LTD and LTP at the developing retinogeniculate synapse.

The purpose of the present study was to determine whether retinal activity can support long-term changes in synaptic strength in the developing dorsal lateral geniculate nucleus (LGN) of thalamus. To test for this we made use of a rodent in vitro explant preparation in which retinal afferents and the intrinsic circuitry of the LGN remain intact. We repetitively stimulated the optic tract with a tetanus protocol that approximated the temporal features of spontaneous retinal waves. We found the amplitude of extracellular field potentials evoked by retinal stimulation changed significantly after tetanus and that the polarity of these alterations was related to postnatal age. At a time when substantial pruning of retinal connections occurs (postnatal day 1 [P1] to P14), high-frequency stimulation led to an immediate and long-term depression (LTD). However, at times when pruning wanes and adult-like patterns of connectivity are stabilizing (P16 to P30), the identical form of stimulation produced a modest form of potentiation (long-term potentiation [LTP]). The LTD was unaffected by the bath application of gamma-aminobutyric acid type A and N-methyl-D-aspartate receptor antagonists. However, both LTD and LTP were blocked by L-type Ca(2+)-channel antagonists. Thus the Ca(2+) influx associated with L-type channel activation mediates the induction of synaptic plasticity and may signal the pruning and subsequent stabilization of developing retinogeniculate connections.