Recruitment of inhibition by enhanced activation of synaptic NMDA responses in the rat cerebral cortex

Intracellular recordings of layer V neurons from rat neocortical slices were obtained to examine the effects of reducing extracellular magnesium on inhibition. Magnesium-free solutions induced interictal and ictal-like events in cortical neurons. Changes in synaptic events underlying epileptogenesis were studied when extracellular calcium was raised (from 2 to 3-7 mM) since this delayed seizure activity. With increasing time of exposure of cells to magnesium-free solutions, there was a significant increase in the size and duration of both the depolarizing and slow synaptic hyperpolarizing responses, but the fast synaptic hyperpolarization significantly declined in amplitude. When cells were recorded with cesium acetate-filled microelectrodes slow hyperpolarizing responses were blocked, but depolarization of cells to 0 mV allowed an isolated fast hyperpolarizing response to be recorded following synaptic stimulation. The amplitude of this response was unchanged after exposure to magnesium-free solutions. Synaptic responses of cells initially bathed in an N-methyl-D-aspartate (NMDA) antagonist (CPP) were unchanged by subsequent exposure to magnesium-free solutions. CPP exposure by itself caused a decrease in depolarization duration, increase in fast hyperpolarizing amplitude, and decrease in slow hyperpolarization amplitude and duration. When the fast hyperpolarization was viewed in isolation (cesium recording electrodes) at 0 mV, the amplitude of this event was unchanged by exposure to CPP. Given these results stimulus-response characteristics of neocortical neurons were reassessed under control conditions. With higher intensity stimuli larger depolarizing and slow hyperpolarizing responses were evoked, but the fast hyperpolarization showed a decremental response. These effects were reversed when CPP was added. When NMDA activity was enhanced by exposure to magnesium-free solutions or electrical stimulation, the amplitude of excitatory events and slow hyperpolarizations increased, but fast inhibitory responses showed limited capacity for incremental recruitment. This suggests fast inhibition is saturated (maximal) at submaximal levels of excitation, and can be overcome by increasing levels of excitation. Such a process is active under physiological conditions, altering the efficacy of inhibition.