[Function of substance-P projection from superior colliculus to lateral geniculate nucleus of rats]

Substance P immunoreactive terminals arising from the superior colliculus have been observed in the dorsal lateral geniculate nucleus of the rat. However, the functional role of this tachykininergic projection is still unknown. We studied the effects of substance P on voltage-dependent ion channels of lateral geniculate neurons in rat's in vitro brain slice with a single-electrode-voltage-clamp technique. It was found that substance P caused a depolarizing shift of membrane potential with a decrease of membrane conductance, suggesting that substance P depressed the linear potassium leak current. In addition, substance P inhibited the slowly inactivated potassium current, low threshold (T-) calcium current and inward rectifier (H- or Q-) current. Probably, the transient potassium (A-) current was also depressed by substance P. Therefore, the effects of substance P were considered to transfer activities of lateral geniculate neurons from burst response mode or oscillatory state to relay response mode, and to facilitate synaptic transmission, so that retinal visual signals could be relayed to the visual cortex with less distortion faithfully.