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Department of Physiology, Kyushu Dental College, Kokurakitaku, Kitakyushu 803 - 8580 Japan
Cholinergic actions
on subfornical organ (SFO) neurons in rat slice preparations were
studied by using whole cell voltage- and current-clamp recordings. In
the voltage-clamp recordings, carbachol and muscarine decreased the
frequency of GABAergic inhibitory postsynaptic currents (IPSCs) in a
dose-dependent manner, with no effect on the amplitudes or the time
constants of miniature IPSCs. Meanwhile, carbachol did not influence
the amplitude of the outward currents induced by GABA. Furthermore,
carbachol and muscarine also elicited inward currents in a
TTX-containing solution. From the current-voltage relationship, the
reversal potential was estimated to be
7.1 mV. These
carbachol-induced responses were antagonized by atropine. In the
current-clamp recordings, carbachol depolarized the membrane with
increased frequency of action potentials. These observations suggest
that acetylcholine suppresses GABA release through muscarinic receptors
located on the presynaptic terminals. Acetylcholine also directly
affects the postsynaptic membrane through muscarinic receptors, by
opening nonselective cation channels. A combination of these
presynaptic and postsynaptic actions may enhance activation of SFO
neurons by acetylcholine.
inhibitory postsynaptic current; synaptic input; acetylcholine; patch clamp
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