Immunocytochemical characterization of the synaptic innervation of a single spinal neuron, the electric catfish electromotoneuron

The complete mitochondrial genome of electric catfish Malapterurus electricus and its phylogeny

Electric catfishes evolved the substantial electric organ that can instantly release powerful high-voltage electricity. To better study the phylogenetic position of the electric fish (Malapterurus electricus) in catfishes, in this study, we presented the complete mitochondrial genome of M. electricus assembled by the next-generation sequencing data. The mitogenome has 16,504 bp and contains 13 protein-coding genes (PCGs), 22 transfer RNA genes, two ribosomal RNA genes, an L-strand replication origin (OL), and a control region (D-loop). The overall base composition is A 31.08%, C 27.54%, G 14.81%, and T 26.57%. Phylogenetic analysis based on 13 PCGs of 43 species from Siluriformes showed M.electricus belonging to the Malapteruridae displayed a close relationship with Siluridae. Taken together, the complete mitochondrial genome of M. electricus would be beneficial for the study of the phylogenetic relationship of Siluriformes.

Gap junction-mediated glycinergic inhibition ensures precise temporal patterning in vocal behavior

Precise neuronal firing is especially important for behaviors highly dependent on the correct sequencing and timing of muscle activity patterns, such as acoustic signaling. Acoustic signaling is an important communication modality for vertebrates, including many teleost fishes. Toadfishes are well known to exhibit high temporal fidelity in synchronous motoneuron firing within a hindbrain network directly determining the temporal structure of natural calls. Here, we investigated how these motoneurons maintain synchronous activation. We show that pronounced temporal precision in population-level motoneuronal firing depends on gap junction-mediated, glycinergic inhibition that generates a period of reduced probability of motoneuron activation. Super-resolution microscopy confirms glycinergic release sites formed by a subset of adjacent premotoneurons contacting motoneuron somata and dendrites. In aggregate, the evidence supports the hypothesis that gap junction-mediated, glycinergic inhibition provides a timing mechanism for achieving synchrony and temporal precision in the millisecond range for rapid modulation of acoustic waveforms.

Unitary giant synapses embracing a single neuron at the convergent site of time-coding pathways of an electric fish,Gymnarchus niloticus

Phase-locking neurons in the electrosensory lateral line lobe (ELL) of a weakly electric fish, Gymnarchus niloticus, fire an action potential in response to each cycle of the sinusoidal electrosensory signal (350-500 Hz) created by the fish's own electric organ. The exact firing times of the phase-locking neurons are altered (time-shifted) by capacitance of electrolocation objects or by electric organ discharges of other individuals. The magnitude of the time shifts depends on the location of the neurons' receptive field on the skin; thus, time disparities arise between the firing of phase-locking neurons. To compute these disparities, there should be a site where these phase-locking neurons converge. In this study we morphologically identified a novel cell type, which we named the "ovoidal cell", that receives the convergent projections of phase-locking neurons in the inner cell layer (ICL) of the ELL. We labeled these neurons with biocytin and examined them by light and electron microscopy. The giant cells and the S-type primary afferents, two types of phase-locking neurons, respectively terminate on the soma via chemical synapses and on the dendrite of the ovoidal cells via mixed synapses. Each terminal of the giant cells embraces the soma of an ovoidal cell, covering as much as 84% of the somatic membrane. The giant cell terminals and ovoidal cell somata were immunoreactive to SV2, a synaptic vesicle protein, but the S-afferent terminals were not, even though they contain numerous synaptic vesicles. The dendrite of the ovoidal cells also contacts the pyramidal cells of the ICL, which are known to be sensitive to time disparities. The anatomical connections of the phase-locking neurons to the ovoidal cells strongly suggest that they are involved in computing time disparity.