ORGANIZATION OF THE CEREBELLAR NUCLEUS OF THE DOGFISH, SCYLIORHINUS-CANICULA L - A LIGHT-MICROSCOPIC, IMMUNOCYTOCHEMICAL, AND ULTRASTRUCTURAL-STUDY

Elasmobranchs possess a well-developed cerebellum with an associated c erebellar nucleus. To determine whether the organization of this nucle us is comparable with that of the deep cerebellar nuclei of mammals, w e studied the dogfish cerebellar nucleus with light microscopic method s (Nissl stain, Golgi method, reduced silver stain, NADPH-diaphorase h istochemistry and immunocytochemistry) and with electron microscopy. W e found the dogfish cerebellar nucleus to consist of about 1,050 large neurons, the ratio of Purkinje cells to cerebellar nucleus neurons be ing 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 r egion of the nucleus. Large glutamatergic neurons corresponded to bipo lar or triangular cells revealed by Golgi methods. Application of hors eradish peroxidase to the cerebellar cortex produced the labelling of beaded fibres of Purkinje cells in the cerebellar nucleus. Unlike in m ammals, GABAergic innervation of the cerebellar nucleus was scarce: Pu rkinje cell axon terminals in the cerebellar nucleus did not appear to be GABA-immunoreactive, most GABAergic fibres being found in the subv entricular neuropile. Some fibres immunoreactive to serotonin and soma tostatin were also observed in the subventricular neuropile of the cer ebellar nucleus. Three neuron types were distinguished with electron m icroscopy (types A to C). Type A cells were abundant and smooth-surfac ed, and appeared to correspond to Golgi-impregnated neurons and large glutamate-immunoreactive cells. Type B neurons were scarce and possess ed dendrites covered by sessile or stalked spines. Type C neurons were small cells located mainly in the medialmost region of the nucleus an d corresponded to subventricular glutamate- and GABA-immunoreactive ce lls. Six types of synaptic bouton were observed (types I to VI). The m ost abundant (type I boutons) made symmetrical contacts and appeared t o correspond to Purkinje cell axons. Type I boutons were the only type observed on perikarya and initial axon segments of type A cells. Type TV and type V boutons made complex glomerular-like asymmetrical conta cts with spines of type B cells. Type VI boutons appeared to correspon d to peptidergic and/or monoaminergic axons. The functional significan ce of these results is discussed. (C) 1996 Wiley-Liss, Inc.