Mice lacking the PACAP type I receptor have impaired photic entrainment and negative masking

Oct 17, 2008American journal of physiology. Regulatory, integrative and comparative physiology

Mice without the PACAP type I receptor have trouble adjusting their internal clock to light and show reduced light-driven activity suppression

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Abstract

PAC1 knockout mice exhibited a significantly decreased phase delay of the endogenous rhythm at early night.

PACAP and its receptor PAC1 are involved in regulating light's effects on circadian rhythms and behavior.
Mice lacking PAC1 showed reduced ability to adapt to light cycles longer than 26 hours.
These mice required more time to readjust to large shifts in light-dark cycles, particularly at low light intensities.
During the phase-advancing section of the phase-response curve, PACAP/PAC1 signaling appeared less critical.
Impaired negative masking behavior was observed in PAC1 knockout mice when exposed to low light levels.

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The retinohypothalamic tract (RHT) is a retinofugal neuronal pathway which, in mammals, mediates nonimage-forming vision to various areas in the brain involved in circadian timing, masking behavior, and regulation of the pupillary light reflex. The RHT costores the two neurotransmitters glutamate and pituitary adenylate cyclase activating peptide (PACAP), which in a rather complex interplay are mediators of photic adjustment of the circadian system. To further characterize the role of PACAP/PACAP receptor type 1 (PAC1) receptor signaling in light entrainment of the clock and in negative masking behavior, we extended previous studies in mice lacking the PAC1 receptor (PAC1 KO) by examining their phase response to single light pulses using Aschoff type II regime, their ability to entrain to non-24-h light-dark (LD) cycles and large phase shifts of the LD cycle (jet lag), as well as their negative masking response during different light intensities. A prominent finding in PAC1 KO mice was a significantly decreased phase delay of the endogenous rhythm at early night. In accordance, PAC1 KO mice had a reduced ability to entrain to T cycles longer than 26 h and needed more time to reentrain to large phase delays, which was prominent at low light intensities. The data obtained at late night indicated that PACAP/PAC1 receptor signaling is less important during the phase-advancing part of the phase-response curve. Finally, the PAC1 KO mice showed impaired negative masking behavior at low light intensities. Our findings substantiate a role for PACAP/PAC1 receptor signaling in nonimage-forming vision and indicate that the system is particularly important at lower light intensities.

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Mice lacking the PACAP type I receptor have impaired photic entrainment and negative masking

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