Damodaran S, Evans RC, Blackwell KT. Synchronized firing of fast-spiking interneurons is critical to maintain balanced firing between direct and indirect pathway neurons of the striatum. J Neurophysiol 111: 836 -848, 2014. First published December 4, 2013 doi:10.1152/jn.00382.2013.-The inhibitory circuits of the striatum are known to be critical for motor function, yet their contributions to Parkinsonian motor deficits are not clear. Altered firing in the globus pallidus suggests that striatal medium spiny neurons (MSN) of the direct (D1 MSN) and indirect pathway (D2 MSN) are imbalanced during dopamine depletion. Both MSN classes receive inhibitory input from each other and from inhibitory interneurons within the striatum, specifically the fast-spiking interneurons (FSI). To investigate the role of inhibition in maintaining striatal balance, we developed a biologically-realistic striatal network model consisting of multicompartmental neuron models: 500 D1 MSNs, 500 D2 MSNs and 49 FSIs. The D1 and D2 MSN models are differentiated based on published experiments of individual channel modulations by dopamine, with D2 MSNs being more excitable than D1 MSNs. Despite this difference in response to current injection, in the network D1 and D2 MSNs fire at similar frequencies in response to excitatory synaptic input. Simulations further reveal that inhibition from FSIs connected by gap junctions is critical to produce balanced firing. Although gap junctions produce only a small increase in synchronization between FSIs, removing these connections resulted in significant firing differences between D1 and D2 MSNs, and balanced firing was restored by providing synchronized cortical input to the FSIs. Together these findings suggest that desynchronization of FSI firing is sufficient to alter balanced firing between D1 and D2 MSNs.Parkinson's disease; striatum; medium-spiny neuron; fast-spiking interneuron; gap junctions DOPAMINE DEPLETION LEADS TO an imbalance in the activation of the direct and indirect pathways of the striatum (Albin et al. 1989;DeLong 1990;Hikosaka et al. 2000;Mallet et al. 2006;Obeso et al. 2004). The direct pathway is composed of striatal medium spiny neurons (MSNs) that express dopamine D1 receptors (D1 MSNs) and projects to both the substantia nigra pars reticulata and the internal segment of the globus pallidus. The indirect pathway is composed of MSNs that express dopamine D2 receptors (D2 MSNs) and projects to the external segment of the globus pallidus. The two populations of MSNs differ in their intrinsic excitability, with D2 MSNs more responsive to somatic current injection (Gertler et al. 2008). Inhibitory circuits of the striatal network, including feedback inhibition from other MSNs and feedforward inhibition from fast-spiking interneurons (FSIs), modulate this excitability (Koós and Tepper 1999;Plenz 2003). The contributions of these circuits to maintaining balanced firing between D1 and D2 MSNs in the network (Cui et al. 2013;Mallet et al. 2006) are not clear.A strong source of GABAergic in...