Short-term (2 to 5 h) dark exposure lowers long-term potentiation (LTP) induction threshold in rat primary visual cortex.

Up- and down-regulation of synaptic strength (i.e., long-term potentiation, LTP, long-term depression) is thought to be the primary mechanism mediating experience-dependent plasticity of cortical networks. Recent evidence indicates that the expression of plastic changes at synapses itself is dynamic and regulated, at least in part, by the recent history of synaptic activity, a concept termed metaplasticity. Here, adult, urethane-anesthetized rats were exposed to light or dark conditions for various durations (1, 2, and 5 h) to influence activity levels in the retinal-dorsal lateral geniculate nucleus (dLGN)-primary visual cortex (V1) pathway. Field potentials, recorded in layer IV of V1, were evoked by light flashes to the retina or single pulse electrical stimulation of the dLGN. Brief (60 s) periods of high frequency (50 Hz) retinal light stimulation results in an increase in visual evoked potential (VEP) amplitude in animals exposed to complete darkness for 2 h, while VEP amplitude failed to show potentiation in animals maintained in darkness for shorter periods. Similarly, weak theta burst stimulation of the dLGN failed to induce LTP in animals maintained under continuous light, but elicited robust LTP after 5 h of dark exposure. These data demonstrate that induction thresholds for sensory- and electrically-induced LTP in the retino-geniculo-cortical pathway of adult rats are dynamically regulated by levels of preceding sensory stimulation. Importantly, such metaplastic adjustments of plasticity in V1 can occur over time-scales significantly shorter than previously recognized.