In common with other areas of the prefrontal cortex, activity in frontopolar area 10 is probably modulated by dopamine. We studied the dopaminergic innervation of monkey prefrontal area 10 by immunostaining with tyrosine hydroxylase (TH) antibodies. TH-positive axons in layer 3 were examined by electron microscopy of series of ultrathin sections. TH-positive boutons containing vesicles were sparse (2 × 10−4 per μm3) and the majority (94%, n = 52) had no identifiable synaptic specialization, which supports the hypothesis that dopamine is released non-synaptically and raises the question of whether the local microenvironment surrounding the boutons is special. Compared with unlabelled boutons TH-positive boutons had a higher proportion of their perimeter in contact with dendritic shafts and were more often in continuous contact with pairs of pre- and postsynaptic structures. However, this may result from exclusion from sites preferred by glutamatergic and GABAergic synapses as the density of all synapses in the closer vicinity was no different from any randomly selected site in the neuropil. This quantitative ultrastructural study presents basic features of the dopaminergic innervation in prefrontal area 10 and provides a more detailed understanding of the structural basis of dopamine signalling in the cortex.
In this study we examined dopaminergic axons in prefrontal Area 10 of macaque monkey. Boutons containing vesicles and axonal segments were examined using electron microscopy in long series of sections, but synapses were rare. Boutons were randomly distributed and nearest neighbours were separated by ˜9 μm in 3-D. Our anatomical observations support the hypothesis of a non-synaptic release of dopamine into the extracellular space, thus broadcasting a signal to receptors distributed anywhere across the cortical neuropil.