Neurons containing gastrin-releasing peptide and vasoactive intestinal polypeptide are involved in the reception of the photic signal in the suprachiasmatic nucleus of the Syrian hamster: an immunocytochemical ultrastructural study

In mammals, the suprachiasmatic nuclei are involved in the generation of biological rhythms and are synchronized by light input coming from the retina. The targets of retinal afferents and the involvement of neurons containing gastrin-releasing and vasoactive intestinal peptides in photic reception were investigated in the suprachiasmatic nuclei of the Syrian hamster by using light- and electron-microscopic immunocytochemistry. Cholera toxin was used to trace retinal fibers and Fos immunoreactivity to visualize cellular response to light stimulation. Ultrastructural observations were made in the intermediate third of the nuclei, the area of highest overlap for the immunoreactivities investigated. Gastrin-releasing peptide and vasoactive intestinal peptide cell bodies were localized in the ventral part of the nuclei; their dense immunoreactive fiber network often displayed synaptic contacts. Both neuropeptides were colocalized in elongated cells observed near the optic chiasm. Following a light pulse in the middle of the subjective night, Fos protein was expressed in most gastrin-releasing peptide perikarya and in some vasoactive intestinal peptide cells. Retinal terminals mostly occurred in the midline zone between the suprachiasmatic nuclei. Symmetrical or asymmetrical retinal synapses were observed on gastrin-releasing peptide-immunoreactive dendrites and somata, but never on vasoactive intestinal peptide neurons. These results are discussed in relation to the photic entrainment of the circadian clock.     

Neurons that express the AMPA receptor GluR2/3 subunits in suprachiasmatic nuclei of Syrian hamsters colocalize either vasoactive intestinal peptide, peptide histidine isoleucine or gastrin-releasing peptide

In mammals, many circadian rhythms are driven by a clock located inside the suprachiasmatic nucleus of the hypothalamus. They are synchronized to environmental light-dark cycles by information coming directly from the retina via glutamatergic afferents. In rodents, retinal fibres make direct synaptic contacts with neurons synthesizing vasoactive intestinal peptide and gastrin-releasing peptide. These two neuropeptides, administered alone or combined with the peptide histidine isoleucine, phase-shift the clock in the same way that light does. Using ICC and light and electron microscopy, our study demonstrates that subunits 2 and 3 of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-type glutamatergic receptors are colocalized in neurons expressing one or other of these three neuropeptides. Double-labelled neurons were located in the ventral and lateral ventral parts and near the symmetrical plane of the intermediate and caudal thirds of the nucleus. In light microscopy, brown and granular blue stainings of chromogens revealing both antigens were easily identifiable and spatially separated in perikarya. In electron microscopy, almost all the cells observed in these zones expressed the receptor subunits. A few labelled dendritic profiles, some of them post-synaptic, were observed; axon terminals were always unlabelled. Colocalization with vasoactive intestinal peptide and gastrin-releasing peptide was confirmed by the immunogold technique in perikarya and some dendrites. The present study suggests that peptidergic neurons expressing the AMPA receptors are involved in photic entrainment of the clock by the retina without excluding some glutamatergic information coming from other hypothalamic nuclei.     

The serotonin-immunoreactive system of the suprachiasmatic nucleus in the hibernating ground squirrel,Spermophilus richardsonii

Summary In the suprachiasmatic nucleus (NSC) of hibernating and non-hibernating ground squirrels, the distribution of serotonin-immunoreactive (5HT-IR) fibers was studied by the use of the peroxidase-antiperoxidase technique. The cytology of perikarya giving rise to these suprachiasmatic 5HT-IR fibers was investigated in the anterior raphe nuclei. Differences in the immunoreactivity of suprachiasmatic fibers between hibernating and non-hibernating ground squirrels were determined by digital image analysis. The cellular activity was determined densitometrically after RNA-staining in anterior raphe neurons and suprachiasmatic perikarya. Abundant 5HT-IR fibers were observed in the medial and ventromedial portions of the NSC. Frequently, the fibers were found in close contact with perikarya of suprachiasmatic neurons. The central portion of the nucleus and the surrounding hypothalamic areas contained only a few scattered 5HT-IR fibers. Inside the raphe nuclei, 5HT-IR fibers and perikarya formed a dense network. In hibernating ground squirrels, the immunoreactivity to serotonin was approximately 45% higher than in non-hibernating controls. This difference is in accordance with signs of higher neuronal activity (40% higher RNA-content, 20% larger cell nuclei) in 5HT-IR perikarya of the raphe nucleus and the persisting activity of the NSC during hibernation; the activity of other brain regions dropped conspicuously in torpid animals.Supported by the Deutsche Forschungsgemeinschaft (Nu 36/2-1)     

Circadian rhythms in the retina of rats with photoreceptor degeneration

Previous studies have demonstrated that the mammalian retina contains a circadian clock system that controls several retinal functions. In mammals the location of the retinal circadian clock is unknown whereas, in non-mammalian vertebrates, earlier work has demonstrated that photoreceptor cells contain the circadian clock. New experimental evidence has suggested that in mammals the retinal circadian clock may be located outside the photoreceptor cells. In this study we report that circadian rhythms in Aa-nat mRNA (in vivo) and melatonin synthesis (in vitro) are still present in the retina of rats lacking photoreceptors. The circadian pacemaker(s) controlling such rhythms is probably located in kainic acid sensitive neurons in the inner retina since kainic acid injections abolished the rhythmicity. These data are the first direct demonstration that circadian rhythmicity in the mammalian retina can be generated independently from the photoreceptors and the suprachiasmatic nuclei of the hypothalamus.     

Immunocytochemical localization of the vasopressinergic and the oxytocinergic neurons in the human hypothalamus

Summary The human hypothalamic-neurohypophysial hormone-producing nuclei were investigated with the unlabeled antibody peroxidase-antiperoxidase complex (PAP) technique at the light microscopic level. The size, shape and location of the supraoptic, paraventricular, accesssory supraoptic and suprachiasmatic nuclei were determined. It was demonstrated in the human hypothalamus, as well as in the hypothalamus of other mammals, that vasopressin and oxytocin are synthesized in separate neurons. In each of the nuclei of the magnocellular neurosecretory system, the distribution, ratios and structural features of the vasopressinergic and oxytocinergic neurons were determined. It was shown that the human suprachiasmatic nuclei contain numerous neurophysin-vasopressin-producing neurons.This investigation was supported by a grant from the Belgian Nationaal Fonds voor Geneeskundig Wetenschappelijk Onderzoek     

Topography of retinal axons in the diencephalon of goldfish

Summary The projections of horseradish peroxidase-filled axons from each quadrant of the retina were studied to determine whether retinal projections of goldfish are topographically organized in diencephalic target nuclei. A distinct topography of the dorsal, nasal, ventral and temporal retina exists in the lateral geniculate nucleus and in the dorsolateral optic nucleus of the thalamus. The projections of retinal quadrants show minimal spatial overlap in each of these nuclei. The suprachiasmatic nucleus of the hypothalamus is extensively innervated by ventral retinal fibers, whereas the nucleus is sparsely innervated by fibers from the other three retinal quadrants. A rudimentary topography also exists in the pretectum where the dorsal pretectal area receives projections primarily from the ventral retina and the ventral pretectal area receives projections mostly from the dorsal retina. These data show that retinal projections to some diencephalic nuclei are topographically organized.This work was supported by Research Grant EY-01426 to S.C.S.     

Distribution of vasoactive intestinal peptide-like and neurophysin-like immunoreactive neurons and acetylcholinesterase staining in the ring dove hypothalamus with emphasis on the question of an avian suprachiasmatic nucleus

Summary Two nuclei, termed here the medial hypothalamic nucleus and the lateral hypothalamic retinorecipient nucleus, are possible homologs of the mammalian suprachiasmatic nucleus. As the mammalian suprachiasmatic nucleus is characterized by a dense concentration of vasoactive intestinal peptide (VIP)-and neurophysin (NP)-immunoreactive neurons and an absence of acetylcholinesterase (AChE) staining, we decided to examine these factors in the ring dove hypothalamus. Neither the medial hypothalamic nucleus nor the lateral hypothalamic retinorecipient nucleus contained either VIP-or NP-like immunoreactive neurons. The lateral hypothalamic retinorecipient nucleus stained darkly for AChE. Although there was some overlap in the distribution of VIP-and NP-like immunoreactive neurons, a clustering of both types into a well defined nucleus was not observed. Therefore, an avian homolog to the mammalian suprachiasmatic nucleus must differ in its chemoarchitecture from that of mammalian species described to date.     

Immunocytochemical localization of neuropeptides in the hypothalamus of the Japanese long-fingered bat,Miniopterus schreibersii fuliginosus

Summary To elucidate the role of hypothalamic neuropeptides in regulation of reproductive phenomena of seasonally breeding feral mammals, we used Japanese long-fingered bats, Miniopterus schreibersii fuliginosus, for immunocytochemical study of distribution of the following neuropeptides in the hypothalamus: arginin vasopressin, oxytocin, luteinizing hormone-releasing hormone, somatostatin, corticotropin-releasing factor, and growth hormone-releasing factor. The size, shape and location of supraoptic, paraventricular, suprachiasmatic, and arcuate nuclei of the bat were determined. Arginin vasopressin-and oxytocin-immunoreactive magnocellular neurons were found in the supraoptic and paraventricular nuclei, where they exhibited separate distribution into two distinct groups. Parvocellular arginin vasopressin neurons occurred only in the suprachiasmatic nucleus. The hibernating bats exhibited slightly increased numbers of vasopressin and oxytocin neurons in the supraoptic and paraventricular nuclei. The pregnant bat displayed further increased numbers of vasopressin and oxytocin neurons in both nuclei. Somatostatin-immunoreactive neurons in the paraventricular nucleus were also immunopositive to anti-oxytocin serum, while those in the ventromedial and arcuate nuclei reacted solely to anti-somatostatin serum. They projected to the anterior median eminence and infundibular stalk. Luteinizing hormone-releasing hormone-immunoreactive perikarya were scattered throughout the basal hypothalamus, being particularly abundant in the arcuate nucleus. They were larger in size in hibernating bats than those in normal (non-pregnant) and pregnant females. They projected fibers mainly to the internal layer of the median eminence and infundibular stalk. A few luteinizing hormone-releasing hormone-reactive fibers were also observed in the organum vasculosum laminae terminalis, lateral habenular nuclei, pineal stalk, retroflexus fasciculus, and olfactory tubercle. Corticotropin releasing factor-immunoreactive perikarya were distributed in the paraventricular nucleus and medial preoptic area and projected into the external layer of the anterior median eminence, while growth hormone-releasing factor-immunoreactive perikarya occurred only in the arcuate nucleus and projected into the posterior part of the median eminence.     

The suprachiasmatic nucleus of the mink (Mustela vison): apparent absence of vasopressin-immunoreactive neurons

The hypothalamic suprachiasmatic nucleus is centrally involved in generation of several circadian rhythms. Neurons of the mammalian suprachiasmatic nucleus express a number of neuropeptides including vasopressin. The suprachiasmatic nucleus of the mink (Mustela vison) is easily distinguished from neighbouring hypothalamic areas and the underlying optic chiasm as a small nucleus containing densely packed parvocellular neurons. A dorsal and ventral subdivision were clearly recognized within the midportion and caudal part of the nuclcus. Using immunohistochemistry, we have identified vasopressin-, neurophysin-, and vasoactive intestinal peptide-immunoreactive neuronal elements in the hypothalamus of the mink. Vasoactive intestinal peptide-immunoreactive neurons can be observed in the ventral aspect of the suprachiasmatic nucleus, but to our surprise, no vasopressin immunoreactive perikarya are found within the suprachiasmatic nucleus, this absence being independent of the experienced annual cycle. The hypothalamic paraventricular and supraoptic nuclei contain large numbers of vasopressin-, neurophysin-and vasoactive intestinal peptide-immunoreactive magnocellular neurons with extensive projections towards the infundibulum and neurohypophysis. A comparative analysis of the distribution of vasopressin-immunoreactive elements in a number of conventional laboratory animals has demonstrated that, in contrast to the rat, golden hamster and Mongolian gerbil, neither vasopressin-containing perikarya in the suprachiasmatic nucleus nor fine calibered immunoreactive fibres entering the adjacent subparaventricular zone are present in the mink. The mink is a photodependent seasonal breeder, and thus vasopressin-immunoreactive neurons in the suprachiasmatic nuclei may not be essential for the photoperiodic regulation of reproduction and seasonal events experienced by this species.     

Demonstration of a different localization of perikarya immunoreactive to oxytocin and vasopressin in the cat hypothalamus

Using immunohistochemical techniques, we demonstrated oxytocin (OT) and vasopressin (AVP) neurons in the cat hypothalamus. The OT immunoreactive neurons were found mainly in the paraventricular nucleus, supraoptic nucleus and dorsal accessory group located lateral to the fornix. In addition to these hypothalamic structures, the AVP immunoreactive neurons were observed in the suprachiasmatic nucleus, ventral accessory group located in the retrochiasmatic area and lateral accessory group, dorsal to the supraoptic nucleus caudally, and ventral to the medial part of the internal capsule rostrally. We further demonstrated a different localization of the OT and AVP immunoreactive neurons in the paraventricular and supraoptic nuclei.     

Changes in RNA content of the hypothalamus of mice following insulin administration.

Cytoplasmic total RNA in neurons of the supraoptic nucleus, paraventricular nucleus, suprachiasmatic nucleus, and arcuate nucleus was determined cytophotometrically following a single intramuscular dose of 4 IU/kg insulin semilente. A significant decrease was observed after 3, 6, and 12 hours. A return towards normal values was evident after 24 hours and was full after 48 hours only in suprachiasmatic nucleus. Other hypothalamic nuclei did not reach the full normal value in the time interval examined, i.e. 48 hours post-injection.     

Immunocytochemical evidence for the presence of a mutant vasopressin precursor in the supraoptic nucleus of the homozygous Brattleboro rat

Summary CP-14, a tetradecapeptide from the predicted mutant vasopressin precursor in the homozygous Brattleboro rat was detected immunocytochemically in the supraoptic nucleus of homozygous Brattleboro but not normal rats. The staining was localized to the periphery of the perikarya. CP-14 immunoreactivity was not found in the neural lobes, paraventricular nuclei, accessory nuclei or suprachiasmatic nuclei of either homozygous Brattleboro or normal rats. Vasopressin immunoreactivity was found in the neural lobe and in the perinuclear region of neurons of the supraoptic, paraventricular, suprachiasmatic and accessory nuclei of normal rats. Vasopressin immunoreactivity was also found in homozygous Brattleboro rats, mainly in the ventral part of the supraoptic nucleus: densely stained solitary cells were found amongst other faintly stained perikarya. In both cell-types the staining was mainly in the periphery of the perikarya. No vasopressin immunoreactivity was detected in the paraventricular nuclei, suprachiasmatic nuclei, accessory nuclei or neural lobe of homozygous Brattleboro rats.CP-14 and vasopressin immunoreactivities were found to be co-localized; both were present in the periphery of the same perikarya of the supraoptic nuclei of homozygous Brattleboro rats. Differential staining was found with antioxytocin serum in both normal rats and homozygous Brattleboro rats: separate neurons were stained for either oxytocin or vasopressin and CP-14. Immunoreactive oxytocin was found mainly in the perinuclear region of the neurons from the supraoptic, paraventricular and accessory nuclei.     

Retinofugal projections of the big brown bat, Eptesicus fuscus and the neotropical fruit bat, Artibeus jamaicensis

Retinal connections were studied in Eptesicus fuscus and Artibeus jamaicensis using anterograde axonal degeneration and autoradiographic techniques following unilateral enucleations and uniocular injections of radioactive amino acids. Although each retina projected bilaterally to the brainstem, the number of silver grains in the emulsion of autoradiographs indicated that nearly all fibers in the optic nerve entered the contralateral optic tract. Ipsilaterally, a major portion of the projection ended in the suprachiasmatic nucleus; caudal to the suprachiasmatic nucleus, the amount of label was so small that individual silver grains were counted to determine the location and quantity of label in other ipsilateral nuclei. In both species the retinal projection terminated bilaterally in the suprachiasmatic, dorsal lateral geniculate, ventral lateral geniculate, and pretectal olivary nuclei and contralaterally in the posterior pretectal nucleus, superficial gray layers of the superior colliculus, and nuclei of the accessory optic system. In Eptesicus the projection to the nucleus of the optic tract ended contralaterally, and in Artibeus it ended in this nucleus bilaterally. The results of this study revealed a basic theme in the optic projection of the two ecologically different microchiropterans. The results differed, however, in that the projection was larger and visually related nuclei were better developed in Artibeus. Such variations are presumed to relate to eye size and the relative use of vision by the two chiropterans.     

Neuropeptide Y phase advances the in vitro hamster circadian clock during the subjective day with no effect on phase during the subjective night

The mammalian daily (circadian) clock is located in the suprachiasmatic nuclei of the hypothalamus. Clock function can be detected by the measurement of the circadian change in spontaneous firing rate of suprachiasmatic nuclei cells in a brain slice preparation in vitro. We investigated the effects of neuropeptide Y on this rhythm of firing rate in hamster suprachiasmatic nuclei neurons. Slices were prepared using standard techniques. On the 1st day in vitro, neuropeptide Y (200 ng/200 nL; 47 pmol) was applied as a microdrop to the suprachiasmatic nuclei region at various times. Spontaneous single-unit firing was measured for 6-12 h on the 2nd day in vitro. Peak firing rate in treated slices was compared with that of untreated control slices to measure phase shifts induced by the peptide. Neuropeptide Y induced phase advances of circa-3h when applied during the subjective day (ZT 2-10) but did not significantly alter phase when applied during the subjective night. The phase shifts to neuropeptide Y in the hamster tissue in vitro are similar in phase dependency and magnitude to shifts measured in vivo.     

Placing ocular mutants into a functional context: a chronobiological approach

The behavior of mammals is characterized by a 24-h cycle of rest and activity which is a fundamental adaption to the solar cycle of light and darkness. The pacemaker of this circadian clock is localized in the ventral part of the hypothalamus, the so-called suprachiasmatic nuclei (SCN), and is entrained by light signals mediated by the eye. The eye is directly connected via the retinohypothalamic tract (RHT) to the SCN. Light that reaches the retina elicits glutamate release at the synaptic terminals of the RHT and influences the neurons in the SCN in a manner that alters the behavioral state of the animal. A light pulse that reaches the retina at the beginning of the night elicits a delay of the clock phase, whereas a light pulse that reaches the retina at the end of the dark period leads to an advance of the clock phase. This advance or delay can be quantified by measuring the change in onset of wheel-running activity. Such measurements have, and continue to provide, a remarkably powerful assay of how light is detected and transduced to regulate circadian rhythms. The methods used for such measurements in mice are described in the following article.     

Distribution of oxytocin and vasopressin neurons in the diencephalon of the Japanese horseshoe bat, Rhinolophus ferrumequinum. An immunohistochemical study.

The distribution of oxytocin (OXT) and vasopressin (VP) neurons in the diencephalon of the hibernating Japanese horseshoe bat, Rhinolophus ferrumequinum, was immunohistochemically investigated by the avidin-biotin complex method. Magnocellular OXT and VP neurons were localized mainly in the paraventricular nucleus and the supraoptic nucleus. In addition to these main nuclei, both kinds of magnocellular neurons were also found in the periventricular nucleus, perifornical area and lateral hypothalamic area. Extensively distributed parvocellular neurons containing only VP were observed in the rostral and middle portions of the suprachiasmatic nucleus. The size of OXT and VP magnocellular neurons was almost equal in the paraventricular and ventromedial supraoptic nuclei, whereas VP neurons were significantly larger than OXT neurons in the dorsolateral supraoptic nucleus. The OXT and VP cells in the ventral supraoptic nucleus showed a distinctive elliptical shape. Both OXT and VP fibers were distributed in the lateral habenular nucleus, stria medullaris thalami, lateral preoptic area, stria terminalis, and medial and supracapsular part of the bed nucleus of the stria terminalis. Moreover, OXT fibers were found in the substantia nigra, and VP fibers were noted in the nucleus reunions and the paraventricular nucleus of the thalamus.     

Immunocytochemical demonstration of separate vasopressin-neurophysin and oxytocin-neurophysin neurons in the human hypothalamus

Summary With the use of immunocytochemistry, it was shown that both the supraoptic and paraventricular hypothalamic nuclei in humans contain at least two different neurophysins. These two human neurophysins are immunologically related to bovine neurophysin I and neurophysin II, respectively. One human neurophysin is associated with vasopressin, the other with oxytocin. Human vasopressin-neurophysin and oxytocin-neurophysin are located separately in two different types of neurons, which correspond respectively to the vasopressinergic and oxytocinergic neurons of both the supraoptic and paraventricular nuclei. The neurophysin of the human vasopressinergic suprachiasmatic neurons appears to be closely related to or identical with neurophysin of the vasopressinergic neurons of the human magnocellular hypothalamic nuclei.This investigation was supported by a grant from the Belgian Nationaal Fonds voor Geneeskundig Wetenschappelijk Onderzoek     

Expression of corticosteroid-binding protein in the human hypothalamus, co-localization with oxytocin and vasopressin.

Corticosteroid-binding globulin, a specific steroid carrier in serum with high binding affinity for glucocorticoids, is expressed in various tissues. In the present study, we describe the immunocytochemical distribution of this protein in neurons and nerve fibers in the human hypothalamus. CBG immunoreactive perikarya and fibers were observed in the paraventricular, supraoptic, and sexual dimorphic nuclei in the perifornical region, as well as in the lateral hypothalamic and medial preoptic areas, the region of the diagonal band, suprachiasmatic and ventromedial nuclei, bed nucleus of the stria terminalis and some epithelial cells from the choroid plexus and ependymal cells. Stained fibers occurred in the median eminence and infundibulum. Double immunostaining revealed a partial co-localization of corticosteroid-binding globulin with oxytocin and, to a lesser extent, with vasopressin in the paraventricular and the supraoptic nuclei. Double immunofluorescence staining showed coexistence of these substances in axonal varicosities in the median eminence. We conclude that neurons of the human hypothalamus are capable of expressing corticosteroid-binding globulin, in part co-localized with the classical neurohypophyseal hormones. The distribution of CBG immunoreactive neurons, which is widespread but limited to specific nuclei, indicates that CBG has many physiological functions that may include neuroendocrine regulation and stress response.     

Neurons of the Paraventricular Hypothalamic Nucleus Under Normal and Modified Illumination Conditions: Immunohistochemical and Morphometric Parallels

Using an immunohistochemical technique and densitometric analysis in experiments on rats, we examined the expression of an early-response protein, c-Fos, in the medial parvicellular subnucleus of the hypothalamic paraventricular nucleus (mpcPVN). During 7 days, the animals were kept under conditions of (i) normal photoperiod (group LD), (ii) continuous illumination (group LL), and (iii) continuous darkness (light deprivation, group DD). It was found that both normal photoperiodicity and artificial modifications of the latter result in significant changes of c-Fos immunopositivity in the nuclei of mpcPVN neurons (cells that synthesize corticotropin-releasing factor). Changes in the indices of concentration and amount of c-Fos in mpcPVN neurons were, from some aspects, similar to those observed under analogous experimental conditions in neurons of a leading pacemaker of the circadian periodicity, the suprachiasmatic nuclei (SChNs) of the hypothalamus [10], but, at the same time, demonstrated noticeable specificity from other aspects. In all experimental groups, the concentration and amount of c-Fos in mpcPVN neurons were greater during daytime than at night. Dramatic increases in the above parameters observed at daytime in rats subjected to light deprivation were the most significant effect of modifications of the photoperiod. Both continuous illumination and light deprivation resulted in some increase of the number (density) of c-Fos-immunopositive units in the mpcPVN. The nuclei of neurons of this structure demonstrated the phenomenon found earlier in SChN neuron; geometrical dimensions of the cell nuclei of these structures depended noticeably on both illumination conditions and periodicity. Probable mechanisms of the dependence of c-Fos expression in the hypothalamic nuclei on the photoperiod and its modifications, interdependence of such changes observed in the circadian pacemaker (SChN) and an important component of the system controlling the stress reactions (mpcPVN), and the relation of the above phenomena to the level of melatonin produced by the epiphysis cerebri (main neuroendocrine intermediator organizing the above-mentioned periodicity) are discussed.     

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.This study was supported by grants from the DFG (Re 644/2-1) and the NMFZ, Mainz (to S.R.).