Seasonal variation of long-term potentiation at a central synapse in the medicinal leech.

Long-term potentiation (LTP) is a persistent increase in synaptic transmission that is thought to contribute to a variety of adaptive processes including learning and memory. Although learning is known to undergo circannual variations, it is not known whether LTP undergoes similar changes despite the importance of LTP in learning and memory. Here we report that synapses in the CNS of the medicinal leech demonstrate seasonal variation in the capacity to undergo LTP following paired presynaptic and postsynaptic stimulation. LTP was observed during the April-October period, but no LTP was observed during the November-March period. Application of forskolin, a technique often used to produce chemical LTP, failed to elicit potentiation during the November-March period. Implementing stimulation patterns that normally result in long term depression (LTD) also failed to elicit any change in synaptic strength during the November-March period. These experiments indicate that LTP and LTD can be influenced by circannual rhythms and also suggest a seasonal influence on learning and memory.