Somatostatin regulates central clock function and circadian responses to light

Abstract

Daily and annual changes in light are processed by central clock circuits that control the timing of behavior and physiology. The suprachiasmatic nucleus (SCN) in the anterior hypothalamus processes daily photic inputs and encodes changes in day length (i.e., photoperiod), but the SCN circuits that regulate circadian and photoperiodic responses to light remain unclear. Somatostatin (SST) expression in the hypothalamus is modulated by photoperiod, but the role of SST in SCN responses to light has not been examined. Our results indicate that SST signaling regulates daily rhythms in behavior and SCN function in a manner influenced by sex. First, we use cell-fate mapping to provide evidence that SST in the SCN is regulated by light via de novo Sst activation. Next, we demonstrate that Sst ^-/- mice display enhanced circadian responses to light, with increased behavioral plasticity to photoperiod, jetlag, and constant light conditions. Notably, lack of Sst ^-/- eliminated sex differences in photic responses due to increased plasticity in males, suggesting that SST interacts with clock circuits that process light differently in each sex. Sst ^-/- mice also displayed an increase in the number of retinorecipient neurons in the SCN core, which express a type of SST receptor capable of resetting the molecular clock. Last, we show that lack of SST signaling modulates central clock function by influencing SCN photoperiodic encoding, network after-effects, and intercellular synchrony in a sex-specific manner. Collectively, these results provide insight into peptide signaling mechanisms that regulate central clock function and its response to light.

Keywords: behavior; circadian; photoperiod; somatostatin; suprachiasmatic nucleus.

Fig. 1.

Spatiotemporal mapping of Sst SCN subpopulation. (A) Genetic approach for labeling Sst–tdT+ cells, with representative images illustrating Sst–tdT and SST expression at midday (ZT06) and midnight (ZT18). (B) Percent cells expressing Sst–tdT with and without SST. (C) Daily rhythm in SST and Sst expression, double-plotted to facilitate visualization. (D) Day–night difference in SCN expression of Sst–tdT and SST. (E) Photoperiodic modulation of SCN SST expression. (F) Photoperiodic modulation of Sst transcription in the hypothalamus. Representative images illustrating SCN Sst–tdT+ cells. Twelve weeks of L20 entrainment increased the number of SCN Sst–tdT+ cells. Scale bars, 100 μm, ZT = Zeitgeber time, ZT12 = time of lights off, a.u. = arbitrary units. ^@Circwave test of rhythmicity, P < 0.05. *post hoc comparisons, P < 0.05.

Fig. 2.

Lack of SST enhances photoperiodic modulation of circadian waveform. (A) Representative double-plotted wheel-running actograms from male mice of each group. Lighting conditions are illustrated with white:black bars and internal shading. (B) Photoperiodic modulation of circadian waveform during entrainment. (C) Photoperiodic modulation of daily siesta time during entrainment. *post hoc comparisons, P < 0.05.

Fig. 3.

Lack of SST enhances circadian responses to light. (A) Representative double-plotted wheel-running actograms from male mice exposed to simulated jetlag (6 h advance) and constant light (LL). Lighting conditions are illustrated with white:black bars and internal shading. *post hoc comparisons, P < 0.05. (B) Days to reentrain to new LD cycle. (C-D) Incidence of LL-induced arrhythmia and power of χ² periodogram. ^†Wild-type sex difference, *Genotype difference, post hoc comparisons, P < 0.05.

Fig. 4.

Lack of SST increases the number of SCN VIP and GRP neurons. (A) Representative, thresholded images illustrating total peptide expression. (B) Average number of cells/sample. (C) Average cellular peptide levels. Male n = 4 to 9 mice/genotype, Female n = 3 mice/genotype. Scale bars, 100 μm, a.u. = arbitrary units. *post hoc comparisons, P < 0.05.

Fig. 5.

SCN expression of SSTR. (A) Representative images of Sstr1-4 expression and levels in the SCN. Gray bar indicates background levels for each probe ± the 95% (CI). (B) Representative image illustrating SCN transcription of Vip, Grp, and Sstr1 at ZT18. At higher magnification, Vip/Grp cells with and without Sstr1 coexpression are indicated by closed and open arrows, respectively. (C) Number of Sstr1-expressing Vip and Grp SCN neurons. Scale bar, 100 μm, Higher-magnification Scale bar, 25 μm *post hoc comparisons, P < 0.05.

Fig. 6.

Lack of SST enhances SCN encoding of long day photoperiods. (A) Representative double-plotted wheel-running actograms and individual SCN phase maps of mice entrained to L20 for 4 wk. (B) Lack of SST increased alpha compression in L20 males. (C) Lack of SST increased SCN reorganization in L20 males. (D) Lack of SST did not affect SCN period. ^†Wild-type sex difference, *Genotype difference, post hoc comparisons, P < 0.05.

Fig. 7.

Lack of SST and sex modulates SCN coupling after exposure to long days. (A) Composite phase maps for L20 SCN over time in culture. (B) Coupling response curves illustrating cellular resynchronization in male SCN. Polar plots along Y-axis illustrate that the sign of cellular responses reflects the direction of change over time in culture (blue: SCN core neurons, yellow: SCN shell neuron, ψ: phase difference angle). (C) Area under the curve for positive and negative regions of the coupling response curve for male SCN. (D and E) Coupling response curves and area under the curve for female SCN. Male n = 6 to 14 mice/genotype, Female n = 5 to 9 mice/genotype. *post hoc comparisons, P < 0.05.