LAUSR

Retinoic acid, the active metabolite of vitamin A, is important for vertebrate cognition and hippocampal plasticity, but few studies have examined its role in invertebrate learning and memory, and its actions in the invertebrate CNS are currently unknown. Using the mollusc Lymnaea stagnalis, we examined operant conditioning of the respiratory behaviour, controlled by a well-defined central pattern generator (CPG), and used citral to inhibit retinoic acid signaling. Both citral and vehicle-treated animals showed normal learning, but citral-treated animals failed to exhibit long-term memory at 24 hrs. Cohorts of citral or vehicle-treated animals were dissected into semi-intact preparations, either 1 hr after training, or after the memory-test 24 hrs later. Simultaneous electrophysiological recordings from the CPG pacemaker cell (RPeD1) and an identified motorneuron (VI) were made whilst monitoring respiratory activity (pneumostome opening). Activity of the CPG pneumostome opener interneuron (IP3) was also monitored indirectly. Vehicle-treated conditioned preparations showed significant changes in network parameters immediately after learning, such as reduced motorneuron bursting activity (from IP3 input), delayed pneumostome opening, and a decoupling of coincident IP3 input within the network. However, citral-treated preparations failed to exhibit these network changes and more closely resembled naïve preparations. Importantly, these citral-induced differences were manifested immediately after training and before any overt changes in the behavioural response (memory impairment). These studies shed light on where and when retinoid signaling might affect a central pattern generating network to promote memory formation during conditioning of a homeostatic behaviour.