THETA-FREQUENCY RESONANCE IN HIPPOCAMPAL CA1 NEURONS IN-VITRO DEMONSTRATED BY SINUSOIDAL CURRENT INJECTION

Sinusoidal currents of various frequencies were injected into hippocam pal CA1 neurons in vitro, and the membrane potential responses were an alyzed by cross power spectral analysis. Sinusoidal currents induced a maximal (resonant) response at a theta frequency (3-10 Hz) in slightl y depolarized neurons. As predicted by linear systems theory, the reso nant frequency was about the same as the natural (spontaneous) oscilla tion frequency. However, in some cases, the resonant frequency was hig her than the spontaneous oscillation frequency, or resonance was found in the absence of spontaneous oscillations. The sharpness of the reso nance (Q), measured by the peak frequency divided by the half-peak pow er bandwidth, increased from a mean of 0.44 at rest to 0.83 during a m ean depolarization of 6.5 mV. The phase of the driven oscillations cha nged most rapidly near the resonant frequency, and it shifted about 90 degrees over the half-peak bandwidth of 8.4 Hz. Similar results were found using a sinusoidal function of slowly changing frequency as the input. Sinusoidal currents of peak-to-peak intensity of >100 pA may ev oke nonlinear responses characterized by second and higher harmonics. The theta-frequency resonance in hippocampal neurons in vitro suggests that the same voltage-dependent phenomenon may be important in enhanc ing a theta-frequency response when hippocampal neurons are driven by medial septal or other inputs in vivo.