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1. Long “summer” days activate reserve pool of SST neurons in three distinct regions of the hypothalamus

Figure 1. Long day length increases the number of neurons that express SST in the hypothalamus. (A-C) Representative images and quantification oftdT expression in the PVN (A), PeVN (B), and SCN (C) after exposure for at least 12 weeks. * Student’s t test, p < 0.05 (+ one-tail Student’s t test, p < 0.05).

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Seasonal Modulation of Novel Brain Circuits Regulating Mood and CognitionAlissa Wuorinen, Deborah A.M. Joye, Adam J. Telega, & Jennifer A. Evans

Department of Biomedical Sciences, Marquette University, Milwaukee, WI 53201 USA

▪ Activation of reserve SST neurons occurs in the

hypothalamus

▪ Findings consistent with studies done in rats1

▪ Increases in the PVN and PeVN

▪ Cell increase indicates neurotransmitter switching, not

upregulation of peptide expression

▪ Genetic model shows that increases occur in cells

that have never previously expressed SST

▪ Increase in SST-tdT+ cells also observed in the SCN

and corresponds to changes in behavior

▪ Increase in SCN SST-tdT+ cells requires at least 4

weeks of long-day exposure

▪ Behavioral adjustment to L20 takes about 6 weeks

▪ Suggests that increase in SST-expressing cells and

behavioral adjustment to long days may be related

▪ Larger increase in SST-tdT+ in females suggests sex

differences in light responsiveness

▪ Males and females exhibit similar cell number under

L12 conditions

▪ Larger increases in females after L20 suggest that

brain plasticity may differ between sex

▪ Females more sensitive to changes in lighting

environment

▪ Larger SST-tdT+ increases seen in the mid-SCN,

corresponds to location of SCN core neurons

▪ Coincides with proximity of retinorecipient neurons

Conclusions

Future Directions

Acknowledgements

References

▪ Neurons in the brain are classified by peptide expression.

▪ Over 100 billion neurons in the brain

▪ Based on these classifications, fixed identity assumed

▪ Light alters the neurochemical profile of hypothalamic neurons1.

▪ Rats exposed to long "summer” days:

▪ Increased levels of Somatostatin (SST) in the hypothalamus

▪ Changes seen in the paraventricular nucleus (PVN) and

periventricular nucleus (PeVN)

▪ The mechanisms for increased SST expression are unclear

▪ De novo transcription of SST in reserve pool of cells

(neurotransmitter switching)

OR

▪ Upregulation of peptide in existing SST neurons

▪ The suprachiasmatic nucleus (SCN) processes light input from

the environment.

▪ Receives direct light input from the eye via retinohypothalamic tract

▪ Transmits light information to other areas of the brain

▪ The SCN expresses SST, although its role is unclear

Introduction

Research Questions:

Do long days activate SST transcription in a reserve pool of neurons?

Does this change happen in the SCN?

When does this change occur?

Is this change region-specific?

▪ How does light cause neurotransmitter switching?

▪ Genetic model suggests switch is due to increase

in Sst transcription

▪ Result of change in chromatin or alteration of

transcriptional machinery?

▪ Do changes occur directly from light or as a signal

from other cells?

▪ Which SCN neurons respond to SST and the

cellular mechanisms of SST signaling?

▪ What are the cellular and genetic components of

the switch?

▪ Why are neurotransmitters switching?

1. Dulcis D, Jamshidi P, Leutgeb S & Spitzer NC 2013 Neurotransmitter switching in the adult brain regulates

behavior. Science 340 449-453.

PVN - L12 PVN - L20

Methods▪ Lighting Conditions:

▪ Mice exposed to 2 different light conditions for

1, 2, 4, 8, and 12 weeks

▪ Control = L12

▪ 12 h of light/day

▪ Long day = L20

▪ 20 h of light/day

▪ Genetic Model:

▪ Mouse #1 -

Cre-recombinase

driven by upstream

Sst transcription

▪ Mouse #2 -

Fluorescent labeling

by Cre-dependent

tdTomato

▪ Offspring -

All SST neurons are

labeled red by

tdTomato

L12

L20

PVN

PeVN

SCN

▪ Regions of

Interest:

▪ PVN

▪ PeVN

▪ SCN

3. Larger SST-tdT+ increases in female SCN

Figure 2. Changes in SST-tdT+ expression occur after at least 7 weeks of exposure. (A) 1, 2, 4, and 6-week

time points show no difference in cell count. (B-C) Wheel-running actograms for L12 and L20 conditions (B) and

quantification (C) show behavior adjusts to long day conditions after 5-7 weeks. * Differs from L12, Post-hoc contrasts, p < 0.05.

A

B

Thanks to Marquette University Animal Resource Center for providing animal care.

Thank you Emily Herff and to the Marquette University Honors Program for funding the

undergraduate summer research fellowships. This work was supported by NIH grant

R01NS091234 and the Whitehall Foundation. For more information, please contact

deborah.joye@marquette.edu and jennifer.evans@marquette.edu.

X

Mouse #2

STOP tdTomato

Mouse #1

CreSst

Offspring

CreSst tdTomato

4. Region-specific dynamics of SCN switching may reflect network-level interactions

Figure 4. SST-tdT+ cells are expressed differently across the SCN. (A) Anterior, middle, and posterior regions defined and

visualized in sagittal representative images of the SCN. (B-D) Cell counts from 1, 2, 4, 8, and 12 weeks observed in the anterior (B), middle (C), and posterior (D) regions of the SCN. * Post-hoc contrasts, p < 0.05.

DAPIA

tdTomato

C D(B) (C) (D)

B

2. SCN switch corresponds to time required for behavioral adjustment

14% increase 13% increase 27% increase

Switching Upregulation

Figure 3. SST-tdT+ changes are more prominent in

females. (A-B) Females show increase in cell number at 8

weeks (A) in comparison to the males at the same time point (B). * Differs from L12, Post-hoc contrasts, p < 0.05.

(B) (C) (D)

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