EXPRESSION OF BRAIN-DERIVED NEUROTROPHIC FACTOR AND ITS COGNATE RECEPTOR, TRKB, IN THE RAT SUPRACHIASMATIC NUCLEUS

Photic entrainment of mammalian circadian rhythms occurs because the p acemaker in the suprachiasmatic nuclei (SCN) of the hypothalamus is en dowed with a rhythmic sensitivity to photic signals conveyed by the re tinohypothalamic tract. Since brain-derived neurotrophic factor (BDNF) has been implicated in the functional modulation of other retinal tar gets, the rat SCN was examined for expression and cellular distributio n of this neurotrophin and TrkB, the tyrosine kinase receptor that pre ferentially binds BDNF. The rat SCN was found to express the mature BD NF peptide and mRNA by Western blotting, enzyme-linked immunosorbent a s say (ELISA), and reverse trans cription-polymerase chain reaction (R T-PCR) analyses. BDNF-immuno-reactivity and hybridization signal for i ts mRNA were coextensively localized within a number of SCN cells thro ughout the rostrocaudal axis of each nucleus. In addition, some cells intercalated within the optic chiasm were distinguished by expression of BDNF immunoreactivity or mRNA. Immunostaining for the TrkB receptor was also evident in the SCN within terminals or fibers predominantly located along the SCN/optic chiasm interface and within scattered peri karya near the medial border of each nucleus. Combined in situ hybridi zation and immunocytochemical analysis revealed that BDNF mRNA-express ing cells within the ventrolateral SCN were often closely apposed to T rkB-positive fibers extending from the optic chiasm. These findings ra ise the possibility that target-derived interactions between BDNF and TrkB receptors could play a role in the circadian modulation of SCN pa cemaker sensitivity to photic input transmitted by the retinohypothala mic tract. (C) 1998 Academic Press.