Presynaptic inhibition of synaptic transmission in the rat hippocampus by activation of muscarinic receptors: involvement of presynaptic calcium influx

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Abstract
1. Modulation of presynaptic voltage-dependent calcium channels (VDCCs) by muscarinic receptors at the CA3-CA1 synapse of rat hippocampal slices was investigated by using the calcium indicator fura-2. Stimulation-evoked presynaptic calcium transients ([Ca(pre)]t) and field excitatory postsynaptic potentials (fe.p.s.ps) were simultaneously recorded. The relationship between presynaptic calcium influx and synaptic transmission was studied. 2. Activation of muscarinic receptors inhibited [Ca(pre)]t, thereby reducing synaptic transmission. Carbachol (CCh, 10 microM) inhibited [Ca(pre)]t by 35% and reduced fe.p.s.p. by 85%. The inhibition was completely antagonized by 1 microM atropine. An approximate 4th power relationship was found between presynaptic calcium influx and postsynaptic responses. 3. Application of the N-type VDCC-blocking peptide toxin omega-conotoxin GVIA (omega-CTx GVIA, 1 microM) inhibited [Ca(pre)]t and fe.p.s.ps by 21% and 49%, respectively, while the P/Q-type VDCC blocker omega-agatoxin IVA (omega)-Aga IVA, 1 microM) reduced [Ca(pre)]t and fe.p.s.ps by 35% and 85%, respectively. 4. Muscarinic receptor activation differentially inhibited distinct presynaptic VDCCs. Omega-CTx GVIA-sensitive calcium channels were inhibited by muscarinic receptors, while omega-Aga IVA-sensitive channels were not. The percentage inhibition of omega-CTx GVIA-sensitive [Ca(pre)]t was about 63%. 5. Muscarinic receptors inhibited presynaptic VDCCs in a way similar to adenosine (Ad) receptors. The percentage inhibition of omega-CTx GVIA-sensitive [Ca(pre)]t by Ad (100 microM) was about 59%. There was no significant inhibition of omega-Aga IVA-sensitive channels by Ad. The inhibitions of [Ca(pre)]t by CCh and Ad were mutually occlusive. 6. These results indicate that inhibition of synaptic transmission by muscarinic receptors is mainly the consequence of a reduction of the [Ca(pre)]t due to inhibition of presynaptic VDCCs.