disinhibition burst can be evoked by removing the background inhibitory conductance. We demonstrate three functional characteristics of network-based disinhibition that promote high-frequency, short-latency bursting in dopaminergic neurons. First, we found that increasing the total background NMDA and GABA(A) synaptic conductances increased the intraburst firing frequency and reduced its latency. Second, we found that the disinhibition burst is sensitive to the proportion of background inhibitory input that is removed. In particular, we found that high-frequency, short-latency bursts were enhanced by increasing the degree of disinhibition. Third, the time course over which inhibition is removed had a large effect on the burst, namely, that synchronous removal of weak inhibitory inputs produces bursts of high intraburst frequency and shorter latency. Our results suggest that fast, more precisely timed bursts can be evoked by complete and synchronous disinhibition of dopaminergic neurons in a high-conductance state.

Fig. 1.

Dopaminergic neurons in a high-chord conductance state with N-methyl-d-aspartate (NMDA)/GABA_A receptor activation. A: concurrent NMDA and GABA_A conductance ramps were applied to a dopaminergic neuron in a whole cell recording. A fixed 3:1 NMDA-to-GABA_A ratio was maintained during the ramp. The ramp began at time t = 5 s and was completed at t = 485 s with a total conductance (g_NMDA + g_GABAA) of 80 nS. Spontaneous activity resumed after both conductances were removed. B: mean single spiking frequency during application of conductance ramps for a series of fixed NMDA-to-GABA_A conductance ratios (number of cells shown in C). C: the conductance at which single spiking failed during the conductance ramp (failure point) is plotted for the different NMDA-to-GABA_A conductance ratios. The numbers of cells for each ratio are shown in parenthesis. D: single spiking persisted after application of 40 μM NMDA and 40 μM isoguvacine (a GABA_A receptor agonist) (I). The steady-state current-voltage (I-V) curve of the dopaminergic neuron is shown (II) for control artificial cerebrospinal fluid (aCSF; black), after application of NMDA and isoguvacine (red), and postdrug aCSF (return to aCSF; blue). The shape of the I-V curve largely remained unchanged with drug application. E: the dopaminergic neuron in the pharmacologically induced high-chord conductance state could generate bursts of action potentials in response to subtraction of a GABA_A receptor conductance with the dynamic clamp (maximum burst frequency 19.4 Hz; mean burst frequency 13.4 Hz). Recordings in D and E are from the same cell.

Fig. 2.

Disinhibition burst frequency increases as the total applied NMDA/GABA_A conductance increases. A: concurrent NMDA and GABA_A conductance ramps (3:1 ratio) were applied to a dopaminergic neuron in a representative example. As the total applied conductance reached 10, 20, ..., 80 nS, the GABA_A conductance was phasically set to zero for 1 s. Insets show the change in firing at total conductance of 20, 40, 60, and 80 nS. B: summary data for the maximum and mean frequency (I), number of spikes (II), and latency to the first spike (III) during the 1-s disinhibition window as a function of the total applied conductance (n = 9). Burst onset was defined as the latency of the first spike in the first interspike interval (ISI) of <80 ms (cells in which this criteria were not met were excluded from the mean calculation for that conductance). C: maximum firing frequency (I), number of spikes (II), and latency to first spike (III) for disinhibition bursts (solid) are shown along with bursts evoked from phasic NMDA receptor activation alone (shaded). The NMDA conductance of the disinhibition burst was 0.75 times the total applied conductance. NMDA-only bursts were evoked by a 1-s NMDA conductance step (5–70 nS, 5-nS increment) in a spontaneously firing dopaminergic neuron.

Fig. 3.

Burst frequency increases as the degree of disinhibition increases. A: concurrent NMDA and GABA_A conductance ramps (3:1 ratio; not shown) were applied to a dopaminergic neuron in a representative example. A total conductance of 40 nS (g_NMDA = 30 nS; g_GABAA = 10 nS) was reached at the end of the ramp and held steady. In 1-s windows, a proportion of g_GABAA was removed, causing a phasic increase in firing. B: summary data for the maximum (Max; solid) and mean (shaded) frequency of spikes (I), number of spikes (II), and latency to first spike (III) as a function of the proportion of g_GABAA removed (n = 9).

Fig. 4.

Modeling the time course of disinhibition of substantia nigra pars compacta dopaminergic neurons. A source of tonically active inhibitory inputs was turned off synchronously (A) or asynchronously (B). This is represented by a step function (A, left) or the complementary cumulative distribution function [f(t) = cCDF (mean μ = 1.0 s, standard deviation σ = 100 ms)] (B, left). Each input is described by the conductance waveform g(t) (middle) with a deactivation time constant (τ) of 6 ms. The time course of disinhibition is determined by their convolution [f*g](t). Thus tonic inhibition decays according to the GABA_A deactivation time constant (A, synchronous) or the complementary cumulative distribution function (B, asynchronous). The time at which an individual GABA_A input in the asynchronous case is removed is taken from a normal distribution.

Fig. 5.

Burst frequency decreases as the time constant of disinhibition increases. A: concurrent NMDA and GABA_A conductance ramps (3:1 ratio) were applied to a dopaminergic neuron in a representative example (as in Fig. 1). A total conductance of 40 nS (g_NMDA = 30 nS; g_GABAA = 10 nS) was achieved at the end of the ramp and maintained. In 1-s intervals, g_GABAA was removed according to a simple exponential decay, 10*e^(−t/τ_d). The disinhibition time constant (τ_d) was varied. B: unlike mean burst frequency (P > 0.05, n = 8), the maximum burst frequency showed a significantly nonzero slope across the range of τ_d values (P < 0.05, n = 8). C: the total number of spikes in the disinhibition window was not changed with different τ_d values (P < 0.05, n = 8). D: the latency to the initiation of the burst, defined as an ISI <80 ms, increased significantly with τ_d values (P < 0.05, n = 8).

Fig. 6.

Disinhibition bursts change in frequency and shape with asynchronous removal of the GABA_A conductance. A: concurrent NMDA and GABA_A conductance ramps (3:1 ratio) were applied to a dopaminergic neuron in a representative example (as in Fig 1). A total conductance of 40 nS (g_NMDA = 30 nS; g_GABAA = 10 nS) was achieved at the end of the ramp and maintained. At 30-s intervals, g_GABAA was removed according to the complementary cumulative distribution function, which describes the asynchronous removal of a population of tonically activated GABA_A receptors. The standard deviation parameter of that function is described by σ. A raster plot of disinhibition spiking as sampled in A is shown in B. The maximum (solid) and mean frequency (shaded) of the burst (C) as well as the number of spikes in the burst (D) decreased significantly with increasing σ (P < 0.05, n = 6). E–G: the shape of the disinhibition bursts changed with increasing σ; changes in the ISI as the burst progressed in all 6 cells. With small σ (i.e., more synchronous g_GABAA removal), disinhibition bursts showed a progressive increase in the ISI of each spike in the burst (B and E, solid). With large σ (more asynchronous g_GABAA removal), disinhibition bursts showed a progressive decrease in the ISI of each spike in the burst (B and E, shaded). The first ISI <80 ms (dashed line in C) defines the onset of the burst. F: there was a significant increase in the ISI up to ISI #3 for σ = 1 ms compared with σ =300 ms (P < 0.05, n = 6). G: the latency at which the burst began increased with σ (P < 0.05, n = 6).