Bidirectional Control of Orienting Behavior by the Substantia Nigra Pars Reticulata: Distinct Significance of Head and Whisker Movements

Abstract

Detection of an unexpected, novel, or salient stimulus typically leads to an orienting response by which animals move the head, in concert with the sensors (e.g., eyes, pinna, whiskers), to evaluate the stimulus. The basal ganglia are known to control orienting movements through tonically active GABAergic neurons in the substantia nigra pars reticulata (SNr) that project to the superior colliculus. Using optogenetics, we explored the ability of GABAergic SNr neurons on one side of the brain to generate orienting movements. In a strain of mice that express channelrhodopsin (ChR2) in both SNr GABAergic neurons and afferent fibers, we found that continuous blue light produced a robust sustained excitation of SNr neurons which generated ipsiversive orienting. Conversely, in the same animal, trains of blue light excited afferent fibers more effectively than continuous blue light, producing a robust sustained inhibition of SNr neurons which generated contraversive orienting. When ChR2 expression was restricted to either GABAergic SNr neurons or GABAergic afferent fibers from the striatum, blue light patterns in SNr produced only ipsiversive or contraversive orienting movements, respectively. Interestingly, whisker positioning and the reaction to an air-puff on the whiskers were incongruent between SNr-evoked ipsiversive and contraversive head movements, indicating that orienting driven by exciting or inhibiting SNr neurons have different behavioral significance. In conclusion, unilateral SNr neuron excitation and inhibition produce orienting movements in opposite directions and, apparently, with distinct behavioral significance.

Keywords: basal ganglia; orienting behavior; substantia nigra; vibrissa; whisker.

Graphical abstract

Figure 1.

Effect of optogenetic blue light patterns on SNr firing in slices. A–C, Whole-cell recordings from SNr cells in slices of Vgat-SNr-ChR2 (A), Vgat-Str-ChR2 (B), and Vgat-ChR2 (C) mice. Each panel overlays five trials showing the effect of continuous blue light (upper) and of a blue light train (lower, 20 Hz; 1-ms pulses) applied in SNr (each panel highlights one trial in red). The schematic indicates in green the expression of ChR2 in GABAergic cells of either the SNr (A), the striatum (B), or both (C). Note that in Vgat-ChR2 mice, the continuous pulse excited the SNr cell, while the train inhibits the SNr cell, which is a combination of the effects evoked in the other mice. D, Effect of continuous blue light at different powers (0.1 vs 0.25 mW; left and middle panels), which reveals an IPSP at the onset of the pulse when the power is increased. Application of a train (20 Hz; 1-ms pulses) at the same 0.25-mW power drives a sustained IPSP that abolishes SNr firing. Each panel highlights one trial in red. E, Overlay of averaged traces at the onset of continuous blue light at the two different powers reveal the IPSP evoked by the light at the higher power. F, Population data from whole-cell recordings showing the effect of different patterns of blue light on the firing of SNr cells in slices from Vgat-ChR2 and Vgat-SNr-ChR2 mice. In Vgat-SNr-ChR2 mice, which express ChR2 selectively in SNr GABAergic cells, blue light produces an increase in SNr firing as a function of train frequency that becomes maximal during continuous blue light. In contrast, in Vgat-ChR2 mice that also express ChR2 in GABAergic afferents to SNr, blue light applied as trains produces a suppression of SNr firing while continuous blue light excites SNr firing.

Figure 2.

ChR2 expression and cannula placements. A, Schematic of the different optogenetic groups of animals employed in the present study. Blue indicates blue light and expression of blue-sensitive opsins (ChR2 and IC++). Green indicates green light and expression of green-sensitive opsin (Arch). As noted, opsin expression occurs in GABAergic afferents that reach the SNr from neurons located in different parts of the striatum (including StrD and/or StrV, or Acb) and/or in GABAergic neurons located in the SNr. Light is always applied in the SNr. B, C, Coronal sections at the level of the midbrain from eight Vgat-ChR2 mice showing the robust expression of ChR2 circumscribed to the SNr and the optical fiber tract by which blue light was applied during behavior. The upper panels (B) blend a dark-field image of the section with the green channel of the eYFP fluorescence image. The lower panels (C) show the fluorescence image alone. The top right numbers are animal IDs. D, Reconstruction of optical fiber track endings in the SNr for all mice in the study. E, Location of AAV injections in the striatum and corresponding projections in the SNr of Vgat-StrDV-ChR2 mice. F, Location of AAV injections in the Acb and corresponding projections in the SNr of Vgat-Acb-ChR2 mice.

Figure 3.

Effect of trains or continuous blue light in the SNr of Vgat-ChR2 mice on head orienting movements (head bias) during exploration of an open field. A, upper, Head bias evoked by blue light in the ipsiversive (positive) or contraversive (negative) direction versus the side of the brain where the stimulated optical fiber is implanted. The traces overlay the effects in Vgat-ChR2 mice and in No Opsin mice. Lower, Change in speed from blue light onset associated with the head bias. B, Effect of continuous blue light (upper) or trains of blue light (lower) at different powers (0.7, 1.6, 3.0 mW) on head bias. C, Population measures of peak head bias, change in peak speed and times to these peaks (if the peaks exist) for Vgat-ChR2 and No Opsin mice. Asterisks show significant differences between the two groups of mice.

Figure 4.

Effect of trains or continuous light on head orienting movements during exploration of an open field in different mice that express opsin only in GABAergic SNr cells or in afferent fibers. Panels show population mean ± SEM. A, Effect of exciting GABAergic SNr neurons using blue light in Vgat-SNr-ChR2 mice on head bias and change in speed. B, Effect of inhibiting SNr neurons by exciting GABAergic afferent fibers in SNr using blue light in Vgat-StrD-ChR2, Vgat-StrV-ChR2, Vgat-StrDV-ChR2, and Vgat-Acb-ChR2 mice on head bias and change in speed. C, Effect of inhibiting GABAergic SNr neurons using green light in Vgat-SNr-Arch and Vgat-Arch mice, and blue light in Vgat-SNr-IC++ mice on head bias and change in speed.

Figure 5.

Population data of peak head bias, change in peak speed, and times to these peaks (if they exist) for the different groups in our study. A, Peak head bias and time to peak for the different groups. Gray asterisks on brackets denote significant differences between trains and continuous blue light within a group. Black or red asterisks at the bottom denote significant differences versus No Opsin mice. B, Same as A but for change in peak speed and time to peak speed.

Figure 6.

Effect of blue light on whisker position during head bias evoked by blue light in Vgat-ChR2 mice. A, EMG recorded from the ipsilateral (green) and contralateral (red) whisker pads. The left panels show the effect of continuous blue light (Cont) while the right panels show the effect of trains of blue light. Panels show population mean ± SEM for all the sessions. B, Shows the position of the whiskers versus the head (whisker bias) evoked by the blue light in the same sessions as A. The green trace tracks the ipsilateral whiskers while the red trace tracks the contralateral whiskers. Overlaid in blue is the head bias. The green and red color lines parallel to the x-axis denote the period when the whisker bias traces are significantly different compared with the prelight period based on a bootstrap comparison.

Figure 7.

Effect of an air puff applied to the ipsilateral or contralateral whiskers on head bias evoked by blue light in Vgat-ChR2 mice. A, Air-puff applied to the ipsilateral (upper) and contralateral (lower) whiskers on head bias evoked by continuous or trains of blue light. The traces show the effect of light alone, an air-puff alone, or the light and air-puff delivered together. Panels show population mean ± SEM for all the sessions. B, Population measures of peak head bias for the different conditions shown in A. The asterisks denote differences between the light alone versus the light and air-puff applied together.

Figure 8.

Effects of blue light in the SNr of Vgat-ChR2 mice on signaled active avoidance. A, Effect of continuous or trains of blue light in the SNr of Vgat-ChR2 mice and No Opsin mice on ACS+LCS trials, which test the effect of the light pattern on the ability to perform signaled active avoidance to an ACS. Note that continuous blue light impaired active avoidance in Vgat-ChR2 mice but not in No Opsin mice. The right panels show trial speed, trial velocity, and intertrial speed for the data in the left panels. Asterisks denote blue light patterns that were significantly different between the two groups. B, Effect of trains of blue light, which inhibit SNr in Vgat-ChR2 mice (but not in No Opsin mice), on ACS alone trials. In ACS alone trials, the SNr inhibition substitutes the regular external ACS to determine whether it has the ability to serve as a CS and trigger active avoidance responses. The right panels show trial speed, trial velocity, and intertrial speed for the data in the left panels. Asterisks denote blue light patterns that were significantly different between the two groups.