Heterosynaptic metaplastic regulation of synaptic efficacy in CA1 pyramidal neurons of rat hippocampus

The induction threshold, and the magnitude and direction of changes in synaptic plasticity may depend on the previous history of neuronal activity. This phenomenon, termed "metaplasticity," could play an important role in integration processes by coordinating the modulation of synapses. Although metaplasticity has been analyzed extensively, its underlying cellular mechanisms remain largely unknown. Using in vitro electrophysiological and computer simulation approaches, we investigated the contribution of the slow Ca2+-dependent afterhyperpolarization (sAHP) in the metaplastic control of the induction of long-term potentiation (LTP) at convergent CA3-CA1 pyramidal neuron synapses. We report that classical conditioning protocols may lead to the simultaneous induction of a sustained homosynaptic LTP and a potentiation of the sAHP that endured approximately 1 h. The sAHP potentiation dramatically altered the spike responses of the CA1 pyramidal neuron. Of particular interest was the reduction of the CA1 neuron excitability and, consequently, of the capacity of a nonpotentiated synaptic input to elicit spikes while the sAHP was potentiated. This reduction in excitability temporarily prevented nonpotentiated synaptic inputs to exhibit an LTP induced by presynaptic tetanization. This metaplasticity was strongly resistant to increases in the magnitude of synaptic tetanization protocols. We propose that this heterosynaptic metaplasticity, mediated by intrinsic cellular mechanisms, triggered by brief periods of activity, and relying on changes of a slow Ca2+-activated K+ current, may contribute to adjusting the efficacy of synaptic connections and shaping network behavior to regulate integration processes.