CELLULAR AND SUBCELLULAR SITES FOR NORADRENERGIC ACTION IN THE MONKEYDORSOLATERAL PREFRONTAL CORTEX AS REVEALED BY THE IMMUNOCYTOCHEMICAL LOCALIZATION OF NORADRENERGIC RECEPTORS AND AXONS

A series of electron microscopic immunocytochemical studies was perfor med to analyze subcellular sites for noradrenergic modulation in monke y prefrontal cortex. One out of 12 noradrenergic varicosities, identif ied by dopamine beta-hydroxylase immunocytochemistry within single ult rathin sections, forms morphologically identifiable junctions with sma ll dendrites and spines. Accordingly, alpha(2)-adrenergic receptors, a lmost all of which are of the A-subtype, that occur in spines are loca lized discretely over postsynaptic membranes. alpha(2)-Adrenergic rece ptors are also found at sites along axons, dendritic shafts and astroc ytic processes lacking morphologically identifiable synaptic junctions , suggesting that these receptors are activated by volume transmission . In particular, axonal alpha(2)-adrenergic receptors occur mostly at pre-terminal regions, suggesting that axo-axonic interactions may medi ate reduction of neurotransmitter release at sites other than axo-spin ous junctions by closing voltage-dependent calcium channels. These res ults indicate that noradrenergic modulation of prefrontal cortex invol ves synaptic interactions at spines of pyramidal neurons and nonsynapt ic volume transmission to glia, dendritic shafts and axons.