INHIBITORY ROLE OF DENTATE HILUS NEURONS IN GUINEA-PIG HIPPOCAMPAL SLICE

1. Current and voltage-clamp recordings of CA3/CA4 pyramidal neurons, hilar neurons, and granule cells or pairs of these neurons were used to study the generation of Cl-dependent and K-dependent inhibitory postsynaptic potentials (IPSPs) in the guinea pig hippocampal slice preparation. 2. A sequence of an early Cl-dependent and a late K-dependent IPSP was evoked in CA3 neurons by electrical stimulation from the stratum moleculare of the dentate gyrus, the hilus, and the stratum oriens/alveus. Blockade of glutamatergic excitation by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 μm) and D(-)-2-amino-5-phosphonovaleric acid (APV, 30 μM) abolished IPSPs evoked from the stratum moleculare of the dentate gyrus, but IPSPs could still be evoked from the hilus and the stratum oriens/alveus. 3. Repetitive giant IPSPs, which consisted of Cl-dependent and K-dependent components, were evoked by bath application of 4-aminopyridine (4-AP, 10-50 μM) in CA3 neurons and in granule cells. Giant IPSPs were blocked by bath-applied tetrodotoxin (TTX). In addition, 4-AP hyperpolarized CA3 neurons in a Cl-dependent and picrotoxin-sensitive way. 4. Focal application of TTX to the dentate gyrus or the hilus considerably reduced the amplitude of giant IPSPs evoked by 4-AP in CA3 neurons. In hilar neurons, 4-AP evoked repetitive bursts, eventually, but not necessarily intermigled with giant IPSPs. Bursts were observed in hilar neurons in presence as well as absence of CNQX and APV. 5. In paired recordings, bursts in hilar neurons induced by 4-AP occurred simultaneously to giant IPSPs in granule cells and CA3 neurons, and giant IPSPs in granule cells occurred simultaneously to giant IPSPs in CA3 neurons. Blockade of glutamatergic excitation by CNQX and APV did not abolish this synchrony. 6. 4-AP-evoked Cl- and K-dependent IPSPs were, unlike electrically evoked IPSPs, not strictly coupled: some 20% of large IPSPs and up to 90% of small IPSPs were either CL or K dependent. In granule cells K-dependent components either preceded or followed Cl-dependent components. 7. K-dependent IPSPs only could be evoked in CA3 neurons by focal application of 4-AP (1 mM) to the hilus, the stratum lacunosum moleculare or the stratum pyramidale. Wash out of Ca for 15-20 min blocked the Cl-dependent but not the K-dependent component of giant IPSPs evoked by bath-applied 4-AP. 8. The γ-aminobutyric acid (GABA)-uptake blocker nipecotic acid potentiated electrically as well as 4-AP-evoked Cl-dependent IPSPs up to 250% but had no significant effect on K-dependent IPSPs. However, nipecotic acid potentiated GABA(B) responses to microdrop application of GABA as well as GABA(A) responses. 9. We conclude that hilar neurons generate Cl-dependent and K-dependent IPSPs in CA3 neurons and granule cells. Cl-dependent and K-dependent IPSPs are obviously mediated by at least two distinct populations of inhibitory neurons, which are coupled rather effectively by a nonglutamatergic synchronizing mechanism extending from the dentate gyrus to the CA3 area.