Intracellular recordings were obtained from pyramidal, granule and hilar cells in transverse slices of guinea-pig hippocampus to examine synaptic interactions between GABAergic neurones. 2. In the presence of the convulsant compound 4-aminopyridine (4-AP), after fast excitatory amino acid (EAA) neurotransmission was blocked pharmacologically, large amplitude inhibitory postsynaptic potentials (IPSPs) occurred rhythmically (every 4-8 s) and synchronously in all principal cell populations (triphasic synchronized IPSPs). In the presence of the GABAA receptor blocker picrotoxin (PTX), a large amplitude IPSP continued to occur spontaneously in all principal cells simultaneously (monophasic synchronized IPSP). 3. Burst firing occurred simultaneously in a group of hilar neurones (synchronized bursting neurones) coincident with triphasic synchronized IPSPs in principal cells. After PTX was added, the bursts and the underlying depolarizing synaptic potentials were completely suppressed in some of the synchronized bursting neurones (type I hilar neurones), while others (type II hilar neurones) continued to fire in bursts coincident with monophasic synchronized IPSPs in principal cells. Intense hyperpolarization blocked burst firing and revealed underlying attenuated spikes of less than 10 mV, but did not uncover any underlying depolarizing synaptic potentials. 4. In type II hilar neurones, during sufficient hyperpolarization, spontaneous activity consisted of attenuated spikes. With depolarization, the small spikes began to trigger full size action potentials. These data suggest the presence of electrotonically remote spike initiation sites. 5. The morphology of synchronized bursting neurones was revealed by intracellular injection of the fluorescent dye Lucifer Yellow. Attempts to inject dye into one type II hilar neurone often resulted in the labelling of two to four cells (dye coupling). Dye coupling was not observed in type I hilar neurones. 6. These findings indicate that excitatory interactions synchronizing the firing of GABAergic neurones can occur in the absence of fast EAA neurotransmission. GABAergic neurones can become synchronized via their recurrent collaterals through the depolarizing action of synaptically activated GABAA receptors. In addition, a subpopulation of GABAergic neurones can become synchronized by a mechanism probably involving electrotonic coupling.
