The fine structural demonstration of monoaminergic synapses in immature rat neocortex

The radial distribution of monoaminergic (MA) synapses has been studied in lateral neocortex (SI) of infant rats using an electron microscopic histochemical technique. Systemically administered 5-hydroxydopamine crosses the blood-brain barrier and produces small granular vesicles (SGV) in 30% of all synaptic terminals in SI. In the primordium of layer IV, at six days postnatal, 70% of the synaptic terminals contain SGVs. Immature cortex thus contains a high proportion of synapses with a vesicular uptake-storage mechanism for catecholamines. It is proposed that in infant rat there is a major projection from brain stem MA nuclei to the neocortex; this projection may exert a potent influence on the activity of neurons in layer IV.