Spontaneous activity of striatal projection neurons supports maturation of striatal inputs to substantia nigra dopaminergic neurons

Spontaneous activity of neurons during early ontogenesis is instrumental for stabilization and refinement of developing neuronal connections. The role of spontaneous activity in synaptic development has been described in detail for cortical-like structures. Yet, very little is known about activity-dependent development of long-range inhibitory projections, such as projections from striatum. Here, we show that striatal projection neurons (SPNs) in dorsal striatum are spontaneously active in P4-P14 mice. Spontaneous activity was detected in both direct-pathway SPNs (dSPNs) and indirect-pathway SPNs (iSPNs). Most of the spontaneously active cells were in striosomes - a chemical compartment in striatum defined by expression of {micro}-opioid receptor. Higher excitability of both striosomal dSPNs and iSPNs was related to their intrinsic excitability properties (higher action potential half-width and IV slope). Tonic activation of muscarinic M1 receptor maintains the spontaneous activity of striosomal SPNs, the effect being stronger in iSPNs and weaker in dSPNs. To investigate if the neonatal spontaneous activity is needed for the stabilization of SPN long-range projections, we chemogenetically inhibited striosomal SPNs in neonatal animals and studied the efficiency of striatonigral projections in adult animals. Inhibition of striosomal SPNs by chronic CNO administration to P6-14 pups caused a reduction in the functional GABAergic innervation and in the density of gephyrin puncta in dopaminergic neurons of substantia nigra pars compacta of the adult (P52-79) animals. Chronic administration of CNO later in development (P21-29), on the contrary, resulted in higher mIPSC frequency in dopaminergic cells of the adult animals. Thus, the activity-dependent stabilization of striosomal projections has different developmental phases, and the long-term outcome of perturbations in these processes depends on the developmental period when they occur. Taken together, our results demonstrate that spontaneous activity of SPNs is essential for the maturation and stabilization of striatal efferents.