‘On‐pituitary’ injection: a novel method for the in vivo investigation of secretory function of the pituitary gland in the common carp, Cyprinus carpio L.

. A new means of in vivo drug administration was developed in order to reach the direct environment of the pituitary gland of common carp, Cyprinus carpio L., without any surgery. This permits decreased dosage of rhe drugs and (heir possible side effects. Injections‘on the pituitary'(o.pit.) were done with a 20G × 23/16 inch needle attached to a microsyringe. The needle, introduced under the eyeball, followed the route of the nervus opticus. The effects of different doses of D-Ala 6 LHRH (GnRH-A) injected o.pit. were compared with those of intraperitoneal (i.p) injection. It was found that a dose of 4mu;g/kg of GnRH-A injected i.p. significantly stimulated GtH2 release starting at 15min post injection, whereas o.pit. injection had no effect. A ten times lower dose of GnRH-A (0.4mu;g/kg) injected o.pit. provoked a significant increase of GtH2 secretion in comparison with i.p. injection at 10 and 15min post injection. Starting from 25min after injection no differences were found between these two groups, however GtH2 levels in these two groups were still significantly higher than in their controls. I.p. injection of 0·04pmu; g/kg of GnRH-A caused no significant rise in GtH2 secretion at any sampling time, whereas o.pit. injection evoked a significant rise in GtH2 secretion in comparison with all the other groups at 10 and 15 min post injection. Our results show that using this new method, the dosage of injected drugs can be highly decreased, down to the level which is ineffective after classic i.p. administration. This is also of economic importance because of the cost of certain drugs.