Complementary and non-matching afferent compartments in the cat's superior colliculus: innervation of the acetylcholinesterase-poor domain of the intermediate gray layer

Evidence for intrinsic expression of enkephalin-like immunoreactivity and opioid binding sites in cat superior colliculus

We have investigated the cellular localization of opioid peptides and binding sites in the cat's superior colliculus by testing the effects of retinal deafferentation and intracollicular excitotoxin lesions on patterns of enkephalin-like immunostaining and opiate receptor ligand binding. In normal cats, enkephalin-like immunoreactivity marks a thin tier in the most dorsal stratum griseum superficiale, small neurons of the stratum griseum superficiale, and patches of fibers in the intermediate and deeper gray layers. Eliminating crossed retinotectal afferents by contralateral eye enucleation had little immediate effect on this pattern, although chronic eye enucleation from birth did reduce immunoreactivity in the superficial layers. By contrast, fiber-sparing destruction of collicular neurons by the excitotoxins N-methyl-D-aspartate and ibotenic acid virtually eliminated enkephalin-like immunoreactivity in the neuropil of the upper stratum griseum superficiale, presumably by killing enkephalinergic cells of the superficial layers. Such lesions did not eliminate the patches of enkephalin-like immunoreactivity in the deeper layers. In normal cats, opiate receptor ligand binding is dense in the stratum griseum superficiale, particularly in its upper tier, and moderately dense in the intermediate gray layer. Contralateral eye removal had no detectable effect on the binding pattern, but excitotoxin lesions of the colliculus dramatically reduced binding in both superficial and deep layers. Some ligand binding, including part of that in the upper stratum griseum superficiale, apparently survived such lesions. Similar effects were observed in the lateral geniculate nucleus: enucleation produced no change in binding, whereas excitotoxin lesions greatly reduced specific opiate binding. We conclude that in the superficial collicular layers, both enkephalin-like opioid peptides and their membrane receptors are largely expressed by neurons of intrinsic collicular origin. The close correspondence between the location of these intrinsic opioid elements and the tier of retinal afferents terminating in the upper stratum griseum superficiale further suggests that opiatergic interneurons may modulate retinotectal transmission postsynaptically.

Parvalbumin in rat superior colliculus

Parvalbumin-like immunoreactivity (PA-LI) has been studied in sections of the superior colliculus (SC) of the rat and its distribution compared to the patterns of acetylcholinesterase (AChE) and cytochrome oxidase (CO) staining. In the intermediate layers it was found that PA-LI is spatially associated with AChE only in the medial part of the SC, but assumes a complementary distribution further laterally. There was a positive correlation between PA-LI and CO. We conclude that the patterns of PA-LI and CO are not systematically related to collicular input known to be associated with the AChE-rich zones, but may reflect adherence to channel separation beyond the terminal fields of clustered afferents.

The projection from different visual cortical areas to the rat superior colliculus

The rat occipital cortex contains a number of morphologically and physiologically distinct visual areas. Retrograde tracing studies have shown that most if not all of these areas project in a topographic fashion to the ipsilateral superior colliculus (SC). In the present study, small amounts of wheat germ agglutinin-conjugated HRP (WGA-HRP) were injected into adult rat occipital cortex to determine how afferents from the different visual cortical areas are distributed within the various layers of the SC. Cytoarchitectonic criteria were used to help establish the location of the WGA-HRP injections in the cortex. As a further aid to identifying the sites of injection, the distribution of retrogradely labelled cells within the thalamus was mapped in each brain. Analysis revealed a surprising range of visuocortical projections to the rat SC, with input to the majority of tectal laminae. Area 17 projected most heavily to the dorsal stratum opticum (SO) and lower half of stratum griseum superficiale (SGS) with lighter label extending up to the collicular surface. Axons and terminals from area 18 formed two horizontal tiers, one in the middle of the stratum griseum intermediale (SGI) and the other at the border between the stratum album intermediale (SAI) and the stratum griseum profundum (SGP). Periodic puffs of label extended between these horizontal tiers, with a periodicity of 300-400 microns. There was some variability in the labelling pattern in the superior colliculus after area 18a injections, perhaps because this cytoarchitectonic area contains multiple representations of the visual field. Generally the projection from the lateromedial, laterointermediate, and laterolateral parts of 18a was heaviest in the lower half of SO and upper regions of SGI. Lighter label extended up into the lower half of SGS. In the SGI it was common to find periodic puffs of terminal label interspersed with areas devoid of innervation. This periodic pattern was particularly noticeable after WGA-HRP injections into anterior 18a. Two horizontal tiers of label were located at the SO/SGI border and SGI/SAI border (the dorsal being the most dense) with patches of label extending between these tiers every 230-250 microns. There did not appear to be a significant corticotectal projection from the posterior part of the lateral extrastriate region.

Output pathways from the rat superior colliculus mediating approach and avoidance have different sensory properties

Neuroanatomical studies have demonstrated that the two major descending pathways from the superior colliculus arise from regionally segregated, distinct, cells of origin. Stimulation and lesion studies have implicated the crossed descending tecto-reticulo-spinal projection in approach movements towards novel stimuli whereas the ipsilateral pathway appears to be involved in the control of avoidance and escape-like behaviours. The present electrophysiological study attempted to characterise the sensory properties of antidromically identified cells of origin of these pathways in anaesthetised rats. We found that the contralaterally projecting predorsal bundle (PDB) efferents were primarily somatosensory while the ipsilateral cuneiform (CNF) projection was primarily visual. PDB cells, mainly found in the intermediate layers, responded principally to vibrissal stimulation with their overlying visual fields optimally stimulated by small dark moving objects in the lower rostral and lateral field. In contrast, most CNF cells were located rostromedially, with the greatest contribution from visual cells responsive to stimuli in the upper rostral field. A significant proportion of these showed no response to small moving dark discs but fired vigorously to 'looming' stimuli. Ethological considerations suggest that these are appropriate stimulus characteristics for a system controlling approach and avoidance behaviour in an animal such as the rat where predators generally appear from above and prey is found on the ground.

Choline acetyltransferase-like immunoreactivity in the superior colliculus of the cat and its relation to the pattern of acetylcholinesterase staining

Choline acetyltransferase, the biosynthetic enzyme for acetylcholine, is thought to be a marker for cholinergic neurons. This report presents an analysis of the pattern of choline acetyltransferase-like immunoreactivity in the superior colliculus of the cat. A dense network of highly varicose immunoreactive fibers pervaded the superficial gray and optical layer. The density of the fiber network in the superficial layers was heterogeneous, forming a mosaic pattern with a period of about 400 microns. The antigen was also located in numerous small perikarya embedded in this network. This neuronal population reached a density of 2,000 cells/mm3 of the superficial gray layer and suggested the presence of a substantial cholinergic system originating in the superior colliculus. A detailed comparison was made between the pattern of choline acetyltransferase-like immunoreactivity and the distribution of acetylcholinesterase activity. By comparisons of adjacent sections, both staining patterns were found to be similar in all collicular layers. In particular, the compartmental distribution of immunoreactivity in the intermediate collicular layers seemed to mimic the pattern of acetylcholinesterase staining. A double-staining technique demonstrated a near-perfect correlation between the two patterns. In conclusion, there was no indication of heightened acetylcholinesterase activity without an associated elevation in choline acetyltransferase-like immunoreactivity throughout the superior colliculus. In this part of the brain, the presence of the putative cholinergic terminals could fully account for the distribution of acetylcholinesterase activity.

Organization of the crossed tecto-reticulo-spinal projection in rat--II. Electrophysiological evidence for separate output channels to the periabducens area and caudal medulla

The previous paper (Redgrave et al., Neuroscience 37, 571-584, 1990) presented anatomical evidence indicating there are at least two largely segregated components of the crossed tecto-reticulo-spinal pathway which project to the periabducens area and caudal medulla. An immediate question arising from this finding is whether tectal cells which project either to the periabducens area or to the caudal medulla have different electrophysiological response properties. An answer to this question would be relevant to the issue of whether different components of the tecto-reticulo-spinal system are specialized for the production of different classes of orienting movement. Accordingly, extracellularly recorded units in the superior colliculus of urethane anaesthetized rats were tested for antidromic activity following electrical stimulation of the periabducens area or the caudal medulla. When antidromic potentials were successfully recorded the sensory properties of the units were tested with a range of unimodal visual, somatosensory and auditory stimuli. The following results were obtained. (i) Tectal cells antidromically activated by stimulation of the caudal medulla were preferentially sensitive to somatosensory stimuli from the perioral region, while cells activated from the periabducens area were more frequently responsive to auditory stimuli. (ii) Tectal fibres activated by stimulation of the caudal medulla had significantly higher conduction velocities than the fibres activated by electrodes in the periabducens region. (iii) More than 90% of antidromically activated cells were located in stratum album intermediale or dorsal stratum profundum. These electrophysiological findings confirm and extend previous anatomical observations which indicate that components of the crossed descending projection of the colliculus may be functionally specialized for the production of different classes of orienting movements.

Cholinergic innervation of the superior colliculus in the cat

The superficial and intermediate gray layers of the superior colliculus are heavily innervated by fibers that utilize the neurotransmitter acetylcholine. The distribution, ultrastructure, and sources of the cholinergic innervation of these layers have been examined in the cat by using a combination of immuno-cytochemical and axonal transport methods. Putative cholinergic fibers and cells were localized by means of a monoclonal antibody to choline acetyltransferase (ChAT). ChAT immunoreactive fibers are distributed throughout the depth of the superior colliculus, with particularly dense zones of innervation in the upper part of the superficial grey layer and in the intermediate grey layer. Within the superficial grey layer, the fibers form a continuous, dense band, whereas within the intermediate grey layer the fibers are arranged in clusters or patches. Although the patches are present throughout the rostrocaudal extent of the superior colliculus, they are most prominent in middle to caudal sections. The structure of the ChAT immunoreactive terminals was examined electron microscopically. The appearance of the terminals is similar in the superficial and intermediate grey layers. They contain closely packed, mostly round vesicles, and form contacts with medium-sized dendrites that exhibit small, but prominent postsynaptic densities; a few of the terminals contact vesicle-containing profiles. To identify the sources of the cholinergic input to the superior colliculus, injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made in the superior colliculus and the sections were processed to demonstrate both the retrograde transport of WGA-HRP and ChAT immunoreactivity. Neurons containing both labels were found in the parabigeminal nucleus, and in the lateral dorsal and pedunculopontine tegmental nuclei of the pontomesencephalic reticular formation. Almost every cell in these nuclei that contained retrograde label was also immunoreactive for ChAT. The similarities between the laminar distributions of the ChAT terminals and the terminations of the pathway from the parabigeminal nucleus (Graybiel: Brain Res. 145:365-374, '78) support the view that the latter nucleus is a source of the cholinergic fibers in the superficial grey layer. The possibility that the pedunculopontine tegmental nucleus is a source of cholinergic fibers in the deep layers was tested by examining the distribution of labeled fibers following injections of WGA-HRP into this region of the tegmentum. Patches of labeled terminals were found in the intermediate grey layer that resemble in distribution the patches of ChAT immunoreactive fibers in this layer.(ABSTRACT TRUNCATED AT 400 WORDS)

Event or emergency? Two response systems in the mammalian superior colliculus

Recent studies of the effects of stimulating the superior colliculus (SC) in rodents suggest that this structure mediates at least two classes of response to novel sensory stimuli. One class contains the familiar orienting response, together with movements resembling tracking or pursuit, and appears appropriate for undefined sensory 'events'. The second class contains defensive movements such as avoidance or flight, together with cardiovascular changes, that would be appropriate for a sudden emergency such as the appearance of a predator, or of an object on collision course. The two response systems appear to depend on separate output projections, and are probably subject to different sensory and forebrain influences. These findings (1) suggest an explanation for the complex anatomical organization of the SC, with multiple output pathways differentially accessed by a very wide variety of inputs, (2) emphasize the similarities between the SC and the optic tectum in non-mammalian species, and (3) suggest that the SC may be useful as a model for studying both the sensory control of defensive responses, and how intelligent decisions can be taken about relatively simple sensory inputs.

Neuroanatomical studies of the nigrotectal projection in the cat

We have used retrograde and anterograde transport methods to analyze the nigrotectal projection in the cat. This projection arises from both pars reticulata (SNr) and pars lateralis (SNl) and distributes to all cellular laminae of the superior colliculus. This extensive nigrotectal innervation is not a simple, single circuit. Rather it appears to consist of several parallel channels, with each taking origin from a particular zone of the substantia nigra and terminating within specific collicular laminae and/or sublaminae. For instance, only neurons within the SNl project to the stratum griseum superficiale; such neurons also project diffusely to all other tectal laminae. Cells in the most lateral portion of the SNr project to a horizontal, patchy tier in the interface region between the stratum opticum and the stratum griseum intermediate (SGI). Finally, more medially placed neurons within the SNr project to a horizontal patchy tier within the middle of the SGI and to a wedge-shaped locus in the stratum griseum profundum. Our findings provide an anatomical substrate for electrophysiological data (Karabelas and Moschovakis: J. Comp. Neurol. 239: 309-329, '85) showing a widespread distribution of nigrorecipient tectal neurons in the cat.

Spatial relation of the acetylcholinesterase-rich domain to the visual topography in the feline superior colliculus

The superior colliculus (SC) of the cat shows a prominent compartmentalized organization at the level of its intermediate layers. The mosaic of these compartments is apparent in the pattern of acetylcholinesterase (AChE) staining. Patches of high AChE-activity are sharply set off from surrounding areas in the caudal SC while they are less distinct anteriorly. The rostral part lacks such obvious compartments. Thus, a structural reorganization apparently cuts across the topographical representations spread out in the SC. In order to test if this compartmental gradient relates to the topographic maps of the colliculus, retinotopic landmarks were visualized in the superficial layers by labeling the retinotectal pathway. In the SC ipsilateral to the eye injected with horseradish peroxidase (HRP) a paucity of labeling indicated the zone representing the ipsilateral visual half-field. Serial reconstructions of collicular sections, cut longitudinally or tangentially, revealed that the non-compartmentalized part of the intermediate layers corresponds to the representation of the ipsilateral visual half-field in the layers above, while an intricate mosaic array of compartments prevail in tectal zones related to the representation of the contralateral visual half-field.

Origin of high acetylcholinesterase activity in the mouse superior colliculus

The acetylcholinesterase activity in the colliculus mainly occurs in two layers and is arranged as a lattice in the intermediate grey layer and as a continuous sheet in the superficial grey layer. Undercutting lesions abolish the lattice in the intermediate grey layer but leave the superficial sheet of activity intact. By contrast the injection of kainic acid into the colliculus leaves the intermediate layer lattice intact while causing a local reduction in the superficial layer. Injections of the retrograde tracer Fluoro-Gold into the colliculus labels cells in the pedunculopontine and laterodorsal tegmental nuclei that contains acetylcholinesterase. Cells in the parabigeminal nucleus are also labelled but these cells contain low levels of cholinesterase. Thus, it is concluded that the lattice in the intermediate layers is mainly dependent on afferents from the laterodorsal tegmental and pedunculopontine nuclei while the sheet in the superficial layers is mainly dependent on intrinsic cells.

Uptake, release, and modulation of release of noradrenaline in rabbit superior colliculus

The noradrenaline content, the uptake of [3H]noradrenaline, and the release of previously incorporated [3H]noradrenaline were studied in slices of rabbit superior colliculus. The concentration of endogenous noradrenaline was higher in superficial than in deep layers of the superior colliculus. Upon incubation with [3H]noradrenaline, tritium was accumulated by a mechanism that was strongly inhibited by oxaprotiline but little inhibited by 6-nitroquipazine. Electrical stimulation at 0.2 or 3 Hz increased the outflow of tritium from slices preincubated with [3H]noradrenaline; the increase was almost abolished by tetrodotoxin or a low calcium medium. Clonidine reduced the evoked overflow of tritium, whereas yohimbine increased it and antagonized clonidine. The evoked overflow was also reduced by the dopamine D2-receptor-selective agonists apomorphine and quinpirole, an effect antagonized by sulpiride. The preferential opioid kappa-receptor agonist ethylketocyclazocine produced an inhibition that was counteracted by naloxone. Nicotine accelerated the basal outflow of tritium; part of the acceleration was blocked by hexamethonium. The muscarinic agonist oxotremorine slightly diminished the electrically evoked overflow, and its effect was abolished by atropine. The oxaprotiline-sensitive uptake of [3H]noradrenaline as well as the tetrodotoxin-sensitive and calcium-dependent overflow of tritium upon electrical stimulation (presumably reflecting the release of [3H]noradrenaline) indicate that noradrenaline is a neurotransmitter in the superior colliculus. The release of [3H]noradrenaline is modulated through alpha 2-adrenoceptors as well as dopamine D2-receptors, opioid kappa-receptors and nicotine and muscarine receptors. No clear evidence was found for modulation through beta-adrenoceptors, D1-receptors, serotonin receptors, opioid mu- or delta-receptors or receptors for GABA or glutamate. Only the alpha 2-adrenoceptors receive an endogenous agonist input, at least under the conditions of these experiments. The pattern of presynaptic modulation resembles that found for noradrenaline release in other rabbit brain regions, suggesting that all noradrenergic axons arising in the locus coeruleus possess similar presynaptic receptor systems.

Lattices of high histochemical activity occur in the human, monkey, and cat superior colliculus

A lattice of high oxidative metabolic activity occurs in the intermediate gray layer of the human, monkey, and cat superior colliculus. It is composed of a matrix of high enzyme activity that surrounds pale islands or bands of lower activity. In the human the pale bands are 300-400 micron wide while in the smaller colliculi of the monkey and cat they are 100-200 micron wide. The lattice was demonstrated by studying either cytochrome oxidase or succinate dehydrogenase. In the cat and monkey the lattice occurs at the same depth as the lattice of intense acetylcholinesterase activity, but the two lattices are not in spatial register. In the human the lattice of high oxidative metabolic activity is in the middle of the intermediate gray layer, whereas the lattice of intensely stained cholinesterase activity is at the base of this layer, but again the two lattices are not in spatial register. However, in the middle of the intermediate gray layer of the human, there are elongated islands and bands of very low acetylcholinesterase activity that coincide with the pale islands and bands of low cytochrome oxidase activity. An additional lattice of high enzyme activity occurs based on the enzyme nicotinamide dinucleotide phosphate (reduced form)-diaphorase. This lattice is prominent in the cat, occurs more faintly in the monkey, but did not appear to be present in the human. In the intermediate gray layer it had a high degree of overlap with the acetylcholinesterase lattice. The lattice of high oxidative metabolism contains loosely knit clusters of large multipolar cells containing high cytochrome oxidase activity and these cells do not occur in the pale islands. By contrast the cell bodies in the intermediate gray layer that contain either acetylcholinesterase or the diaphorase occur both between and within the patches of corresponding, high enzyme activity. It is suggested that the acetylcholinesterase and diaphorase lattices are mainly associated with afferent fibers while the lattice of high oxidative metabolism is mainly associated with intrinsic cells. The lattices occur in all mammals studied to date and appear to represent a fundamental principle in the organization of the mammalian colliculus. It is concluded that the lattices will provide a useful basis for further studies of the relationship between the many afferent and efferent modules thought to exist in this structure.

The nigrotectal projection and tectospinal neurons in the rat. A light and electron microscopic study demonstrating a monosynaptic nigral input to identified tectospinal neurons

In order to investigate the nigro-tecto-spinal pathway in the rat, the pattern of termination of nigrotectal fibres and the distribution of tectospinal neurons have been investigated in a light and electron microscopic study of the superior colliculus. In addition, the pattern of termination of nigrotectal fibres was compared to the pattern of acetylcholinesterase staining. The light microscopic studies showed that the nigrotectal fibres, which had been identified by anterograde transport of horseradish peroxidase from the substantia nigra, terminated in a distinctive clustered pattern throughout the rostrocaudal extent of the stratum griseum intermedium, stratum album intermedium and adjacent dorsal portion of the stratum griseum profundum of the ipsilateral superior colliculus. The clusters of nigrotectal terminals formed a series of branching, interconnected longitudinal columns which largely corresponded with the pattern of acetylcholinesterase staining. The tectospinal neurons, which had been identified by retrograde transport of horseradish peroxidase from the spinal cord, had mainly large-sized somata, were stellate in shape with multiple long dendrites, and formed variable-sized clusters of 4-15 neurons within lateral regions of the ventral stratum album intermedium and dorsal stratum griseum profundum. In experiments where both the nigrotectal terminals and the tectospinal neurons were labelled by the transport of horseradish peroxidase, the clusters of tectospinal neurons largely corresponded with the regions of densest nigrotectal fibre termination in the lateral regions of the superior colliculus. In addition, a small contralateral nigrotectal projection was localized in the rostrolateral region of the superior colliculus where the crossed fibres terminated in a clustered pattern in alignment with clusters of tectospinal neurons in this region. Electron microscopic examination of the superior colliculus following ibotenic acid lesions in the substantia nigra and horseradish peroxidase injections in the spinal cord showed multiple degenerating nigrotectal boutons in synaptic contact with the soma and the mainstem and secondary dendrites of labelled tectospinal neurons in the lateral regions of the stratum album intermedium and stratum griseum profundum of the superior colliculus. The majority of the degenerating nigrotectal boutons showed electron-lucent degenerative changes and were in axodendritic contact. All of the identified nigrotectal synapses were of the symmetrical type.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural study of large efferent neurons in the superior colliculus of the cat after retrograde labeling with horseradish peroxidase

The ultrastructure of large neurons in the stratum griseum intermedium of the cat superior colliculus was examined following injections of horseradish peroxidase (HRP) into the dorsal tegmental decussation. Four HRP-labeled cells were selected, and the synaptology of their cell bodies and selected regions of proximal and distal dendrites was examined. The four neurons represent four morphologically distinct cell types: multipolar radiating, tufted, large vertical, and medium-sized trapezoid radiating. These four neurons correspond with cell types X1, X2, X3, and T1 respectively, according to the recent classification of neurons in the superior colliculus of the cat by Moschovakis and Karabelas (J. Comp Neurol. 239:276-308, '85). The three X type neurons are similar in having 83% of their somata and over 74% of their proximal dendrites contacted by synaptic profiles. Distal dendrites of the X type neurons, however, receive fewer synaptic contacts. In contrast, in the T1 cell, only 69% of the soma membrane is contacted by synaptic profiles, and the synaptic coverage on proximal and distal dendrites does not vary much from this. Of the eight types of synaptic terminals described in the stratum griseum intermedium of the cat superior colliculus by Norita (J. Comp. Neurol. 190:29-48, '80), only five are found in contact with the X and T type efferent neurons described here. There are some regional differences in terminal distribution, although each terminal is represented on each cell. Type III terminals (small, contain mostly pleomorphic vesicles, and make symmetrical contacts) are the most abundant on cell bodies and dendrites of all four cell types. Terminal types II (medium-sized, containing round and flattened vesicles, and making asymmetrical contacts), and IV (medium to large in size, containing flattened vesicles, and making symmetrical contacts) are well represented. In general, terminal types I (small, containing densely packed round vesicles, and making asymmetrical contacts) and VI (small and irregular in shape, containing flattened vesicles and making symmetrical contacts) are found infrequently. The identity of different types of synaptic terminal is discussed.

Organization of the nigrotectospinal pathway in the cat: a light and electron microscopic study

The relationships between nigrotectal terminals and tectospinal neurons in the deep layers of the superior colliculus have been examined light and electron microscopically by using antero- and retrograde transport of tracer, and anterograde degeneration. For light microscopy, horseradish peroxidase (HRP) was injected into the cervical spinal cord, and HRP conjugated to wheat germ agglutinin (WGA-HRP) into the substantia nigra at the same time. It appeared that about 22% of the retrogradely labeled tectospinal neurons were totally or partially in the anterogradely labeled nigral terminal clusters. This coexistence was particularly manifest in the dorso-lateral part of the intermediate gray layer. For electron microscopy, nigral lesions and HRP injections into the cervical spinal cord were combined in the same animals. Degenerating nigrotectal terminals were found in both axosomatic and axodendritic symmetrical synaptic contacts with retrogradely labeled tectospinal neurons in the intermediate gray layer. Present experiments show tectospinal neurons receiving monosynaptic input from the substantia nigra do exist in the intermediate gray layer of the superior colliculus, especially in its dorsolateral portion.

Evidence for a neurotransmitter function of acetylcholine in rabbit superior colliculus

Acetylcholinesterase staining and studies on the uptake of [3H]choline into the subsequent efflux of tritium from collicular slices were carried out in order to provide evidence for a neurotransmitter function of acetylcholine in rabbit superior colliculus. Acetylcholinesterase staining was dense and homogeneous in superficial layers whereas the staining was arranged in patches with slightly higher density caudally than rostrally in the intermediate layers. The accumulation of tritium in slices incubated with [3H]choline depended on time, temperature and concentration, and was inhibited by hemicholinium-3. Accumulation was slightly higher in caudal than in rostral slices. Electrical stimulation enhanced tritium outflow from slices preincubated with [3H]choline. Tetrodotoxin and a low calcium medium inhibited the evoked overflow whereas hemicholinium-3 caused an enhancement. Oxotremorine decreased the evoked overflow; atropine prevented this effect. The opioids [D-Ala2, MePhe4, Glycol5]enkephalin, [D-Ala2, D-Leu5]enkephalin and ethylketocyclazocine caused an inhibition. The effects of the latter two agonists were antagonized by naloxone. The GABAB-receptor-agonist (-)-baclofen decreased the evoked overflow at lower concentrations than GABA, whereas the GABAA-receptor-agonist muscimol was ineffective. Serotonin produced an inhibition which was prevented by metitepin, alpha- and beta-adrenoceptor as well as dopamine-receptor ligands caused no change. It is concluded that in the rabbit superior colliculus the pattern of acetylcholinesterase staining is comparable, but not identical to the distribution in other species. The accumulation of [3H]choline, as well as the tetrodotoxin-sensitive and calcium-dependent overflow of tritium upon electrical stimulation (reflecting presumably release of [3H]acetylcholine) indicate that acetylcholine has a neurotransmitter function in this tissue. The release of [3H]acetylcholine was modulated by various transmitter substances and related compounds. The pattern of modulation of release differed from the pattern in other cholinergically innervated tissues.