Limits...
Using light to tell the time of day: sensory coding in the mammalian circadian visual network.

Brown TM - J. Exp. Biol. (2016)

Bottom Line: In mammals, these changes are exclusively detected in the retina and are relayed by direct and indirect neural pathways to the master circadian clock in the hypothalamic suprachiasmatic nuclei.Recent work reveals a surprising level of complexity in this sensory control of the circadian system, including the participation of multiple photoreceptive pathways conveying distinct aspects of visual and/or time-of-day information.In this Review, I summarise these important recent advances, present hypotheses as to the functions and neural origins of these sensory signals, highlight key challenges for future research and discuss the implications of our current knowledge for animals and humans in the modern world.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, UK timothy.brown@manchester.ac.uk.

No MeSH data available.


Related in: MedlinePlus

Geniculohypothalamic inputs to the central clock. (A) Schematic of the geniculohypothalamic pathway. The IGL receives dense excitatory input (+) from mRGCs, and GABAergic neurons in this region [including a population that co-expresses neuropeptide Y (NPY)] provide inhibitory input (−) to VIP-rich portions of the SCN. (B) The normalised (according to maximal firing) sensitivity of cells in the mouse IGL region to light steps providing equal activation of all photoreceptor classes. The data reveal substantially larger responses to low light levels than those shown by SCN cells tested under equivalent conditions. (C) Responses of ‘antagonistic’ cells in the IGL region (similar to those reported for some SCN-projecting cells) that selectively respond to illumination of just one eye, with little response to binocular illumination. Periods of darkness are indicated by grey backgrounds and periods of light are indicated by white backgrounds. IGL data in B and C are derived from data presented in Howarth et al. (2014) and SCN sensitivity from Walmsley and Brown (2015).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4920240&req=5

JEB132167F6: Geniculohypothalamic inputs to the central clock. (A) Schematic of the geniculohypothalamic pathway. The IGL receives dense excitatory input (+) from mRGCs, and GABAergic neurons in this region [including a population that co-expresses neuropeptide Y (NPY)] provide inhibitory input (−) to VIP-rich portions of the SCN. (B) The normalised (according to maximal firing) sensitivity of cells in the mouse IGL region to light steps providing equal activation of all photoreceptor classes. The data reveal substantially larger responses to low light levels than those shown by SCN cells tested under equivalent conditions. (C) Responses of ‘antagonistic’ cells in the IGL region (similar to those reported for some SCN-projecting cells) that selectively respond to illumination of just one eye, with little response to binocular illumination. Periods of darkness are indicated by grey backgrounds and periods of light are indicated by white backgrounds. IGL data in B and C are derived from data presented in Howarth et al. (2014) and SCN sensitivity from Walmsley and Brown (2015).

Mentions: IGL cells send inhibitory GABAergic projections to the SCN [Fig. 6A; the geniculohypothalamic tract (GHT)]; these projections include a population of cells that co-express neuropeptide Y (NPY; Harrington, 1997; Morin and Allen, 2006). In nocturnal rodents, electrical activation of the GHT (or intra-SCN microinjection of NPY) exerts phase-resetting effects that are opposite to those of light (Huhman and Albers, 1994; Rusak et al., 1989). Importantly, however, although this pathway is known to mediate circadian responses to arousal-inducing stimuli (Janik and Mrosovsky, 1994; Johnson et al., 1988; Marchant et al., 1997; Wickland and Turek, 1994), its role in photoentrainment remains elusive.Fig. 6.


Using light to tell the time of day: sensory coding in the mammalian circadian visual network.

Brown TM - J. Exp. Biol. (2016)

Geniculohypothalamic inputs to the central clock. (A) Schematic of the geniculohypothalamic pathway. The IGL receives dense excitatory input (+) from mRGCs, and GABAergic neurons in this region [including a population that co-expresses neuropeptide Y (NPY)] provide inhibitory input (−) to VIP-rich portions of the SCN. (B) The normalised (according to maximal firing) sensitivity of cells in the mouse IGL region to light steps providing equal activation of all photoreceptor classes. The data reveal substantially larger responses to low light levels than those shown by SCN cells tested under equivalent conditions. (C) Responses of ‘antagonistic’ cells in the IGL region (similar to those reported for some SCN-projecting cells) that selectively respond to illumination of just one eye, with little response to binocular illumination. Periods of darkness are indicated by grey backgrounds and periods of light are indicated by white backgrounds. IGL data in B and C are derived from data presented in Howarth et al. (2014) and SCN sensitivity from Walmsley and Brown (2015).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4920240&req=5

JEB132167F6: Geniculohypothalamic inputs to the central clock. (A) Schematic of the geniculohypothalamic pathway. The IGL receives dense excitatory input (+) from mRGCs, and GABAergic neurons in this region [including a population that co-expresses neuropeptide Y (NPY)] provide inhibitory input (−) to VIP-rich portions of the SCN. (B) The normalised (according to maximal firing) sensitivity of cells in the mouse IGL region to light steps providing equal activation of all photoreceptor classes. The data reveal substantially larger responses to low light levels than those shown by SCN cells tested under equivalent conditions. (C) Responses of ‘antagonistic’ cells in the IGL region (similar to those reported for some SCN-projecting cells) that selectively respond to illumination of just one eye, with little response to binocular illumination. Periods of darkness are indicated by grey backgrounds and periods of light are indicated by white backgrounds. IGL data in B and C are derived from data presented in Howarth et al. (2014) and SCN sensitivity from Walmsley and Brown (2015).
Mentions: IGL cells send inhibitory GABAergic projections to the SCN [Fig. 6A; the geniculohypothalamic tract (GHT)]; these projections include a population of cells that co-express neuropeptide Y (NPY; Harrington, 1997; Morin and Allen, 2006). In nocturnal rodents, electrical activation of the GHT (or intra-SCN microinjection of NPY) exerts phase-resetting effects that are opposite to those of light (Huhman and Albers, 1994; Rusak et al., 1989). Importantly, however, although this pathway is known to mediate circadian responses to arousal-inducing stimuli (Janik and Mrosovsky, 1994; Johnson et al., 1988; Marchant et al., 1997; Wickland and Turek, 1994), its role in photoentrainment remains elusive.Fig. 6.

Bottom Line: In mammals, these changes are exclusively detected in the retina and are relayed by direct and indirect neural pathways to the master circadian clock in the hypothalamic suprachiasmatic nuclei.Recent work reveals a surprising level of complexity in this sensory control of the circadian system, including the participation of multiple photoreceptive pathways conveying distinct aspects of visual and/or time-of-day information.In this Review, I summarise these important recent advances, present hypotheses as to the functions and neural origins of these sensory signals, highlight key challenges for future research and discuss the implications of our current knowledge for animals and humans in the modern world.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, UK timothy.brown@manchester.ac.uk.

No MeSH data available.


Related in: MedlinePlus