Behavioural NeuroscienceResearch PaperEffects of 17β-estradiol on glutamate synaptic transmission and neuronal excitability in the rat medial vestibular nuclei

We investigated the effects of the neurosteroid 17β-estradiol (E2) on the evoked and spontaneous activity of rat medial vestibular nucleus (MVN) neurons in brainstem slices. E2 enhances the synaptic response to vestibular nerve stimulation in type B neurons and depresses the spontaneous discharge in both type A and B neurons. The amplitude of the field potential, as well as the excitatory post-synaptic potential (EPSP) and current (EPSC), in type B neurons, are enhanced by E2. Both effects are long-term phenomena since they outlast the drug washout. The enhancement of synaptic response is mainly due to facilitation of glutamate release mediated by pre-synaptic n-methyl-d-aspartate receptors (NMDARs), since the reduction of paired pulse ratio (PPR) and the increase of miniature EPSC frequency after E2 are abolished under D-(−)-2-amino-5-phosphonopentanoic acid (AP-5). E2 also facilitates post-synaptic NMDARs, but it does not affect directly α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and group I-metabotropic glutamate receptors (mGluRs-I). In contrast, the depression of the spontaneous discharge of type A and type B neurons appears to depend on E2 modulation of intrinsic ion conductances, as the effect remains after blockade of glutamate, GABA and glycine receptors (GlyRs). The net effect of E2 is to enhance the signal-to-noise ratio of the synaptic response in type B neurons, relative to resting activity of all MVN neurons. These findings provide evidence for a novel potential mechanism to modulate the responsiveness of vestibular neurons to afferent inputs, and so regulate vestibular function in vivo.