Regulation of prokineticin 2 expression by light and the circadian clock

BACKGROUND: The suprachiasmatic nucleus (SCN) contains the master circadian clock that regulates daily rhythms of many physiological and behavioural processes in mammals. Previously we have shown that prokineticin 2 (PK2) is a clock-controlled gene that may function as a critical SCN output molecule responsible for circadian locomotor rhythms. As light is the principal zeitgeber that entrains the circadian oscillator, and PK2 expression is responsive to nocturnal light pulses, we further investigated the effects of light on the molecular rhythm of PK2 in the SCN. In particular, we examined how PK2 responds to shifts of light/dark cycles and changes in photoperiod. We also investigated which photoreceptors are responsible for the light-induced PK2 expression in the SCN. To determine whether light requires an intact functional circadian pacemaker to regulate PK2, we examined PK2 expression in cryptochrome1,2-deficient (Cry1-/-Cry2-/-) mice that lack functional circadian clock under normal light/dark cycles and constant darkness. RESULTS: Upon abrupt shifts of the light/dark cycle, PK2 expression exhibits transients in response to phase advances but rapidly entrains to phase delays. Photoperiod studies indicate that PK2 responds differentially to changes in light period. Although the phase of PK2 expression expands as the light period increases, decreasing light period does not further condense the phase of PK2 expression. Genetic knockout studies revealed that functional melanopsin and rod-cone photoreceptive systems are required for the light-inducibility of PK2. In Cry1-/-Cry2-/- mice that lack a functional circadian clock, a low amplitude PK2 rhythm is detected under light/dark conditions, but not in constant darkness. This suggests that light can directly regulate PK2 expression in the SCN. CONCLUSION: These data demonstrate that the molecular rhythm of PK2 in the SCN is regulated by both the circadian clock and light. PK2 is predominantly controlled by the endogenous circadian clock, while light plays a modulatory role. The Cry1-/-Cry2-/- mice studies reveal a light-driven PK2 rhythm, indicating that light can induce PK2 expression independent of the circadian oscillator. The light inducibility of PK2 suggests that in addition to its role in clock-driven rhythms of locomotor behaviour, PK2 may also participate in the photic entrainment of circadian locomotor rhythms

Role of the hypothalamic paraventricular nucleus in neuroendocrine responses to daylength in the golden hamster

Daylength regulates reproduction in golden hamsters through a mechanism which involves the pineal indoleamine, melatonin. Retinal input to the suprachiasmatic nucleus of the hypothalamus (SCN) and sympathetic innervation of the pineal are critical to the inhibition of reproduction by short photoperiods. Since the hypothalamic paraventricular nucleus (PVN) receives extensive input from the SCN in the rat, and may influence autonomic function via its brainstem and spinal cord projections, we studied the role of this nucleus in photoperiodically induced gonadal regression in the hamster. Bilateral electrolytic destruction of either the paraventricular nucleus (PVN) or suprachiasmatic nucleus (SCN) of the hypothalamus completely blocked testicular regression induced by either blinding or exposure to short days (10L:14D). Lesions in the retrochiasmatic hypothalamus (RCA) which may have interrupted the pathway of previously identified efferents from the SCN to the PVN were also effective in preventing short day-induced gonadal regression. Pineal melatonin content was measured in intact and lesioned hamsters sacrificed 3-5 h before lights on, at the time of the expected nocturnal peak. While SCN and RCA lesions significantly reduced pineal melatonin content, PVN lesions were still more effective in this regard. We conclude that the hamster's neuroendocrine response to photoperiod is mediated by neural pathways which include retinohypothalamic input to the SCN and efferents from this nucleus to the PVN which travel dorsocaudally through the retrochiasmatic area of the hypothalamus. Effectiveness of lesions restricted to the PVN suggests that direct projections from the PVN to spinal autonomic centers convey photoperiodic information which regulates pineal, and hence gonadal, function.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24726/1/0000148.pd

Total pinealectomy by an occipital parasagittal approach in sheep

A reliable method of total pinealectomy in sheep without a resulting significant neurological deficit has been developed and is described. The known effect of light on sheep gonadotropin levels helps validate these studies of its pineal gland. Special attention is given to delineation of the surgical anatomy of the gland and its relationship under magnification to the central draining veins of the brain. In this large mammal this relationship differs considerably from that of humans. The application of an intrahemispheric occipital exposure using magnification, as well as the preoperative administration of steroids and osmotic agents, helped to minimize the effects of retraction and resultant cerebral edema. We have thus been able to increase the rate of successful pinealectomy to 83%, allowing the development of groups of animals suitable for studies of the neuroendocrine function of the pineal and pituitary glands.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23818/1/0000057.pd

Effect of genotype on circadian period and its variability in male hamsters.

<p>Mean (±SEM) τ_DD (top) and phase variation (bottom) of the time of activity onset (black bars), offset (hatched bars) or acrophase (white bars) of the first-order regression line used to determine τ_DD in male hamsters bearing wild type, <i>tau</i>, and duper alleles in various combinations. Posthoc analysis indicated statistically significant differences in τ_DD of activity onsets at the <i>P</i><0.05 level: TTDd = TTDD>TTdd>TtDD>ttDD>Ttdd>ttdd (Tukey HSD test; TtDd did not differ from TTdd and TtDD). Statistical differences between genotypes in τ_DD of activity offset and acrophase showed similar patterns. Note that while strains differ markedly in τ_DD, phase variation is generally similar between genotypes. For phase rariation of activity onset and offset, only ttdd and TTdd groups differed from each other at the <i>P</i><0.05 level; for phase variation of acrophase, ttdd>TTDD = TTdd. In all genotypes, phase variation of τ_DD based on activity offset is greater than phase variation of τ_DD assessed for onsets or acrophase.</p

Circadian period does not predict activity:rest ratio.

<p>The relationship between the activity/rest ratio and the period of activity onsets (τ_onset) in constant darkness was similar in TTDD, TTdd, ttDD and ttdd hamsters.</p

Relationships between activity or rest duration and circadian period differ between genotypes.

<p>Correlations between the period of activity onset (τ_DD) and α (top) or ρ (bottom) in TTDD (diamonds), TTdd (squares), ?tDD (triangles), and ttdd (circles) Syrian hamsters. Both α and ρ correlated with τ_DD across all genotypes (linear and quadratic fits both <i>P</i><0.0001) but in none of the individual strains was the fit of the linear or quadratic regression statistically significant.</p

All strains show coherent rhythmicity despite differences in precision.

<p>Actograms depicting locomotor activity rhythms in DD in the individuals showing the median value of circadian precision among groups of (A) TTDD, (B)TTdd, (C) ttDD, and (D) ttdd male hamsters. In each panel the top actogram is plotted modulo 24h. The same data are replotted in the bottom actogram modulo τ_DD.</p

Compensatory changes in variability of α and ρ occur to minimize variability of τ_DD.

<p>Summed variance of activity and rest is greater than the variance of free running period of activity onsets in male TTDD (diamonds), TTdd (squares), ttDD (triangles) and ttdd (circles) hamsters.</p

Mean (± SEM) period of activity onsets and precision, and linear and quadratic regression analysis in male wild type and mutant Syrian hamsters.

<p>Groups with different superscripts differ significantly at <i>P</i><0.05 level. Linear and quadratic R² values reflect first and second order regression fits of τ_DD vs. precision within the respective group, respectively. Even in cases in which regression fit was statistically significant, the maximum precision did not occur at the mean τ_DD. When all male hamsters were combined and analyzed across genotypes, linear and quadratic fits were both statistically significant at the <i>P</i><0.0001 level, but the R² value was low (0.09 and 0.118, respectively).</p