MUSCARINIC CHOLINERGIC AGONISTS STIMULATE ARACHIDONIC-ACID RELEASE FROM MOUSE STRIATAL NEURONS IN PRIMARY CULTURE

In cultured striatal neurons from embryonic mice, carbachol was found to stimulate the release of arachidonic acid (AA) EC(50) = 87 mu M) and formation of inositol phosphates (IPs) (EC(50) = 54 mu M). Both responses were reproduced by muscarinic but not nicotinic agonists, and both exhibited the same pharmacological profile toward four muscarinic antagonists. Furthermore, both responses were insensitive to pertussis toxin, providing additional evidence for the involvement of the same muscarinic receptor(s), most probably of the m1 subtype. Both carbachol-evoked responses were also highly sensitive to the presence of external calcium. The calcium ionophore ionomycin, ineffective alone on AA release, strongly potentiated the carbachol response. In contrast, ionomycin alone stimulated the formation of IPs but did not significantly modify the carbachol response. Protein kinase C activation positively regulated the carbachol-evoked release of AA because this response was markedly potentiated by phorbol 12-myristate 13-acetate (PMA) and was abolished by sphingosine and Ro 31-8220. In contrast, PMA markedly inhibited the carbachol-evoked formation of IPs. The carbachol-evoked release of AA was not mimicked by the combined applications of ionomycin and PMA, which suggests that phospholipase C stimulation alone is not sufficient to trigger AA release. Taken together, these results suggest that the coupling of m1 receptors to a putative phospholipase A2 that is positively regulated by protein kinase C and by calcium is necessary for the carbachol-evoked release of AA. Finally, a cyclic AMP-dependent process could also regulate the carbachol-evoked release of AA, because this response was potentiated by 8-bromocyclic AMP, by isoproterenol and by pretreatment of the cells with cholera toxin.