Organisation of the cerebellar nucleus of the dogfish,Scyliorhinus canicula L.: A light microscopic, immunocytochemical, and ultrastructural study

Abstract

Elasmobranchs possess a well-developed cerebellum with an associated cerebellar nucleus. To determine whether the organization of this nucleus is comparable with that of the deep cerebellar nuclei of mammals, we studied the dogfish cerebellar nucleus with light microscopic methods (Nissl stain, Golgi method, reduced silver stain, NADPH-diaphorase histochemistry and immunocytochemistry) and with electron microscopy. We found the dogfish cerebellar nucleus to consist of about 1,050 large neurons, the ratio of Purkinje cells to cerebellar nucleus neurons being about 17:1. Immunocytochemistry showed large glutamatergic neurons in the main portions of the nucleus and small glutamate- and/or alpha-aminobutyric acid (GABA)-immunoreactive cells in the subventricular region of the nucleus. Large glutamatergic neurons corresponded to bipolar or triangular cells revealed by Golgi methods. Application of horseradish peroxidase to the cerebellar cortex produced the labelling of beaded fibres of Purkinje cells in the cerebellar nucleus. Unlike in mammals, GABAergic innervation of the cerebellar nucleus was scare: Purkinje cell axon terminals in the cerebellar nucleus did not appear to be GABA-immunoreactive, most GABAergic fibres being found in the subventricular neuropile. Some fibres immunoreactive to serotonin and somatostatin were also observed in the subventricular neuropile of the cerebellar nucleus. Three neuron types were distinguished with electron microscopy (types A to C). Type A cells were abundant and smooth-surfaced, and appeared to correspond to Golgi-impregnated neurons and large glutamate-immunoreactive cells. Type B neurons were scarce and possessed dendrites covered by sessile or stalked spines. Type C neurons were small cells located mainly in the medialmost region of the nucleus and corresponded to subventricular glutamate- and GABA-immunoreactive cells. Six types of synaptic bouton were observed (types I to VI). The most abundant (type I boutons) made symmetrical contacts and appeared to correspond to Purkinje cell axons. Type I boutons were the only type observed on perikarya and initial axon segments of type A cells. Type IV and type V boutons made complex glomerular-like asymmetrical contacts with spines of type B cells. Type VI boutons appeared to correspond to peptidergic and/or monoaminergic axons. The functional significance of these results is discussed.

Afferent and efferent connections of the cerebellum of the chondrostean Acipenser baeri: A carbocyanine dye (DiI) tracing study

“…Our results reveal that cerebellar efferent neurons are concentrated in three granular‐layer regions occupying a peripheral position in the three main parts of the cerebellum: the valvula, corpus, and auricles. In these locations they do not form an organized cerebellar nucleus, like that found in elasmobranchs (Paul and Roberts, 1984; Alvarez‐Otero et al, 1996). The efferent neurons of the teleost cerebellum are eurydendroid cells (Pouwels, 1978; Finger, 1978b; Meek et al, 1986b; Murakami and Morita, 1987; Torres et al, 1992; Díaz‐Regueira and Anadón, 2000), often located just below the Purkinje cell layer, unlike the cerebellar efferent cells of sturgeon.…”

Organization of the nitrergic neuronal system in the primitive bony fishesPolypterus senegalusandErpetoichthys calabaricus(Actinopterygii: Cladistia)

“…This study has identified a putative nitrergic cerebellar nucleus in cladistians according to its localization. Elasmobranches possess a well‐developed cerebellar nucleus positive for the NADPH‐d reaction (Álvarez‐Otero et al, ; Pose‐Mendez et al, ). Although no similar group has been described for teleosts, it was identified in some species of amphibians (Lázár and Losonczy, ; Huynh and Boyd, ), reptiles (Brüning et al, ; Smeets et al, ), birds (Cozzi et al, ; Atoji et al, ), and mammals (Vincent and Kimura, ), representing a variable feature that appears several times in evolution.…”

Evolutionary mechanisms that generate morphology and neural‐circuit diversity of the cerebellum

“…The DCN projection neurons receive afferents from PCs and send their efferent axons outside the cerebellum, including the red nuclei (Ebbesson & Campbell ; Paul & Roberts ; Alvarez‐Otero et al . ; Butler & Hodos ). These features indicate that the cartilaginous fish have cerebellar neural circuits similar to those of amniotes, although there are some differences between them.…”

“…The cartilaginous fish cerebellum contains DCNs that include projection neurons and interneurons, located in the cerebellar peduncles that connect the cerebellum with other regions of the CNS. The DCN projection neurons receive afferents from PCs and send their efferent axons outside the cerebellum, including the red nuclei (Ebbesson & Campbell 1973;Paul & Roberts 1984;Alvarez-Otero et al 1996;Butler & Hodos 2005). These features indicate that the cartilaginous fish have cerebellar neural circuits similar to those of amniotes, although there are some differences between them.…”

“…Ptf1a expression in the shark cerebellum has not been reported yet. However, since the shark cerebellum has well-differentiated PCs, GCs, and DCN neurons (projection and interneurons), and forms neural circuits similar to those of amniote cerebella (Alvarez-Otero et al 1996;Butler & Hodos 2005;Paul & Roberts 1984;Pose-Mendez et al 2016b;Rodriguez-Moldes et al 2008), these neurons are likely to be generated from Atoh1and Ptf1a-expressing neuronal progenitors by mechanisms shared by other amniotes. The GCs in the shark cerebellum do not form a broad layer but instead are concentrated in the eminentiae granularis and the upper and lower auricular leafs (Chaplin et al 2010;Pose-Mendez et al 2016b; Fig.…”