Anatomically discrete functional effects of adenoviral clostridial light chain gene-based synaptic inhibition in the midbrain

The gene for the Light Chain fragment of Tetanus Toxin (LC) induces synaptic inhibition by preventing the release of synaptic vesicles. The present experiment applied this approach within the rat midbrain in order to demonstrate that LC gene expression can achieve functionally and anatomically discrete effects within a sensitive brain structure. The deep layers of the superior colliculus/deep mesencephalic nucleus (dSC/DpMe) that are located in the rostral midbrain has been implicated in fear-induced increase of the acoustic startle reflex (fear potentiated startle) but exists in close proximity to neural structures important for a variety of critical functions. The dSC/DpMe of adult rats was injected bilaterally with adenoviral vectors for LC, green fluorescent protein, or vehicle. Synaptobrevin was depleted in brain regions of adenoviral LC expression. LC gene expression in the dSC/DpMe inhibited the increase in startle amplitude seen with the control viral infection, and blocked context-dependent potentiation of startle induced by fear conditioning. Although LC gene expression reduced the absolute amount of cue-specific fear potentiated startle, it did not decrease percent potentiated startle to a cue, nor did it reduce fear-induced contextual freezing, nonspecific locomotor activity, or general health, indicating that its effects were functionally and anatomically specific. Thus, vector-driven LC expression inhibits the function of deep brain nuclei without altering the function of surrounding structures supporting its application to therapeutic neuromodulation.