The mormyrid rhombencephalon: I. Light and EM investigations on the structure and connections of the lateral line lobe nucleus with HRP labelling.

The rhombencephalic posterior lateral line lobe nucleus (nLLL) and its connections were investigated in the mormyrid fish Gnathonemus petersii, at light and electron microscopical levels using HRP tracing. The nLLL, constituted on each side of about 1500 large, round shaped, adendritic cells, is located in the intermediate cell and fibre layer exclusively, in the ventrolateral zone of the posterior lateral line lobe. The cells show a complex synaptology: boutons with chemical synapses cover the largest part of the soma and the long initial segment of the axons. In addition each nLLL cell bears generally two club endings which form gap junctions with the postsynaptic membrane. On the unmyelinated portion of the club ending, a particular synaptic complex (= serial synaptic connections) was observed; large endings, bearing boutons with chemical synapses, contact the club ending with gap junctions. HRP injection into either lateral line nerve showed that the club endings represent the peripheral input of the nLLL. This input is exclusively ipsilateral; the anterior and posterior lateral line nerve projection does not seem to overlay at this level. Retrograde labelling of nLLL cells after HRP injection into the anterior mesencephalic exterolateral nucleus (nELa) confirms the electrophysiological results according to which the nLLL projects directly and bilaterally to the nELa. This rhombo-mesencephalic connection is established through club endings of the nLLL axons which form gap junctions with the large cells of the nELa. About two thirds of the nLLL axons form a crossed and one third an uncrossed bundle within the lateral lemniscal pathway. Anterograde transport in the axon collaterals shows that some of the nLLL cells project simultaneously to the ipsi- and to the contralateral nELa permitting this system a high degree of electrosensory information processing.