Shared calcium signaling pathways in the induction of long-term potentiation and synaptic disinhibition in CA1 pyramidal cell dendrites

1. Calcium signaling pathways were examined in the induction of long-term synaptic disinhibition following tetanization. Effects of tetanization on γ- aminobutyric acid-A (GABA(A))-receptor mediated inhibitory responses were measured and compared with excitatory responses under experimental conditions previously used for examining induction mechanisms of N-methyl-D-aspartate (NMDA)-dependent long-term potentiation (LTP). Intracellular recordings were performed in current-clamp and discontinuous single-electrode voltage-clamp (dSEVC) modes in CA1 pyramidal cell apical dendrites in hippocampal slices of adult guinea pigs with the use of sharp electrodes. Test pulses and tetanic stimuli were applied to the Schaffer collateral fibers in stratum radiatum. 2. Under standard control conditions [3 M K Ac in the recording pipette and artificial cerebrospinal fluid as extracellular solution], tetanization- induced sustained increases of excitatory responses were accompanied by marked decreases of parameters of GABA(A)-mediated synaptic inhibition: at 40 min after tetanization [posttetanus 40 (PT 40)], orthodromically evoked excitatory postsynaptic potential (EPSP) peak amplitudes were on average 195 ± 15% (mean ± SE) and excitatory postsynaptic currents (EPSCs) were 166 ± 10% of pretetanus controls. Peak amplitudes of orthodromically evoked inhibitory postsynaptic potentials (IPSPs) were 30 ± 5% and inhibitory postsynaptic currents (IPSCs) were 21 ± 4% at PT 40. Synaptic GABA(A) conductances (measured as chord conductances) were reduced to 22 ± 4% at PT 40. Iontophoretic GABA(A) responses measured as conductance changes were 28 ± 4% of pretetanus controls at PT 40. 3. A role of NMDA receptors in induction of long-term synaptic disinhibition was tested by preventing NMDA receptor activation 1) by pharmacological means and 2) by holding the membrane clamped at -80 mV (in dSEVC) during tetanization. In the presence of the NMDA-receptor antagonist D-2 amino-5-phosphonopentanoic acid (D-AP5) (10- 40 μM), orthodromically evoked EPSP amplitudes were 107 ± 9%, EPSCs were 104 ± 6%, GABA(A)-mediated IPSPs were 88 ± 8%, IPSCs were 97 ± 8%, synaptic GABA(A) conductances were 84 ± 9%, and iontophoretic GABA(A) conductances were 102 ± 13% at PT 40. In recordings in which the dendritic membrane potential was clamped at -80 mV during tetanization, orthodromically evoked peak amplitudes of EPSPs were 105 ± 11%, EPSCs were 102 ± 8, IPSPs were 103 ± 4%, IPSCs were 102 ± 5%, GABA(A) chord conductances were 101 ± 9%, and iontophoretically evoked GABA(A) conductances were 105 ± 5% at PT 40. 4. In recordings in which the intracellular pipette was preloaded with the Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid (BAPTA) (5 mM), long-term changes of synaptic transmission (increases of excitation, decreases of synaptic inhibition) were prevented. At PT 40, EPSP peak amplitudes were 93 ± 7%, EPSCs were 115 ± 6%, IPSPs were 115 ± 9%, IPSCs were 117 ± 8%, and synaptic GABA(A) conductances were 108 ± 17%. Iontophoretic conductances at PT 40 were 109 ± 9% over pretetanus controls when recorded with BAPTA-containing electrodes. 5. In recordings in which the intracellular pipette was preloaded with cypermethrin, a potent and selective inhibitor of phosphatase 2B, respective long-term changes of synaptic transmission (increases of excitation, decreases of synaptic inhibition) were prevented. At PT 40, EPSP peak amplitudes were 98 ± 6%, EPSCs were 105 ± 10%, IPSPs were 99 ± 5%, IPSCs were 104 ± 7%, synaptic GABA(A) conductances were 97 ± 6% and iontophoretic GABA(A) conductances were 113 ± 18% over pretetanus controls in cypermethrin-containing recordings. 6. In conclusion, the data presented demonstrate shared cellular pathways in the induction of both LTP and long-term synaptic disinhibition in apical dendrites of CA1 pyramidal cells after tetanization of the Schaffer collaterals. The close temporal long-term correlation between potentiation of excitatory synaptic responses and decreases of inhibitory synaptic responses under various experimental conditions provides evidence for the notion of synaptic disinhibition contributing to NMDA-dependent pyramidal cell LTP in CA1.