Evidence for a direct retinal projection to the anterior pretectal nucleus in the cat

A Novel Tectal/Pretectal Population of Premotor Lens Accommodation Neurons

Purpose

Under real-world conditions, saccades are often accompanied by changes in vergence angle and lens accommodation that compensate for changes in the distance between the current fixation point and the next target. As the superior colliculus directs saccades, we examined whether it contains premotor neurons that might control lens compensation for target distance.

Methods

Rabies virus or recombinant rabies virus was injected into the ciliary bodies of Macaca fascicularis monkeys to label circuits controlling lens accommodation via retrograde transsynaptic transport. In addition, conventional anterograde tracers were used to confirm the rabies findings with respect to projections to preganglionic Edinger–Westphal motoneurons.

Results

At time courses that rabies virus labeled lens-related premotor neurons in the supraoculomotor area and central mesencephalic reticular formation, labeled neurons were not found within the superior colliculus. They were, however, found bilaterally in the medial pretectal nucleus continuing caudally into the tectal longitudinal column, which lies on the midline, between the colliculi. A bilateral projection by this area to the preganglionic Edinger–Westphal nucleus was confirmed by anterograde tracing. Only at longer time courses were cells labeled in the superior colliculus.

Conclusions

The superior colliculus does not provide premotor input to preganglionic Edinger–Westphal nucleus motoneurons, but may provide input to lens-related premotor populations in the supraoculomotor area and central mesencephalic reticular formation. There is, however, a novel third population of lens-related premotor neurons in the tectal longitudinal column and rostrally adjacent medial pretectal nucleus. The specific function of this premotor population remains to be determined.

Connections between the zona incerta and superior colliculus in the monkey and squirrel

The zona incerta contains GABAergic neurons that project to the superior colliculus in the cat and rat, suggesting that it plays a role in gaze changes. However, whether this incertal connection represents a general mammalian pattern remains to be determined. We used neuronal tracers to examine the zona incerta connections with the midbrain tectum in the gray squirrel and macaque monkey. Collicular injections in both species revealed that most incertotectal neurons lay in the ventral layer, but anterogradely labeled tectoincertal terminals were found in both the dorsal and ventral layers. In the monkey, injections of the pretectum also produced retrograde labeling, but mainly in the dorsal layer. The dendritic fields of incertotectal and incertopretectal cells were generally contained within the layer inhabited by their somata. The macaque, but not the squirrel, zona incerta extended dorsolaterally, within the external medullary lamina. Zona incerta injections produced retrogradely labeled neurons in the superior colliculus of both species. In the squirrel, most cells inhabited the lower sublamina of the intermediate gray layer, but in the monkey, they were scattered throughout the deeper layers. Labeled cells were present among the pretectal nuclei in both species. Labeled terminals were concentrated in the lower sublamina of the intermediate gray layer of both species, but were dispersed among the pretectal nuclei. In summary, an incertal projection that is concentrated on the collicular motor output layers and that originates in the ventral layer of the ipsilateral zona incerta is a common mammalian feature, suggesting an important role in collicular function.

The ventral portion of the anterior pretectal nucleus controls descending mechanisms that initiate neuropathic pain in rats

Stimulating the dorsal anterior pretectal nucleus (dAPtN) in rats is more effective than stimulating the ventral APtN (vAPtN) at reducing tail-flick latency, whereas stimulation of the vAPtN is more effective at reducing postoperative pain behaviour. This study examines whether a cell lesion caused by injecting N-methyl-D-aspartate into the dAPtN or vAPtN changes the withdrawal threshold of a rat hind paw during different phases of the tactile hypersensitivity induced by a chronic constriction injury (CCI) of the contralateral sciatic nerve. The number of Fos immunoreactive cells in the APtN was also evaluated. The rats whose vAPtN was lesioned 2 days before CCI had more intense tactile hypersensitivity 2 days after CCI than that of the control group, but the groups were not different 7 days after the CCI. The rats whose vAPtN was lesioned 5 days after CCI had withdrawal thresholds that did not differ significantly 7 days after the CCI. The tactile hypersensitivity of the rats whose dAPtN was lesioned 2 days before or 5 days after CCI was not different from that of the control on the second and seventh days after the CCI. The number of Fos immunoreactive cells in the vAPtN and dAPtN increased 2 days after CCI, but did not differ from that in the control 7 days after CCI. We conclude that vAPtN and dAPtN cells are activated by nerve injury; the vAPtN exerts inhibitory control of the initial phase of neuropathic pain whereas the dAPtN does not appear to exert an inhibitory effect in neuropathic processing.

Autonomic control of the eye

The autonomic nervous system influences numerous ocular functions. It does this by way of parasympathetic innervation from postganglionic fibers that originate from neurons in the ciliary and pterygopalatine ganglia, and by way of sympathetic innervation from postganglionic fibers that originate from neurons in the superior cervical ganglion. Ciliary ganglion neurons project to the ciliary body and the sphincter pupillae muscle of the iris to control ocular accommodation and pupil constriction, respectively. Superior cervical ganglion neurons project to the dilator pupillae muscle of the iris to control pupil dilation. Ocular blood flow is controlled both via direct autonomic influences on the vasculature of the optic nerve, choroid, ciliary body, and iris, as well as via indirect influences on retinal blood flow. In mammals, this vasculature is innervated by vasodilatory fibers from the pterygopalatine ganglion, and by vasoconstrictive fibers from the superior cervical ganglion. Intraocular pressure is regulated primarily through the balance of aqueous humor formation and outflow. Autonomic regulation of ciliary body blood vessels and the ciliary epithelium is an important determinant of aqueous humor formation; autonomic regulation of the trabecular meshwork and episcleral blood vessels is an important determinant of aqueous humor outflow. These tissues are all innervated by fibers from the pterygopalatine and superior cervical ganglia. In addition to these classical autonomic pathways, trigeminal sensory fibers exert local, intrinsic influences on many of these regions of the eye, as well as on some neurons within the ciliary and pterygopalatine ganglia.

The pretectum: connections and oculomotor-related roles

Research over the past two decades in mammals, especially primates, has greatly improved our understanding of the afferent and efferent connections of two retinorecipient pretectal nuclei, the nucleus of the optic tract (NOT) and the pretectal olivary nucleus (PON). Functional studies of these two nuclei have further elucidated some of the roles that they play both in oculomotor control and in relaying oculomotor-related signals to visual relay nuclei. Therefore, following a brief overview of the anatomy and retinal projections to the entire mammalian pretectum, the connections and potential roles of the NOT and the PON are considered in detail. Data on the specific connections of the NOT are combined with data from single-unit recording, microstimulation, and lesion studies to show that this nucleus plays critical roles in optokinetic nystagmus, short-latency ocular following, smooth pursuit eye movements, and adaptation of the gain of the horizontal vestibulo-ocular reflex. Comparable data for the PON show that this nucleus plays critical roles in the pupillary light reflex, light-evoked blinks, rapid eye movement sleep triggering, and modulating subcortical nuclei involved in circadian rhythms.

Retinal projections in the cat: A cholera toxin B subunit study

The B fragment of cholera toxin (CTb) is a highly sensitive anterograde tracer for the labelling of retinal axons. It can reveal dense retinofugal projections to well-known retinorecipient nuclei along with sparse but distinct input to target areas that are not commonly recognized. Following a unilateral injection of CTb into the vitreous chamber of seven adult cats, we localized the toxin immunohistochemically in order to identify direct retinal projections in these animals. Consistent with previous findings, the strongest projections were observed in the superficial layers of the superior colliculus, the dorsal and ventral lateral geniculate nuclei, the pretectal nuclei, the accessory optic nuclei, and the suprachiasmatic nucleus of the hypothalamus. However, we also found labelled terminals in several other brain areas, including the zona incerta, the medial geniculate nucleus, the lateral posterior-pulvinar complex, the lateral habenular nucleus, and the anterior and lateral hypothalamic regions. The morphological characteristics of the retinal axon terminals in most of the identified novel target sites are described.

Neuropeptide Y and enkephalin immunoreactivity in retinorecipient nuclei of the hamster pretectum and thalamus

This investigation was stimulated by the historical confusion concerning the identity of certain pretectal nuclei and by large differences reported between species with respect to which nuclei receive retinal innervation. Subcortical visual nuclei were studied using immunohistochemistry to identify retinal projections labeled following intraocular injection of cholera toxin, b fragment. In addition, neuropeptide Y (NPY) or enkephalin (ENK) immunoreactive cells and fibers were also evaluated in the retinorecipient pretectal and thalamic areas. The results confirm the established view that the retina directly innervates the nucleus of the optic tract (NOT), posterior (PPT), and olivary pretectal (OPT) nuclei. However, the retina also innervates the hamster medial (MPT) and anterior (APT; dorsal division) pretectal nuclei, results not previously reported in rodents. A commissural pretectal area (CPT) sparsely innervated by retina is also described. The data show for the first time that the posterior limitans nucleus (PLi) receives a moderately dense, direct retinal input. The PLi does not project to the cortex and appears to be a pretectal, rather than thalamic, nucleus. All retinal projections are bilateral, although predominantly contralateral. The PLi contains a moderately dense plexus of NPY- and ENK-IR fibers and terminals. However, peptidergic fibers also traverse the ATP and connect with the dorsomedial pretectium. The OPT contains ENK- and NPY-IR neurons and fibers, but is specifically identifiable by a moderately dense plexus of ENK-IR terminals. Numerous ENK-IR neurons are found in the NOT and PPT. The latter also has moderate numbers of ENK-IR fibers and terminals, but few NPY-IR neurons or fibers. The MPT contains modest numbers of ENK-IR fibers. The APT has no NPY-IR neurons or terminals, but an occasional ENK-IR neuron is seen and there is sparse ENK-IR innervation. Peptidergic innervation of the visual nuclei does not appear to be derived from the retina. The results show a set of retinally innervated, contiguous nuclei extending from the thalamic ventrolateral geniculate nucleus dorsomedially to the midbrain CPT. These nuclei plus the superior colliculus comprise a dorsal "visual shell" embracing a central core of caudal thalamus and rostral midbrain.

Reciprocal connections between the zona incerta and the pretectum and superior colliculus of the cat

The goal of the present experiments was to examine the relationships of the zona incerta with two structures associated with visuomotor behavior, the superior colliculus and pretectum. The experiments were carried out in the cat, a species commonly used in studies of visuomotor integration, and utilized wheat germ agglutinin horseradish peroxidase and biocytin as retrograde and anterograde neuronal tracers. Retrograde axonal transport demonstrated that most cells in the ventral subdivision of the zona incerta project to the superior colliculus. Anterograde tracers demonstrated that the incertotectal terminal field is most dense in the intermediate gray layer, which is the primary source of the descending pathway from the superior colliculus to brainstem gaze centers. Further experiments showed that scattered cells within the intermediate gray layer give rise to a reciprocal pathway that terminates in both the dorsal and ventral subdivisions of the zona incerta. The distribution of both labeled incertotectal cells and tectoincertal terminals extends dorsolateral to the zona incerta proper, between the reticular thalamic nucleus and the external medullary lamina. Electron microscopic examination of labeled tectoincertal terminals demonstrated that they contain mainly spherical vesicles and have slightly asymmetric to symmetric synaptic densities. Labeled terminals were observed contacting labeled cells in the zona incerta, suggesting that the reciprocal pathway may be monosynaptic. The zona incerta is also reciprocally interconnected with the pretectum. The anterior pretectal nucleus provides a dense projection to the ventral part of the zona incerta and receives a sparse reciprocal projection. The posterior pretectal nucleus and nucleus of the optic tract may also project to the zona incerta. The pretectoincertal fibers form terminals that contain primarily spherical vesicles and make distinctly asymmetric synaptic contacts. In summary, these results indicate that the deep layers of the superior colliculus, which are important for controlling saccades, are the target of a projection from the ventral subdivision of the zona incerta. Like the substantia nigra, the zona incerta may play a permissive role in the tectal initiation of saccadic eye movements. The incertotectal terminal field in the cat is less dense than that observed previously in the rat, suggesting species differences in the development of this pathway. An additional finding of this study is that one of the main sources of input to these incertotectal cells is the anterior pretectal nucleus. This pretectal incertal tectal pathway is likely to play a role in the guidance of tectally initiated saccades by somatosensory stimuli.

Efferent connections from the anterior pretectal nucleus to the diencephalon and mesencephalon in the rat

The anterior pretectal nucleus has been described as part of the visual pretectal complex. However, several electrophysiological and behavioural studies showed that this area is involved in somatosensory modulation, more specifically, antinociception. The efferents of the anterior pretectal nucleus have not been identified taking into account the different function of this nucleus in relation to the rest of the pretectal complex. In the study herein described, a sensitive anterograde tracer Phaseolus vulgaris leucoagglutinin was used to trace the mesencephalic and diencephalic efferents of the anterior pretectal nucleus in the rat. The majority of the connections were ipsilateral. Fibres with varicosities were observed in discrete areas of the thalamus (central lateral, posterior complex), hypothalamus (lateral, posterior and ventromedial), zona incerta, parvocellular red nucleus, intermediate and deep layers of the superior colliculus, central grey, deep mesencephalon, pontine parabrachial region, and pontine nuclei. Fibres en passant were detected in the medial lemniscus, from the level of the injection site to rostral medullary levels. Some labelled axons were seen coursing to the contralateral side through the posterior commissure and the decussation of the superior cerebellar peduncle. These results show that the anterior pretectal nucleus projects principally to areas involved in somatosensory and motor control in a manner that permits sensory modulation at higher and lower levels of the brain. These connections may explain the antinociceptive and antiaversive effects of stimulating the anterior pretectal nucleus in freely moving animals.

Direct projections from the anterior pretectal nucleus to the ventral medulla oblongata in rats

The anterior pretectal nucleus has recently been implicated in the descending modulation of nociception. Electrical stimulation of the nucleus was found to reduce the nociceptive responses of deep dorsal horn neurons and to inhibit spinally integrated withdrawal reflexes. It is believed that at least part of the descending inhibitory effects of the anterior pretectal nucleus are mediated by reticulospinal cells of the ventrolateral medulla. The purpose of the present study was to trace the direct medullary projections of the anterior pretectal nucleus, to describe their topographical organization and to reveal the chemical nature of some of their putative target cells. The connections were studied using anterograde tract-tracing with Phaseolus vulgaris leucoagglutinin. Direct projections from the anterior pretectal nucleus to the ipsilateral rostral ventral medulla were found in all cases. A dense innervation of the dorsal inferior olive, the gigantocellular reticular nucleus pars ventralis and pars alpha and the ventral pontine reticular nucleus was found from all aspects of the anterior pretectal nucleus. Descending labelled terminals were also observed in the gigantocellular reticular nucleus proper and, laterally, in the lateral paragigantocellular nucleus and in the region of the A5 noradrenergic cell group. A relatively lower density of labelled terminals was noted in the medullary raphe nuclei and in the rostroventrolateral reticular nucleus. Following tract-tracer injections into five distinct subregions of the anterior pretectal nucleus, the topographical organization of the projection was examined and the relatively highest density and most widespread projection was found to originate from the caudoventral part of the anterior pretectal nucleus. A combined tract-tracing and immunolabelling study revealed that some of the descending, labelled terminals were in close proximity of tyrosine hydroxylase-immunoreactive dendrites in the C1 and A5 cell groups. Some labelled fibres were also noted among the serotonin-immunoreactive cells in the lateral extension of the B3 cell population. The existence of direct projections to the ventral medulla and pons correlates well with physiological data which showed that the descending, antinociceptive effects of the anterior pretectal nucleus are relayed via the rostral ventrolateral medulla. The data are also in keeping with pharmacological studies that suggested the role of catecholaminergic cells in the mediation of these descending effects. It is proposed that the rostral ventral medullary projections provide a path through which antinociceptive effects of the anterior pretectal nucleus are mediated to the spinal cord.

Pattern of striate cortical projections to the pretectal complex in the guinea pig

The primary goal of this study was to determine whether the striate cortex (Oc 1) of the guinea pig projects to the pretectal nucleus of the optic tract (NOT), the first postretinal station of the horizontal optokinetic pathway, and, if so, to analyze the anatomical organization of this cortico-NOT projection. Other goals of this investigation are to identify other pretectal nuclear projections from the visual cortex in the guinea pig, and to determine whether there is any visuotopic organization in this pathway. Axonal tracers (biocytin or 3H-leucine) were injected into the striate cortex (Oc 1), and the tissue processed with histochemical or light autoradiographic techniques. All subcortical terminal labeling is ipsilateral in the basal ganglia and thalamic nuclei. Furthermore, projections are traced to the ipsilateral brainstem, including two areas of the pretectal complex: (1) one in the NOT, extending in some cases to the adjacent lateral portion of the posterior pretectal nucleus (PPN), and (2) one in the pars compacta of the anterior pretectal nucleus (APNc). The terminal fields in the APN are consistently located rostrally in the dorsolateral portion of the nucleus, independently of the injection site in Oc 1, whereas in the NOT the terminal fields shift slightly after injections placed in different locations in the striate cortex. A correlation of the injection sites in Oc 1 and terminal fields in the NOT reveals a loose topographic organization in the cortico-NOT projection; accordingly, the rostrocaudal axis of the striate cortex projects to the lateromedial axis of the NOT, with a 90 degrees rotation, whereas lateral parts of the striate cortex project diffusely throughout the rostrocaudal extent of the NOT. These data show for the first time that the NOT in the guinea pig receives a substantial projection from the visual cortex. Given the fact that in the guinea pig the optokinetic nystagmus shares some of the characteristics found in cat and monkey (i.e., consistent initial fast rise in the slow phase velocity and reduced asymmetry in monocular stimulation), the present findings lend support to the hypothesis that a cortical input to the NOT is a necessary condition for these oculomotor properties to be present.

The anterior pretectal nucleus: a proposed role in sensory processing

Four nuclei of the pretectal complex, the olivary pretectal nucleus, the medial pretectal nucleus, the nucleus of the optic tract and the posterior pretectal nucleus, all have a demonstrated role in visual function. In contrast, the anterior pretectal nucleus (APtN) has no inputs from retina and has few outputs to visual accessory nuclei. The APtN has connections with areas associated with sensory functions and it has been suggested that this nucleus may have a role to play in somatosensory processing. An increasing number of behavioural and electrophysiological studies support this view. Brief low-intensity electrical or chemical stimulation of the APtN causes antinociception in the tail flick test in both unanaesthetised and anaesthetised animals. This inhibition of the tail flick response is attenuated by naloxone, alpha-adrenoceptor antagonists and muscarinic cholinergic receptor antagonists. Electrical stimulation of the APtN is similarly effective in the paw pressure and formalin tests. APtN stimulation also causes a brief inhibition of the tooth pulp-evoked jaw opening reflex. studies with [C14]2-deoxyglucose indicate that peripheral noxious stimuli will cause an increase in metabolic activity within the APtN. Animals with electrodes placed in the APtN will self-administer electrical stimulation and this can reduce the aversive and autonomic effects of stimulating the ventromedial hypothalamus. Part of the antinociceptive effects of stimulating the APtN are due to a descending inhibition of spinal dorsal horn projection neurones. Multireceptive neurones deep in the dorsal horn are inhibited by APtN stimulation. In contrast, superficial projection neurones that respond to intense cutaneous stimuli are excited by APtN stimulation. The APtN receives an excitatory input from low-threshold afferents via the dorsal column pathway and a high-threshold excitatory drive from superficial cells projecting through the dorsolateral funiculus. The excitatory input from the dorsal columns may well participate in the long-term inhibition of spinal projection neurones evoked by dorsal column stimulation. These ascending excitatory pathways may also be important to the long-term activation of descending inhibition from the APtN.

Retinal projections to the pretectum, accessory optic system and superior colliculus in pigmented and albino ferrets

Retinal projections to the pretectal nuclei, accessory optic system and superior colliculus in pigmented and albino ferrets were studied using anterograde tracing techniques. Both Nissl- and myelin-stained material was used to identify the pretectal nuclei, nuclei of the accessory optic system and the layers of the superior colliculus. Following monocular injection of either horseradish peroxidase or rhodamine-B-isothiocyanate, four pretectal nuclei, including the nucleus of the optic tract, posterior pretectal nucleus, anterior pretectal nucleus and the olivary pretectal nucleus, could be identified to receive direct retinal input in both pigmented and albino strains. In the accessory optic system, retinal terminals were observed in the dorsal, lateral and medial terminal nuclei as well as in the interstitial nucleus of the superior fasciculus, posterior fibres. The retinal projection to the superior colliculus was found to innervate the three superficial layers. The retinal projections to the pretectal nuclei and nuclei of the accessory optic system in the pigmented animals were bilateral, although the label was most dense contralateral to the injected eye. Ipsilateral retinal projections to the pretectal nuclei and nuclei of the accessory optic system appeared to be absent in albino ferrets, i.e. they were invisible with our methods. In both pigmented and albino ferrets retinal terminals in the contralateral superior colliculus densely innervated the three superficial layers. In both strains the ipsilateral projection appeared as clusters which were absent in rostral and caudal poles. In pigmented animals the ipsilateral projection was much denser and more extensive than in albinos. Following injection of retrograde tracers into the brainstem at the level of the dorsal cap of the inferior olive, retrogradely labelled neurons in the pretectum were found in the ipsilateral nucleus of the optic tract. Their somata overlapped mainly with scattered retinal terminals close to the pretectal surface and rarely or not all with the deeper prominent terminal clusters. In the accessory optic system, inferior olive projecting neurons were observed in all four ipsilateral nuclei and fully coincided with the retino-recipient zones. In the superior colliculus, retrogradely labelled neurons were found contralateral to the injection site in the deep layers.