1,721,217 research outputs found
Effetto del Bario e di alcuni bloccanti dei canali Potassio sulla trasmissione sinaptica afferente nel labirinto isolato di rana
No full textNel labirinto isolato di rana é stato esaminato l'effetto del blocco selettivo delle correnti Ca (ICa) e K (IKCa e IKA) delle cellule sensoriali sull'attività spontanea (A.S.) del canale posteriore. Il blocco della ICa da parte del Ba (1mM) e della IKCa operato da Ba o TEA (20mM), produce un consistente aumento della frequenza spontanea degli EPSPs e dei potenziali d'azione. La soppressione della IKA da parte della 4-AP (20mM) riduce la frequenza di scarica. Per verificare se in presenza di Ba, TEA o 4-AP l'A.S. sia modulabile dalla stimolazione meccanica dei recettori e definire l'eventuale relazione tra livello di scarica ed entità della risposta, il canale era sottoposto a variazioni sinusoidali di velocità (0.1-0.3 Hz; massima accelerazione:39-638 deg/s2). Nel controllo esiste una relazione lineare tra livello di A.S. e valore di picco della risposta. In Ba e TEA la risposta dei recettori é ben evidente e mostra di essere una funzione lineare della nuova frequenza di scarica spontanea. In presenza di 4-AP si é invece osservata una riduzione della risposta. L'analisi delle equazioni lineari che descrivono l'andamento dei dati sperimentali dimostra che in presenza di TEA la risposta si riduce, probabilmente a causa di una diminuzione dell'ampiezza del potenziale del recettore. Tale riduzione é imputabile alla depolarizzazione delle cellule conseguente al blocco della IKCa, che controlla quindi una frazione consistente del potenziale di membrana. Nonostante l'effetto TEA-simile il Ba produce durante accelerazioni moderate un aumento della risposta, dimostrando di essere più efficace del Ca nel sostenere il processo di neurosecrezione. I risultati suggeriscono inoltre che il livello di A.S. consente ad ogni unità di codificare una risposta diversa da quella di altre unità simili pur in presenza di stimoli della stessa intensità
THE EFFECT OF BARIUM AND SOME K-CHANNEL BLOCKERS ON THE AFFERENT SYNAPTIC TRANSMISSION IN THE ISOLATED FROG LABYRINTH
No full textThe effect of Ba, TEA and 4-AP on the afferent synaptic transmission has been investigated in the isolated frog labyrinth by intracellularly recording the resting and the evoked receptor discharge from the posterior canal. Ba (1mM; Ca-free Ringer solution) and TEA (20mM) elicited a clear-cut increase in the spontaneous EPSP and spike discharges. On the other hand, 4-AP (20mM) application resulted in a decrease of the sensory activity. During sinusoidal rotation at 0.1 and 0.3 Hz consistent responses were observed in TEA and Ba solutions, while the mechanically evoked discharge was greatly reduced in the presence of 4-AP. Under the drug treatments, as well as in normal saline, the evoked responses proved to increase linearly with the resting discharge frequency. The present results suggest that: 1) the unit response is determined by its resting level; 2) Ba may substitute for Ca in the transmitter release process at the cytoneural junction, its ability being even greater than that of Ca itself. The effects of TEA and 4-AP are discussed in the light of their possible interaction with the presynaptic hair cell K-currents
Attività sensoriale nel labirinto di rana: effetto del bario
No full textE' 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
EFFETTO DI ELEVATE CONCENTRAZIONI PERILINFATICHE DI K+ E Rb+ SULLA ATTIVITÀ' SPONTANEA DEI RECETTORI LABIRINTICI DI RANA.
No full textNelle 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
Effect of barium ions on the sensory activity of the isolated frog labyrinth
No full textBarium ions are known to interact with the K+ conductance in several preparations as well as to substitute for Ca2+ in resting and evoked transmitter release. The primary afferent resting activity ìn the frog labyrinth is a Ca2+-dependent process, which is extremely sensitive to the external K level; it seems thus interesting to investigate the effect of Ba2+ on the EPSP and spike discharges recorded from single fibres of the posterior nerve in the isolated preparation. Control recordings were performed in Ringer solution of the following ionic composition (mM): NaCl 116, KCl 2.5, CaCl2 1.8, TrisCl 5. The changes in bathing saline were usually accompanied by appropriate modifications to the NaCI concentration to keep the solution isoosmolar.
When BaCl2 was added to a Calcium free Ringer solution the resting activity was preserved; moreover, on increasing Ba2+ concentration (0.9 - 4 mM), an obvious facílitation of the EPSP and spike discharges was observed after 3 - 5 minutes. The spike shape and duration were unaffected by this treatment. In 15 mM Mg2+- Calcium-free Ringer solution the resting activity was completely abolished and no recovery was observed by gradually rising the Ca2+ concentration (1.8 - 18.6 mM). However, when Ba2+ (1.8 mM) was added to the bathing medium the Mg2+ block was fully releaved, independently of the Ca2+ levels, though no facilitatìon was observed. A sìmìlar recovery was not obtaìned in 15 mM Mg2+ - 18.6 mM Ca2+ - 7.5 mM K+ solution, whích rules out the possibility that recovery in spike activity might be simply related to a Ba2+ induced depolarization of the hair cell.
Finally, when the resting activity was completely abolished by a 3 - 5 mM CoCl2-Calcium-free Ringer solution, some recovery was obtained in the presence of BaCl2 (1.8 mM), whereas an isosmolar amount of CaCl2 was completely uneffective. The time of recovery increases and facilitation decreases on increasing CoCl2 concentration.
It follows that Ba2+ions have a powerful and persistent effect on the transmitter release that may reflect mainly their ability to penetrate the Ca-channel more easily than Ca2+ ions. Ba2+ action might also be related to a depolarizing effect on the hair cell membrane, possibly due to the block of a Ca2+- dependent K+ conductance. On the other hand, Ba2+ ions proved not to interfere with the K+ channels of the myelinated sensory fibres
THE EFFECT OF HIGH POTASSIUM AND RUBIDIUM CONCENTRATIONS ON EPSP AND SPIKE DISCHARGES OF THE FROG LABYRINTH POSTERIOR CANAL
No full textThe effects of high perilymphatic K and Rb concentrations (20 mM) on the posterior canal resting discharge have been examined in the isolated frog labyrinth. Both K and Rb usually increase the resting EPSP and spike frequencies recorded intra-axonally at the level of the posterior nerve. The afferent K-Rb facilitation is accompanied by a reduction in size and broadening of the spikes; no repetitive action potentials, however, occur in the fibre. Therefore, the higher resting receptor activity is exclusively related to the increase in the EPSP emission rate. The afferent discharge facilitation was inversely related to the fibre's initial resting activity. The K effect was larger than the Rb effect, and both K and Rb actions were more conspicuous in some units (high sensitive) than in others (low sensitive). High K and Rb media steadily depolarize the basolateral receptor membrane. The increase in EPSP rate observed under these conditions indicates that the reduction in hair celi membrane potential sustains an increased transmitter release at the junction. This presynaptic effect is most likely related to larger amounts of Ca entering the cell during maintained depolarization
MOLECULAR CHARACTERIZATION AND IN VIVO ACTIVITY OF PSEUDOMONAS FLUORESCENS STRAIN 4, A POTENTIAL BIOCONTROL AGENT AGAINST RHIZOCTONIA SOLANI ROOT ROT ON LAMB’S LETTUCE.
No full textThe effect of barium and some channel blockers on sensory discharge of the frog labyrinth posterior canal recorded at rest and during rotation
No full textThe effect on the afferent synaptic transmission of Ba2+, Sr2+, tetraethylammonium (TEA) and 4-aminopyridine (4-AP) has been investigated in the isolated frog labyrinth by intracellularly recording the posterior canal resting and evoked receptor discharge. BaCl2 (0.3 mM) or SrCl2 (1.8 mM) substitution for normal external CaCl2 restored the afferent activity without affecting the membrane potential of the sensory fibres. On further increasing Ba2+ concentration (0.5-5 mM) a dose-dependent increase in the EPSP and spike discharges was observed in all the units examined. Ba2+ (1.8-4 mM) removed the depression of the sensory activity operated by CoCl2 (3 mM), while its facilitatory effect was completely antagonized by raising Ca2+ concentration (up to 10 mM). TEA (20 mM) elicited a clear-cut increase in the EPSP and spike discharges which, however, was less consistent than that produced by Ba2+ (1 mM). The increment in spike frequency produced by TEA and Ba2+ proved to be inversely related to the initial resting firing level of the different units. The 4-AP (4-20 mM) effect resulted in a decrease of the sensory activity, which was fully restored by TEA or Ba2+. In normal saline a linear relationship was found between the mean unit resting discharge and the respective excitatory peak response during sinusoidal rotation (0.1-0.3 Hz). This result suggest that the mechanical response is mainly determined by the unit resting level. Consistent evoked responses were obtained under TEA and Ba2+ treatment which proved to depend linearly on the new mean resting discharge of the different units. Conversely, a reduced evoked response was invariably observed in all the fibres tested in the presence of 4-AP. The present results suggest that Ba2+ and Sr2+ may substitute for Ca2+ in the transmitter release process at the cyto-neural junction, the ability of Ba2+ being even larger than that of Sr2+ and Ca2+ itself. The effects of TEA and 4-AP are discussed in the light of their possible interaction with the presynaptic K+-currents recently described in hair cells
High perilymphatic potassium and rubidium levels modulate the sensory resting discharge in the isolated frog labyrinth
No full textThe effects of high perilymphatic K+ and Rb+ concentrations (20 mM) on the posterior canal resting discharge have been examined in the isolated frog labyrinth. Both K+ and Rb+ produced an increase in spike frequency which is sustained by a parallel increase in the excitatory postsynaptic potential (EPSP) emission rate. High K+ and Rb+ levels reduce the size of the afferent spike, broaden its time course but do not induce repetitive activity at the axon level. The K+ and Rb+ facilitation proved to be inversely related to the fibre's initial resting activity. The facilitation produced by increased K+ was usually larger than that in Rb+ solution. High sensitive and low sensitive units with similar low resting discharge were detected in relation to their response either to K+ or Rb+. The effects of both ions can be explained on the basis of their interactions with the presynaptic hair cell currents
A quantitative analysis of efferent inhibition and facilitation of frog vestibular afferents
No full textIn 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
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