Full methyl jacket? A complete library of the N‐methylated somatostatin cyclopeptidic analogue Veber–Hirschmann peptide cyclo(‐PFwKTF‐) has been prepared with the aim of improving its bioavailability. Several analogues from the library were found to bind to the somatostatin receptor in the nanomolar range and one of them shows a significant oral bioavailability of 10 %. Conformational analysis shows that N‐methylation is allowed at specific positions without affecting the bioactive conformation.

Somatostatin-14 [somatotropin release-inhibiting factor (SRIF)] reduces hippocampal epileptiform activity but the contribution of its specific receptors (sst1-5) is poorly understood. We have focused on the role of sst1 and sst2 in mediating SRIF modulation of epilepsy using hippocampal slices of wild-type (WT) and sst1 or sst2 knockout (KO) mice. Recordings of epileptiform discharge induced by Mg2+ -free medium with 4-aminopyridine were performed from the CA3 region before and after the application of SRIF compounds. In WT mice, SRIF and the sst1 agonist CH-275 reduce epilepsy whereas sst1 blockade with its antagonist SRA-880 increases the bursting discharge. Activation of sst2 does not affect the bursting frequency unless its agonist octreotide is applied with SRA-880, indicating that sst1 masks sst2-mediated modulation of epilepsy. In sst1 KO mice: (i) the bursting frequency is lower than in WT; (ii) SRIF, CH-275 and SRA-880 are ineffective on epilepsy and (iii) octreotide is also devoid of effects, whereas blockade of sst2 with the antagonist D-Tyr8 Cyn 154806 increases the bursting frequency. In sst2 KO mice, the SRIF ligand effects are similar to those in WT. In the whole hippocampus of sst1 KO mice, sst2 mRNA, protein and binding are higher than in WT and reverse transcription-polymerase chain reaction of the CA3 subarea confirms an increase of the sst2 messenger. We conclude that sst1 mediates inhibitory actions of SRIF and that interactions between sst1 and sst2 may prevent sst2 modulation of epilepsy. We suggest that, in sst1 KO mice, activation of over-expressed sst2 reduces the bursting frequency, indicating that sst2 density represents the rate-limiting factor for ss(2-mediated modulation of epilepsy.

No abstract

Somatostatin-14 (SRIF) co-localizes with gamma-aminobutyric acid (GABA) in the hippocampus and regulates neuronal excitability. A role of SRIF in the control of seizures has been proposed, although its exact contribution requires some clarification. In particular, SRIF knockout (KO) mice do not exhibit spontaneous seizures, indicating that compensatory changes may occur in KO. In the KO hippocampus, we examined whether specific SRIF receptors and/or the cognate peptide cortistatin-14 (CST) compensate for the absence of SRIF. We found increased levels of both sst2 receptors (sst2) and CST, and we explored the functional consequences of sst2 compensation on bursting activity and synaptic responses in hippocampal slices. Bursting was decreased by SRIF in wild-type (WT) mice, but it was not affected by either CST or sst2 agonist and antagonist. sst4 agonist increased bursting frequency in either WT or KO. In WT, but not in KO, its effects were blocked by agonizing or antagonizing sst2, suggesting that sst2 and sst4 are functionally coupled in the WT hippocampus. Bursting was reduced in KO as compared with WT and was increased upon application of sst2 antagonist, while SRIF, CST and sst2 agonist had no effect. At the synaptic level, we observed that in WT, SRIF decreased excitatory postsynaptic potentials which were, in contrast, increased by sst2 antagonist in KO. We conclude that sst2 compensates for SRIF absence and that its upregulation is responsible for reduced bursting and decreased excitatory transmission in KO mice. We suggest that a critical density of sst2 is needed to control hippocampal activity.

Nervous System Research, S-386-745, Novartis Pharma AG, CH-4002 Basel N-Acyl-b 2 /b 3 -dipeptide-amide somatostatin analogs, 5 ± 8, with b 2 -HTrp-b 3 -HLys (×natural× sequence) and b 2 -HLys-b 3 -HTrp (retro-sequence) have been synthesized (in solution). Depending on their relative configurations and on the nature of the terminal N-acyl and terminal C-amino group, the linear b-dipeptide derivatives have affinities for the human receptor hsst 4, ranging from 250 to > 10000 nanomolar (Fig. 3). Also, N-Actetrapeptide amides 9 and 10, which contain one a-and three b-amino acid residues (N-b-a-b-b-C), have been prepared (solid-phase synthesis), with the natural (Phe, Trp, Lys, Thr) and the retro-sequence (Thr, Lys, Trp, Phe) of side chains and with two different configurations, each, of the two central amino acid residues. The novel −mixed×, linear a/b-peptides have affinities for the hsst 4 receptor ranging from 23 to > 10000 nanomolar (Fig. 4), and, like −pure× b-peptides, they are completely stable to a series of proteolytic enzymes. Thus, the peptidic turn of the cyclic tetradecapeptide somatostatin (Fig. 1) can be mimicked by simple linear di-and tetrapeptides. The tendency of b-dipeptides for forming hydrogen-bonded rings is confirmed by calculations at the B3LYP/6-31G(d,p) level (Fig. 2). The reported results open new avenues for the design of low-molecularweight peptidic drugs. b-Peptides have recently emerged as a promising class of peptidomimetics for medicinal chemistry. The ability of b-peptides (for reviews, see [1]) to adopt secondary structures such as helices [1], sheets [2a] [3], and, especially, turns [3] suggests that these compounds might be structural and functional mimics of natural peptides. The potential of such mimics is evident from the facts that a) these secondary structures can be readily designed [1 ± 5], and that b) the b-peptides are completely stable against proteolytic degradation in vitro and in vivo [2a] [6].So far, b-peptides, designed to form amphiphilic helices, have been shown to inhibit an intestinal membrane-bound cholesterol-and lipid-transporting protein [5]. Also, similar peptides have been shown to possess antimicrobial, and sometimes, hemolytic activities [7].