ULTRASTRUCTURAL EVIDENCE OF GABAERGIC INHIBITION AND GLUTAMATERGIC EXCITATION IN THE PACEMAKER NUCLEUS OF THE GYMNOTIFORM ELECTRIC FISH, HYPOPOMUS

The medullary pacemaker nucleus of Hypopomus triggers each electric or gan discharge (EOD) by a single command pulse. It consists of electrot onically coupled 'pacemaker' cells, which generate the rhythm, and 're lay' cells, which follow the pacemaker cells and excite the spinal mot oneurons of the electric organ. The pacemaker cells receive two inputs from the complex of the diencephalic prepacemaker nucleus (PPn), a GA BA-ergic inhibition and a glutamatergic excitation. Relay cells, on th e other hand, receive two glutamatergic inputs, one from a subnucleus of the PPn, the PPn-C, and a second from the sublemniscal prepacemaker nucleus (SPPn). We have labelled afferents to the pacemaker nucleus b y injecting HRP to specific sites of the prepacemaker complex. By usin g immunogold-labelled antibodies and en-grid staining techniques, we d emonstrated GABA and glutamate immunoreactivity in labelled synaptic p rofiles of ultra-thin sections of the pacemaker nucleus. The two types of synapses were interspersed on the surfaces of pacemaker cells, wit h GABA-immunoreactive synapses apparently representing the GABA-mediat ed input of the 'PPn-I', an inhibitory subdivision of the PPn, and glu tamate-immunoreactive synapses representing the input of the 'PPn-G', an excitatory subdivision of the PPn. Only glutamate-immunoreactive sy napses were found on, relay cells.