Myelinated dendrites in the mormyrid electrosensory lobe

This is the third paper in a series on the morphology, immunohistochemistry , and synaptology of the mormyrid electrosensory lateral line lobe (ELL). T he ELL is a highly laminated, cerebellum-like structure in the rhombencepha lon that subserves an active electric sense: Objects in the nearby environm ent are detected on the basis of changes in the reafferent electrosensory s ignals that are generated by the animal's own electric organ discharge. Thi s paper concentrates on the intermediate (cell and fiber) layer of the medi al zone of the ELL and pays particular attention to the large multipolar ne urons of this layer (LMI cells). LMI cells are gamma -aminobutyric acid (GA BA)ergic and have one axon and three to seven proximal dendrites that all b ecome myelinated after their last proximal branching point. The axon projec ts to the contralateral homotopic region and has ipsilateral collaterals. B oth ipsilaterally and contralaterally, it terminates in the deep and superf icial granular layers. The myelinated dendrites end in the deep granular la yer, where they most likely do not make postsynaptic specializations, but d o make presynaptic specializations, similar to those of the LMI axons. Beca use it is not possible to distinguish between axonal and dendritic LMI term inals in the granular layer, the authors refer to both as LMI terminals. Th ese are densely filled with small, flattened vesicles and form large apposi tions with ELL granular cell somata and dendrites with symmetric synaptic m embrane specializations. LMI cells do not receive direct electrosensory inp ut on their somata, but electrophysiological recordings suggest that they n evertheless respond strongly to electrosensory signals (Bell [1990] J. Neur ophysiol. 63:303-318). Consequently, the authors speculate that the myelina ted dendrites of LMI cells are excited ephaptically (i.e., by electric fiel d effects) by granular cells, which, in turn, are excited via mixed synapse s by mormyromast primary afferents. The authors suggest that this ephaptic activation of the GABAergic presynaptic terminals of the myelinated dendrit es may trigger immediate synaptic release of GABA and, thus, may provide a very fast local feedback inhibition of the excited granular cells in the ce nter of the electrosensory receptive field. Subsequent propagation of the d endritic excitation down the myelinated dendrites to the somata and axon hi llocks of LMI cells probably generates somatic action potentials, resulting in the spread of inhibition through axonal terminals to a wide region arou nd the receptive held center and in the contralateral ELL. Similar presynap tic myelinated dendrites that subserve feedback inhibition, until now, have not been described elsewhere in the brain of vertebrates. (C) 2001 Wiley-L iss, Inc.