Properties of LTD and LTP of retinocollicular synaptic transmission in the developing rat superior colliculus

The developing retinocollicular pathway undergoes synaptic refinement in order to form the precise retinotopic pattern seen in adults. To study the mechanisms which underlie refinement, we investigated long-term changes in retinocollicular transmission in rats aged P0-P25. Field potentials (FPs) in the superior colliculus (SC) were evoked by stimulation of optic tract fibers in an in vitro isolated brainstem preparation. High intensity stimulation induced long-term depression (LTD) in the SC after both low (1000 stimuli at 1 Hz) and higher (1000 stimuli at 50 Hz) frequency stimulation. The induction of LTD was independent of activation of NMDA and GABA(A) receptors, because d-APV (100 muM) and bicuculline (10 muM) did not block LTD. Induction of LTD was dependent upon activation of l-type Ca2+ channels as 10 muM nitrendipine, an l-type Ca2+ channel blocker, significantly decreased the magnitude of LTD. LTD was down-regulated during development. LTD magnitude was greatest in rats aged P0-P9 and significantly less in rats aged P10-P25. Long-term potentiation (LTP) was induced by low intensity stimulation and only after high frequency tetanus (1000 stimuli at 50 Hz). LTP was NMDA receptor dependent because d-APV (100 muM) completely abolished it. LTP induction was also blocked by the l-type Ca2+ channel blocker nitrendipine. The magnitude of LTP first increased with age, being significantly greater at P7-P13 than at P0-3 and then decreased at P23-25. In summary, both LTD and LTP are present during retinocollicular pathway refinement, but have different transmitter and ionic mechanisms and time courses of expression.