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 formation of whisker-related patterns in trigeminothalamic afferents and the onset of synapse formation between afferents and cells in the ventroposteromedial nucleus (VPM) of the marsupial mammal, the wallaby, by labelling afferents with a carbocyanine dye. A parallel in vitro study was made of the functional development of the trigeminothalamic pathway to the VPM. Evoked synaptic responses could be recorded in the VPM from the time that afferents first reached the VPM at postnatal day 15 (P15). At all stages, the excitatory response comprised both N-methyl-D-aspartate- and non-N-methyl-D-aspartate-mediated components. At P40, the response decreased markedly in duration, coinciding with the onset of synaptogenesis. This implies that transmission is occurring prior to synapse formation, probably through transmitter release from growth cones. At P50, synaptic responses became dominated by a fast, non-N-methyl-D-aspartate potential, and this coincided with the first appearance of whisker-related patterns in the VPM. A gamma-aminobutyric acid (subtype A)-mediated, inhibitory component was also present from the time of afferent arrival. These findings support the idea that functional interactions between afferents and their targets may play a role in pattern formation in the somatosensory thalamus.