The induction mechanism of long-term potentiation (LTP) in developing visual cortex was studied by recording intracellular responses from layer III-IV cells in slice preparations of kitten visual cortex at 30-40 days after birth. 2. Strong stimulation of white matter produced a late depolarizing response after an orthodromic action potential. This depolarizing response was abolished by membrane depolarization or hyperpolarization caused by current injection through the recording electrode. In addition, this response was reduced by bath application of a low concentration (100 microM) of Ni2+ without any changes in the rising slope of the excitatory postsynaptic potential (EPSP) or orthodromic action potential. This suggests that this response is mediated by low-threshold Ca2+ channels (LTCs). 3. The involvement of LTCs in the induction of LTP was tested. White matter was stimulated at 2 Hz for 15 min as a conditioning stimulus to induce LTP, and the resultant changes in EPSPs were tested by low-frequency (0.1 Hz) stimulation of white matter. Conditioning stimulation produced a large N-methyl-D-aspartate (NMDA) receptor-mediated depolarizing response in these cells, which obscured the presence of the late depoliarzation. Therefore the test was conducted in a solution containing an NMDA antagonist 2-amino-5-phosphonovalerate (APV). 4. Weak conditioning stimulation, which evoked no LTC responses, never induced LTP; whereas strong conditioning stimulation, which evoked LTC responses, always induced LTP. Strong conditioning stimulation failed to induce LTP when LTC responses were prevented either by membrane depolarization or hyperpolarization or by a bath application of 100 microM Ni2+. 5. In a solution without APV, the application of Ni2+ also prevented the induction of LTP. 6. When cells were impaled by an electrode containing a Ca2+ chelator 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), LTP was never induced, even though LTC responses were evoked by conditioning stimulation. These results indicate that Ca2+ influx into postsynaptic cells through LTCs induces the LTP. 7. The responses mediated by LTCs, which were evoked by the injection of current pulses into the cells, were maximum at the critical period of visual cortical plasticity, suggesting that LTCs in postsynaptic cells regulate the plastic changes in developing visual cortex.
