Retinohypothalamic tract symmetry and phase shifts of circadian rhythms in rats and hamsters

Abstract

The role of symmetrical innervation of the suprachiasmatic nuclei (SCN) by the retinohypothalamic tract (RHT) in the entrainment of circadian rhythms was investigated in rats and golden hamsters. In the rat, the RHT is asymmetric and unilateral binding delays re-entrainment of drinking rhythms to 6 hr phase advances, 6 hr phase delays and 3 hr phase advances of the light-dark cycle by several days. In constant light, unilaterally blinded rats have free-running rhythms with higher amplitudes and slightly shorter periods. In hamsters, the RHT projects symmetrically to both SCN and unilateral blinding does not affect the rate of re-entrainment. The results are consistent with the hypothesis that asymmetric innervation of the SCN, rather than a reduction of retinohypothalamic input, interferes with re-entrainment of circadian rhythms in unilaterally blinded rats. A comparison of hamster activity and rat drinking rhythms shows that most hamsters respond to phase shifts of the light-dark cycle by a reduction in activity time (alpha compression) with gradual decompression during the transients, whereas in the rat activity time is not as readily compressed. Both species show re-entrainment asymmetry in that phase advances require more transients than phase delays.