Cortical slow oscillatory activity is reflected in the membrane potential and spike trains of striatal neurons in rats with chronic nigrostriatal lesions

Abstract

Neurons in the basal ganglia output nuclei display rhythmic burst firing after chronic nigrostriatal lesions. The thalamocortical network is a strong endogenous generator of oscillatory activity, and the striatum receives a massive projection from the cerebral cortex. Actually, the membrane potential of striatal projection neurons displays periodic shifts between a very negative resting potential (down state) and depolarizing plateaus (up states) during which they can fire action potentials. We hypothesized that an increased excitability of striatal neurons may allow transmission of cortical slow rhythms through the striatum to the remaining basal ganglia in experimental parkinsonism. In vivo intracellular recordings revealed that striatal projection neurons from rats with chronic nigrostriatal lesions had a more depolarized membrane potential during both the down and up states and an increased firing probability during the up events. Furthermore, lesioned rats had significantly fewer silent neurons than control rats. Simultaneous recordings of the frontal electrocorticogram and membrane potential of striatal projection neurons revealed that the signals were oscillating synchronously in the frequency range 0.4-2 Hz, both in control rats and rats with chronic nigrostriatal lesions. Spreading of the slow cortical rhythm is limited by the very low firing probability of control rat neurons, but a slow oscillation is well reflected in spike trains of approximately 60% of lesioned rat neurons. These findings provide in vivo evidence for a role of dopamine in controlling the flow of cortical activity through the striatum and may be of outstanding relevance for understanding the pathophysiology of Parkinson's disease.

Fig. 1.

A, B, Coronal sections of the forebrain immunolabeled with antibodies directed against tyrosine hydroxylase. The microphotographs depict the typical aspect of the striatum in sham-lesioned (A) and 6-OHDA-lesioned (B) rats. Scale bar, 1.5 mm.C, The behavioral effect of the lesion was evaluated with the “stepping test.” The 6-OHDA-lesioned rats had a significant impairment in the test (p < 0.001; main group effect in a two-way ANOVA) involving both the contralateral forelimb (** p < 0.05; Tukey's test) and, although in a smaller degree, the ipsilateral forelimb (*p < 0.05; Tukey's test). D, Schematic diagram showing the placement of the concentric bipolar electrodes used to record the electrocorticogram (lateral electrode) and to stimulate the frontal cortex (medial electrode). Scale bar, 1.2 mm. E, F, All the neurons that were successfully injected with Neurobiotin had the typical morphology of striatal medium spiny neurons. In sham-lesioned rats, all injections yielded a single labeled cell (E), whereas in 6-OHDA-lesioned rats, ∼60% of the injections yielded two neurons (F). Insets, Segments of dendrites showing spines. Scale bar, 25 μm. ECoG, Electrocorticogram; FCx-S, frontal cortex stimulation;C, contralateral; I, ipsilateral.

Fig. 2.

Representative recordings of striatal neurons from sham-lesioned rats (top trace) and 6-OHDA-lesioned rats (bottom trace). In both experimental groups, most neurons had a fluctuating membrane potential. Histograms depicting the time spent at any given membrane potential typically had bimodal profiles with the distributions fitting dual-Gaussian functions.

Fig. 3.

Sham-lesioned and 6-OHDA-lesioned rats differed in several measurements related to their two-state membrane potential.Top, The membrane potential was more depolarized during both the down states (*p= 0.027) and up states (*p = 0.004; two-tailed Student'st test) in rats with nigrostriatal lesions. The box-plots include data from both silent and active neurons. Median (black line), 25th and 75th percentiles (bar limits), 10th and 90th percentiles (error bars), and outliers (black circles) are shown. Bottom left, Down states were shorter in 6-OHDA-lesioned rats (*p < 0.001; two-tailed Student'st test). The duration of the up events was not significantly different between groups. Bottom right, In a given time window, neurons from 6-OHDA-lesioned rats spent a smaller proportion of time in the down state (*p = 0.0044; ANOVA by ranks) and a higher proportion of time shifting from one state to the next (fluctuating around the transitional voltage or reaching short-lived steady states) (NUD) (*p = 0.011, ANOVA by ranks). There was a trend toward an increased proportion of time spent in the up state that did not reach statistical significance (*p = 0.10; ANOVA by ranks).

Fig. 4.

Box-plots summarizing information regarding the firing rate and firing probability of striatal neurons. Median (black line), 25th and 75th percentiles (bar limits), 10th and 90th percentiles (error bars), and outliers (black circles) are shown. Left, Seventy-five percent of the spontaneously active neurons recorded from 6-OHDA-lesioned rats had firing rates >3 Hz, whereas 75% of the spontaneously active neurons recorded from control rats had rates <3 Hz (*p = 0.002; Mann–Whitney Utest). Right, The spontaneously active neurons recorded from control rats had a significantly lower probability of firing at least one spike during an up event than those recorded from rats with nigrostriatal lesions (*p = 0.011; Mann–WhitneyU test).

Fig. 5.

Typical examples of the response evoked by cortical stimulation in sham-lesioned (top) and 6-OHDA-lesioned rats (bottom). Several events were superimposed in each graphic. Note the shorter latency of the depolarizing plateau and the more depolarized membrane potential during the hyperpolarizing phase of the response in the 6-OHDA-lesioned rats.

Fig. 6.

Visual inspection of simultaneous recordings of the frontal electrocorticogram and the membrane potential of striatal neurons indicated that the two waveforms were oscillating synchronously at ∼1 Hz. The signals are displayed as they were recorded, but note that they were down-sampled, smoothed, and standardized before analysis. The belief that the signals were synchronized was substantiated through the analysis of cross-correlograms and by means of coherence analysis. Each row of graphics shows the cross-correlogram, cross-spectrum, coherence spectrum, and phase-spectrum corresponding to the signal pairs displayed in the upper part of the figure (top row of graphics, sham-lesioned rat;bottom row of graphics, rat with nigrostriatal lesion). In each graphic, the results of the analysis of several 30 sec epochs from the same signal pair were superimposed. The four disjoined 30 sec epochs depicted for the sham-lesioned rat were chosen from a recording session elapsing 15 min. A 12 min recording session from a 6-OHDA-lesioned rat provided the three 30 sec epochs that were chosen for the bottom row of graphics. Both signal pairs showed strong correlations. The cross-spectra revealed a powerful common frequency component of ∼0.7 Hz (SHAM) or ∼0.9 Hz (6-OHDA), and the signals displayed a very high coherence and a lineal phase relationship at the dominant frequency.

Fig. 7.

Each column depicts the power spectra of the electrocorticogram (ECoG), membrane potential (Vm), and spike trains of all the well correlated signal pairs recorded from sham (n = 13) and 6-OHDA-lesioned (n = 12) rats. Theinsets are averages of all the spectra contained in the corresponding graphic. Only one neuron in the pairs recorded from sham-lesioned rats displayed enough spontaneous discharge (>1 Hz) to compute its spike train spectrum. For 6-OHDA-lesioned rats, ∼70% of these recordings showed firing rates higher than ∼1 Hz.

Fig. 8.

Postulated mechanism for the generation of rhythmic modulations of firing rate in the basal ganglia output nuclei. The ∼1 Hz cortical rhythm is propagated to the striatum, where it produces a rhythmic fluctuation in the membrane potential of striatal projection neurons. During the depolarizing phase of the membrane potential fluctuation, striatal neurons are very close to threshold, but they only rarely discharge action potentials when the nigrostriatal system is intact (left). In animals having chronic nigrostriatal lesions, striatal projection neurons show a more depolarized up state and an increased firing probability (right). These changes may allow transmission of a coarse representation of the cortical rhythm to the striatal target nuclei.