COLOCALIZATION OF PROSOMATOSTATIN-DERIVED PEPTIDES IN THE CAUDATE-PUTAMEN OF THE RAT

In the striatum of rat, somatostatin 14, somatostatin 28, and somatost atin 28(1-12) have previously been localized within a small population of medium aspiny local circuit neurons. Because all three peptide fra gments are generated through the cleavage of prosomatostatin by differ ent converting enzymes, the possibility for differential expression of these peptides exists. In order to investigate this possibility froze n sections were collected from the brains of adult female Wistar rats fixed with 4% paraformaldehyde and double labelled using immunocytoche mistry and in situ hybridization. Sections were first processed for so matostatin 14, somatostatin 28, or somatostatin 28(1-12) by using the avidin-biotin complex immunocytochemical technique followed by in situ hybridization using S-35-labelled antisense riboprobes to somatostati n mRNA. The results of such analysis revealed that somatostatin 28 and somatostatin mRNA are 100% colocalized. Somatostatin 14 and somatosta tin 28(1-12), in contrast, are only present within 66% of the neurons that express somatostatin mRNA. Examination of the anatomical distribu tion of neurons that express both somatostatin mRNA and somatostatin 1 4 or somatostatin 28(1-12) protein reveals that these neurons are pres ent throughout the caudate-putamen of rat but are more prevalent in th e ventromedial regions. Neurons that express somatostatin mRNA but not somatostatin 14 or somatostatin 28(1-12) are also present throughout the caudate-putamen but are most numerous within a dorsolateral strip just beneath the corpus callosum. These results suggest that the somat ostatin neuron population within the rat caudate-putamen is actually c omposed of two smaller subpopulations based on neuropeptide content. T he first subpopulation contains somatostatin 28 and constitutes one-th ird of the total somatostatin population, whereas the other contains s omatostatin 28, somatostatin 14, and somatostatin 28(1-12) and represe nts the remaining two-thirds of the cells that express somatostatin mR NA. (C) 1994 Wiley-Liss, Inc.