Carbachol-induced synchronized rhythmic bursts in CA3 neurons of guinea pig hippocampus in vitro.

1. Carbachol effects on CA3 hippocampal cells were studied in the absence of ionotropic glutamatergic and GABAergic transmission with intracellular and extracellular recordings from guinea pig septohippocampal slices. 2. In all experiments the perfusing solution contained ionotropic glutamate and gamma-aminobutyric acid (GABA) receptor blockers [6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10-20 microM), 3-((+/-)-2-carboxypiperazin-4-il)propyl-1-phosphonic acid (CPP, 10-20 microM), and picrotoxin (50 microM), respectively]. Under these conditions, the excitatory and early inhibitory postsynaptic potentials, evoked in CA3 pyramidal cells by mossy fiber stimulation before the addition of the blockers, were completely suppressed. 3. Carbachol (50 microM) introduced via bath perfusion or pulse application elicited a series of rhythmic bursts with overriding action potentials. Each rhythmic burst lasted up to 30 s and repeated at intervals of 0.7-6 min. Rhythmic bursts were blocked by atropine (1 microM). 4. At membrane potentials more depolarized than -70 mV, carbachol also elicited a sustained depolarization associated with an increase in membrane input resistance and action-potential firing. This response was blocked by atropine (1 microM). 5. Carbachol can induce both rhythmic bursts and sustained depolarizations in the same cell. Rhythmic bursts were elicited when the membrane potential of the cell was more hyperpolarized than -70 mV; sustained depolarizing responses were activated by carbachol when the cell membrane potential was more depolarized than -70 mV. 6. Extracellular field potential responses in the CA3 region occurred simultaneously with rhythmic bursts, indicating the synchronization of the event in the CA3 field. Dual intracellular recordings confirmed that rhythmic bursts occurred simultaneously in CA3 hippocampal pyramidal cells.(ABSTRACT TRUNCATED AT 250 WORDS)