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The neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3), and NT4/5 are all found in the developing cerebellum. Granule cells, the major target neurons of mossy fibers, express BDNF during mossy fiber synaptogenesis. To determine whether neurotrophins contribute to the development of cerebellar afferent axons, we characterized the effects of neurotrophins on the growth of mossy fiber neurons from mice and rats in vitro. For a mossy fiber source, we used the basilar pontine nuclei (BPN), the major source of cerebellar mossy fibers in mammals. BDNF and NT4/5 increased BPN neuron survival, neurite outgrowth, growth cone size, and elongation rate, while neither NT3 nor NGF increased survival or outgrowth. In addition, BDNF and NT4/5 reduced the size of neurite bundles. Consistent with these effects, in situ hybridization on cultured basilar pontine neurons revealed the presence of mRNA encoding the TrkB receptor which binds both BDNF and NT4/5 with high affinity. We detected little or no message encoding the TrkC receptor which preferentially binds NT3. BDNF and NT4/5 also increased TrkB mRNA levels in BPN neurons. In addition to previously established functions as an autocrine/paracrine trophic factor for granule cells, the present results indicate that cerebellar BDNF may also act as a target-derived trophic factor for basilar pontine mossy fibers. © 1999 John Wiley & Sons, Inc. J Neurobiol 40: 254–269, 1999

In vitro methods for studying interactions between axons and their target cells are presented. The methods maximize the number of cultures that can be produced by limiting the volume and area of the cultures. Small cultures promote cell-cell Interactions and permit rapid conditioning of medium. In addition, valuable reagents added to these microcultures are conserved. The methods include: (a) the manufacture of 40-µl well-volume, coverslip-bottomed culture dishes with plating area of less than 24 mm2 the dishes allow the small working distances of high-resolution light microscopy; (b) a micromethod to test for the Involvement of secreted factors in cell-cell interactions; cells on different surfaces are cocultured in shared medium; (c) a method to plate explant sources of neurites at a controlled distance from target cells to facilitate neurite identification and to control the timing of growth cone-target cell contacts; and (d) nonisotopic in situ hybridization for chamber-slide cultures combined with immunolabeling of cells in the hybridized culture. These methods can be used in culture assays to identify cell types or molecules involved in a variety of neuronal or, more generally, cell-cell interactions.Copyright 1994, 1999 Academic Press

MY attention has been directed to a letter appearing in the issue of NATURE for May 6 dealing with the subject of scientific apparatus. Your correspondents are extremely moderate in tone, but they do not state the class of apparatus to which they are referring.