From 25 1

Circadian Clock in Mammals

Nikolai I. Kononenko

Department of General Physiology of Nervous System, Bogomoletz Institute of Physiology,

Kiev, Ukraine

From 25 2

INTRODUCTION

As a consequence of the Earth’s rotation about its axis approximately every 24 hours, most organisms on our planet are subjected to fluctuations of light and temperature. A diverse range of species, from cyanobacteria to humans, developed endogenous biological clocks that allow for the anticipation of these daily variations. Thus, our internal physiology and function are fundamentally intertwined with this geophysical cycle.

From 25 3

The mammalian circadian timing system

Hypothalamus

The paired suprachiasmatic nuclei (SCN) of the hypothalamus is the primary biological clock regulating circadian rhythms in mammals.

It has been show by lesion of these nuclei, destroying circadian behavior, with subsequent its restoration by implantation of SCN

from donor animal.

Retino-hypothalamictract

From 25 4

Coronal and horizontal slices of suprachiasmatic nuclei

Retino-hypothalamicglutamate tract

~ 700

From 25 5

Suprachiasmatic nuclei of the mouse

1 mm

From 25 6

SCN neurons of the mouse at infrared differential interference contrast microscopy

100 m

From 25 7

Multiple-unit activity (MUA) from the rat SCN in vitro.

from ~100 SCN neurons

From 25 8

1. The paired suprachiasmatic nuclei (SCN) of the hypothalamus is the primary biological clock regulating

circadian rhythms in mammals

From 25 9

Multielectrode array dish (MED)

0

6

Hz

0 1 2 3 4 5(Days)

1 min

1 mm

From 25 10

Activity of dispersed SCN neurons in MED and MUA of SCN in slice preparation

Firi

ng r

ate

(Hz)

From 25 11

Individual SCN neurons express self-sustained circadian oscillations. As a result of internal coupling, the SCN generates a coherent output signal

From 25 12

Three main components of clock:a) oscillator, b) gear, and c) hands or digitals

From 25 13

Circadian-clockcore

Messenger

Membrane target

0

6

Hz

From 25 14

Minimal mammalian circadian clockwork model (circadian oscillator)

From Scheper et al., J Neurosci 1999

The delay and nonlinearity in the protein production and the cooperativity in the negative feedback were found to be necessary

and sufficient to generate robust circadian oscillations

Period

Period

Period*

From 25 15

Temperature compensation

The effect of changes in temperature on the rate of most biochemical reactions is measured by a Q10 value, which is defined as the ratio of the rate of a given process at one temperature to the rate at a temperature 10°C lower.

Q10 = Vt/Vt-10o

C

for known biochemical reaction 2 < Q10 < 3,

for period of circadian rhythms Q10~1

0

6

From 25 16

Light production in fireflies is due to a type of chemical reaction called bioluminescence. This process occurs in specialised light-emitting organs, usually on a firefly's lower abdomen. The enzyme luciferase acts on the luciferin, in the presence of magnesium ions, ATP, and oxygen to produce light. Genes coding for these substances have been inserted into many different organisms

Firefly

From 25 17

Circadian rhythms of PER2::LUCbioluminescence recorded from knock-in mouse SCN neurons in dispersed culture

From D.K.Welsh et al., Annu. Rev. Physiol., 2010

From 25 18

Activity of dispersed SCN neurons in MED and MUA of SCN in slice preparation

Firi

ng r

ate

(Hz)

From 25 19

Circadian rhythms of PER2::LUC bioluminescence recorded from mouse SCN neurons in slice preparation

From 25 20

1. Nothing is known about messenger(s) between circadian-clockcore and membrane target(s) responsible for circadian peaks of

firing rate

2. Nature of membrane target(s) responsible for circadian peaks of firing rate enigmatic and contradictory

From 25 21

Nature, 2002, 16, 286-290

From 25 22

From 25 23

Nature Neuroscience, 2005, 8, 650-656

From 25 24

Nature Neuroscience, 2006, 9, 1041-1049

From 25 25

There are two common features for presented above hypotheses:

1. All experiments were done employing whole cell recordings

2. Spike-associated currents are key targets for circadian modulation of firing rate

Our approach was based on on-cell (i.e. cell-attached) recordings of electrical events in SCN neurons