Cerebellar outputs contribute to spontaneous and movement-related activity in the motor cortex of monkeys
Introduction
The cerebellum plays an essential role in smooth and accurate limb movements (Holmes, 1917, 1939; Dow and Moruzzi, 1958; Kitazawa, 2002; Manto et al., 2012). Cerebellar impairment results in deficits in limb movements in humans, such as delay in movement initiation (Beppu et al., 1984), slowness of movements (Hore et al., 1991), increased variability of movement trajectory (Brown et al., 1990), and consistent decrease in muscle tone (Holmes, 1917, 1939). In non-human primates, ablation or transient inactivation of the cerebellum shows similar impairments in motor behaviors (Gilman, 1969; Meyer-Lohmann et al., 1977; Goodkin and Thach, 2003; Nashef et al., 2019). However, how cerebellar activity controls normal motor behaviors is unclear.
The main targets of cerebellar outputs are the motor cortices (Sasaki et al., 1976; Dum and Strick, 2003; Hashimoto et al., 2010), especially the primary motor cortex (M1), which plays an essential role in voluntary limb movements (Evarts, 1968; Cheney and Fetz, 1980; Dum and Strick, 1991; Lemon, 2008). Neurons in the cerebellar dentate nucleus (DN), one of the output nuclei of the cerebellum, send prominent di-synaptic excitatory projections to the M1 (Sasaki et al., 1976; Shinoda et al., 1985; Steriade, 1995; Na et al., 1997) through the motor thalamus (Asanuma et al., 1983; Nambu et al., 1991), and this pathway is well established as the cerebello-thalamo-cortical pathway (Allen and Tsukahara, 1974; Horne and Butler, 1995). Electrical stimulation of the cerebello-thalamo-cortical pathway evokes local field potentials in the M1 and causes both facilitation and/or suppression in most M1 neurons with short latencies (Holdefer et al., 2000; Nashef et al., 2018). Thus, the cerebello-thalamo-cortical pathway can immediately and broadly affect motor cortical activity. Furthermore, previous studies showed that firing rate changes of motor cortical neurons in the cerebello-thalamo-cortical pathway are involved in movement initiation (Meyer-Lohmann et al., 1977; Nashef et al., 2018). Taken together, these results suggest that DN outputs to the M1 are involved in the timing control for initiation of movement. However, it remains unclear how cerebellar outputs contribute to the motor cortical activity in other phases of motor tasks, such as inter-trial intervals (ITIs), preparation, and movement periods.
To address this question, we identified M1 neurons involved in the cerebello-thalamo-cortical pathway by recording responses to DN stimulations and neuronal activity while monkeys performed an arm-reaching task, which includes ITIs, preparation, and execution. We found that the firing rate of M1 neurons responding to the DN stimulation was significantly higher than that of the other neurons. These results suggest that cerebellar outputs contribute to M1 activity not only in initiation of movement, but also during ITIs, movement preparation, and execution. Reversible pharmacological inactivation of the DN supports this notion, in that DN inactivation induced a reduction of mean firing rates of M1 neurons during not only ITIs and movement preparation, but also the execution period.
Section snippets
Materials and Methods
Three male Japanese monkeys (Macaca fuscata; monkey 1, 11.0 kg; monkeys 2 and 3, 10.0 kg) were used in this study. Two monkeys (monkeys 1 and 2) were used for the DN stimulation experiments. One monkey (monkey 3) was used for the DN inactivation experiments. All experimental protocols were approved by the Ethical Committee for Animal Research at Tokyo Metropolitan Institute of Medical Science, Japan (Approved No. 14070). The monkeys were housed and handled in accordance with both the guidelines
Responses of M1 neurons to DN stimulations
We recorded neuronal activity within the forearm region of the M1 (Fig. 1C), and analyzed these activities during successful trials. In total, we sampled the activity of 526 neurons in the M1 (346 in monkey 1 and 180 in monkey 2).
DN stimulations induced facilitation and/or suppression in M1 neurons (Fig. 2A, B). We classified neurons into the following three types based on initial responses to DN stimulations: 1) facilitation-type, 2) suppression-type, and 3) no-response-type (see Materials and
Discussion
In the present study, we identified M1 neurons that were involved in the cerebello-thalamo-cortical pathway and systematically analyzed their activity during voluntary arm movements in macaque monkeys. We revealed that firing rates of M1 neurons of facilitation- and suppression-types were higher than those of no-response-type during entire periods, including periods of voluntary arm reaching. Further, pharmacological inactivation of the DN revealed a causal relationship between the DN outputs
Authors contribution
E.H. and A.N. conceived and designed the experiments. N.S., H.I., E.H., S.C. and A.N. performed the surgery and experiments. S.N. and Y.Na. analyzed the data and prepared figures. S.N., E.H., Y.Na., S.C., Y.Ni. and A.N. contributed to data interpretation. S.N., H.I., Y.Na., S.C., A.N. and Y.Ni. wrote the paper. All authors critically reviewed the manuscript.
Funding
This work was supported by the Japan Society for Promotion of Science (JSPS) KAKENHI Grant-in-Aid for Research Fellow18J11989 (to N.S.), Scientific Research (B) 26290012 (to E.H.), and Scientific Research (A) 26250009 (to A.N.), AMED-CREST from the Japan Agency for Medical Research and Development (to E.H. and A.N.), and MEXT KAKENHI “Non-linear Neuro-oscillology”15H05873 (to A.N.).
Declaration of Competing Interest
The authors declare no competing interests.
Acknowledgments
We thank N. Hashimoto, Dr. M. Hashimoto, K. Kondo, S. Nagai, T. Ogata, Dr. T. Yamagata, and Dr. O. Yokoyama for their technical assistance. We are also grateful to Drs. T. Minamimoto and Y. Nagai for acquiring magnetic resonance imaging. A monkey used in this study (monkey 1) was supplied from the National Bio-resource Project (Japanese Monkeys) supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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