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

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.

Loss of photic entrainment and altered free-running circadian rhythms in math5-/- mice

Mammalian free-running circadian rhythms are entrained to the external light/dark cycle by photic signaling to the suprachiasmatic nuclei via the retinohypothalamic tract (RHT). We investigated the circadian entrainment and clock properties of math5-/- mutant mice. math5 is a critical regulator of retinal ganglion cell development; math5-/- mice show severe optic nerve hypoplasia. By anterograde cholera toxin B tracing, we find that math5-/- mice do not develop an identifiable RHT pathway. This appears to be attributable to agenesis or dysgenesis of the majority of RHT-projecting retinal ganglion cells. math5-/- mice display free-running circadian rhythms with a period approximately 1 hr longer than B6/129 controls (24.43 +/- 0.10 vs 23.62 +/- 0.19 hr; p < 0.00001). The free-running period of heterozygote mice is indistinguishable from that of controls. math5-/- mice show no entrainment to light/dark cycles, whereas heterozygote mice show normal entrainment to both 12 hr light/dark cycles and to a 1 hr skeletal photoperiod. math5-/- mice show reduced ability to entrain their rhythms to the nonphotic time cue of restricted running wheel access but demonstrate both free-running behavior and entrained anticipation of wheel unlocking in these conditions, suggesting the presence of a second diurnal oscillatory system in math5-/- animals. These results demonstrate that retinal ganglion cell input is not necessary for the development of a free-running circadian timekeeping system in the suprachiasmatic nucleus but is important for both photic entrainment and determination of the free-running period.

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.

Suprachiasmatic nucleus and intergeniculate leaflet in the diurnal rodent Octodon degus: retinal projections and immunocytochemical characterization

The neural connections and neurotransmitter content of the suprachiasmatic nucleus and intergeniculate leaflet have been characterized thoroughly in only a few mammalian species, primarily nocturnal rodents. Few data are available about the neural circadian timing system in diurnal mammals, particularly those for which the formal characteristics of circadian rhythms have been investigated. This paper describes the circadian timing system in the diurnal rodent Octodon degus, a species that manifests robust circadian responses to photic and non-photic (social) zeitgebers. Specifically, this report details: (i) the distribution of six neurotransmitters commonly found in the suprachiasmatic nucleus and intergeniculate leaflet; (ii) the retinohypothalamic tract; (iii) the geniculohypothalamic tract; and (iv) retinogeniculate projections in O. degus. Using immunocytochemistry, neuropeptide Y-immunoreactive, serotonin-immunoreactive and [Met]enkephalin-immunoreactive fibers and terminals were detected in and around the suprachiasmatic nucleus; vasopressin-immunoreactive cell bodies were found in the dorsomedial and ventral suprachiasmatic nucleus; vasoactive intestinal polypeptide-immunoreactive cell bodies were located in the ventral suprachiasmatic nucleus; [Met]enkephalin-immunoreactive cells were located sparsely throughout the suprachiasmatic nucleus; and substance P-immunoreactive fibers and terminals were detected in the rostral suprachiasmatic nucleus and surrounding the nucleus throughout its rostrocaudal dimension. Neuropeptide Y-immunoreactive and [Met]enkephalin-immunoreactive cells were identified in the intergeniculate leaflet and ventral lateral geniculate nucleus, as were neuropeptide Y-immunoreactive, [Met]enkephalin-immunoreactive, serotonin-immunoreactive and substance P-immunoreactive fibers and terminals. The retinohypothalamic tract innervated both suprachiasmatic nuclei equally; in contrast, retinal innervation to the lateral geniculate nucleus, including the intergeniculate leaflet, was almost exclusively contralateral. Bilateral electrolytic lesions that destroyed the intergeniculate leaflet depleted the suprachiasmatic nucleus of virtually all neuropeptide Y- and [Met]enkephalin-stained fibers and terminals, whereas unilateral lesions reduced fiber and terminal staining by approximately half. Thus, [Met]enkephalin-immunoreactive and neuropeptide Y-immunoreactive cells project equally and bilaterally from the intergeniculate leaflet to the suprachiasmatic nucleus via the geniculohypothalamic tract in degus. This is the first report examining the neural circadian system in a diurnal rodent for which formal circadian properties have been described. The data indicate that the neural organization of the circadian timing system in degus resembles that of the most commonly studied nocturnal rodents, golden hamsters and rats. Armed with such data, one can ascertain differences in the functional organization of the circadian system between diurnal and nocturnal mammals.

Anterograde axonal tracing with the subunit B of cholera toxin: a highly sensitive immunohistochemical protocol for revealing fine axonal morphology in adult and neonatal brains

We report an improved immunohistochemical protocol for revealing anterograde axonal transport of the subunit B of cholera toxin (CTB) which stains axons and terminals in great detail, so that single axons can be followed over long distances and their arbors reconstructed in their entirety. Our modifications enhance the quality of staining mainly by increasing the penetration of the primary antibody in the tissue. The protocol can be modified to allow combination in alternate sections with tetramethylbenzidine (TMB) histochemical staining of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Using the protocol, we tested the performance of CTB as an anterograde tracer under two experimental paradigms which render other anterograde tracers less sensitive or unreliable: (1) labeling the entire retinofugal projection to the brain after injections into the vitreal chamber of the eye, and (2) labeling developing projections in the cortex and thalamus of early postnatal mammals. Qualitative comparisons were made with other tracers (Phaseolus vulgaris leucoagglutinin, dextran rhodamine, biotinylated dextran, free WGA, or WGA-HRP) that were used to label these same projections. From these observations it is clear that CTB, visualized with our protocol, provides more sensitive anterograde labeling of retinofugal projections as well as of axonal connections in the neonatal forebrain.

Hypophysiotrophic systems in the brain of the Atlantic salmon. Neuronal innervation of the pituitary and the origin of pituitary dopamine and nonapeptides identified by means of combined carbocyanine tract tracing and immunocytochemistry

The neuroanatomical organization of neurons projecting to the pituitary and the origin of pituitary dopamine and nonapeptides were investigated in the brain of the Atlantic salmon (Salmo salar). Carbocyanine tract tracing in combination with tyrosine hydroxylase, arginine vasotocin and isotocin immunocytochemistry for double labelling revealed a previously unknown organization of hypophysiotrophic cell groups and their extrahypothalamic projections, and provide the first direct identification in a teleost fish of the origin of the dopaminergic and nonapeptidergic innervation of the pituitary. The present data include identification of (1) hypophysiotrophic neurons in the ventral telencephalon and in the periventricular preoptic nucleus, (2) large (magnocellular) vasotocinergic hypophysiotrophic neurons in the most rostral extension of the preoptic area, (3) a distinct neuronal group located in a supraoptic/suprachiasmatic position in the anterior periventricular nucleus, that seems to be the major source of dopaminergic innervation of the pituitary, (4) the nonapeptidergic hypophysiotrophic neurons in the preoptic nucleus, (5) hypophysiotrophic neurons in the ventral and posterior hypothalamus of which some are of liquor-contacting type, (6) projections from hypophysiotrophic and non-hypophysiotrophic neurons in the preoptic nucleus to extrahypothalamic areas such as thalamic and periventricular pretectal nuclei, and (7) subdivisions within the preoptic nucleus that exhibit different combinations of hypophysiotrophic and extrahypothalamic efferent connections. Together with previous studies of retinohypothalamic projections and neurochemical organization of hypothalamic/preoptic areas, the present data suggest that the preoptic nucleus and the anterior periventricular nucleus in teleosts possess functional subdivisions with features that resemble those of the paraventricular, supraoptic and suprachiasmatic nuclei of other vertebrates. In the Atlantic salmon, specific dopaminergic and nonapeptidergic neuronal subdivisions are proposed to play a role for photoperiod control of endocrine activity.

Effects of continuous environmental illumination on the albino rat hypothalamo-neurohypophysial system

Continuous environmental illumination or constant light (LL) exposure causes a suppression of daily water intake, and long-term exposure of greater than 19 days produces a hypertrophy of magnocellular neuroendocrine cells (MNCs) in the hypothalamus. These findings led Glantz to hypothesize that LL increases the secretion of vasopressin (VP). We wanted to determine whether LL could trigger morphological changes within the hypothalamo-neurohypophysial system (HNS) seen with other manipulations that result in enhanced hormone release. The posterior pituitary of male albino rats that were exposed to LL for 24 or 48 h were examined ultrastructurally for evidence of enhanced hormone release. In addition, water intake, plasma VP levels, and MNC size within the supraoptic nucleus (SON) were measured. After LL exposure, the posterior pituitary morphology was different, suggesting enhanced hormone release. LL exposure did not affect plasma VP or the size of SON MNCs, but did suppress drinking behavior. These data show that posterior pituitary morphology is affected rapidly by LL exposure. The HNS response to LL exposure may consist of changes within the first 24 h of LL found within the posterior pituitary followed later by hypertrophy of the SON MNCs.

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.

Photic induction and circadian expression of Fos-like protein. Immunohistochemical study in the retina and suprachiasmatic nuclei of hamster

Fos-immunohistochemistry was performed in the retina and at four rostro-caudal levels of the suprachiasmatic nuclei (SCN) in hamsters. Animals were sacrificed at four circadian times (CT) relative to activity onset (CT12): CT07, 11, 14, 19 either in permanent darkness (DD) or 1 h after light stimulation. Quantification of immunoreactive nuclei showed (i) endogenous CT related changes exclusively within the rostral SCN with maximum immunoreactivity at CT07, (ii) CT related responses to light in retinal displaced amacrines, ganglion cells and caudal SCN (maximum at CT19), (iii) expression differences in four subsets of SCN cells according to CT. The rostral subset could be implicated in the endogenous clock mechanism since it exhibited a fluctuation of Fos immunoreactivity in DD and expression of Fos protein at CTs 06 and 18 when light provokes transients in the free-running period. In the caudal SCN, a ventro-laterally localized set responded to light at CTs 13 and 18, a dorsal crescent of cells responded only at CT18 and a group located laterally between these two responded at CT18. These cellular subsets may have different functions in the light-entrainment mechanism since light stimuli at CT13 induced phase-delays and, at CT18, phase-advances in the onset of the free-running locomotor activity rhythm.