Dopamine innervation of monkey entorhinal cortex: postsynaptic targets of tyrosine hydroxylase-immunoreactive...

Dopamine (DA) has been demonstrated to play an important role in regulating cortical activity in both neocortical and periallocortical regions. However, marked differences between these two types of cortices in the laminar pattern of DA axons, the types and distribution of DA receptors, and the postnatal development of the DA innervation suggest that DA may have region-specific effects. Such regional specialization may also include the types of cortical cells apposed to DA terminals. In neocortical regions, such as the prefrontal and motor cortices, the majority of structures apposed to DA terminals appear to be the dendritic spines and shafts of pyramidal cells, and a minority are dendrites immunoreactive for gamma-amino butyric acid (GABA). However, the identity of the neural elements apposed to DA terminals in the entorhinal cortex, a periallocortical region, is unknown. In this study, we used immunocytochemical techniques and antibodies against tyrosine hydroxylase (TH) and GABA, visualized with preembedding immunoperoxidase and immunogold-silver labels, respectively, to examine DA terminals and their targets with electron microscopy. In the superficial layers of the monkey entorhinal cortex, TH-immunoreactive (IR) terminals varied greatly in size and formed thin, symmetric synapses. The majority of dendritic structures apposed to these TH-terminals were not GABA-IR, and included both dendritic shafts (64%) and spines (14%). A minority (22%) of the apposed dendrites were GABA-IR. A similar distribution of targets was observed for the subset of TH-IR terminals with identifiable synaptic specializations. In addition, the proportions of GABA-labeled and unlabeled dendrites apposed to TH terminals did not differ from those previously reported for monkey prefrontal cortex. These findings indicate that DA terminals provide direct input to both excitatory and inhibitory cells in the monkey entorhinal cortex and suggest that the effects of DA are mediated through a set of targets that are common to both neo- and periallocortex.