Shosaku Obara

Differential blocking effects of a spider toxin on synaptic and glutamate responses in the afferent synapse of the acoustico-lateralis receptors of Plotosus.

The hypothesis that glutamate is the afferent transmitter in the acoustico-lateralis receptors was examined in Plotosus electroreceptors. JSTX , a spider toxin known to specifically block glutamate receptors, irreversibly abolished afferent impulse discharges induced by iontophoretically applied glutamate, whereas those induced synaptically by focal stimulation of receptor cells were little affected. Such differential blocking effects by JSTX , complementary to other biochemical data, further provide pharmacological evidence against the glutamate hypothesis.

Ionic compositions of the media across the sensory epithelium in the ampullae of Lorenzini of the marine catfish,Plotosus

Summary1.In an electroreceptor, ampullae of Lorenzini of the marine catfishPlotosus, the jelly in the ampullary ducts was studied in order to determine the ionic composition of the microenvironment over the sensory epithelium of the electroreceptor. In comparison, the blood plasma of the fish and the sea water were similarly analysed.2.The ionic composition of the plasma, Na 194, K 4.0, Ca 2.9, Mg 1.5 and Cl 169 (in mM/l), was similar to those in other marine teleosts despite the species difference.3.The ampullary jelly was approximately isotonic to sea water, but contained much more K (62.6±22.0 mM/l, 580% of sea water), with Na, Ca and Mg all lower than those in sea water. The K concentration was higher in the proximal half of the ampullary duct than in the distal half, suggesting a secretion of the jelly from the ampulla.4.The high electrical sensitivity of thePlotosus ampulla is maintained by a steady bias current which flows inwards through the apical and outwards through the basal membranes of the receptor cells in the sensory epithelium. It is suggested that this bias current is carried by K.

Effects of bath-appliedl-glutamate and related chemicals on the afferent synapse of thePlotosus electroreceptor

Abstract Tonic electroreceptors of the marine catfishPlotosus were isolated, and effects of chemicals applied in the bath were examined in terms of firing rate (F) responses in single unit afferent nerve. l -Glutamate ( l -Glu) and agonists caused marked F increase in the spontaneous discharge. Their potencies, estimated from concentrations for 50% of max F increase, were in the order of quisqualate (2 μM), kainate (7 μM), l -Glu (0.4 mM), l -homocysteate (0.4 mM), d -Glu (3 mM) and l -aspartate ( l -Asp, > 10mM).N-Methyl- d,l -aspartate (10 mM) had no effect. l -Glu induced F increase also in the receptors fully suppressed either by cathodal pulses or by high Mg (15 mM), which indicated the postsynaptic action. The synaptic responses were often affected differently in the fast and slow phases, here termed as the peak F and the adapted F, respectively. l -Asp potentiated only the adapted F. Kynurenic acid (Kyn) suppressed only the adapted F, but incompletely and rather dose-independently. Kyn, however, competitively antagonized the amino the amino acid-induced responses. The present results suggest the presence of two distinct postsynaptic receptors, one a Kyn-sensitive Glu receptor that is responsible for part of the adapted F, and the other still undetermined that is responsible for most of the synaptic responses.

Damped oscillation in the ampullary electroreceptors ofPlotosus involves Ca-activated transient K conductance in the basal membrane of receptor cells

K-blockers suppressed damped oscillation of Plotosus electroreceptors in situ. Ca-blockers abolished V-dependent non-linear responses, as well, and shifted the DC level in the ampulla by ca. -1 mV. Thus, the in situ receptor is held depolarized with maintained Ca current in the basal membrane of receptor cells. The oscillation involves Ca-activated transient K current in the same membrane, which contributes to the initial sensory adaptation, and presumably also to stabilizing the sensitive receptors.

Ionic currents in the sensory epithelium examined in isolated electroreceptors ofPlotosus under simulated in situ conditions

The hypothesis of steady epithelial current in an electrically biased sensory epithelium was examined in Plotosus electroreceptors. The in situ conditions were simulated reversibly by electronic shunting of isolated receptors. The epithelial current had an N-shaped property due to Ca current superimposed over passive bias current, with net current close to zero when unstimulated. The high-gain synaptic transfer proved to be the result of modulation of steady Ca component.

Chloride current observed as calcium-gated tail current in trigeminal root ganglion neurons of the marine catfish, Plotosus

Isolated trigeminal ganglion (TRG) neurons of Plotosus in primary culture were studied with patch electrodes in a whole-cell recording configuration. When Ca currents were isolated using electrodes filled with CsCl and Ca buffer in Na- and K-free saline, a large tail current was induced. Both Ca and tail currents were blocked by 2 mM Co2+ in the bath. Reversal potential of the tail current was close to the equilibrium potential for Cl-. The reversal potential was altered by substitution of external Cl- with Br- or methanesulphonate- (MSA-). The anion permeability ratios were estimated as PBr/PCl = 1.95 and PMSA/PCl < 0.05. These results suggested that the tail current was the Ca-gated Cl current. In Plotosus neurons, two types of Ca current, high- and low-voltage activated (HVA and LVA), have been described. Both types of Ca currents were able to induce a tail current. Ba currents through both HVA and LVA channels proved to be ineffective. Upon repetitive activation, the tail current progressively increased in the presence of a nearly constant peak Ca current, and showed a more prolonged complex decay phase. When the electrode was filled with 20 mM EGTA, no tail current was induced, not even after repetitive stimulation. Therefore, the tail current seemed to be regulated by local Ca activity in the vicinity of the membrane. The Ca-gated Cl tail current may regulate the membrane excitability of TRG neurons during repetitive firing.

The membrane properties and Ca-currents of the trigeminal root ganglion cells in primary culture of the marine catfish, Plotosus, studied with whole-cell recordings

Neurons of the trigeminal root ganglion (TRG) were isolated from the marine catfish Plotosus. Collagenase treatment and culture in L15 medium, modified for higher tonicity, were required to remove their myelin sheath. TRG neurons were spherical 15-20 microns in diameter after 1-4 days culture, although they later developed extensive neurites. The membrane properties were studied by whole-cell recording technique. The resting potential was about -63 mV. The specific membrane resistance and capacitance, 5.9 K omega.cm2 and 1.2 microF/cm2, were similar to those of mouse dorsal root ganglion (DRG). The action potential, however, was usually humped, and followed by a long afterhyperpolarization. The maximum firing rate reached only about 70 Hz. Voltage-clamp study revealed TTX-sensitive Na current and TEA-sensitive K current, and in addition, two types of Ca currents: low- and high-voltage activated (LVA and HVA). The HVA current seemed to be involved in hump formation. The LVA current was similar in kinetics to T-type current of chick DRG, and was presumably inactivated at the resting potential, which might be removed during the afterhyperpolarization.