THE NITRIC-OXIDE SYNTHASE INHIBITOR, N-MONOMETHYL-L-ARGININE BLOCKS INDUCTION OF A LONG-TERM POTENTIATION-LIKE PHENOMENON IN RAT MEDIAL FRONTAL CORTICAL-NEURONS IN-VITRO

1. Nitric oxide has been implicated in the production of long-term depression (LTD) in the cerebellum and in the production of long-term potentiation (LTP) and LTD in the hippocampus. We now provide evidence of its involvement in the induction of long-term synaptic potentiation in in vitro slices in the cerebral cortex of the rat. 2. Intracellular recordings were made from layer V neurons in the medial frontal cortex, and excitatory synaptic potentials (EPSPs) were evoked by electrical stimulation of layers II/III. Tetanic stimulation of this pathway may induce LTD or LTP or no change at these synapses. First we established experimental conditions under which a long lasting potentiation could be induced with a high incidence (>60%), namely perfusion of slices with 1 muM bicuculline methiodide, second the use of increased shock duration in the tetanic conditioning stimuli, third and most important the addition of QX-314 to the microelectrode to reduce potassium conductances. Because the potentiation of the mean EPSP slope was significantly greater than the control at 40-min postconditioning, but was declining throughout this period, we refer to it for brevity as LTP, but strictly class it as an LTP-like phenomenon. 3. The nitric oxide (NO) synthase inhibitor interfered with the production of LTP. In the control group of neurons (n = 13) the mean depolarizing slope of the EPSP at 30-min post-conditioning was 142.7 +/- 2% (mean +/- SE) of the prestimulation control. In contrast, in the group of cells (n = 12) in slices preincubated with the nitric oxide synthase inhibitor, N-monomethyl-L-arginine (L-NMMA) at 100 muM for >45 min, the mean slope of the EPSP at 30-min post-tetanus was 109% (+/- 1.7%). The difference between the control and the L-NMMA groups was significant at P < 0.01 (t test, two-tailed). The probability of induction of LTP was also greatly reduced by the synthase inhibitor while the probability of the conditioning producing no change or LTD was increased and that of producing LTD was also increased. The change in incidence of LTP was significantly different between the two groups at P < 0.05 (chi2 test). 4. Following unsuccessful attempts to induce long-term potentiation in the presence of the nitric oxide synthase inhibitor, in five slices the L-NMMA was replaced with perfusate containing the substrate L-arginine. Subsequently, long-term potentiation was induced in 2 of the 5 slices, thus reversing the effect of blockade in those two cells. 5. We conclude that nitric oxide synthase is involved in the production of a LTP like change at synapses on layer V cells in medial frontal cortex and implicate nitric oxide as a transcellular messenger in synaptic plasticity in the neocortex, as in the cerebellum and hippocampus.