Two mechanisms underlying the induction of long-term potentiation in motor cortex of adult cat in vitro

Long-term potentiation of synaptic transmission (LTP), as documented by the enhancement of evoked field potentials in layer III following stimulation of the underlying white matter, has been studied in slices of motor cortex from adult cats. With a 1 M NaCl-filled recording electrode, LTP was induced only in one out of eight slices. When the recording electrode in addition contained 5 mM bicuculline metiodide, LTP was obtained with a much higher rate of success (15/19), suggesting that reduction of GABA_A receptor-mediated inhibition facilitated the induction of LTP in the motor cortex. Bath application of dl-2-amino-5-phosphonovaleric acid (APV, 100 μM) or Ni²⁺ (100 μM) significantly reduced the success rate for LTP occurrence (6/16 and 5/16, respectively); but when LTP was induced, it did not show significant change in magnitude and time course. In slices perfused with APV (100 μM) plus Ni²⁺ (100 μM), LTP induction was completely blocked (0/12). These results suggest that two different mechanisms may subserve LTP induction in the cat motor cortex: one is mediated by N-methyl-d-aspartate receptors and can be blocked by APV; the other may be mediated by low-threshold calcium channels and can be blocked by Ni²⁺.