Figure 1.
Histological controls of the 6-OHDA induced nigral denervation.
Photomicrographs of TH-immunoreactivity at the SNc (A and C) and VTA (B) levels in unilaterally 6-OHDA lesioned rats. Note the sparing of the DA cell bodies in the VTA (B) and the complete loss of the DA cell bodies in the SNc ipsilateral to the lesion (C). Asterisks (*) indicate the loss of DA cell bodies in the 6-OHDA lesioned SNc. Scale bar in C (200 µm) applies in A, B. SNR: substantia nigra pars reticulata; VTA: ventral tegmental area.
Table 1.
Discharge rate and pattern of cortical cells recorded using single-unit extracellular or intracellular recordings in control and SNc-lesioned animals were similar.
Figure 2.
Effect of SNc lesion and impact of STN HFS on the spontaneous activity of cortico-STN cells and on the electrocorticogram (ECoG) of the orofacial motor cortex.
The spontaneous activity of a cortico-STN neuron was extracellularly recorded simultaneously with the ECoG signal of the orofacial cortex in control rats (A), and in SNc-lesioned animals, before (B) and during STN HFS (C). Note that the SNc lesion induces an increased firing rate of the cortico-STN neuron (B1) that is accompanied by an increased number of bursts and by the appearance of an excessive synchronization in beta frequency band in the ECoG (B2, inset) compared to control situation (A1, A2, inset). These SNc lesions-induced changes were abolished during STN HFS (C). In panels A1, B1, and C1, the different traces correspond, from the top to the bottom, to the ECoG signal, the simultaneous extracellular recording of the cortico-STN cell, the bursting discharge detected by Poisson Surprise analysis (S≤2) and the discharge of the cell represented as a sequence of spikes. In panels A2, B2, and C2, histograms display the corresponding interspike intervals and insets show the power spectrum (FFT) of the corresponding ECoGs. In C1, arrows indicate the stimulation artifacts and the asterisk an action potential. Abbreviation: ECoG: electrocorticogram; FFT: Fast Fourier Transform; HFS: high-frequency stimulation; ISI: interspike interval; SNc: substantia nigra pars compacta; STN: subthalamic nucleus.
Figure 3.
Effect of SNc lesion and impact of STN HFS on the spontaneous activity of cortico-striatal cells and the electrocorticogram (ECoG) of the orofacial motor cortex.
The spontaneous activity of a cortico-striatal neuron was extracellularly recorded simultaneously with the ECoG signal of the orofacial cortex in control rats (A), and in SNc-lesioned animals, before (B) and during STN HFS (C). Note that the SNc lesion induced an increased firing rate of the cortico-striatal neuron (B1) that was accompanied by an increased number of bursts and by the appearance of an excessive hypersynchronisation in the beta frequency band in the ECoG (B2, inset) compared to control situation (A1, A2, inset). These SNc lesions-induced changes were abolished during STN HFS (C). In panels A1, B1, and C1, the different traces correspond, from the top to the bottom, to the ECoG signal, the simultaneous extracellular recording of the cortico-striatal cell, the bursting discharge detected by Poisson Surprise analysis (S≤2) and the discharge of the cell represented as a sequence of spikes. In panels A2, B2, and C2, histograms display the corresponding interspike intervals and insets show the power spectrum (FFT) of the corresponding ECoGs. In C1, arrows indicate the stimulation artifacts and the asterisk an action potential. Abbreviation: ECoG: electrocorticogram; FFT: Fast Fourier Transform; HFS: high-frequency stimulation; ISI: interspike interval; SNc: substantia nigra pars compacta; STN: subthalamic nucleus.
Figure 4.
Intracellularly recorded spontaneous activity of identified pyramidal cells and simultaneously recorded ECoG in the orofacial motor cortex in intact and SNc-lesioned rats, before and during STN HFS.
Activity of cortico-STN (A) and cortico-striatal (B) neurons were recorded simultaneously with the ECoG from the orofacial motor cortex of intact rats (A1, B1) and of SNc-lesioned animals, before (A2, B2) and during (A3, B3) STN HFS. In each panel A1, A2, A3, B1, B2 and B3, the top trace is the ECoG signal and the bottom trace is the simultaneous intracellular recording of the pyramidal cell. Note that changes of firing rate and pattern were similar to those observed using extracellular recordings. In A1, A2, A3, B1, B2 and B3, the values of the membrane potential are indicated on the left. Note that SNc lesion resulted in a more depolarized membrane potential. (C, D) Example of the impact of STN HFS on the distribution of the membrane potential of the cortico-STN (C) and the cortico-striatal (D) neurons recorded in SNc lesioned animals and illustrated in (A2, A3) and (B2, B3), respectively. In lesioned rats (black bars), the membrane potential of this cortico-STN neuron was unimodally distributed around a mean value of −61.8 mV that was best fitted by a Gaussian function (r2 = 0.98; n = 6.25×105 values; bin size: 0.5 mV) and shifted to more hyperpolarized values during STN HFS (light gray bars) (mean, −64.0 mV; n = 6.25×105 values; bin size, 0.5 mV; Gaussian fit, r2 = 0.95). In lesioned rats (black bars), the membrane potential of this cortico-striatal neuron was unimodally distributed around a mean value of −58.0 mV that was best fitted by a Gaussian function (r2 = 0.99; n = 6.25×105 values; bin size: 0.5 mV) and shifted to more hyperpolarized values during STN HFS (light gray bars) (mean, −62.5 mV; n = 6.25×105 values; bin size, 0.5 mV; Gaussian fit, r2 = 0.97). (E) Box plot summary of the impact of STN HFS on the value “w” that was estimated for each Gaussian-Laplace fit of the distribution of the membrane potentials in all pyramidal neurons (see materials and methods) indicating a narrowing of the distribution of membrane potential during STN HFS. Abbreviation: ECoG: electrocorticogram; HFS: high-frequency stimulation; SNc: substantia nigra pars compacta; STN: subthalamic nucleus.
Figure 5.
Electrophysiological identifications of pyramidal neurons.
Cortico-STN and cortico-striatal neurons were identified by their antidromic activation following electrical stimulation of the ipsilateral STN and contralateral striatum, respectively. Intracellular characterization of the antidromic spike evoked in a pyramidal cell by ipsilateral STN stimulation: The antidromic latency (1.5 ms), which was measured as indicated by the double-headed arrows, was not affected by DC-induced hyperpolarization (from −64 to −73 mV; iDC −0.5 nA). The top trace shows no collision between orthodromic and antidromic spikes when the STN stimulation is applied 3.3 ms after a spontaneous orthodromic action potential. The middle trace shows a collision of an antidromic spike (asterisk) with a spontaneous orthodromic action potential occurring at an appropriate time interval. In each trace, the values of the membrane potential are indicated on the left.
Table 2.
Impact of SNc lesions on the discharge rate and pattern of cortico-STN and cortico-striatal cells.
Table 3.
Impact of STN HFS on the discharge rate and pattern of cortico-STN and cortico-striatal cells recorded in SNc-lesioned rats.
Figure 6.
Long lasting antidromic activation of cortico-subthalamic neurons during STN HFS.
(A) Recording of one cortico-STN neuron during STN HFS indicating that this cell was reliably antidromically driven from the STN during the entire period of the stimulation. Note that the antidromic spikes were evoked independently of the cell membrane potential. (B) Magnified view of the recording illustrating the fixed latency of the antidromic spikes (latency = 1.3 ms) as marked by the double-headed arrows.
Figure 7.
Impact of SNc lesion on electrical membrane properties of cortico-subthalamic (A, C and E) and cortico-striatal (B, D and F) neurons in control and SNc-lesioned situations, before and during STN HFS. A1, B1, C1
and D1, Voltage responses of pyramidal neurons (top traces) to intracellular injection of positive and negative square current pulses (bottom traces) in control (A1, B1) and SNc-lesioned (C1, D1) situations. The responses to the negative current pulses are an average of 10 successive trials. A2, B2, C2 and D2, Plots of the voltage changes (ΔV) (bottom) and of the mean firing frequency (<F>) (top) as a function of current intensity in control (A2, B2) and SNc-lesioned (C2, D2) rats. E and F are the voltage responses and the corresponding plots of the voltage changes and mean firing frequency as a function of current intensity obtained in the same cells shown in C and D, respectively, but during STN HFS. The apparent input resistance was measured from the linear portion of the V-I curve. The F-I relationship was best fitted by a sigmoidal function (r2 >0.99). Abbreviation: HFS: high-frequency stimulation; STN: subthalamic nucleus; SNc: substantia nigra pars compacta.
Table 4.
Comparison of membrane properties of pyramidal cells in control and SNc-lesioned rats.
Table 5.
Comparison of membrane properties of cortico-STN and cortico-STR cells in control and SNc-lesioned rats.