IMMUNOLOCALIZATION OF NMDA RECEPTORS IN THE CENTRAL-NERVOUS-SYSTEM OFWEAKLY ELECTRIC FISH - FUNCTIONAL IMPLICATIONS FOR THE MODULATION OF A NEURONAL OSCILLATOR

Using a monoclonal antibody raised against the R1 subunit of the rat N MDA receptor, we mapped the distribution of NMDA receptors in the brai ns of three genera of electric fish. On Western blots, the antibody re cognized a glycoprotein of approximately 105 kDa throughout the CNS. O n tissue sections, it strongly labeled a number of neuronal somata and dendrites in the medulla, with weaker immunoreactivity in the forebra in and across much of the rest of the nervous system. At the ultrastru ctural level, reaction product was localized, though not exclusively, to the postsynaptic region of synapses. To study the role of NMDA rece ptors in a specific neural circuit, we focused on the medullary pacema ker nucleus. Neurons in this nucleus, which fire action potentials reg ularly and trigger each electric organ discharge (EOD), receive glutam atergic input from identified premotor areas. Activity in these areas can cause the pacemaker nucleus to produce outputs with distinct tempo ral dynamics, which are observed in the behaving animal as modulations of the EOD. The projection cells of the pacemaker nucleus, the relay cells, were heavily labeled with the anti-NMDA R1 antibody in all gene ra studied. These results are consistent with the previous finding tha t a particular EOD modulation mediated by the connection from one prem otor area of the brain to the relay cells is blocked by application to the pacemaker nucleus of NMDA receptor blockers. Our results compleme nt ongoing efforts to study this nucleus and provide additional eviden ce for the role of NMDA receptors in diverse neural circuits.