Long-term plasticity at excitatory synapses on aspinous interneurons in area CA1 lacks synaptic specificity

The synaptic specificity of long-term potentiation (LTP) was examined at synapses formed on aspinous dendrites of interneurons whose somata were located in the pyramidal cell layer of hippocampal area CA1. Intracellular recordings from slices prepared from rats were used to monitor excitatory postsynaptic potentials (EPSPs) elicited by extracellular stimulation in stratum radiatum. Two synaptic inputs were evoked at 0.5 Hz by stimulating axons adjacent to stratum pyramidale and s. lacunosum-moleculare. After obtaining baseline recordings (greater than or equal to 10 min), one of the EPSPs was conditioned. The protocol involved tetanic stimulation, sometimes combined with somatic depolarization. Low-frequency stimulation of the two pathways was then resumed and EPSPs were recorded for <30 min. We observed both homosynaptic and heterosynaptic changes in synaptic strength. LTP and long-term depression (LTD) were seen in both pathways and all possible combinations of changes in the two EPSPs were observed, including heterosynaptic LTP associated with either homosynaptic LTP or LTD. Intracellular 1,2-bis (2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (10 mM) abolished alterations in synaptic strength. When axons in s. radiatum synapse onto a spiny pyramidal cell, synaptic specificity of LTP is preserved. However the results obtained from aspinous interneurons show that synaptic specificity of LTP is lost. These results are consistent with the hypothesis that spines provide postsynaptic mechanism(s) for conferring specificity to LTP.