Ultrastructural localization of somatostatin‐like immunoreactivity in the rat dentate gyrus
Authors: Milner Teresa A. , Bacon Catherine E. 
Published: January 2005

Neurons containing somatostatin (SOM) are enriched in the dentate gyrus. We sought to establish the ultrastructural localization of this peptide in the dentate gyrus of the rat brain with a double‐bridged peroxidase‐antiperoxidase (PAP) method localizing antisera directed against somatostatin (SOM)‐28 and SOM‐281–12. Initial light microscopic observations confirmed that the majority of perikarya and thick varicose processes with intense SOM‐like immunoreactivity (SOM‐LI) were observed in the hilus. Fine varicose processes with SOM‐LI were found throughout all layers of the dentate gyrus but were most intense in the outer third of the molecular layer (ML), where an occasional perikaryon with SOM‐LI was seen.

By electron microscopy, SOM‐LI was found in neuronal perikarya, dendrites, axons, and axon terminals. Two types of SOM‐containing perikarya were observed. The first type was small (6–10 μm), round or ovoid, and had a labeled cytoplasm with abundant Golgi complexes and a dense accumulation of PAP‐reaction product. The second type of perikarya was larger (11–16 μm) and had a more abundant cytoplasm than the first type, but the Golgi complexes did not appear labeled. Most (96% of 374) of the synapses on the SOM‐labeled perikarya and dendrites were from terminals without SOM‐LI which formed nearly equal proportions of asymmetric and symmetric junctions. The remainder of the presynaptic terminals contained SOM‐LI and made primarily symmetric synapses. Synaptic junctions from both unlabeled and labeled terminals were primarily on the shafts of the small (0.5–1.5 μm) SOM‐immunoreactive dendrites.

The terminals with SOM‐LI (0.25‐1.3 μm) contained many small, clear vesicles and from zero to four large dense‐core vesicles. Terminals with SOM‐LI were associated (1) with one unlabeled perikaryon or dendrite (49% of 215 in the hilus; 76% of 326 in the ML); (2) with two unlabeled perikarya or dendrites simultaneously (5% hilus; 4% ML); and (3) with one SOM‐containing perikaryon or dendrite (6% hilus; 3% ML). In all three types of associations, synaptic contacts on perikarya were few while the majority were with small (distal) dendrites. Moreover, most of the terminals with SOM‐LI formed symmetric junctions or lacked membrane specializations but were without any apparent glial intervention in the plane of section analyzed. The remaining SOM‐labeled terminals (40% hilus; 17% ML) were without any apparent synaptic relations. However, a few of these terminals were in direct apposition to other terminals, some of which were also SOM‐immunoreactive. These results provide cellular substrates in the dentate gyrus for (1) direct synaptic modulation of SOM‐containing neurons by terminals with SOM as well as unidentified transmitters and (2) for primarily inhibition (symmetric synapses) of granule cells and interneurons by SOM‐containing terminals. Moreover, they support the concept that SOM‐containing neurons in the dentate gyrus may differ in terms of the involvement of the Golgi saccules in the synthesis and/or packaging of the peptide.