MORPHOLOGICAL DEVELOPMENT OF AFFERENT SEGREGATION AND ONSET OF SYNAPTIC TRANSMISSION IN THE TRIGEMINOTHALAMIC PATHWAY OF THE WALLABY (MACROPUS-EUGENII)

A light and electron microscopic study has been made of the time of fo rmation of whisker-related patterns in trigeminothalamic afferents and the onset of synapse formation between afferents and cells in the ven troposteromedial nucleus (VPM) of the marsupial mammal, the wallaby, b y labelling afferents with a carbocyanine dye. A parallel in vitro stu dy was made of the functional development of the trigeminothalamic pat hway to the VPM. Evoked synaptic responses could be recorded in the VP M from the time that afferents first reached the VPM at postnatal day 15 (P15). At all stages, the excitatory response comprised both N-meth yl-D-aspartate- and non-N-methyl-D-aspartate-mediated components. At P 40, the response decreased markedly in duration, coinciding with the o nset of synaptogenesis. This implies that transmission is occurring pr ior to synapse formation, probably through transmitter release from gr owth cones. At P50, synaptic responses became dominated by a fast, non -N-methyl-D-aspartate potential, and this coincided with the first app earance of whisker-related patterns in the VPM. A gamma-aminobutyric a cid (subtype A)-mediated, inhibitory component was also present from t he time of afferent arrival. These findings support the idea that func tional interactions between efferents and their targets may play a rol e in pattern formation in the somatosensory thalamus. J. Comp. Neurol. 399:47-60, 1998. (C) 1998 Wiley-Liss, Inc.