An autoradiographic and electron microscopic study of retino-hypothalamic connections

Is there a direct retina-raphe-suprachiasmatic nucleus pathway in the rat?

Possible pathways from the retina to the suprachiasmatic nucleus (SCN) relaying in the dorsal raphe nucleus (DRN) were investigated in rats using combined anterograde and retrograde tracing with immunohistochemistry. After injection of wheat germ agglutinin-conjugated horseradish peroxidase-colloidal gold complex into the SCN, many neurons were retrogradely labeled in the middle levels of the DRN. Approximately one half of these neurons contained serotonin. After injection of cholera toxin B subunit into the eyes, a few anterogradely labeled afferent fibers were detected in the rostral DRN, however, not in contact with retrogradely labeled neurons. Our findings provide direct evidence that serotonergic projections to the rat SCN stem from the DRN nuclei. They also suggest that retina-raphe-SCN projections, a presumed third visual input to the mammalian circadian pacemaker, may include further neuronal connections or brain sites.

Ventral lateral geniculate nucleus afferents to the suprachiasmatic nucleus in the cat

The lateral geniculate complex innervates the hypothalamic suprachiasmatic nucleus (SCN). The location of neurons in the cat ventral lateral geniculate nucleus (vLGN) that give rise to the geniculohypothalamic tract has not been described. In this study, retrogradely labeled neurons were noted throughout the rostrocaudal extent of the medial vLGN following tracer injection into the SCN region. In addition, neuropeptide Y immunoreactive processes were also observed in the vLGN in this same medial zone and in the SCN. The data suggest that the medial zone of the cat vLGN may be homologous to the rodent intergeniculate leaflet (IGL).

Bicuculline increases and muscimol reduces the phase-delaying effects of light and VIP/PHI/GRP in the suprachiasmatic region

The present study investigated the effects of gamma-amino butyric acid (GABA)A-active drugs on the ability of light or coadministration of vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI), and gastrin-releasing peptide (GRP) to phase delay hamster activity rhythms. Microinjection of the GABAA agonist, muscimol, significantly (p < .01) reduced the phase-delaying effect of light administered at circadian time (CT) 13.5. By contrast, microinjection of the GABAA antagonist, bicuculline, significantly (p < .01) increased the phase-delaying effect of light administered at CT 13.5. Microinjection of muscimol or bicuculline into the suprachiasmatic nucleus (SCN) produced little or no effect on circadian phase when no light pulses were provided. Coadministration of muscimol with VIP/PHI/GRP significantly (p < .01) reduced the phase-delaying effect of VIP/PHI/GRP, whereas administration of bicuculline with VIP/PHI/GRP significantly (p < .05) increased the phase-delaying effect of these peptides. These data indicate that changes in GABAA activity within the SCN can modulate the phase-delaying effects of light and VIP/PHI/GRP during the early portion of subjective night.

Photic regulation of c-fos gene expression in the suprachiasmatic nucleus and the circadian rhythm of photosensitivity in the mink

The relationship between the photic stimulation of the c-fos gene product in cells of the suprachiasmatic nuclei and the photoperiodic control of testicular activity were examined in mink. Mink were kept in a short photoperiod (control, LD4:20), or in 'asymmetric skeleton photoperiods' (groups A and B). The light period for groups A and B was divided into two fractions (3 h 30 min and 30 min); the shorter fraction occurred in the night, 4 h (group A) or 8 h (group B) after the end of the main light period. There was no photic activation of the proto-oncogene c-fos on the control or group A, and 4 weeks on this photoperiodic treatment produced marked testicular development. In contrast, in group B, c-fos mRNA was induced 30 min after the end of the secondary photofraction, was maximal 30 min later and then decreased. Fos-like immunoreactivity was detected 2 h after the end of the secondary photofraction, with activity peaking 1 h later. The animals of this group remained sexually quiescent. These results suggest that photo-induction of the proto-oncogene c-fos is implicated in the gonadal inhibition induced in this species when the light period, extends into the photosensitive phase of the circadian rhythm of photosensitivity.

Morphological changes in the hypothalamic suprachiasmatic nucleus and circadian rhythm of locomotor activity in hereditary microphthalmic rats

Analysis of circadian locomotor activity, Golgi-Cox impregnation, and immunohistochemistry were studied on the hereditary microphthalmic rat which congenitally lacked the optic nerve. These blind rats showed free-running circadian rhythms in their locomotor activities. Both the normal and microphthalmic rats had similar ultradian rhythms in addition to circadian rhythms. The neuronal cell population and volume of the hypothalamic suprachiasmatic nucleus (SCN) of the microphthalmic rats were 66% and 71% of those in normal rats, respectively. The number of SCN neurons containing vasoactive intestinal peptide-like immunoreactive substance was dramatically decreased to 35% of that in normal rats. Golgi-Cox impregnation revealed that three types of neurons in the SCN of the microphthalmic rats were consistently distinguished as observed in normal rats. Although there were no changes in the numbers of primary dendrites of the SCN neurons in the microphthalmic and normal rats, the number of secondary and tertiary dendrites in the SCN of the microphthalmic rats was smaller than that of normal rats. These observations suggest that the retinal input may be important for normal morphological formation of the SCN during development, but not for the generation of circadian rhythms and ultradian rhythms.

Presence of LHRH (luteinizing hormone-releasing hormone) fibers in the optic nerve, optic chiasm and optic tract of the adult rat

In mammals LHRH (luteinizing hormone-releasing hormone) is synthesized and released by a set of neurons that have their embryonic origin in the olfactory placode. We have observed that, besides their classical location, LHRH fibers can also be seen in the optic nerve and optic chiasm. Some LHRH fibers could also be traced in the optic tract. The possible course of these projections, and their functional significance are discussed.

Demonstration of direct input from the retina to the lateral habenular nucleus in the albino rat

The projection from the retina to the habenular complex was studied using fluorescent retrograde tracers in the albino rat (Wistar, Japan Clea). Following separate unilateral injections of Fluoro-Gold (FG), Fluoro-Ruby (FR), or 4-acetamido,4- isothiocyanostilbene-2,2'-disulfonic acid (SITS) into the lateral habenular nucleus (LHB), a small population of ganglion cells was labeled sporadically, predominantly those in the nasal retina contralateral to each injection site. Most of them were small cells, ranging from 9 to 16 mu m in diameter, roughly corresponding to the type III ganglion cell in the rat retina. Additionally, all of the structures previously described as regions projecting to the LHB were confirmed. Upon re-examination of previous brain sections of albino rats which had undergone monocular enucleation, degenerating retinal nerve axons and/or their terminals, stained by a modified selective silver impregnation method, were observed in the well-documented end regions of retinal afferents as well as the LHB. The degenerating retino-habenular nerve terminals were distributed sparsely and restricted mainly to the caudal part of the LHB contralateral to the side of ocular enucleation. The present experimental data provide evidence for the existence of a non-image forming retino-habenular pathway in the albino rat. We suggest that, besides serving as a point of convergence for some of the major conduction channels of the limbic and striatal systems, the LHB may play more general integrative roles, including participation in the integration of visual information.

Demonstration of retinal afferents in the RCS rat, with reference to the retinohypothalamic projection and suprachiasmatic nucleus

In the Royal College of Surgeons (RCS) rat, characterized by inherited retinal dystrophy, retinal projections to the brain were studied using anterograde neuronal transport of cholera toxin B subunit upon injection into one eye. The respective immunoreactivity was found predominantly contralateral to the injection site in the lateral geniculate nucleus, superior colliculus, nucleus of the optic tract, medial terminal nucleus of the accessory optic tract, and bilateral hypothalamic suprachiasmatic nuclei. Although terminal density was somewhat reduced in dystrophic rats, the projection patterns in these animals appeared similar to those seen in their congenic controls and were comparable to the visual pathways described for the rat previously. In dystrophic rats, the number of cell bodies exhibiting immunoreactivity to vasoactive intestinal polypeptide, viz. a population of suprachiasmatic neurons receiving major retinohypothalamic input, was reduced by one-third, and some differences were observed in the termination pattern of the geniculohypothalamic tract, as revealed by immunoreactivity to neuropeptide Y in the suprachiasmatic nucleus.

Morphological diversity and glutamate immunoreactivity of retinal terminals in the suprachiasmatic nucleus of the cat

Although the cat visual system has been the subject of intensive investigation, little attention has been given to the morphological features of ganglion cell projections to the suprachiasmatic nucleus. The present study has utilized anterograde transport of horseradish peroxidase and wheat germ agglutinin-conjugated horseradish peroxidase to label ganglion cell terminals in the cat suprachiasmatic nucleus. Visualization of the reaction product was facilitated through the use of gold-substituted silver intensification. Ganglion cell terminals were found to be morphologically diverse, making both asymmetric and symmetric contacts with postsynaptic processes. Synaptic vesicles were either scattered or densely packed, sometimes forming paracrystalline arrays. In contrast to other retinorecipient areas in which ganglion cell terminals have been characterized by the presence of lightly staining mitochondria, many of the retinal terminals in the suprachiasmatic nucleus were seen to contain darkly stained mitochondria. Postembedding antiglutamate immunocytochemistry was used to evaluate the level of endogenous glutamate in these ganglion cell terminals. Although morphologically diverse, all of the retinal terminals in the suprachiasmatic nucleus were glutamate positive, consistent with the postulated role of glutamate as the neurotransmitter of retinal ganglion cells.

Seasonal variation in NPY immunoreactivity in the suprachiasmatic nucleus of the jerboa (Jaculus orientalis), a desert hibernator

Using immunocytochemical techniques the seasonal variation in NPY immunoreactive fibers was investigated in the suprachiasmatic nucleus of both male and female jerboas. During the period of sexual quiescence (autumn), the amount of NPY immunoreactive fibers in the suprachiasmatic nucleus of both male and female jerboas was higher than in the period of sexual activity (spring-middle of summer). Compared with the respective control groups, castration during the period of sexual activity and testosterone or estrogen supplementation in sexually inactive animals did not affect NPY immunolabeling. These results indicate that the seasonal variation observed in NPY immunoreactivity in the suprachiasmatic nucleus of the jerboa is independent of circulating levels of steroid hormones. The possible influence of another hormonal system or a direct influence of an external factor such as photoperiod on NPY content in the suprachiasmatic nucleus remains to be determined.

Bicuculline blocks neuropeptide Y-induced phase advances when microinjected in the suprachiasmatic nucleus of Syrian hamsters

Microinjection of neuropeptide Y (NPY) into the suprachiasmatic nucleus of the hypothalamus (SCN) during the middle of the subjective day (i.e. circadian time 6) causes large phase advances in circadian rhythms. The present study demonstrates that microinjection of the gamma-aminobutyric acid (GABA) antagonist, bicuculline, completely blocks NPY-induced phase advances. These data indicate that GABAA activity within the SCN may mediate the phase shifting effects of some stimuli on the circadian pacemaker.

Topographical organization of the rat suprachiasmatic-paraventricular projection

The suprachiasmatic nucleus (SCN) is a dominant pacemaker involved in the generation of circadian rhythms in mammals. Surprisingly, the expression of the many rhythms appears to be mediated via a limited efferent projection system of the pacemaker, of which the largest pathway terminates in the subparaventricular area and in the paraventricular nucleus of the hypothalamus. In order to investigate a possible topographical organization of this major outflow pathway of the SCN, microiontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) or the retrograde tracer cholera toxin subunit B (ChB) were centered in distinct subparts of the SCN (PHA-L) or in the subparaventricular area-paraventricular nucleus (ChB), respectively. PHA-L injections involving the entire SCN revealed not only a major projection to the subparaventricular area, but also one directed towards the medial and dorsal parvicellular subnuclei of the paraventricular nucleus. As opposed to injections involving the entire nucleus, injections of PHA-L centered in the dorsomedial subdivision of the SCN resulted in a relatively larger number of PHA-L-immunoreactive fibers in the parvicellular subdivisions of the PVN, whereas the terminal field in the subparaventricular area was less substantial. A topography of the SCN efferent output system was also revealed by the retrograde tracing with ChB. Injections of ChB in the dorsal part of the paraventricular hypothalamic nucleus, not involving the underlying subparaventricular area, gave rise to a population of retrogradely labeled cells in the dorsomedial part of the SCN. In contrast, ChB injections in the subparaventricular area resulted in labeling of neurons clustered in a more ventrolateral aspect of the SCN. The present data provide evidence for a topography in the major efferent projection system from the SCN, implying that different subparts of the rat SCN, presumably containing partly different potential neurotransmitter substances, may regulate different circadian rhythms.

The retinohypothalamic tract originates from a distinct subset of retinal ganglion cells

The retinal ganglion cells giving rise to retinohypothalamic projections in the rat were identified using retrograde transport of horseradish peroxidase (HRP) or FluoroGold injected into the suprachiasmatic nucleus (SCN), and using transneuronal transport of the Bartha strain of the swine herpesvirus (PRV-Bartha). When PRV-Bartha is injected into one eye, it is taken up by retinal ganglion cells, replicated, transported to axon terminals in the SCN, and released. There the virus may take one, or both, of two paths to retinal ganglion cells in the contralateral eye: 1) uptake by SCN neurons, replication, and release from the neurons with uptake and retrograde transport in retinal afferents originating in the contralateral retina; 2) transneuronal passage through axo-axonic appositions between retinal afferents in the SCN with subsequent retrograde transport of virus to the contralateral retina. The ganglion cells thus labeled are a homogeneous population of small neurons (mean diameter, 12.8 +/- 2.2 microns and mean area, 81.8 +/- 21.8 microns 2) with sparsely branching dendrites that are widely distributed over the retina. This population is best identified when virus labeling of retinal projections in areas beyond the hypothalamus is eliminated by lateral geniculate lesions that transect the optic tract at its entry into the geniculate complex. The same population is labeled with retrograde tracers but, with both HRP and FluoroGold, other ganglion cells are labeled, presumably from uptake by fibers of passage, indicating that the virus is a more reliable marker for ganglion cells giving rise to retinohypothalamic projections. The ganglion cells identified correspond to a subset of type III, or W, cells.

Nitric oxide-synthesizing neurons in the hamster suprachiasmatic nucleus: a combined NOS- and NADPH- staining and retinohypothalamic tract tracing study

Neuronal nitric oxide (NO), thought to be a neuroactive substance of high potency, is produced by the enzyme nitric oxide synthase (NOS) which has been demonstrated to additionally exhibit a so-called NADPH-diaphorase (NADPH-d) activity. Since physiological results pointed to the involvement of NO in circadian regulation, and morphological descriptions are not available, we sought to study the distribution of NO-producing cells in the hypothalamic suprachiasmatic nucleus (SCN) in Djungarian hamsters (Phodopus sungorus) by means of histochemistry and immunohistochemistry (IHC). In the SCN, NADPH-d stained perikarya of varying intensity and number were found predominantly in the ventrolateral subdivision. Diaphorase staining combined with the IHC demonstration of NOS revealed a complete overlapping of both. The combination of NADPH-d staining with the demonstration of the retinohypothalamic tract using the anterograde neuronal transport of cholera toxin B (CTB) following intraocular injection showed CTB terminals accumulating at NADPH-d cell bodies mainly in the ventrolateral region of the SCN. These data provide morphological evidence for the involvement of nitric oxide in the mediation of photic stimulation of the circadian oscillator located in the SCN.

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.

Intergeniculate leaflet lesions and behaviorally-induced shifts of circadian rhythms

The aim of this work was to assess the effect of lesions of the intergeniculate leaflet on nonphotic phase shifts produced by confining hamsters to novel running wheels for 3 h in the middle of the subjective day. In intact hamsters this procedure produces large phase advances provided that the hamsters maintain high levels of wheel running during the confinement. Intergeniculate leaflet lesions blocked or reduced phase shifts after confinement to a novel wheel. However, for most animals these lesions also reduced both the amount of activity during the 3 h pulse in the novel wheel and the amount of daily wheel running in the home cage. To boost activity of lesioned hamsters to levels associated with large phase shifts, the animals were confined to novel wheels at low ambient temperature. The lesioned hamsters still failed to show large phase shifts. The benzodiazepine triazolam also failed to induce phase shifts in lesioned animals, but it induced less activity in lesioned animals as compared to sham-operated controls. The data support the hypothesis that the intergeniculate leaflet conveys information about nonphotic phase-shifting to the circadian pacemaker in the suprachiasmatic nucleus. They also raise the possibility that some effects of intergeniculate leaflet lesions previously interpreted as having a photic basis, might be due to the activity-lowering effect of the lesions.

Projections of the suprachiasmatic nuclei, subparaventricular zone and retrochiasmatic area in the golden hamster

The patterns of projections from the hamster suprachiasmatic nucleus, retrochiasmatic area and subpraventricular hypothalamic zone were examined using anterograde tracing with the plant lectin, Phaseolus vulgaris leucoagglutinin. Suprachiasmatic nucleus efferents comprise four major fiber groups: (i) an anterior projection to the ventral lateral septum, the bed nucleus of the stria terminalis and anterior paraventricular thalmus; (ii) a periventricular hypothalamic projection extending from the preoptic region to the premammillary area; (iii) a lateral thalamic projection to the intergeniculate leaflet and ventral lateral geniculate; and (iv) a posterior projection to the posterior paraventricular thalamus, precommissural nucleus and olivary pretectal nucleus. The retrochiasmatic area showed a similar projection pattern with several major exceptions. There are projections to endopiriform cortex, fundus striati, ventral pallidum, horizontal limb of the nucleus of the diagonal band and three separate routes to the amygdala. There are also projections laterally with fibers of the supraoptic commissures, which enter the superior thalamic radiation and innervate the caudal dorsomedial thalamic nuclei. Other fibers traveling with the commissures terminate in the ventral zona incerta. The subparaventricular zone projects to most targets of the suprachiasmatic nucleus, but not to the intergeniculate leaflet. There is a substantial input to both the subparaventricular zone and retrochiasmatic area from the suprachiasmatic nucleus, but little apparent reciprocity. There is extensive overlap of suprachiasmatic nuclei and retrochiasmatic efferents, and between retrochiasmatic and known medial amygdaloid efferents. The anatomical information is discussed in the context of circadian rhythm regulation, photoperiodism and chemosensory pathways controlling male hamster reproductive behavior.

Anterograde neuronal tracing of retinohypothalamic projections in the hamster--possible innervation of substance P-containing neurons in the suprachiasmatic nucleus

The retinohypothalamic tract (RHT) in the Djungarian hamster Phodopus sungorus was studied using anterograde neuronal transport of cholera toxin subunit B (CTB) following unilateral intraocular injection. A major projection terminates bilaterally in the hypothalamic suprachiasmatic nuclei (SCN). In the anterior ventral SCN, a light ipsilateral predominance was evident. In the medial SCN, labelling was concentrated laterally where it was seen over the dorso-ventral extension of the nuclei, pronounced contralaterally to the site of CTB injection, which was even more characteristic in the posterior aspects of the nuclei. Labelled fibers and terminals were observed in the supraoptic nuclei, but not in lateral and paraventricular hypothalamic regions. Additional experiments utilizing double immunofluorescence of CTB and of substance P (SP) in the SCN showed that SP-containing perikarya were particularly observed in a central portion of the nucleus, where CTB-stained terminals were accumulated in the vicinity of immunoreactive cell bodies, fibers and terminals. Our data provide preliminary morphological evidence for the regulation of SCN function by retinal afferents and may explain the circadian and photoperiodic fluctuations in the amount of SP in the SCN.

The control of circadian rhythms and the levels of vasoactive intestinal peptide mRNA in the suprachiasmatic nucleus are altered in spontaneously hypertensive rats

Vasoactive intestinal peptide (VIP) has been localized within the suprachiasmatic nucleus of the hypothalamus (SCN) and appears to play an important role in the entrainment of circadian rhythms with the light-dark (LD) cycle. The spontaneously hypertensive rat (SHR), an inbred strain used extensively in research on primary hypertension, has significantly more VIP mRNA in its brain than normotensive Wistar-Kyoto control (WKY) rats. Because VIP levels are abnormally high in SHR rats the present study examined whether the mechanisms controlling circadian rhythms are also altered in SHR rats. When entrained to a 24 h LD cycle, SHR rats began their wheel-running rhythm approximately 1.5 h earlier than WKY controls. SHR rats re-entrained to a phase delay in the LD cycle more slowly than did WKY rats, but tended to re-entrain to a phase advance more rapidly. The free-running period of SHR rats in both constant light and constant dark was significantly shorter than that of WKY rats. In SHR rats, phase delays produced by 1-h pulses of light were less than one-half the magnitude of the delays seen in WKY rats; however, the phase advances were nearly twice that of WKY rats. Using in situ hybridization, the SCN levels of mRNA encoding VIP were found to be significantly greater in SHR rats, but the mRNA levels of another peptide important for entrainment, gastrin releasing peptide, did not differ between SHR and WKY rats. These data indicate that the mechanisms controlling circadian rhythms in SHR rats differ significantly from those controlling rhythms in WKY rats and that VIP mRNA is significantly elevated within the SCN of SHR rats. The role of VIP in the entrainment of circadian rhythms is discussed.

Direct and indirect retinohypothalamic projections to the supraoptic nucleus in the female albino rat

Earlier studies have shown that retinohypothalamic projections terminate extensively within the hypothalamus of the rat. Recently, we identified a light retinal projection to the supraoptic nucleus as well as a larger, well-focused projection resulting in a peri-supraoptic nucleus terminal field. In this study, we employed a double labeling method with cholera toxin conjugated to horseradish peroxidase (CT-HRP) and pseudorabies virus, a transsynaptic neural tracer, to evaluate retinorecipient neurons in both the supraoptic nucleus and peri-supraoptic nucleus terminal field. In addition, we looked for evidence that cells in the peri-supraoptic nucleus terminal field project into the supraoptic nucleus. Three strains of pseudorabies virus were compared. A direct retinosupraoptic nucleus circuit was confirmed with all three strains. Retinorecipient neurons in the peri-supraoptic nucleus terminal field were also confirmed. However, there was a strain-based difference in the identification of these neurons. The wild-type Becker strain labeled cells in the peri-supraoptic nucleus terminal field in a manner paralleling the early, intermediate and late stages of infection of the suprachiasmatic nucleus. This parallel time course suggests that retinal ganglion cells terminate directly on cells in the peri-supraoptic nucleus terminal field. Conversely, the Bartha and PRV-91 strains showed appreciable labeling of peri-supraoptic neurons only at long survival times. This longer time course suggests that these mutant strains label neurons in the peri-supraoptic nucleus terminal field indirectly, after passing through additional neurons. In addition, experiments with monocular injection of CT-HRP and posterior pituitary injection of pseudorabies virus showed retrogradely labeled second-order cells in the peri-supraoptic nucleus amidst the CT-HRP labeled terminal field of the retinohypothalamic tract. These results demonstrate a direct projection from the retina to the supraoptic nucleus and provide evidence for an indirect circuit from the retina to the supraoptic nucleus via neurons located in the peri-supraoptic nucleus terminal field. The strain-based differences imply that those retinal ganglion cells that project to the peri-supraoptic nucleus terminal field differ from those that project to the suprachiasmatic nucleus. In addition, these results suggest a neuroanatomic basis for photic effects on physiological mechanisms that are not mediated by the circadian timing system.

Retinohypothalamic projections and immunocytochemical analysis of the suprachiasmatic region of the desert iguana Dipsosaurus dorsalis

Two separate and distinct retinal projections to the hypothalamus in the iguanid lizard Dipsosaurus dorsalis were described using horseradish peroxidase and cobalt-filling techniques. Both of the projections were unilateral and completely crossed; one terminated in the supraoptic nucleus and the other in the suprachiasmatic nucleus. Immunocytochemical analysis showed that the supraoptic nucleus contained cell bodies and fibers that cross-react with antibodies raised against arginine vasopressin, while the suprachiasmatic nucleus contained arginine vasopressin-like immunoreactive fibers emanating from cells in the nearby paraventricular nucleus. The suprachiasmatic nucleus contained a dense plexus of fibers that cross-reacted with neuropeptide-Y antibody. Antiserum against vasoactive intestinal polypeptide showed no reactivity in any part of the forebrain, while antiserum against serotonin showed sparse and uniform reactivity throughout the forebrain, including the suprachiasmatic nucleus. These results, together with other data, indicate that the suprachiasmatic nucleus of D. dorsalis is homologous to the suprachiasmatic nuclei of rodents, structures known to contain circadian pacemakers. We suggest that the suprachiasmatic nucleus may play a similar role in the circadian system of D. dorsalis.

Structural differences between optic synapses in the suprachiasmatic nucleus of hooded and albino rats

Synapses of retinal afferents (optic synapses) in the suprachiasmatic nucleus (SCN) were morphometrically compared between hooded and albino rats. Optic synapses from albino rats show larger boutons with larger mitochondria, synaptic appositions with less postsynaptic density material, relatively more symmetric (presumably inhibitory) contacts, and relatively less asymmetric (presumably excitatory) contacts, compared with hooded rats. There are no differences in the numbers of clear and dense core vesicles, the size of synaptic appositions, and their numerical density in area of micrograph. Possible differences in activity and/or metabolism between optic synapses from hooded and albino rats are discussed.

The circadian visual system

The retina transduces photic stimuli and transmits that information centrally for further processing. This review emphasizes the fact that the nervous system components governing circadian rhythmicity constitute a specialized subdivision of the vertebrate visual system. The brain houses different targets for retinal efferents parcellated according circadian or non-circadian function. Although the suprachiasmatic nucleus (SCN), being the site of the master circadian clock, is necessary for the generation of circadian rhythmicity, precise phase regulation of any rhythm is subject to modulation by SCN-afferent processes. Photic information necessary for entrainment arrives at the SCN via the retinohypothalamic tract. The geniculohypothalamic tract, originating in the intergeniculate leaflet (IGL), provides a secondary route by which photic information can reach the SCN. It also projects extensively to the contralateral IGL and receives reciprocal input from the SCN region. An interaction between the circadian and non-circadian visual systems may exist through connections of the superior colliculus with ventrolateral geniculate leaflet (VLG) and IGL. The SCN, IGL, VLG and superior colliculus are all innervated by serotonin-containing fibers. The following observations are likely to have an impact beyond the rhythm field itself: certain transneuronal tracers label only the circadian visual system; c-fos protein synthesis is induced in the circadian, but not non-circadian, visual system by a phasically active stimulus; blockade of SCN action potentials is unable to alter circadian rhythmicity; transplantation of dispersed fetal SCN cells to arrhythmic adults restores circadian periodicity, but not phase response to light; and the IGL is actually a very extensive part of the lateral geniculate complex.

Neuropeptide Y microinjected into the suprachiasmatic region phase shifts circadian rhythms in constant darkness

The geniculohypothalamic tract (GHT) is a projection from the intergeniculate leaflet to the suprachiasmatic nucleus (SCN). The GHT exhibits neuropeptide Y (NPY) immunoreactivity and appears to communicate photic information to the SCN. Microinjection of NPY into the SCN has been found to phase shift circadian rhythms of hamsters housed in constant light in a manner similar to the phase shifts produced by pulses of darkness or triazolam injections. In the present study, NPY was injected into the SCN of Syrian hamsters housed in constant darkness and was found to produce phase shifts similar to those seen in hamsters housed in constant light. Microinjections were not followed by wheel running during the subjective day (the time when NPY microinjections are followed by significant phase advances). These data suggest that NPY produces phase shifts by some mechanism other than by inducing wheel running or by inhibiting the response of SCN neurons to light and supports a role for NPY in nonphotic shifting of the circadian clock.

Retinohypothalamic tract development in the hamster and rat

The development of the retinohypothalamic tract (RHT) in the albino rat and golden hamster was studied using anterograde transport of cholera toxin conjugated to horseradish peroxidase (CT-HRP). The RHT has three components in the adult: (1) a dense projection to the ventrolateral subdivision of the suprachiasmatic nucleus (SCN) with some fibers extending into the dorsomedial SCN; (2) a projection to adjacent areas, the anterior hypothalamic area (AHA) and retrochiasmatic area (RCA) and in the hamsters, into the preoptic area (POA); (3) a projection to the lateral hypothalamic area (LHA). In the rat, the projection to the SCN and adjacent areas first appears as scattered varicosities at the ventral border of the SCN at postnatal day 1 (P1) and gradually increases until the adult pattern is achieved at approximately P10. The projections to the AHA and RCA are seen first at P2-P3 and gradually increase to the adult appearance by P15. Both the projection to the SCN and adjacent areas and to the LHA, initially are more extensive than in the adult. Many of the axons extend well beyond the zone of the adult pattern but these anomalous fibers are eliminated by P6-P10. The LHA projection first appears at embryonic day 21-22 (E 21-22) and gradually increases in density from P1-P6. In the hamster the projections to the SCN, AHA and LHA appear first on P4 and gradually increase in density to reach the adult pattern by P15. The projections to the RCA and POA are present by P6 and reach the adult pattern by P15. None of the RHT projections in the hamster has the initial extended growth followed by pruning back that characterizes RHT development in the rat. Thus, the development of the RHT in both the rat and the hamster is complex with components of the projection appearing at different times with differing patterns of development that indicate specialized interactions of the developing axons with their target neurons. Synaptogenesis in the hamster hypothalamus was analyzed using an antiserum to synapsin I. Few synapses are present at E16, the last day of gestation, in the LHA, SCN and AHA. From P1-P3, synaptogenesis proceeds rapidly and the adult pattern is achieved in all three areas by P4.

Development of the hamster serotoninergic system: cell groups and diencephalic projections

Nuclei of the circadian visual system are extensively innervated by serotoninergic neurons and rhythmicity is modulated by the serotoninergic system. This study investigated the temporal relationships between prenatal origins of serotoninergic cell groups and perinatal innervation of structures in the hamster circadian visual system as well as in the remaining diencephalon. Serotonin-immunoreactive (5-HT-IR) neurons of the B4-B9 complex were first seen on embryonic day 8 (E8). The number of neurons increases sharply by E10 when the first 5-HT-IR cells are evident in the medulla (B1-B3 complex). The distribution of serotoninergic neurons in the hamster brainstem is generally adult-like by E14. Thick 5-HT-IR fibers arch around the mesencephalic flexure at E10 and reach more rostral mesencephalic areas at E11. A branch of the medial forebrain bundle (MFB) projects ventrally toward the retrochiasmatic area; a second branch ascends along the fasciculus retroflexus. Fibers cross the midline in the supraoptic commissure by E12, other arrive in the lateral geniculate region, and a branch of the MFB extends toward the mammillary area. At E13, a periventricular medial thalamic branch of the MFB is seen, axons appear in the supramammillary commissure, and a fine fasciculus between the medial thalamus and intergeniculate leaflet is visible. Lateral, paraventricular, and retrochiasmatic hypothalamic areas and centro- and dorsomedial thalamus are densely innervated at E14. The mammillary area and lateral geniculate body are moderately innervated, and the first fibers appear in the deep laminae of the superior colliculus. The innervation of the suprachiasmatic nuclei, periventricular hypothalamus, and superficial layers of the superior colliculus occurs postnatally. The results are consistent with serotoninergic system development in other species.

[Work of the inner clock. Neuroanatomy of circadian systems of mammals]

Many aspects of mammalian life exhibit distinct alterations throughout the 24-h cycle. Morphological, physiological, and biochemical parameters display circadian rhythms which are thought to be generated by an endogenous pacemaker and regulated by environmental factors. The morphological substrates of the endogenous circadian system have been studied extensively during the last two decades. Although knowledge is far from complete, there is general agreement that the pathways involved consist mainly of retina, hypothalamus, spinal cord, sympathetic trunk, and pineal gland. This review characterizes the anatomical structures and tracts responsible for generation and maintenance of circadian rhythmicity and discusses functional implications of neurotransmitter involvement and the selectivity of connections.

Cellular communication in the circadian clock, the suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus functions as the circadian clock in the mammalian brain. Communication between the cells of the suprachiasmatic nucleus is likely to be responsible for the generation and accuracy of this biological clock. Communication between many cells of the brain is mediated by action potentials that pass down the axon and cause release of neurotransmitters at the neuronal synaptic junction. Additional mechanisms of cellular communication appear to operate in the suprachiasmatic nucleus. Several lines of evidence point to multiple modes of cellular communication: these include the continuing operation of the clock after Na(+)-mediated action potentials have been blocked, the orchestrated metabolic rhythms of suprachiasmatic nucleus cells prior to synaptogenesis, the entrainment of fetal to maternal rhythms, and the rapid recovery of function after suprachiasmatic nucleus transplants into arrhythmic rodents. Possible alternative means of intercellular communication in the suprachiasmatic nucleus are examined, including calcium spikes in presynaptic dendrites, ephaptic interaction, paracrine communication, glial mediation, and gap junctions. This paper identifies and examines some of the unanswered questions related to intercellular communication of suprachiasmatic nucleus cells.

Effects of light stimulation on the activity of the autonomic nerves in anesthetized rats

The existence of the retino-hypothalamic pathway suggests that light stimulation may influence the activity of the autonomic outflows. Efferent activities of the pancreatic, hepatic, and gastric branches of the vagus nerve and those of pancreatic, hepatic, splenic, adrenal, and renal branch of the splanchnic nerve were recorded. Light stimulation with 2000 1x for 10 min to the left eye increased the splanchnic (sympathetic) outflows and suppressed the vagal outflows. The effects lasted for several hours. The minimal effective stimulation was 20 1x for 1 min or 200 1x for 0.1 min. These responses were observed in the light period as well as dark period. However, in the suprachiasmatic nucleus (SCN) lesioned rat, the changes in autonomic outflows following light stimulation were absent. The observations suggest that light stimulation modulates visceral functions through changes in the autonomic nervous system activities via the SCN.

Suprachiasmatic circadian pacemaker of rat shows two windows of sensitivity to neuropeptide Y in vitro

The geniculohypothalamic tract carries visual information from the intergeniculate leaflet to the suprachiasmatic circadian pacemaker. NPY, found in this projection, has been shown to affect the phase of behavioral rhythms and influence photic entrainment. We now demonstrate that NPY, when briefly applied to the geniculate projection sites of rat SCN in vitro, induces permanent phase-shifts in the rhythm of neuronal electrical activity at two separate phases of the circadian cycle.

Substance P in the suprachiasmatic nucleus of the rat: an immunohistochemical and in situ hybridization study

Using a biotin-streptavidin-horseradish peroxidase (HRP) immunohistochemical technique the distribution of substance P-immunoreactive neuronal elements was investigated in the rat suprachiasmatic nucleus (SCN). Substance P-immunoreactive nerve fibres and varicosities were distributed throughout the suprachiasmatic nucleus, with the largest accumulation in its ventral part. Because this location overlaps with the innervation of retinal afferents, the distribution and density of substance P-immunoreactive fibres in bilaterally enucleated rats were compared to normal rats. The density of substance P-immunoreactive fibres and nerve terminals in the ventral part of the suprachiasmatic nuclei was reduced in the rats with bilateral destruction of the optic nerves, whereas the density of fibres and nerve terminals in the dorsal part as well as other retinal target areas in the thalamus and mesencephalon was unaffected. In rats pretreated with an intraventricular injection of colchicine several substance P-immunoreactive perikarya were identified in the suprachiasmatic nucleus. The immunoreactive neurons, measuring 9.7 microns +/- 1.1 microns in diameter, were frequently observed in the central core of the nucleus and to a lesser extent in the dorsomedial and ventrolateral subparts. Using in situ hybridization histochemistry pre-protachykinin-A mRNA was found in the same part of the SCN indicating that synthesis of substance P takes place in SCN neurons. Using a double immunohistochemical approach applying diaminobenzidine and benzidinedihydrochloride as chromagens substance P-, vasoactive intestinal peptide (VIP)-, and vasopressin/neurophysin-immunoreactivities were identified in the same brain section. The substance P-immunoreactive perikarya constituted a separate population of SCN neurons, which were not vasopressin-, neurophysin- or VIP-immunoreactive. Taken together, these observations show that substance P is contained in the retinohypothalamic pathway and within a group of SCN cell bodies, indicating that substance P may play a role in the generation and entrainment of circadian rhythmicity.

A cholinergic antagonist, mecamylamine, blocks light-induced fos immunoreactivity in specific regions of the hamster suprachiasmatic nucleus

Recent studies have shown that light-induced phase shifts of the circadian rhythm of locomotor activity are associated with c-fos expression in the suprachiasmatic nucleus (SCN) of rodents. In order to explore further the importance of c-fos expression for the phase-shifting effects of light, we examined the effects of mecamylamine on light-induced Fos-like immunoreactivity (Fos-lir) in the SCN. Mecamylamine was chosen because it is a cholinergic antagonist that blocks the phase-shifting effects of light on the circadian activity rhythm in the golden hamster. Golden hamsters were entrained for at least 14 days to a 14 h light: 10 h dark photoperiod. Animals were then placed in constant darkness (DD) and during exposure to DD were subjected to one or more of the following treatments: (1) vehicle alone; (2) mecamylamine alone (450 micrograms, i.c.v.) at circadian time (CT) 19; (3) vehicle plus a light pulse at CT 19; (4) mecamylamine 10 min prior to the light pulse at CT 19. Mecamylamine blocked the phase-shifting effects of light on the circadian rhythm of locomotor activity when compared to vehicle-treated animals (P < 0.001). A light pulse at CT 19 induced Fos-lir in the SCN within 1 h, whereas treatment with vehicle or mecamylamine had no noticeable effect on Fos-lir in the SCN. Mecamylamine pretreatment dramatically reduced light-induced Fos-lir in the SCN by 75%. The most striking observation was the clear inhibition of Fos-lir by mecamylamine in the dorsomedial region of the SCN while there was little inhibition of Fos-lir in the most ventral portions of the SCN.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in the retinohypothalamic tract in albino Lewis versus brown Norway rat strains

Differences in sleep-wake patterns in response to light-dark stimulation have been observed between albino Lewis and pigmented Brown Norway strains of rats, which may be associated with albinism. Since several anatomical differences have been demonstrated in the visual pathways of albino and pigmented mammals, the present study was undertaken to determine whether additional differences in visual pathways of these rat strains exist that might account for their behavioral differences. Using anterograde tracing techniques and image analysis, we have investigated the retinal projections of Lewis and Brown Norway rats. Our results demonstrate that the distribution of retinal terminals in the hypothalamic suprachiasmatic nucleus extends over a greater area in Lewis compared to Brown Norway rats. This zone of termination corresponds to a cytoarchitectonically definable ventrolateral subdivision of the suprachiasmatic nucleus (SCN), which is also greater in Lewis than in Brown Norway rats. These results may have implications for behaviors related to the SCN.

Membrane properties of rat suprachiasmatic nucleus neurons receiving optic nerve input

1. The electrophysiological properties of suprachiasmatic nucleus (SCN) neurons (n = 33) receiving optic nerve input were studied with intracellular recordings in rat hypothalamic slices maintained in vitro. Our major goal was to provide baseline data concerning the intrinsic membrane properties of these neurons and to test the hypothesis that the neurons are homogeneous electrophysiologically. 2. Action potentials were short in duration and followed by a pronounced hyperpolarizing after-potential. Spike amplitude (58.2 +/- 1.1 mV, mean +/- S.E.M.; measured from threshold), spike duration (0.83 +/- 0.03 ms; measured at half amplitude) and hyperpolarizing after-potential amplitude (23.9 +/- 1.0 mV; measured from threshold) appeared unimodally distributed and did not co-vary. 3. Intracellular injection of depolarizing current pulses evoked spike trains, and spike inactivation, spike broadening and frequency accommodation were always present. An after-hyperpolarization followed the spike train in all but one neuron. 4. Membrane time constant ranged from 7.5 to 21.1 ms (11.4 +/- 0.7 ms, n = 27), and its distribution appeared to be unimodal with the peak at approximately 10 ms. Input resistance ranged from 105 to 626 M omega (301 +/- 23 M omega, n = 33); the distribution also appeared unimodal with its peak at approximately 250 M omega. 5. A subpopulation (16 of 33, 48%) of the neurons exhibited slight (6-29%) time-dependent inward rectification in their voltage responses to hyperpolarizing current injection. Of the neurons lacking the time-dependent rectification, some (n = 5) exhibited time-independent inward rectification of 6-20% and others showed no (or < 3%) such rectification. The degree of inward rectification was correlated with neuronal excitability (r = 0.60, P < 0.002; assessed by measuring the steepness of the primary slope of the frequency-current plot) and with the spontaneous firing rate (r = 0.49, P < 0.007). Furthermore, the neurons with > 6% inward rectification (neurons with time-dependent and -independent rectification were combined) were more excitable (362 +/- 43 Hz/nA (n = 15) vs. 221 +/- 37 Hz/nA (n = 9), P < 0.05) and had a higher spontaneous firing rate (11.1 +/- 1.9 Hz (n = 19) vs. 3.9 +/- 1.5 Hz (n = 11), P < 0.02) than the neurons with no or negligible (i.e. < 3%) inward rectification. The two groups, however, were not significantly different in membrane time constant and input resistance. 6. When adequately hyperpolarized, twelve of seventeen (71%) neurons generated small low-threshold spike (LTS) potentials in response to depolarizing current pulses.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Retinal projection to the olfactory tubercle and basal telencephalon in primates

The retinal projection to the basal telencephalon was studied in eight species of primates from the suborders Strepsirhini and Haplorhini, including one anthropoid primate, the gibbon. Animals received an intraocular injection of tritiated amino acids and the distribution of retinal fibers and terminals was demonstrated by autoradiographic techniques in horizontal and coronal sections. In all species a discrete group of labeled retinal fibers is observed to branch off from the dorsolateral aspect of the optic tract at the level of the suprachiasmatic nucleus. These fibers, destined to the basal telencephalon, are topographically distinct from the retinal fibers which innervate the suprachiasmatic nucleus and medial hypothalamic regions. The fibers of the retinotelencephalic tract course dorsally above the supraoptic nucleus through the lateral hypothalamic area and then proceed further rostrally and laterally below the diagonal band of Broca towards the olfactory tubercle. Within the olfactory tubercle, terminal distribution of label is observed in the mediocaudal region along the granular cell layer II. In the macaque this cellular layer shows a characteristic thickening in the region of retinal terminals which is evident in both coronal and horizontal section. In some species this labeled region is seen within the superficial bulge of the tubercle on the ventral aspect of basal telencephalon. In all primates the retinal projection to olfactory tubercle is bilateral. In prosimians label is predominantly contralateral to the injected eye, in New World monkeys label is equally distributed on both sides of the brain and in Old World monkeys label is mainly found ipsilaterally. Retinal fibers were also seen in the periamygdaloid region but never extended as far as piriform cortex. These results, in addition to previous studies in other mammalian orders, confirm that the basal telencephalon, and in particular the olfactory tubercle, constitutes a region of visual and olfactory convergence. This sensory integration may be related to photic and chemosensory modulation of reproductive physiology and behavior.

Effect of substance P on circadian rhythms of firing activity and the 2-deoxyglucose uptake in the rat suprachiasmatic nucleus in vitro

The suprachiasmatic nuclei (SCN) have been identified as a pacemaker for many circadian rhythms in mammals. Although substance P (SP) fibers from retina are found to terminate the SCN, the physiological role of this peptide is uncertain. The 2-deoxyglucose (2-DG) uptake and firing activity in the SCN show a robust circadian change. SP causes an increase in 2-DG uptake by SCN during the subjective night but not during subjective day. SP-induced increase in 2-DG uptake is blocked by co-treatment with the SP receptor antagonist, spantide. Treatment with SP produces phase shifts of circadian rhythm in spontaneous neural activity in SCN neurons with a phase-response curve that is similar to the effect of light pulses to animals under constant darkness. SP-induced phase change is also blocked by pretreatment with spantide. SP-induced increase in 2-DG uptake and phase changes in firing activity occur only during subjective night, at circadian times when photic phase shifting of activity occurs. The present results suggest that SP may be an important transmitter for conveying environmental light-dark information from retina to the SCN.

Melatonin influences Fos expression in the rat suprachiasmatic

Administration of the pineal hormone melatonin to rats induces expression of Fos, the protein product of the c-fos proto-oncogene, in the suprachiasmatic nucleus (SCN), the putative biological clock of mammals. Expression of the Fos protein is dependent on circadian phase: injections in the late subjective night (circadian time (CT) 22) induce Fos expression in cells within the ventral SCN whereas injections during the subjective day are ineffective. Since melatonin injections in the late subjective day have previously been shown to phase advance circadian rhythms, these results indicate that phase-advances of the circadian system can occur without increased expression of Fos protein in the SCN, at least at levels detectable by immunohistochemistry. In support of in situ hybridization histochemical evidence obtained previously, immunocytochemical data from vehicle-injected control rats suggest that the Fos protein undergoes an endogenous fluctuation with peak levels in the SCN occurring during the subjective night. These observations indicate that melatonin can affect immediate early gene expression within the SCN.

Effect of orbital enucleation on glucose homeostasis and morphology of the suprachiasmatic nucleus

In rats there is a direct neural connection called the retinohypothalamic tract (RHT) from retinal ganglion cells to the ventrolateral part of the suprachiasmatic nucleus (SCN), which has neurons containing vasoactive intestinal polypeptide (VIP)-like substance. Previously, we observed that bilateral orbital enucleation (blinding) caused temporary suppression of the hyperglycemic response to intracranial injection of 2-deoxy-D-glucose (2DG) from week 4 to 6 after blinding. Moreover, bilateral lesions of the SCN had a similar effect. From these findings, we supposed that the neurons responsible for the hyperglycemic response to 2DG were present in the SCN, that after blinding these neurons temporarily lost their activity, and that this functional change was reflected in the morphology of the SCN. To investigate this possibility, we examined the morphological changes of the SCN by Nissl staining and immunohistochemical studies with anti-VIP and anti-peptide histidine isoleucine (PHI) antibodies in blinded rats, and the relationship between these morphological changes and the hyperglycemic response to 2DG. After surgical blinding, we observed following changes. (1) The optic chiasm became thinner. (2) The SCN became displaced rostrally. (3) The density of neurons in the middle to caudal part of the SCN, where the retinal ganglion cells projected, decreased markedly without change in cell number during the period when the hyperglycemic response to intracranial injection of 2DG was temporarily suppressed after blinding. The first and second changes seemed to reflect reduction of fibers and axon terminals of retinal ganglion cells and their innervation, respectively. As the third change was parallel with suppression of the hyperglycemic response to 2DG injection, it may reflect functional change of the neurons in the SCN that are responsible for the hyperglycemia due to 2DG.

Light enhances sympathetic and suppresses vagal outflows and lesions including the suprachiasmatic nucleus eliminate these changes in rats

Neurons in the suprachiasmatic nucleus (SCN) are suggested to be involved in the mechanism of glucose homeostasis. This mechanism was examined by studies on the effect of illumination on the activity of autonomic efferents to the adrenals, pancreas and liver. Exposure of one eye of anesthetized rats to light enhanced the efferent activity of the adrenal nerve and suppressed that of vagal pancreatic and hepatic nerves. No change in efferent activities of these nerves was observed on light-stimulation of rats with lesions that included the bilateral SCN. These findings indicate that light signals modulate visceral functions including metabolic processes through the retinohypothalamic tract probably via the SCN to autonomic efferent pathways innervating visceral organs.

VIP-like immunoreactivity in the suprachiasmatic nuclei of a mutant anophthalmic mouse

The suprachiasmatic nuclei (SCN) of mutant anophthalmic (strain ZRDCT-An) and sighted control mice was examined using light microscopic methods for cytoarchitecture and immunocytochemistry for vasoactive intestinal polypeptide (VIP). The SCN of the anophthalmic mice were asymmetrical, and quite variable in cytoarchitecture. Immunocytochemistry for VIP revealed strong staining of cells and fibers. Within the SCN, the distribution of VIP-immunoreactive cells was more diffuse in anophthalmic mice than in the controls; immunoreactive neurons were frequently observed in ectopic locations in the mutant animals.

Demonstration of a direct projection from the retina to the hypothalamic supraoptic nucleus of the hamster

Hamsters were injected in the left eye with unconjugated cholera toxin subunit B (CHB) and the tracer was localized using immunohistochemistry. A large number of immunoreactive fibers was found in the suprachiasmatic nuclei and the contralateral lateral hypothalamic area. Labeled fibers coursed around the supraoptic nucleus and nerve terminals accumulated in a zone dorsally and laterally to the nucleus. Single fibers from this plexus penetrated into the supraoptic nucleus, where few fibers arborized into delicate immunoreactive profiles possessing varicosities. Labeled fibers were identified only in the dorsal and lateral parts of the nucleus, and mostly at the caudal level of the optic chiasm. These results show a direct retinal innervation of the magnocellular neurons of the supraoptic nucleus, and indicate a direct photic influence on the hypothalamo-neurohypophysial system.

Unilateral enucleation alters vasoactive intestinal polypeptide-like immunoreactivity in the suprachiasmatic nucleus of the rat

Adult Sprague-Dawley rats were unilaterally enucleated under deep anesthesia and sacrificed 3-11 months post-operatively. The brains were removed and the suprachiasmatic nuclei were evaluated for vasoactive intestinal polypeptide (VIP)-like immunoreactivity. Our data show that expression of VIP is reduced in the nucleus that is deprived of retinal input.

Calretinin in rat brain: an immunohistochemical study

Calretinin is a calcium-binding protein related to calbindin-D28k; both are present in different though overlapping sets of neurons in brains of birds and mammals. We describe in detail the pattern of calretinin immunoreactivity in the rat brain. As in chick brain, calretinin immunoreactivity is abundant in various sensory pathways (particularly certain cells and fibres of the cochlear nuclei and olfactory bulb), in the heterogeneous parts of the brainstem and in parts of the hypothalamus. Many primary sensory fibres are strongly positive. Major groups of calretinin-positive neurons also include the thalamic reticular nucleus, triangular septal nucleus, lateral mammillary nucleus and substantia nigra pars compacta. Many other calretinin-positive cells are recognizable as local inhibitory neurons. Calretinin is absent from all but a few cells in the cerebral cortex, and is never found in motor neurons. There are also some distinctive positive structures whose identity is uncertain, notably irregular "shells" of cells and fibres around the thalamus and in the amygdala and an unnamed cell type in the vestibulocerebellum.