Calretinin-like immunoreactivity in mormyrid and gymnarchid electrosensory and electromotor systems

Calretinin-like immunoreactivity was examined in the electrosensory and electromotor systems of the two families of mormyriform electric fish. Mormyrid fish showed the strongest immunoreactivity in the knollenorgan electroreceptor pathway; in the nucleus of the electrosensory lateral line lobe (ELL) and the big cells of the nucleus exterolateralis pars anterior. Mormyromast and ampullary zones of the ELL showed calretinin-like immunoreactivity in the ganglion, granule, and intermediate cell and fiber layers. Mormyromast zones additionally showed labeling of apical dendrites and commissural cells, but the ampullary zone did not. In the electromotor system, two nuclei in the corollary discharge pathway showed labeling: in the paratrigeminal command-associated nucleus and the juxtalobar nucleus. Gymnarchus niloticus (Gymnarchidae) showed strongest calretinin-like immunoreactivity in part of the phase-coding pathway; in S-type electroreceptor afferents. Zones of the ELL not receiving phase-coder input had weak labeling. The electromotor system showed labeling in the lateral relay nucleus and less strongly in the medullary relay nucleus, but none in the pacemaker. The concentration of calcium-binding proteins in mormyrid and gymnarchid time-coding electrosensory pathways is consistent with the hypothesis that they play a role in preserving temporal information across synapses. Cell types that encode temporal characteristics of stimuli in precise spike times have high levels of calcium-binding proteins, but cells that re-code temporal information into presence or magnitude of activity have low levels. Some cell types in the electromotor pathways and early in the time-coding electrosensory pathways do not follow this hypothesis, and therefore preserve temporal information using a mechanism independent of calcium-binding proteins. In particular, electromotor systems may use extensive electrotonic coupling within nuclei to ensure precise timing.