Chemogenetic silencing reveals presynaptic Gi/o protein-mediated inhibition of developing hippocampal synchrony in vivo

Recent advances in understanding how neuronal activity shapes developing brain circuits increasingly rely on G_i/o-dependent inhibitory chemogenetic tools (G_i-DREADDs). However, their mechanisms of action and efficacy in neurons with immature G_i/o signaling are elusive. Here, we express the G_i-DREADD hM4Di in glutamatergic telencephalic neurons and analyze its impact on CA1 pyramidal neurons in neonatal mice. Using acousto-optic two-photon Ca²⁺ imaging, we report that activation of hM4Di leads to a complete arrest of spontaneous synchrony in CA1 <i>in vitro</i>. We demonstrate that hM4Di does not cause somatic hyperpolarization or shunting but rather mediates presynaptic silencing of glutamatergic neurotransmission. <i>In vivo</i>, inhibition through hM4Di potently suppresses early sharp waves (eSPWs) and discontinuous oscillatory network activity in CA1 of head-fixed mice before eye opening. Our findings provide insights into the role of G_i/o signaling in synchronized activity in the neonatal hippocampus and bear relevance for applying chemogenetic silencing at early developmental stages.