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Acetylcholine-induced back-firing in the preganglionic trunk of the rat superior cervical ganglion
No full textACh (5 X 10(-4) M), when applied to isolated ganglion preparations elicited an apparently antidromic discharge in the cervical sympathetic trunk. The intensity of this back-firing was found to be about 10 times lower than that of the postganglionic discharge evoked by ACh in the internal carotid nerve. Both responses, however, displayed a similar time course and consisted of an early and a late component. In the back-firing the early component died out in a few seconds, while the late one lasted for 20-30 s. The two components were cancelled by d-tubocurarine (5 X 10(-6) M) and atropine (10(-6) M), respectively, suggesting that both nicotinic and muscarinic cholinoceptive sites are involved. In chronically decentralized preparations ACh evoked a clear back-firing response not substantially different from that elicited in normal ganglia. Therefore it is likely that the back-firing phenomenon is not due to antidromic activation of preganglionic fibers. The back-firing observed in the rat superior cervical ganglion was interpreted as being due to activation of sympathetic neurons known to give rise to recurrent axons in the cervical sympathetic trunk
Glutamate excitatory effects on ampullar receptors of the frog
No full textThe results are consistent with the hypothesis that glutamate is the afferent transmitter in the vestibular organs and indicate that receptors mainly of the non-NMDA type are present not only at postsynaptic level but also in hair cells
Intracellular recordings from "recurrent neurons" in the rat superior cervical ganglion
No full textIntracellular recordings in the isolated superior cervical ganglion of the rat showed that electrical stimulation of the cervical sympathetic trunk elicited in a cluster of neurons localized in the caudal part of the ganglion synaptically driven action potentials, and propagated potentials having the features of typical antidromic spikes. The results demonstrate that these neurons, besides synapsing with common preganglionic fibres, project their axons to the cervical sympathetic trunk. The recurrent neurons showed a very low threshold to direct intracellular stimulation and a high input resistance, suggesting that they have a small size. Almost all recurrent neurons were activated synaptically also by stimulating the postganglionic trunks, indicating that they are innervated by collaterals of preganglionic through-fibres which are known to sustain a direct pathway between pre- and postganglionic nerves. Moreover, some recurrent neurons could also be activated antidromically following stimulation of the external carotid nerve, indicating that their axons divide into collaterals which project not only to the preganglionic trunk but also to a postganglionic nerve. The presence of recurrent neurons in the superior cervical ganglion of the rat provides further evidence for the concept that sympathetic ganglia consist of discrete cell subpopulations which are segregated in different regions and probably subserve different functions
Preganglionic discharge induced by acetylcholine in the superior cervical ganglia of the rat
No full textACh (5.10(-4) M), when applied to isolated ganglion preparations elicited an apparently antidromic discharge in the cervical sympathetic trunk. The intensity of this back-firing was found to be about 10 times lower than that of the postganglionic discharge evoked by ACh in the internal carotid nerve. Both responses however displayed a similar time course consisting mainly of an early and a late component. In the back-firing the early component died out in few seconds, while the late one lasted 20-30 seconds. The two components were cancelled by d-tubocurarine (5.10(-6) M) and atropine (10(-6) M) respectively, suggesting that both nicotinic and muscarinic cholinoceptive sites are involved. In chronically decentralized preparations ACh evoked a clear back-firing response not substantially different from that elicited in normal ganglia. Therefore it is likely that the back-firing phenomenon is not due to antidromic activation of preganglionic fibers. The back-firing observed in the rat superior cervical ganglion was interpreted as being due to activation of sympathetic neurons, known to give rise to recurrent axons in the cervical sympathetic cord
Nature and expression of L and non-L calcium currents by frog crista hair cells
No full textVariations in the electrophysiological properties of hair cells appear to be a general feature of the vertebrate vestibular sensory organs. Here we studied Ca2+ currents in hair cells of frog crista ampullaris using the whole cell variant of the patch clamp technique. Currents were recorded in situ from hair cells in peripheral, intermediate and central regions of the sensory epithelium. Two types of Ca2+ currents were found: a large nifedipine-sensitive current and a small nifedipine insensitive current. The latter was insensitive to R, P/Q and N-type Ca2+ channels antagonists. The amplitude of nifedipine sensitive and insensitive currents varied in cells along the peripheral region. The present results demonstrate the expression of L and non-L Ca2+ channels in frog crista ampullaris that show intraregional variations in magnitude
Pre- and postsynaptic recovery of semicircular canal activity in the frog following ototoxic insult
No full textAim: Inner ear hair cells of lower vertebrates regenerate following physical or chemical insults. In the present study we aimed to define the damage and recovery of the pre and postsynaptic activity in the frog crista ampullaris after ototoxic insult with the antibiotic, gentamicin.
Methods: Frogs were treated with a single dose of gentamicin sulphate (GM-5mM) administered intraotically. The presynaptic activity was monitored by recording in isolated canal preparations the endoampullar (receptor) potential (Adc). The postsynaptic activity was monitored by recording the resting (R) and evoked spike activity (E) from the whole ampullar nerve and from single canal afferents.
Results: We found that Adc was abolished 5 days after GM administration and that it progressively recovered close to control values 20 days after treatment. These results well fitted with the morphological damage and recovery of hair cells in the crista epithelium. Consistently with the presynaptic damage, the canal afferent activity was abolished 5 days after GM treatment, while the recovery of the background and the evoked afferent activity showed a different sequence. Background activity (R) was detected 7-8 days after GM treatment and it was not modulated by mechanical stimulation of the canal. In addition, R reached control values (14/15 days post-treatment), before those of the evoked activity (E-20 days post-treatment). Intracellular recordings confirmed the above mentioned results and revealed that the evoked EPSP discharge, together with the spike activity, reached normal values close to 20 days post-treatment.
Conclusion: The present results show that frog semicircular canal rapidly and completely recovers its pre -and postsynaptic function following severe ototoxic insult and that during the recovery, the synaptic function of regenerating hair cells becomes functional before the mechano-transduction function
Differential recovery of background and evoked afferent activity in the frog semicircular canal following ototoxic insult
No full textThe present study was aimed to define the damage and recovery of pre- and postsynaptic activity in the frog crista ampullaris after ototoxic insult with gentamicin (GM). We found that the endoampullar potential (receptor potential) recorded from the isolated canal was abolished 6 days after GM administration and that it recovered close to the control 20 days after treatment. Consistently with the presynaptic damage, the afferent activity from the ampullar nerve was abolished 6 days after GM treatment, while the recovery of background and evoked afferent activity showed a different sequence. Background activity reappeared 7-8 days after GM treatment and in this period it was not driven by canal mechanical stimulation. In addition, background activity reached control values around 15 days after the ototoxic insult, while the evoked activity was detected 9-10 days after GM treatment and appeared normal close to 20 days. Intracellular recordings from single canal afferents confirmed the above mentioned results. The present study show that the frog semicircular canal rapidly and completely recovers its pre -and postsynaptic function following severe ototoxic insult and that during the recovery afferent transmission at rest becomes functional before the mechano-transduction function
Isolation of A-type K+ current in hair cells of the frog crista ampullaris
No full textDifferent procedures to isolate the K+ A-type current (IA) from other membrane currents were tested on the complex inactivating outward K+ current generated in hair cells from the peripheral regions of the frog crista ampullaris. Experiments were performed in thin slices of epithelium using the whole-cell configuration of the patch-clamp technique. The conventional conditioning voltage protocol did not allow a satisfactory isolation of IA, due to the presence of other K+ currents showing overlapping steady-state inactivation properties. An attempt to block other K+ currents using calcium-free saline containing 50 mM TEA also failed to provide a satisfactory isolation of IA, due to contamination by a residual sustained current, probably consisting of a slow delayed outward K+ current (IK). Use of the selective A-channel blocker 4-aminopyridine (4-AP) at concentrations < 12 mM was also unsatisfactory because at these concentrations 4-AP produced a voltage-dependent blockade. Conversely, use of 4-AP at concentrations of 15-20 mM allowed a good separation of an uncontaminated IA. These results indicate that IA in hair cells of vestibular epithelium can be isolated most effectively by the 4-AP procedure, provided that sufficiently high concentrations of the A-channel blocker are used
Differential expression of L type and non-L type calcium currents by hair cells of frog crista ampullaris
No full textNon-NMDA receptors mediate glutamate-induced depolarization in frog crista ampullaris
No full textThe effect of glutamate on frog crista ampullaris was investigated in order to assess the potential role of this agent as an afferent transmitter in inner ear organs. Intracellular recordings from single afferent axons in the isolated labyrinth showed that, after blocking synaptic transmission with high concentrations of Mg2+, micro-injections of glutamate elicit a dose-dependent postsynaptic depolarization. The amplitude of depolarization was reduced dose-dependently by the competitive non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. When Na+ concentration in the bath was progressively reduced, depolarization decreased gradually and disappeared almost completely in Na(+)-free Ringer. On the contrary, complete substitution of Ca2+ ions in the bath was without apparent effects. These results indicate that the postsynaptic depolarization induced by glutamate in frog semicircular canals involves the activation of non-NMDA amino acid receptors
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