The thalamic intergeniculate leaflet modulates photoperiod responsiveness in Siberian hamsters

Nov 3, 2004Brain research

A thalamus region controls how Siberian hamsters respond to day length changes

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Abstract

Neurochemical lesions of the intergeniculate leaflet (IGL) alter the circadian entrainment and photoperiodic responsiveness of Siberian hamsters to a simulated natural photoperiod.

Siberian hamsters use changes in day length to time their reproductive efforts.
The suprachiasmatic nucleus (SCN) in the hypothalamus is crucial for measuring and responding to day length.
Two pathways transmit light information to the SCN: the retinohypothalamic tract (RHT) and the geniculohypothalamic tract (GHT).
Ablating the IGL/GHT leads to modest changes in entrainment to static day lengths but does not affect seasonal responses to shifts between long and short day lengths.
Lesions of the IGL affect both circadian entrainment patterns and the ability to respond to a simulated natural photoperiod.
Both IGL-intact and IGL-lesioned hamsters showed testicular regression in response to shortening day lengths, but only IGL-intact hamsters molted seasonally.

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Siberian hamsters are seasonal breeders that use changes in day length to synchronize their reproductive effort with those times of the year most favorable for successful reproduction. The ability of Siberian hamsters to measure and respond to changes in day length depends upon accurate photoentrainment of the circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus. Two pathways have been characterized through which entraining stimuli reach the SCN: the retinohypothalamic tract (RHT), which transmits light information from the retinae, and the geniculohypothalamic tract (GHT) from the intergeniculate leaflet of the thalamus (IGL), which is involved in transmitting both photic and nonphotic cues. Ablating the IGL/GHT results in only modest alterations in entrainment to static day lengths and fails to interfere with seasonal responses induced by transfer from static long day to static short day lengths. Because several studies suggest that the IGL may be involved in tracking the time of dusk and dawn, we sought to determine whether an intact IGL is necessary for hamsters to respond to a simulated natural photoperiod (SNP) in which the time of dusk and dawn gradually changes in a pattern approximating the rate of change in day length that occurs during autumn at the latitude this species inhabits in nature. The results indicate that neurochemical lesions of the IGL alter both the pattern of circadian entrainment and photoperiodic responsiveness of Siberian hamsters to an SNP. Both intact and IGL-lesioned hamsters exhibited testicular regression in shortening day lengths, but only IGL-intact hamsters exhibited seasonal pelage molt.

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The thalamic intergeniculate leaflet modulates photoperiod responsiveness in Siberian hamsters

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