Yoshiko Sugawara

The Mormyrid Electrosensory Lobe In Vitro: Physiology and Pharmacology of Cells and Circuits

This paper is concerned with the electrosensory lobe (ELL) of mormyrid electric fish as examined in in vitro slices. Intracellular recordings from morphologically identified cells and field potential recordings were used to characterize the physiology and pharmacology of ELL cells. Most intracellular recordings were from the Purkinje-like interneurons that are known as medium ganglion cells and from the two types of efferent neurons, large ganglion and large fusiform cells. Stimulation of primary afferent fibers elicits both excitatory and inhibitory effects in these cells, with the excitatory effects being mediated by both the AMPA and NMDA types of glutamate receptors and the inhibitory effects being mediated by both GABAA and glycine receptors. Parallel-fiber stimulation evokes an EPSP–IPSP sequence, with the EPSPs being mediated by both AMPA and NMDA receptors and the IPSPs being mediated by GABAA receptors only. The parallel fiber-evoked EPSPs and IPSPs show marked paired-pulse facilitation. A large and unusually broad spike is recorded inside medium ganglion cells, and field potential responses suggest that this spike is propagated into the apical dendrites. The results provide essential information for understanding how peripheral and central inputs are integrated in ELL.

Disturbance of neural respiratory control in neonatal mice lacking gaba synthesizing enzyme 67-kda isoform of glutamic acid decarboxylase

To examine the role of GABA in the respiratory rhythm and pattern generation in neonatal mice, we analyzed the function of the respiratory control system of 67-kDa isoform of glutamic acid decarboxylase (GAD67)-deficient neonatal mice. In these mutant (GAD67-/-) mice, GABA levels in the brainstem were reduced to about 30% of those in wild-type (GAD67+/+) mice. In in vivo preparations, ventilatory parameters were analyzed by whole body plethysmography and electromyography of intercostal muscles. GAD67-/- mice exhibited abnormal respiratory patterns, i.e. irregular respiratory rhythm, and periodic gasp-like respiration followed by shallow breathing with short inspiratory duration and apnea. In in vitro GAD67-/- brainstem-spinal cord preparations, inspiratory C4 burst duration was shorter than that in GAD67+/+ preparations. Whole cell recordings revealed that activities of inspiratory neurons in the ventral medulla of GAD67-/- mice were characterized by a short depolarization period and a paucity of firing during the inspiratory phase. Superfusion of the in vitro GAD67-/- preparation with 10 microM GABA prolonged C4 burst duration and partly restored a normal pattern of inspiration, although the restoration was limited. These results indicate that reduced GABA levels during the perinatal period induce malfunction in the respiratory control system. We suggest that GABAergic transmission is not essential for basic respiratory rhythm generation but plays an important role in the maintenance of regular respiratory rhythm and normal inspiratory pattern in neonatal mice.

Mormyrid electrosensory lobe in vitro: morphology of cells and circuits.

The electrosensory lobe (ELL) of mormyrid electric fish is a cerebellum‐like brainstem structure that receives the primary afferent fibers from electroreceptors in the skin. The ELL and similar sensory structures in other fish receive extensive input from other central sources in addition to the peripheral input. The responses to some of these central inputs are adaptive and serve to minimize the effects of predictable sensory inputs. Understanding the interaction between peripheral and central inputs to the mormyrid ELL requires knowledge of its functional circuitry, and this paper examines this circuitry in the in vitro slice preparation and describes the axonal and dendritic morphology of major ELL cell types based on intracellular labeling with biocytin. The cells described include medium ganglion cells, large ganglion cells, large fusiform cells, thick‐smooth dendrite cells, small fusiform cells, granule cells, and primary afferent fibers. The medium ganglion cells are Purkinje‐like interneurons that terminate on the two types of efferent cells, i.e., large ganglion and large fusiform cells, as well as on each other. These medium ganglion cells fall into two morphologically distinct types based on the distributions of basal dendrites and axons. These distributions suggest hypotheses about the basic circuit of the ELL that have important functional consequences, such as enhancement of contrast between “on” elements that are excited by increased afferent activity and “off” elements that are inhibited. J. Comp. Neurol. 404:359–374, 1999.

Dendritic backpropagation and synaptic plasticity in the mormyrid electrosensory lobe.

This study is concerned with the origin of backpropagating action potentials in GABAergic, medium ganglionic layer neurones (MG-cells) of the mormyrid electrosensory lobe (ELL). The characteristically broad action potentials of these neurones are required for the expression of spike timing dependent plasticity (STDP) at afferent parallel fibre synapses. It has been suggested that this involves active conductances in MG-cell apical dendrites, which constitute a major component of the ELL molecular layer. Immunohistochemistry showed dense labelling of voltage gated sodium channels (VGSC) throughout the molecular layer, as well as in the ganglionic layer containing MG somata, and in the plexiform and upper granule cell layers of ELL. Potassium channel labelling was sparse, being most abundant in the deep fibre layer and the nucleus of the electrosensory lobe. Intracellular recordings from MG-cells in vitro, made in conjunction with voltage sensitive dye measurements, confirmed that dendritic backpropagation is active over at least the inner half of the molecular layer. Focal TTX applications demonstrated that in most case the origin of the backpropagating action potentials is in the proximal dendrites, whereas the small narrow spikes also seen in these neurones most likely originate in the axon. It had been speculated that the slow time course of membrane repolarisation following the broad action potentials was due to a poor expression of potassium channels in the dendritic compartments, or to their voltage- or calcium-sensitive inactivation. However application of TEA and 4AP confirmed that both A-type and delayed rectifying potassium channels normally contribute to membrane repolarisation following dendritic and axonal spikes. An alternative explanation for the shape of MG action potentials is that they represent the summation of active events occurring more or less synchronously in distal dendrites. Coincidence of backpropagating action potentials with parallel fibre input produces a strong local depolarisation that could be sufficient to cause local secretion of GABA, which might then cause plastic change through an action on presynaptic GABA(B) receptors. However, STP depression remained robust in the presence of GABAB receptor antagonists.

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.

Role of GABA in central respiratory control studied in mice lacking GABA-synthesizing enzyme 67-kDa isoform of glutamic acid decarboxylase.

In in vivo adult mammals, the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) has been shown to play an essential role in the termination of the respiratory phase in the central respiratory rhythm generator.1, 2, 3 On the other hand, works with in vitro brainstem-spinal cord preparations have revealed that the respiratory rhythm of neonatal rats is unaffected by blockade of GABAergic and glycinergic receptors.4—6 These results suggest that either GABAergic or glycinergic synaptic inhibition is not essential for the generation of respiratory rhythm in neonatal mammals. We recently analyzed the role of GABA in the generation of respiratory rhythm and pattern and reported that GABA plays an important role in the maintenance of regular respiratory rhythm and normal inspiratory pattern in neonatal mice7. However, the precise role of GABA in the generation of respiratory rhythm and pattern in neonatal mammals is not well understood at the level of respiratory neurons.