The corticostriatal input to giant aspiny interneurons in the rat: a candidate pathway for synchronising the response to reward-related cues.

Tonically active neurons (TANs) in the mammalian striatum show a pause in their ongoing firing activity in response to an auditory cue that is paired with a reward. This response to reward-related cues develops through learning and becomes expressed synchronously by TANs located throughout the striatum. The pause response is abolished by inactivating the thalamic inputs to the striatum but a short-latency excitatory response to reward-related cues remains, which may originate in the cortex. We investigated the cortical inputs to striatal neurons to determine the electrophysiological properties of their cortical projections. We made in vivo intracellular recordings from 14 giant aspiny interneurons (which correspond to the TANs) and from a control group of spiny projection neurons (n=18) in urethane-anaesthetised rats. All giant aspiny interneurons were tonically active (firing rate: 3.0+/-1.5 Hz) and displayed small-amplitude subthreshold fluctuations in membrane potential. These fluctuations in membrane potential were correlated with the cortical electroencephalogram (EEG). Test stimulation of the contralateral cortex induced postsynaptic potentials (PSPs) in giant aspiny interneurons. These PSPs were significantly shorter in latency (5.1+/-1.6 ms) than those measured in spiny projection neurons (9.3+/-2.8 ms; p<0.01), whereas the latencies of ipsilaterally evoked PSPs did not differ. Taken together, these observations suggest that giant aspiny interneurons are under the significant influence of spontaneous excitatory inputs and receive specialised input from either faster conducting or less branching cortical fibres than spiny projection neurons. These inputs may be involved in the synchronised convergence of reward-related cues from spatially distinct cortical areas onto giant aspiny interneurons.