Monoaminergic synapses in infant mouse neocortex: Comparison of cortical fields in seizure-prone and resistant mice

In a quantitative electron microscopic study, 5-hydroxydopamine was utilized to identify putative monoamine-containing synaptic boutons in the temporal and in the somatosensory cortex of 6-day-old mice. In Swiss mice, a stratiform distribution of a synapses is seen in both fields. Although the overall synaptic density is slightly lower in the temporal cortex, the relative density of small granular vesicles synapses is equivalent in both fields. In the temporal cortex of seizure-prone DBA mice, only 4% of the synapses are labeled by 5-hydroxydopamine compared with 20% in the temporal cortex of normal Swiss mice; the degree of labeling in the somatosensory cortex is comparable in both strains. Morphometric analysis of the structure of the labeled synapses suggests that the typical monoamine-containing synapse in DBA mice differs from that in Swiss mice. Assuming that 5-hydroxydopamine is a reliable marker for monoamine-containing synapses, we conclude that the temporal cortex differs in several key ways from other fields in the cortex of DBA mice and from the same region in non-seizure-prone Swiss mice.