Neuronal diversity in the subiculum: correlations with the effects of somatostatin on intrinsic properties and on GABA-mediated IPSPs in vitro.

1. We used intracellular current-clamp techniques to record from 33 ventral subicular neurons in slices or rat hippocampal formation. Presumed pyramidal neurons were characterized by their responses to depolarizing current pulses as either intrinsically burst firing (IB) or regular spiking (RS). Within the subiculum, IB cells were encountered most frequently in the deep cell layer, whereas RS cells were encountered most frequently in the superficial cell layer. IB cells had more depolarized resting potentials, lower input resistances, and more sag in their voltage responses to hyperpolarizing current pulses. 2. Somatostatin (5 microM) applied in the bathing medium caused a hyperpolarization and reduction in input resistance. These effects were of greater magnitude in IB cells. Somatostatin had no effect on sag in either cell type. These effects of somatostatin were unchanged in the presence of gamma-aminobutyric acid (GABA) receptor antagonists. 3. In a series of experiments conducted in RS cells only, somatostatin reduced the amplitude of the late but not the early component of evoked biphasic inhibitory postsynaptic potentials (IPSPs). 4. A second series of experiments was conducted in RS and IB cells. Somatostatin reduced the amplitude of pharmacologically isolated GABAA IPSPS in both cell types. In IB cells but not RS cells there was a correlation between this effect and the somatostatin-induced hyperpolarization. Somatostatin also reduced the amplitude of isolated GABAB IPSPS in both cell types, but more so in IB cells. 5. Somatostatin had no effect on the reversal potential of either IPSP in either cell type and no effect on the GABAA-mediated conductance in either cell type. In contrast, the GABAB-mediated conductance was reduced, especially in IB cells. 6. The effects of somatostatin on GABAA IPSPS are principally a result of membrane shunting and reductions in ionic driving force, but these mechanisms do not account for the reduction in GABAB IPSPS. 7. We suggest that the combined effects of somatostatin are likely to alter the balance between fast and slow inhibition and to do so more in IB cells than in RS cells.