Comparative study of mechanism of antiparasitic and toxic action of gabaergic and cholinergic antihelmintics

GABAergic and cholinergic systems in the neuromuscular system of the parasitic nematodes are the main target of action for anthelmintic drugs. In the nematode cholinergic system pharmacological importance has primarily nicotinic-acetylcholine receptor (nAChR). On the other hand, the GABA-receptor was originally identified as the main site of action for the Avermectins and Milbemicins. However, later was shown that these drugs work on a new, previously never described glutamate-gated chloride ion channel, located in the nematode pharynx. The most important problems that are endangering the success of antiparasitic therapy are the development of resistance in the parasites and often occurrences of the toxic effects of antiparasitic drugs in host. In the order to elucidate the mechanisms of action of GABAergičkih and cholinergic anthelmintic, we examined the pharmacological characteristics of the representatives of these two groups of drugs in the isolated nerve-muscle preparation of large pig nematode Ascaris suum. Also, it was important to examine comparatively differences in the effects of cholinergic and GABAergic anthelmintic on the corresponding receptors in mammals (investigations were performed on isolated rat diaphragm and ileum), and thus to analyze the mechanisms of their adverse effects. In presented investigations we measured the effects of contraction or relaxation on isolated nematode or mammalian preparations and analyzed the results by using appropriate statistical methods (non-linear regression, ANOVA, t-test). Based on the results obtained in our investigations the following conclusions may be presented: (1) Agonists of L, N and B type of nematode nicotinic acetylcholine receptor (nAChR) studied on a model of neuromuscular preparations of A. suum, showed variable efficacy. Highest efficiency in the first group of tested agonists has pyrantel (agonist of L-type nAChR, EC50=0.010 μM, Emax=2.5g), then bephenium (agonist of B-type nAChR, EC50=0.37μM, Ema
Gabaergički i holinergički sistem u neuro-mišićnom sistemu parazitskih nematoda su glavna ciljna mesta delovanja antihelmintika. U okviru holinergičkog sistema nematoda, farmakološki značaj ima pre svega nikotinski-acetilholinski receptor (nAChR). S druge strane, GABA-receptor parazitskih nematoda je prvobitno označen kao glavno mesto dejstva avermektina i milbemicina. Međutim, vremenom se pokazalo da ovi lekovi deluju i na jedan potpuno nov, do tada neopisani glutamat-zavisni hloridni jonski kanal u farinksu nematoda. Pored toga, značajno je da neki aktivni principi etarskih ulja imaju dokazana antiparazitska svojstva, i da postoje indicije da deluju upravo preko ova dva receptorska sistema. Osnovni problemi koji danas ugrožavaju antiparazitsku terapiju su razvoj rezistencije i često ispoljavanje toksičnih efekata antiparazitskih lekova. Da bi se bolje razumeli mehanizmi dejstva gabaergičkih i holinergičkih antihelmintika ispitali smo farmakološke karakteristike predstavnika ove dve grupe lekova na neuro-mišićnom preparatu velike nematode svinja Ascaris suum. Takođe, značajno je bilo komparativno ispitati razlike u dejstvu gabaergičkih i holinergičkih antihelmintika na odgovarajuće receptore sisara (ispitivanja su izvršena na izolovanoj dijafragmi i ileumu pacova) i na taj način analizirati mehanizme njihovih neželjenih efekata. U našim istražvanjima mereni su efekti kontrakcije ili relaksacije izolovanih preparata i odgovarajućim statističkim metodama obrađivani dobijeni rezultati (nelinearna regresija, ANOVA, t-tets). Na osnovu rezultata dobijenih u ispitivanjima doneti su sledeći zaključci: (1) Agonisti L, N i B tipa nikotinskog-acetilholinskog receptora (nAChR) nematoda, ispitivani na modelu neuro-mišićnog preparata A. suum, ispoljili su različitu efikasnost. Najvišu efikasnost u prvoj grupi ispitivanih agonista, ispoljio je pirantel (agonista L-tipa nikotinskog receptora EC50=0.010μM, Emax=2.5g), zatim befinijum (agonista B-tipa nikotinskog receptora EC50=0.37