Plasticity at hippocampal to prefrontal cortex synapses is impaired by loss of dopamine and stress: Importance for psychiatric diseases

The direct hippocampal to prefrontal cortex pathway and its changes in synaptic plasticity is a useful framework for investigating the functional operations of hippocampal-prefrontal cortex communication in cognitive functions. Synapses on this pathway are modifiable and synaptic strength can be turned up or down depending on specific patterns of activity in the pathway. The objective of this review will be to summarize the different studies carried out on this topic including very recent data and to underline the importance of animal models for the development of new and effective medications in psychiatric diseases. We have shown that long-term potentiation (LTP) of hippocampal-prefrontal synapses is driven by the level of mesocortical dopaminergic (DA) activity and more recently that stress is also an environmental determinant of LTP at these cortical synapses. Stimulation of the ventral tegmental area at a frequency known to evoke DA overflow in the prefrontal cortex produces a long-lasting enhancement of the magnitude of hippocampal-prefrontal cortex LTP whereas a depletion of cortical DA levels generates a dramatic decrease in this LTP. Moreover, hippocampal stimulation induces a transient but significant increase in DA release in the prefrontal cortex, and an optimal level of D, receptor activation is essential for LTP expression. We recently investigated the impact of stress on hippocampal-prefrontal LTP and demonstrated that exposure to an acute stress causes a remarkable and long-lasting inhibition of LTP. Furthermore, we demonstrated that tianeptine, an antidepressant which has a unique mode of action, and clozapine, an atypical antipsychotic when administered at doses normally used in human testing, are able to reverse the impairment in LTP. Stressful life events have a substantial causal association with psychiatric disorders like schizophrenia and depression and recent imaging studies have shown an important role of the limbic-cortical circuit in the pathophysiology of these illnesses. Therefore, we proposed that agents capable of reversing the impairment of plasticity at hippocampal to prefrontal cortex synapses have the potential of becoming new therapeutic classes of antidepressant or antipsychotic drugs.