Functional Distinctions between Spine and Dendritic Synapses Made onto Parvalbumin-Positive Interneurons in Mouse Cortex

Dendritic spines influence synapse function by boosting synaptic potentials and sequestering synaptically generated second messengers. Spines have been extensively studied in densely spiny principal neurons, but little is known about how they expand the information-gathering capabilities of sparsely spiny interneurons (INs). We find in the mouse primary visual cortex, parvalbumin-positive INs have a low density of spines that enclose functional glutamatergic synapses. Both spine and dendritic synapses contain calcium-permeable AMPA receptors (CP-AMPARs) and NMDA receptors (NMDARs), but NMDARs are enriched at spine synapses. Glutamate-receptor-mediated Ca influx at proximal dendritic sites is bidirectionally modulated by the timing of action potentials (APs). Surprisingly, spine synapses are largely insensitive to APs, but coincident activity originating in the adjacent dendrite strongly influences spine NMDAR-mediated calcium influx. Thus, while glutamate receptors on spines and dendrites are modulated by the activity of the neuron, they are distinctive in the type of coincident activity detected.