Male gonadotrophic factor in brain and blood of photoperiodically stimulated slugs.

A pulmonate male gonadotrophic factor (MGF) has been described that is released from cerebral ganglia of male-phase slugs (Limax maximus). This factor produces, directly or indirectly, an increase in spermatogonial proliferation as determined by in vivo incorporation of [3H]thymidine into gonadal DNA. In the present investigation MGF activity was demonstrated in saline homogenates of male-phase cerebral ganglia by injecting homogenates into immature slugs for 5 consecutive days and assaying gonadal [3H]thymidine incorporation on Day 7. Dose-response data indicate that daily administration of as little as 0.1 brain equivalent can produce a significant stimulation in incorporation. Comparison of brain homogenates from immature (short-day) and male-phase (long-day) animals has shown that male-phase cerebral ganglia contain substantially more MGF activity than immature ganglia. Similar injection experiments using slug blood plasma showed that activity is present in male-phase blood but not in the blood of short-day immatures. MGF activity in long-day brain homogenates and blood plasma was found to be associated with a molecular weight fraction of 50 to 100 kDa obtained by ultrafiltration. Activity could be reduced or destroyed by treatment with trypsin or by heating. The present findings suggest that MGF is a proteinaceous factor of substantial size. It appears that both the synthesis and the secretion of MFG are stimulated in slugs that are in their male developmental phase as a result of prior exposure to long-day photoperiods.