Biochemical evidence for enhanced sensitivity to N-methyl-D-aspartate in the hippocampal formation of kindled rats

NMDA-sensitive L-[3H]glutamate binding in cerebral cortex of El mice

NMDA-sensitive L-[3H]glutamate binding was examined in the brains of El mice, a genetic animal model of epilepsy, and in ddY mice. In whole brain, Scatchard analysis showed that both stimulated and unstimulated El mice had significantly lower Bmax values for binding than did ddY mice. In regional studies, the binding of NMDA-sensitive L-[3H]glutamate was significantly less in the cerebral cortex of both stimulated and unstimulated El mice than in that of ddY mice. These data suggest that NMDA receptors may be involved in the genetic susceptibility of El mice to seizures.

Inhibition of muscarinic-coupled phosphoinositide hydrolysis by N-methyl-D-aspartate is dependent on depolarization via channel activation

The intent of this work was to elucidate the mechanism by which N-methyl-D-aspartate (NMDA) receptor agonists inhibit a second messenger system, namely, the stimulation of phosphoinositide (PI) hydrolysis activated by muscarinic cholinergic receptor agonists. NMDA inhibited cholinergic stimulation of PI hydrolysis in a dose- and time-dependent manner. NMDA exerts this effect indirectly through channel activation, because both MK-801 and N-[1-(2-thienyl)cyclohexyl]piperidine (TCP) prevented this action. Prevention of the NMDA effect by removal of sodium, but not calcium, from the incubation buffer suggested that depolarization may be the responsible mechanism. Depolarization alone proved sufficient to inhibit cholinergic activation of PI hydrolysis, because both veratridine and an elevated extracellular potassium level inhibited cholinergic stimulation of PI hydrolysis. The effect of NMDA appeared to require sodium flux through NMDA channels rather than through voltage-dependent sodium channels, because tetrodotoxin failed to inhibit the effect of NMDA. In correlative electrophysiologic experiments, NMDA profoundly inhibited evoked excitatory postsynaptic potentials and population action potentials of CA1 neurons, an effect almost certainly due to depolarization. The dose and time course of the electrophysiologic effects correlated well with the biochemical effects. Taken together, the data support the assertion that NMDA receptor activation inhibits PI hydrolysis by depolarization mediated by sodium flux through NMDA channels.

Kainate and quisqualate receptor autoradiography in rat brain after angular bundle kindling

The kainate and quisqualate types of excitatory amino acid receptor were visualized autoradiographically in brain sections from rats kindled by stimulating the angular bundle. Kainate receptors were labeled with [3H]kainate and quisqualate receptors with L-[3H]glutamate. When assayed one day after the last evoked seizure, kainate receptor binding had declined by 24-29% in stratum lucidum of hippocampal area CA3 and by 12-14% in the inner third of the dentate molecular layer, but was unchanged in the neocortex and basolateral amygdala. Saturation binding curves revealed that, under the conditions of these experiments, [3H]kainate labeled a single class of binding sites with a KD of 33-36 nM. In stratum lucidum of area CA3, kindling reduced the density of kainate receptors without altering their affinity for kainate. At the same time, quisqualate receptor binding had declined by 20-35% in many layers of the hippocampal formation and neocortex, but remained unchanged in the basolateral amygdala. Repeated stimulation or repeated seizures were required to produce these effects, since both kainate and quisqualate receptor binding were unchanged one day after a single afterdischarge. These receptor changes largely or completely reversed during a 28-day period without further stimulation. Thus maintenance of the kindled state probably cannot be explained by a long-lasting change in the expression of kainate or quisqualate receptors. The transient, regionally-selective down-regulation of these receptors may represent a compensatory response of forebrain neurons to repeated stimulation or seizures.

Kindling, prenatal exposure to ethanol and postnatal development selectively alter responses of hippocampal pyramidal cells to NMDA

Our studies suggest that treatments, such as kindling and exposure to ethanol in utero, which produce irreversible pathological changes in brain function also selectively alter neuronal responses to NMDA. We have identified increases in agonist potency, which may result from enhanced NMDA receptor expression, as well as both positive and negative modifications of Mg2+ regulation. There is no proof as yet that NMDA receptor plasticity accounts for the kindling phenomenon, for the cognitive deficits associated with fetal exposure to ethanol or for developmental events. However, all the effects we obtained are in the appropriate direction. The same functional parameters altered by kindling and prenatal exposure to ethanol are also modified during the normal development of the brain. It is tempting to speculate that these pathological stimuli produce their effects through the same mechanisms that would normally be activated by developmental signals.