In:
Canadian Journal of Physiology and Pharmacology, Canadian Science Publishing, Vol. 58, No. 4 ( 1980-04-01), p. 373-383
Abstract:
(1) Hemicholinium-3 (HC-3) reduces the size of the depolarization obtained by iontophoretic application of acetylcholine (ACh) on cell R15 in Aplysia. This effect of HC-3 is not antagonized by choline and is attributed to a blockade of the postsynaptic cholinergic excitatory receptor as HC-3 has no effect on the membrane potential and resistance of R15.(2) HC-3 does not affect the desensitization observed by giving successive pulses of exogenous ACh.(3) HC-3 reduces the cholinergic excitatory postsynaptic potentials (EPSPs) recorded in cell R15 following stimulation of the right connective (synapse RC1-R15). This effect cannot be accounted for solely by the postsynaptic action of HC-3, as this effect is partially antagonized by choline and is increased by a higher stimulation frequency of the synapse or by a higher extracellular Ca 2+ concentration.(4) Part of the action of HC-3 on the EPSP size is therefore accounted for by a reduction of choline reuptake by the presynaptic terminal. This would reduce the size of the storage pool of neurotransmitter and indirectly the size of the available pool of neurotransmitter.(5) Synapse RC1-R15 was stimulated for a long period (30 min) at 2 Hz with or without HC-3 in the perfusate. Following this initial phase, the rate of transmitter mobilization was evaluated by giving a short train of 100 stimuli at several frequencies. In the presence of HC-3, the rate of mobilization is reduced more at the higher frequencies. As the depletion of the storage pool produced by HC-3 during the initial phase is the same in all cases, it is concluded that the process of mobilization is more sensitive to a reduction of the storage pool when the mobilization rate is higher.(6) HC-3 increases the posttetanic potentiation (PTP) and the synaptic depression observed at synapse RC1-R15 when perfused in the presence of elevated extracellular Ca 2+ (2 × Ca 2+ , 22 mM), but HC-3 reduces the amplitudes of these phenomena when perfused in reduced Ca 2+ (½ × Ca 2+ , 5.5 mM).(7) HC-3 has been perfused alone or with choline, with or without continuous stimulation at 2 Hz in a medium containing a high concentration of Ca 2± . Following this initial phase, PTP and depression were tested by a short train of stimulation. When there is stimulation during the initial phase, choline significantly antagonizes the increase of PTP and depression produced by HC-3 alone. However, when there is no stimulation during the initial phase, choline increases the augmentation of PTP and depression produced by HC-3 alone.(8) Similar experiments were also done in low Ca 2± concentration. With stimulation during the initial phase, choline does not antagonize the reduction of PTP and depression produced by HC-3. However, without stimulation during the initial phase, choline significantly antagonizes the reduction of PTP and depression produced by HC-3.
Type of Medium:
Online Resource
ISSN:
0008-4212
,
1205-7541
Language:
English
Publisher:
Canadian Science Publishing
Publication Date:
1980
detail.hit.zdb_id:
2004356-9
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