Histrionicotoxins: effects on binding of radioligands for sodium, potassium, and calcium channels in brain membranes

Seasonal changes in diet and chemical defense in the Climbing Mantella frog (Mantella laevigata)

Poison frogs acquire chemical defenses from the environment for protection against potential predators. These defensive chemicals are lipophilic alkaloids that are sequestered by poison frogs from dietary arthropods and stored in skin glands. Despite decades of research focusing on identifying poison frog alkaloids, we know relatively little about how environmental variation and subsequent arthropod availability impacts alkaloid loads in poison frogs. We investigated how seasonal environmental variation influences poison frog chemical profiles through changes in the diet of the Climbing Mantella (Mantella laevigata). We collected M. laevigata females on the Nosy Mangabe island reserve in Madagascar during the wet and dry seasons and tested the hypothesis that seasonal differences in rainfall is associated with changes in diet composition and skin alkaloid profiles of M. laevigata. The arthropod diet of each frog was characterized into five groups (i.e. ants, termites, mites, insect larvae, or 'other') using visual identification and cytochrome oxidase 1 DNA barcoding. We found that frog diet differed between the wet and dry seasons, where frogs had a more diverse diet in the wet season and consumed a higher percentage of ants in the dry season. To determine if seasonality was associated with variation in frog defensive chemical composition, we used gas chromatography / mass spectrometry to quantify alkaloids from individual skin samples. Although the assortment of identified alkaloids was similar across seasons, we detected significant differences in the abundance of certain alkaloids, which we hypothesize reflects seasonal variation in the diet of M. laevigata. We suggest that these variations could originate from seasonal changes in either arthropod leaf litter composition or changes in frog behavioral patterns. Although additional studies are needed to understand the consequences of long-term environmental shifts, this work suggests that alkaloid profiles are relatively robust against short-term environmental perturbations.

A Formal Synthesis of (-)-Perhydrohistrionicotoxin Using a Cross Metathesis-Hydrogenation Approach

The development of an efficient, high yielding six-step convergent synthesis of the semisynthetic alkaloid (-)-perhydrohistrionicotoxin is described. The key transformations include the cross metathesis of a Brønsted-acid masked primary homoallylic amine with a vinyl cyclohexenone and a regioselective palladium catalyzed hydrogenation. This sequence generated the advanced Winterfeldt spirocyclic precursor in 47% overall yield, with a longest linear sequence of five steps.

Calcium influx mediated by nicotinic receptors and voltage sensitive calcium channels in SK-N-SH human neuroblastoma cells

When SK-N-SH human neuroblastoma cells were exposed to nicotine (NIC) or KCl they showed a dose-dependent transient increase (2- to 4-fold) in intracellular Ca2+ concentration ([Ca2+])i as detected by quin-2 fluorescence, with half maximal effects (EC50) observed at 13 microM and 26 mM, respectively. Tubocurarine and 1-isodihydrohistrionicotoxin potently blocked the NIC-evoked (IC50 congruent to 1 microM and 0.3 microM, respectively), but not the high [K+]o-evoked [Ca2+]i accumulation. The KCl-induced response was inhibited by verapamil and diltiazem (IC50 = 1.4 and 10.9 microM, respectively). Tetrodotoxin (3 microM) and tetraethylammonium (10 microM) had no effect on [Ca2+]i accumulation induced by either agent. Increases in [Ca2+]i could be evoked sequentially by NIC and KCl in the same cells suggesting independent mechanisms of Ca2+ entry. In a Ca2+-free medium, no response to either KCl or NIC was observed. However, when Ca2+ ions were restored, [Ca2+]i accumulation was enhanced to the same extent as cells suspended in a Ca2+-containing buffer. Long-term (18 hr) pretreatment of SK-N-SH cells with pertussis (100 ng/ml) or cholera toxins (10 nM) had no effect on NIC or KCl-induced [Ca2+]i accumulation. Together, these data demonstrate the presence of NIC receptors and voltage-sensitive Ca2+ channels on SK-N-SH neuroblastoma cells, through which [Ca2+]i may be modulated.

High-affinity and selectivity of neosurugatoxin for the inhibition of 22Na influx via nicotinic receptor-ion channel in cultured bovine adrenal medullary cells: comparative study with histrionicotoxin

In cultured bovine adrenal medullary cells, neosurugatoxin and histrionicotoxin inhibited carbachol-induced influx of 22Na, 45Ca and secretion of catecholamines with IC50 of 27 nM and 3 microM, respectively. The inhibitory effects of neosurugatoxin were reversed by the increased concentrations of carbachol, whereas those of histrionicotoxin were not. Histrionicotoxin at concentrations higher than 10 microM also reduced veratridine-induced influx of 22Na, 45Ca and secretion of catecholamines, while neosurugatoxin had no effects. High K-induced 45Ca influx and catecholamine secretion were not altered by either neosurugatoxin or histrionicotoxin. The present findings suggest (1) neosurugatoxin competitively inhibits nicotinic receptor-ion channel complex at nanomolar concentrations, but has no effects on voltage-dependent Na channel and voltage-dependent Ca channel; (2) histrionicotoxin at micromolar concentrations non-competitively suppresses nicotinic receptor-ion channel complex. Higher concentrations of histrionicotoxin also interferes with voltage-dependent Na channel, but has no effect on voltage-dependent Ca channel; (3) neosurugatoxin, due to its high-affinity and selectivity, may be a useful probe for studying nicotinic receptors in nervous tissues.