SPONTANEOUS SUBTHRESHOLD MEMBRANE-POTENTIAL FLUCTUATIONS AND ACTION-POTENTIAL VARIABILITY OF RAT CORTICOSTRIATAL AND STRIATAL NEURONS IN-VIVO

We measured the timing of spontaneous membrane potential fluctuations and action potentials of medial and lateral agranular corticostriatal and striatal neurons with the use of in vivo intracellular recordings in urethan-anesthetized rats. All neurons showed spontaneous subthresh old membrane potential shifts from 7 to 32 mV in amplitude, fluctuatin g between a hyperpolarized down state and depolarized up state. Action potentials arose only during the up state. The membrane potential sta te transitions showed a weak periodicity with a peak frequency near 1 Hz. The peak of the frequency spectra was broad in all neurons, indica ting that the membrane potential fluctuations were not dominated by a single periodic function. At frequencies >1 Hz, the log of magnitude d ecreased linearly with the log of frequency in all neurons. No serial dependence was found for up and down state durations, or for the time between successive up or down state transitions, showing that the up a nd down state transitions are not due to superimposition of noisy inpu ts onto a single frequency. Monte Carlo simulations of stochastic syna ptic inputs to a uniform finite cylinder showed that the Fourier spect ra obtained for corticostriatal and striatal neurons are inconsistent with a Poisson-like synaptic input, demonstrating that the up state is not due to an increase in the strength of an unpatterned synaptic inp ut. Frequency components arising from state transitions were separated from those arising from the smaller membrane potential fluctuations w ithin each state. A larger proportion of the total signal was represen ted by the fluctuations within states, especially in the up state, tha n was predicted by the simulations. The individual state spectra did n ot correspond to those of random synaptic inputs, but reproduced the s pectra of the up and down state transitions. This suggests that the pr ocess causing the state transitions and the process responsible for sy naptic input may be the same. A high-frequency periodic component in t he up states was found in the majority of the corticostriatal cells in the sample. The average size of the component was not different betwe en neurons injected with QX-314 and control neurons. The high-frequenc y component was not seen in any of our sample of striatal cells. Corti costriatal and striatal neurons' coefficients of variation of interspi ke intervals ranged from 1.0 to 1.9. When interspike intervals includi ng a down state were subtracted from the calculation, the coefficient of variation ranged from 0.4 to 1.1, indicating that a substantial pro portion of spike interval variance was due to the subthreshold membran e potential fluctuations.