ACTION OF RESIN ACIDS IN NERVE ENDING FRACTIONS-ISOLATED FROM FISH CENTRAL-NERVOUS-SYSTEM

The actions of three resin acids (abietic acid, dehydro abietic acid, and 12,14-dichlorodehydroabietic acid) were investigated using synapto somes isolated from the whole brain of the rainbow trout (Oncorhynchus mykiss). Exposure of synaptosomes to resin acids caused a concentrati on-dependent increase in synaptosomal free [Ca2+] that was unaffected by tetrodotoxin; 12,14-Dichlorodehydroabietic acid (EC50 = 12 mu M) wa s five- to sixfold more potent as a calcium mobilizing agent compared to its nonchlorinated analog dehydroabietic acid (IC50 = 68 mu M) and maximum increases in synaptosomal free [Ca2+] were estimated at approx imately 325 nM and 260 nM, respectively. Although closer to dehydroabi etic acid in potency, abietic acid was more efficacious, increasing sy naptosomal free [Ca2+] by 500 nM at 133 mu M. The requirement for extr acellular Ca2+ was greatest for dehydroabietic acid compared to abieti c acid or 12,14-dichlorodehydroabietic acid; however, none of the stud y compounds was capable of stimulating Ca-45(2+) uptake or reducing Ca -45(2+) efflux from synaptosomes. For 12,14-dichlorodehydroabietic and dehydroabietic acids only, the rises in synaptosomal free [Ca2+] were accompanied by significant reductions in synaptosomal adenosine triph osphate levels and increases in synaptosomal oxygen consumption. Elect ron microscopic studies have shown that the vesicular content of synap tosomes was dramatically reduced by treatment, with all three study co mpounds. Our investigation suggests that resin acids have a strong cal cium mobilizing action in nerve endings isolated from fish brain and c hlorination increases potency. We postulate that resin acids mobilize Ca2+ from intracellular stores to facilitate neurotransmitter release and that for dehydroabietic acid (and to a lesser extent for abietic a nd 12,14-dichlorodehydroabietic acids), extracellular calcium is requi red for efficient penetration of the plasma membrane. The bioenergetic and respiratory disruption we observe is not closely linked to change s in cytosolic free [Ca2+].