Retinal projection to the olfactory tubercle and basal telencephalon in primates

Exposure to Blue Wavelength Light Is Associated With Increases in Bidirectional Amygdala-DLPFC Connectivity at Rest

Blue wavelength light has been used successfully as a treatment method for certain mood disorders, but, the underlying mechanisms behind the mood enhancing effects of light remain poorly understood. We investigated the effects of a single dose of 30 min of blue wavelength light (n = 17) vs. amber wavelength light (n = 12) exposure in a sample of healthy adults on subsequent resting-state functional and directed connectivity, and associations with changes in state affect. Individuals who received blue vs. amber wavelength light showed greater positive connectivity between the right amygdala and a region within the left dorsolateral prefrontal cortex (DLPFC). In addition, using granger causality, the findings showed that individuals who received blue wavelength light displayed greater bidirectional information flow between these two regions relative to amber light. Furthermore, the strength of amygdala-DLPFC functional connectivity was associated with greater decreases in negative mood for the blue, but not the amber light condition. Blue light exposure may positively influence mood by modulating greater information flow between the amygdala and the DLPFC, which may result in greater engagement of cognitive control strategies that are needed to perceive and regulate arousal and mood.

An Extreme Value Theory Model of Cross-Modal Sensory Information Integration in Modulation of Vertebrate Visual System Functions

We propose a computational model of vision that describes the integration of cross-modal sensory information between the olfactory and visual systems in zebrafish based on the principles of the statistical extreme value theory. The integration of olfacto-retinal information is mediated by the centrifugal pathway that originates from the olfactory bulb and terminates in the neural retina. Motivation for using extreme value theory stems from physiological evidence suggesting that extremes and not the mean of the cell responses direct cellular activity in the vertebrate brain. We argue that the visual system, as measured by retinal ganglion cell responses in spikes/sec, follows an extreme value process for sensory integration and the increase in visual sensitivity from the olfactory input can be better modeled using extreme value distributions. As zebrafish maintains high evolutionary proximity to mammals, our model can be extended to other vertebrates as well.

Morphological properties of medial amygdala-projecting retinal ganglion cells in the Mongolian gerbil

The amygdala is a limbic structure that is involved in many brain functions, including emotion, learning and memory. It has been reported that melanopsin-expressing retinal ganglion cells (ipRGCs) innervate the medial amygdala (MeA). However, whether conventional RGCs (cRGCs) project to the MeA remains unknown. The goal of this study was to determine if cRGCs project to the MeA and to determine the morphological properties of MeA-projecting RGCs (MeA-RGCs). Retrogradely labeled RGCs in whole-mount retinas were intracellularly injected to reveal their dendritic morphologies. Immunohistochemical staining was performed to selectively label ipRGCs (MeA-ipRGCs) and cRGCs (MeA-cRGCs). The results showed that 95.7% of the retrogradely labeled cells were cRGCs and that the rest were ipRGCs. Specifically, MeA-cRGCs consist of two morphological types. The majority of them exhibit small but dense dendritic fields and diffuse ramification patterns as previously reported in RG_B2 (95%), while the rest exhibit small but sparse dendritic branching patterns resembling those of RG_B3 cells (5%). MeA-ipRGCs consist of M1 and M2 subtypes. The MeA-RGCs showed an even retinal distribution patterns. The soma and dendritic field sizes of the MeA-RGCs did not vary with eccentricity. In conclusion, the present results suggest that MeA-RGCs are structurally heterogeneous. These direct RGCs that input to the MeA could be important for regulating amygdala functions.

The Antidepressant Effect of Light Therapy from Retinal Projections

Observations from clinical trials have frequently demonstrated that light therapy can be an effective therapy for seasonal and non-seasonal major depression. Despite the fact that light therapy is known to have several advantages over antidepressant drugs like a low cost, minimal side-effects, and fast onset of therapeutic effect, the mechanism underlying light therapy remains unclear. So far, it is known that light therapy modulates mood states and cognitive functions, involving circadian and non-circadian pathways from retinas into brain. In this review, we discuss the therapeutic effect of light on major depression and its relationship to direct retinal projections in the brain. We finally emphasize the function of the retino-raphe projection in modulating serotonin activity, which probably underlies the antidepressant effect of light therapy for depression.

24-h monitoring devices and nyctohemeral rhythms of intraocular pressure

Intraocular pressure (IOP) is not a fixed value and varies over both the short term and periods lasting several months or years. In particular, IOP is known to vary throughout the 24-h period of a day, defined as a nyctohemeral rhythm in humans. In clinical practice, it is crucial to evaluate the changes in IOP over 24 h in several situations, including the diagnosis of ocular hypertension and glaucoma (IOP is often higher at night) and to optimize the therapeutic management of glaucoma. Until recently, all evaluations of 24-h IOP rhythm were performed using repeated IOP measurements, requiring individuals to be awakened for nocturnal measurements. This method may be imperfect, because it is not physiologic and disturbs the sleep architecture, and also because it provides a limited number of time point measurements not sufficient to finely asses IOP changes. These limitations may have biased previous descriptions of physiological IOP rhythm. Recently, extraocular and intraocular devices integrating a pressure sensor for continuous IOP monitoring have been developed and are available for use in humans. The objective of this article is to present the contributions of these new 24-h monitoring devices for the study of the nyctohemeral rhythms. In healthy subjects and untreated glaucoma subjects, a nyctohemeral rhythm is consistently found and frequently characterized by a mean diurnal IOP lower than the mean nocturnal IOP, with a diurnal bathyphase - usually in the middle or at the end of the afternoon - and a nocturnal acrophase, usually in the middle or at the end of the night.

FoxP2 and olfaction: divergence of FoxP2 expression in olfactory tubercle between different feeding habit bats

FoxP2 is a member of the winged helix/forkhead class of transcription factors. Despite FoxP2 is found to have particular relevance to speech and language, the role of this gene is broader and not yet fully elucidated. In this study, we investigated the expression of FoxP2 in the brains of bats with different feeding habits (two frugivorous species and three insectivorous species). We found FoxP2 expression in the olfactory tubercle of frugivorous species is significantly higher than that in insectivorous species. Difference of FoxP2 expression was not observed within each of the frugivorous or insectivorous group. The diverse expression patterns in olfactory tubercle between two kinds of bats indicate FoxP2 has a close relation with olfactory tubercle associated functions, suggesting its important role in sensory integration within the olfactory tubercle and such a discrepancy of FoxP2 expression in olfactory tubercle may take responsibility for the different feeding behaviors of frugivorous and insectivorous bats.

Nostril-specific olfactory modulation of visual perception in binocular rivalry

It is known that olfaction and vision can work in tandem to represent object identities. What is yet unclear is the stage of the sensory processing hierarchy at which the two types of inputs converge. Here we study this issue through a well established visual phenomenon termed binocular rivalry. We show that smelling an odor from one nostril significantly enhances the dominance time of the congruent visual image in the contralateral visual field, relative to that in the ipsilateral visual field. Moreover, such lateralization-based enhancement extends to category selective regions so that when two images of words and human body, respectively, are engaged in rivalry in the central visual field, smelling natural human body odor from the right nostril increases the dominance time of the body image compared with smelling it from the left nostril. Semantic congruency alone failed to produce this effect in a similar setting. These results, taking advantage of the anatomical and functional lateralizations in the olfactory and visual systems, highlight the functional dissociation of the two nostrils and provide strong evidence for an object-based early convergence of olfactory and visual inputs in sensory representations.

Binaral rivalry in the presence of visual perceptual and semantic influences

When two different odorants are presented simultaneously to the two nostrils, we experience alternations in olfactory percepts, a phenomenon called binaral rivalry. Little is known about the nature of such alternations. Here we investigate this issue by subjecting unstable and stable olfactory percepts to the influences of visual perceptual or semantic cues as participants engage in simultaneous samplings of either two different odorants (binaral) or a single odorant and water (mononaral), one to each nostril. We show that alternations of olfactory percepts in the binaral setting persist in the presence of visual perceptual and semantic modulations. We also show that perceptual cues have a stronger effect than semantic cues in the binaral case, whereas their effects are comparable in the mononaral setting. Our findings provide evidence that an inherent, stimulus-driven process underlies binaral rivalry despite its general susceptibility to top-down influences.

Sniffing out the contributions of the olfactory tubercle to the sense of smell: hedonics, sensory integration, and more?

Since its designation in 1896 as a putative olfactory structure, the olfactory tubercle has received little attention in terms of elucidating its role in the processing and perception of odors. Instead, research on the olfactory tubercle has mostly focused on its relationship with the reward system. Here we provide a comprehensive review of research on the olfactory tubercle-with an emphasis on the likely role of this region in olfactory processing and its contributions to perception. Further, we propose several testable hypotheses regarding the likely involvement of the olfactory tubercle in both basic (odor detection, discrimination, parallel processing of olfactory information) and higher-order (social odor processing, hedonics, multi-modal integration) functions. Together, the information within this review highlights an understudied yet potentially critical component in central odor processing.

Retinal afferents to the thalamic mediodorsal nucleus in the rock cavy (Kerodon rupestris)

The MD has reciprocal connections with the ventromedial prefrontal cortex (PFC) and with limbic cortices and appears to participate in learning and memory-related processes. In this study, we report the identification of a hitherto not reported direct retinal projection to the MD of the rock cavy, a typical rodent species of the Northeast region of Brazil. After unilateral intravitreal injections of cholera toxin subunit B (CTb), anterogradely transported CTb-imunoreactive fibers and presumptive terminals were seen in the MD. A few labeled retinal fibers/terminals detected in the MD of the rock cavy brain show clear varicosities, suggesting terminal fields. The present work is the first to show a direct retinal projection to the MD of rodents and may contribute for elucidating the anatomical substrate of the functional involvement of this thalamic nucleus in the modulation of the visual recognition, emotional learning and object-reward association memory.

Segregated hemispheric pathways through the optic chiasm distinguish primates from rodents

At the optic chiasm retinal fibers either cross the midline, or remain uncrossed. Here we trace hemispheric pathways through the marmoset chiasm and show that fibers from the lateral optic nerve pass directly toward the ipsilateral optic tract without any significant change in fiber order and without approaching the midline, while those from medial regions of the nerve decussate directly. Anterograde labeling from one eye shows that the two hemispheric pathways remain segregated through the proximal nerve and chiasm with the uncrossed confined laterally. Retrograde labeling from the optic tract confirms this. This clearly demonstrates that hemispheric pathways are segregated through the primate chiasm. Previous chiasmatic studies have been undertaken mainly on rodents and ferrets. In these species there is a major change in fiber order pre-chiasmatically, where crossed and uncrossed fibers mix, reflecting their embryological history when all fibers approach the midline prior to their commitment to innervate either hemisphere. This pattern was thought to be common to placental mammals. In marsupials there is no change in fiber order and uncrossed fibers remain confined laterally through nerve and chiasm, again, reflecting their developmental history when all uncrossed fibers avoid the midline. Recently it has been shown that this distinction is not a true dichotomy between placental mammals and marsupials, as fiber order in tree shrews and humans mirrors the marsupial pattern. Architectural differences in the mature chiasm probably reflect different developmental mechanisms regulating pathway choice. Our results therefore suggest that both the organization and development of the primate optic chiasm differ markedly from that revealed in rodents and carnivores.

Dissociated representations of irritation and valence in human primary olfactory cortex

Irritation and negative valence are closely associated in perception. However, these perceptual aspects can be dissociated in olfaction where irritation can accompany both pleasant and unpleasant odorants. Whereas the sensation of odor reflects transduction at olfactory receptors, irritation reflects concurrent transduction of the odorant at trigeminal receptors. Thus a stimulus can be either a pure olfactant activating the olfactory receptors only or a bimodal odorant activating both types of receptors. Using event-related functional magnetic resonance imaging and a 2 x 2 experimental design contrasting odorant valence (pleasant/unpleasant) and odorant type (pure olfactant/bimodal) we found activity in piriform cortex to be associated with valence, and not type, of odors. In contrast, activity in the olfactory tubercle was associated with type, and not valence, of odors. Importantly, this was found when perceived intensity was held equal across odorants. These findings suggest that dissociable neural substrates subserve the encoding of irritation and valence in olfaction.

Central projections of melanopsin-expressing retinal ganglion cells in the mouse

A rare type of ganglion cell in mammalian retina is directly photosensitive. These novel retinal photoreceptors express the photopigment melanopsin. They send axons directly to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus (OPN), thereby contributing to photic synchronization of circadian rhythms and the pupillary light reflex. Here, we sought to characterize more fully the projections of these cells to the brain. By targeting tau-lacZ to the melanopsin gene locus in mice, ganglion cells that would normally express melanopsin were induced to express, instead, the marker enzyme beta-galactosidase. Their axons were visualized by X-gal histochemistry or anti-beta-galactosidase immunofluorescence. Established targets were confirmed, including the SCN, IGL, OPN, ventral division of the lateral geniculate nucleus (LGv), and preoptic area, but the overall projections were more widespread than previously recognized. Targets included the lateral nucleus, peri-supraoptic nucleus, and subparaventricular zone of the hypothalamus, medial amygdala, margin of the lateral habenula, posterior limitans nucleus, superior colliculus, and periaqueductal gray. There were also weak projections to the margins of the dorsal lateral geniculate nucleus. Co-staining with the cholera toxin B subunit to label all retinal afferents showed that melanopsin ganglion cells provide most of the retinal input to the SCN, IGL, and lateral habenula and much of that to the OPN, but that other ganglion cells do contribute at least some retinal input to these targets. Staining patterns after monocular enucleation revealed that the projections of these cells are overwhelmingly crossed except for the projection to the SCN, which is bilaterally symmetrical.

Ambient Odors Influence the Amplitude and Time Course of Visual Distraction

Behavioral performance was examined in a task of attentional capture by luminance under conditions of ambient odors (phenyl ethyl alcohol [PEA], olfactory stimulus, and allyl isothiocyanate [AIC], mixed olfactory/trigeminal stimulus). The AIC increased the amplitude and duration of capture, whereas the presence of PEA led capture to disappear. Furthermore, the PEA caused a general slowing in the speed of information processing. The amplitude and time course of capture were correlated to the irritating components of these odorants, whereas a control experiment showed that the general slowing caused by the PEA was correlated to a drop-off of the subjects' arousal level. These results suggest that ambient odors may exert differential influence of visual-attentional processes and that this influence may depend on the odor's properties.

Direct visual and circadian pathways target neuroendocrine cells in primates

The effect of light on neuroendocrine functions is thought to be mediated through retinal inputs to the circadian pacemaker, the hypothalamic suprachiasmatic nucleus (SCN). The present studies were conducted to provide experimental evidence for this signaling modality in non-human primates. In the St. Kitts vervet monkey, anterograde tracing of SCN efferents revealed a monosynaptic pathway between the circadian clock and hypothalamic neurons producing luteinizing hormone-releasing hormone (LHRH). Using a variety of tracing techniques, direct retinal input was found to be abundant in the SCN and in other hypothalamic sites. Strikingly, in hypothalamic areas other than the SCN, primary visual afferents established direct contacts with neuroendocrine cells including those producing LHRH and dopamine, neurons that are the hypothalamic regulators of pituitary gonadotrops and prolactin. Thus, our data reveal for the first time in primates that light stimuli can reach the hypothalamo-pituitary-gonadal axis, directly providing a pathway independent of but parallel to that of the circadian clock for the photic modulation of hormone release.

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.

Optic afferents to the parabrachial nucleus

Following intraocular injection of cholera toxin subunit B (CTB), optic afferents to the dorsal pontine region were observed in Mongolian gerbils, Chilean degus, and laboratory rats. CTB-positive optic axons emerge at the caudal pole of the superior colliculus, descend through the periaqueductal gray, and innervate the lateral parabrachial nucleus. This projection appears to be a continuation of the retinal pathway that innervates the dorsal raphe nucleus in these same species.

Retinal projection to the dorsal raphe nucleus in the Chilean degus (Octodon degus)

A substantial projection from the retina to the dorsal raphe nucleus (DRN) has been demonstrated in the Chilean degus, a diurnal/crepuscular hystricomorph rodent. Following intraocular injection of cholera toxin subunit B (CTB), immunocytochemically labeled CTB-positive axons and terminals were observed in all major retinorecipient nuclei as well as in the DRN and periaqueductal gray (PAG) of the mesencephalon. Two streams of optic axons to the DRN were observed: one descending from the optic tract at the level of the pretectum and anterior superior colliculus, the other emerging as a small fascicle at the anterior pole of the inferior colliculus and descending bilaterally through the PAG. Contralateral retinal afferents in the DRN appeared to terminate primarily in the dorsomedial and lateral subdivisions of the DRN, and a less extensive ipsilateral component also was observed. Axonal arborizations were characterized by short branches and multiple varicosities, both in the DRN and in the PAG. The extent and density of DRN retinal afferents were not as extensive as previously observed in Mongolian gerbils using identical techniques, but the retinal-DRN projection is considerably larger in degus than in rats. The functional significance of the retinal-DRN pathway remains to be determined, although a variety of evidence indicates that light may directly affect the activity of neurons and serotonin levels in the DRN.

Retinal afferents to the dorsal raphe nucleus in rats and Mongolian gerbils

A direct pathway from the retina to the dorsal raphe nucleus (DRN) has been demonstrated in both albino rats and Mongolian gerbils. Following intraocular injection of cholera toxin subunit B (CTB), a diffuse stream of CTB-positive, fine-caliber optic axons emerged from the optic tract at the level of the pretectum/anterior mesencephalon. In gerbils, CTB-positive axons descended ventromedially into the periaqueductal gray, moving caudally and arborizing extensively throughout the DRN. In rats, the retinal-DRN projection comprised fewer, but larger caliber, axons, which arborized in a relatively restricted region of the lateral and ventral DRN. Following injection of CTB into the lateral DRN, retrogradely labeled ganglion cells (GCs) were observed in whole-mount retinas of both species. In gerbils, CTB-positive GCs were distributed over the entire retina, and a nearest-neighbor analysis of CTB-positive GCs showed significant regularity (nonrandomness) in their distribution. The overall distribution of gerbil GC soma diameters ranged from 8 to 22 micrometer and was skewed slightly towards the larger soma diameters. Based on an adaptive mixtures model statistical analysis, two Gaussian distributions appeared to comprise the total GC distribution, with mean soma diameters of 13 (SEM +/-1.7) micrometer, and 17 (SEM +/-1.5) micrometer, respectively. In rats, many fewer CTB-positive GCs were labeled following CTB injections into the lateral DRN, and nearly all occurred in the inferior retina. The total distribution of rat GC soma diameters was similar to that in gerbils and also was skewed towards the larger soma diameters. Major differences observed in the extent and configuration of the retinal-DRN pathway may be related to the diurnal/crepuscular vs. nocturnal habits of these two species.

Caffeine and the olfactory bulb

Caffeine, a popular CNS stimulant, is the most widely used neuroactive drug. Present in coffee, tea, chocolate, and soft drinks as well as over-the-counter and prescription medications, it influences millions of users. This agent has achieved recent notoriety because its dependency consequences and addictive potential have been re-examined and emphasized. Caffeine's central actions are thought to be mediated through adenosine (A) receptors and monoamine neurotransmitters. The present article suggests that the olfactory bulb (OB) may be an important site in the brain that is responsible for caffeine's central actions in several species. This conclusion is based on the extraordinarily robust and selective effects of caffeine on norepinephrine (NE), dopamine (DA), and particularly serotonin (5HT) utilization in the OB of mice. We believe that these phenomena should be given appropriate consideration as a basis for caffeine's central actions, even in primates. Concurrently, we review a rich rodent literature concerned with A, 5HT, NE, and DA receptors in the OB and related structures along with other monoamine parameters. We also review a more limited literature concerned with the primate OB. Finally, we cite the literature that treats the dependency and addictive effects of caffeine in humans, and relate the findings to possible olfactory mechanisms.

Olfactory bulbectomy impedes social but not photic reentrainment of circadian rhythms in female Octodon degus

Recent studies demonstrated that nonphotic (social) cues markedly accelerate reentrainment to large phase shifts of the light-dark (LD) cycles in female Octodon degus and that such changes are likely effected by chemosensory stimuli. This experiment investigated the effects of olfactory bulbectomies on (1) socially facilitated reentrainment rates of circadian rhythms following a 6-h phase advance of the LD cycle, (2) photic reentrainment rates of circadian rhythms following a 6-h advance of the LD cycle, (3) photic entrainment, and (4) the circadian period (tau) of activity rhythms in constant darkness (DD). olfactory bulbectomies (BX) blocked socially facilitated reentrainment rates but did not alter reentrainment rates of circadian rhythms to photic cues alone. In addition, BX lowered mean daily locomotor activity levels and decreased the amplitude of the activity rhythm in degus housed in entrained (LD 12:12) conditions but did not alter the phase of activity onset or offset, duration (alpha) of activity, or mean daily core body temperature. Bulbectomies also failed to modify tau of free-running activity rhythms. This experiment confirms that the olfactory bulbs and chemosensory cues are necessary for socially facilitated reentrainment. In contrast to their effects in nocturnal rodents, BX do not produce significant circadian photic changes in diurnal degus. This is the first experiment to determine that chemosensory stimuli modulate the circadian system in a diurnal rodent.

The suprachiasmatic nucleus in the sheep: retinal projections and cytoarchitectural organization

The retinal innervation, cytoarchitectural, and immunohistochemical organization of the suprachiasmatic nucleus (SCN) was studied in the domestic sheep. The SCN is a large elongated nucleus extending rostrocaudally for roughly 3 mm in the hypothalamus. The morphology is unusual in that the rostral part of the nucleus extends out of the main mass of the hypothalamus onto the dorsal aspect of the optic chiasm. Following intraocular injection of wheat-germ agglutinin-horseradish peroxidase or tritiated amino acids, anterograde label is distributed throughout the SCN. Retinal innervation of the SCN is bilaterally symmetric or predominantly ipsilateral. Quantitative image analysis demonstrates that, although the amount of autoradiographic label is greatest in the ventral and central parts of the nucleus, density varies progressively between different regions. In addition to the SCN, retinal fibers are also seen in the medial preoptic area, the anterior and lateral hypothalamic area, the dorsomedial hypothalamus, the retrochiasmatic area, and the basal telencephalon. Whereas the SCN can be identified using several techniques, complete delineation of the nucleus requires combined tract tracing, cytoarchitectural, and histochemical criteria. Compared with the surrounding hypothalamic regions, the SCN contains smaller, more densely packed neurons, and is largely devoid of myelinated fibers. Cell soma sizes are smaller in the ventral SCN than in the dorsal or lateral parts, but an obvious regional transition is lacking. Using Nissl, myelin, acetylcholinesterase, and cytochrome oxidase staining, the SCN can be clearly distinguished in the rostral and medial regions, but is less differentiated toward the caudal pole. Immunohistochemical demonstration of several neuropeptides shows that the neurochemical organization of the sheep SCN is heterogeneous, but that it lacks a distinct compartmental organization. Populations of different neuropeptide-containing cells are found throughout the nucleus, although perikarya positive for vasoactive intestinal polypeptide and fibers labeled for methionine-enkephalin are predominant ventrally; neurophysin-immunoreactive cells are more prominent in the dorsal region and toward the caudal pole. The results suggest that the intrinsic organization of the sheep SCN is characterized by gradual regional transitions between different zones.

Visual system of the fossorial mole-lemmings, Ellobius talpinus and Ellobius lutescens

Ocular regression in subterranean species has been shown to be associated with a number of alterations in the retina and in retinal pathways. In order to examine the consequences of eye reduction, the visual system was studied in two species of the murine genus, Ellobius, a specialized fossorial rodent. The axial length of the eye is only 2.2 mm in E. lutescens and 2.9 mm in E. talpinus. The mean soma size of ganglion cells in Nissl-stained flatmounts is approximately 10 microns in E. lutescens and 12 microns in E. talpinus. The soma size distribution in both species appears unimodal and falls within a range of 6-17 microns in diameter. The topographic distribution of ganglion cells shows a weak centroperipheral gradient, and an area centralis cannot be distinguished. The total number of neurons in the ganglion cell layer in Nissl-stained flat mounts is 12,000 in E. lutescens and 28,500 in E. talpinus and, following injection of retrograde tracers in the superior colliculus, is, respectively, 3,600 and 20,000. Based on the axial length and maximum ganglion cell density, the calculated retinal magnification factor (20-26 microns/degree) and spatial resolution (0.4-0.9 cycles/degree) of these minute eyes are extremely reduced. Retinofugal projections, demonstrated by autoradiography and horseradish peroxidase histochemistry, are similar to those in other rodents. The superior colliculus is well developed and receives a predominantly contralateral projection. Ganglion cells projecting to the contralateral colliculus are distributed over the entire retina, while cells that project ipsilaterally are restricted to the ventrotemporal region. The dorsal lateral geniculate nucleus has clearly defined binocular and monocular segments, including a partial segregation of regions receiving ipsilateral or contralateral retinal innervation. In addition, a localized region of label is observed medial to the geniculate nucleus. The retina also sends a bilateral projection to the suprachiasmatic nucleus; the intergeniculate leaflet; the pretectum; and the medial, lateral, and dorsal terminal nuclei of the accessory optic system. Sparse retinal projections were also seen in the bed nucleus of the stria terminalis, the anterior thalamus, and the inferior colliculus. A substantial retinal projection is observed in the basal telencephalon, including the cortical amygdaloid region, the diagonal band of Broca, the olfactory tubercle, and the piriform cortex. The results suggest that the morphological constraints of reduced eye size are reflected in the retina by a generally homogeneous organization but that central visual projections are not substantially modified as in some more specialized, strictly subterranean rodents.

Neuroanatomical pathways linking vision and olfaction in mammals

Retinal projections to several telencephalic structures have been demonstrated in a wide range of mammalian species following intraocular injections of tritiated amino acids and cholera toxin subunit-B conjugated to horseradish peroxidase. Since these regions are also innervated by olfactory fibers, we investigated the distribution of convergent projections using simultaneous injections of different anterograde tracers in the eye and olfactory bulbs. Convergent projections from the retina and from the olfactory bulbs were observed in the piriform cortex, olfactory tubercle, the cortical region of the medial amygdala, lateral hypothalamus, and the bed nucleus of the stria terminalis. A few retinal fibers also invade the nucleus of the lateral olfactory tract, the bed nucleus of the accessory olfactory bulb and the diagonal band of Broca. Injections of retrograde tracers in the medial amygdala, the bed nucleus or the lateral hypothalamus shows that the visuo-olfactory convergence mainly involves projections originating from the accessory olfactory bulb, and to a lesser extent from the ventromedial region of the main olfactory bulb. Fewer than 20 retinotelencephalic ganglion cells were identified in the retina, mainly located contralateral to the injection site. Ganglion cells were medium sized and possessed two long slender opposing dendrites. These retinal and olfactory projections could provide an anatomical substrate for the modulation of gonadotropin hormone levels and the olfactory influence on light mediated rhythms related to reproductive physiology.

Visual system of a naturally microphthalmic mammal: the blind mole rat, Spalax ehrenbergi

Retinal projections and visual thalamo-cortical connections were studied in the subterranean mole rat, belonging to the superspecies Spalax ehrenbergi, by anterograde and retrograde tracing techniques. Quantitative image analysis was used to estimate the relative density and distribution of retinal input to different primary visual nuclei. The visual system of Spalax presents a mosaic of both regressive and progressive morphological features. Following intraocular injections of horseradish peroxidase conjugates, the retina was found to project bilaterally to all visual structures described as receiving retinal afferents in non-fossorial rodents. Structures involved in form analysis and visually guided behaviors are reduced in size by more than 90%, receive a sparse retinal innervation, and are cytoarchitecturally poorly differentiated. The dorsal lateral geniculate nucleus, as defined by cyto- and myelo-architecture, cytochrome oxidase, and acetylcholinesterase distribution as well as by afferent and efferent connections, consists of a narrow sheet 3-5 neurons thick, in the dorsal thalamus. Connections with visual cortex are topographically organized but multiple cortical injections result in widespread and overlapping distributions of geniculate neurons, thus indicating that the cortical map of visual space is imprecise. The superficial layers of the superior colliculus are collapsed to a single layer, and the diffuse ipsilateral distribution of retinal afferents also suggests a lack of precise retinotopic relations. In the pretectum, both the olivary pretectal nucleus and the nucleus of the optic tract could be identified as receiving ipsilateral and contralateral retinal projections. The ventral lateral geniculate nucleus is also bilaterally innervated, but distinct subdivisions of this nucleus or the intergeniculate leaflet could not be distinguished. The retina sends a sparse projection to the dorsal and lateral terminal nuclei of the accessory optic system. The medial terminal nucleus is not present. In contrast to the above, structures of the "non-image forming" visual pathway involved in photoperiodic perception are well developed in Spalax. The suprachiasmatic nucleus receives a bilateral projection from the retina and the absolute size, cytoarchitecture, density, and distribution of retinal afferents in Spalax are comparable with those of other rodents. A relatively hypertrophied retinal projection is observed in the bed nucleus of the stria terminalis. Other regions which receive sparse visual input include the lateral and anterior hypothalamic areas, the retrochiasmatic region, the sub-paraventricular zone, the paraventricular hypothalamic nucleus, the anteroventral and anterodorsal nuclei, the lateral habenula, the mediodorsal nucleus, and the basal telencephalon.(ABSTRACT TRUNCATED AT 400 WORDS)