Abstract:The toxin fraction (FIX) and peptide to-Aga-WA from the venom of the funnel-web spider Agelenopsis aperta, as well as a synthetic analogue ofFX, speciically block the P-type voltage-dependent Cam channel (VDCC). The effects of these toxins on synaptic transmission were studied in the neuromuscular synapses of the crayfish opener muscle, which has a single excitatory and a single inhibitory motoneuron. FIX selectively and reversibly blocked excitatory and inhibitory stsynaptic currents and potentials in a dosed… Show more
“…The predominant role of P/Q-like channels in transmitter release observed in our study is in accord with results from crayfish and crab (Araque et al, 1994;Blundon et al, 1995;Wright et al, 1996;Hong and Lnenicka, 1997;Hurley and Graubard, 1998) and mammals, but in the rat, motor terminals at some muscles also contain a small fraction of N-type channels (Westenbroek et al, 1998). In frog and lizard neuromuscular synapses, N-or L-type channels mediate transmission (Lindgren and Moore, 1989;Katz et al, 1995;Arenson and Gill, 1996).…”
Section: Different Ca 2؉ Channel Types Are Differentially Colocalizedsupporting
confidence: 80%
“…Although its physiological type was not stated, it is likely a fast-type neuron because of its high output terminals. Effects of -CgTX were not observed in recent studies of motor neurons in crustaceans (Araque et al, 1994;Wright et al, 1996;Hurley and Graubard, 1998). This led to the conclusion that N-like channels are not involved in neuromuscular transmission in crustaceans.…”
Section: Different Ca 2؉ Channel Types Are Differentially Colocalizedmentioning
confidence: 72%
“…In crayfish, additional subtypes are present that are pharmacologically different from channels characterized in vertebrate neurons (Richmond et al, 1995(Richmond et al, , 1996Hong and Lnenicka, 1997). At crustacean neuromuscular junctions, transmitter release is thought to be mediated through P-type channels, with no contribution by N-, Q-, or L-type (Araque et al, 1994;Blundon et al, 1995;Wright et al, 1996;Hurley and Graubard, 1998).…”
Section: Abstract: P/q-type Ca 2ϩ Channels; N-type Ca 2ϩ Channels; Rmentioning
confidence: 99%
“…There is evidence for L-, N-, P/Q-, or T-like channels in molluscs (for review, see Kits and Mansvelder, 1996), insects (for review, see Wicher et al, 2001), and crustaceans (Araque et al, 1994;Blundon et al, 1995;Chrachri, 1995;Wright et al, 1996;Hong and Lnenicka, 1997;Hurley and Graubard, 1998;GarciaColunga et al, 1999). In crayfish, additional subtypes are present that are pharmacologically different from channels characterized in vertebrate neurons (Richmond et al, 1995(Richmond et al, , 1996Hong and Lnenicka, 1997).…”
Section: Abstract: P/q-type Ca 2ϩ Channels; N-type Ca 2ϩ Channels; Rmentioning
Most crustacean muscle fibers receive double excitatory innervation by functionally different motor neurons termed slow and fast. By using specific -toxins we show that the terminals of the slow closer excitor (SCE) and the fast closer excitor (FCE) at a crab muscle are endowed with different sets of presynaptic Ca 2ϩ channel types. -Agatoxin, a blocker of vertebrate P/Q-type channels, reduced the amplitude of EPSCs by decreasing the mean quantal content of transmitter release in both neurons by 70-85%, depending on the concentration. We provide the first evidence that -conotoxin-sensitive channels also participate in transmission at crustacean neuromuscular terminals and are colocalized with -agatoxin-sensitive channels in an axon-type-specific distribution. -Conotoxin, a blocker of vertebrate N-type channels, inhibited release by 20-25% only at FCE, not at SCE endings. Low concentrations of Ni 2ϩ , which block vertebrate R-type channels, inhibited release in endings of the SCE by up to 35%, but had little effects in FCE endings.We found that two neuropeptides, the FMRFamide-like DF 2 and proctolin, which occur in many crustaceans, potentiated evoked transmitter release differentially. Proctolin increased release at SCE and FCE endings, and DF 2 increased release only at FCE endings. Selective blocking of Ca 2ϩ channels by different -toxins in the presence of peptides revealed that the target of proctolin-mediated modulation is the -agatoxin-sensitive channel (P/Q-like), that of DF 2 the -conotoxin-sensitive channel (N-like). The differential effects of these two peptides allows fine tuning of transmitter release at two functionally different motor neurons innervating the same muscle.
“…The predominant role of P/Q-like channels in transmitter release observed in our study is in accord with results from crayfish and crab (Araque et al, 1994;Blundon et al, 1995;Wright et al, 1996;Hong and Lnenicka, 1997;Hurley and Graubard, 1998) and mammals, but in the rat, motor terminals at some muscles also contain a small fraction of N-type channels (Westenbroek et al, 1998). In frog and lizard neuromuscular synapses, N-or L-type channels mediate transmission (Lindgren and Moore, 1989;Katz et al, 1995;Arenson and Gill, 1996).…”
Section: Different Ca 2؉ Channel Types Are Differentially Colocalizedsupporting
confidence: 80%
“…Although its physiological type was not stated, it is likely a fast-type neuron because of its high output terminals. Effects of -CgTX were not observed in recent studies of motor neurons in crustaceans (Araque et al, 1994;Wright et al, 1996;Hurley and Graubard, 1998). This led to the conclusion that N-like channels are not involved in neuromuscular transmission in crustaceans.…”
Section: Different Ca 2؉ Channel Types Are Differentially Colocalizedmentioning
confidence: 72%
“…In crayfish, additional subtypes are present that are pharmacologically different from channels characterized in vertebrate neurons (Richmond et al, 1995(Richmond et al, , 1996Hong and Lnenicka, 1997). At crustacean neuromuscular junctions, transmitter release is thought to be mediated through P-type channels, with no contribution by N-, Q-, or L-type (Araque et al, 1994;Blundon et al, 1995;Wright et al, 1996;Hurley and Graubard, 1998).…”
Section: Abstract: P/q-type Ca 2ϩ Channels; N-type Ca 2ϩ Channels; Rmentioning
confidence: 99%
“…There is evidence for L-, N-, P/Q-, or T-like channels in molluscs (for review, see Kits and Mansvelder, 1996), insects (for review, see Wicher et al, 2001), and crustaceans (Araque et al, 1994;Blundon et al, 1995;Chrachri, 1995;Wright et al, 1996;Hong and Lnenicka, 1997;Hurley and Graubard, 1998;GarciaColunga et al, 1999). In crayfish, additional subtypes are present that are pharmacologically different from channels characterized in vertebrate neurons (Richmond et al, 1995(Richmond et al, , 1996Hong and Lnenicka, 1997).…”
Section: Abstract: P/q-type Ca 2ϩ Channels; N-type Ca 2ϩ Channels; Rmentioning
Most crustacean muscle fibers receive double excitatory innervation by functionally different motor neurons termed slow and fast. By using specific -toxins we show that the terminals of the slow closer excitor (SCE) and the fast closer excitor (FCE) at a crab muscle are endowed with different sets of presynaptic Ca 2ϩ channel types. -Agatoxin, a blocker of vertebrate P/Q-type channels, reduced the amplitude of EPSCs by decreasing the mean quantal content of transmitter release in both neurons by 70-85%, depending on the concentration. We provide the first evidence that -conotoxin-sensitive channels also participate in transmission at crustacean neuromuscular terminals and are colocalized with -agatoxin-sensitive channels in an axon-type-specific distribution. -Conotoxin, a blocker of vertebrate N-type channels, inhibited release by 20-25% only at FCE, not at SCE endings. Low concentrations of Ni 2ϩ , which block vertebrate R-type channels, inhibited release in endings of the SCE by up to 35%, but had little effects in FCE endings.We found that two neuropeptides, the FMRFamide-like DF 2 and proctolin, which occur in many crustaceans, potentiated evoked transmitter release differentially. Proctolin increased release at SCE and FCE endings, and DF 2 increased release only at FCE endings. Selective blocking of Ca 2ϩ channels by different -toxins in the presence of peptides revealed that the target of proctolin-mediated modulation is the -agatoxin-sensitive channel (P/Q-like), that of DF 2 the -conotoxin-sensitive channel (N-like). The differential effects of these two peptides allows fine tuning of transmitter release at two functionally different motor neurons innervating the same muscle.
“…The calcium current-voltage relation, in voltageclamped terminals, has onset activation, peak, and reversal potentials of -35, -10, and calcium equilibrium potential (EC,) = +30 mV, respectively (Wright et al 1994). Funnel web spider toxin is the effective blocker in this preparation, as is the case for vertebrate P-type Ca, channels (Araque et al 1994;Usowicz et al 1992;Wright et al 1994). The distinctive properties of the P-type channel were initially defined in cerebellar Purkinje cells.…”
SUMMARYAND CONCLUSIONS1. In a model synaptic system, the excitatory neuromuscular junction of the freshwater crayfish, the nerve terminals possess synapses that vary in structural complexity, with numbers of active zones ranging from zero to five. Active zones on individual synapses show a wide range of separation distances. We tested the hypothesis that two active zones of a single synapse in close proximity can enhance the localized increase in free calcium ion concentration, thus enhancing the probability of neurotransmission at that synapse. We evaluated the increase in calcium ion concentration as a function of distance between adjacent active zones.2. To test this hypothesis, a reaction-diffusion model for Ca2+ entering the presynaptic terminals was used. This test was used because 1) present measurement techniques are inadequate to resolve quantitatively the highly localized, transient calcium microdomains at synaptic active zones; and 2) there is presently no suitable preparation for physiological recording from isolated synapses with varying distances between active zones. Included in the model were intracellular buffer and a typical distribution of voltage-activated Ca2+ channels for an active zone, estimated from freeze-fracture micrographs.3. The model indicated that localized Ca2+ clouds from discrete active zones can overlap to create spatial enhancement of Ca2+ concentration. The degree of interaction between two active zones depends on the distance between them. When two typical active zones are separated by 5200 nm, the maximum intracellular Ca2+ concentration ( [ Ca2'li) is greater at 1) the midpoint between them, and 2) the center of each one, than at the corresponding positions for a single isolated active zone. Enhanced [ Ca2+]i at the edge of the active zone where "docked' ' synaptic vesicles occur would be expected to have an effect on transmitter release.4. When the model includes no intracellular buffer, the increase in [Ca"]i is a linear function of calcium channel current, but is a nonlinear function of the number of conducting calcium channels in an active zone. With immobile buffer included, the increase in [ Ca2+li is nonlinear with respect to both channel current and number of conducting channels.5. Inclusion of immobile buffer in the model provides "released" residual calcium that slowly accumulates during a train of current pulses. Released residual calcium accumulates more rapidly at paired active zones separated by 5200 nm than at single isolated active zones.6. We propose that the probability of release is enhanced at synapses with closely associated active zones. Synapses of this type ( "complex' ' synapses) could be selectively recruited when the neuron is active at low frequencies. At higher frequencies of neuronal activity, more distant active zones may interact and acquire a greater probability of releasing quanta. This would provide the nerve terminal with one component of a mechanism for frequency facilitation, because the number of quanta released by the terminal as a...
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