Abstract
In rodent cerebral cortex, vasoactive intestinal peptide (VIP) is contained in a homogeneous population of radially oriented bipolar interneurons. We have observed that 4-aminopyridine (4-AP), a K+-channel blocker, promotes a concentration- and Ca2+-dependent release of VIP from mouse cerebral cortical slices, with a significant effect already observed at 50 microM. Over 70% of VIP release elicited by 4-AP is blocked by 2 microM tetrodotoxin (TTX). Mepacrine, an inhibitor of phospholipase A2 (PLA2) activity and hence of arachidonic acid (AA) formation from membrane phospholipids, markedly inhibits (IC50 of approximately 15 microM) the release of VIP evoked by 4-AP. The inhibitory effect of mepacrine is not additive to that of TTX, thus indicating an involvement of PLA2 activation in the TTX-sensitive component of the 4-AP-evoked release. As a corollary, melittin (0.1-10 micrograms/ml), a PLA2 activator, promotes VIP release. Inhibition of AA metabolites of the lipoxygenase pathway by nordihydroguaiaretic acid, 5,8,11,14-eicosatetranoic acid, and caffeic acid results in a concentration-dependent inhibition of VIP release evoked by 4-AP. This set of observations indicates for the first time that the formation of AA metabolites of the lipoxygenase pathway plays a role in the release of a peptide in the mammalian CNS. Furthermore, these observations together with the previously reported potentiation by prostaglandins of the increase in cyclic AMP elicited by VIP in mouse cerebral cortex (Schaad et al., 1987) indicate that AA metabolites may act at both the presynaptic (lipoxygenase metabolites) and the postsynaptic (cyclooxygenase metabolites) levels to increase the "throughput" or "strength" of VIP-containing circuits in the rodent neocortex.