Neurons in the suprachiasmatic area are labelled after intravenous injections of horseradish peroxidase.

Intravenous injections of horseradish peroxidase (HRP) produced labelled neurons in the suprachiasmatic area of house mice (Mus musculus), deer mice (Peromyscus maniculatus bairdi), and white-footed mice (Peromyscus leucopus). In the two species of Peromyscus, a significant proportion of the labelled cells was found within the suprachiasmatic nucleus (SCN). These results suggest that when HRP is injected in the eyes to trace retinal inputs to the hypothalamus, some of the reaction product seen in the SCN may represent blood-borne transport of the enzyme.