Immunohistochemical detection of dopamine D2 receptors in neuroendocrine tumours

Recently, dopamine D2 receptors (RD2) have been found to be expressed in neuroendocrine tumours (NET), the tumours which arise from the diffuse neuroendocrine cells. Moreover, successful trials of the treatment of NET with cabergoline - D2 agonist, have been reported. These findings increase the interest of investigating RD2 expression in NET.

The expression of RD2 was investigated immunohistochemically using the antibody which recognises both short (S) and long (L) isoforms of the receptor in 17 NET samples taken from 15 patients.

In 17 NET samples, a positive reaction with the anti-RD2 antibody occurred in 11 cases. In six cases, the localisation of the immunostaining was cytoplasmic and in nine cases it was nuclear. Only in one case was the receptor cell membrane-located, and in two cases the immunoreaction was also localised in the blood vessels walls. The relation between RD2 expression and the grade of malignancy examined by means of Ki-67 antigen expression needs further study. However, preliminary observations indicate that the nuclear localisation of RD2 is linked to higher tumour malignancy. The next investigated question was the co-expression of somatostatin and dopamine receptors. This question seems important because of the perspectives of somatostatin-dopamine chimeras application in NET treatment. In the samples examined by us, RD2 were co-expressed in 5/10 cases with sstr1, in 3/10 with sstr2A, in 2/9 with sstr2B, in 3/10 with sstr3, and in 5/10 with sstr5.

Dopamine D2 receptors are revealed by means of immunohistochemistry in the majority of NET. They exhibit cytoplasmic and/or nuclear localisations, the latter being possibly linked to a higher grade of malignancy, and are often co-expressed with somatostatin receptors (mostly with subtypes1 and 5).