The injury resistant ability of melanopsin-expressing intrinsically photosensitive retinal ganglion cells

Neurons in the mammalian retina expressing the photopigment melanopsin have been identified as a class of intrinsically photosensitive retinal ganglion cells (ipRGCs). This discovery more than a decade ago has opened up an exciting new field of retinal research, and following the initial identification of photosensitive ganglion cells, several subtypes have been described. A number of studies have shown that ipRGCs subserve photoentrainment of circadian rhythms. They also influence other non-image forming functions of the visual system, such as the pupillary light reflex, sleep, cognition, mood, light aversion and development of the retina. These novel photosensitive neurons also influence form vision by contributing to contrast detection. Furthermore, studies have shown that ipRGCs are more injury-resistant following optic nerve injury, in animal models of glaucoma, and in patients with mitochondrial optic neuropathies, i.e., Leber’s hereditary optic neuropathy and dominant optic atrophy. There is also an indication that these cells may be resistant to glutamate-induced excitotoxicity. Herein we provide an overview of ipRGCs and discuss the injury-resistant character of these neurons under certain pathological and experimental conditions.

Subcellular distribution of 5-HT(1B) and 5-HT(7) receptors in the mouse suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN), a circadian oscillator, receives glutamatergic afferents from the retina and serotonergic (5-HT) afferents from the median raphe. 5-HT(1B) and 5-HT(7) receptor agonists inhibit the effects of light on SCN circadian activity. Electron microscopic (EM) immunocytochemical procedures were used to determine the subcellular localization of 5-HT(1B) and 5-HT(7) receptors in the SCN. 5-HT(1B) receptor immunostaining was associated with the plasma membrane of thin unmyelinated axons, preterminal axons, and terminals of optic and nonoptic origin. 5-HT(1B) receptor immunostaining in terminals was almost never observed at the synaptic active zone. To a much lesser extent, 5-HT(1B) immunoreaction product was noted in dendrites and somata of SCN neurons. 5-HT(7) receptor immunoreactivity in gamma-aminobutyric acid (GABA), vasoactive intestinal polypeptide (VIP), and vasopressin (VP) neuronal elements in the SCN was examined by using double-label procedures. 5-HT(7) receptor immunoreaction product was often observed in GABA-, VIP-, and VP-immunoreactive dendrites as postsynaptic receptors and in axonal terminals as presynaptic receptors. 5-HT(7) receptor immunoreactivity in terminals and dendrites was often associated with the plasma membrane but very seldom at the active zone. In GABA-, VIP-, and VP-immunoreactive perikarya, 5-HT(7) receptor immunoreaction product was distributed throughout the cytoplasm often in association with the endoplasmic reticulum and the Golgi complex. The distribution of 5-HT(1B) receptors in presynaptic afferent terminals and postsynaptic SCN processes, as well as the distribution of 5-HT(7) receptors in both pre- and postsynaptic GABA, VIP, and VP SCN processes, suggests that serotonin plays a significant role in the regulation of circadian rhythms by modulating SCN synaptic activity.

Serotonergic modulation of retinal input to the mouse suprachiasmatic nucleus mediated by 5-HT1B and 5-HT7 receptors

Serotonin (5-HT) and 5-HT receptor agonists can modify the response of the mammalian suprachiasmatic nucleus (SCN) to light. It remains uncertain which 5-HT receptor subtypes mediate these effects. The effects of 5-HT receptor activation on optic nerve-mediated input to SCN neurons were examined using whole-cell patch-clamp recordings in horizontal slices of ventral hypothalamus from the male mouse. The hypothesis that 5-HT reduces the effect of retinohypothalamic tract (RHT) input to the SCN by acting at 5-HT1B receptors was tested first. As previously described in the hamster, a mixed 5-HT(1A/1B) receptor agonist, 1-[3-(trifluoromethyl)phenyl]-piperazine hydrochloride (TFMPP), reduced the amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) evoked by selectively stimulating the optic nerve of wild-type mice. The agonist was negligibly effective in a 5-HT1B receptor knockout mouse, suggesting minimal contribution of 5-HT1A receptors to the TFMPP-induced reduction in the amplitude of the optic nerve-evoked EPSC. We next tested the hypothesis that 5-HT also reduces RHT input to the SCN via activation of 5-HT7 receptors. The mixed 5-HT(1A/7) receptor agonist, R(+)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT), reduced the evoked EPSC amplitude in both wild-type and 5-HT1B receptor knockout mice. This effect of 8-OH-DPAT was minimally attenuated by the selective 5-HT1A receptor antagonist WAY 100635 but was reversibly and significantly reduced in the presence of ritanserin, a mixed 5-HT(2/7) receptor antagonist. Taken together with the authors' previous ultrastructural studies of 5-HT1B receptors in the mouse SCN, these results indicate that in the mouse, 5-HT reduces RHT input to the SCN by acting at 5-HT1B receptors located on RHT terminals. Moreover, activation of 5-HT7 receptors in the mouse SCN, but not 5-HT1A receptors, also results in a reduction in the amplitude of the optic nerve-evoked EPSC. The findings indicate that 5-HT may modulate RHT glutamatergic input to the SCN through 2 or more 5-HT receptors. The likely mechanism of altered RHT glutamatergic input to SCN neurons is an alteration of photic effects on the SCN circadian oscillator.

Pseudorabies virus expressing enhanced green fluorescent protein: A tool for in vitro electrophysiological analysis of transsynaptically labeled neurons in identified central nervous system circuits

Physiological properties of central nervous system neurons infected with a pseudorabies virus were examined in vitro by using whole-cell patch-clamp techniques. A strain of pseudorabies virus (PRV 152) isogenic with the Bartha strain of PRV was constructed to express an enhanced green fluorescent protein (EGFP) from the human cytomegalovirus immediate early promoter. Unilateral PRV 152 injections into the vitreous body of the hamster eye transsynaptically infected a restricted set of retinorecipient neurons including neurons in the hypothalamic suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) of the thalamus. Retinorecipient SCN neurons were identified in tissue slices prepared for in vitro electrophysiological analysis by their expression of EGFP. At longer postinjection times, retinal ganglion cells in the contralateral eye also expressed EGFP, becoming infected after transsynaptic uptake and retrograde transport from infected retinorecipient neurons. Retinal ganglion cells that expressed EGFP were easily identified in retinal whole mounts viewed under epifluorescence. Whole-cell patch-clamp recordings revealed that the physiological properties of PRV 152-infected SCN neurons were within the range of properties observed in noninfected SCN neurons. Physiological properties of retinal ganglion cells also appeared normal. The results suggest that PRV 152 is a powerful tool for the transsynaptic labeling of neurons in defined central nervous system circuits that allows neurons to be identified in vitro by their expression of EGFP, analyzed electrophysiologically, and described in morphological detail.

A 5-HT(1B) receptor agonist inhibits light-induced suppression of pineal melatonin production

Serotonin (5-HT) modulates the phase adjusting effects of light on the mammalian circadian clock through the activation of presynaptic 5-HT(1B) receptors located on retinal terminals in the suprachiasmatic nucleus (SCN). The current study was conducted to determine whether activation of 5-HT(1B) receptors also alters photic regulation of nocturnal pineal melatonin production. Systemic administration of the 5-HT(1B) receptor agonist TFMPP attenuated the inhibitory effect of light on pineal melatonin synthesis in a dose-related manner with an apparent ED(50) value of 0.9 mg/kg. The effect of TFMPP on light-induced melatonin suppression was blocked by the 5-HT(1) receptor antagonist, methiothepin, but not by the 5-HT(1A) antagonist, WAY 100,635, consistent with the involvement of 5-HT(1B) receptors. The results are consistent with the interpretation that activation of presynaptic 5-HT(1B) receptors on retinal terminals in the SCN attenuates the effect of light on pineal melatonin production, as well as on circadian phase.

5-HT1B receptor-mediated presynaptic inhibition of retinal input to the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) receives glutamatergic afferents from the retina and serotonergic afferents from the midbrain, and serotonin (5-HT) can modify the response of the SCN circadian oscillator to light. 5-HT1B receptor-mediated presynaptic inhibition has been proposed as one mechanism by which 5-HT modifies retinal input to the SCN (Pickard et al., 1996). This hypothesis was tested by examining the subcellular localization of 5-HT1B receptors in the mouse SCN using electron microscopic immunocytochemical analysis with 5-HT1B receptor antibodies and whole-cell patch-clamp recordings from SCN neurons in hamster hypothalamic slices. 5-HT1B receptor immunostaining was observed associated with the plasma membrane of retinal terminals in the SCN. 1-[3-(Trifluoromethyl)phenyl]-piperazine HCl (TFMPP), a 5-HT1B receptor agonist, reduced in a dose-related manner the amplitude of glutamatergic EPSCs evoked by stimulating selectively the optic nerve. Selective 5-HT1A or 5-HT7 receptor antagonists did not block this effect. Moreover, in cells demonstrating an evoked EPSC in response to optic nerve stimulation, TFMPP had no effect on the amplitude of inward currents generated by local application of glutamate. The effect of TFMPP on light-induced phase shifts was also examined using 5-HT1B receptor knock-out mice. TFMPP inhibited behavioral responses to light in wild-type mice but was ineffective in inhibiting light-induced phase shifts in 5-HT1B receptor knock-out mice. The results indicate that 5-HT can reduce retinal input to the circadian system by acting at presynaptic 5-HT1B receptors located on retinal axons in the SCN.

Restoration of circadian behavior by anterior hypothalamic grafts containing the suprachiasmatic nucleus: graft/host interconnections

Destruction of the hypothalamic suprachiasmatic nucleus (SCN) disrupts circadian behavior. Transplanting SCN tissue from fetal donors into SCN-lesioned recipients can restore circadian behavior to the arrhythmic hosts. In the transplantation model employing fetal hamster donors and SCN-lesioned hamsters as hosts, the period of the restored circadian behavior is hamster-typical. However, when fetal rat anterior hypothalamic tissue containing the SCN is implanted into SCN-lesioned rats, the period of the restored circadian rhythm is only rarely typical of that of the intact rat. The use of an anterior hypothalamic heterograft model provides new approaches to donor specificity of restored circadian behavior and with the aid of species-specific markers, provides a means for assessing connectivity between the graft and the host. Using an antibody that stains rat and mouse neuronal tissue but not hamster neurons, it has been demonstrated that rat and mouse anterior hypothalamic heterografts containing the SCN send numerous processes into the host (hamster) neuropil surrounding the graft, consistent with graft efferents reported in other hypothalamic transplantation models in which graft and host tissue can be differentiated (i.e., Brattleboro rat and hypogonadal mouse). Moreover, SCN neurons within anterior hypothalamic grafts send an appropriately restricted set of efferent projections to the host brain which may participate in the functional recovery of circadian locomotor activity.

Serotonergic innervation of the hypothalamic suprachiasmatic nucleus and photic regulation of circadian rhythms

Converging lines of evidence have firmly established that the hypothalamic suprachiasmatic nucleus (SCN) is a light-entrainable circadian oscillator in mammals, critically important for the expression of behavioral and physiological circadian rhythms. Photic information essential for the daily phase resetting of the SCN circadian clock is conveyed directly to the SCN from retinal ganglion cells via the retinohypothalamic tract. The SCN also receives a dense serotonergic innervation arising from the mesencephalic raphe. The terminal fields of retinal and serotonergic afferents within the SCN are co-extensive, and serotonergic agonists can modify the response of the SCN circadian oscillator to light. However, the functional organization and subcellular localization of 5HT receptor subtypes in the SCN are just beginning to be clarified. This information is necessary to understand the role 5HT afferents play in modulating photic input to the SCN. In this paper, we review evidence suggesting that the serotonergic modulation of retinohypothalamic neurotransmission may be achieved via at least two different cellular mechanisms: 1) a postsynaptic mechanism mediated via 5HT1A or 5ht7 receptors located on SCN neurons; and 2) a presynaptic mechanism mediated via 5HT1B receptors located on retinal axon terminals in the SCN. Activation of either of these 5HT receptor mechanisms in the SCN by specific 5HT agonists inhibits the effects of light on circadian function. We hypothesize that 5HT modulation of photic input to the SCN may serve to set the gain of the SCN circadian system to light.

TFMPP, a 5HT1B receptor agonist, inhibits light-induced phase shifts of the circadian activity rhythm and c-Fos expression in the mouse suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus (SCN) receives afferents from the retina and the midbrain raphe. The retinal innervation mediates photic entrainment of the SCN circadian oscillator whereas the serotonergic input arising from the midbrain raphe nuclei appears to modulate retinohypothalamic neurotransmission. We hypothesized that serotonergic innervation of the SCN may modulate retinal input by activation of 5HT1B presynaptic receptors on retinal axon terminals in the SCN. We tested this hypothesis using the 5HT1B receptor agonist, 1-[3-(trifluoromethyl)phenyl]-piperazine (TFMPP). Systemic administration of TFMPP prior to light stimulation significantly attenuated light-induced phase shifts of the circadian activity rhythm and Fos expression in the SCN. These results in the mouse support our earlier findings in the hamster [Pickard, G.E., Weber, E.T., Scott, P.A., Riberdy, A.F. and Rea, M.A., J. Neurosci., 16 (1996) 8208-8220] and are consistent with the interpretation that 5HT1B presynaptic receptors participate in the regulation of photic input to the SCN.

5HT1B receptor agonists inhibit light-induced phase shifts of behavioral circadian rhythms and expression of the immediate-early gene c-fos in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is a circadian oscillator and a critical component of the mammalian circadian system. It receives afferents from the retina and the mesencephalic raphe. Retinal afferents mediate photic entrainment of the SCN, whereas the serotonergic afferents originating from the midbrain modulate photic responses in the SCN; however, the serotonin (5HT) receptor subtypes in the SCN responsible for these modulatory effects are not well characterized. In this study, we tested the hypothesis that 5HT1B receptors are located presynaptically on retinal axon terminals in the SCN and that activation of these receptors inhibits retinal input. The 5HT1B receptor agonists TFMPP and CGS 12066A, administered systemically, inhibited light-induced phase shifts of the circadian activity rhythm in a dose-dependent manner at phase delay and phase advance time points. This inhibition was not affected by previous systemic application of either the selective 5HT1A receptor antagonist (+)WAY 100135 or by the 5HT2 receptor antagonist mesulergine, whereas pretreatment with the nonselective 5HT1 antagonist methiothepin significantly attenuated the effect of TFMPP. TFMPP also produced a dose-dependent reduction in light-stimulated Fos expression in the SCN, although a small subset of cells in the dorsolateral aspect of the caudal SCN were TFMPP-insensitive. TFMPP (1 mM) infused into the SCN produced complete inhibition of light-induced phase advances. Finally, bilateral orbital enucleation reduced the density of SCN 5HT1B receptors as determined using [125I]-iodocyanopindolol to define 5HT1B binding sites. These results are consistent with the interpretation that 5HT1B receptors are localized presynaptically on retinal terminals in the SCN and that activation of these receptors by 5HT1B agonists inhibits retinohypothalamic input.

The tau mutation shortens the period of rhythmic photoreceptor outer segment disk shedding in the hamster

The outer segments of vertebrate retinal photoreceptors undergo periodic shedding of membrane from their distal tips. This circadian rhythm of disk shedding persists with a period of about 24 h in the absence of external time cues. A circadian oscillator controlling photoreceptor disk shedding may exist in the eye, but in addition, the circadian clock in the hypothalamic suprachiasmatic nucleus (SCN) may also influence ocular rhythms including that of disk shedding. The tau mutation directly affects the SCN, and shortens the period of locomotor activity from 24 h in wild-type hamsters to 20 h in homozygous mutants. Here we show that homozygous tau-mutant hamsters in a 20-h light/dark cycle exhibit a 20-h oscillation in the rate of disk shedding, with peak phagosome numbers in the retinal pigmented epithelium occurring just after light onset. The numbers of phagosomes are significantly elevated from mid-dark levels prior to light onset, indicating that the disk shedding cycle anticipates dawn. Under conditions of constant darkness, the disk shedding rhythm in tau-mutant hamsters persists with a period of approximately 20 h. These results indicate that a rhythm of retinal photoreceptor outer segment disk shedding exists in the hamster eye, and that the period of this rhythm is shortened by the tau mutation.

Thyrotropin-releasing hormone phase shifts circadian rhythms in hamsters

The role of thyrotropin-releasing hormone (TRH) in regulating circadian rhythms was investigated by assessing the ability of TRH microinjections into the suprachiasmatic nucleus (SCN) to induce phase shifts in hamster wheel-running behavior. TRH injected into the SCN at 10 and 100 nM doses produced phase advances in wheel-running activity of 18.3 +/- 1.9 and 34.8 +/- 2.9 minutes, respectively, when administered at circadian time (CT) 6. Injections at CT 18 produced no effects. The temporal sensitivity of the SCN to TRH administration was examined by administering TRH at specific circadian times. TRH produced significant phase advances at CT 4, 6, and 8, while no significant changes in wheel-running onset were observed at other CT times. These studies represent the first evidence of TRH's ability to affect circadian function.

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).

Altered circadian rhythmicity in the Wocko mouse, a hyperactive transgenic mutant

Induced changes in the level of daily activity can alter the period of the mammalian circadian clock. In this report, we examined the period of the circadian rhythm of wheel-running activity in a transgenic neurological mouse mutant, Wocko. Wocko mice display a dominant behavioral phenotype that consists of hyperactivity, circling and head tossing. The period of the circadian rhythm of wheel-running activity in constant dark conditions was significantly shorter in mice expressing the Wocko mutation than in their normal littermates. Total activity, monitored by the interruption of an array of infrared beams, was significantly elevated in Wocko mice. These findings support the view that spontaneous exercise can modulate the circadian timekeeping system.

Mutagenesis and behavioral screening for altered circadian activity identifies the mouse mutant, Wheels

The molecular processes underlying the generation of circadian behavior in mammals are virtually unknown. To identify genes that regulate or alter circadian activity rhythms, a mouse mutagenesis program was initiated in conjunction with behavioral screening for alterations in circadian period (tau), a fundamental property of the biological clock. Male mice of the inbred BALB/c strain, treated with the potent mutagen N-ethyl-N-nitrosourea were mated with wild-type hybrids. Wheel-running activity of approximately 300 male progeny was monitored for 6-10 weeks under constant dark (DD) conditions. The tau DD of a single mouse (#187) was longer than the population mean by more than three standard deviations (24.20 vs. 23.32 +/- 0.02 h; mean +/- S.E.M.; n = 277). In addition, mouse #187 exhibited other abnormal phenotypes, including hyperactive bi-directional circling/spinning activity and an abnormal response to light. Heterozygous progeny of the founder mouse, generated from outcrossings with wild-type C57BL/6J mice, displayed lengthened tau DD although approximately 20% of the animals showed no wheel-running activity despite being quite active. Under light:dark conditions, all animals displaying circling behavior that ran in the activity wheels exhibited robust wheel-running activity at lights-ON and these animals also showed enhanced wheel-running activity in constant light conditions. The genetic dissection of the complex behavior associated with this mutation was facilitated by the previously described genetic mapping of the mutant locus causing circling behavior, designated Wheels (Whl), to the subcentromeric portion of mouse chromosome 4. In this report, the same locus is shown to be responsible for the abnormal responses to light and presumably for the altered circadian behavior. Characterization of the gene altered in the novel Whl mutation will contribute to understanding the molecular elements involved in mammalian circadian regulation.

Vasoactive intestinal peptide efferent projections of the suprachiasmatic nucleus in anterior hypothalamic transplants: correlation with functional restoration of circadian behavior

Circadian rhythmicity can be restored by transplantation of fetal anterior hypothalamic (AH) tissue containing the suprachiasmatic nucleus (SCN) into hosts rendered arrhythmic by SCN ablation. However, the nature of the SCN effector pathways mediating functional recovery has remained elusive. To examine implant-derived SCN innervation of the host, AH homografts (hamster-to-hamster) and heterografts (mouse- or rat-to-hamster) were employed and the distribution of vasoactive intestinal peptide (VIP) within the SCN terminal fields was evaluated. A comparison was made between cases where circadian locomotor activity was restored and cases where circadian rhythmicity remained disrupted following AH transplantation. A dense aggregation of VIP neurons and processes was identified in each transplant that restored behavioral rhythmicity in the host. In these cases, SCN-derived VIP fibers were integrated with the host brain and could be identified in host terminal fields typically innervated by SCN-VIP fibers. A correlation was noted between VIP innervation of the host paraventricular thalamic nucleus (PVT) and restoration of circadian rhythmicity. Neither qualitative nor quantitative differences in transplant VIP projections were noted between AH homografts and heterografts. These results demonstrate that SCN VIP neurons in AH transplants send an appropriately restricted set of efferent projections to the host brain and suggest that SCN efferent projections to the PVT may participate in mediating the functional recovery of circadian locomotor activity.

Visual responses and connectivity in the turtle pretectum

1. Using an isolated turtle brain preparation, we made extracellular spike recordings in the dorsal midbrain during visual stimulation. Single units were isolated by their response to a slow-moving full-field visual pattern imaged on the contralateral retina. This stimulus elicits responses from the basal optic nucleus (BON) and the cerebellar cortex using a similar preparation. Direction and speed tuning were then analyzed, as well as the size and position of the receptive field. 2. In one brain stem region, anterior to the optic tectum and deep to the dorsal surface, all of the visually responsive neurons were direction sensitive (DS) to contralateral retinal stimulation. The location and properties of these cells indicate that they are in the mesencephalic lentiform nucleus (nLM). Anterograde transport of intravitreally injected horseradish peroxidase revealed that this pretectal nucleus receives direct input from the contralateral eye. 3. All but 2 of the 48 cells of the nLM were strongly DS. The most effective stimulus was a slowly moving complex visual pattern that drifted nasally in the contralateral visual field. Brief flashes of spots, patterns, or diffuse light were much less effective. Receptive fields were large and usually (9 of 13 cells) centered in the superior visual field near the horizon and nasal to the blind spot. 4. The visual responses of nLM cells were compared to those of cells in the superficial layers of the optic tectum. In contrast to nLM, the responses of tectal cells were heterogeneous and frequently not DS. Neither tectum or nLM cells had much spontaneous spike activity during darkness or stationary patterns. On the other hand, visual responses of nLM cells were very similar to those of the BON, where neurons also had low spontaneous activity, preferred slow-moving patterns, and were DS. However, nLM and BON exhibit different distributions of preferred directions. Most nLM cells preferred temporal-to-nasal motion, whereas BON cells preferred almost any direction, although few preferred the nasal direction. nLM cell responses were not affected by removal of the ventral brain stem including the BON. 5. The visual properties of nLM cells recorded in vitro were very similar to those that were recorded in intact turtles.(ABSTRACT TRUNCATED AT 400 WORDS)

Heterozygosity mapping of partially congenic lines: mapping of a semidominant neurological mutation, Wheels (Whl), on mouse chromosome 4

We identified a semidominant, chemically induced, mouse mutation with a complex array of abnormal behaviors including bidirectional circling and hyperactivity, abnormal circadian rhythmicity and abnormal responses to light. In this report, we genetically and phenotypically characterized the circling/waltzing component of the abnormal behavior. We mapped the locus controlling this trait by heterozygosity mapping of partially congenic lines carrying the mutagenized chromosome outcrossed to different inbred strains for three generations. Analysis of 68 PCR-based markers in 13 affected individuals indicated that the mutant locus, named Wheels (Whl), resides in the subcentromeric portion of mouse chromosome 4. The statistical evaluation of data obtained by heterozygosity mapping validates this efficient mapping approach. Further characterization of the Whl mutation demonstrated that Whl/Whl homozygotes die during embryonic life and that the penetrance of circling behavior depends on genetic background. Morphological analysis of the inner ears of Whl/+ mice revealed a variable number of abnormalities in the sensory and nonsensory portions of their semicircular canals. Abnormalities ranged from slight atrophy of one or more cristae to complete absence of the lateral crista and canal. The molecular characterization of the gene disrupted in the Whl mutation will provide insight into developmental mechanisms involved in inner ear formation.

Restoration of circadian behavior by anterior hypothalamic heterografts

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus (AH) is a circadian oscillator and an important component of the mammalian circadian system. To determine whether the SCN is the dominant circadian pacemaker responsible for generating a species-typical characteristic of circadian rhythms [i.e., period length (tau)], neural transplantation was conducted using fetal AH donors of different species and SCN-lesioned (SCNx) hosts. The circadian behavior of each of the three donor species is clearly distinguishable by its species-typical tau. The extent of SCN pacemaker autonomy was assessed by noting whether the period of the restored circadian rhythm following heterograft transplantation was characteristic of the donor or the host, or whether an atypical circadian period was established. Hamsters rendered arhythmic by SCN ablation were implanted with AH tissue from fetal hamsters (E13-E14, homograft controls) or fetal mice or rats (E15-E17). The AH homografts restored circadian activity rhythms with a tau similar to that of intact hamsters, and fetal mouse AH heterografts restored circadian rhythmicity with a tau similar to that of the donor mouse strain. However, fetal rat AH tissue implanted into SCNx hamsters renewed circadian rhythmicity with a period significantly shorter than either the species-typical tau of the rat donor or the hamster host. In both the mouse and rat AH heterograft experiments, immunocytochemical analysis performed with species-specific monoclonal antibodies revealed extensive fiber outgrowth from the implant into the host hypothalamus, evident up to 7 months postimplantation. The rat implants were consistently larger, more fully vascularized and exhibited less necrosis than the implanted mouse tissue. The histological appearance of the grafts, thus, provides no explantation for the difference in efficacy of the grafts to restore species-typical behavior. However, several interpretations are considered that are consistent with the combined behavioral results observed.

Intergeniculate leaflet ablation alters circadian rhythms in the mouse

The intergeniculate leaflet (IGL) receives retinal input and sends afferents to the hypothalamic suprachiasmatic nucleus (SCN), a circadian oscillator. The effect of IGL ablation on the circadian rhythm of wheel-running activity was examined in mice maintained in constant dark (DD) and in two intensities of constant light (LL). IGL-lesioned animals demonstrated significantly longer free-running rhythms in DD conditions than IGL-intact mice similarly housed; there was no effect of IGL ablation on the period of the free-running activity rhythm in LL of either 10 or 100 lux intensity. A change in the period of the free-running activity rhythm in DD following IGL destruction is evidence that the IGL exerts an endogenous influence on the SCN circadian system.

Pineal photoreceptors rhythmically secrete melatonin

Individual pineal cells secrete melatonin with a circadian period, reducing a vertebrate circadian system to the level of a single cell [Brain Res., 627 (1993) 141-146]. In the present study, dissociated pineal cells were identified as melatonin-secreting by a reverse hemolytic plaque assay (RHPA) and all melatonin-secreting cells were immuno-positive when analyzed for the photoreceptor protein S-antigen. The results are the first direct evidence that isolated photoreceptor cells secrete melatonin and taken together with our previous findings indicate that single pineal cells contain: (1) a circadian oscillator; (2) a photoreceptive capacity; and (3) the ability to secrete melatonin rhythmically.

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.

Individual pineal cells exhibit a circadian rhythm in melatonin secretion

Pineal glands and dissociated pineal cells exhibit a circadian rhythm of melatonin secretion in vitro which persists for several cycles under constant conditions. It is not known whether individual, physically isolated pineal cells are capable of generating a circadian oscillation in melatonin release. This question was addressed by utilizing a reverse hemolytic plaque assay for the detection of melatonin secretion from individual pineal cells. Dissociated pineal cells from the anole lizard, maintained in short term culture, displayed a marked variation in melatonin secretion for up to 72 h under both cyclic lighting conditions and in constant dark. The persistence of daily fluctuations of melatonin secretion from individual cells strongly suggests that individual pineal cells can function as circadian oscillators.

Light intensity and splitting in the golden hamster

Hamster circadian activity rhythms split into two components during prolonged exposure to conditions of constant light (LL). Several aspects of this phenomenon were examined in this study. The frequency of splitting was significantly greater among animals exposed to LL of 100 lux intensity (LL100) compared with animals in LL10. Animals that split had significantly longer free-running periods (tau) compared to nonsplitters and the decrease in tau associated with splitting was highly correlated with the presplit tau. Splitting was also observed under continuous dim light which fluctuated rhythmically from 5-10 lux. Thus, splitting of the circadian rhythm of activity is positively correlated with LL intensity with an LL intensity threshold for the induction of splitting in the range of 3-5 lux.

Time course of fiber outgrowth from fetal anterior hypothalamic heterografts

Fetal anterior hypothalamic (AH) heterografts can restore circadian rhythmicity to animals rendered arrhythmic following ablation of the suprachiasmatic nucleus (SCN). Behavioral restoration of circadian activity typically begins between two and six weeks post-implantation. The time course of fiber outgrowth from fetal AH heterografts was examined to determine whether neuronal outgrowth from the implants precedes the typically observed effects of such implants upon circadian behavior. Fetal mouse or rat AH tissue containing the SCN was implanted into the third ventricle of SCN-lesioned hamsters. Using species-specific monoclonal antibodies generated against mouse or rat neuronal elements, fiber outgrowth into the host hypothalamus was examined at 2, 4, 7, 14, 30 and 45 days after implantation. Fibers were observed to have emerged from the implant at the earliest time point examined. Four days after surgery, individual fibers had extended up to 0.6 mm into the host neuropil. By 14 days post-implantation, outgrowth from the implant had formed a dense fiber plexus in the host hypothalamus. This observation demonstrates that neuronal integration of the implant with the host brain begins within 48 hours of implantation, and is extensively established well before a restoration of rhythmicity is typically observed. Thus, on the basis of the time course of fiber outgrowth, it is clear that neuronal contact between graft and host may mediate the observed restoration of circadian rhythmicity.

Circadian locomotor rhythms in the desert iguana. II. Effects of electrolytic lesions to the hypothalamus

Desert iguanas, Dipsosaurus dorsalis, displaying freerunning circadian locomotor rhythms in conditions of constant darkness and temperature received electrolytic lesions to the hypothalamus. The locomotor activity of those lizards (N = 9) which sustained 80% or more damage to the suprachiasmatic nucleus (SCN) became arrhythmic whereas all animals that sustained less than 35% damage to the SCN remained rhythmic, even though they sustained significant damage to nearby regions of the hypothalamus and preoptic area. These results suggest strongly that the SCN plays a role in the regulation of circadian rhythms in the desert iguana. Taken together with other evidence, they support the view that this structure is homologous to the mammalian SCN, which acts as a pacemaker in the circadian system.

Entrainment of the circadian rhythm of wheel-running activity is phase shifted by ablation of the intergeniculate leaflet

The phase angle of entrainment of golden hamster wheel-running activity was determined before and after bilateral destruction of the intergeniculate leaflet (IGL). Animals were maintained under continuous dim light which rhythmically varied in intensity (10-5 lux) in the form of a sine wave with a 24 h period. The phase angle of entrainment changed significantly following IGL ablation suggesting a role for the IGL in entrainment.

Circadian rhythm of nociception in the golden hamster

The response latency of golden hamsters to nociceptive stimuli was measured under cyclic lighting conditions and during constant illumination. A day-night rhythm of nociception was demonstrated; response latencies were significantly longer during the day. A circadian rhythm of nociception was displayed by hamsters maintained for 30 days in constant dim light. Short response latencies noted under these conditions were associated with the inactive period of the animals circadian cycle (subjective day). The experiments provide data which indicate the phase relationship between the circadian rhythms of nociception and locomotor activity differs under entrained and free-running conditions.

The intergeniculate leaflet partially mediates effects of light on circadian rhythms

Photic signals affect circadian activity rhythms by both phasic and tonic mechanisms that modulate pacemaker phase and period. In mammals, the effects of light on circadian activity are mediated by the retina, which communicates with the suprahiasmatic nucleus (SCN) by two different anatomical routes: the retino-hypothalamic tract (RHT), originating in the retina, and the geniculo-hypothalamic tract (GHT), arising from a retino-recipient nucleus, the intergeniculate leaflet (IGL). We assessed the roles of these two afferent systems in mediating phasic and tonic effects of light on circadian activity in IGL-lesioned animals. Destruction of the IGL significantly affected phase shifts produced by brief light pulses (phasic effect) and modified the change in period (tau) of the free-running activity rhythm produced by changing the level of constant light (LL) (tonic effect). Phase advances produced by brief light pulses were decreased in amplitude while phase delays were increased in IGL-lesioned animals as compared to controls. The free-running period in constant dark (tau DD) of IGL-lesioned animals was greater than tau DD of controls, and the lengthening of tau normally produced by LL was not observed or was greatly reduced in IGL-lesioned animals. Entrainment to light-dark cycles was unaffected by the lesions, as were other aspects of the circadian activity rhythm that normally change in response to LL (e.g., activity-rest ratio, total activity, splitting). Our data support the interpretation that the IGL plays a significant role in relaying information regarding illumination intensity to the SCN.

Influence of deuterium oxide on circadian activity rhythms of hamsters: role of the suprachiasmatic nuclei

The period of the free-running circadian activity rhythm of Syrian hamsters was measured before and during treatment with 10% deuterium oxide (D2O). Deuteration increased period length by approximately 0.5 h per cycle both pre- and postoperatively in hamsters sustaining complete, incomplete or no unilateral lesions of the suprachiasmatic nuclei (SCN). Neither coupling between the bilaterally paired SCN, nor elimination of 50% of SCN tissue affected period length during D2O treatment. However, variability of the response to D2O was much greater in lesioned than in intact hamsters. We propose that a small percentage of the normal complement of SCN neurons is sufficient to permit full responsiveness of the circadian system to D2O and that there is substantial redundancy in the neural system that responds to deuterium. Stability of the circadian system appears to be increased by the full complement of SCN neurons.

Persistence of circannual rhythms in ground squirrels with lesions of the suprachiasmatic nuclei

Golden-mantled ground squirrels (Spermophilus lateralis) sustained complete ablation of the suprachiasmatic nuclei (SCN) while being maintained under constant conditions of photoperiod, temperature and food availability. After SCN ablation, most squirrels observed for up to 3 years postsurgically underwent normal circannual cycles in body mass. Other squirrels with similar SCN damage evidenced various anomalies in their circannual body weight cycles. Response of squirrels to SCN ablation was not related to the phase of the annual cycle in which lesions were produced. The variable nature of the effect of SCN ablation on circannual body mass cycles suggests that the SCN are non-essential components of the system underlying the generation or expression of circannual rhythms.

Bifurcating axons of retinal ganglion cells terminate in the hypothalamic suprachiasmatic nucleus and the intergeniculate leaflet of the thalamus

At least some retinal axons afferent to the hypothalamic suprachiasmatic nucleus (SCN; a circadian oscillator) bifurcate in the optic chiasm (O.E. Millhouse, Brain Res., 137 (1977) 351-355). The termination site(s) of the axonal branch that continues in the optic tract is unknown. Injection of the fluorescent tracer, True Blue, into the SCN and the fluorescent dye, Nuclear Yellow, into the lateral geniculate complex resulted in the labeling of individual retinal ganglion cells with both tracers. However, only Nuclear Yellow injections which included the intergeniculate leaflet (IGL) resulted in double-labeled ganglion cells in the retinae. These results indicate that individual retinal ganglion cells innervate both the hypothalamic SCN and the IGL of the thalamus by means of divergent axonal collaterals. Moreover, neurons of the IGL are afferent to the SCN, thereby forming a complex circuit within which photic information from the same retinal ganglion cell may influence the SCN both directly and after thalamic processing.

Splitting of the circadian rhythm of body temperature in the golden hamster

The circadian rhythm of hamster locomotor activity "splits" into two distinct circadian components during conditions of constant illumination. To determine if the circadian rhythm of body temperature also splits under these conditions, body temperature and locomotor activity were monitored concurrently in animals housed in constant illumination. Splitting of the body temperature rhythm into two circadian components was observed in animals manifesting split activity rhythms. Concurrent splitting of both rhythms suggests a common mechanism either for the generation or coupling of these rhythms.

Immunohistochemical localization of luteinizing hormone-releasing hormone (LHRH) in the hypothalamus of adult female hamsters treated neonatally with monosodium glutamate or hypertonic saline

The immunohistochemical localization of luteinizing hormone-releasing hormone (LHRH) was studied in paraffin and vibratome-sectioned tissue from adult female hamsters that were treated neonatally with monosodium glutamate (MSG) or hypertonic saline. There appeared to be a reduction in LHRH-positive fibers in the median eminence of animals with an MSG-induced lesion of the arcuate nucleus in paraffin-embedded tissue. However, when unembedded tissue was cut on a vibratome, the distribution of LHRH-positive fibers and perikarya was similar in both groups of animals. Fibers were seen coursing through the periventricular area and lateral hypothalamus to the median eminence. In addition, LHRH-positive fibers were seen in the organum vasculosum of the lamina terminalis, subfornical organ, septal and preoptic areas, fasciculus retroflexus, habenular complex, and several regions in the basal forebrain. Animals that were pretreated with colchicine had LHRH-positive perikarya in the medial habenular nucleus, diagonal band of Broca, and the medial olfactory tract.

The hypothalamic paraventricular nucleus mediates the photoperiodic control of reproduction but not the effects of light on the circadian rhythm of activity

Photoperiods of less than 12.5 h of light/24 h induce gonadal regression in the golden hamster. Photic information is relayed from the retina to the hypothalamic suprachiasmatic nucleus (SCN), a structure responsible for the generation of many circadian rhythms including the circadian rhythms in locomotor activity and pineal melatonin synthesis and release. Although pineal melatonin mediates the photoperiodic-neuroendocrine response, the complete neural circuit from the SCN to the pineal gland is unknown. Complete destruction of the hypothalamic paraventricular nucleus (PVN) prevented short-day-induced testicular regression without affecting the circadian rhythm of locomotor activity. The results indicate that the PVN plays an important role in the photoperiodic-neuroendocrine circuit and is responsible for relaying information from the SCN to the pineal. A different efferent pathway connects the SCN to structures in the brain responsible for locomotor activity.

The suprachiasmatic nuclei: two circadian clocks?

Unilateral lesions of the bilaterally paired suprachiasmatic nuclei of the hypothalamus were made in golden hamsters maintained in constant light (LL). Prior to surgery, the circadian rhythm of locomotor activity had dissociated or 'split' into two distinct components in 19 out of 30 hamsters, while in the remaining 11 animals (i.e. non-splitters) a normal rhythm with a single bout of activity was maintained. Following destruction of a major portion (i.e. 50-100%) of a single suprachiasmatic nucleus (SCN) in 6 'split' and 5 'non-split' hamsters, a single circadian bout of activity was observed in all animals. In 9 of these 11 animals the circadian period of the activity rhythm was altered following unilateral SCN lesions. In two split animals sustaining lesions which missed the SCN, but disrupted efferents caudal to the SCN, the split condition was also abolished. Lesions which destroyed less than 50% of a single SCN did not abolish the split condition (n = 6) and had little effect on the circadian pattern of the non-splitters (n = 5). In contrast, bilateral destruction of the suprachiasmatic nuclei eliminated the circadian pattern of activity in both 'split' (n = 5) and 'non-split' (n = 1) animals. These results demonstrate that each SCN is capable by itself of maintaining circadian rhythmicity and that at least two circadian oscillators, or oscillating systems, reside within the suprachiasmatic nuclei.

Retinohypothalamic pathway in the dove demonstrated by anterograde HRP

The hypothalamic suprachiasmatic nucleus (SCN) of the ring dove was demonstrated by the presence of labeled retinal processes. Horseradish peroxidase or tritiated proline were injected into the vitreous of the eye and were transported in an anterograde manner to label the SCN. After identifying the retinorecipient area of the hypothalamus by anterograde tracing procedures, a diffusely organized nucleus corresponding to this region was noted in Nissl material.

The afferent connections of the suprachiasmatic nucleus of the golden hamster with emphasis on the retinohypothalamic projection

The afferent connections of the hypothalamic suprachiasmatic nucleus (SCN) of the golden hamster were examined using horseradish peroxidase (HRP) as the retrograde tracer molecule. Unilateral iontophoretic deposition of HRP into the SCN labeled ganglion cells bilaterally in the retinae. The labeled ganglion cells all had large somata and were randomly distributed across the retina. A similar number were labeled in each retina, which contrasted with the findings from injections into the optic chiasm and lateral geniculate body. Chiasm and geniculate injections both labeled three classes of ganglion cell (small, large, and giant) predominantly in the contralateral retinae. Telencephalic afferent projections to the SCN included the ventral subicular cortex and the septum. Notable diencephalic afferents included the dorsal lamina of the internal division of the ventral lateral geniculate nucleus (vLGN); the ipsilateral input was twice that of the contralateral projection. The same region of the vLGN was also noted to be reciprocally connected to the contralateral vLGN. The thalamic paraventricular nucleus was also heavily labeled but only ipsilaterally. Of functional significance, the SCN was discovered to innervate its contralateral homologue. Other less numerous afferents in the midbrain included the dorsal and median raphe nuclei and the dorsal nucleus of the lateral lemniscus. The afferent projections to the SCN determined in this study are discussed in regard to the known physiological role of the SCN as part of the circadian clock system.

Splitting of the circadian rhythm of activity is abolished by unilateral lesions of the suprachiasmatic nuclei

The circadian rhythm of activity in vertebrates often splits into two components after continuous exposure to constant light. This observation suggests that at least two circadian pacemakers underlie the activity rhythm. After unilateral ablation of the hypothalamic suprachiasmatic nuclei in hamsters, the splitting phenomenon was eliminated and a single rhythm of activity was established. The period of the new circadian activity rhythm different from the periods of the split rhythm and that preceding the split. These results suggest an interaction between the bilaterally paired suprachiasmatic nuclei in the generation of the circadian rhythm of activity.

Direct retinal projections to the hypothalamus, piriform cortex, and accessory optic nuclei in the golden hamster as demonstrated by a sensitive anterograde horseradish peroxidase technique

The central projections of the retinal ganglion cells of the golden hamster were examined using horseradish peroxidase (HRP) as the anterograde tracer molecule. Following monocular injections of HRP into the vitreous, retinofugal fibers were histochemically demonstrated using the chromagen tetramethylbenzidine. This procedure, being more sensitive than the 3H-amino acid radioautographic technique, provided a clear demonstration of previously controversial retinal projections, clearer definition of established projections, and the discovery of new retinal pathways. An inferior accessory optic system was shown to be unequivocally present in this species and to consist of both crossed and uncrossed components. A direct retinal projection to the suprachiasmatic nucleus (SCN) of the hypothalamus was confirmed in this study. But the distribution of terminals as seen by this procedure was substantially different than previously reported; both rostrocaudal and mediolateral asymmetries in the distribution of label between the ipsilateral and contralateral SCN were observed. Substantial differences in the retinal projection to the SCN in the hamster and the rat were also noted. It is suggested that these differences may reflect the different effects photic input has on the neuro-endocrine-gonadal axis in these two species. Finally, labeled retinal axons were followed leaving the optic tract and coursing anteriorly through the plexiform layer of the piriform cortex; other labeled fibers were seen to enter the septal region. The physiological significance of these previously undescribed retinal projections is not known.

Effects of photoperiod on hypothalamic luteinizing hormone releasing hormone in the male hamster

Male hamsters were maintained on long (14 h light : 10 h darkness; 14L : 10D) or short (6L : 18D) photoperiods. Animals on short-days had reduced levels of LH in the serum and anterior pituitary gland, decreased androgen in the circulation and regressed testes and accessory sex organs. These same hamsters had significantly raised concentrations of hypothalamic luteinizing hormone releasing hormone (LH-RH). There was no significant difference in the response to exogenous LH-RH between groups maintained on long- and short-days. Castration significantly reduced levels of LH-RH in the hypothalamus in the long-day animals but had little effect on this parameter in short-day animals which had already undergone testicular regression. The increased levels of LH-RH in the hyothalami of both intact and castrated hamsters on non-stimulatory photoperiods is interpreted as a decreased release of the neurohormone which subsequently results in a decreased release of LH.