Aim: 

Exogenous and neural 17b-Estradiol (E2) affects excitability, glutamate synaptic transmission and plasticity of neurons in the medial vestibular nucleus (MVN). Since brain production of E2 is due to aromatase conversion of testosterone (T), a role of T on neuronal activity may be suggested. This study analysed the possible direct and E2-mediated effects of T on synaptic responses and excitability of MVN neurons.

Methods: 

In male rat brainstem slices, field potential (FP) evoked by vestibular afferent stimulation and spontaneous neuron firing rate (FR) were analysed in the MVN. The direct effect of T was examined by bath infusion of T (1 nM), while the E2-mediated effect, by T under block of aromatase (letrozole, 100 nM), or E2 receptors (ERa, ERb) (ICI 182,780, 100 nM).

Results: 

T caused rapid and long-term increase of FP (124.5 ‡ 5.2%, 85% of the cases) and decrease of FR (86.4 ‡ 3.2%, 80% of the cases), like observed by E2. Under letrozole or ICI the occurrences of T induced FP potentiation and FR depression decreased to 30% and 55%, respectively, and the FP potentiation was smaller and slower. Moreover, reverse effects: FP depression (30%) and FR potentiation (40%) occurred.

Conclusion: 

Our results show that T modifies evoked and spontaneous activity of MVN neurons in the same way of E2, maybe by its conversion to E2. The residual effects, after block of E2, and the presence of opposite events may depend on a direct action of T or its conversion to other neurosteroids.