FMRFamide reverses protein phosphorylation produced by 5-HT and cAMP in Aplysia sensory neurons

Abstract

NEUROTRANSMITTERS can modulate neuronal activity through a variety of second messengers that act on ion channels and other substrate proteins^1–3. The most commonly described effector mechanism for second messengers in neurons depends on protein phosphorylation mediated by one of three sets of kinases: the cyclic AMP-dependent protein kinases^4,5, the Ca²⁺–calmodulin-dependent protein kinases^3,6, and the Ca²⁺–phospholipid-dependent protein kinases⁷. In addition, some neurotransmitters and second messengers can also inhibit protein phosphorylation by lowering cAMP levels (either by inhibiting adenylyl cyclase⁸ or activating phosphodiesterases⁹). This raises the question: can neurotransmitters also modulate neuronal activity by decreasing protein phosphorylation that is independent of cAMP? Various biochemical experiments show that a decrease in protein phosphorylation can arise through activation of a phosphatase^10,11 or inhibition of kinases^12,13. In none of these cases, however, is the physiological role for the decrease in protein phosphorylation known. Here we report that in Aplysia sensory neurons, the presynaptic inhibitory transmitter FMRFamide decreases the resting levels of protein phosphorylation without altering the level of cAMP. Furthermore, FMRFamide overrides the cAMP-mediated enhancement of transmitter release produced by 5-hydroxytryptamine (5-HT), and concomitantly reverses the cAMP-dependent increase in protein phosphorylation produced by 5-HT. These findings indicate that a receptor-mediated decrease in protein phosphorylation may play an important part in the modulation of neurotransmitter release.