Muscarinic activation of a cation current and associated current noise in entorhinal-cortex layer-II neurons

The effects of muscarinic stimulation on the membrane potential and current of in situ rat entorhinal-cortex layer-II principal neurons were analyzed using the whole cell, patch-clamp technique. In current-clamp experiments, application of carbachol (CCh) induced a slowly developing, prolonged depolarization initially accompanied by a slight decrease or no significant change in input resistance. By contrast, in a later phase of the depolarization input resistance appeared consistently increased. To elucidate the ionic bases of these effects, voltage-clamp experiments were then carried out. In recordings performed in nearly physiological ionic conditions at the holding potential of -60 mV, CCh application promoted the slow development of an inward current deflection consistently associated with a prominent increase in current noise. Similarly to voltage responses to CCh, this inward-current induction was abolished by the muscarinic antagonist, atropine. Current-voltage relationships derived by applying ramp voltage protocols during the different phases of the CCh-induced inward-current deflection revealed the early induction of an inward current that manifested a linear current/voltage relationship in the subthreshold range and the longer-lasting block of an outward K+ current. The latter current could be blocked by 1 mM extracellular Ba2+, which allowed us to study the CCh-induced inward current (I CCh) in isolation. The extrapolated reversal potential of the isolated I-CCh was approximate to0 mV and was not modified by complete substitution of intrapipette K+ with Cs+. Moreover, the extrapolated I-CCh reversal shifted to approximately -20 mV on removal of 50% extracellular Na+. These results are consistent with I-CCh being a nonspecific cation current. Finally, noise analysis of I-CCh returned an estimated conductance of the underlying channels of similar to13.5 pS. We conclude that the depolarizing effect of muscarinic stimuli on entorhinal-cortex layer-II principal neurons depends on both the block of a K+ conductance and the activation of a "noisy" nonspecific cation current. We suggest that the membrane current fluctuations brought about by I-CCh channel noise may facilitate the "theta" oscillatory dynamics of these neurons and enhance firing reliability and synchronization.