REGULATION OF ACTION-POTENTIAL FIRING IN SPINY NEURONS OF THE RAT NEOSTRIATUM IN-VIVO

Both silent and spontaneously firing spiny projection neurons have bee n described in the neostriatum, but the reason for their differences i n firing activity are unknown. We compared properties of spontaneously firing and silent spiny neurons in urethan-anesthetized rats. Neurons were identified as spiny projection neurons after labeling by intrace llular injection of biocytin. The threshold for action-potential firin g was measured under three different conditions: I) electrical stimula tion of the contralateral cerebral cortex, 2) brief directly applied c urrent pulses, and 3) spontaneous action-potentials occurring during s pontaneous episodes of depolarization (up state). The average membrane potential and the amplitude of noiselike fluctuations of membrane pot ential in the vp state were determined by fitting a Gaussian curve to the membrane-potential distribution. All neurons in the sample exhibit ed spontaneous membrane potential shifts between a hyperpolarized DOWN state and a depolarized up state, but not all fired action potentials while in the up state. The difference between, the spontaneously firi ng and the silent spiny neurons was in the average membrane potential in the up state, which was significantly more depolarized in the spont aneously firing than in the silent spiny neurons. There were no signif icant differences in the threshold, the amplitude of the noiselike flu ctuations of membrane potential in the vp state, or in the proportion of time that the membrane potential was in the up state. Ln both spont aneously firing and silent neurons, the threshold for action potential s evoked by current pulses was significantly higher than for those evo ked by cortical stimulation. Application of more intense current pulse s that reproduced the excitatory postsynaptic potential rate of rise p roduced firing at correspondingly lower thresholds. Because the membra ne potential in the vp state is mainly determined by the balance betwe en the synaptic drive and the outward potassium conductances activated in the subthreshold range of membrane potentials, either or both of t hese factors may determine whether firing occurs in response to sponta neous afferent activity.