Somalostatin-IR neurons are a major subpopulation of the cuneothalamic neurons in the rat cuneate nucleus

This study was aimed to localize and characterize the somatostatin-immunore active (SOM-IR) neurons in the rat cuneate nucleus (CN). By immuno-histoche mistry, the SOM-IR neurons, which were widely distributed in the nucleus, w ere round, spindle or multiangular in shape (mean area=226.1+/-3.1 mu m(2), n=1016). By electron microscopy, the neurons shared all the ultrastructura l features of the cuneothalamic neurons (CTNs) which showed a slightly inde nted nucleus and a fairly rich cytoplasm containing well-developed Golgi ap paratuses and rough endoplasmic reticulum (rER). The SOM immunoreaction pro duct filled the cytoplasm of the neurons extending from the soma to the pro ximal and distal dendrites, which were postsynaptic to unlabeled boutons. I n addition to soma and dendrites, SOM-IR boutons were also identified which made axodendritic synaptic contacts with SOM-IR dendrites. The SOM-IR neur ons were characterized by using anti-SOM pre-embedding immunolabeling coupl ed with horseradish peroxidase (HRP) retrograde method, or SOM immunolabeli ng along with anti-glutamate, gamma-aminobutyric acid (GABA) or glycine pos t-embedding immunolabeling for identification of CTNs, glutamate-IR, GABA-I R and glycine-IR neurons, respectively. It was shown that more then 80% of the CTNs contained SOM and, furthermore, they contained glutamate but not G ABA or glycine. On the basis of present findings, it is suggested the major ity of the SOM-IR neurons in the rat CN are CTNs and that they may be invol ved in modulation of somatosensory synaptic transmission. (C) 2000 Elsevier Science Ireland Ltd and the Japan Neuroscience Society. All rights reserve d.