Evidence indicates that some regulatory peptides (endothelins, cholecystokinin and VIP) are involved in the control of thymus growth, and we have investigated whether galanin may be included in this group of peptides. In fact, galanin, a 29-amino acid peptide acting through three subtypes of G protein-coupled receptors (GalR1, GalR2 and GalR3), seems to play a role in the control of the immune system. Reverse transcription (RT)-polymerase chain reaction (PCR) allowed the detection of galanin, GalR1 and GalR3 mRNAs in the thymus cortex of immature (20-day-old) rats, while GalR2 expression was very weak or absent. Immature rats were given three subcutaneous injections (28, 16 and 4 h before sacrifice) of 2 nmol/100 g galanin and or the galanin-receptor antagonist (galanin-A) [D-Thr6,D-Trp8,9,15-ol]-galanin1-15, and 0.1 mg/100 g vincristine 3 h before sacrifice. Thymuses were processed for light microscopy and the percentage of metaphase-arrested cells (mitotic index) was evaluated. Galanin-A increased the thymus mitotic index, while galanin was ineffective, thereby suggesting that endogenous galanin exerts a maximal tonic inhibitory effect on the proliferative activity of thymocytes in immature rats. Immature rat thymocytes were cultured in vitro for 12 h in the presence of 10-6 M galanin and/or galanin-A. Hoechst 33342 and propidium iodide were added to the cultures, and the percentage of apoptotic and necrotic cells was determined under fluorescence microscope. Galanin increased apoptotic index, and the effect was prevented by galanin-A. Neither galanin nor galanin-A altered necrotic index. Collectively, our findings indicate that galanin, probably acting through GalR1 and GalR3, exerts antiproliferative and proapoptotic effects on immature rat thymocytes, which makes it likely that this peptide plays a role in the autocrine/paracrine functional regulation of immune system in the rat.

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