Choline-mediated depression of hippocampal synaptic transmission

Choline is a micronutrient essential for the structural integrity of cellular membranes, and its presence at synapses follows either depolarization-induced pre-synaptic release or degradation of acetylcholine. Previous studies using whole-cell recording have shown that choline can modulate inhibitory input to hippocampal pyramidal neurons by acting upon nicotinic acetylcholine receptors (nAChRs) found on interneurons. However, little is known about how choline affects neuronal activity at the population level; therefore, we used extracellular recordings to assess its influence upon synaptic transmission in acutely prepared hippocampal slices. Choline caused a reversible depression of evoked field excitatory post-synaptic potentials (fEPSPs) in a concentration-dependent manner (10, 500, and 1000 µM). When applied after the induction of long-term potentiation, choline-mediated depression (CMD) was still observed, and potentiation returned on wash-out. Complete blockade of CMD could not be achieved with antagonists for the α7 nAChR, to which choline is a full agonist, but was possible with a general nAChR antagonist. The ability of choline to increase paired-pulse facilitation, and the inability of applied gamma-aminobutyric acid (GABA) to mediate further depression of fEPSPs, suggests that the principal mechanism of choline's action was on the facilitation of neurotransmitter release. Our study provides evidence that choline can depress population-level activity, quite likely by facilitating the release of GABA from interneurons, and may thereby influence hippocampal function.