TrkB-like immunoreactivity is present on geniculocortical afferents in layer IV of kitten primary visual cortex

Exogenous administration of the neurotrophins brain-derived neurotrophic fa ctor (BDNF) or neurotrophin-4/5 (NT-4/5), or blockade of their endogenous a ctions, have been reported to affect the anatomic organization and physiolo gical responses of neurons in developing mammalian primary visual cortex. E xperimental alteration of levels of these neurotrophic factors can also inf luence the morphology of the geniculocortical afferents that project from t he lateral geniculate nucleus (LGN) to primary visual cortex. BDNF and NT-4 /5 are ligands of the TrkB tyrosine kinase receptor. Although multiple popu lations of cortical neurons express TrkB, it is not known whether geniculoc ortical afferents express this receptor on their axon branches in visual co rtex. We have anatomically labeled geniculocortical afferents of postnatal day 40 kittens with the anterograde neuronal tracer Phaseolus vulgaris leuc oagglutinin (PHA-L) and performed double-label immunofluorescence with a pa nel of anti-TrkB antibodies. Confocal microscopy and object-based colocaliz ation analysis were used to measure levels of TrkB-like immunoreactivity (I R) on geniculocortical afferents in layer IV of primary visual cortex. By u sing a conservative analysis involving a comparison of measured colocalizat ion with the amount of colocalization expected based on random overlap of T rkB puncta and PHA-L-labeled afferents, 3 of 5 anti-TrkB antibodies tested showed significant colocalization with the geniculocortical axons. Results for the other two antibodies were indeterminate. The indices obtained for c olocalization of TrkB and geniculocortical afferents were also compared wit h the equivalent index obtained for GAD65, a protein that has a similar ove rall expression pattern to that of TrkB but is not expressed on geniculocor tical axons, This analysis indicated that TrkB was present on geniculocorti cal axons for all five TrkB antibodies tested. TrkB-like IR was also observ ed on neuronal somata in the LGN. These results indicate that TrkB receptor s on geniculocortical afferents are potential mediators of the actions of B DNF and NT-4/5 in developing visual cortex. J. Comp. Neurol. 436:391-398, 2 001. (C) 2001 Wiley-Liss, Inc.