Interaction between cortico-tectal and retino-tectal inputs on single neurons of the cat's superior colliculus

The corticotectal projection of the rat in vitro: development, anatomy and physiological characteristics

In this study, the formation of the corticotectal projection of the rat in organotypic slice culture was investigated, using both anatomical and physiological approaches. The establishment of fibre connections from visual cortex to superior colliculus explants was monitored after 3, 6, 14, 20 and 30 days in vitro by cortical injections of Dil. As in cortical cultures without cocultured colliculus, fibres anterogradely labelled by this procedure spread radially from the injection site into the surroundings of the explant, without displaying any directional preference. Especially, layer V pyramidal cells could be seen to extend processes not only to the collicular target, but also in the opposite direction, suggesting that no axonal guidance was exerted by the projection target. The total number of fibres projecting in the direction of the colliculus was not higher than of those projecting in the opposite direction. However, fibres projecting into the colliculus were significantly longer. This was also the case when the colliculus was placed next to the pial side of the cortical explant, indicating that outgrowth direction was not related to this observation. We therefore assume a chemotrophic rather than a chemotactic influence of the projection target on cortical axons, which is based on direct contact of axons to the target tissue. It cannot be excluded, however, that the failure to detect chemotactic guidance was caused by the lack of diffusion gradients in our culture system. Innervation of the collicular slice exclusively originated from layer V pyramidal cells, irrespective of the position of the collicular target. Fibre courses suggested that discrimination of the projection target was achieved upon encounter with the collicular surface by direct membrane contact. Inside the collicular tissue, fibre arborizations occurred preferredly in up to three layers perpendicular to the surface. Even after the smallest tracer injections, termination fields were diffusely distributed over the collicular slice. Also, the spatial distribution of retrogradely stained projection neurons did not differ statistically from an equal distribution. Thus, a high degree of convergence and divergence was observed anatomically in the corticotectal projection formed in vitro, corresponding to the immature state in vivo. The functionality of the corticotectal projection was assessed by intracellular recordings from collicular neurons. Electrophysiological properties, such as membrane potential (-68 +/- 11 mV), membrane resistance (35.4 +/- 27.7 M omega) and the time constant (3.0 +/- 2.1 ms) were comparable to reference values, confirming the viability of our culture preparation. The functionality of corticotectal transmission was revealed by intracellularly recorded responses of collicular cells to extracellular cortical stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of kainic acid and other excitotoxins in the rat superior colliculus: relations to glutamatergic afferents

In this study we have performed surgical, chemical and combined surgical/chemical lesions in order to elucidate neurotransmitter mechanisms in the superior colliculus (SC) of albino rats. Visual cortex (VC) ablation reduced high affinity (HA) uptake of D-Asp by 32% in the deafferented SC. Local injection of kainic acid (KA) into SC reduced HA D-Asp uptake selectively in the lower dose range (less than 1 nmol) by 50-60%. The GABAergic marker glutamate decarboxylase (GAD) was decreased by maximally 60% only at doses exceeding 2 nmol. Choline acetyltransferase (ChAT), however, was not affected at any of the doses administered. VC ablation provided an almost complete protection against 1 nmol KA. When KA was injected 2 days prior to VC ablation an additive effect on HA D-Asp uptake of the two lesions was observed. From these observations we infer that the notion of a glutamatergic projection from VC to SC has been strengthened. Moreover, local neurons in intermediate layers account for about 60% of the HA D-Asp uptake in SC, and these are most likely impinged upon by the glutamatergic afferents. The neurotoxic effects of KA were compared with those of some suspected endogenous excitotoxins, i.e. N-methyl tetrahydrofolic acid (Me-THF), other folates and the tryptophan metabolite quinolinic acid (QA). N-methyl tetrahydrofolic acid, Me-THF (4 and 10 nmol) reduced HA D-Asp uptake by about 50%, only when coinjected with ascorbic acid. GAD and ChAT were not affected at either of the doses. QA was about 100-fold less potent than KA on a molar basis, and the maximal reduction of GAD was similar in QA and KA injected animals, whereas the maximal reduction of HA D-Asp was only 40% after QA injection in SC. We conclude that Me-THF, QA and KA exert their neurotoxic actions by different mechanisms as judged by the behavioral, histopathological and biochemical sequelae seen after local injections of the respective substances in intermediate layers of SC and corroborate data obtained from other brain areas.

An EM-autoradiographic analysis of the projection from cortical areas 17, 18, and 19 to the superior colliculus in the cat

The electron microscopic autoradiographic method was used to identify terminals of axons from cortical areas 17, 18, and 19 in the superficial layers of the superior colliculus. The results show that terminals of area 17 neurons contain round vesicles and made asymmetrical synaptic contacts predominantly onto one or more dendrites or dendritic appendages. Some profiles postsynaptic to labeled terminals contain vesicles and presumably are involved in serial synaptic arrangements. Terminals of area 18 and 19 neurons in the superficial collicular layers appear to comprise two populations, one similar in most respects to area 17 terminals, containing round vesicles and making asymmetrical contacts. The other contains pleomorphic vesicles and makes symmetrical contacts upon dendrites and dendritic appendages. These terminals rarely contact more than one postsynaptic profile, and rarely do the postsynaptic profiles contain vesicles. The two populations of area 18 and 19 terminals containing round and pleomorphic vesicles, respectively, are present in the ratio of approximately 3:1, although this ratio varies throughout the sublaminae of the superficial collicular layers. The presence of two distinct types of cortical terminals in the colliculus suggests that cortical modulation of collicular processing is more complex than was previously conceived.

Somatotopic organization of SI corticotectal projections in rats

The somatotopic organization of somatosensory (SI) cortical projections to the deep layers of the superior colliculus (SC) in rats was examined using neurophysiological micromapping methods. Electrical stimulation of SI produced SC unit responses at the same depths as did peripheral tactile stimulation, in SC laminae IV and VI; mapping of corticotectal projections was performed only in lamina IV. Antidromic activation of the pathway revealed that corticotectal cell bodies are located in SI layer V. Corticotectal projections were found to interconnect only those SI and collicular areas which have similar tactile receptive fields.

Response suppression induced by afferent stimulation in the superficial and deep layers of the hamster's superior colliculus

Stimulation of the optic chiasm (OX) or visual cortex (VC) elicited a burst of impulses from visual cells in the superficial layers of the hamster's superior colliculus which was followed by a period of response suppression which lasted from 50-200 ms. During this period responses to normally suprathreshold OX and VC shocks, spontaneous activity and even injury discharges were markedly attenuated. For approximately 50% of the visual cells tested VC stimulation also reduced responses to visual stimuli. No correlations between receptive field properties and whether or not VC shocks diminished a given cell's visual responses were noted. Stimulation of either the cervical spinal cord (SC) or somatic sensory cortex (SMCTX) evoked action potentials from somatosensory neurons in the deep tectal laminae. These responses were followed by a period of suppression identical to that seen in the superficial layers after OX or VC shocks. SMCTX stimulation attenuated responses to tactile stimuli for 30% of the cells tested in the deep layers. Again, no correlation was observed between somatosensory response characteristics and whether or not a given cell exhibited response suppression.

Cortical suppression of the ritino-collicular pathway in the monocularly deprived cat

1. In nine cats monocularly deprived from birth, the responses of single neurones in the superior colliculus contralateral to the deprived eye were studied. 2. In six animals most units could be driven by visual stimuli only through the ipsilateral (experienced) eye despite the fact that this colliculus receives a major input from the contralateral (deprived) retina. 3. Immediately following removal of visual cortex, including areas 17, 18 and 19, the collicular units could be driven by the deprived eye. 4. We conclude that the cortex must exert a powerful suppression of the retino-collicular input, and we argue that this suppression occurs in normal as well as in monocularly deprived animals. 5. In three animals the retinal input from the deprived eye was not suppressed but instead dominated many collicular celld, apparently to the exclusion of the cortical input from the experienced eye.

Influence of the visual cortex upon collicular evoked responses in the rabbit

In rabbit, the depression of the visual cortex by KCl showed an enhancement of the collicular evoked potentials. This enhancement revealed that the visual cortex had exerted a tonic type influence upon the superior colliculus.