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Termination and initial branch formation of SNAP‐25‐deficient thalamocortical fibres in heterochronic organotypic co‐cultures

We are interested in the role of neural activity mediated through regulated vesicular release in the stopping and early branching of the thalamic projections in the cortex. Axon outgrowth, arrival at the cortical subplate, side‐branch formation during the waiting period and cortical plate innervation of embryonic thalamocortical projections occurs without major abnormalities in the absence of regulated release in Snap25 −/− null mutant mice [ Washbourne et al. (2002) Nat. Neurosci. 5:19–26; Molnár et al. (2002) J. Neurosci. 22:10313–10323]. The fact that Snap25 −/− null mutant mice die at birth limited our previous experiments to the prenatal period. We therefore investigated the behaviour of thalamic projections in co‐culture paradigms by using heterochronic thalamic [embryonic day (E)16–E18] and cortical [postnatal day (P)0–P3] explants, in which the stopping and branching behaviour has been previously documented. Our current co‐culture experiments established that thalamic projections from E16–E18 Snap25+/+ or Snap25 −/− explants behaved in an identical fashion in P0–P3 Snap25 +/+ cortical explants after 7 days in vitro. Thalamic projections from Snap25 −/− explants developed similar patterns of fibre ingrowth to the cortex, and stopped and formed branches at a similar depth in the Snap25+/+ cortical slice as in control cultures. These results imply that thalamic projections can reach their ultimate target cells in layer 4, stop, and start to develop branches in the absence of regulated vesicular transmitter release from their own terminals.