Static and dynamic properties of synaptic transmission at the cyto-neural junction of frog labyrinth posterior canal

The properties of synaptic transmission have been studied at the cyto-neural junction of the frog labyrinth posterior canal by examining excitatory postsynaptic potential (EPSP) activity recorded intraaxonally from the afferent nerve after abolishing spike firing by tetrodotoxin. The waveform, amplitude, and rate of occurrence of the EPSPs have been evaluated by means of a procedure of fluctuation analysis devised to continuously monitor these parameters, at rest as well as during stimulation of the semicircular canal by sinusoidal rotation at 0.1 Hz, with peak accelerations ranging from 8 to 87 deg.s-2. Responses to excitatory and inhibitory accelerations were quantified in terms of maximum and minimum EPSP rates, respectively, as well as total numbers of EPSPs occurring during the excitatory and inhibitory half cycles. Excitatory responses were systematically larger than inhibitory ones (asymmetry). Excitatory responses were linearly related either to peak acceleration or to its logarithm, and the same occurred for inhibitory responses. In all units examined, the asymmetry of the response yielded nonlinear two-sided input-output intensity functions. Silencing of EPSPs during inhibition (rectification) was never observed. Comparison of activity during the first cycle of rotation with the average response over several cycles indicated that variable degrees of adaptation (up to 48%) characterize the excitatory response, whereas no consistent adaptation was observed in the inhibitory response. All fibers appeared to give responses nearly in phase with angular velocity, at 0.1 Hz, although the peak rates generally anticipated by a few degrees the peak angular velocity. From the data presented it appears that asymmetry, adaptation, and at least part of the phase lead in afferent nerve response are of presynaptic origin, whereas rectification and possible further phase lead arise at the encoder. To confirm these conclusions a simultaneous though limited study of spike firing and EPSP activity has been attempted in a few fibers

Microgravity on frog labyrinth sensory function

none1IL PROGETTO HA DURATA TRIENNALE, E' INIZIATO il 1° GENNAIO 2006 E TERMINERA' il 31 DICEMBRE 2009. Vestibular information is directly concerned with gravity; thus changes in gravity conditions are expected to interfere with vestibular function. The project collects several researchers who have long been involved in studying the structural, morphological and functional aspects of vestibular organs, and is aimed at evaluating possible effects of microgravity on vestibular organ function. The project focusses on interference with: (i) sensory signal processing (transfer function and mathematical modelling of signal processing steps); (ii) ion distribution and membrane currents, especially calcium currents; (iii) structural and subcellular organization of the cytoneural junction; (iv) correlations between structural and functional alterations. The effects of microgravity are studied in the labyrinth, isolated from the frog head after exposing the animal to a conditioning period (4-6 hr) in simulated zero-gravity condition (random positioning machine).noneMaria Lisa RossiRossi, Marialis

Proprietà idrodinamiche e neurofisiologiche dei canali semicircolari nel labirinto di rana

Il 33% dei neuroni del I° ordine del canale posteriore nel labirinto di rana presenta una relazione logaritmica tra accelerazione e frequenza di scarica.Questa non-linearità può insorgere sia a livello pre- che post-sinaptico. Per verificare se le accelerazioni imposte al canale portino alla saturazione del potenziale del recettore sono state valutate le corrispondenti deflessioni cupolari e cigliari. Inoltre, per stabilire se il maggior guadagno dei neuroni del I° ordine nella rana sia dovuto a fattori idrodinamici, si sono confrontate le deflessioni cupolari.nella rana e nel gatto. La misura diretta del raggio di curvatura del canale e dei raggi dell'ampolla e del dotto semicircolare nella rana ed i valori corrispondenti per il gatto hanno permesso di calcolare, utilizzando l'equazione di Steinhausen, lo spostamento dell'endolinfa nei canali dei due animali. La soluzione dell'equazione di Bernard ha consentito inoltre il calcolo della costante A2 .si é così dimostrato che la massima accelerazione impiegata (64 deg/s2; t=1.2 s) produceva nella rana un valore di 180X10-3 deg corrispondente ad un piegamento ciliare di 280 A°; tale valore garantisce una relazione lineare tra stimolo ed ampiezza del potenziale del recettore. Il comportamento non-lineare dei neuroni afferenti nasce quindi a livello post-sinaptico, probabilmente nell'encoder. Il confrontò tra l'entità delle deflessioni cupolari nella rana e nel gatto ha rivelato che il sistema idrodinamico vestibolare in un piccolo animale, rispetto ad uno di dimensioni maggiori, non mantiene un guadagno costante poiché il guadagno diminuisce con il perdurare dello stimolo. Se ne deduce che la maggior sensibilità dei neuroni del 1° ordine nella rana dipende dalla trasduzione meccano-elettrina e dalle proprietà sinaptiche

Attività sensoriale nel labirinto di rana: effetto del bario

E' stato esaminato l'effetto del Ba2+ sulla liberazione di mediatore alla giunzione citoneurale del canale posteriore nel labirinto isolato di rana. Gli EPSPs ed i potenziali d'azione, derivati dalle fibre afferenti utilizzando convenzionali tecniche elettrofisiologiche, rimangono inalterati in una soluzione di Ringer priva di Calcio e contenente 0.5 mM di Ba2+; l'aumento progressivo della concentrazione del Ba2+ (0.9-4 mM) produce un aumento della frequenza dei potenziali d'azione senza modificarne l'ampiezza e la durata. Questo effetto é sostenuto dall'aumento della frequenza degli EPSPs. La facilitazione prodotta dalla medesima concentrazione di Ba2+ risulta inversamente proporzionale al livello di scarica delle diverse unità, presentando un evidente fenomeno di saturazione. Se l'effetto di concentrazioni crescenti di Ba2+ viene esaminato su fibre aventi un simile livello di attività a riposo, l'effetto facilitante si dimostra correlato positivamente alla concentrazione dello ione. La facilitazione prodotta da 1 mM di Ba2+ viene abolita aumentando a 8 mM la concentrazione del Calcio esterno. Infine, quando la scarica a riposo viene drasticamente depressa dalla presenza nel Ringer di Co2+ (3 mM) o Mg2+ (15 mM), l'aggiunta di Ba2+ (1.8 mM) ripristina parzialmente l'attività della fibra. Questi risultati dimostrano che alla giunzione citoneurale il Ba2+ é in grado di sostituire. il Ca2+ nel processo di liberazione di mediatore. L'effetto antagonista esercitato da Ca2+ , Mg2+ , Co2+ indica che presumibilmente il Ba2+ utilizza lo stesso canale del Ca2+ per attraversare la membrana delle cellule sensoriali. La presenza di un fenomeno di facilitazione della scarica ed il parziale recupero dell'attività sinaptica in presenza di Co2+ o di Mg2+ suggeriscono inoltre che il Ba2+ passa attraverso il canale del Ca2+ più agevolmente del Ca2+ stesso

A quantitative analysis of efferent inhibition and facilitation of frog vestibular afferents

In the isolated frog labyrinth, inhibition or facilitation of the posterior canal sensory discharge can be achieved by antidromic electrical shocks to the anterior-horizontal nerves. The existence of a peripheral inhibitory efferent pathway is well established, but the efferent nature of facilitation is still controversial. To clarify the modalities of functioning of the efferent system following different activation patterns we have analysed the changes in posterior canal spike rate produced by various protocols of stimulation of the anterior nerve. Results may be summarized as follows: 1) the latency of any effect, estimated from the modifications in EPSP rate, is 15 ms; 2) the degree of inhibition or facilitation of the afferent discharge is positively related to the frequency of stimulation (10-50/s); 3) during long stimulation periods (10 s) a decrease of both responses (adaptation) is observed; 4) the rebound spike discharge at the end of the inhibitory period is a linear function of the preceding stimulation rate; 5) with short, high frequency stimulus trains (250-624 ms; 200-80/s) stimulation time, instead of frequency, is crucial in determining the magnitude of the response. Inhibition and facilitation do not coexist in the same unit and are equally dependent on thestimulation pattern; this suggests that the effects are sustained by two different groups of efferent fibres acting in an antagonistic way on the receptor

Risposta dei neuroni del I ordine ad accelerazioni angolari controllate nel labirinto isolato di rana

EPSPs and spike discharge were recorded intracellularly from 30 fibres of the posterior canal in the isolated frog labyrinth. The resting discharge was 6.6 +/- 7 spikes/sec (Mean +/-SD). The response properties of single units were described during repeated angular acceleration steps (0.3-20 deg/sec2) of 6-12 sec duration. Only one unit exhibited a clearcut adaptation, 7 units were slightly and irregularly adapting, while 22 units did not adapt at all. The response was linearly related to acceleration, with a gain of 2.4 +/- 1.7 spikes.sec-1/deg.sec-2 (Mean +/- SD) in 17 of the units tested, while the discharge frequency of 6 units was logarithmically related to acceleration. Threshold for frequency increase ranged from 0.2 to 2 deg/sec2 and latency from 50 msec to 2 sec. The response timecourse to a single acceleration step markedly deviated from the exponential expected from the torsion pendulum model. The observed nonlinearities can be ascribed to the absence of the efferent control. However, distortions from the model may also be due both to the mechanoelectrical transduction at the receptor level and the properties of the afferent synapse; in fact, an exponential decay of the discharge is consistently observed when the cupola passively shifts to the zero position, at the end of the acceleration step

EFFETTO DI ELEVATE CONCENTRAZIONI PERILINFATICHE DI K+ E Rb+ SULLA ATTIVITÀ' SPONTANEA DEI RECETTORI LABIRINTICI DI RANA.

Nelle hair cells é dimostrabile la presenza di una corrente K+ Ca2+-dipendente che contribuisce al mantenimento ed alla stabilità del potenziale di membrana. Per valutare gli effetti postsinaptici del blocco della IKCa, la scarica di EPSPs e di potenziali d'azione è stata derivata da singole fibre del canale posteriore in presenza di elevate concentrazioni perilinfatiche di K+ o Rb+ (20 mM). In Ringer normale la frequenza di scarica delle unità era 0.5-25 spikes/s. In presenza di soluzioni ad elevato contenuto di K+ o Rb+ si osservava una facilitazione della scarica di EPSPs e di potenziali d'azione accompagnata da una riduzione dell'ampiezza (5-10 mV) e da un aumento della durata degli spikes. Non essendo tuttavia presenti spikes ripetitivi a livello di fibra. la nuova frequenza di scarica era sostenuta esclusivamente dall'aumento dell'attività sinaptica. Le unità a bassa attività spontanea (0.5 spike/s) raggiungevano una frequenza di 40-50 spikes/s in presenza di alte concentrazioni di Rb+ o K+; nelle unità ad elevata frequenza iniziale (20-25 spikes/s) l'effetto facilitante di entrambi gli ioni era meno evidente. L'analisi dei dati ha dimostrato infatti che il grado di facilitazione è inversamente proporzionale al livello iniziale di scarica. I risultati ottenuti suggeriscono che il blocco dell'IKCa produce una stabile depolarizzazione del polo sinaptico delle cellule. Questa depolarizzazione probabilmente favorisce l'influsso di Ca2+. L'aumento della concentrazione del Ca2+ intracellulare infatti può sostenere un potenziamento della liberazione di trasmettitore che si manifesta con l'aumento dell'attività sensoriale nelle fibre afferenti

Le dimensioni fisiche influenzano le proprietà funzionali dei canali semicircolari

The functional dependence of the semicircular canal upon its physical dimensions was evaluated by measuring the internal radius (r), the radius of curvature (R) and the cupula radius (rC ) of the posterior canal in 10 freshly dissected frog labyrinths. These values have been compared to the same parameters of the cat labyrinth. The coefficiente I, B, K in the Steinhausen equation were determined for both animals. The A ratio between cupula deflection and endolymph displacement was also calculated by utilizing the Bernard equation. The A ratio is three times larger in the frog than in the cat. It follows that if the same acceleration produces similar endolymph displacements in the posterior canal of both animals, the cupula deflection will be larger in the frog. The solution of the Steinhausen equation in the presence of a constant acceleration, however, reveals that the same stimulus intensity will result in a larger endolymph displacement in the cat posterior canal; similarly, the endolymph displacement directly depends on the duration of the stimulating period in both animals. Contrary to the Bernard assumption, these effects generate a Q ratio (fi frog / fi cat) which is less than the Q ratio (A frog/A cat). Moreover, G decreases on increasing the duration of the stimulating period. For stimuli of short duration the semicircular canal of a small animal is expected to exhibit a higher sensitivity than that of a larger one. However, the definitive primary afferent discharge will be largely controlled by the receptor/generator potential properties

Afferent activity recorded during rotation from single fibres of the posterior nerve in the isolated frog labyrinth

none2EPSPs and spikes were recorded at rest and during rotation from single fibres of the posterior nerve in the isolated frog labyrinth. The spike discharge properties of 57 units were examined at rest and during repetitive acceleratory-velocity steps. Forty of these units were subjected to excitatory steps of 5-12 s duration and 45% displayed an evident discharge adaptation. In the non-adapting units, the excitatory response also deviated from that expected on the basis of the torsion-pendulum model and exhibited an exponential time-course in only 36% of the fibres examined. The time constant T2 of the response rising phase was significantly longer than that of the decay (2.5 s versus 1.7 s). When all the 57 units were considered, a linear behaviour was found in 67%. The average gain in these linear units was 1.9 +/- 1.4 spikes X s-1/deg X s-1. Adaptive fibres exhibited a lower resting firing rate and a higher gain (3.8 spikes/s and 2.3 spikes X s-1/deg X s-2, respectively) when compared with the non-adapting ones (7.1 spikes/s and 1.5 spikes X s-1/deg X s-2). An undershoot was present in 57% of the units; it increased with acceleration and was not strictly related to adaptation. Fifteen of the 40 units tested with the 5-12 s duration excitatory steps survived repeated inhibitory accelerations of the same duration. In these units a marked response asymmetry was evident since their resting activity could be abolished by accelerations not larger than 10 deg/s2.noneROSSI ML; M. MARTINIRossi, Marialisa; Martini, Mart

Azione modulatrice del bario e di altri ioni sull'attività sensoriale del labirinto di rana

The effect of Ba2+, TEA, 4-AP and CoCl2 on the EPSP and spike discharges recorded from single fibres of the posterior nerve in the isolated frog labyrinth has been investigated. In Ca-free solution Ba2+ preserved, at low concentration (0.3 mM), the resting activity and at higher levels (up to 6 mM) it resulted in a pronounced facilitation of the EPSP and spike discharges. Facilitation increased on increasing Ba2+ concentration up to 4-5 mM and it was more evident in those units exhibiting a low resting spike firing. The effect of Ba2+ (1 mM) was completely antagonized by 10 mM Ca2+. CoC12 (3 mM) suppressed the resting rate at the normal external Ca2+ concentration; the Co2+ block was partially releaved by 1.8 mM Ba2+ . TEA (20 mM) evoked a clear-cut increase in the EPSP and spike discharges which, however, was less consistent than that produced by Ba2+. By comparing the effect of TEA on the spike frequency with that obtained at different Ba2+ levels, the Ba2+ capacity to carry the Ca2+ current was dissected. Such an effect is dose-dependent and it is more evident in low-frequency units. Conversely, 4-AP did not affect the resting discharge frequency. These results indicate that either the Ca2+ or the Ba2+ current sustain the transmitter release at the cytoneural junction. The effect of TEA suggests that the Ca2+-dependent K+ current may play an important role in supporting the neurosecretory process by controlling the membrane potential of the hair cells