MUSCARINIC INHIBITION OF M-CURRENT AND A POTASSIUM LEAK CONDUCTANCE IN NEURONS OF THE RAT BASOLATERAL AMYGDALA

1. Voltage-clamp recordings using a single microelectrode were obtained from pyramidal neurones of the basolateral amygdala (BLA) in slices of the rat ventral forebrain. Slow inward current relaxations during hyperpolarizing voltage steps from a holding potential of -40 mV were identified as the muscarinic-sensitive M-current (I(M)), a time- and voltage-dependent potassium current previously identified in other neuronal cell types. 2. Activation of I(M) was voltage dependent with a threshold of approximately -70 mV. At membrane potentials positive to this, the steady-state current-voltage (I-V) relationship showed substantial outward rectification, reflecting the time- and voltage-dependent opening of M-channels. The underlying conductance (g(M)) also increased sharply with depolarization. 3. The reversal potential for I(M) was - 84 mV in medium containing 3.5 mm K+. This was shifted positively by 27 mV when the external K+ concentration was raised to 15 mm. 4. The time courses of M-current activation and deactivation were fitted by a single exponential. The time constant for I(M) decay, measured at 24-degrees-C, was strongly dependent on membrane potential, ranging from 330 ms at -40 mV to 12 ms at -100 mV. 5. Bath application of carbachol (0.5-40 muM) inhibited I(M), as evidenced by the reduction or elimination of the slow inward M-current relaxations evoked during hyperpolarizing steps from a holding potential of -40 mV. The outward rectification of the steady-state I-V relationship at membrane potentials positive to -70 mV was also largely eliminated. The inhibition of Im by carbachol was dose dependent and antagonized by atropine. 6. Carbachol produced an inward current shift at a holding potential of -40 mV that was only partially attributable to inhibition of I(M). An inward current shift was also produced by carbachol at membrane potentials negative to -70 mV, where I(M) is inactive. These effects were dose dependent and antagonized by atropine. They were attributed to the muscarinic inhibition of a voltage-insensitive potassium leak conductance (I(Leak)). 7. In most cells, carbachol reduced the slope of the instantaneous I-V relationship obtained from a holding potential of -70 mV so that it crossed the control I-V plot at the reversal potential for I(Leak). This was found to be -108 mV in 3-5 mm K+ saline, shifting to -66 mV in 15 mm K+ saline. However, in a few neurones, carbachol produced a parallel shift of the instantaneous I-V relationship and no reversal potential could be obtained, possibly due to the activation by carbachol of a non-specific cation conductance. 8. In unclamped BLA pyramidal neurones, carbachol produces a slow membrane depolarization accompanied by an increase in input resistance. It is concluded that these actions are mediated by muscarinic receptor activation, resulting in inhibition of both the M-current and a voltage-independent K+ leak current.