Physiology of electrosensory lateral line lobe neurons in Gnathonemus petersii

In mormyrid electric fish, sensory signals from electroreceptors are relaye d to secondary sensory neurons in a cerebellum-like structure known as the electrosensory lateral line lobe (ELL). Efferent neurons and interneurons o f the ELL also receive inputs of central origin, including electric organ c orollary discharge signals, via parallel fibers and via fibers from the jux talobar nucleus. To understand the cellular mechanisms of the integration o f sensory inputs and central inputs in the ELL, the intracellular activity and ionic properties of the efferent projection neurons and interneurons we re examined in an in vitro slice preparation, We focus here on the electrophysiological properties of the efferent neuron s of the ELL network, the large fusiform cells and large ganglion cells, an d on a class of gamma-aminobutyric acid (GABA)-ergic interneurons known as medium ganglion (MG) cells, In response to current injection through a reco rding pipette, both types of efferent neuron fire a large narrow spike foll owed by a large hyperpolarizing afterpotential. The MG cells fire a complex spike which consists of small narrow spikes and a large broad spike, Altho ugh the forms of the action potentials in efferent neurons and in MG cells are different, all spikes are mediated by tetrodotoxin (TTX)-sensitive Naconductances and spike repolarization is mediated by tetraethylammonium (TE A(+))-sensitive K+ conductances. In the presence of TEA(+), substitution of Ba2+ for Ca2+ in the bath revealed the presence of a high-voltage-activate d Ca2+ conductance, Stimulation of parallel fibers conveying descending input intrinsic to the ELL network. to the ELL molecular layer in vitro evokes an excitatory posts ynaptic potential (EPSP), generally followed by an inhibitory postsynaptic potential (IPSP), in the efferent neurons, In MG cells, the same stimulatio n evokes an EPSP, often followed by small IPSP. Synaptic transmission at pa rallel fiber synapses is glutamatergic and is mediated via both N-methyl-D- aspartate (NMDA)- and (AMPA)-type glutamate receptors, The inhibitory compo nent of the parallel fiber response is GABAergic, It is probably mediated v ia the stellate neurons and the MG cells, which are themselves GABAergic in terneurons intrinsic to the ELL network. A hypothetical neural circuit of the intrinsic connections of the ELL, base d on the known morphology of projection neurons and medium ganglion interne urons, is presented, This circuit includes an excitatory and an inhibitory submodule. The excitatory submodule is centered on a large fusiform cell an d appears to relay the sensory input as a positive 'ON' image of an object, The inhibitory submodule is centered on a large ganglion cell and relays a negative 'OFF' image to the nest higher level, We suggest that MG cells ex ert an inhibitory bias on efferent neuron types and that the ELL network ou tput is modulated by the dynamically plastic integration of central descend ing signals with sensory input.