Electron microscopic dual labeling of high-affinity neurotensin and dopamine D2 receptors in the rat nucleus accumbens shell

The dopamine D2 receptor (D2R) in the nucleus accumbens (NAc) shell is implicated in schizophrenia and in psychostimulant‐induced drug‐seeking behavior, both of which are affected by activation of the functionally opposed high‐affinity neurotensin receptor (NTS1). To determine the functionally relevant sites, we examined the dual electron microscopic immunocytochemical localization of D2R and NTS1 in the NAc shell of rat brain. Immunolabeling for each receptor was seen in association with cytoplasmic organelles, or more rarely, on the plasma membrane of both axonal and somatodendritic profiles. Some of the axonal and many of the dendritic processes colocalized the two receptors. The dually labeled axon terminals often formed symmetric synapses or appositional contacts with unlabeled dendritic profiles. The morphology of these terminals suggests that they contain either inhibitory amino acids or dopamine. Other axonal profiles expressing exclusively NTS1 or D2R were without synaptic specializations or formed asymmetric, excitatory‐type synapses mainly on unlabeled dendritic spines. In addition, however, several D2R‐immunoreactive terminals were observed presynaptic to dendrites containing NTS1. The somatodendritic profiles immunolabeled for NTS1 and/or D2R had morphological features typical of inhibitory spiny projection neurons in the NAc. These results suggest that activation of NTS1 and D2R can dually modulate transmitter release from the same or separate phenotypically distinct axon terminals in the NAc shell. These presynaptic receptors as well as the postsynaptic NTS1 distribution in neurons that also contain or receive input from terminals containing D2R may mediate the opposing actions of neurotensin and dopamine in the NAc. Synapse 52:176–187, 2004. © 2004 Wiley‐Liss, Inc.