Impaired Circadian Photosensitivity in Mice Lacking Glutamate Transmission from Retinal Melanopsin Cells

Dec 17, 2014Journal of biological rhythms

Reduced daily light sensitivity in mice without glutamate signals from light-sensing eye cells

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Abstract

Mice lacking the ability to package glutamate in intrinsically photoreceptive retinal ganglion cells (ipRGCs) exhibited disrupted circadian rhythms under a standard light-dark cycle.

The circadian rhythms of ipRGCs unable to package glutamate did not synchronize with a 12:12 light-dark cycle.
These mice did not exhibit a phase delay in their rhythms following a 45-minute light pulse at circadian time 14.
A small subset of these mice showed potential for entrainment to the 12:12 light-dark cycle with a positive phase angle to lights-off.
Increased light intensity (6.53 W/m²) allowed for some weak entrainment of free-running mice despite impaired glutamate transmission.
Glutamate transmission from ipRGCs is essential for normal light entrainment of the circadian system at moderate light levels (0.35 W/m²).
Residual signaling through other neurotransmitters like PACAP may provide limited entrainment capabilities at very high light levels but may not effectively suppress locomotor activity.

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Intrinsically photoreceptive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin and convey retinal light inputs to the circadian system via the retinohypothalamic tract (RHT) projection to the suprachiasmatic nucleus (SCN). The principal neurotransmitter of this projection is glutamate, and ipRGCs use the vesicular glutamate transporter 2 (VGLUT2) to package glutamate into synaptic vesicles. However, these neurons contain other potential neurotransmitters, such as pituitary adenylate cyclase activating polypeptide (PACAP). To test the role of glutamate in mediating ipRGC light inputs into the SCN, we crossed mice in which Cre-recombinase expression is driven by the melanopsin promotor (Opn4(Cre/+)) with mice in which the second exon of VGLUT2 is flanked by loxP sites (VGLUT2(fl/fl)), producing ipRGCs that are unable to package glutamate into synaptic vesicles. Such mice had free-running circadian rhythms that did not entrain to a 12:12 light-dark (12:12 LD) cycle, nor did they show a phase delay after a 45-min light pulse administered at circadian time (CT) 14. A small subset of the mice did appear to entrain to the 12:12 LD cycle with a positive phase angle to lights-off; a similar entrainment pattern could be achieved in free-running mice if they were exposed to a 12:12 LD cycle with light of a greater intensity. Glutamate transmission from the ipRGCs is necessary for normal light entrainment of the SCN at moderate (0.35 W/m(2)) light levels, but residual transmission (possibly by PACAP in ipRGCs or by other RGCs) can weakly entrain animals, particularly at very high (6.53 W/m(2)) light levels, although it may be less effective at suppressing locomotor activity (light masking).

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Impaired Circadian Photosensitivity in Mice Lacking Glutamate Transmission from Retinal Melanopsin Cells

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