Fiber Connections of the Caudal Corpus Cerebelli, with Special Reference to the Intrinsic Circuitry, in a Teleost (<b><i>Oreochromis niloticus</i></b>)

Abstract

The caudal part of the corpus cerebelli of Nile tilapia can be divided into dorsal and ventral regions. The granule cell layer of the dorsal (dGL) and ventral (vGL) regions of the caudal corpus cerebelli is known to receive indirect inputs from the telencephalon relayed by the nucleus paracommissuralis. The descending pathways are topographically organized, and the dGL and vGL receive inputs from different dorsal telencephalic parts. The caudal corpus cerebelli, in turn, projects extracerebellar efferents. However, it remains unknown how the descending telencephalic inputs are processed within the cerebellum. Therefore, the present study investigated intrinsic connections of the caudal corpus cerebelli by injecting neural tracers into the molecular layer of dorsal and ventral regions. Injections of tracers into the ventral molecular layer resulted in labeled cells in the vGL and the ganglionic layer of the ventral corpus. The axonal trajectories from labeled cells in the ganglionic layer were analyzed in detail via single-axon reconstructions, which suggested that the terminal portions were confined to the ganglionic layer of the dorsal corpus. No labeled terminals were observed outside the caudal corpus cerebelli. Tracer applications to the dorsal molecular layer resulted in labeled cells not only in the ganglionic layer and the granule cell layer of the dorsal corpus but also in the ganglionic layer of the ventral corpus. The latter finding confirms the presence of intrinsic projections from the ventral region to the dorsal region in the caudal corpus cerebelli. We further revealed that the intrinsic projection neurons are Purkinje cells by immunohistochemistry for zebrin II (aldolase C), which is a marker of Purkinje cells, combined with tracer injections into the dorsal corpus. Unlike injections into the ventral corpus, injections into the dorsal corpus resulted in labeled terminals in extracerebellar structures, such as the nucleus of the medial longitudinal fascicle and reticular formation. The present study suggests that indirect inputs from different telencephalic parts received and processed by distinct regions of caudal corpus cerebelli are sent out of the corpus through the efferent neurons in the dorsal corpus.

Morphological analysis of the cerebellum and its efferent system in a basal actinopterygian fish, Polypterus senegalus

“…Their axons ran medially and crossed the midline (Figure 5c’). These features are consistent with those of neurons in the inferior olivary nucleus of teleosts (Bae et al., 2009; Finger, 1978b; Imura et al., 2017; Takeuchi et al., 2015; Xue et al., 2008). By neural tracer injection into the cerebellum, some of parvalbumin‐ir neurons were retrogradely labeled contralaterally (Figure 5c”).…”

“…Most retrogradely labeled efferent cells lie in a restricted area the ventrolateral region of the auCe (Figure 6). Since tracer injections were limited to the midbrain which is but one of the efferent targets of teleost cerebellum, our experiment might not label all efferent cells, such as neurons, that project to caudal brain areas including the medulla (Ikenaga et al., 2002; Imura et al., 2017). However, neurons expressing vglut2 , which is considered as a marker for the cerebellar efferent neurons, distributed in regions identical to retrogradely labeled neurons, but not near the Purkinje cells in Polypterus cerebellum (Figure 9).…”

Afferent and efferent connections of the nucleus prethalamicus in the yellowfin goby Acanthogobius flavimanus

“…The present study revealed the presence of cerebello‐prethalamic projections in the yellowfin goby. Although connections of the cerebellum have been reported for various species of teleosts (e.g., catfish: Finger, 1978; false kelpfish: Murakami & Morita, 1987; goldfish and green sunfish: Wullimann & Northcutt, 1988; Nile tilapia: Imura et al, 2017; rainbow trout: Folgueira, Anadón, & Yáñez, 2006), a cerebello‐prethalamic projection has been found only in holocentrids (Xue et al, 2003). In the holocentrids, the cerebellar corpus is composed of dorsal and ventral lobes, and only the ventral lobe appears to project to the PTh, and only to the contralateral nucleus.…”

Teleost Fish

“…The corpus cerebelli is separated into two subdivisions (dorsal and ventral) in holocentrid (Xue et al 2004). In the Nile tilapia, caudal region of corpus cerebelli can be subdivided into dorsa and ventral regions based on their cytoarchitechture and fiberconnections (Imura et al 2017). Mormyrid corpus was considered to be consist with four subdivisions traditionally (C1 to C4, Nieuwenhuys and Nicholson 1969a), but recent studies suggested that part of them belongs to the corpus and others to the valvula (see details in Meek and Nieuwenhuys 1998).…”