Anticholinergic agents do not block light-induced circadian phase shifts

A Symphony of Signals: Intercellular and Intracellular Signaling Mechanisms Underlying Circadian Timekeeping in Mice and Flies

The central pacemakers of circadian timekeeping systems are highly robust yet adaptable, providing the temporal coordination of rhythms in behavior and physiological processes in accordance with the demands imposed by environmental cycles. These features of the central pacemaker are achieved by a multi-oscillator network in which individual cellular oscillators are tightly coupled to the environmental day-night cycle, and to one another via intercellular coupling. In this review, we will summarize the roles of various neurotransmitters and neuropeptides in the regulation of circadian entrainment and synchrony within the mammalian and Drosophila central pacemakers. We will also describe the diverse functions of protein kinases in the relay of input signals to the core oscillator or the direct regulation of the molecular clock machinery.

Light pulse duration differentially regulates mouse locomotor suppression and phase shifts

Brief exposure of mice to nocturnal light causes circadian rhythm phase shifts, simultaneously inducing locomotor suppression, a drop in body temperature, and associated sleep. The exact nature of the relationship between these light-induced responses is uncertain, although locomotor suppression and phase shift magnitudes are related to stimulus irradiance. Whether stimulus duration has similar effects is less clear. Here, the relationship between stimulus duration and response magnitude was evaluated further using 100 µW/cm(2) white light-emitting diode pulses administered for 30, 300, 1200, or 3000 sec. The results show that, in general, shorter pulses yielded smaller responses and larger pulses yielded larger responses. However, the 300-sec pulse failed to augment locomotor suppression compared with the effect of a 30-sec pulse (44.7 ± 4.8 vs 40.6 ± 2.0 min) but simultaneously induced much larger phase shifts (1.28 ± 0.20 vs 0.52 ± 0.11 h). The larger phase shifts induced by the 300-sec stimulus did not differ from those induced by either the 1200- or 3000-sec pulses (1.43 ± 0.10 and 1.30 ± 0.17 h, respectively). The results demonstrate differential photic regulation of the two response types. Pulses ranging from 300 to 3000 sec produce equal phase shifts (present data); pulses ranging from 30 to 600 sec produce equal locomotor suppression levels. Greater suppression can occur additively in response to pulses of 1200 sec or more (present data), but this is not true for phase shifts. Nocturnal light appears to trigger a fixed duration event, locomotor suppression, or phase shift, with the latter followed by a light-refractory interval during which locomotor suppression can additively increase. The results also provide further support for the view that temporal integration of photic energy applies, at best, across a limited set of stimulus durations for both light-induced locomotor suppression/sleep and phase shift regulation.

Signals from the brainstem sleep/wake centers regulate behavioral timing via the circadian clock

Sleep-wake cycling is controlled by the complex interplay between two brain systems, one which controls vigilance state, regulating the transition between sleep and wake, and the other circadian, which communicates time-of-day. Together, they align sleep appropriately with energetic need and the day-night cycle. Neural circuits connect brain stem sites that regulate vigilance state with the suprachiasmatic nucleus (SCN), the master circadian clock, but the function of these connections has been unknown. Coupling discrete stimulation of pontine nuclei controlling vigilance state with analytical chemical measurements of intra-SCN microdialysates in mouse, we found significant neurotransmitter release at the SCN and, concomitantly, resetting of behavioral circadian rhythms. Depending upon stimulus conditions and time-of-day, SCN acetylcholine and/or glutamate levels were augmented and generated shifts of behavioral rhythms. These results establish modes of neurochemical communication from brain regions controlling vigilance state to the central circadian clock, with behavioral consequences. They suggest a basis for dynamic integration across brain systems that regulate vigilance states, and a potential vulnerability to altered communication in sleep disorders.

Nicotine and nicotinic receptors in the circadian system

Considerable data support a role for cholinergic influences on the circadian system. The extent to which these influences are mediated by nicotinic acetylcholine receptors (nAChRs) has been controversial, as have the specific actions of nicotine and acetylcholine in the suprachiasmatic nucleus (SCN) of the hypothalamus. In this article we review the existing literature and present new data supporting an important role for nAChRs in both the developing and adult SCN. Specifically, we present data showing that nicotine is capable of causing phase shifts in the circadian rhythms of rats. Like light and carbachol, nicotine appears to cause phase delays in the early subjective night and phase advances in the late subjective night. In the isolated SCN slice, however, only phase advances are seen, and, surprisingly, nicotine appears to cause the inhibition rather than the excitation of neurons. Among nAChR subunit mRNAs, alpha 7 appears to be the most abundant subunit in the adult SCN, whereas in the perinatal period, the more typical nAChRs with higher affinity for nicotine predominate in the SCN. This developmental change in subunit expression may explain the dramatic sensitivity of the perinatal SCN to nicotine that we have previously observed. The effects of nicotine on the SCN may contribute to alterations caused by nicotine in other physiological systems. These effects might also contribute to the dependence properties of nicotine through influences on arousal.

Muscarinic receptors mediate carbachol-induced phase shifts of circadian activity rhythms in Syrian hamsters

Carbachol, a non-specific cholinergic agonist, when administered intraventricularly or directly into the suprachiasmatic nucleus (SCN), causes phase-dependent shifts in circadian rhythms of wheel-running activity in rodents. The cholinergic receptor subtype involved in mediating these carbachol-induced phase shifts, however, remains uncertain. In order to investigate this issue we injected carbachol into the SCN through indwelling cannulas at circadian times (CT) 6, 14 and 22 in Syrian hamsters (Mesocricetus auratus) maintained in constant darkness. Carbachol elicited large phase advances at CT 6 (69.8 +/- 15.7 min; mean +/- S.E.M.) and CT 22 (83.9 +/- 24.8 min) and phase delays at CT 14 (59.7 +/- 18 min). We attempted to block the carbachol-induced phase shifts at these three phases using specific antagonists of nicotinic and muscarinic receptors. Mecamylamine, a nicotinic receptor antagonist, did not block carbachol-induced phase shifts at any of the phases tested. Atropine, a muscarinic receptor antagonist, blocked carbachol-induced phase shifts at CT 6 (-11.6 +/- 4.8 min; Mean phase shift +/- S.E.M.) and CT 22 (-20 +/- 6.6 min), suggesting that carbachol mediates its phase-shifting effects at these phases through muscarinic receptors.

The role of glutamate in the photic regulation of the suprachiasmatic nucleus

Endogenous circadian rhythms govern most aspects of physiology and behaviour in mammals, including body temperature, autonomic and endocrine function, and sleep-wake cycles. Such rhythms are generated by the suprachiasmatic nucleus of the hypothalamus (SCN), but are synchronised to the environmental light-dark cycle by photic cues perceived by the retina and conveyed to the SCN via the retinohypothalamic tract (RHT). This review considers many lines of evidence from diverse experimental approaches indicating that the RHT employs glutamate (or a related excitatory amino acid) as a neurotransmitter. Ultrastructural studies demonstrate the presence of glutamate in presynaptic terminals within the SCN. In situ hybridisation and immunocytochemical studies reveal the presence of several NMDA (NMDAR1, NMDAR2C), non-NMDA (GluR1, GluR2, GluR4) and metabotropic (mGluR1) glutamate receptor subunits in the SCN. Messenger RNA encoding a glutamate transporter protein is also present. In behavioural tests, glutamate antagonists can block the effects of light in phase-shifting circadian rhythms. Such treatments also block the induction of c-fos within SCN cells by light, whereas a glutamate agonist (NMDA) induces c-fos expression. In hypothalamic slice preparations in vitro, electrical stimulation of the optic nerves induces release of glutamate and aspartate, and glutamate antagonists block field potentials in the SCN evoked by stimulation of the optic nerve. Circadian rhythms of electrical activity which persist in vitro are phase shifted by application of glutamate in a manner which mimics the phase shifting effects of light in vivo. This wide range of experimental findings provides strong support for the hypothesis that glutamate is the principal neurotransmitter within the RHT, and thus conveys photic cues to the circadian timing system in the SCN.

The circadian visual system

The retina transduces photic stimuli and transmits that information centrally for further processing. This review emphasizes the fact that the nervous system components governing circadian rhythmicity constitute a specialized subdivision of the vertebrate visual system. The brain houses different targets for retinal efferents parcellated according circadian or non-circadian function. Although the suprachiasmatic nucleus (SCN), being the site of the master circadian clock, is necessary for the generation of circadian rhythmicity, precise phase regulation of any rhythm is subject to modulation by SCN-afferent processes. Photic information necessary for entrainment arrives at the SCN via the retinohypothalamic tract. The geniculohypothalamic tract, originating in the intergeniculate leaflet (IGL), provides a secondary route by which photic information can reach the SCN. It also projects extensively to the contralateral IGL and receives reciprocal input from the SCN region. An interaction between the circadian and non-circadian visual systems may exist through connections of the superior colliculus with ventrolateral geniculate leaflet (VLG) and IGL. The SCN, IGL, VLG and superior colliculus are all innervated by serotonin-containing fibers. The following observations are likely to have an impact beyond the rhythm field itself: certain transneuronal tracers label only the circadian visual system; c-fos protein synthesis is induced in the circadian, but not non-circadian, visual system by a phasically active stimulus; blockade of SCN action potentials is unable to alter circadian rhythmicity; transplantation of dispersed fetal SCN cells to arrhythmic adults restores circadian periodicity, but not phase response to light; and the IGL is actually a very extensive part of the lateral geniculate complex.

Classical acetylcholine receptors do not play a direct role in neuronal transmission of photic information in the suprachiasmatic nucleus in rats

Acetylcholine is said to be involved in the neuronal transmission of photic information in the suprachiasmatic nucleus (SCN). The purpose of this study was to specify what subtypes of acetylcholine receptors play a major role using in vivo system and to examine the interaction of acetylcholine and excitatory amino acid receptors. To evaluate the effect of a selective agonist or antagonist for the nicotinic or muscarinic receptor on the neuronal transmission in the SCNT, N-acetyltransferase (NAT) activity in the pineal gland was measured after microinjection at this site. In the case of pretreatment with an antagonist, light stimulation was given after 20 min. Carbamilcholine chloride (carbachol) mimicked and only alpha-bungarotoxin (alpha-BTX) blocked the light effect; however, more selective agonists or antagonists were not effective. As for the interaction of these two cholinergic agents with an agonist or antagonist for N-methyl-D-aspartate (NMDA) receptor, alpha-BTX or D-2-amino-5-phosphonovalerate (D-APV) significantly blocked the suppressive effect of NMDA or carbachol, respectively. These data suggest that classical acetylcholine receptors do not play a direct role in neuronal transmission of photic information in the SCN in rats.

N-methyl-D-aspartate, quisqualate and kainate receptors are all involved in transmission of photic stimulation in the suprachiasmatic nucleus in rats

In order to clarify the neuronal transmission mechanism of photic stimulation in the suprachiasmatic nucleus (SCN), the effects of agonists and antagonists for excitatory amino acid receptors on N-acetyltransferase (NAT) activity in the pineal gland were observed following the microinjection of drugs into both sides of the nuclei. N-Methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, and kainate (which are selective agonists for three different subtypes, i.e. NMDA, quisqualate and kainate receptors, respectively) significantly decreased NAT activity similarly to the suppressive effect of light. Moreover, compared with a control group, all the groups pretreated with a selective competitive antagonist for NMDA receptor (D-2-amino-5-phosphonovalerate or 3-((+-)-2-carboxypiperazine-4-yl)-propyl-1-phosphonate), or a selective non-competitive antagonist for non-NMDA receptors (Joro spider toxin-3 or 1-naphthylacetyl spermine) partially blocked the suppressive effect of photic stimulation on NAT activity. These results suggest that NMDA, quisqualate and kainate receptors are all involved in mediating photic stimulation in the SCN.

The effects of intraventricular carbachol injections on the free-running activity rhythm of the hamster

The effects of light on the circadian pacemaker in the suprachiasmatic nucleus (SCN) are mediated by the retinohypothalamic tract (RHT) and by the retinogeniculosuprachiasmatic tract (RGST). The neurotransmitter of the RGST is neuropeptide Y. The RHT may contain glutamate and aspartate. Recent evidence indicates that acetylcholine could also be involved in phase shifting by light. We determined that intraventricular injections with an acetylcholine agonist, carbachol, induces phase advances during the subjective day and phase delays during the early subjective night. No differences were observed between phase shifts induced in constant darkness and those induced in continuous light. A dose-response curve for carbachol was described at circadian time 6 (CT6). Injections at CT14 with various dosages of carbachol indicated the same dose dependency for this circadian time. Finally, carbachol injections in split animals resulted in similar responses of the two components of the split activity rhythm.

Autoradiographic localization of alpha-bungarotoxin binding sites in the suprachiasmatic region of rat brain

High affinity alpha-bungarotoxin (alpha-BTX) binding sites of the hypothalamus in and near the suprachiasmatic nuclei (SCN) were mapped by in vitro macroautoradiographic analysis. Adult male rats were killed at specific circadian phases. Their brains were rapidly dissected out and frozen sections were made at a thickness of 16 micron. After having been mounted on slides, the sections were incubated with iodinated alpha-BTX (3 nM), washed and exposed to X-ray film. Analyses of binding were performed with the aid of a digital video densitometer system. Autoradiographic loci that bound alpha-BTX were traced and the image of the SCN histology from the adjacent section was superimposed. In this way the exact relationships of the topography of areas that bound alpha-BTX and those that contained SCN cell bodies (and other hypothalamic landmarks) could be observed. Non-specific binding was tested by incubation in the presence of 3 microM unlabelled alpha-BTX and was found to be very low and uniform throughout the sections. Hypothalamic areas that bound alpha-BTX included the SCN, supraoptic, periventricular, lateral and anterior hypothalamic nuclei. In the rostral SCN, alpha-BTX binding coincided with the nucleus proper. Caudally this relationship dissociated so that at mid-SCN alpha-BTX bound dorsally and laterally both within and outside the SCN and by the most caudal portion of the nucleus, alpha-BTX binding was entirely outside the SCN in a vertical band dorsal to the SCN. This topography suggests that alpha-BTX binding may be coincident with a major output pathway that courses dorsally and caudally from the SCN.

Glutamate phase shifts circadian activity rhythms in hamsters

The suprachiasmatic nuclei (SCN) have been identified as a pacemaker for many circadian rhythms in mammals. Photic entrainment of this pacemaker can be accomplished via the direct retino-hypothalamic tract (RHT). Glutamate is a putative transmitter of the RHT. In the present study it is demonstrated that glutamate injections in the SCN cause phase shifts of the circadian activity rhythm of the hamster. In contrast, glutamate injections outside the SCN or vehicle injections inside the SCN did not affect the circadian phase. These data suggest that glutamate could be involved in photic entrainment of the circadian pacemaker.

Perturbations of locomotor activity rhythms following suprachiasmatic bungarotoxin infusion

The actions of intracranial injections of alpha bungarotoxin (BTX) on locomotor activity rhythms were examined in male rats. The hypothalamic suprachiasmatic region is known to be a locus of high affinity BTX binding although the potential roles of this receptor system are unknown. Rats were stereotaxically implanted with cannulae aimed just dorsal to the SCN (or cortex for control injections). Free-running locomotor activity rhythms in darkness were constantly monitored. Seventy-eight percent (78%) of the animals injected with BTX in the SCN region had phase shifts that were outside the 99% confidence limits of control animals. Infusion of either saline (into the SCN) or BTX into cortex were without effects. Doses of BTX varied from 125 fmol to 600 pmol. At the highest dose a substantial fraction of the animals showed both period changes and loss of rhythmicity as well as phase shifts. Although nearly all of the animals injected with BTX experienced phase shifts the direction of the shifts were not consistently correlated with the circadian time of injection. However, the sensitivity of the animals varied systematically with the smallest shifts resulting after injection at CT12 and CT16. These results argue that BTX does not influence the SCN pacemaker as an entraining signal but does potently perturb the circadian system.

A cholinergic antagonist, mecamylamine, blocks the phase-shifting effects of light on the circadian rhythm of locomotor activity in the golden hamster

Despite the well known role of the light-dark cycle in the entrainment of circadian rhythms, very little is known about the neurochemical events that mediate the effects of light on the mammalian circadian clock. Recent anatomical and pharmacological data support the hypothesis that acetylcholine may be involved in relaying light-dark information from the retina to, or within, the circadian clock of rodents. If acetylcholine is required for this response, it should be possible to block the phase-shifting effects of a light pulse by blocking cholinergic neurotransmission. To test this possibility, hamsters free-running in constant darkness received an intraventricular injection of the anticholinergic drug, mecamylamine (450 micrograms), 10 min before being exposed to a 5-min pulse of light known to induce sub-maximal phase shifts in the circadian rhythm of wheel-running behavior. Compared to vehicle-injected control animals, mecamylamine treatment blocked or reduced both the phase-advancing and phase-delaying effects of light. These results support the hypothesis that acetylcholine is involved in mediating the phase-shifting effects of light on the mammalian circadian clock.

Structure and function in circadian timing systems: evidence for multiple coupled circadian oscillators

Considerable progress has been made in elucidating the mechanisms underlying the generation of circadian rhythmicity. This review describes several distinct lines of evidence which converge on the general hypothesis that circadian timing systems consist of multiple circadian oscillators, coordinated by both hierarchical and non-hierarchical coupling relationships. Such a view is supported by the complex phenomenology of circadian systems, as well as by physiological considerations. We have attempted wherever possible to integrate these two sources of evidence, in order to define the current "state of the art" in bridging the gap between structure and function in the analysis of circadian timing systems. While we concentrate mainly on the mammalian, and particularly the rodent, circadian system, we also incorporate comparative evidence obtained from a variety of vertebrate and invertebrate species.