Consequences of synaptic plasticity at inhibitory synapses in mouse hippocampal area CA2 under normal and pathological conditions

The hippocampus is a region of critical importance for memory formation. Recent studies have shown that the long-overlooked hippocampal region CA2 plays a role in certain forms of memory, including social recognition. Furthermore, post-mortem studies of schizophrenic patients have revealed specific changes in area CA2. As yet, the role of CA2 neurons in the hippocampal circuitry remains poorly understood under both normal physiological and pathological conditions. By combining pharmacology, mouse genetics and electrophysiology, we investigated how CA2 pyramidal neurons (PNs) could be recruited in hippocampal circuits in mice hippocampal slices following an activity-dependent change in the strength of their inhibitory inputs. We further investigated how subsequent recruitment of CA2 PNs could modulate hippocampal output. Moreover, we examined the functional alterations of area CA2 in the Df(16)A+/- mouse model of the 22q11.2 microdeletion, a spontaneous chromosomal deletion that is the highest known genetic risk factor for developing schizophrenia. In area CA2, inhibitory synapses exert a powerful control of Schaffer collateral (SC) inputs and undergo a unique long-term depression (iLTD) mediated by delta-opioid receptor (DOR) activation. Unlike SC-CA1 synapses, SC-CA2 excitatory synapses fail to express long-term potentiation after classical induction protocols. However, we found that different patterns of activity persistently increase both the SC and the distal input net excitatory drive onto CA2 PNs via a modulation of the balance between excitation and inhibition. We demonstrated that increases in the excitatory/inhibitory ratio are direct consequences of the DOR-mediated iLTD. Interestingly, we found that the inhibition in area CA2 completely preventing CA3 PNs to activate CA2 PNs, and following iLTD, SC stimulation allows CA2 PNs to fire action potentials. Moreover, the recruitment of CA2 PNs by SC intra-hippocampal inputs after their activity-dependent disinhibition adds a delayed SC-CA2-CA1 response to the SC-CA1 monosynaptic post-synaptic potential (PSP) in CA1 and increases CA1 PN activity. Furthermore, pharmaco-genetic silencing of parvalbumin-expressing interneurons revealed that these inhibitory cells control the PSP amplitude and the firing of CA2 PNs in response to SC stimulation and are necessary for the DOR-mediated increase in excitatory/inhibitory balance between CA3 and CA2. Finally, we found several age-dependent alterations in area CA2 in Df(16)A+/- mouse model of the 22q11.2 microdeletion. These included a reduction in inhibition, an impaired activity-dependent modulation of the excitatory drive between CA3 and CA2 and a more hyperpolarized CA2 PN resting potential. These cellular disruptions may provide a potential mechanism for the social memory impairment that we observe in Df(16)A+/- adult mice. Altogether, our studies highlight the role of CA2 neurons in hippocampal circuitry. To conclude, we postulate that the recruitment of CA2 neurons in neuronal networks underlies key aspects of hippocampal function.