Mercer, Audrey; (2005) Hippocampal circuitry and characterisation of interneurones in the CA2 subfield of the rat hippocampus. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The CA2 subfield of the hippocampus has been, so far, largely ignored or taken together with the CA3 region. This region, unlike the other CA regions, receives inputs from neurones of the supramammillary nucleus of the hypothalamus which are known to fire phase locked to the hippocampal theta activity. The CA2 region may, therefore, be involved in the modulation of theta oscillations and play an important role in the hippocampal circuitry. Dual intracellular recordings and biocytin filling were performed in the CA2 subfield of the adult rat hippocampus to study synaptic connections involving two types of morphologically characterised interneurones (basket and bistratified cells). Like CA1 basket cells, one subset of CA2 basket cells whose dendrites and axons were confined to the CA2 region were classical fast spiking cells which received depressing EPSPs from neighbouring pyramidal cells. In contrast, unlike CA1 basket cells but like CA1 OLM cells, the majority of CA2 basket cells had horizontally oriented dendrites which extended into all three CA subfields, had an adapting firing pattern, presented a "sag", indicative of Ih, in their voltage responses to hyperpolarising current injection and received facilitating EPSPs. CA2 bistratified cells displayed similar synaptic responses. Those that had narrow dendritic arbours confined to CA2 received depressing EPSPs while those that had dendrites extending into all three subfields received facilitating EPSPs. These anatomical and electrophysiological data reveal features of CA2 interneurones that are quite distinct from those described in CA1 in this and previous studies and suggest that CA2 interneurones that extend across subfields may perform a unique function. In addition, intracellular recordings in the CA1 region demonstrated convincing evidence for direct electrical coupling between pyramidal cells that is thought to underlie hippocampal oscillations.

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