Presynaptic inhibition at excitatory hippocampal synapses: Development and role of presynaptic Ca²⁺ channels

1. Presynaptic inhibition of excitatory postsynaptic currents (EPSCs) induced by activation of adenosine receptors was examined at hippocampal synapses in cell culture. Changes in the degree of presynaptic inhibition during development were examined. The results were then used to test the role of presynaptic Ca²⁺ channels in presynaptic inhibition. 2. Application of the selective Al adenosine receptor agonist N⁶-cyclopentyladenosine (CPA) reduced EPSCs measured with the use of whole cell voltage-clamp procedures. In cells grown in culture for <15 days, CPA (100 nM) inhibited EPSCs by 74 ± 2%. In cells grown in culture for >20 days, the same concentration of CPA inhibited EPSCs by 47 ± 3%. 3. In mature cells (grown in culture for >20 days), application of the selective N-type Ca²⁺ channel blocker ω- conotoxin GVIA (ω-CTx GVIA; 2.5 μM) partially occluded the effects of CPA. In contrast, the P/Q channel blocker ω-Aga IVA enhanced the effects of CPA. Both toxins reduced the amplitude of the EPSC. 4. ω-CTx GVIA was applied to the EPSC that remained after application of 100 nM CPA. Under these conditions, ω-CTx GVIA reduced the EPSC by less than when ω-CTx GVIA was applied under control conditions. In contrast, when ω-Aga IVA was applied in the presence of CPA, the toxin reduced the EPSC to a greater extent than when it was applied under control conditions. 5. Somatic Ca²⁺-channel currents were inhibited by CPA. This effect was partially occluded by pretreatment with ω-CTx GVIA but was unaffected by pretreatment with ω-Aga IVA (1 μM). Both toxins blocked part of the somatic Ca²⁺-channel current. 6. The results indicate that inhibition of presynaptic N-type Ca²⁺ channels accounted for 40-50% of presynaptic inhibition, another type of Ca²⁺ channel may participate as well. In addition, the efficacy of presynaptic inhibition declined during synapse maturation due in part to a developmental decline in the relative contribution of N-type channels to transmitter release.