Somatostatin-28

The term somatostatin refers to a family of peptides, mainly somatostatin-14, somatostatin-28 and somatostatin-28 (1-12), which are the cleavage products of a single 116 amino acid-long preprosomatostain molecule. The production of antibodies to these peptides allows their localization in a number of neuronal populations throughout the entire neuroaxis in many mammals. The dog has been pointed out as an extremely useful animal model for studying age-related cognitive dysfunction and other neuronal changes associated with aging in which somatostatin appears to be involved. However, only very scanty information is available with regard to the distribution of somatostatin in the brain of the dog. In the present work we have determined the pattern of the distribution of somatostatin-28 (1-12) immunoreactivity in the diencephalon and the brainstem of the dog. High to moderate densities of labeled perikarya were found in the anterior periventricular and arcuate hypothalamic nuclei, the reticular thalamic nucleus, in delimited parts of the nucleus of the brachium inferior colliculus, the retrorubral area, the dorsal raphe nucleus, the myelencephalic reticular formation and the dorsal motor nucleus of the vagus. Less dense population of somatostatin cells were localized in other diencephalic and brainstem nuclei. The distribution of labeled fibers was even broader as in addition to those above mentioned there were a number of areas that appeared devoid of labeled perikarya. Many of the findings were similar to those reported in earlier works while others underlined the existence of inconsistencies in the distribution pattern of this peptide in the brain of mammals.

The search for a peptide corresponding to the NH2-terminus of somatostatin-28 (SS-28) in tissues has led to the isolation and characterization of somatostatin-28[1-12] from pancreas and hypothalamus. Somatostatin-28[1-12]-like immunoreactivity [SS-28 [1-12]-LI] is widely distributed throughout the central nervous system and the digestive system of rodents and primates, reaching levels comparable to those of somatostatin-14 (SS-14). Antibodies directed against the C-terminal end of the dodecapeptide are more specific and constitute excellent markers for the "prosomatostatin" system in mammalian tissues. In rat brain, SS-28[1-12]-LI material is highly concentrated in nerve fibres and terminals, especially in the median eminence, layer I of neocortex, the outer molecular layer of the dentate gyrus and the striatum. Additionally, immunoreactivity is observed in large multipolar or occasionally pyramidal-like neurons of the neocortex. SS-28[1-12] is secreted from hypothalamus and amygdala by a calcium dependent mechanism. No biological role is presently known for the dodecapeptide. Two other peptides of Mr = 8000 (8 K) and Mr = 5000 (5 K) which contain SS-28[1-12] at their carboxy-termini are present in acid extracts from rat pancreas, brain and spinal cord. These two peptides were isolated from an acid extract of rat brains using ion-exchange chromatography, gel permeation chromatography and reverse-phase HPLC. Results from amino acid analysis and partial sequencing were compared to the sequence of the cDNA encoding rat pre-prosomatostatin (prepro-SS) and revealed that the 8 K peptide is a 76 amino acid molecule corresponding to prepro-SS[25-100] and that the 5K peptide, which contains 44 amino acids, corresponds to prepro-SS [57-100]. The 5 K peptide was generated after cleavage of a Leu-Leu bond at position 56-57 of prepro-SS. The four most predominant peptides of the "prosomatostatin system" presently characterized are: SS-14, SS-28[1-12], SS-28 and prepro-SS[25-100]. Studies on pooled perfusates from rat hypothalamic tissue show that prepro-SS[25-100] is released with SS-28[1-12] in vitro and accounts for 22% of the total SS-28[1-12]-like immunoreactive material released during depolarization. The 5 K peptide is apparently not secreted. The presence of prepro-SS[25-100] in brain implies that, first, prosomatostatin can serve as an immediate precursor for SS-14 without going through SS-28 as an intermediate step and second, other peptides could conceivably be derived from the cryptic portion of the precursor.

Five somatostatin receptors (SSTRs) bind somatostatin-14 (S-14) and somatostatin-28 (S-28), but SSTR5 has the highest affinity for S-28. To determine whether S-28 acting through SSTR5 mediates inhibition of glucagon-like peptide-1 (GLP-1), fetal rat intestinal cell cultures were treated with somatostatin analogs with relatively high specificity for SSTRs 2-5. S-28 dose-dependently inhibited GLP-1 secretion stimulated by gastrin-releasing peptide more potently than S-14 (EC(50) 0.01 vs. 5.8 nM). GLP-1 secretion was inhibited by an SSTR5 analog, BIM-23268, more potently than S-14 and nearly as effectively as S-28. The SSTR5 analog L-372,588 also suppressed GLP-1 secretion equivalent to S-28, but a structurally similar peptide, L-362,855 (Tyr to Phe at position 7), was ineffective. An SSTR2-selective analog was less effective than S-28, and an SSTR3 analog was inactive. Separate treatment with GLP-1-(7-36)-NH(2) increased S-28 and S-14 secretion by three- and fivefold; BIM-23268 abolished S-28 without altering S-14, whereas the SSTR2 analog was inactive. The results indicate that somatostatin regulation of GLP-1 secretion occurs via S-28 through activation of SSTR5. GLP-1-stimulated S-28 secretion is also autoregulated by SSTR5 activation, suggesting a feedback loop between GLP-1 and S-28 modulated by SSTR5.