Both silent and spontaneously firing silent spiny neurons are not observed to fire in extracellular spiny projection neurons have been described in the neostriatum, records made before penetration, their silence is not thought to but the reason for their differences in firing activity are unknown. be from the effects of impalement (Wilson and Groves 1981) . We compared properties of spontaneously firing and silent spiny Extracellular recording combined with iontophoretic applicaneurons in

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... n-anesthetized rats. Neurons were identified as tion of excitatory neurotransmitters has also revealed a large spiny projection neurons after labeling by intracellular injection of population of silent spiny neurons in awake behaving animals biocytin. The threshold for action-potential firing was measured under three different conditions: 1) electrical stimulation of the (Kiyatkin and Rebec 1996). contralateral cerebral cortex, 2) brief directly applied current Membrane potential shifts from a hyperpolarized DOWN pulses, and 3) spontaneous action-potentials occurring during state to a depolarized UP state appear to be necessary for spontaneous episodes of depolarization (UP state). The average action-potential firing in striatal neurons (Wilson and Groves membrane potential and the amplitude of noiselike fluctuations of 1981; Wilson and Kawaguchi 1996) . Several pieces of evimembrane potential in the UP state were determined by fitting a dence suggest that these UP state transitions are brought Gaussian curve to the membrane-potential distribution. All neurons about by synaptic input from the cerebral cortex and the in the sample exhibited spontaneous membrane potential shifts thalamus. UP state transitions do not occur following removal between a hyperpolarized DOWN state and a depolarized UP state, or deactivation of the cortex (Wilson et al. 1983) or in brain but not all fired action potentials while in the UP state. The difference between the spontaneously firing and the silent spiny neurons slices in which most cortical inputs have been disconnected was in the average membrane potential in the UP state, which was (Arbuthnott et al. 1985; Kawaguchi et al. 1989 ). On the significantly more depolarized in the spontaneously firing than in other hand, cortical stimulation in the intact animal can the silent spiny neurons. There were no significant differences in evoke depolarizing events very similar to the UP state transithe threshold, the amplitude of the noiselike fluctuations of memtions that occur spontaneously (Wilson 1995; brane potential in the UP state, or in the proportion of time that the Kawaguchi 1996) . Thus corticostriatal inputs are necessary membrane potential was in the UP state. In both spontaneously and sufficient for UP state transitions. UP state transitions, firing and silent neurons, the threshold for action potentials evoked however, do not necessarily lead to action-potential firing by current pulses was significantly higher than for those evoked by cortical stimulation. Application of more intense current pulses and occur in silent as well as spontaneously firing cells (Wilthat reproduced the excitatory postsynaptic potential rate of rise son and Groves 1981).