A novel nitric oxide (NO)-dependent ‘molecular switch’ mediates LTP in the Octopus vulgaris brain through persistent activation of nitric oxide synthase (NOS)

Cephalopod mollusks provide a striking example of the independent evolution of advanced cognitive abilities in invertebrates. In octopuses, learning and memory are mediated by the vertical lobe (VL), whose visual input synapses undergo robust activity-dependent long-term potentiation (LTP). Here, we used Octopus vulgaris of either sex to show that persistent activation of nitric oxide synthase (NOS) mediates both LTP expression and maintenance, independently of de novo protein synthesis. Contrary to the prevailing view, NO signaling is not required for LTP induction, as LTP can still be induced in the presence of NOS inhibitors. Remarkably, however, NOS inhibitors blocked presynaptic LTP expression, while NO donors induced only short-term potentiation, together suggesting that LTP expression depends on persistently elevated NO levels. The effective NO concentrations, estimated both pharmacologically and amperometrically, were more than three orders of magnitude higher than those required for activation of the canonical sGC–cGMP pathway, suggesting that NO acts through direct covalent modification of proteins, such as S-nitrosylation. In a subset of synapses, the NOS inhibitor L-NNA rapidly and completely erased established LTP; however, following washout, full potentiation could be reinstated by renewed tetanic stimulation. We therefore propose a ‘molecular switch’ mechanism in which an NO-dependent positive feedback loop persistently reactivates NOS, thereby maintaining LTP. These findings reveal an unprecedented form of convergent evolution in which molecular innovations equip the octopus VL network with the computational properties required for associative learning. Significance Statement Long-term potentiation (LTP) is a central cellular mechanism of learning and memory. While extensively studied in vertebrates, its mechanisms in octopuses—animals that independently evolved sophisticated cognitive abilities—remain poorly understood. Here, we uncover a novel form of LTP involving activity-dependent activation of nitric oxide synthase (NOS) in the postsynaptic cell. The produced nitric oxide (NO) mediates LTP maintenance through persistent NO-dependent reactivation of NOS (a “molecular switch”) and simultaneously mediates LTP expression by retrogradely facilitating presynaptic transmitter release. These findings reveal a striking example of convergent evolution in which distinct molecular mechanisms independently evolved to implement associative (Hebbian) learning rules within a neural network.