Network oscillations are a prominent phenomenon in neuronal signal processing, yet their functional role remains elusive. Three main classes of cellular mechanism have been suggested to contribute to network oscillations: (1) rhythmic activity in external pacemaker neurones could drive the network oscillation (Petsche et al. 1962). (2) Network oscillations could be generated by a synaptic feedback loop between excitatory and inhibitory neurones (Freeman, 1968). Alternatively, or additionally, a subnetwork of synaptically interconnected neurones could express a frequency preference based on the kinetic properties of the synaptic connections (Whittington et al. 1995). (3) Intrinsic frequency properties of neurones within the network could support the emergence of population oscillations when the network is activated (Llinas, 1988). The pacemaker current (Ih) is a mechanism that has been implicated in rhythmic activity in excitable cells including neurones. The aim of the present project was to investigate the contribution by Ih to frequency preference at physiologically relevant frequencies in hippocampal CA1 pyramidal neurones.
Transverse hippocampal slices were made from P12-P18 Wistar rats, decapitated under isoflurane anaesthesia in accordance with the UK Animals (Scientific Procedures) Act, 1986. Following a recovery period of at least 1 h, whole-cell patch-clamp recordings were obtained from individual neurones at room temperature (23-25 °C) under visual guidance by infra-red video-microscopy using standard procedures. The patch pipette solution contained (mM): potassium gluconate (100), NaCl (4), Hepes (40), Mg-ATP (4), GTP (0.3) and biocytin (5 mg ml-1); pH 7.2-7.4.
To investigate the intrinsic resonance properties at subthreshold membrane potentials, a sinusoidal current with linearly increasing frequency (0-30 Hz) was applied. The impedance magnitude, estimated as the magnitude of the ratio of the fast fourier transform (FFT) of the voltage response to the FFT of input current, showed a maximal response around 2-4 Hz (n = 14) at resting membrane potential. The peak frequency increased with hyperpolarisation (0.03 Hz mV-1) along with a reduction of the peak impedance magnitude (-3.3 MΩ mV-1), suggesting involvement of h conductance. Consistent with this hypothesis, the resonance peak was completely abolished by a blocker of the h conductance, ZD7288 (10 mM; n = 6). Dynamic clamp experiments, mimicking electronically the h conductance, whose properties were determined in independent voltage-clamp experiments, confirmed that this conductance could generate resonance (n = 3).
To study to what extent the h conductance could explain spike frequency preference in these neurones, we determined the effect of oscillation frequency on spike generation using sinusoidal current at discrete frequencies superposed on a current step. In pyramidal cells, at low rates of firing (< 1 Hz), input frequency preference was demonstrated at ~2 Hz. The input frequency at peak firing rate increased steeply with depolarization. The addition of 10 mM ZD7288 abolished the input frequency preference only at the lowest firing rates, leaving the frequency preference at higher rates largely unaffected (n = 4). Conversely, blocking a component of the spike after-hyperpolarisation (AHP) with apamin (50 nM), altered the input frequency preference only at higher firing rates (n = 6). Thus the control of subthreshold resonance and suprathreshold firing entrainment can be experimentally disassociated and is mediated by distinct mechanisms.
These results suggest that distinct intrinsic membrane properties support resonance in physiologically relevant frequency ranges, and that these may contribute to signal processing in individual neurones.
This work was supported by the BBSRC, the MRC and The Wellcome Trust.