Department of Pharmacology, The School of Pharmacy, University of London, London, UK
The CA2 subfield, if considered at all, has been seen as a transitional zone between CA1 and CA3. It is, however, the only CA region receiving direct input from the supramammilary body, indicating a role for this subfield in the generation of theta rhythms. Moreover, its direct inputs from entorhinal cortex and amygdala can directly activate CA1 independently of the CA3 subfield and in parallel with the trisynaptic pathway. In addition, CA2 is resistant to temporal lobe epilepsy, but exhibits particular interneuronal loss early in the onset of schizophrenia. The most thoroughly studied CA2 interneurones, the basket and bistratified cells, display striking features that are quite distinct from those of their equivalents in CA1, ie. spiny horizontal dendrites, a pronounced ”sag”, indicative of Ih, in response to hyperpolarising current injection and they receive facilitating EPSPs from neighbouring pyramidal cells. Dual intracellular recordings were performed in CA2 to investigate the contribution made by Ih to the electrophysiological and synaptic properties of these cells. The ”sag” seen during negative current injection in CA2 interneurones was completely blocked after 25 min exposure to the Ih blocker ZD7288 and hyperpolarised the cells, resulting in an increase in spike threshold. ZD7288 produced only a small decrease in the amplitude and an increase in the duration of EPSPs, and however no change in paired pulse ratios. CA2 interneurones also presented a higher level of spontaneous synaptic activity after application of ZD7288.