The hippocampal cornu ammonis 2 (CA2) region is unique in being the only CA region receiving inputs from the hypothalamic supramammillary nucleus, of importance in modulating hippocampal theta rhythm, and is seizure resistant in temporal lobe epilepsy. CA2 has, however, been little studied, possibly because of its small size and difficulty encountered in defining its borders. To investigate the properties of CA2 interneurons, intracellular recordings with biocytin filling were made in adult hippocampal slices. Two types of basket cells were identified. A minority resembled those in CA1, with fast spiking behavior, vertically oriented dendrites, and axons confined to the region of origin. In contrast, the majority of parvalbumin-immunopositive CA2 basket and bistratified cells had long, horizontally oriented, sparsely spiny dendrites extending into all CA subfields in stratum oriens, adapting firing patterns and a pronounced "sag" in voltage responses to hyperpolarizing current, indicative of I h . Broad CA2 basket cells innervated all three CA subfields and could thus provide CA1 and CA2 with feedforward and CA3 with feedback inhibition. In contrast, CA2 bistratified cell axons displayed striking subfield preference, innervating stratum oriens and stratum radiatum of CA2 and CA1 but stopping abruptly at the CA2/CA3 border, implying feedforward inhibition of CA2 and CA1. These unique features suggest that CA2 is more than a transitional region between CA1 and CA3. The pronounced slow sag current of many CA2 interneurons may contribute to coordination of pyramidal cell firing during theta, whereas the fast spiking behavior of a smaller population of interneurons supports more localized gamma.

There is a growing recognition that the CA2 region of the hippocampus has its own distinctive properties, inputs, and pathologies. The dendritic and axonal patterns of some interneurons in this region are also strikingly different from those described previously in CA1 and CA3. The local circuitry in this region, however, had yet to be studied in detail. Accordingly, using dual intracellular recordings and biocytin-filling, excitatory and inhibitory connections involving CA2 parvalbumin-positive basket cells were characterized for the first time. CA2 basket cells targeted neighboring pyramidal cells and received excitatory inputs from them. CA2 basket cells that resembled those in CA1 with a fast spiking behavior and dendritic tree confined to the region of origin received depressing excitatory postsynaptic potentials (EPSPs). In contrast, unlike CA1 basket cells but like CA1 Oriens-Lacunosum Moleculare (OLM) cells, the majority of CA2 basket cells had horizontally oriented dendrites in Stratum Oriens (SO), which extended into all three CA subfields, had an adapting firing pattern, presented a "sag" in their voltage responses to hyperpolarizing current injection, and received facilitating EPSPs. The expression of I(h) did not influence the EPSP time courses and paired pulse ratios (PPR). Estimates of the probability of release (p) for the depressing and facilitating EPSPs were correlated with the PPR. Connections with low probabilities of release had higher PPR. Quantal amplitude (q) for the facilitating connections was larger than q at depressing inputs onto fast spiking basket cells.