Suprachiasmatic Nucleus and Photic Entrainment of Circannual Rhythms in Ground Squirrels

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Abstract
The efficacy of photoperiod as a zeitgeber for entrainment of circannual body weight and estrous rhythms was tested in female golden-mantled ground squirrels maintained for 3 or more years in either a simulated natural photoperiod (SNP) or a fixed LD 14:10 photoperiod (FP). The role of the retinohypothalamic tract—suprachiasmatic nucleus (RHT-SCN) projection in photic entrainment was assessed in animals that sustained destruction of the SCN (SCNx ). Circannual rhythms were lengthened by the SNP as compared to the FP. Mean periods (τ's) for neurologically intact animals in the third year of testing were 49.6 ± 0.3 weeks and 43.1 ± 1.2 weeks ( p < 0.001) for the SNP and FP groups, respectively; furthermore, 56% and 7% of animals in these groups had τ's not significantly different from 365 days ( p < 0.005), and within-group variability was lower for SNP than for FP squirrels ( p < 0.01). SCNx squirrels differed from animals with the SCN intact (SCNc), as evidenced by higher within-group variability ( p < 0.001); only 29% of SCNx squirrels had τ's not different from 365 days ( p < 0.03 compared to the SCNc group). The coupling between estrous and body weight rhythms that was evident in SCN-intact SNP and FP squirrels was disrupted in SCNx animals. The RHT-SCN pathway is implicated in entrainment and in maintenance of normal phase relations among the several circannual rhythms. In a second experiment, female squirrels were maintained for 2.5 years in an accelerated SNP that compressed two normal annual photocycles into each calendar year. Of 12 squirrels, 3 had τ's that did not differ significantly from 6 months; 6 had τ's equivalent to 12 months; and 3 had τ's significantly different from both 6 months and 12 months. The data suggest that photoperiod is a major zeitgeber for entrainment of golden-mantled ground squirrels' circannual rhythms.