Insect larvae contain substances toxic to adults: the discovery of paralysins.

Acidic methanolic extracts of larvae of nine different insect species were found to contain substances that cause a lethal effect in the adult stage of the same species and of other species. These endogenous toxic substances, apparently being widely spread over the class of insects, were designated as paralysins, because of their immediate and observable paralytic effect upon injection. The developmental concentration curves of five different species of insects (Galleria mellonella (Lepidoptera), Neobellieria bullata (Diptera), Spodoptera frugiperda (Lepidoptera), Tenebrio molitor (Coleoptera) and Schistocerca gregaria (Orthoptera) indicate that the toxins are not present throughout all the developmental stages in the same concentration. The strongest paralytic activity was found in late instar larvae or in the early pupal stage. The temporal distribution of paralysins during development suggests that they might be involved in metamorphosis.

Evidence of a thyrotropin-releasing activity of ovine corticotropin-releasing factor in the domestic fowl (Gallus domesticus).

Ovine corticotropin-releasing factor (oCRF) administered to 19-day-old chicken embryos (E19) increased plasma concentration of pituitary glycoprotein alpha-subunit concentrations within 15 min for at least 4 hr. Follicle stimulating hormone levels were unchanged, while plasma luteinizing hormone concentrations only began to increase 1 hr after the oCRF treatment. Calculation of circulating thyrotropin (TSH) indicator values revealed a rapid elevation in TSH plasma levels following oCRF. Concentrations of thyroxine (T4), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3), and corticosterone increased from 1 hr postinjection. Hypothalamic outer ring deiodinating type II increased and hepatic inner ring deiodinating type III fell after 2 and 4 hr, explaining at least in part the plasma T3 increase at the end of the experiment. In a second experiment, using E18 chicks, a comparison was made between the effects of a single injection of 2 micrograms oCRF and 20 mlU bovine TSH. Both hormones increased T4, T3, and rT3 plasma concentrations, supporting the hypothesis of a TSH-releasing activity for oCRF in the embryonic chicken. The proposed TSH-mediated effect of CRF on thyroid function was further confirmed in two in vitro experiments in which oCRF did not directly influence the thyroidal T4 secretion but, when applied to pituitaries, clearly increased the alpha-subunit release. In chickens CRF is concluded to not only control the adrenal axis, but also to participate in the coordination of avian TSH release.

Plasma thyroid hormone levels and iodothyronine deiodinase activity following an acute glucocorticoid challenge in embryonic compared with posthatch chickens.

Embryonic chickens (Day 18 of incubation) and 8-day-old posthatch chicks were subjected to an acute glucocorticoid challenge by a single iv injection of corticosterone (B), dexamethasone, or porcine adrenocorticotropin. Plasma samples were analyzed for changes in T4, T3, rT3, alpha-subunit, LH, GH, and B levels; iodothyronine deiodinase activity was measured in liver, kidney, and hypothalamus at several time points after injection. The effects of the different treatments were broadly similar within one age group, but differed clearly between pre- and post-hatch animals. In 18-day-old embryos glucocorticoids increased plasma T3 and decreased plasma T4, rT3, and the calculated TSH index, within hours after injection. These changes were accompanied by an immediate (1-4-24 hr after injection) decrease in hepatic inner ring deiodinating type III enzyme (IRD-III) activity and a delayed (24-48 hr after injection) increase in hepatic outer ring deiodinating type I enzyme (ORD-1) activity. Glucocorticoid challenge in 8-day-old chicks similarly decreased plasma T4 and the TSH index but tended to also lower plasma T3. Hepatic ORD-I activity decreased within 1 hr after injection, while the already very low hepatic IRD-III activity was for the most part unaffected. Acute increases in glucocorticoids lower thyroidal T4 secretion in both pre- and posthatch chickens but have clearly different effects on peripheral thyroid hormone deiodination and hence on circulating T3 at both developmental stages.