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Structure–activity relationships studies have established the minimal sequence of melanin-concentrating hormone (MCH) that retains full agonist potency at the MCH 1 , to be the dodecapeptide MCH(6-17). The α-amino function is not required for activity since arginine 6 can be replaced by p -guanidinobenzoyl, further improving activity. We report that the deletion of glycine in this short potent agonist (EC 50 3.4 nM) turns it into a potent and new MCH 1 antagonist (S38151, K B 4.3 nM in the [ 35 S]-GTPγS binding assay), which is selective versus MCH 2 . A compared Ala-scan of the agonist and antagonist sequences reveals major differences in the residues that are mandatory for affinity, including arginine 11 and tyrosine 13 for the agonist and leucine 9 for the antagonist, whereas methionine 8 was necessary for both agonist and antagonist activities. A complete molecular study of the antagonist behavior is described in the present report, with a particular focus on the description of several analogues, attempting to find structure–activity relationships. Finally, S38151 antagonizes food intake when injected intra-cerebroventricularly in the rat. This is in agreement with the in vitro data and with our previous demonstration of a good correlation between in vitro and in vivo data on MCH 1 agonists.

We have identified from human liver eight alpha(1A)-adrenoceptor (alpha(1A)-AR) splice variants that were also expressed in human heart, prostate and hippocampus. Three of these alpha(1A)-AR isoforms (alpha(1A-1)-AR, alpha(1A-2a)-AR and alpha(1A-3a)-AR) gave rise to receptors with seven transmembrane domains (7TMalpha(1A)-AR). The other five (alpha(1A-2b)-AR, alpha(1A-2c)-AR, alpha(1A-3c)-AR, alpha(1A-5)-AR and alpha(1A-6)-AR) led to truncated receptors lacking transmembrane domain VII (6TMalpha(1A)-AR). The 7TMalpha(1A)-AR isoforms transiently expressed in COS-7 cells bound [(3)H]prazosin with high affinity (K(d) 0.2 nM) and mediated a noradrenaline (norepinephrine)-induced increase in cytoplasmic free Ca(2+) concentration, whereas the 6TMalpha(1A)-AR isoforms were incapable of ligand binding and signal transduction. Immunocytochemical studies with N-terminal epitope-tagged alpha(1A)-AR isoforms showed that the 7TMalpha(1A)-AR isoforms were present both at the cell surface and in intracellular compartments, whereas the 6TMalpha(1A)-AR isoforms were exclusively localized within the cell. Interestingly, in co-transfected cells, each truncated alpha(1A)-AR isoform inhibited [(3)H]prazosin binding and cell-surface trafficking of the co-expressed 'original' 7TMalpha(1A-1)-AR. However, there was no modification of either the [(3)H]prazosin-binding affinity or the pharmacological properties of alpha(1A-1)-AR. Immunoblotting experiments revealed that co-expression of the alpha(1A-1)-AR with 6TMalpha(1A)-AR isoforms did not impair alpha(1A-1)-AR expression. Therefore the expression in human tissues of many truncated isoforms constitutes a new regulation pathway of biological properties of alpha(1A)-AR.