An isolated, synthetic or recombinant X-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 MrIA, MrIB, Mar2, CMrVIA, Bn1.5, Mr1.3 or Au1.4; or a salt, ester, amide, prodrug or cyclised derivative thereof.