Visually driven regulation of intrinsic neuronal excitability improves stimulus detection in vivo.

Neurons adapt their electrophysiological properties to maintain stable levels of electrical excitability when faced with a constantly changing environment. We find that exposing freely swimming Xenopus tadpoles to 4-5 hr of persistent visual stimulation increases the intrinsic excitability of optic tectal neurons. This increase is correlated with enhanced voltage-gated Na+ currents. The same visual stimulation protocol also induces a polyamine synthesis-dependent reduction in Ca2+-permeable AMPAR-mediated synaptic drive, suggesting that the increased excitability may compensate for this reduction. Accordingly, the change in excitability was prevented by blocking polyamine synthesis during visual stimulation and was rescued when Ca2+-permeable AMPAR-mediated transmission was selectively reduced. The changes in excitability also rendered tectal cells more responsive to synaptic burst stimuli, improving visual stimulus detection. The synaptic and intrinsic adaptations function together to keep tectal neurons within a constant operating range, while making the intact visual system less responsive to background activity yet more sensitive to burst stimuli.