Pregeniculate Nucleus Research Articles

The postnatal development of retinal projections in strepsirrhine galagos (Otolemur garnettii).

Here, we describe the postnatal development of retinal projections in galagos. Galagos are of special interest as they represent the understudied strepsirrhine branch (galagos, pottos, lorises, and lemurs) of the primate radiations. The projections of both eyes were revealed in each galago by injecting red or green cholera toxin subunit B (CTB) tracers into different eyes of galagos ranging from postnatal day 5 to adult. In the dorsal lateral geniculate nucleus, the magnocellular, parvocellular, and koniocellular layers were clearly labeled and identified by having inputs from the ipsilateral or contralateral eye at all ages. In the superficial layers of the superior colliculus, the terminations from the ipsilateral eye were just ventral to those from the contralateral eye at all ages. Other terminations at postnatal day 5 and later were in the pregeniculate nucleus, the accessory optic system, and the pretectum. As in other primates, a small retinal projection terminated in the posterior part of the pulvinar, which is known to project to the temporal visual cortex. This small projection from both eyes was most apparent on day 5 and absent in mature galagos. A similar reduction over postnatal maturation has been reported in marmosets, leading to the speculation that early retinal inputs to the pulvinar are responsible for the activation and early maturation of the middle temporal visual area, MT.

Central projections of intrinsically photosensitive retinal ganglion cells in the macaque monkey

ABSTRACTCircadian rhythms generated by the suprachiasmatic nucleus (SCN) are entrained to the environmental light/dark cycle via intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing the photopigment melanopsin and the neuropeptide pituitary adenylate cyclase‐activating polypeptide (PACAP). The ipRGCs regulate other nonimage‐forming visual functions such as the pupillary light reflex, masking behavior, and light‐induced melatonin suppression. To evaluate whether PACAP‐immunoreactive retinal projections are useful as a marker for central projection of ipRGCs in the monkey brain, we characterized the occurrence of PACAP in melanopsin‐expressing ipRGCs and in the retinal target areas in the brain visualized by the anterograde tracer cholera toxin subunit B (CtB) in combination with PACAP staining. In the retina, PACAP and melanopsin were found to be costored in 99% of melanopsin‐expressing cells characterized as inner and outer stratifying melanopsin RGCs. Two macaque monkeys were anesthetized and received a unilateral intravitreal injection of CtB. Bilateral retinal projections containing colocalized CtB and PACAP immunostaining were identified in the SCN, the lateral geniculate complex including the pregeniculate nucleus, the pretectal olivary nucleus, the nucleus of the optic tract, the brachium of the superior colliculus, and the superior colliculus. In conclusion, PACAP‐immunoreactive projections with colocalized CtB represent retinal projections of ipRGCs in the macaque monkey, supporting previous retrograde tracer studies demonstrating that melanopsin‐containing retinal projections reach areas in the primate brain involved in both image‐ and nonimage‐forming visual processing. J. Comp. Neurol. 522:2231–2248, 2014. © 2014 Wiley Periodicals, Inc.

Retinal projections and neurochemical characterization of the pregeniculate nucleus of the common marmoset (Callithrix jacchus)

In mammals, the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) are the main components of the circadian timing system. The SCN is the site of the endogenous biological clock that generates rhythms and synchronizes them to environmental cues. The IGL is a key structure that modulates SCN activity and is responsible for the transmission of non-photic information to the SCN, thus participating in the integration between photic and non-photic stimuli. Both the SCN and IGL receive projections of retinal ganglion cells and the IGL is connected to the SCN through the geniculohypothalamic tract. Little is known about these structures in the primate brain and the pregeniculate nucleus (PGN) has been suggested to be the primate equivalent of the rodent IGL. The aim of this study was to characterize the PGN of a primate, the common marmoset (Callithrix jacchus), and to analyze its retinal afferents. Here, the marmoset PGN was found to be organized into three subsectors based on neuronal size, pattern of retinal projections, and the distribution of neuropeptide Y-, GAD-, serotonin-, enkephalin- and substance P-labeled terminals. This pattern indicates that the marmoset PGN is equivalent to the IGL. This detailed description contributes to the understanding of the circadian timing system in this primate species considering the importance of the IGL within the context of circadian regulation.

Immunocytochemical characterization of the pregeniculate nucleus and distribution of retinal and neuropeptide Y terminals in the suprachiasmatic nucleus of the Cebus monkey

Circadian rhythms generated by the suprachiasmatic nucleus (SCN) are modulated by photic and non-photic stimuli. In rodents, direct photic stimuli reach the SCN mainly through the retinohypothalamic tract (RHT), whereas indirect photic stimuli are mainly conveyed by the geniculohypothalamic tract (GHT). In rodents, retinal cells form a pathway that reaches the intergeniculate leaflet (IGL) where they establish synapses with neurons that express neuropeptide Y (NPY), hence forming the GHT projecting to the SCN. In contrast to the RHT, which has been well described in primates, data regarding the presence or absence of the IGL and GHT in primates are contradictory. Some studies have suggested that an area of the pregeniculate nucleus (PGN) of primates might be homologous to the IGL of rodents, but additional anatomical and functional studies on primate species are necessary to confirm this hypothesis. Therefore, this study investigated the main histochemical characteristics of the PGN and the possible existence of the GHT in the SCN of the primate Cebus, comparing the distribution of NPY immunoreactivity, serotonin (5-HT) immunoreactivity and retinal terminal fibers in these two structures. The results show that a collection of cell bodies containing NPY and serotonergic immunoreactivity and retinal innervations are present within a zone that might be homologous to the IGL of rodents. The SCN also receives dense retinal innervations and we observed an atypical distribution of NPY- and 5-HT-immunoreactive fibers without regionalization in the ventral part of the nucleus as described for other species. These data may reflect morphological differences in the structures involved in the regulation of circadian rhythms among species and support the hypothesis that the GHT is present in some higher primates (diurnal animals).

The presence and absence of prosencephalic cell groups relaying striatal information to the medial and lateral thalamus in tenrec

Although there are remarkable differences regarding the output organization of basal ganglia between mammals and non-mammals, mammalian species with poorly differentiated brain have scarcely been investigated in this respect. The aim of the present study was to identify the pallidal neurons giving rise to thalamic projections in the Madagascar lesser hedgehog tenrec (Afrotheria). Following tracer injections into the thalamus, retrogradely labelled neurons were found in the depth of the olfactory tubercle (particularly the hilus of the Callejal islands and the insula magna), in subdivisions of the diagonal band complex, the peripeduncular region and the thalamic reticular nucleus. No labelled cells were seen in the globus pallidus. Pallidal neurons were tentatively identified on the basis of their striatal afferents revealed hodologically using anterograde axonal tracer substances and immunohistochemically with antibodies against enkephalin and substance P. The data showed that the tenrec's medial thalamus received prominent projections from ventral pallidal cells as well as from a few neurons within and ventral to the cerebral peduncle. The only regions projecting to the lateral thalamus appeared to be the thalamic reticular nucleus (RTh) and the dorsal peripeduncular nucleus (PpD). On the basis of immunohistochemical data and the topography of its thalamic projections, the PpD was considered to be an equivalent to the pregeniculate nucleus in other mammals. There was no evidence of entopeduncular (internal pallidal) neurons being present within the RTh/PpD complex, neuropils of which did not stain for enkephalin and substance P. The ventrolateral portion of RTh, the only region eventually receiving a striatal input, projected to the caudolateral rather than the rostrolateral thalamus. Thus, the striatopallidal output organization in the tenrec appeared similar, in many respects, to the output organization in non-mammals. This paper considers the failure to identify entopeduncular neurons projecting to the rostrolateral thalamus in a mammal with a little differentiated cerebral cortex, and also stresses the discrepancy between this absence and the presence of a distinct external pallidal segment (globus pallidus).

Calcium-binding proteins in the circadian centers of the common marmoset ( Callithrix jacchus) and the rock cavy ( Kerodon rupestris) brains

The hypothalamic suprachiasmatic nucleus (SCN) and the thalamic intergeniculate leaflet (IGL) are considered to be the main centers of the mammalian circadian timing system. In primates, the IGL is included as part of the pregeniculate nucleus (PGN), a cell group located mediodorsally to the dorsal lateral geniculate nucleus. This work was carried out to comparatively evaluate the immunohistochemical expression of the calcium-binding proteins calbindin D-28k (CB), parvalbumin (PV), and calretinin (CR) into the circadian brain districts of the common marmoset and the rock cavy. In both species, although no fibers, terminals or perikarya showed PV-immunoreaction (IR) into the SCN, CB-IR perikarya labeling was detected throughout the SCN rostrocaudal extent, seeming to delimit its cytoarchitectonic borders. CR-IR perikarya and neuropil were noticed into the ventral and dorsal portions of the SCN, lacking immunoreactivity in the central core of the marmoset and filling the entire nucleus in the rock cavy. The PGN of the marmoset presented a significant number of CB-, PV-, and CR-IR perikarya throughout the nucleus. The IGL of the rocky cavy exhibited a prominent CB- and CR-IR neuropil, showing similarity to the pattern found in other rodents. By comparing with literature data from other mammals, the results of the present study suggest that CB, PV, and CR are differentially distributed into the SCN and IGL among species. They may act either in concert or in a complementary manner in the SCN and IGL, so as to participate in specific aspects of the circadian regulation.

Descending projections of the hamster intergeniculate leaflet: Relationship to the sleep/arousal and visuomotor systems

The intergeniculate leaflet (IGL), homolog of the primate pregeniculate nucleus, modulates circadian rhythms. However, its extensive anatomical connections suggest that it may regulate other systems, particularly those for visuomotor function and sleep/arousal. Here, descending IGL-efferent pathways are identified with the anterograde tracer, Phaseolus vulgaris leucoagglutinin, with projections to over 50 brain stem nuclei. Projections of the ventral lateral geniculate are similar, but more limited. Many of the nuclei with IGL afferents contribute to circuitry governing visuomotor function. These include the oculomotor, trochlear, anterior pretectal, Edinger-Westphal, and the terminal nuclei; all layers of the superior colliculus, interstitial nucleus of the medial longitudinal fasciculus, supraoculomotor periaqueductal gray, nucleus of the optic tract, the inferior olive, and raphe interpositus. Other target nuclei are known to be involved in the regulation of sleep, including the lateral dorsal and pedunculopontine tegmentum. The dorsal raphe also receives projections from the IGL and may contribute to both sleep/arousal and visuomotor function. However, the locus coeruleus and medial vestibular nucleus, which contribute to sleep and eye movement regulation and which send projections to the IGL, do not receive reciprocal projections from it. The potential involvement of the IGL with the sleep/arousal system is further buttressed by existing evidence showing IGL-efferent projections to the ventrolateral preoptic area, dorsomedial, and medial tuberal hypothalamus. In addition, the great majority of all regions receiving IGL projections also receive input from the orexin/hypocretin system, suggesting that this system contributes not only to the regulation of sleep, but to eye movement control as well.

Connections of the medial posterior parietal cortex (area 7m) in the monkey

The afferent and efferent cortical and subcortical connections of the medial posterior parietal cortex (area 7m) were studied in cebus (Cebus apella) and macaque (Macaca fascicularis) monkeys using the retrograde and anterograde capabilities of the horseradish peroxidase (HRP) technique. The principal intraparietal corticocortical connections of area 7m in both cebus and macaque cases were with the ipsilateral medial bank of the intraparietal sulcus (MIP) and adjacent superior parietal lobule (area 5), inferior parietal lobule (area 7a), lateral bank of the IPS (area 7ip), caudal parietal operculum (PGop), dorsal bank of the caudal superior temporal sulcus (visual area MST), and medial prestriate cortex (including visual area PO and caudal medial lobule). Its principal frontal corticocortical connections were with the prefrontal cortex in the shoulder above the principal sulcus and the cortex in the shoulder above the superior ramus of the arcuate sulcus (SAS), the area purported to contain the smooth eye movement-related frontal eye field (FEFsem) in the cebus monkey by other investigators. There were moderate connections with the cortex in the rostral bank of the arcuate sulcus (purported to contain the saccade-related frontal eye field; FEFsac), supplementary eye field (SEF), and rostral dorsal premotor area (PMDr). Area 7m also had major connections with the cingulate cortex (area 23), particularly the ventral bank of the cingulate sulcus. The principal subcortical connections of area 7m were with the dorsal portion of the ventrolateral thalamic (VLc) nucleus, lateral posterior thalamic nucleus, lateral pulvinar, caudal mediodorsal thalamic nucleus and medial pulvinar, central lateral, central superior lateral, and central inferior intralaminar thalamic nuclei, dorsolateral caudate nucleus and putamen, middle region of the claustrum, nucleus of the diagonal band, zona incerta, pregeniculate nucleus, anterior and posterior pretectal nuclei, intermediate layer of the superior colliculus, nucleus of Darkschewitsch and dorsomedial parvicellular red nucleus (macaque cases only), dorsal, dorsolateral and lateral basilar pontine nuclei, nucleus reticularis tegmenti pontis, locus ceruleus, and superior central nucleus. The findings are discussed in terms of the possibility that area 7m contains a "medial parietal eye field" and belongs to a neural network of oculomotor-related structures that plays a role in the control of eye movement.

The retinal projection to the pregeniculate nucleus in normal and destriate monkeys.

Taking advantage of the segregation of different classes of ganglion cell fibres in the optic tract, we investigated the ganglion cell class that projects to the pregeniculate nucleus (PGN) in the normal macaque monkey (Macaca mulatta and Macaca fascicularis) and following long-standing removal of striate cortex in one hemisphere. Confining small pellets of horseradish peroxidase and biocytin into ventral or dorsal parts of the tract unilaterally, or placing several pellets throughout the tract, we labelled the retina retrogradely and the PGN anterogradely. Classification of ganglion cells according to soma size and dendritic morphology showed that implants in the dorsal part of the tract labelled predominantly, and perhaps exclusively, P beta cells, and produced dense anterograde label in the parvocellular lateral geniculate nucleus as well in the PGN. Labelling of the PGN was sparse or absent following implants into the ventral aspect of the tract, which labelled the magnocellular geniculate nucleus anterogradely and chiefly P alpha and P gamma cells retrogradely. The finding of a projection to the PGN from P beta cells has implications for the permanent selective sparing of a subpopulation of these cells after removal of striate cortex and their contribution to wavelength processing in blindsight.

Retinal projections to the pregeniculate nucleus in the hemispherectomized monkey

Intraocular injections of tritiated proline were used to test the hypothesis that unilateral removal of all visual cortical areas results in increased distribution of retinal terminals in the pregeniculate nucleus (PGN) of the thalamus in monkeys. Following hemispherectomy, retinal input to the ipsilateral PGN was reduced by an average of 18.5% when compared to its contralateral homologue, which corresponded to the reduction in nuclear volume (19.3%). Our results show that removal of cortical afferents to the external layer of the PGN does not induce invasion of retinal projections into this region of the nucleus.

The anatomical organization of the macaque pregeniculate complex

Saccade-related activity recorded in the primate pregeniculate nucleus, and its anatomical connections with the pretectal nucleus of the optic tract (NOT) and superior colliculus (SC), suggest that it plays a role in visual-ocular motor integration. To study this role, a clearer understanding of pregeniculate organization is required. Based on its connectivity and neurotransmitter immunocytochemistry, we demonstrate that this nucleus is composed of several subnuclei, suggesting the term, pregeniculate complex (PrGC). The PrGC includes a weakly developed dorsal lamina, rostrally, and a well-developed ventral lamina. The ventral lamina includes the retinorecipient and superior sublayers, rostrally, and the medial division, caudally. A thin lamina of cells lateral to the dorsal lateral geniculate nucleus is contiguous with the PrGC; we term this the lateral division. The PrGC and the lateral division each project to the SC/NOT; the superior sublayer and medial division of the PrGC are connected reciprocally to the SC/NOT. Immunocytochemistry for gamma-aminobutyric acid (GABA) and substance P (SP) further delineate the PrGC subnuclei. The retinorecipient sublayer stains most intensely for GABA and SP. The superior sublayer and medial division also stain strongly for GABA and SP. Essentially all neurons in the lateral division are GABA-positive. The combination of tract tracing and immunocytochemistry demonstrate differences in the connectivity of the PrGC subnuclei and the lateral division with the SC/NOT. This, combined with the differential localization of GABA in the PrGC, provides a basis for further study of its functional role.

Comparison of cholinergic and histaminergic axons in the lateral geniculate complex of the macaque monkey

The cholinergic and histaminergic projections have important neuromodulatory functions in the ascending visual pathways, so we compared the pattern and mode of innervation of the two projections in the lateral geniculate complex (dorsal lateral geniculate nucleus and pregeniculate nucleus) of the macaque monkey. Brain tissue from macaques was immunoreacted by means of antibodies to choline acetyltransferase (ChAT) or to histamine and processed for light and electron microscopy. A dense plexus of thin, highly branched ChAT-immunoreactive axons laden with varicosities was found in all layers of the dLGN including the koniocellular laminae and in the pregeniculate nucleus. ChAT label was more dense in magnocellular layers 1 and 2 than in parvocellular layers 3–6 and relatively sparse in the interlaminar zones. Varicosities associated with the cholinergic axons had an average of three conventional asymmetric synapses per varicosity, and these appeared to contact dendrites of both thalamocortical cells and interneurons. Histamine-immunoreactive axons were distributed homogeneously throughout all laminar and interlaminar zones of the dLGN, but were denser in the pregeniculate nucleus than in the dLGN. Histaminergic axons branched infrequently and were typically larger in caliber than cholinergic axons. The overwhelming majority of varicosities were found en passant and rarely displayed conventional synapses, despite the abundance of synaptic vesicles, and were not associated preferentially with specific cellular structures. The innervation of the macaque dLGN complex by cholinergic and histaminergic systems is consistent with their proposed role in state dependent modulation of thalamic activity. The dense and highly synaptic innervation by cholinergic axons supports the proposal of additional involvement of these axons in functions related to eye movements. Anat Rec 255:295–305, 1999. © 1999 Wiley-Liss, Inc.

Characterization of the Circadian System in a Brazilian Species of Monkey (Callithrix jacchus): Immunohistochemical Analysis and Retinal Projections

The hypothalamic suprachiasmatic nucleus (SCN) and the thalamic pregeniculate nucleus (PGN), which appears to include the intergeniculate leaflet (IGL), comprise circadian related centers in the primate brain. In this study, these centers were analysed in respect to their cytoarchitecture, retinal afferents and chemical of major cells and axon terminals with tract tracers and immunohistochemical techniques to define cytoarchitecture and connections, in the common marmoset. The SCN was shown to be a triangularly shaped cluster of compact cells just dorsal to the optic chiasm and lateral to the third ventricle. It is innervated in its ventral portion by terminals from the retina, and NPY-ergic fibers. Serotonergic and SP-staining processes are distributed throughout. VIP-neurons form a dorsolateral group of cells and CB-immunoreactive neurons fill much of the nucleus. The PGN was shown to be a wedge-shaped cluster of cells located dorsomedially to the dorsal lateral geniculate nucleus. It appears to comprise a ventral portion which receives a bilateral retinal projection and contains NPY-neurons, suggesting that this portion may correspond to IGL. The PGN also contains CB-neurons, PV-neurons and fibers, and SP- and 5-HT-fibers. These results in marmoset show that, beside a common plan revealed for most mammals, there are significant interspecific variations in the circadian timing system. Future studies are needed in order to elucidate the circadian organization in this primate species.

Transient features of the thalamic reticular nucleus in the human foetal brain.

The architectonic organization and neuronal types of the human foetal reticular nucleus (RN)--with special reference to transient characteristics--have been investigated using antisera against calretinin, parvalbumin and neurofilament epitopes of somata and dendrites (SMI 311). The RN consists of four subdivisions (clearly distinguishable in the 6/7th gestational month): The main portion appears as a prominent structure on account of its extension and high packing density of neurons which coexpress calretinin and parvalbumin. These two calcium-binding proteins are also expressed by the perireticular nucleus forming a conspicuous grey within the internal capsule. Perireticular cells form clusters which are in continuity with the main portion, globus pallidus, ganglionic eminence and pregeniculate nucleus. In double-labellings, a medial subnucleus stands out distinctly as it only expresses calretinin. SMI 311-immunopreparations show neurons revealing a high degree of diversification and elaborated dendritic trees. Several transient characteristics become obvious: the perireticular nucleus, not visible in the adult, represents a distinct entity in the human foetal brain. The main portion and the pregeniculate nucleus appearing as prominent greys are dramatically reduced in size later on. The percentage of RN-neurons expressing calretinin, the diversity of neuronal types and elaborated dendritic trees are reduced. The transient features can be correlated with the RN's putative functional roles in development: early RN-afferents to the dorsal thalamus may represent pioneer fibres providing guiding cues for outgrowing axons from or into the thalamus. Moreover, the RN may serve as an intermediate target for growing axons which are sorted and directed towards different final targets.

Is there a geniculohypothalamic tract in primates? A comparative immunohistochemical study in the circadian system of strepsirhine and haplorhine species

In rodents, the circadian rhythm generated by the hypothalamic suprachiasmatic nucleus (SCN) is modulated by two types of phenomena: photic phase-shifts, mediated by the retinohypothalamic pathway and non-photic phase-shifts mediated by the projection of the intergeniculate leaflet (IGL) to the SCN which contains the neuropeptide Y (NPY). In primates, the retinohypothalamic pathway has been well-demonstrated but very little is known about the geniculohypothalamic tract. This prompted us to study NPY immunoreactivity in both the SCN and the IGL in species representative of the three main primate lineages: prosimians ( Microcebus), New World monkeys ( Callithrix) and Old World monkeys ( Macacca). In species studied, we found a region in the pregeniculate nucleus containing both NPY immunopositive cells and substance P immunopositive fibres that we identified as the IGL. During evolution, this structure has moved from a ventral to a dorsomedial position relative to the adjacent dorsal lateral geniculate nucleus. By contrast, NPY-IP fibres in the SCN are dense in prosimians, but are sparse or absent in other primate species. We suggest that either the geniculohypothalamic projection is absent in higher primates as is the case in humans, or is absent in diurnal mammals, or contains a different peptide, or that NPY immunoreactivity varies according to other parameters.

Calbindin Immunoreactivity Delineates the Circadian Visual Centers of the Brain of the Common Marmoset (Callithrix jacchus)

The hypothalamic suprachiasmatic nucleus and the thalamic pregeniculate nucleus (which includes the intergeniculate leaflet) comprise the circadian visual system in the primate brain. In this study, we used intraocular injections of cholera toxin subunit B to identify those nuclei in the common marmoset brain, and demonstrated that calbindin D-28k immunoreactivity apparently labels most neurons in both the suprachiasmatic and pregeniculate nuclei. These data suggest that calbindin D-28k could represent a reliable neuronal marker for structures of the circadian visual system in marmosets and provide anatomical information on the primate equivalent of the rodent intergeniculate leaflet.

Response properties of pretectal omnidirectional pause neurons in the behaving primate.

We have identified a region in the pretectum of rhesus monkeys (Macaca mulatta) that contains units that evince a complete cessation in firing immediately after saccades. The pause occurs for saccades to target steps and catch up saccades during smooth pursuit, spontaneously in complete darkness or after quick phases of nystagmus. Because the pause in unit firing always follows saccade onset, we call these neurons following omnidirectional pause neurons (FOPNs). Because the pause also occurs with saccades in the dark, it is related to the saccade per se and is not a visually contingent response. The duration of the pause in firing exceeded the duration of all saccades up to 40 deg. For targeting saccades, the start of the pause was locked rather tightly to the beginning of the saccade but began an average of 51 ms after the saccade did. The end of the pause was linked only loosely to either the beginning or end of the saccade. About half (54%) of our 59 FOPNs also discharged a distinct burst of firing that preceded the pause. In different units, the burst preceded saccade onset by from 0 to 20 ms with an average of 11 ms and therefore could signal the occurrence of an impending saccade. The presaccadic burst was not correlated with any parameter of the saccade. Most FOPNs were found 278 microns, on average, dorsal to the direction-selective units characteristic of the pretectal nucleus of the optic tract (NOT) and occasionally slightly beyond the anterior-posterior and medial-lateral borders of the NOT. The FOPN region does not coincide with any known anatomically or functionally delineated pretectal nucleus. Because the characteristics of the FOPN pause are not reflected in the characteristics of the saccade and the FOPN pause occurs well after the saccade is over, it is unlikely that the pause in pretectal FOPNs is involved with saccade generation. On the other hand, the leading burst exhibited by the majority of FOPNs reliably signals that a saccade is occurring but neither its size nor direction. Perhaps this signal indicating the occurrence of all saccades is routed to visual relay neurons to effect saccadic modification of visual pathways. The substantial efferent connections of the FOPN/NOT region to the pregeniculate nucleus and the saccadic discharge or pregeniculate cells are discussed in the context of this suggestion.

Anatomical connections of the primate pretectal nucleus of the optic tract.

The pretectal nucleus of the optic tract (NOT) plays an essential role in optokinetic nystagmus, the reflexive movements of the eyes to motion of the entire visual scene. To determine how the NOT can influence structures that move the eyes, we injected it with lectin-conjugated horseradish peroxidase and characterized its afferent and efferent connections. The NOT sent its heaviest projection to the caudal half of the ipsilateral dorsal cap of Kooy in the inferior olive. The rostral dorsal cap was free of labeling. The NOT sent lighter, but consistent, projections to other visual and oculomotor-related areas including, from rostral to caudal, the ipsilateral pregeniculate nucleus, the contralateral NOT, the lateral and medial terminal nuclei of the accessory optic system bilaterally, the ipsilateral dorsolateral pontine nucleus, the ipsilateral nucleus prepositus hypoglossi, and the ipsilateral medial vestibular nucleus. The NOT received input from the contralateral NOT, the lateral terminal nuclei bilaterally, and the ipsilateral pregeniculate nucleus. Although our injections involved the pretectal olivary nucleus (PON), there was neither orthograde nor retrograde labeling in the contralateral PON. Our results indicate that the NOT can influence brainstem preoculomotor pathways both directly through the medial vestibular nucleus and nucleus prepositus hypoglossi and indirectly through both climbing and mossy fiber pathways to the cerebellar flocculus. In addition, the NOT communicates strongly with other retino-recipient zones, whose neurons are driven by either horizontal (contralateral NOT) or vertical (medial and lateral terminal nuclei) fullfield image motion.

Preoccipital cortex receives a differential input from the frontal eye field and projects to the pretectal olivary nucleus and other visuomotor-related structures in the rhesus monkey

The bidirectional axonal transport capabilities of the horseradish peroxidase (HRP) technique facilitated the study of the frontal-eye-field (FEF) input and pretectal output of two regions of extrastriate preoccipital cortex (POC). Following horseradish peroxidase (HRP) gel implants into the middle and dorsal POC in two rhesus monkeys, the middle POC implant demonstrated retrograde frontal cortical labeling largely restricted to the inferior frontal eye field (iFEF) and adjacent inferior prefrontal convexity, whereas the dorsal POC implant showed labeling in the caudal ventral bank of the superior ramus of the arcuate sulcus (sas) and middle-to-dorsal region of the rostral bank of the concavity of the arcuate sulcus (dorsal FEF). Prominent anterogradely labeled efferent preoccipital projections were observed to the ipsilateral pretectal olivary nucleus (PON) and to a lesser extent the anterior pretectal nucleus. Although the middle POC case had heavier projections to the lateral PON, the dorsal case projected more heavily to the medial PON. In addition, both implants demonstrated subcortical connections with the lateral and dorsal inferior pulvinar nuclei, central superior lateral thalamic intralaminar nucleus, caudate nucleus, and middle-to-ventral claustrum. However, while the middle POC implant had efferent projections to the superficial superior colliculus (SC), pregeniculate nucleus (PGN), lateral terminal accessory optic nucleus (LTN), and dorsolateral pontine nucleus (DLPN), resembling those previously reported for the middle temporal (MT) visual area (Maunsell & Van Essen, 1982; Ungerleider et al., 1984), the dorsal implant had projections to the lateral intermediate SC, zona incerta (ZI), PGN, a notably lesser projection to the LTN, and basilar pontine projections to the lateral and lateral dorsal pontine subnuclei (not including the extreme dorsolateral DLPN). These preliminary results suggest that the preoccipital cortex, which reportedly functions in pupillary constriction, accommodation, and convergence, entertains connections with the PON and other visuomotor-related structures, and thus could act as an intermediary in the pathway between the iFEF and PON, and provide a possible explanation for pupillary effects that occur with stimulation of the FEF (Jampel, 1960) and within the contex of other oculomotor activities. The findings shed light on certain differences in connections of middle vs. dorsal POC with visuomotor-related nuclei, and appear to suggest that the middle region, which receives input from the iFEF, has greater access to the optokinetic (OKN) system by virtue of its projection to the LTN, and to the smooth-pursuit system b

The geniculohypothalamic tract in monkey and man

The intergeniculate leaflet (IGL) of the lateral geniculate complex in rodents contains neuropeptide Y-immunoreactive (NPY-IR) neurons which project to the suprachiasmatic nucleus (SCN) of the hypothalamus. In the macaque monkey and human brain, a large portion of the pregeniculate nucleus contains NPY-IR neurons indicating that this is the primate homologue of the rodent IGL. The monkey SCN has a dense plexus of NPY-IR axons identical in location and appearance to that in rodents. As in other mammals, no NPY-IR neurons are found in the monkey SCN. In contrast, in the human SCN, the NPY-IR plexus is less dense than in the monkey and there are numerous NPY-IR neurons. This suggests that the human SCN differs in organization from that of other mammals and that the functional homologue of the mammalian geniculohypothalamic tract is local circuit NPY+ neurons within the nucleus.