In vincristine (VCR) chemotherapy the dose-limiting factor is neurotoxicity, a side-effect which probably is caused by the action of the drug on axonal microtubules. VCR transforms them into paracrystalline structures. ORG 2766, an ACTH(4-9)/MSH(4-9), analogue, might have protective properties against this toxicity, because of its potency to induce the formation of microtubules. Therefore, the effects were studied of co-treatment with VCR and ORG 2766 on paracrystals and on microtubules in axons of the cerebral commissure (CC) of the snail Lymnaea stagnalis. In vitro experiments were carried out, incubating cerebral ganglia including the CC, for various periods of time (5-25 h) in Ringer's solution, in ORG 2766 (10(-6) M), in VCR (10(-4) M), or in VCR+ORG 2766. Furthermore, the effects of VCR (5 h) were studied in CC, pre- and/or post-treated with ORG 2766 (10 h). In another experiment VCR was used at a concentration at which no paracrystals were formed (2x10(-6) M). In this experiment VCR-treatment (5, 15 h) was compared to pre treatment and pre- and post-treatment with ORG 2766 (10 h). The numbers of microtubules (per mu m(2) axoplasm) and paracrystals (number encountered along perpendicular axes of the CC) were counted and the size of the paracrystals was measured in cross-sections of the CC, using electron microscopy. Irrespective of the type of additional treatment, significant differences were not observed in the numbers of paracrystals (32+/-10) after VCR-treatment for 5 h. Between 5 and 10 h of incubation the number increased (to 55+/-21) and remained approximately at this level. The number of microtubules decreased during the first 5 h of VCR treatment from 128+/-16 (control) to 37+/-10, and decreased further 19+/-5 between 5 and 10 h of incubation. The size of the paracrystals of all groups treated for 5 h with either VCR or VCR+ORG 2766 were similar (x=1.7 mu m(2)) However, the paracrystals were significantly larger in all groups co-treated for longer periods (10, 15 or 20 h) with ORG 2766 (VCR: x=2.3-3.0 mu m(2); VCR+ORG 2766: x=3.0-3.4 mu m(2)). This indicates that after an initial period of formation of paracrystals, tubulin is added to the existing paracrystals. At the low VCR concentration studied no paracrystals were formed. However, the number of microtubules had decreased significantly (from 159+/-10 to 84+/-17 after 15 h treatment). Pre- as well as pre- and post-treatment with ORG 2766 resulted in microtubule numbers which were not different from control values, indicating the positive effect of ORG 2766. In the groups pre- and/or post-treated with the high concentration of ORG 2766 (especially in the pre-treated group), the remaining number of microtubules was higher than in those co-treated with VCR and ORG 2766, but lower than in the controls. Although translation of the present results into clinical terms is difficult, they suggest that pre-and/or post-treatment with ORG 2766 may have a benificial effect on VCR-induced neurotoxicity. This conclusion is based on the observation that when ORG 2766 is given before VCR administration or after VCR clearance, larger numbers of microtubules were found in the nervous tissue than after treatment with VCR alone.

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