Parvocells: A novel interneuron type in the pacemaker nucleus of a weakly electric fish

Gymnotiform weakly electric fish produce electric organ discharges (EODs) t hat function in electrolocation and communication. The command signal for t he EOD is produced by the medullary pacemaker nucleus, which contains two w ell-characterized neuron types: pacemaker cells and relay cells. In this st udy, we characterized a third neuron type in the pacemaker nucleus. These n eurons, which we have named parvocells, were smaller (7-15 mu m in diameter ) than relay and pacemaker cells. The parvocells were labeled with an antib ody against the neuronal calcium-binding protein, parvalbumin, and were not labeled with several glial-specific antibodies. Parvocells had one to thre e fine processes that often terminated at the periphery of relay and pacema ker cell bodies. The parvalbumin-positive terminals of the parvocells coloc alized with immunoreactivity for SV-2, suggesting that the parvocells form chemical synapses on the relay and pacemaker cells. Parvalbumin-positive ne urons are frequently gamma-aminobutyric acid (GABA)ergic or glycinergic, an d the cytoplasm of the parvocell somata was immunoreactive with a glycine a ntibody. Antibodies against glycine receptors and gephyrin, however, did no t label any cells in the pacemaker nucleus, suggesting that the pacemaker n ucleus does not contain glycine or GABA((A)) receptors. Electron microscopy revealed gap junctions between the membranes of parvocells and adjacent te rminal-like structures. Furthermore, neurobiotin injected into individual p acemaker or relay cells labeled parvocells as well as other pacemaker and r elay cells, demonstrating that the parvocells are dye-coupled to the other neuron types in the pacemaker nucleus. These findings indicate that the par vocells are histochemically distinct from relay and pacemaker cells and tha t they receive electrotonic inputs from and make chemical synapses back ont o pacemaker and relay cells. Further study is needed to investigate the fun ction of these neurons in regulating the EOD. (C) 2000 Wiley-Liss, Inc.