EXCITATORY AMINO-ACID RECEPTORS MODULATE HABITUATION OF THE RESPONSE TO VISUAL-STIMULATION IN THE CAT SUPERIOR COLLICULUS

In visual neurones of the superficial layers of the superior colliculu s (SSC), repetitive stimulation causes a progressive decline in the si ze of the response to the stimulus, usually known as response habituat ion or response adaptation. A mechanism has been proposed in which hab ituation results from coactivation of excitatory and inhibitory neuron es, and the responses of the inhibitory neurones block the response to subsequent stimulus presentations. Excitatory amino acid (EAA) neurot ransmitters mediate visual responses via NMDA and non-NMDA receptors i n cat SSC. We have investigated the role of these receptors in the gen eration of response habituation. Following the iontophoretic applicati on of the EAA antagonists CNQX, AP5 or CPP, repetitive visual stimulat ion paradigms which normally produce response habituation no longer do so. Indeed the response to each presentation of the stimulus is simil ar. Intravenous administration of the dissociative anesthetic ketamine (2-10 mg/kg) had similar actions to iontophoretically applied NMDA an tagonists. The data imply that intracollicular mechanisms activated by NMDA and non-NMDA receptors contribute to the generation of the inhib itory responses in SCC which lead to response habituation. Furthermore , the effects seen with ketamine anesthesia suggest that the use of ke tamine in studies of sensory systems may result in the lack of habitua tion.