VGCCs are multisubunit complexes that play a crucial role in neuronal signaling. Auxiliary a2d subunits of VGCCs modulate trafficking and biophysical properties of the pore-forming a1 subunit and trigger excitatory synaptogenesis. Alterations in the expression level of a2d subunits were implicated in several syndromes and diseases, including chronic neuropathic pain, autism, and epilepsy. However, the contribution of distinct a2d subunits to excitatory/inhibitory imbalance and aberrant network connectivity characteristic for these pathologic conditions remains unclear. Here, we show that a2d1 overexpression enhances spontaneous neuronal network activity in developing and mature cultures of hippocampal neurons. In contrast, overexpression, but not downregulation, of a2d3 enhances neuronal firing in immature cultures, whereas later in development it suppresses neuronal activity. We found that a2d1 overexpression increases excitatory synaptic density and selectively enhances presynaptic glutamate release, which is impaired on a2d1 knockdown. Overexpression of a2d3 increases the excitatory synaptic density as well but also facilitates spontaneous GABA release and triggers an increase in the density of inhibitory synapses, which is accompanied by enhanced axonal outgrowth in immature interneurons. Together, our findings demonstrate that a2d1 and a2d3 subunits play distinct but complementary roles in driving formation of structural and functional network connectivity during early development. An alteration in a2d surface expression during critical developmental windows can therefore play a causal role and have a profound impact on the excitatory-to-inhibitory balance and network connectivity.