RAPID COMMUNICATION Theta-Frequency Resonance in Hippocampal CA1 Neurons In Vitro Demonstrated by Sinusoidal Current Injection

Leung, L. Stan and Hui-Wen Yu. Theta-frequency resonance in frequency, and the resonant frequency is just below the natuhippocampal CA1 neurons in vitro demonstrated by sinusoidal ral oscillation frequency for a linear system. However, resocurrent injection. J. Neurophysiol. 79: 1592–1596, 1998. Sinusoinance at the theta frequency has not been clearly demondal currents of various frequencies were injected into hippocampal strated in hippocampal neurons (cf. Yu and Leung 1993). CA1 neurons in vitro, and the membrane potential responses were Garcia-Munoz et al. (1993), with sinusoidal currents of analyzed by cross power spectral analysis. Sinusoidal currents inú100 pA, showed that hippocampal neurons responded duced a maximal (resonant) response at a theta frequency (3–10 more to 7 than 2 or 14 Hz, but responses were not quantified Hz) in slightly depolarized neurons. As predicted by linear systems and resonance was not mentioned. Jahnsen and Karnup theory, the resonant frequency was about the same as the natural (1994) showed little resonance in power spectra of MPOs (spontaneous) oscillation frequency. However, in some cases, the resonant frequency was higher than the spontaneous oscillation of hippocampal neurons driven by band-pass white noise. frequency, or resonance was found in the absence of spontaneous In this study, a sharp resonance at the theta frequency was oscillations. The sharpness of the resonance (Q) , measured by observed in many CA1 neurons driven by sinusoidal currents the peak frequency divided by the half-peak power bandwidth, in vitro. A sinusoidal input is selected because hippocampal increased from a mean of 0.44 at rest to 0.83 during a mean theta rhythm in vivo is almost sinusoidal (Leung and Yim depolarization of 6.5 mV. The phase of the driven oscillations 1986; Leung et al. 1982). Systems analysis using sinusoidal changed most rapidly near the resonant frequency, and it shifted inputs is an established engineering technique (D’Azzo and about 907 over the half-peak bandwidth of 8.4 Hz. Similar results Houpis 1966; Gwinn and Westervelt 1986). However, resowere found using a sinusoidal function of slowly changing frenance in other central neurons was demonstrated by an imquency as the input. Sinusoidal currents of peak-to-peak intensity pedance (Z) amplitude profile (ZAP) function input (Gutof ú100 pA may evoke nonlinear responses characterized by second and higher harmonics. The theta-frequency resonance in hippofreund et al. 1995; Puil et al. 1986), and thus the ZAP campal neurons in vitro suggests that the same voltage-dependent function (a sinusoidal function of slowly changing frephenomenon may be important in enhancing a theta-frequency quency) was also applied to CA1 neurons. response when hippocampal neurons are driven by medial septal or other inputs in vivo. M E T H O D S