Ionophoretically applied substance P activates hamster suprachiasmatic nucleus neurons

Specializations of gastrin-releasing peptide cells of the mouse suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus regulates daily rhythms in physiology and behavior. It is composed of a heterogeneous population of cells that together form the circuits underlying its master clock function. Numerous studies suggest the existence of two regions that have been termed core and shell. At a gross level, differences between these regions map to distinct functional differences, although the specific role(s) of various peptidergic cellular phenotypes remains unknown. In mouse, gastrin-releasing peptide (GRP) cells lie in the core, are directly retinorecipient, and lack detectable rhythmicity in clock gene expression, raising interest in their role in the SCN. Here, we provide evidence that calbindin-expressing cells of perinatal mouse SCN express GRP, identified by a green fluorescent protein (GFP+), but lack detectable calbindin later in development. To explore the intra-SCN network in which GRP neurons participate, individual GFP+ cells were filled with tracer and their morphological characteristics, processes, and connections, as well as those of their non-GFP-containing immediate neighbors, were compared. The results show that GFP+ neurons form a dense network of local circuits within the core, revealed by appositions on other GFP+ cells and by the presence of dye-coupled cells. Dendrites and axons of GFP+ cells make appositions on arginine vasopressin neurons, whereas non-GFP cells have a less extensive fiber network, largely confined to the region of GFP+ cells. The results point to specialized circuitry within the SCN, presumably supporting synchronization of neural activity and reciprocal communication between core and shell regions.

Circadian trafficking of calbindin-ir in fibers of SCN neurons

Calbindin-D28K (CalB)-containing cells form a distinct cluster within the core of the hamster suprachiasmatic nucleus (SCN). These cells are directly retinorecipient but lack detectable rhythms in clock gene expression or electrical activity. In studies exploring SCN connectivity using double-label immunochemistry, we previously reported an absence of contacts among CalB fibers and vasopressin (VP) cells in animals sacrificed during the day. Here, we explored circadian variations in CalB-immunoreactivity (-ir) and re-examined the connections between CalB and other SCN cell types at zeitgeber times (ZT) 4 and 14. The results reveal a circadian rhythm of CalB-ir in fibers of SCN cells with high expression during the night and subjective night and low expression during the day and subjective day. This circadian difference is not seen in the other brain regions studied. Significantly more appositions were detected between CalB fibers and VP cells during the night than during the day, while circadian variation in numbers of contacts was not seen between CalB fibers and vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), or gastrin-releasing peptide (GRP) cells. There was no detectable variation in appositions from any peptidergic fiber type onto CalB cells. The present findings suggest that CalB cells relay photic information to VP oscillator cells of the SCN shell in a temporally gated manner.

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

Substance P and neurokinin-1 immunoreactivities in the neural circadian system of the Alaskan northern red-backed vole, Clethrionomys rutilus

The suprachiasmatic nucleus (SCN) of the hypothalamus houses the main mammalian circadian clock. This clock is reset by light-dark cues and stimuli that evoke arousal. Photic information is relayed directly to the SCN via the retinohypothalamic tract (RHT) and indirectly via the geniculohypothalamic tract, which originates from retinally innervated cells of the thalamic intergeniculate leaflet (IGL). In addition, pathways from the dorsal and median raphe (DR and MR) convey arousal state information to the IGL and SCN, respectively. The SCN regulates many physiological events in the body via a network of efferent connections to areas of the brain such as the habenula (Hb) in the epithalamus, subparaventricular zone (SPVZ) of the hypothalamus and locus coeruleus of the brainstem-areas of the brain associated with arousal and behavioral activation. Substance P (SP) and the neurokinin-1 (NK-1) receptor are present in the rat SCN and IGL, and SP acting via the NK-1 receptor alters SCN neuronal activity and resets the circadian clock in this species. However, the distribution and role of SP and NK-1 in the circadian system of other rodent species are largely unknown. Here we use immunohistochemical techniques to map the novel distribution of SP and NK-1 in the hypothalamus, thalamus and brainstem of the Alaskan northern red-backed vole, Clethrionomys rutilus, a species of rodent currently being used in circadian biology research. Interestingly, the pattern of immunoreactivity for SP in the red-backed vole SCN was very different from that seen in many other nocturnal and diurnal rodents.

Tachykinins and the control of prolactin secretion

Tachykinins are present in the pituitary gland and in brain areas involved in the control of the secretion of pituitary hormones. Tachykinins have been demonstrated to stimulate prolactin release acting directly on the anterior pituitary gland. These peptides have also been revealed to be able to act at the hypothalamic level, interacting with neurotransmitters and neuropeptides that have the potential to affect prolactin secretion. Tachykinins seem to act by stimulating or inhibiting the release of the factors that affect prolactin secretion. Among them, tachykinins have been demonstrated to stimulate oxytocin and vasopressin release, which in turn results in prolactin release. Tachykinins also potentiated the response to vasoactive intestinal peptide (VIP) and reinforced the action of glutamate, which in turn result in prolactin release. They have also been shown to interact with serotonin, a neurotransmitter involved in the control of prolactin secretion. In addition, tachykinins have been shown to inhibit GABA release, a neurotransmitter with prolactin-release inhibiting effect. This inhibition may result in an increased prolactin secretion by removal of the GABA inhibition. On the other hand, tachykinins have also been shown to stimulate dopamine release by the hypothalamus, an action that results in an inhibition of prolactin release. Dopamine is a well known inhibitor of prolactin secretion. In conclusion, although tachykinins have been shown to have a predominantly stimulatory effect on prolactin secretion, especially at the pituitary level, under some circumstances they may also exert an inhibitory influence on prolactin release, by stimulating dopamine release at the hypothalamic level.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

The selective tachykinin neurokinin 1 (NK1) receptor antagonist, GR 205,171, stereospecifically inhibits light-induced phase advances of hamster circadian activity rhythms

Circadian rhythms in mammals are generated by master pacemaker cells located within the suprachiasmatic nucleus of the hypothalamus. In hamsters, the suprachiasmatic nucleus contains a small collection of cells immunoreactive for substance P, the endogenous ligand of tachykinin neurokinin 1 (NK1) receptors. In addition, two other nuclei which form part of the circadian system, the intergeniculate leaflet of the thalamus and the raphe nuclei, also contain fibers and/or cell bodies immunoreactive for substance P. In light of these observations, we evaluated the influence of the selective tachykinin NK1 receptor antagonist, GR 205,171, upon circadian activity rhythms in the hamster. Systemic injection of GR 205,171 dose-dependently (2.5-40.0 mg/kg, i.p.) inhibited light-induced phase advances in hamster circadian wheel running activity rhythms by approximately 50%. In contrast, GR 226,206, the less active enantiomer of GR 205,171, failed to affect light-induced phase advances. In addition, we examined the potential ability of GR 205,171 to induce non-photic phase shifts in hamster wheel running rhythms when injected at mid-day to late night circadian times. However, GR 205,171 (40 mg/kg) did not elicit non-photic phase shifts at these times indicating that tachykinin NK1 receptor antagonists are only effective when a light stimulus is applied to the pacemaker. Although GR 205,171 may, in theory, activate several sites within the circadian system, we suggest that GR 205,171 acts in the raphe nuclei to increase inhibitory serotonergic input to pacemaker cells in the suprachiasmatic nuclei, thereby suppressing photic modulation of the pacemaker. These findings have important implications for the use of tachykinin NK1 receptor antagonists in the treatment of depression and other central nervous system disorders.

Distribution of substance P and neurokinin-1 receptor immunoreactivity in the suprachiasmatic nuclei and intergeniculate leaflet of hamster, mouse, and rat

The circadian pacemaker in the hypothalamic suprachiasmatic nuclei (SCN) receives photic information directly via the retinohypothalamic tract (RHT) and indirectly from retinally innervated cells in the thalamic intergeniculate leaflet (IGL) that project to the SCN. Using standard immunohistochemical methods, we examined the presence and distribution of substance P (SP) and the neurokinin-1 receptor (NK-1) in the SCN and IGL of rat and determined whether the patterns of immunostaining generalized to the SCN and IGL of Syrian hamster, Siberian hamster, and mouse. Terminals immunoreactive for SP were sparse within the SCN of Siberian and Syrian hamsters and mouse but were intense in the ventral, retinally innervated portion of the rat SCN. Immunostaining for the NK-1 receptor was mainly absent from the SCN of hamster and mouse. In contrast, a plexus of NK-1-ir cells and processes that was in close proximity to SP-ir terminals was found in the ventral SCN of the rat. Substance P-ir terminals were observed in the IGL of all four species, as were NK-1-ir cells and fibres. Double-labelled IGL sections of hamster or rat revealed SP-ir terminals in close apposition to NK-1-immunostained cells and/or fibres. These data indicate that SP could be a neurotransmitter of the RHT in rat, but not in hamster or in mouse, and they highlight potential species differences in the role of SP within the SCN circadian pacemaker. Such species differences do not appear to exist at the level of the IGL, where SP-ir and NK-1-ir were similar in all species studied.

Effects of neurotensin on discharge rates of rat suprachiasmatic nucleus neurons in vitro

The neuropeptide neurotensin and two classes of its receptors, the neurotensin receptor-1 and 2, are present in the suprachiasmatic nucleus of the mammalian hypothalamus. The suprachiasmatic nucleus houses the mammalian central circadian pacemaker, but the effects of neurotensin on cellular activity in this circadian pacemaker are unknown. In this study, we examined the effects of neurotensin on the spontaneous discharge rate of rat SCN cells in an in vitro slice preparation. Neurotensin (1-10 microM) increased cell firing rate in approximately 50% of cells tested, while approximately 10% of suprachiasmatic cells tested showed a decrease in firing rate in response to neurotensin. These effects of neurotensin were not altered by the GABA receptor antagonist bicuculline (20 microM) or the glutamate receptor antagonists, D-aminophosphopentanoic acid (50 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). The neurotensin receptor selective antagonists SR48692 and SR142948a (10 microM) failed to antagonise neurotensin responses in the majority of cells examined. Compounds that function as agonists selective for the neurotensin-receptor subtypes 1 and 2, JMV-510 and JMV-431 respectively, elicited neurotensin-like responses in approximately 90% of cells tested. Six out of seven cells tested responded to both JMV-510 and JMV-431. Neuropeptide Y (100nM) treatment of suprachiasmatic nucleus slices was found to elicit profound suppression of neuronal firing rate. Co-application of neurotensin with neuropeptide Y significantly (P<0.05) reduced the duration of the response, as compared to that elicited with neuropeptide Y alone. Together, these results demonstrate for the first time the actions of neurotensin in the suprachiasmatic nucleus and raise the possibility that this neuropeptide may play a role in modulating circadian pacemaker function.

Substance p plays a critical role in photic resetting of the circadian pacemaker in the rat hypothalamus

Glutamate is considered to be the primary neurotransmitter in the retinohypothalamic tract (RHT), which delivers photic information from the retina to the suprachiasmatic nucleus (SCN), the locus of the mammalian circadian pacemaker. However, substance P (SP) also has been suggested to play a role in retinohypothalamic transmission. In this study, we sought evidence that SP from the RHT contributes to photic resetting of the circadian pacemaker and further explored the possible interaction of SP with glutamate in this process. In rat hypothalamic slices cut parasagittally, electrical stimulation of the optic nerve in early and late subjective night produced a phase delay (2.4 +/- 0.5 hr; mean +/- SEM) and advance (2.6 +/- 0.3 hr) of the circadian rhythm of SCN neuronal firing activity, respectively. The SP antagonist L-703,606 (10 microm) applied to the slices during the nerve stimulation completely blocked the phase shifts. Likewise, a cocktail of NMDA (2-amino-5-phosphonopentanoic acid, 50 microm) and non-NMDA (6,7-dinitroquinoxaline-2,3-dione, 10 microm) antagonists completely blocked the shifts. Exogenous application of SP (1 microm) or glutamate (100 microm) to the slices in early subjective night produced a phase delay ( approximately 3 hr) of the circadian firing activity rhythm of SCN neurons. Coapplication of the NMDA and non-NMDA antagonist cocktail (as well as L-703,606) resulted in a complete blockade of the SP-induced phase delay, whereas L-703,606 (10 microm) had no effect on the glutamate-induced delay. These results suggest that SP, as well as glutamate, has a critical role in photic resetting. Furthermore, the results suggest that the two agonists act in series, SP working upstream of glutamate.

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.

Electrophysiological evidence for the role of substance P in retinohypothalamic transmission in the rat

The retinohypothalamic tract (RHT) is a neural pathway through which photic time cues are delivered directly to the mammalian circadian pacemaker in the suprachiasmatic nucleus (SCN). Although the excitatory amino acid glutamate is the primary neurotransmitter in the RHT, other substances such as substance P (SPq also have been suggested to play a role. The present study tested the hypothesis that SP participates in retinohypothalamic transmission and selectively modulates either N-methyl-D-aspartate (NMDA) or non-NMDA receptor-mediated neurotransmission. The SP antagonist L-703,606 depressed the excitatory postsynaptic current (EPSC) evoked by optic nerve stimulation in SCN neurons in rat hypothalamic slices. The SP antagonist also had a similar depressive effect on the NMDA and non-NMDA receptor-mediated components of the EPSC. These results suggest that SP is an excitatory neuromodulator contributing to the expression of both the NMDA and non-NMDA receptor-mediated components of retinohypothalamic transmission.

Electrophysiological effects of opioid receptor activation on Syrian hamster suprachiasmatic nucleus neurones in vitro

Entrainment of the dominant circadian pacemaker localised to the hypothalamic suprachiasmatic nuclei (SCN) is mediated partially via the indirect retino-geniculo-hypothalamic projection to the SCN, which is presumed to utilise enkephalin and other neurotransmitters, to modulate circadian rhythmicity. In the present study, we have investigated electrophysiologically the currently unknown functional effects of enkephalin, and another opioid receptor agonist morphine, on hamster SCN neuronal activity in vitro. Basal or N-methyl-D-aspartate-evoked firing rates of SCN neurones were generally unresponsive (86%) to the opioid receptor agonists leucine-enkephalin, methionine-enkephalin, or morphine. Washout of the enkephalins or morphine resulted in a rebound excitatory response ("withdrawal activation") in 39% of neurones tested. Withdrawal activation was also elicited by administration of the opioid receptor antagonist naloxone, following pre-exposure to morphine, in 59% of neurones tested. These withdrawal responses were blocked or attenuated by the alpha2-adrenoceptor agonist clonidine, results which suggest a functional interaction exists between opioid receptors and alpha2-adrenoceptors in the SCN. Our observations show that opioid receptor agonists are largely devoid of actions on normal hamster SCN circadian pacemaker activity, while the occurrence of withdrawal responses may have implications on circadian function during withdrawal from opiate abuse.

Involvement of glutamate release in substance P-induced phase delays of suprachiasmatic neuron activity rhythm in vitro

The suprachiasmatic nucleus (SCN) has been identified as a mammalian circadian rhythm clock. Treatment with substance P (SP) at zeitgeber time 13-14 produced phase delays of circadian rhythm in spontaneous neural activity in SCN neurons in vitro. SP-induced phase delays are blocked by treatment with not only SP receptor antagonist, spantide, but N-methyl-D-aspartate receptor antagonist, MK-801. In the biochemical experiment, we demonstrated that SP-induced glutamate release from the SCN slices was observed by the high-performance liquid chromatography assay. The present results suggest that glutamate release may be involved in SP-induced phase delays.

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

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

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.

Histamine phase shifts the hamster circadian pacemaker via an NMDA dependent mechanism

The SCN acts as the central pacemaker for circadian rhythms in mammals. Histamine has been shown to affect circadian rhythms both in vivo and in vitro. We investigated the mechanism by which histamine phase shifts circadian rhythms in vitro. Hypothalamic slices containing the SCN were prepared from golden hamsters, and spontaneous firing rates of individual cells were recorded on the second day in vitro. Application of histamine (1 microM-10 mM) at the extrapolated time of 2 h after lights off (ZT 14) on day 1 in vitro delayed the time of peak firing in a dose-dependent manner. Pre-exposure to the N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)-2-amino-5-phosphonopentanoic acid (AP-5; 100 microM-1 mM) 5 min before histamine (1 microM) was applied to the slice blocked the phase-delaying effects of histamine. Application of the H1 blocker mepryamine (100 nM) or the H2 blocker cimetidine (10 microM) followed by histamine had no effect on the phase delay induced by histamine. In whole cell recordings from acutely dissociated neurons of hamster SCN, histamine (50 microM) was shown to potentiate NMDA-evoked currents by 52 +/- 12%. These experiments demonstrate that histamine phase shifts of the circadian clock are dependent on NMDA receptor activation and that histamine can directly potentiate NMDA currents in SCN neurons. Histamine may alter circadian clock function by acting directly on NMDA receptors, possibly via binding to the polyamine site.

An NK1 receptor antagonist affects the circadian regulation of locomotor activity in golden hamsters

Substance P (SP) is a neuromodulator which may participate in the photic regulation of the circadian timing system in mammals. The biological effects of SP are mediated by interaction with specific receptors, designated as NK1, NK2, and NK3. The NK1 subtype receptor is expressed in the circadian system. Experiment 1 was designed to test whether an NK1 antagonist mimics the effects of dark pulses. Hamsters were housed in constant lighting conditions, either constant darkness or constant light (around 250 lx), and they received an i.p. injection of either the specific NK1 receptor antagonist, L-760,735 (5 mg/kg), or saline during the mid-subjective day, a time when dark pulses cause a phase-advance in circadian rhythm of locomotor activity. After treatment with the NK1 antagonist, significant phase-advances of wheel-running activity rhythm were found in constant light, but not in constant darkness. Experiment 2 was designed to test the ability of the NK1 antagonist to block the phase-delaying and/or the phase-advancing effects of light in animals kept in constant darkness. Phase-advances of locomotor activity rhythm that can normally be induced by light pulses given during the late subjective night were markedly reduced by pre-treatment with the NK1 antagonist. By contrast, phase-delays that can be induced by lights pulses given during the early subjective night were unaffected by the NK1 antagonist. These data support the hypothesis that SP within the circadian system may, by interacting with NK1 receptors, modulate photic responses of the SCN pacemaker.

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.