Whole-cell clamp recordings of the compound synaptic current elicited by afferent stimulation of Schaffer collaterals showed that blockade of the NMDA, AMPA and GABAA receptor-mediated components by 6-nitro-7-sulphamoyl- benzo(f)quinoxaline-2,3-dione (NBQX), 3-((R)-2-carboxypiperazine-4-yl)propyl-1-phosphonate (R-CPP) and picrotoxin, respectively, left a small residual current in 39 out of 41 CA1 pyramidal neurones in organotypic cultures and 9 out of 16 CA1 cells in acutely prepared slices. 2. This current represented 2. 9 +/- 0.4 % of the compound evoked synaptic response in organoypic cultures and 1.4 +/- 0.5 % in slices. It was characterized by a slightly rectifying I-V curve and a reversal potential of 3.4 +/- 5. 1 mV. 3. This residual current was insensitive to blockers of GABAB, purinergic, muscarinic and 5-HT3 receptors, but it was essentially blocked by the nicotinic receptor antagonist d-tubocurarine (91 +/- 4 % blockade; 20 microM), and partly blocked by alpha-bungarotoxin (200 nM) and methyllycaconitine (10 nM), two antagonists with a higher selectivity for alpha7 subunit-containing nicotinic receptors (48 +/- 3 % and 55 +/- 11 % blockade, respectively). 4. The residual current was of synaptic origin, since it occurred after a small delay; its amplitude depended upon the stimulation intensity and it was calcium dependent and blocked by the sodium channel antagonist tetrodotoxin. 5. We conclude that afferent stimulation applied in the stratum radiatum evokes in some hippocampal neurones a small synaptic current mediated by activation of neuronal nicotinic receptors.
