Neurons in the ventral subiculum, amygdala and entorhinal cortex which project to the nucleus accumbens: their input from somatostatin-immunoreactive boutons

Neurons in the hippocampus, amygdala and entorhinal cortex which project to the nucleus accumbens were labelled retrogradely following injection of horseradish peroxidase. The injections were targetted on the medial part of the nucleus accumbens, but some injection sites included the whole nucleus. Projection neurons in all three areas were found to be spiny, and from the entorhinal cortex and ventral subiculum of the hippocampus they were pyramidal neurons. Somatostatin (S28(1-12)-immunoreactive neurons were found in all parts of the three limbic areas examined. They were found to have various morphologies, but in the electron microscope all had the ultrastructural characteristics of interneurons. In the hippocampus the stratum lacunosum was found to contain the most immunoreactive fibres while most cells lay in the stratum oriens. In the amygdala the densest staining for both cells and fibres was in the central nucleus. In the entorhinal cortex somatostatin-immunoreactive fibres and cells seemed to have no preferential distribution. Examination of somatostatin-immunoreactive profiles in the electron microscope revealed that the majority of synaptic contacts were made with dendrites, many of which were spine-bearing. In the light microscope somatostatin-immunoreactive fibres could be seen to lie near the somata and proximal dendrites of neurons that projected to the nucleus accumbens. In the electron microscope it was found that somatostatin-immunoreactive boutons were in symmetrical synaptic contact with the somata and proximal dendrites of neurons in the ventral subiculum, entorhinal cortex and amygdala which project to the nucleus accumbens.