Blockade of GABAA receptors facilitates induction of NMDA receptor-independent long-term potentiation.

An N-methyl-D-aspartate (NMDA)-independent form of long-term potentiation (LTP), which depends on postsynaptic, voltage-dependent calcium channels (VDCCs), has been demonstrated in area CA1 of hippocampus. GABA acting at GABAA receptors limits postsynaptic depolarization during LTP induction. Blockade of GABAA receptors should therefore enhance activation of postsynaptic VDCCs and facilitate the induction of this NMDA receptor-independent, VDCC-dependent LTP. In agreement with this hypothesis, pharmacological blockade of GABAA receptors in the in vitro rat hippocampal slice increased the magnitude of LTP resulting from a normally effective, high-frequency (200 Hz) tetanic stimulation protocol. In addition, GABAA receptor blockade allowed a lower frequency (25 Hz) and normally ineffective tetanic stimulation protocol to induce this form of LTP. Intracellular recordings from CA1 pyramidal cells revealed that blocking GABAA receptors during tetanic stimulation allowed greater postsynaptic depolarization, increased the number of postsynaptic action potentials fired during the tetanization, and also increased the duration of synaptically evoked action potentials. To mimic the increased action potential firing observed when GABAA receptors were blocked, we paired 25-Hz antidromic stimulation with 25-Hz orthodromic stimulation. Paired antidromic + orthodromic 25-Hz stimulation induced NMDA receptor-independent LTP, whereas neither antidromic nor orthodromic stimulation alone induced LTP. Increased action potential firing can therefore at least partially account for the facilitation of NMDA receptor-independent LTP caused by blockade of GABAA receptors. This conclusion is consistent with prior studies demonstrating that action potentials are particularly effective stimuli for the gating of VDCCs in CA1 pyramidal cell dendrites.