Differential effects on substance P immunoreactivity in rat suprachiasmatic and olivary pretectal nuclei

Calcium imaging reveals a network of intrinsically light-sensitive inner-retinal neurons

BACKGROUND

Mice lacking rod and cone photoreceptors (rd/rd cl) are still able to regulate a range of responses to light, including circadian photoentrainment, the pupillary light reflex, and suppression of pineal melatonin by light. These data are consistent with the presence of a novel inner-retinal photoreceptor mediating non-image-forming irradiance detection.

RESULTS

We have examined the nature and extent of intrinsic light sensitivity in rd/rd cl retinae by monitoring the effect of light stimulation (470 nm) on intracellular Ca(2+) via FURA-2 imaging. Using this approach, which does not rely on pharmacological or surgical isolation of ganglion cells from the rod and cone photoreceptors, we identified a population of light-sensitive neurons in the ganglion cell layer (GCL). Retinal illumination induced an increase of intracellular Ca(2+) in approximately 2.7% of the neurons. The light-evoked Ca(2+) fluxes were dependent on the intensity and duration of the light stimulus. The light-responsive units formed an extensive network that could be uncoupled by application of the gap junction blocker carbenoxolone. Three types of light-evoked Ca(2+) influx were observed: sustained, transient, and repetitive, which are suggestive of distinct functional classes of GCL photoreceptors.

CONCLUSIONS

Collectively, our data reveal a heterogeneous syncytium of intrinsically photosensitive neurons in the GCL coupled to a secondary population of light-driven cells, in the absence of rod and cone inputs.

The selective neurokinin 1 receptor antagonist R116301 modulates photic responses of the hamster circadian system

The recent development of selective NK(1) receptor antagonists that are active in vivo provides an important research tool to examine the role of substance P in the regulation of circadian rhythmicity. First, we tested whether R116301 [(2R-trans)-4-[1-[3,5-bis(trifluoromethyl)benzoyl]-2-(phenylmethyl)-4-piperidinyl]-N-(2,6-dimethylphenyl)-1-acetamide (S) hydroxybutanedioate], a new selective NK(1) antagonist, alters the phase-shifting effects of light. Hamsters housed in constant darkness were injected with different doses of R116301, just before being exposed to a light pulse during the subjective night. The results were compared with those obtained with the NK(1) antagonist L-760,735 [2-(R)-(1-(R)-3,5-bis(trifluoromethyl)phenyl)ethoxy)-4-(5-(dimethylaminomethyl)-1,2,3-trioazol-4-yl)methyl-3-(5)-phenyl)morpholine]. Second, the effects of the NK(1) antagonists R116301 or L-760,735 injected immediately after exposure to a light pulse were similarly determined. Third, we investigated whether R116301 or L-760,735 injected during the mid-subjective day or the late subjective night can phase-shift the circadian rhythm of locomotor activity in hamsters housed in constant light. Both compounds reduced, by more than 30%, the phase-advancing effects of a light pulse in hamsters otherwise maintained in constant darkness, only when the drugs were administered before the light pulse. Under constant light conditions, both NK(1) receptor antagonists induced significant phase-advances when injected during the subjective day, but not during the subjective night. The present results indicate that tachykinergic neurotransmission modulates the photic responses of the circadian system upstream of phase resetting mechanisms and suggest that an inhibition of the NK(1) receptor signals "darkness" to the circadian clock.

Photic entrainment of circadian rhythms in rodents

Photic entrainment of circadian rhythms occurs as a consequence of daily, light-induced adjustments in the phase and period of the suprachiasmatic nuclei (SCN) circadian clock. Photic information is acquired by a unique population of retinal photoreceptors, processed by a distinct subset of retinal ganglion cells, and conveyed to the SCN through the retinohypothalamic tract (RHT). RHT neurotransmission is mediated by the release of the excitatory amino acid glutamate and appears to require the activation of both NMDA- and non-NMDA-type glutamate receptors, the expression of immediate early genes (IEGs), and the synthesis and release of nitric oxide. In addition, serotonin appears to regulate the response of the SCN circadian clock to light through postsynaptic 5-HT1A or 5-ht7 receptors, as well as presynaptic 5-HT1B heteroreceptors on RHT terminals.

Neuropeptide Y and enkephalin immunoreactivity in retinorecipient nuclei of the hamster pretectum and thalamus

This investigation was stimulated by the historical confusion concerning the identity of certain pretectal nuclei and by large differences reported between species with respect to which nuclei receive retinal innervation. Subcortical visual nuclei were studied using immunohistochemistry to identify retinal projections labeled following intraocular injection of cholera toxin, b fragment. In addition, neuropeptide Y (NPY) or enkephalin (ENK) immunoreactive cells and fibers were also evaluated in the retinorecipient pretectal and thalamic areas. The results confirm the established view that the retina directly innervates the nucleus of the optic tract (NOT), posterior (PPT), and olivary pretectal (OPT) nuclei. However, the retina also innervates the hamster medial (MPT) and anterior (APT; dorsal division) pretectal nuclei, results not previously reported in rodents. A commissural pretectal area (CPT) sparsely innervated by retina is also described. The data show for the first time that the posterior limitans nucleus (PLi) receives a moderately dense, direct retinal input. The PLi does not project to the cortex and appears to be a pretectal, rather than thalamic, nucleus. All retinal projections are bilateral, although predominantly contralateral. The PLi contains a moderately dense plexus of NPY- and ENK-IR fibers and terminals. However, peptidergic fibers also traverse the ATP and connect with the dorsomedial pretectium. The OPT contains ENK- and NPY-IR neurons and fibers, but is specifically identifiable by a moderately dense plexus of ENK-IR terminals. Numerous ENK-IR neurons are found in the NOT and PPT. The latter also has moderate numbers of ENK-IR fibers and terminals, but few NPY-IR neurons or fibers. The MPT contains modest numbers of ENK-IR fibers. The APT has no NPY-IR neurons or terminals, but an occasional ENK-IR neuron is seen and there is sparse ENK-IR innervation. Peptidergic innervation of the visual nuclei does not appear to be derived from the retina. The results show a set of retinally innervated, contiguous nuclei extending from the thalamic ventrolateral geniculate nucleus dorsomedially to the midbrain CPT. These nuclei plus the superior colliculus comprise a dorsal "visual shell" embracing a central core of caudal thalamus and rostral midbrain.

Effects of microinjections of substance P into the suprachiasmatic nucleus region on hamster wheel-running rhythms

The suprachiasmatic nucleus (SCN) receives a direct retinal projection, which in rats includes substance P (SP)-immunoreactive retinal ganglion cells. While SP has been shown to have neurophysiological effects on SCN cells in Syrian hamsters and rats, it is not known what effects SP in the SCN has on circadian rhythms in hamsters. We examined this question using male Syrian hamsters that were implanted with cannulas aimed at the SCN region and maintained in constant dim red lighting conditions. Hamsters received 0.5 microl microinjections of saline or SP (500 pmol in saline) at a variety of circadian times (CT). Saline injections had little or no phase-shifting effects at any phase tested. SP had no significant effects at CT4-8, 16-20, or 20-24, but did cause small phase delays of -23.7 +/- 7 min (mean +/- sem) at CT12-16. In order to examine the dose-response relations of this effect, hamsters were also microinjected with 50 and 2500 pmol of SP at CT12-16. Both the 50 and 2500 pmol doses induced very small phase delays (-14.2 +/- 7 min and -18.2 +/- 5 min, respectively), indicating no obvious dose dependence within this range. These results do not suggest that SP alone in the SCN mimics light effects on circadian rhythms or is a key neurotransmitter involved in photic entrainment. It remains to be determined whether SP interacts with other transmitters in the SCN to modulate their effects on rhythm phase.

Strain differences in the distribution of arginine-vasopressin- and neuropeptide Y-immunoreactive neurons in the suprachiasmatic nucleus of laboratory rats

We have studied the number of arginine-vasopressin (AVP)-immunoreactive (ir) somata and the area size of AVP- and neuropeptide Y (NPY)-ir fibers in the suprachiasmatic nuclei (SCN) of three strains of laboratory rats exhibiting a strong unimodal (ACI), a bimodal (BH), and a weak multimodal pattern (LEW) of wheel running activity. In all three strains, AVP-ir somata and fibers were located predominantly in the dorsomedial SCN. Significant strain-differences were found for the area size of AVP-ir fibers as well as for the number and density of AVP-ir somata. The total number of AVP-ir somata was significantly higher in strain ACI (2238 +/- 164) than in strains BH (1552 +/- 137) and LEW (1426 +/- 110), whereas the mean area of AVP-ir fibers was significantly larger in strain LEW (50779 +/- 2202 microns2) than in strains ACI (39034 +/- 2095 microns2) and BH (28052 +/- 1728 microns2). Consequently, the density of AVP-ir somata was significantly lower in LEW rats, which have a weak multimodal activity pattern, than in BH and ACI rats, which have a bimodal and unimodal activity pattern, respectively. These data suggest that AVP neurons may be part of SCN output pathways controlling circadian activity rhythms. NPY-ir fibers have been identified mainly in the ventral part of the SCN. The mean area of NPY-ir fibers was smallest in BH rats (26100 +/- 1822 microns2), which show a rather scattered activity onset, and larger in ACI (29934 +/- 2468 microns2) and LEW rats (31889 +/- 2728 microns2), which have rather precise activity onsets. The inbred strains ACI, BH, and LEW may prove to be suitable models to further study distinct neuronal substrates of the SCN functionally correlated with characteristic parameters of circadian rhythms.

Substance P receptor regulates the photic induction of Fos-like protein in the suprachiasmatic nucleus of Syrian hamsters

Substance P (SP) is a candidate neurotransmitter or neuromodulator for conveying light information from the retina to the hypothalamic suprachiasmatic nucleus (SCN) where a circadian oscillator(s) is located in mammals. Immediate early gene c-fos has been demonstrated to be induced in the SCN with a brief light exposure at the subjective night, and suggested to play an important role in the photic entrainment of the oscillator. To clarify the possibility of an involvement of the SP receptor in the photic-induction of c-fos in the SCN, we examined effects of a SP receptor antagonist, spantide, on the light-induced Fos-like protein immunoreactivity (Fos-lir) in the SCN of Syrian hamster. The light-induced Fos-lir was inhibited with the pretreatment of spantide in a dose-related manner and in an anatomically distinctive way. The higher dose of spantide (8 nmol) blocked light-induced Fos-lir substantially in the rostral and central areas of the SCN, and in the dorsal portion of the caudal SCN. However, it blocked Fos-lir only slightly in the ventral portion of the caudal SCN. These results suggest that the SP is involved in conveying light information to induce Fos protein in the hamster SCN, and that different neurotransmitter systems are involved in the light-induced Fos-lir in the different portions of hamster SCN.

A relationship between substance P receptor and retinal fibers in the rat suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) in the hypothalamus controls many of the circadian rhythms in mammalian species. In the present study, we investigated the location of substance P receptor (SPR)-containing neurons in the rat SCN, using a specific antibody against SPR, which corresponds to the NK-1 subtype of tachykinin receptors, and also examined the synaptic relationship between SPR-containing neurons and retinal fibers at the ultrastructural level. An SPR-immunoreactive meshwork of labeled somata and dendrites was identified in the SCN. The strongest SPR-immunoreactivity was observed in the dorsal and lateral parts of the SCN. Many labeled somata were identified there and their dendrites protruded ventrally from their somata. A few SPR-immunoreactive somata were observed also in the ventral part of the SCN and within the optic tract. In the SCN of eye-enucleated animals, degenerating retinal fibers were shown to terminate on SPR-immunoreactive dendrites forming asymmetrical axo-dendritic contacts.

Ionophoretically applied substance P activates hamster suprachiasmatic nucleus neurons

ionophoretic ejection of substance P (SP) activated 31% and suppressed 9% of hamster suprachiasmatic nucleus (SCN) cells in vitro. Hamster SCN cells did not demonstrate variation in sensitivity to SP across the circadian phases tested. SP modulated the response of 47% of hamster SCN cells to the excitatory amino acid (EAA) agonists glutamate and N-methyl-D-aspartate (NMDA). The results indicate that SP can alter both the spontaneous and EAA-evoked firing rate characteristics of hamster SCN neurons.

Neurochemical organization of circadian rhythm in the suprachiasmatic nucleus

The circadian rhythm in mammals is under control of the pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus. This tiny nucleus contains a number of neurochemicals, including peptides, amines and amino acids. Heterogeneous distribution of these neurochemicals defines the substructures of the SCN. In the present review, functional significance of such neurochemical heterogeneity in the SCN is discussed in the light of circadian patterns of the concentrations of these neurochemicals in the SCN and their effects on SCN neurons in in vitro slice preparation. In particular, the hypothesis that the dorsomedial SCN is involved in maintaining the circadian rhythm, while the ventrolateral SCN is involved in adjusting the phase of the rhythm, is critically discussed. These considerations suggest that distinct sub-components of the SCN as marked by neurochemicals, interact with each other and this organizational architecture could be the basis of the proper operation of the circadian time keeping system in this nucleus.

Marginal topography of neurons expressing the substance P receptor in the rat suprachiasmatic nucleus

Neurons expressing the substance P (SP) receptor (NK1 receptor) in the suprachiasmatic nucleus of the hypothalamus (SCN) have been topographically identified using radioactive in situ hybridization histochemistry. In the anterior hypothalamic area, clustered labeled neurons of small size and exhibiting low levels of gene expression are observed exclusively at the dorsolateral margin of the SCN, straddling cytoarchitectural boundaries of the nucleus. The marginal topography of neurons putative target of a SP-containing retinal input to the ventral SCN indicates that their dendrites bearing the receptor extend towards the retinorecipient part of the nucleus, where they can be modulated by overlapping inputs from the intergeniculate leaflet and the raphe. Eventual interactions between glutamatergic and putative tachykininergic retinal pathways for a coherent photic control of circadian rhythms may therefore occur mainly via intrinsic neuronal connections between their distinct target populations. In addition, since glutamate and SP induce electrophysiological responses in ventrolateral neurons with no interactive effect, neurons integrating both chemical signals, subsequently to their modulation by several influences, may be not located within the ventrolateral SCN. Alternatively but not exclusively, marginal neurons could be a target of SP-containing neurons within the SCN or nearby the nucleus, or from ascending projections from the raphé where serotonin and SP colocalize. The marginal topography of neurons expressing the SP receptor supports the view of the involvement of neurons located in the vicinity of the nucleus in the regulation of circadian rhythms.