Izumi Sugihara

The mesodiencephalic junction (MDJ) is located in a caudorostrally elongated, column‐shaped region encompassing the medial accessory oculomotor nucleus and perirubral area. It relays cerebral projections to the rostral subnuclei of the inferior olive (IO), which, in turn, send climbing fibers to the cerebellar hemisphere and receive nucleo‐olivary projections from the cerebellar nuclei (CN), forming a parallel, loop‐shaped, modular circuitry. However, the topographical organization of the MDJ–IO projection, which may contribute to functional localization within the cerebellar hemisphere, has not been fully elucidated. Here, we analyzed the distribution of labeled neurons in the MDJ in relation to those in the CN by injecting a Lumafluor retrograde tracer into various sites within the rostral IO subnuclei of mice. Injections mainly targeting the lateral or medial parts of the rostral IO subnuclei labeled neurons predominantly in the ventral or dorsal CN, respectively (referred to as v‐ and d‐CN‐predominant injection cases). Correspondingly, v‐CN‐predominant injection cases labeled neurons in the rostromedial, centroventral, and caudolateral regions of the MDJ, collectively termed the “sheathing subarea,” whereas d‐CN‐predominant injection cases labeled neurons primarily in the central “shaft subarea” of the MDJ. Considering the previously reported lobule‐related topography among the IO, cerebellar cortex, and CN, the sheathing and shaft subareas of the MDJ are likely to project preferentially to crus I and to other neighboring cerebellar hemispheric lobules, respectively. We speculate that the sheathing subarea of the MDJ is more strongly involved in non‐somatosensorimotor cognitive functions than the shaft subarea.

The cerebellar projection from the trigeminal nuclear complex is one of the major populations of the cerebellar inputs. Although this projection is essential in cerebellar functional processing and organization, its morphological organization has not been systematically clarified. The present study addressed this issue by lobule‐specific retrograde neuronal labeling and single axonal reconstruction with anterograde labeling. The cerebellar projection arose mainly from the interpolaris subdivision of the spinal trigeminal nucleus (Sp5I) and the principal trigeminal sensory nucleus (Pr5). Although crus II, paramedian lobule, lobule IX, and simple lobule were the major targets, paraflocculus, and other lobules received some projections. Reconstructed single trigeminocerebellar axons showed 77.8 mossy fiber terminals on average often in multiple lobules but no nuclear collaterals. More terminals were located in zebrin‐negative or lightly‐positive compartments than in zebrin‐positive compartments. While Pr5 axons predominantly projected to ipsilateral crus II, Sp5I axons projected either predominantly to crus II and paramedian lobule often bilaterally, or predominantly to lobule IX always ipsilaterally. Lobule IX‐predominant‐type Sp5I neurons specifically expressed Gpr26. Gpr26‐tagged neuronal labeling produced a peculiar mossy fiber distribution, which was dense in the dorsolateral lobule IX and extending transversely to the dorsal median apex in lobule IX. The projection to the cerebellar nuclei was observed in collaterals of ascending Sp5I axons that project to the diencephalon. In sum, multiple populations of trigeminocerebellar projections showed divergent projections to cerebellar lobules. The projection was generally complementary with the pontine projection and partly matched with the reported orofacial receptive field arrangement.

The parallel closed‐loop topographic connections between subareas of the inferior olive (IO), cerebellar cortex, and cerebellar nuclei (CN) define the fundamental modular organization of the cerebellum. The cortical modules or zones are organized into longitudinal zebrin stripes which are extended across transverse cerebellar lobules. However, how cerebellar lobules, which are related to the cerebellar functional localization, are incorporated into the olivo‐cortico‐nuclear topographic organization has not been fully clarified. In the present study, we analyzed the lobular topography in the CN and IO by making 57 small bidirectional tracer injections in the lateral zebrin‐positive stripes equivalent with C2, D1, and D2 zones in every hemispheric lobule in zebrin stripe‐visualized mice.C2, D1, and D2 zones were connected to the lateral part of the posterior interpositus nucleus (lPIN), and caudal and rostral parts of the lateral nucleus (cLN, rLN), respectively, and from the rostral part of the medial accessory olive (rMAO), and ventral and dorsal lamellas of the PO (vPO, dPO), respectively, as reported. Within these areas, crus I was specifically connected to the ventral parts of the lPIN, cLN, and rLN, and from the rostrolateral part of the rMAO and the lateral parts of the vPO and dPO.The results indicated that the cerebellar modules have lobule‐related subdivisions and that crus I is topographically distinct from other lobules. We speculate that crus I and crus I‐connected subdivisions in the CN and IO are involved more in nonmotor functions than other neighboring areas in the mouse.

The olivocerebellar projection is organized into an intricate topographical connection from the inferior olive (IO) subdivisions to the longitudinally-striped compartments of cerebellar Purkinje Cells (PCs), to play an essential role in cerebellar coordination and learning. However, the central mechanisms for forming topography need to be clarified. IO neurons and PCs are generated during overlapping periods of a few days in embryonic development. Therefore, we examined whether their neurogenic timing is specifically involved in the olivocerebellar topographic projection relationship. First, we mapped neurogenic timing in the entire IO by using the neurogenic-tagging system of neurog2-CreER (G2A) mice and specific labeling of IO neurons with FoxP2. IO subdivisions were classified into three groups depending on their neurogenic timing range. Then, we examined the relationships in the neurogenic-timing gradient between IO neurons and PCs by labeling topographic olivocerebellar projection patterns and PC neurogenic timing. Early, intermediate, and late groups of IO subdivisions projected to late, intermediate, and early groups of the cortical compartments, respectively, except for a few particular areas. The results indicated that the olivocerebellar topographic relationship is essentially arranged according to the reverse neurogenic-timing gradients of the origin and target.

The olivocerebellar projection is organized into an intricate topographical connection from the inferior olive (IO) subdivisions to the longitudinally-striped compartments of cerebellar Purkinje Cells (PCs) to play an essential role in cerebellar coordination and learning. The central mechanisms for the formation of this topography have been unclear. PCs and IO neurons are generated during overlapping periods of a few days in embryonic development. Here we examined whether their generation timing is specifically involved in the formation of the topographical relationship of the olivocerebellar projection. We mapped neuronal generation timing in the entire IO by using the neurogenic-tagging system of Neurog2-CreER (G2A) mice and specific labeling of IO neurons with FoxP2. IO subdivisions were classified into three groups depending on their neurogenic timing range. Then, we examined the relationships in the neurogenic-timing gradient between PCs and IO neurons based on the reported topographic olivocerebellar projection pattern and PC generation timing. Early, intermediate, and late groups of IO subdivisions projected to late, intermediate, and early groups of the cortical compartments, respectively, except for a few particular areas. The results indicated that the olivocerebellar topographic relationship is essentially arranged according to the reverse neurogenic-timing gradients of the origin and target.

The cerebellar cortex encodes sensorimotor adaptation during skilled locomotor behaviors, however the precise relationship between synaptic connectivity and behavior is unclear. We studied synaptic connectivity between granule cells (GCs) and Purkinje cells (PCs) in murine acute cerebellar slices using photostimulation of caged glutamate combined with patch-clamp in developing or after mice adapted to different locomotor contexts. By translating individual maps into graph network entities, we found that synaptic maps in juvenile animals undergo critical period characterized by dissolution of their structure followed by the re-establishment of a patchy functional organization in adults. Although, in adapted mice, subdivisions in anatomical microzones do not fully account for the observed spatial map organization in relation to behavior, we can discriminate locomotor contexts with high accuracy. We also demonstrate that the variability observed in connectivity maps directly accounts for motor behavior traits at the individual level. Our findings suggest that, beyond general motor contexts, GC-PC networks also encode internal models underlying individual-specific motor adaptation.