Corticofugal influences on visual responses in cat superior colliculus: The role of NMDA receptors

The role of N-methyl-D-aspartate (NMDA) receptors in the mediation of cortical inputs to visual neurones in the superficial layers of the superior colliculus (SSC) has been investigated. Extracellular recording with iontophoresis in the SSC of cortically intact cats has demonstrated that visual responses of most neurones were reduced by iontophoretic application of the NMDA receptor antagonist D-2-amino-5-phosphonopentanoate (APS). Following inactivation of areas 17 and 18 of the visual cortex with topical lignocaine, the visual responses of 11, previously AP5-sensitive, neurones were no longer reduced by APS ejection. The cortical input is generally assumed to influence the directional responses of visual neurones in SSC. However, detailed study of the directional bias showed that the degree of directional tuning in SSC neurones was similar to that of retinal ganglion cells, as previously described by others. Moreover, inactivation of the visual cortex with topical lignocaine did not alter the directional bias of SSC neurones. Likewise, the directional bias of SSC neurones was not reduced by iontophoretic ejection of APS in the SSC. These data imply that NMDA receptors have an important role in mediating the cortical input to the SSC. However, cortical input does not appear to be responsible for conferring directional bias upon SSC neurones' visual responsiveness.

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 colliculus (SSC), repetitive stimulation causes a progressive decline in the size of the response to the stimulus, usually known as response habituation or response adaptation. A mechanism has been proposed in which habituation results from coactivation of excitatory and inhibitory neurones, and the responses of the inhibitory neurones block the response to subsequent stimulus presentations. Excitatory amino acid (EAA) neurotransmitters mediate visual responses via NMDA and non-NMDA receptors in cat SSC. We have investigated the role of these receptors in the generation of response habituation. Following the iontophoretic application of the EAA antagonists CNQX, APS or CPP, repetitive visual stimulation paradigms which normally produce response habituation no longer do so. Indeed the response to each presentation of the stimulus is similar. Intravenous administration of the dissociative anesthetic ketamine (2–10 mg/kg) had similar actions to iontophoretically applied NMDA antagonists. The data imply that intracollicular mechanisms activated by NMDA and non-NMDA receptors contribute to the generation of the inhibitory responses in SCC which lead to response habituation. Furthermore, the effects seen with ketamine anesthesia suggest that the use of ketamine in studies of sensory systems may result in the lack of habituation.

SENSORY INPUTS MEDIATING TWO OPPOSITE BEHAVIOURAL RESPONSES TO LIGHT IN THE CRAYFISH PROCAMBARUS CLARKII

Crustaceans are attracted out of their burrows by dim light whereas they withdraw in response to light of high intensity. We have studied attraction and withdrawal responses in freely walking crayfish (Procambarus clarkii) and in animals tethered above a treadmill device. Electrophysiological recordings have been made of retinal and extraretinal visual units to determine their possible inputs Attraction was induced by light of 0.17-1.41x; the crayfish walked forward with the abdomen extended. Withdrawal was observed at intensities above 5.6lx; the crayfish walked backwards with a cyclic pattern of flexion and extension of the tail. A group of sustaining visual neurones of the optic nerve with low thresholds was found to respond to light in the range of intensities that produced attraction. Two groups of neurones responded in the range of intensities that produced withdrawal: sustaining visual neurones with high thresholds and the caudal photoreceptors. In animals in which the visual fields of the high-threshold fibres had been covered with black paint, the attraction response was not affected, while withdrawal was only observed in response to light stronger than 11 lx. When tested on a treadmill, the latency of the responses to light and the direction of walking of crayfish reflected attraction and withdrawal responses. Moreover, under these conditions, illumination of the eye induced forward walking at intensities that produce attraction and backward walking at intensities that produce withdrawal. In contrast, illumination of the tail induced only backward walking

Development of visual inhibitory interactions in kittens

This study was designed to monitor the development of inhibitory interactions elicited in the cat visual system by oriented visual stimuli. Steady-state visual-evoked potentials (VEPs) were recorded from the scalp of 11 behaving and alert kittens while they viewed contrast-reversed sinusoidal gratings. In adult cats, the form of VEP contrast-response curves (the amplitude of second harmonic modulation as a function of stimulus contrast) was modified by superimposing a mask grating on the test. Parallel masks displaced the curves to a higher contrast region (probably via contrast gain-control mechanisms), increasing contrast threshold without affecting the slope of the curve. Orthogonal gratings, on the other hand, decrease the slope of the curve without affecting threshold (so called cross-orientation inhibition: Morrone et al., 1981). These effects are similar to those previously reported in human VEPs (Morrone & Burr, 1986; Burr & Morrone, 1987) and single cortical cat cells (Morrone et al., 1982). For young kittens of 20 days, the orthogonal mask had no effect whatsoever on the response curves, and the effect of the parallel mask was much less than for adult cats. At about 40 days, the orthogonal mask began to attenuate responses multiplicatively, and by 50 days the amount of multiplicative attenuation had reached adult levels. The effect of the parallel mask (as indicated by the increase in threshold elevation) increased gradually from 20–50 days. The results are consistent with the existence of at least two types of inhibition in cat visual neurones that develop at different rates.

Calcium-Dependent Action Potentials in the Second-Order Neurones of Cockroach Ocelli

The ionic basis of the action potential in the large second-order neurones (L-neurones) of the ocellus of the cockroach, Periplaneta americana, was studied. L-neurones generated action potentials, usually once, at the off-set of hyperpolarizing light responses, or at the termination of hyperpolarizing current stimuli. The action potential was blocked by replacing saline Ca2+ with Mg2+ but maintained when Ba2+ was substituted. A block was produced by 2 mmoll l−1 Cd2+ or 20 mmol l−l Co2+. The peak amplitude of the action potential increased by 26 mV for a 10-fold increase in external Ca2+ concentration, at concentrations below 1.8 mmol l−1. The action potential was not affected by sodium-free saline or by 3×10−6mol l−1 tetrodotoxin (TTX). These observations suggest that calcium ions are the major carrier for the inward current of the action potential. This finding supports the suggestion that the off-set responses of hyperpolarizing visual neurones of both vertebrates and invertebrates have a common ionic mechanism, including voltage-sensitive calcium currents.