Light and electron microscopic immunocytochemical analysis of the noradrenaline innervation of the rat visual cortex.

Immunocytochemistry with an antiserum against noradrenaline was used to examine the organization and morphology of noradrenergic axons in the rat visual cortex. Observations with the light microscope confirmed earlier reports concerning the distribution pattern of noradrenergic fibres, and provided some further clues about their intracortical organization. Particularly striking was the finding of fibres which followed an oscillating course within the boundaries of layers II-IV as they ran in the mediolateral direction. Examination of the morphological characteristics of noradrenaline-containing axon terminals in serial ultrathin sections has provided further evidence that the vast majority (87.6%) form conventional synapses in the visual and frontoparietal cortex, and has given clues about the postsynaptic elements involved in these synaptic contacts; they are, in decreasing frequency, spines, dendritic shafts of various diameters, and pyramidal and non-pyramidal somata. In addition, a few labelled terminals were visualized in close association with intracerebral capillaries.