Membrane and action potential responses evoked by excitatory amino acids acting at N-methyl-D-aspartate receptors and non-N-methyl-D-aspartate receptors in the rat thalamus in vivo.

The membrane potential responses and firing patterns of rat thalamic neurons evoked by iontophoretically applied excitatory amino acids were recorded in vivo. All excitatory amino acids, including N-methyl-D,L-aspartate, evoked a membrane depolarization and a repetitive, regular pattern of action potential firing in the thalamus. Both non-nociceptive and nociceptive thalamic neurons responded to all agonists tested. Iontophoretic application of magnesium ions selectively antagonized responses to N-methyl-D,L-aspartate but did not convert the repetitive firing pattern into a burst firing pattern. In contrast, in the hippocampus, N-methyl-D,L-aspartate evoked a burst pattern of action potential firing associated with rhythmic depolarizing membrane potential shifts, similar to those seen by other workers in the hippocampus and in other brain regions. These findings are discussed in relation to the possibility that the regular firing pattern of spikes evoked by excitatory amino acids in the thalamus is primarily determined by the intrinsic membrane properties of thalamic neurons.