Effects of Inhibitory Synapses on Dendritic Spine Clustering in Adult Rat Hippocampus

Synaptic clustering can serve as a computational unit for the distribution of synaptic resources across dendritic segments. In this study, we investigate whether inhibitory synapses influence the frequency or size of excitatory synapses (whether spine or shaft synapses) in a cluster, and whether the induction of long-term potentiation (LTP) affects this relationship in the stratum radiatum of the CA1 of adult rat hippocampus. We induced LTP through theta-burst stimulation (TBS) in one of two stimulating electrodes through test pulses, while the second stimulating electrode was given test pulses without TBS. We identified symmetric, presumably inhibitory synapses by their equally thin presynaptic and postsynaptic densities, as well as by the pleiomorphic vesicles in the associated axonal bouton. Synaptic clusters were delineated by surrounding asynaptic regions of at least 120 nm. Our preliminary analyses includes 84 clusters and 5 clusters in the LTP and control conditions, respectively. Our preliminary findings show that there are fewer clusters with symmetric synapses two hours post-LTP, and that the surface areas of their symmetric synapses are larger. On one hand, clusters with symmetric synapses had lower asymmetric spine densities than those without. On the other hand, asymmetric synapse densities were consistent between LTP and control if a symmetric synapse is present. These findings suggest that symmetric synapses influences the local spines and synapses in their cluster, and thus serves as an additional layer of analysis in treating synaptic clusters as a computational unit.