The low-molecular-mass, cyclic analog of neuropeptide Y, y -G l~~-~-L y s~~] NPY (YfiSK-Ahx-RHYINK'ITRQRY; Ahx, 6-aminohexanoic acid; NPY, neuropeptide Y), was synthesized and investigated for receptor binding, inhibition of forskolin-stimulated cAMP accumulation, inhibition of electrically stimulated rat vus deferens contractions and ability to increase blood pressure. Like the linear peptide [A~x'-'~] NPY (YPSK-Ahx-RHYINLITRQRY), the more rigid, cyclic analog showed good correlation between receptor binding to rabbit kidney membranes and biological activity in the vas deferens assay. Binding of this peptide to a new Y2-receptor-expressing cell line was slightly reduced, compared to the linear peptide [ A~x~-'~] NPY, however inhibition of cAMP accumulation was even more efficient.Unlike the linear peptide [ A~x~-'~] NPY, the cyclic analog did not induce a blood pressure increase in rats. Reduced binding to Y1 receptor-expressing SK-N-MC cells, as well as the loss of capability of signal transduction, suggest that only Y,-mediated activity is preserved after cyclization. The selectivity of the cyclic compound for Y subtypes of NPY receptors with respect to inhibition of cAMP accumulation is more than fortyfold increased, as compared to the linear NPY-(13 -36) peptide, which has been used to determine Y2 selectivity so far.Neuropeptide Y (NPY) is a 36-amino-acid peptide amide, which belongs to the pancreatic polypeptide hormone family [1, 21. It was found in the central and peripheral nervous system of mammalian, including man [3,4]. In the periphery, NPY mediates an increase in blood pressure by a direct effect . At the intracellular level, the most consistent effect of NPY is certainly its capacity to inhibit adenylate cyclase activity. This effect, however, is mostly observed in situations in which adenylate cyclase is stimulated and is known to be pertussis sensitive [lo, 111. So far at least two subtypes of NPY receptors have been described using analogs of NPY [12]. The so-called Y1 receptor requires the full-length NPY, or large parts of the Nterminus and C-terminus tivities predominantly mediated by Y1 [17]. Y2 receptors are involved in the inhibition of transmitter release and the enhanced memory retention [17]. Presynaptic activity seems to be only mediated by Y2 receptors, whereas on postsynaptic membranes both types of receptor can be found [18 -201. Besides NPY, long C-terminal segments, such as NPY -( 13 -36), were shown to bind to the Yz receptor. Although these analogs mainly show Y2 activity, they still possess some pressor activity [19, 21, 221. Based on the use of the NPY-(1 3 -36), Y2-subtype-expressing cells have been described [17]. Stimulation of these Yz receptors result also in an inhibition of adenylate cyclase activity.We recently developed a new type of NPY analogs, consisting of discontinuous, truncated hormone peptide segments [23]. These compounds were synthesized by coupling an Nterminal segment of NPY via its C-terminal end to a larger Cterminal segment by a spacer molecule. ...