A series of electron microscopic immunocytochemical studies was performed to analyze subcellular sites for noradrenergic modulation in monkey prefrontal cortex. One out of 12 noradrenergic varicosities, identified by dopamine β-hydroxylase immunocytochemistry within single ultrathin sections, forms morphologically identifiable junctions with small dendrites and spines. Accordingly, α2-adrenergic receptors, almost all of which are of the A- subtype, that occur in spines are localized discretely over postsynaptic membranes, α2-Adrenergic receptors are also found at sites along axons, dendritic shafts and astrocytic processes lacking morphologically identifiable synaptic junctions, suggesting that these receptors are activated by volume transmission. In particular, axonal α2-adrenergic receptors occur mostly at pre-terminal regions, suggesting that axo-axonic interactions may mediate reduction of neurotransmitter release at sites other than axo-spinous junctions by closing voltage-dependent calcium channels. These results indicate that noradrenergic modulation of prefrontal cortex involves synaptic interactions at spines of pyramidal neurons and nonsynaptic volume transmission to glia, dendritic shafts and axons.
All Science Journal Classification (ASJC) codes