Stereochemistry of the Aplysia neuronal 12-lipoxygenase: specific potentiation of FMRFamide action by 12( S)-HPETE

Abstract

Nervous tissue of the marine mollusc, Aplysia californica, generates arachidonic acid metabolites in response to neurotransmitters such as histamine or FMRFamide. In addition, identified neurons of Aplysia respond to the pharmacologic application of some of these products, particularly those of the 12-lipoxygenase pathway. We investigated the chirality of the initial Aplysia 12-lipoxygenase product, 12-HPETE, in preparation for more detailed metabolic studies and for the analysis of the physiological activity of the endogenous lipid. Neural homogenates and intact ganglia exclusively generate 12( S)-HPETE as do the better characterized mammalian lipoxygenases. The direct application of 12( S)-HPETE to cultured sensory neurons induced a hyperpolarization which averaged 2.6 mV. We did not find any difference between the response to the naturally-occurring 12( S)-HPETE and its diastereomer, 12( R)-HPETE which is not generated in Aplysia. Both isomers were significantly more effective than 15( S)-HPETE. In contrast, 12( S)-HPETE, but not 12( R)-HPETE, was a potent modulator of the action of the molluscan neuropeptide, FMRFamide. Prior application of 12( S)-HPETE to cultured sensory neurons increased the subsequent response to a submaximal dose of FMRFamide by 60%. On the other hand, 12( R)-HPETE reduced the subsequent response to the peptide by 30%. The lack of stereospecificity in the direct effect of the lipids differs markedly from their stereospecific effects as modulators of FMRFamide action. This suggests that there may be an important neurophysiologic role for these lipid modulators which is distinct from their direct effects, and also indicates that there are multiple sites and mechanisms by which lipid hydroperoxides act on neurons in Aplysia.