Laminar distribution of noradrenergic markers in rat visual cortex

The laminar distribution of ?(1)-, ?(2)- and ?-adrenoreceptors was studied in the visual cortex of adult rat together with an investigation of noradrenaline uptake sites. The different layers of the visual cortex were separated by cutting serial cryostat sections and binding studies were performed in slide-mounted tissue sections of 10?m thickness collected from one individual cortical layer. [(3)H]desipramine binding, assumed to label noradrenaline uptake sites, was found to be highest in layer I by about 37%, whereas binding in the remaining layers was uniformly distributed. The laminar distribution of the ?(1)- and ?(2)-adrenoreceptors studied using [(3)H]prazosin and [(3)H]clonidine as radioligands, was similar to that of the noradrenaline uptake sites: markedly higher binding was detectable in layer I compared to the remaining layers. In contrast, the density of ?-adrenoreceptors, as revealed by [(3)H]dihydroalprenolol binding, was highest in layers I and IV, followed by layer II/III, (58% of that in layer I and IV). Lowest binding was observed in layers V and VI (36%). The similarity in laminar distributions of ?-adrenoreceptors and noradrenaline uptake sites suggests a close correlation of receptor localization and fibre termination, whereas the localization of ?-adrenoreceptors cannot be easily related to the pattern of noradrenergic fibres and terminals.