Circadian activity rhythms in the spiny mouse, Acomys cahirinus.

Circadian locomotor rhythms were examined in adult common spiny mice, Acomys cahirinus. Spiny mice demonstrated nocturnal activity, with onset of activity coinciding promptly with onset of darkness. Re-entrainment to 6-h delays of the light-dark cycle was accomplished faster than to 6-h advances. Access to running wheels yielded significant changes in period and duration of daily activity. Novelty-induced wheel running had no effect on phase of activity rhythms. Circadian responses to light at various times of the circadian cycle were temporally similar to those observed in other nocturnal rodent species. No gender differences were observed in any of the parameters measured.

Local administration of serotonin agonists blocks light-induced phase advances of the circadian activity rhythm in the hamster.

Circadian rhythms in mammals are synchronized to environmental light-dark cycles through a direct retinal projection to the suprachiasmatic nucleus (SCN), a circadian clock. This process is thought to be modulated by other afferents to the SCN, including a dense serotonergic projection from the midbrain raphe. Previous work from this laboratory demonstrated that a systemically administered 5-hydroxytryptamine1A/7 (5-HT1A/7) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) dose dependently attenuates light-induced phase shifts of the circadian activity rhythm of the Syrian hamster. In this study, we demonstrate that local injections (1-100 microM) of the 5-HT1A/7 agonists 8-OH-DPAT or 5-carboxamidotryptamine into the region of the SCN inhibit light-induced phase advances of the circadian wheel-running rhythm. In addition, the inhibitory effects of systemically administered 8-OH-DPAT were unaffected by either radiofrequency-induced lesions of the intergeniculate leaflet or 5,7-dihydroxytryptamine-induced lesions of serotonergic projections to the SCN. These findings support a modulatory role of serotonin in photic regulation of circadian phase through an action at the level of the SCN.

Neuropeptide Y blocks light-induced phase advances but not delays of the circadian activity rhythm in hamsters.

In mammals, the suprachiasmatic nuclei (SCN) are the anatomical site of localization of the light-entrainable circadian clock responsible for the generation of daily rhythms in physiology and behavior. In addition to direct retinohypothalamic innervation, the SCN receive a prominent projection of fibers from the intergeniculate leaflet (IGL) of the thalamus, the geniculohypothalamic tract (GHT), some of which contain the neurotransmitter, neuropeptide Y (NPY). Since the GHT has been suggested to play a role in the modulation of photic entrainment of the SCN circadian clock in rodents, we investigated the effects of local administration of NPY into the region of the SCN on light-induced phase shifts of the free-running activity rhythm in hamsters. Injection of 60 nmol of NPY into the SCN region 10 min prior to light exposure at circadian time 19 completely blocked light-induced phase advances. Similar treatment at circadian time 14 had no significant effect on the magnitude of light-induced phase delays. Injection of NPY at either time point without light exposure did not alter circadian phase. The findings support a modulatory role for NPY in the photic entrainment of the SCN circadian clock.

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.

Nitric oxide synthase inhibitor blocks light-induced phase shifts of the circadian activity rhythm, but not c-fos expression in the suprachiasmatic nucleus of the Syrian hamster.

Circadian rhythms in mammals are entrained to the environmental light cycle by daily adjustments in the phase of the circadian pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Brief exposure of hamsters maintained under constant darkness to ambient light during subjective nighttime produces both phase shifts of the circadian activity rhythm and characteristic patterns of c-fos protein (Fos) immunoreactivity in the SCN. In this study, we demonstrate that light-induced phase shifts of the circadian activity rhythm are blocked by intracerebroventricular (i.c.v.) injection of the competitive nitric oxide synthase (NOS) inhibitor, N-nitro-L-arginine methyl ester (L-NAME), but not by the inactive isomer, D-NAME. The effects of L-NAME are reversible and dose-related, and are countered by co-injection of arginine, the natural substrate for NOS. While effects on behavioral rhythms are pronounced, similar treatment does not alter the pattern of light-induced Fos immunoreactivity in the SCN. These results suggest that nitric oxide is a component of the signal transduction pathway that communicates photic information to the SCN circadian pacemaker, and that nitric oxide production is either independent of, or downstream from, pathways involved in induction of c-fos expression.

cGMP-dependent protein kinase inhibitor blocks light-induced phase advances of circadian rhythms in vivo.

The suprachiasmatic nucleus (SCN) contains the primary mammalian circadian clock. Light synchronizes these circadian rhythms through a mechanism involving the release of excitatory amino acids (EAA) and synthesis of nitric oxide (NO) in the SCN. In the current study, we investigated whether cGMP-mediated activation of cGMP-dependent protein kinase (PKG) is associated with light-induced phase shifts of the circadian oscillator. Local administration of the specific PKG inhibitor, KT-5823, significantly attenuated light-induced advances in the phase of activity rhythms when administered during late subjective night (CT 19). Similar treatment at CT 14 had no significant effect on light-induced phase delays. These results are the first to implicate PKG in the biochemical pathway(s) responsible for photic phase advances, and suggest a divergence in biochemical pathways involved in photic phase shifts.

Intrinsic neuronal rhythms in the suprachiasmatic nuclei and their adjustment.

The central role of the suprachiasmatic nuclei in regulating mammalian circadian rhythms is well established. We study the temporal organization of neuronal properties in the suprachiasmatic nucleus (SCN) using a rat hypothalamic brain slice preparation. Electrical properties of single neurons are monitored by extra-cellular and whole-cell patch recording techniques. The ensemble of neurons in the SCN undergoes circadian changes in spontaneous activity, membrane properties and sensitivity to phase adjustment. At any point in this cycle, diversity is observed in individual neurons' electrical properties, including firing rate, firing pattern and response to injected current. Nevertheless, the SCN generate stable, near 24 h oscillations in ensemble neuronal firing rate for at least three days in vitro. The rhythm is sinusoidal, with peak activity, a marker of phase, appearing near midday. In addition to these electrophysiological changes, the SCN undergoes sequential changes in vitro in sensitivities to adjustment. During subjective day, the SCN progresses through periods of sensitivity to cyclic AMP, serotonin, neuropeptide Y, and then to melatonin at dusk. During the subjective night, sensitivities to glutamate, cyclic GMP and then neuropeptide Y are followed by a second period of sensitivity to melatonin at dawn. Because the SCN, when maintained in vitro, is under constant conditions and isolated from afferents, these changes must be generated within the clock in the SCN. The changing sensitivities reflect underlying temporal domains that are characterized by specific sets of biochemical and molecular relationships which occur in an ordered sequence over the circadian cycle.

Resetting the biological clock: mediation of nocturnal circadian shifts by glutamate and NO.

Circadian rhythms of mammals are timed by an endogenous clock with a period of about 24 hours located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Light synchronizes this clock to the external environment by daily adjustments in the phase of the circadian oscillation. The mechanism has been thought to involve the release of excitatory amino acids from retinal afferents to the SCN. Brief treatment of rat SCN in vitro with glutamate (Glu), N-methyl-D-aspartate (NMDA), or nitric oxide (NO) generators produced lightlike phase shifts of circadian rhythms. The SCN exhibited calcium-dependent nitric oxide synthase (NOS) activity. Antagonists of NMDA or NOS pathways blocked Glu effects in vitro, and intracerebroventricular injection of a NOS inhibitor in vivo blocked the light-induced resetting of behavioral rhythms. Together, these data indicate that Glu release, NMDA receptor activation, NOS stimulation, and NO production link light activation of the retina to cellular changes within the SCN mediating the phase resetting of the biological clock.

Post-diversion pre-cystectomy irradiation for carcinoma of the bladder.

In 47 patients treated sequentially with urinary diversion and laparotomy staging, irradiation therapy and total cystectomy for invasive carcinoma of the bladder, the mortality rate was 2 per cent and the complication rate was 20 per cent. The survival rate was most favorable in patients in whom the pathologic specimen revealed no tumor after irradiation. The lower mortality and morbidity rates are attributed to staging of surgical procedures and the challenge remains to increase the number of patients in whom carcinoma of the bladder is converted by irradiation therapy to a lower stage or no tumor.