Ablation of the geniculo-hypothalamic tract alters circadian activity rhythms of hamsters housed under constant light.

Retino-recipient cells in the hamster lateral geniculate nucleus project to the suprachiasmatic nucleus via the geniculo-hypothalamic tract (GHT). GHT-ablation alters phase advance shifts to light pulses in a hamster's late subjective night. In this study, the effects of GHT-ablation on wheel-running rhythms of hamsters housed under continuous illumination (LL) were assessed. In the first experiment, hamsters received GHT-ablation or sham surgery while under a light:dark schedule and were subsequently exposed to 250 days of LL. GHT-ablated hamsters showed rhythms with shorter periods and were less likely to show split activity rhythms than sham-operated or partial-lesion controls. In the second experiment, hamsters were housed under LL until rhythms split into two components; hamsters then received either GHT-ablation or sham surgery. Four of seven GHT-ablated hamsters showed re-fusion of their activity pattern into one component, while none of the eight sham-operated animals showed such re-fusion. The results of these two experiments indicate that GHT-ablation alters the responsiveness of the activity rhythm pacemaker to LL exposure.