Each of these interneurons comprises only 1–3% of all striatal neurons, impeding their thorough examination. Nonetheless, cholinergic interneurons are well studied due to their large size aiding identification [10]. Cholinergic interneurons are tonically active pacemaking neurons, even in the absence of synaptic inputs [11]. In primates, cholinergic interneurons demonstrate distinct pauses in their tonic firing during motor learning and reward-related behaviors [12,13]. These dopamine-dependent-pauses are hypothesized to serve a “teaching” role in associative and motor learning [14], presumably by altering the strength of corticostriatal glutamatergic synapses. Thalamic activities, from the intralaminar thalamic neurons, give rise to the burst-and-pause firing of cholinergic interneurons [15,16]. This pattern of cholinergic activity produces dichotomic modulation of corticostriatal synaptic transmission through pre- and postsynaptic mechanisms, providing a neural substrate for attentional shifts with appearance of salient environmental stimuli [15]. Previous studies also suggest that sodium currents [13], slow afterhyperpolarization (sAHP) [17] and Ih [18] are involved in the decrease in firing rate or pause in tonic spiking.
