Facilitation of low-frequency stimulation-induced long-term potentiation by endogenous noradrenaline and serotonin in developing rat visual cortex

T-type Ca2+ channel-dependent long-term potentiation (LTP) occurs predominantly during the critical period of ocular dominance plasticity in rat and cat visual cortex. Noradrenaline and serotonin are known to facilitate ocular dominance plasticity. In this study using rat visual cortical slices, we tested whether this LTP is modulated by these neuromodulators in the same way as ocular dominance plasticity. Extracellular field potentials evoked by layer 4 stimulation were recorded from layer 2/3 and LTP was induced by low-frequency (2 Hz) stimulation continued for 15 min. The induction of LTP was suppressed by β, but not α, adrenergic receptor antagonists. LTP induction was also inhibited by selective antagonists for 5-HT1A or 5-HT2 receptors. In slices prepared from rats in which noradrenaline or serotonin was depleted by selective neurotoxins, the magnitude of LTP was significantly smaller compared with control slices. These results indicate that the same types of adrenergic and serotonergic receptors facilitate both LTP and ocular dominance plasticity, supporting our hypothesis that T-type Ca2+ channel-dependent LTP mediates experience-dependent enhancement of visual responses of cortical neurons during the critical period.