Effects of excitatory amino acid antagonists on the activity of inferior colliculus neurons during sleep and wakefulness.

The contribution of N-methyl-D-aspartate to the response to sound of guinea pig inferior colliculus neurons was analyzed by recording single-unit activity before and after iontophoretic injection of a receptor specific antagonist, 2-amino-5-phosphonovaleric acid (AP5), during the sleep-waking cycle. The AP5 produced a significant firing decrease in most of the units recorded, while some neurons exhibited a particular decrease in the later part of the response. A latency reduction in one out of three units in paradoxical sleep was observed. A low proportion of them exhibited a significant firing increase. These actions were observed in wakefulness (W) as well as during sleep phases. We compared the action of kynurenic acid (Kyn) and the electrical stimulation of the auditory cortex on the same inferior colliculus neuron in anesthetized animals and during W. Both Kyn iontophoresis and cortical stimulation evoked similar changes, decreased firing rate in most inferior colliculus units, whereas a low proportion of them increased their discharge, in anesthetized guinea pigs and in W. Ascending as well as descending - efferent - glutamatergic fibers impinging on inferior colliculus neurons contribute to sound-evoked responses. The enhanced unitary activity observed in some neurons with after glutamatergic receptor blocking may indicate that polysynaptic pathways involving inhibitory neurons decreased their activity. These effects were observed in anesthetized and in behaving animals.