Overlap in the distribution of TrkB immunoreactivity and retinohypothalamic tract innervation of the rat suprachiasmatic nucleus.

Brain-derived neurotrophic factor (BDNF) may regulate the circadian sensitivity of the clock in the hypothalamic suprachiasmatic nucleus (SCN) to light, possibly by modulating retinohypothalamic tract (RHT) input. In the present study, the anatomical distribution of the cognate receptor for BDNF, the TrkB tyrosine kinase, in RHT fibers and the SCN of rats was analyzed using combined immunohistochemical and anterograde tracing methods. Fluorescent immunostaining for the TrkB receptor was evident in fibers and cell bodies throughout the SCN. Dual labeling analysis revealed that there was substantial overlap in the distribution of TrkB immunostaining and cholera toxin subunit B (CTB)-labeling within RHT terminals and fibers projecting from the optic chiasm to the ventrolateral SCN. The present results suggest that RHT fibers may express TrkB receptors and thus provide a paracrine target for BDNF-mediated regulation of photic input to the SCN.