Real-time imaging of hippocampal network dynamics reveals trisynaptic induction of CA1 LTP and "circuit-level" effects of chronic stress and antidepressants

Today’s pervasive presence of stress renders stress-related psychiatric disorders (SRPDs), a relevant global health problem. Memory impairment is a major symptom likely mediated by the hippocampus (HIP), a limbic brain region highly vulnerable to stress. Recent evidence suggests that information processing problems within specific neuronal networks might underlie SRPDs. However, the precise functional neurocircuitry that mediates hippocampal CA1 long-term potentiation (LTP), a putative correlate of mammalian learning and memory, remains unknown at present. Furthermore, valuable assays for studying stress and drug effects on polysynaptic activity flow through the classical input/output circuit of the HIP are missing. To engage a circuit-centered approach, voltage-sensitive dye imaging was applied in mouse brain slices. Single pulse entorhinal cortex (EC) to dentate gyrus (DG) input, evoked by perforant path stimulation, entailed strong neuronal activity in the DG, but no distinct neuronal activity in the CA3 and CA1 subfield of the HIP. In contrast, a thetafrequency (5 Hz) stimulus train induced waves of neuronal activity percolating through the entire hippocampal trisynaptic circuit (HTC-waves). Spatially restricted blocking of glutamate release at CA3 mossy fiber synapses caused a complete disappearance of HTC-waves, suggesting frequency facilitation at DG to CA3 synapses the pivotal gating mechanism. In turn, non-theta frequency stimulations (0.2/1/20 Hz) proved much less effective at generating HTC-waves. CA1 long-term potentiation (CA1 LTP) is the best understood form of synaptic plasticity in the brain, but predominantly at the monosynaptic level. Here, HTC-waves comprise high-frequency firing of CA3 pyramidal neurons (>100 Hz), inducing NMDA receptordependent CA1 LTP within a few seconds. Detailed examination revealed the existence of an induction threshold for LTP. Consequently, baseline recordings with a reduced number of HTC-waves were carried out to test the effects of memory enhancing drugs and HPA axis hormones on hippocampal network dynamics. Bath application of caffeine (5 mM), corticosterone (100 nM) and corticotropin-releasing hormone (5 & 50 nM) rapidly boosted HTC-waves. Cognitive processes taking place within the HIP are challenged by stress exposure, but whether and how chronic stress shapes "net" neuronal activity flow through the HIP remains elusive. The HTC-wave assay, refined for group comparisons, revealed that chronic stress markedly lowers the strength of evoked neuronal activity propagation through the hippocampal trisynaptic circuit. In contrast, antidepressants (ADs) of several classes, the mood stabilizer lithium, the anesthetic ketamine, and the neurotrophin brainderived neurotrophic factor amplified HTC-waves. An opposite effect was obtained with the antipsychotic haloperidol and the anxiolytic diazepam. The tested ADs exert this effect at low micromolar concentrations, but not at 100 nM, and nearly always, also not at 500 nM. Furthermore, the AD fluoxetine was found to facilitate LTP of HTC-waves. Finally, pharmacological blockade of the tyrosine-related kinase B receptor abolished fluoxetine effects on HTC-waves. These results highlight a circuit-centered approach suggesting evoked synchronous theta rhythmical firing of EC principal cells as a valuable tool to investigate several aspects of neuronal activity flow through the HIP. The physiological relevance is emphasized by the finding that the resulting HTC-waves, which likely occur during EC theta oscillations, evoke NMDA receptor-dependent CA1 LTP within a few seconds. Furthermore, HTC-waves allow to integrate molecular, cellular and structural adaptations in the HIP, pointing to a monoaminergic neurotransmission-independent, "circuit-level" mechanism of ADs, to balance the detrimental effects of chronic stress on HIP-dependent cognitive abilities.