Glutamatergic synaptic inputs to mouse supraoptic neurons in calcium-free medium in vitro

The effects of Ca2+-free perfusion medium on excitatory postsynaptic currents (EPSCs) and potentials (EPSPs) were studied by whole-cell recordings from neurons of the supraoptic nucleus (SON) in trimmed slice preparations of mouse hypothalamus. EPSCs evoked with either focal stimulation to the SON or perfusion of slices with high K+-medium, spontaneous EPSCs (sEPSCs) and miniature EPSCs (mEPSCs) recorded from neurons of the SON were blocked by the glutamate receptor antagonist kynurenic acid (1 mM). While EPSCs evoked by focal stimulation were abolished in the presence of Ca2+-free perfusion medium; sEPSCs and mEPSCs remained. Neither the frequency nor the amplitude of the sEPSCs and mEPSCs significantly changed during the application of Ca2+-free perfusion medium. Perfusion of slices with high K+-medium increased the mEPSC frequency compared with that recorded in normal Ca2+-containing perfusion medium. In contrast, mEPSC frequency did not change during perfusion with Ca2+-free high K+-medium. In current-clamp mode sEPSPs were observed during the perfusion with Ca2+-free medium. Some sEPSPs recorded in Ca2+-free medium were sufficiently large to evoke action potentials. These results imply that spontaneous glutamatergic synaptic inputs to the hypothalamic neurosecretory cells exist in Ca2+-free perfusion medium. Thus, the present study suggests that Ca2+-free medium does not always block the synaptic transmission in hypothalamic slice preparations.