Exp Brain Res (1988) 7i: 658.662 Ex mental Bran Research �9 Springer-Verlag 1988

Research Note

Projection on the motor cortex of thalamic neurons with pallidal input in the monkey

A. Nambu 1, S. Yoshida 1, and K. Jinnai 2

a Department of Physiology, Institute for Brain Research, Faculty of Medicine, Kyoto University, Kyoto 606, Japan 2 Department of Physiology, Shiga University of Medical Sciences, Ohtsu, Shiga 520-21, Japan

Summary. The cortical projection areas of thalamic neurons with basal ganglia and/or cerebellar inputs were studied electrophysiol0gically in unanesthetized monkeys. Thalamic neurons which receive inhibition from the pallidum were found to project to the motor cortex (area 4) as well as to premotor cortex. The neurons with pallidal input and motor cortical projec- tion were located mainly in VLo. This result indicates that the basal ganglia innervate the motor cortex through the thalamus. Thus the basal ganglia can modify the cortical output for controlling movements directly through this pathway as compared with its influence through the prefrontal and premotor cor- tices.

Key words: Pallidum - Cerebellar nuclei - Thalamus - Motor cortex - Monkey

It has been generally accepted that both pallidal and cerebellar output reach the motor cortex (area 4) via the thalamus and play an important role in control of movements (Kemp and Powell 1971). But Schell and

Strick (1984) have suggested that the major basal ganglia output is directed to the supplementary motor area (SMA), while motor cortex receives its main input from the cerebellar nuclei. The present electrophysiological study clarifies whether pallidal output projects via the thalamus exclusively to the SMA or also projects to the motor cortex.

Two adult Japanese monkeys (Macaca fuscata) were used. Under sodium pentobarbital anesthesia, three concentric electrodes were implanted stereo- taxically in the cerebellar nuclei (CN), and five electrodes were placed in the pallidal complex (i.e. internal (GPi) and external (GPe) segment of the globus pallidus). In the first monkey, nine pairs of

Offprint requests to: A. Nambu (address see above)

silver wire electrodes (inter-tip distance: 2 mm) were implanted in the prefrontal (PF), premotor (PM), and motor cortex (area 4) to activate thalamocortical neurons antidromically as shown in Fig. 1D. In t h e second monkey, electrodes were also placed in SMA and the caudal bank of the arcuate sulcus (arcuate premotor area, APA). A recording chamber was placed over an opening of the skull. After full recovery from the operation, recordings were started. During experimental sessions the monkey was seated in a monkey chair with its head restrained. Using a pulse motor drive, a glass-coated elgiloy microelectrode was inserted through the dura vertically into the thalamus to record neural activity. Inputs from CN to a thalamic neuron were examined by orthodromic activation after CN-stimulation (0.3 ms duration and less than 2 mA strength). Inputs from the pallidal complex were examined by changes in spontaneous discharge rate after GPi- stimulation (0.3 ms duration and 0.5 mA strength) which were observed using peristimulus time histo- grams (PSTHs). The cortical projection area of the neuron was identified by testing for antidromic responses to stimulation of the cerebral cortex (0.3 ms duration and less than 0.5 mA strength). Several points were marked by passing DC current through the recording electrodes. At the end of the experiments, the animals were perfused with forma- lin under deep anesthesia. The recording sites were reconstructed according to marks, and the positions of the stimulating electrodes were checked histologi- cally.

Three typical thalamocortical neurons are pre- sented in Fig. 1. Figure 1A shows a neuron which, received inhibitory input from GPi and projected on the motor cortex. This neuron was activated anti- dromically b Z stimulation of the hand area of the' motor cortex (Fig. 1A, 1), but did not respond orthodromically to CN-stimulation (not shown). To

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Fig. 1A-F. Three examples (A-C) of thalamocortical neurons responding to stimulation of GPi or CN. A 1: Antidromic response induced by stimulation of the hand area of the motor cortex. Several sweeps were superposed. Stimulus artefacts are indicated by triangles. 2: Collision of the antidromic response by preceding spontaneous spikes (average of 10 sweeps). When the interval between a spontaneous discharge and the cortical stimulus was longer than the sum of the antidromic conduction time (C) and the refractory period (R), antidromic responses were observed (lower trace). When the interval was shorter than C+R, the antidromic responses collided with spontaneous spikes (upper trace). Such procedures assured that only spontaneous discharges of this thalamocortical neuron were sampled. 3: PSTH showing suppression of spontaneous discharges after GPi-stimulation at time zero (N = 55 sweeps, bin width: 1 ms). B 1: Antidromic response induced by PM-stimulation. 2: Collision between spontaneous discharges and antidromic response. 3: PSTH (N = 190) showing suppression of spontaneous discharges after GPi-stimulation. C 1: Antidromic response induced by stimulation of the hand area of the motor cortex. 2: CN induced trans-synapfic discharges of the same neuron. 3: Collision extinction same as in A, 2, 4: PSTH (N = 100) shows no effect of GPi-stimulation. D Surface view of cerebral hemisphere showing the location of stimulating electrodes in PF (triangle), PM (square) and the motor cortex (circle). S. c., sulcus centralis; S. a., sulcus arcuatus. E The location of a stimulating electrode (closed circle) in the hand area of the motor cortex shown in a parasagittal section (right side, rostral). Small dots represent giant pyramidal neurons in layer V. F The location of stimulating electrodes (closed circle) in the pallidal complex shown in a frontal section

ensure that we were sampl ing discharges of one and the same n e u r o n exclusively and to rule out trans- synaptic responses to mo to r cortical s t imula t ion , the collision test was rou t ine ly employed (Fig. 1A, 2). Figure 1A, 3 shows that GPi - s t imula t ion suppressed the spon taneous discharges of this n e u r o n (latency: less than 2 ms, dura t ion: abou t 7 ms). The suppres- sion was followed by a slight increase of discharge rate.

Figure 1B presents a tha lamocor t ica l n e u r o n which was found to pro jec t to PM (Fig. 1B, 1) and was not act ivated by CN-s t imula t ion . F igure 1B, 3 shows that GPi - s t imula t ion suppressed the spontane- ous discharges of this n e u r o n (latency: less than 3 ms, durat ion: abou t 9 m s ) . Figure 1C il lustrates a thalamic n e u r o n which was or thodromica l ly act ivated by CN-s t imula t ion (Fig. 1C, 2) and pro jec ted on the motor cortex (Fig. 1C, 1). These o r thodromic and ant idromic responses were checked to be from the

Table 1. Numbers of the thalamic neurons which received input from GPi, CN or both (GPi + CN) and projected to different cortical areas. PF, prefrontal area; PM, premotor area; SMA, supplementary motor area; APA, arcuate premotor area; M, motor cortex (area 4). Numbers in parentheses indicate neurons sampled from the first monkey

(Cortical projection area) M PM APA PF SMA

(Input) GPi 18 (8) 16 (12) 2 2 (2) 7 GPi + CN 3 (2) CN 32 (26) 3 3

same n e u r o n by collision test (not shown). GPi- s t imulat ion did no t affect the spon taneous discharge rate of this n e u r o n (Fig. 1C, 4).

The activity of 86 tha lamocor t ica l neu rons was recorded and analyzed. Tab le 1 shows the n u m b e r s of neurons responding synaptical ly to GPi and/or CN

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+10.5 +9.9 Cd

GPi+CN (~ CNO

+8.7 +8.1

Fig. 2. Distribution of the thalamocortical neurons influenced by GPi- or CN-stimulation. The thalamic neurons are shown by different symbols in six .frontal planes. The symbols, open, filled and dotted in center, indicate GPi-inhibited neurons, CN-excited neurons and neurons with inputs from both GPi and CN, respectively. The triangle, square, and circle symbols indicate neurons projecting to PF, PM and the motor cortex, respectively. The nomenclature and abbreviations follow Olszewski (1952)

stimulation, and their project ion pat tern to different cortical areas. All the cells were tested for responses to stimulation of all the cortical sites. Spontaneous discharges of 45 thalamocort ical neurons were sup- pressed by GPi-stimulation but were not activated by CN-stimulation. The latencies of the suppression were less than 2.5 ms in most (80%) of the 45 neurons, and the mean duration of inhibition was about 8 ms. Of these 45 neurons, 18 neurons pro- jected on the motor cortex.

Thirty eight of the remaining 41 thalamocortical neurons were orthodromical ly excited by CN-stimu- lation, but were not inhibited by GPi-stimulation. Mean latency of CN-induced discharges was 2.3 ms. Only three thalamocortical neurons received inputs from both CN and GPi.

The recording sites of these neurons are plotted

in Fig. 2. Because the second monkey with SMA- and APA-stimulat ing electrodes has not been sac- rificed yet, Fig. 2 presents only the data f rom the first monkey with electrodes in the motor , PM and PF cortices. GPi-inhibited neurons which projected to the motor cortex (open circles) were found mainly in the nucleus ventralis lateralis pars oralis (VLo), while CN-activated neurons projecting on the motor cortex (filled circles) were located mainly in the nucleus ventralis posterior lateralis pars oralis (VPLo) and the nucleus ventralis lateralis pars caudalis (VLc). GPi-inhibited neurons projecting to PM (open squares) were situated mainly in the nucleus ventralis anterior pars parvoceUularis (VApc).

The latency of antidromic responses t o motor cortical stimulation of the neurons with GPi-input (mean + S.D.: 2.1 + 0.70 ms) was significantly

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longer than that of the neurons with CN-input (1.1 _+ 0.32 ms), and it was significantly shorter than the latency of antidromic responses of the neurons with GPi-input activated by PM-stimulation (3.2 + 0.73 ms).

For the following reasons, it can be assumed that the GPi-induced inhibition of thalamocortical neurons is caused by stimulation within the GPi. First, GPi stimulation neither induced any obvious movements and muscle twitches, nor evoked any cortical potentials in the motor cortex. Therefore, the possibility of current spread of GPi-stimulation to internal capsular fibers may be discarded. Secondly, a stimulating electrode in the center of GPi (GP2 in Fig. 1F) inhibited more thalamic neurons than one in the vicinity of the internal capsule (GP~ in Fig. 1F). Among the thalamic neurons which were tested by both GP~- and GP2-stimulation, 15 neurons responded to GP1-, GP2-, or both. GP2-stimulation inhibited more neurons (13/15) than GPl-stimulation (7/15).

The effect of GPi-stimulation on the thalamic neurons in this study was very similar to that in previous studies. Thalamocortical neurons suppres- sed by GPi-stimulation were found mainly in VLo and VApc, whereas CN-activated neurons were located mainly in VPLo and VLc in this study. These distributions are in good accordance with those reported previously (Asanuma et al. 1983; DeVito and Anderson 1982; Kalil 1981; Kim et al. 1976; Kuo and Carpenter 1973; Stanton 1980; Yamamoto et al. 1983). Our present study shows that the pallidal input to thalamic neurons is inhibitory, and that conver- gence of pallidal and cerebellar inputs on a single thalamic neuron occurred infrequently. Previous studies showed the same results (Uno et al. 1970, 1978; Yamamoto et al. 1983, 1984).

Three pairs of electrodes for motor cortical stimulation were checked to be located in the rostral bank of the central sulcus as exemplified in Fig. 1E. Although the exact border between the motor and premotor cortices is a matter of controversy, it is generally accepted that the rostral bank of the central sulcus is area 4. Moreover it may be assumed that the motor cortical stimulation could hardly affect thalamocortical fibers to SMA, PM or PF, as argued below. First, some GPi-inhibited neurons were anti- dromically activated by stimulating the hand area of the motor cortex (Fig. 1D) at low threshold stimulus current (less than 0.1 mA). Stimulation of this area at the same current strength (single pulse) induced movement only of the wrist joint. Absence of evoked movements in other upper limb joints was confirmed by recording EMG. Therefore, stimulation at this current strength excited only a small cortical area.

Secondly, none of the neurons which were excited antidromically by stimulation of the hand motor area with 0.5 mA intensity was activated by stimulation of other cortical areas with the same intensity, i.e., the face and the lower limb areas of the motor cortex, PF and PM. The distance between the stimulating elec- trode in the hand area and that in the face area was less than 3 mm. Therefore, stimulation of the motor cortex with that intensity did not effectively spread more than 1.5 mm and would not activate the other cortical areas.

It has been generally accepted that the motor cortex is influenced by both pallidal and cerebellar outflow (Kemp and Powell 1971), since the motor cortex receives afferent fibers from VLo as well as from VPLo (Kievit and Kuypers 1977; Miyata and Sasaki 1983; Strick 1976), and each subnucleus receives afferent fibers from GPi (DeVito and Anderson 1982; Kim et al. 1976; Kuo and Carpenter 1973) and CN (Asanuma et al. 1983; Kalil 1981; Stanton 1980), respectively. However, Schell and Strick (1984) proposed that the motor cortex receives its main input from the cerebellar nuclei but not from the basal ganglia. This assumption was based on the following findings. According to their definition, VLo was characterized by dense clusters of medium- to large-sized neurons, whereas the region of more loosely packed neurons which lacked dense cell clusters was considered a rostral extension of VPLo. After injections of WGA-HRP into the motor cor- tex, labeled neurons were largely confined to VPLo, and no labeled neurons were found in their VL0. They considered that the region of more loosely packed neurons, i.e., the rostral extension of VPLo, receives no pallidal outflow. However, terminal distribution of pallidothalamic fibers has not been studied so far with the definition of VLo and VPLo according to Schell and Strick (1984). In this study, our data reveals direct electrophysiological evidence that thalamic neurons with pallidal input also project on the motor cortex.

We have not discussed thalamic neurons project- ing on SMA, APA and PF, because the number of the recorded neurons projecting on these areas remains limited. Activity of such neurons should be investigated further. Nevertheless, we suggest that the motor cortex is influenced through the thalamus, by both pallidal and cerebellar outflow. This implies that the basal ganglia participate in control of move- ments by direct influence on the motor cortex.

Acknowledgements. The authors thank Dr. N. Mano for valuable information about elgiloy electrodes. We also thank Prof. K. Sasaki for critical reading of the manuscript and constant encour- agement.

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Received September 29, 1987 / Accepted February 26, 1988