The Impact of Stimulation Induced Short-Term Synaptic Plasticity on Firing Patterns in the Globus Pallidus of the Rat

Electrical stimulation in the globus pallidus (GP) leads to complex modulations of neuronal activity in the stimulated nucleus. Multiple in vivo studies have demonstrated the modulation of both firing rates and patterns during and immediately following the GP stimulation. Previous in vitro studies, together with computational studies, have suggested the involvement of short-term synaptic plasticity (STP) during the stimulation. The aim of the current study was to explore in vitro the effects of STP on neuronal activity of GP neurons during local repetitive stimulation. We recorded synaptic potentials and assessed the modulations of spontaneous firing in a postsynaptic neuron in acute brain slices via a whole-cell pipette. Low-frequency repetitive stimulation locked the firing of the neuron to the stimulus. However, high-frequency repetitive stimulation in the GP generated a biphasic modulation of the firing frequency consisting of inhibitory and excitatory phases. Using blockers of synaptic transmission, we show that GABAergic synapses mediated the inhibitory and glutamatergic synapses the excitatory part of the response. Furthermore, we report that at high stimulation frequencies both types of synapses undergo short-term depression leading to a time dependent modulation of the neuronal firing. These findings indicate that STP modulates the dynamic responses of pallidal activity during electrical stimulation, and may contribute to a better understanding of the mechanism underlying deep brain stimulation like protocols

Electrophysiological Characteristics of Globus Pallidus Neurons

Extracellular recordings in primates have identified two types of neurons in the external segment of the globus pallidus (GPe): high frequency pausers (HFP) and low frequency bursters (LFB). The aim of the current study was to test whether the properties of HFP and LFB neurons recorded extracellularly in the primate GPe are linked to cellular mechanisms underlying the generation of action potential (AP) firing. Thus, we recorded from primate and rat globus pallidus neurons. Extracellular recordings in primates revealed that in addition to differences in firing patterns the APs of neurons in these two groups have different widths (APex). To quantitatively investigate this difference and to explore the heterogeneity of pallidal neurons we carried out cell-attached and whole-cell recordings from acute slices of the rat globus pallidus (GP, the rodent homolog of the primate GPe), examining both spontaneous and evoked activity. Several parameters related to the extracellular activity were extracted in order to subdivide the population of recorded GP neurons into groups. Statistical analysis showed that the GP neurons in the rodents may be differentiated along six cellular parameters into three subgroups. Combining two of these groups allowed a better separation of the population along nine parameters. Four of these parameters (Fmax, APamp, APhw, and AHPs amplitude) form a subset, suggesting that one group of neurons may generate APs at significantly higher frequencies than the other group. This may suggest that the differences between the HFP and LFB neurons in the primate are related to fundamental underlying differences in their cellular properties

Spontaneous firing of GP neurons in acute rat brain slices.

<p><b>A</b>, Population average of the spontaneous firing frequency recorded from GP neurons in the cell-attached mode (n = 76). <b>B</b>, Population average of the spontaneous firing frequency recorded from GP neurons in the whole-cell mode (n = 76). <b>C</b>, Distribution of the average firing frequency recorded in the cell-attached mode across the population. The frequency value used to construct the histogram was taken 200 s after the start of the recording to ensure stability. <b>D</b>, Distribution of the average firing frequency recorded in the whole-cell mode across the population. The frequency value used to construct the histogram was taken 200 s after the start of the recording to ensure stability. <b>E</b>, Correlation between the values used to generate C and D.</p

Representative intracellular and extracellular APs.

<p><b>A</b>, Spontaneous AP recorded in the whole-cell mode superimposed at an extended time scale for each cell type ( ─ type A; ─ type B; ─ type C). <b>B</b>, Spontaneous AP recorded in cell-attached mode superimposed at an extended time scale for each cell type (─ type A; ─ type B; ─ type C).</p

Histograms of several extracellular and intracellular properties of GP cells.

<p><b><i>A</i></b>, Half-width of the extracellular AP (AP_ex) calculated as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012001#pone-0012001-g004" target="_blank">figure 4</a>. <b><i>B</i></b>, Half-width of the intracellular AP (AP_in) calculated as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012001#pone-0012001-g004" target="_blank">figure 4</a>. <b><i>C</i></b>, The intracellular AP adaptation ratio calculated as the ratio between the amplitude of the last AP and that of the first AP in a train of APs generated by current injection. <b><i>D</i></b>, The slow phase of the AHP (AHP_s). AHP amplitude was measured from threshold. <b><i>E</i></b>, Sag of membrane potential induced by activation of I_h. Sag was calculated as the difference between the maximal deflection of the membrane potential following a hyperpolarizing step and the deflection at the end of the pulse. <b>F</b>, Maximal frequency calculated for each cell from F-I curves, similar to those in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012001#pone-0012001-g006" target="_blank">figure 6</a>.</p

Extracellular properties of different cellular populations of the primate GPe.

<p><b>A</b>. Traces from representative neurons from the two major neuron types in the GPe: high frequency pauser (HFP–left) and low frequency burster (LFB–right), shown at long and short time scales (top, 1 s trace; bottom, 100 ms trace). <b>B</b>. Autocorrelation functions of the neurons in A (maximal offset ±1 s). <b>C</b>. Mean firing rate of the two groups. <b>D</b>. Mean ISI distribution coefficient of dispersion of the two groups. <b>E</b>. Mean spike shape of the neurons shown above (HFP–black, LFB–gray). <b>F</b>. Mean spike duration of the two groups, Error bars indicate SEM, ** p≪0.01 Mann-Whitney U-test.</p

Representative recordings from GP neurons of three visually separated subgroups.

<p><i>Ai</i>, <i>Bi</i>, and <i>Ci</i>, responses of GP cells to depolarizing current steps (50 pA increment, 600 ms) applied from 0 to 550 pA using whole-cell configuration of the patch-clamp technique. Representative membrane potentials recorded in response to 100 pA are given for each cell type. Sampled at 20 kHz and filtered at 10 kHz. <i>Aii</i>, <i>Bii</i>, and <i>Cii</i>, responses of GP cells to hyperpolarizing current steps applied from 0 to −450 pA (50 pA increment, 600 ms) using the whole-cell configuration of the patch-clamp technique.</p

Statistical analysis of the intracellular and extracellular physiological parameters of GP cells.

<p>Kruskal-Wallis one- way analysis of variance by ranks and Mann-Whitney-Wilcoxon tests were used for the intracellular and extracellular properties of GP cells.</p><p>*P-value≤0.05.</p

Electrophysiological properties of GP cells.

<p>Data are expressed as mean ± S.D. <i>n</i> =  number of recorded GP neurons in each group.</p