Pregnenolone sulfate increases glutamate release at neonatal climbing fiber-to-Purkinje cell synapses

Development of cerebellar Purkinje cells (PCs) is modulated by neuroactive steroids. Developing hippocampal pyramidal neurons retrogradely release a pregnenolone sulfate (PregS)-like neurosteroid that may contribute to glutamatergic synapse stabilization. We hypothesized that PregS could exert a similar effect on developing PCs. To test this hypothesis, we performed whole-cell patch-clamp recordings from PCs in acute cerebellar vermis slices from neonatal rats. PregS induced a robust (∼3000%) and reversible increase in AMPA receptor-mediated miniature excitatory postsynaptic current (AMPA-mEPSC) frequency without affecting the amplitude, time-to-rise, or half-width of these events. PregS also increased the frequency of GABA(A) receptor-mediated miniature postsynaptic currents but to a significantly lesser extent (<100%). The PregS-induced increase of AMPA-mEPSC frequency was not significantly decreased by antagonists of receptors (NMDA, glycine, α7 nicotinic acetylcholine and σ1) that have been shown to modulate glutamatergic transmission at PCs and/or mediate the actions of PregS on neurotransmitter release. Ca(2+) chelation experiments suggested that PregS acts by increasing presynaptic terminal [Ca(2+)](i), an effect that is independent of voltage-gated Ca(2+) channels, but is blocked by the antagonist of transient receptor potential (TRP) channels, La(3+). PregS also increased the amplitude of EPSCs evoked by climbing fiber (CF) stimulation and decreased the paired-pulse ratio of these events. Neither CF nor parallel fiber-evoked EPSCs were affected by PregS in slices from juvenile rats. These results suggest that glutamate release at CF-to-PC synapses is an important target of PregS in the neonatal cerebellar cortex, an effect that may play a role in the refinement of these synapses.