Fig. 1. Carbachol depolarizes LM interneurons and induces voltage-dependent membrane potential oscillations.A,B, Membrane potential recording from an LM interneuron exposed to the cholinergic agonist carbachol (CCh,solid bar; 25 μm). Letters in A indicate times from which expanded traces in B were taken, and action potentials are truncated in this and subsequent figures. Voltage-dependent oscillations were induced in the LM cell with positive current injection before carbachol application (B, top left trace). Application of carbachol at resting membrane potential (a) was followed by a depolarization that induced oscillations that paced action potentials (b, d). The depolarization was preceded by a small transient hyperpolarization (see also Fig. 2A). Oscillations induced by carbachol were eliminated when the cell was hyperpolarized with steady negative current injection (c) or when the cell repolarized after washout of carbachol (e), indicating that the oscillations are voltage-dependent. C, Power spectral analysis of oscillations in six LM cells in which membrane potential relative to spike threshold was varied using steady current injection. Frequency and amplitude of oscillations, and their voltage dependence, were not significantly different in carbachol (dashed lines) and in normal ACSF (solid lines). D, Voltage responses to positive and negative current pulses that ranged from −100 to 60 pA in 10 pA steps. For this cell, carbachol increased input resistance and decreased afterhyperpolarization amplitude.E, Camera lucida tracing of the LM interneuron from which recordings in A, B, andD were obtained. The axon is indicated by anarrow. Abbreviations in this figure and in Figure 2:Or, stratum oriens; Pyr, stratum pyramidale; Rad, stratum radiatum; L-M, stratum lacunosum-moleculare.