Cellular mechanisms of long-lasting adaptation in visual cortical neurons in vitro

The cellular mechanisms of spike-frequency adaptation during prolonged disc harges and of the slow afterhyperpolarization (AHP) that follows, as occur in vivo with contrast adaptation, were investigated with intracellular reco rdings of cortical neurons in slices of ferret primary visual cortex. Intra cellular injection of 2 Hz sinusoidal or constant currents for 20 sec resul ted in a slow (tau = 1-10 sec) spike-frequency adaptation, the degree of wh ich varied widely among neurons. Reducing either [Ca2+](o) or [Na+](o) redu ced the rate of spike-frequency adaptation. After the prolonged discharge w as a slow (12-75 sec) AHP that was associated with an increase in membrane conductance and a rightward shift in the discharge frequency versus injecte d current relationship. The reversal potential of the slow AHP was sensitiv e to changes in [K+](o), indicating that it was mediated by a K+ current. B lockade of transmembrane Ca2+ conductances did not reduce the slow AHP. In contrast, reductions of [Na+](o) reduced the slow AHP, even in the presence of pronounced Ca2+ spikes. We suggest that the activation of Na+-activated and Ca2+-activated K+ currents plays an important role in prolonged spike- frequency adaptation and therefore may contribute to contrast adaptation an d other forms of adaptation in the visual system in vivo.