The forward primers were designed to predicted exon 13 and reverse primers to exon 2 (Fig. 2A–C; Supplementary Material, Table S4). RT–PCR results showed that the human exon 13 is ∼0.8 kb longer than the mouse exon 13 and carries alternative acceptor sites of splicing (Supplementary Material, Fig. S4) similar to OPRM1 exons 1, 3 and 5 (35,43). The sequencing results of the RT–PCR product amplified from the frontal lobe, nucleus accumbens, medulla oblongata and spinal cord identified a 5′-UTR and a partial coding region of novel OPRM1 splicing isoforms, MOR-1K1 and MOR-1K2 (GenBank accession nos EU362990 and EU360599). A longer MOR-1K1 variant is preferentially expressed in the medulla oblongata, while a shorter MOR-1K2 variant preferentially expressed in the spinal cord. Both these variants are present in the frontal lobe and in nucleus accumbens (Supplementary Material, Table S5). We found no evidence of the expression of an isoform containing a short exon 13 in humans or an isoform with a long exon 13 in mouse.
