An isolated, synthetic or recombinant .chi.-conotoxin peptide having the ability to inhibit neuronal amine transporter comprising the following sequence of amino acids: Cys Cys Gly Tyr Lys Leu Cys Xaa5 Xaa6 Cys, SEQ ID NO. 3, where Xaa5 and Xaa6 are independently absent or represent any amino acid residue except Cys; or a sequence in which Gly, Tyr, Lys or Leu are subject to conservative amino acid substitution or side chain modification; with the proviso that the peptide is not .chi.-MrIA, .chi.-MrIB, Mar2, CMrVIA, Bn1.5, Mr1.3 or Au1.4; or a salt, ester, amide, prodrug or cyclised derivative thereof.