Ionic mechanisms of intrinsic oscillations in neurons of the basolateral amygdaloid complex.

Ionic mechanisms underlying low-threshold (LTO) and high-threshold (HTO) oscillations occurring in a class of spiny neurons within the basolateral amygdaloid complex (see companion paper) were investigated in slice preparations of the guinea pig amygdala in vitro. LTOs were abolished through local application of tetrodotoxin (TTX, 10-20 microM) or a decrease in the extracellular sodium concentration ([Na+]o) from 153 to 26 mM, whereas HTOs were more readily elicited under these conditions. The effects of TTX and low [Na+]o were accompanied by a hyperpolarizing shift of the membrane potential by 3 +/- 1 mV and a decrease in apparent input resistance by 14 +/- 11 MOmega. LTOs were not observed during intracellular recording with QX 314 (50 microM) or Cs-acetate (2 M) containing micropipettes. At membrane potentials associated with LTO generation, voltage responses to sinusoidal current input with changing frequency between 0 and 10 Hz were characterized by a peak in the response (resonance) at 2.4 +/- 1 Hz, largely corresponding to the frequency range of the LTOs. Resonance behavior was evident as a peak in the impedance amplitude plot (ZA-plot) and a maximum in the fast Fourier transformation (FFT). Resonance and LTOs were concomitantly reduced by TTX and barium (Ba2+;2-10 mM) and were preserved during action of extracellular cesium (Cs+; 10-30 mM) or tetraethylammonium chloride (TEA; 20-50 mM), although the peak in the frequency domain tended to shift to lower values in TEA. Application of carbachol (50-200 microM) significantly reduced or blocked LTOs, whereas 4-aminopyridine (4-AP; 10 mM), iberiotoxin (Ibtx, 10 microM), and apamin (20 microM) had no effect. Slow depolarizing/repolarizing current ramps (12.5-125 pA/s) evoked HTOs as rhythmic deflections in membrane potential at either phase of the current ramp. Substitution of extracellular calcium (Ca2+) by magnesium and addition of cobalt chloride (2-4 mM) blocked HTOs but had no measurable effect on the propensity of the cells to produce LTOs. HTOs were abolished within approximately 10 min after impalement of the cells with a bis-(2-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA; 200 mM)-containing micropipette. Intracellular Cs+, extracellular Ba2+ (2-10 mM), or extracellular TEA (20-50 mM) induced an increase in amplitude of the rhythmic discharges and an increasingly slowed time course of repolarization at successive oscillatory events, until a steady depolarization was reached at -20 to -10 mV. Application of Ibtx (10 microM) reversibly abolished rhythmic activity during the repolarizing phase of the current ramp, whereas charybdotoxin (2-10 microM) and apamin (20 microM) had no effect. Changes in the chloride (Cl-) equilibrium potential by approximately +30 mV through intracellular recording with a KNO3 (3 M)-containing micropipette or lowering [Cl-]o from 128 to 4 mM, or blockade of Cl- conductances through niflumic acid (100 microM), did not significantly effect LTOs or HTOs. The generation of repetitive spike patterns on membrane depolarization was substantially influenced through removal of extracellular Ca2+ and associated blockade of HTOs, in that the initial high frequent discharge was abolished, frequency adaptation toward slow-rhythmic firing was delayed, and firing occurred at a more irregular pattern during strong depolarizing stimuli. It is concluded that a TTX-sensitive Na+ conductance and the M current contribute to generation of the LTOs, although their exact role in rhythmogenesisremains to be determined. HTOs seem to largely depend on a functional coupling between high-voltage-activated Ca2+ conductances, a Ca2+-activated K+ current presumably carried through BKCa channels, and additional voltage-dependent K+ conductances. In functional terms, the HTOs are important in determining spike frequency adaptation toward a slow-rhythmic firing pattern during maintained depolarizing influence.