Presynaptic GABAA receptors enhance transmission and LTP induction at hippocampal mossy fiber synapses

Presynaptic GABAA receptors (GABAARs) occur at hippocampal mossy fiber synapses. Whether and how they modulate orthodromic signaling to postsynaptic targets is poorly understood. We found that an endogenous neurosteroid that is selective for high-affinity δ subunit–containing GABAARs depolarized rat mossy fiber boutons, enhanced action potential–dependent Ca2+ transients and facilitated glutamatergic transmission to pyramidal neurons. Conversely, blocking GABAARs hyperpolarized mossy fiber boutons, increased their input resistance, decreased spike width and attenuated action potential–dependent presynaptic Ca2+ transients, indicating that a subset of presynaptic GABA receptors are tonically active. Blocking GABAARs also interfered with the induction of long-term potentiation at mossy fiber–CA3 synapses. Presynaptic GABAARs therefore facilitate information flow to the hippocampus both directly and by enhancing LTP.

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