A role for nerve growth factor (NGF) in the mammalian circadian system
was assessed in mice using a line of gene targeted animals carrying a
null mutation at the low affinity p75(NGFR) receptor locus. Receptor-
deficient mice exhibited positive phase angles of entrainment to 24 h
light-dark cycles. In constant dark, circadian rhythms of behavior wer
e normal but phase shifts induced by brief pulses of light were signif
icantly decreased. These results support the hypothesis that NGF, thro
ugh an action mediated at least in part by p75(NGFR) receptors in the
SCN, is involved in the regulation of circadian responses to light.