Electrophysiological studies on the ascaris muscle gaba receptor
Electrophysiological studies on the ascaris muscle gaba receptor
Intracellular microelectrode recordings were made from the cell bellies of muscle cells of the nematode Ascaris lumbricoides. The putative neu-rotransmitter substance gamma- aminobutyric acid (GABA), and related analogues, were applied to the cell by superfusion. A rank order of relative potencies was obtained. A number of know GABA antagonists were tested for activity in Ascaris using a pressure eject system of applying GABA. The actions of the anthelmintic avermectin on the muscle cells was studied.
GABA was found to hyperpolarise the membrane through an increase in chloride conductance. Subsequent responses sometimes faded by as much as 69% due to intracellular accumulation of chloride. The relative potencies of the agonists tested were: muscimol 0.25; isoguvacine 0.125; CACA 0.064; THIP 0.006 (taking GABA as 1). These relative potency values agreed with those obtained in organ bath experiments measuring length changes. Piperazine had a relative potency of 0.03 which was increased to 0.01 in the presence of 25mM bicarbonate. P4S, 3APS and baclofen were inactive.
The antagonists picrotoxin, bicuculline, securinine, pitrazepin and SR95331 were tested and found to be inactive. Pitrazepin was found to have some direct actions, and depolarized the membrane through a reduction in input conductance. Avermectin antagonised the response to GABA by 25%. There was no evidence for any benzodiazepine receptor, chloride/bicarbonate exchange system or nipecotic acid sensitive uptake system associated with this receptor.
Spontaneous depolarizations could be recorded from the cells at 37° G. These were abolished in calcium-free media but not by nifedipine. In chloride-free media the activity occasionally adopted a bursting pattern. Acetylcholine depolarized the muscle cells and increased the frequency of the activity, picrotoxin antagonised these effects slightly. Application of picrotoxin alone sometimes hyperpolarised the membrane and reduced the firing frequency. GABA reversibly blocked this activity and hyperpolarised the membrane potential. Avermectin irreversibly blocked the activity over a much longer time-course (26 mins) without hyperpolarising the membrane. Actions of both GABA and avermectin were abolished in chloride-free media but not by picrotoxin. GABA continued to irreversibly hyperpolarise the membrane in the presence of avermectin.
The phorbol ester TPA did not affect the activity of the cells.
University of Southampton
1987
Hewitt, Graham Malcolm
(1987)
Electrophysiological studies on the ascaris muscle gaba receptor.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
Intracellular microelectrode recordings were made from the cell bellies of muscle cells of the nematode Ascaris lumbricoides. The putative neu-rotransmitter substance gamma- aminobutyric acid (GABA), and related analogues, were applied to the cell by superfusion. A rank order of relative potencies was obtained. A number of know GABA antagonists were tested for activity in Ascaris using a pressure eject system of applying GABA. The actions of the anthelmintic avermectin on the muscle cells was studied.
GABA was found to hyperpolarise the membrane through an increase in chloride conductance. Subsequent responses sometimes faded by as much as 69% due to intracellular accumulation of chloride. The relative potencies of the agonists tested were: muscimol 0.25; isoguvacine 0.125; CACA 0.064; THIP 0.006 (taking GABA as 1). These relative potency values agreed with those obtained in organ bath experiments measuring length changes. Piperazine had a relative potency of 0.03 which was increased to 0.01 in the presence of 25mM bicarbonate. P4S, 3APS and baclofen were inactive.
The antagonists picrotoxin, bicuculline, securinine, pitrazepin and SR95331 were tested and found to be inactive. Pitrazepin was found to have some direct actions, and depolarized the membrane through a reduction in input conductance. Avermectin antagonised the response to GABA by 25%. There was no evidence for any benzodiazepine receptor, chloride/bicarbonate exchange system or nipecotic acid sensitive uptake system associated with this receptor.
Spontaneous depolarizations could be recorded from the cells at 37° G. These were abolished in calcium-free media but not by nifedipine. In chloride-free media the activity occasionally adopted a bursting pattern. Acetylcholine depolarized the muscle cells and increased the frequency of the activity, picrotoxin antagonised these effects slightly. Application of picrotoxin alone sometimes hyperpolarised the membrane and reduced the firing frequency. GABA reversibly blocked this activity and hyperpolarised the membrane potential. Avermectin irreversibly blocked the activity over a much longer time-course (26 mins) without hyperpolarising the membrane. Actions of both GABA and avermectin were abolished in chloride-free media but not by picrotoxin. GABA continued to irreversibly hyperpolarise the membrane in the presence of avermectin.
The phorbol ester TPA did not affect the activity of the cells.
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Published date: 1987
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Local EPrints ID: 460908
URI: http://eprints.soton.ac.uk/id/eprint/460908
PURE UUID: ebbc1014-598f-4419-9831-d7982666ac11
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Date deposited: 04 Jul 2022 18:31
Last modified: 04 Jul 2022 18:31
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Author:
Graham Malcolm Hewitt
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