Abstract
In terms of its sub-regional differentiation, the hippocampal CA1 region receives cortical information directly via the perforant (temporoammonic) path (pp-CA1 synapse) and indirectly via the tri-synaptic pathway where the last relay station is the Schaffer collateral-CA1 synapse (Sc-CA1 synapse). Research to date on pp-CA1 synapses has been conducted predominantly in vitro and never in awake animals, but these studies hint that information processing at this synapse might be distinct to processing at the Sc-CA1 synapse. Here, we characterized synaptic properties and synaptic plasticity at the pp-CA1 synapse of freely behaving adult rats. We observed that field excitatory postsynaptic potentials at the pp-CA1 synapse have longer onset latencies and a shorter time-to-peak compared to the Sc-CA1 synapse. LTP (>24 h) was successfully evoked by tetanic afferent stimulation of pp-CA1 synapses. Low frequency stimulation evoked synaptic depression at Sc-CA1 synapses, but did not elicit LTD at pp-CA1 synapses unless the Schaffer collateral afferents to the CA1 region had been severed. Paired-pulse responses also showed significant differences. Our data suggest that synaptic plasticity at the pp-CA1 synapse is distinct from the Sc-CA1 synapse and that this may reflect its specific role in hippocampal information processing.
Highlights
Accumulated knowledge derived from behavioral and electrophysiological research has enabled more detailed insights into the role of the different hippocampal subfields in learning and memory (Kemp and Manahan-Vaughan, 2004, 2008; Vago et al, 2007)
Having first established methods to enable the characterization of the pp-CA1 synapse, we studied the conditions under which synaptic plasticity is expressed and compared this to synaptic plasticity at the Schaffer collateralCA1 (Sc-CA1) synapse
ESTABLISHMENT OF ELECTROPHYSIOLOGICAL PROCEDURES FOR RECORDINGS OF pp-CA1 POTENTIALS IN FREELY BEHAVING RATS The electrophysiological evaluation of the pp-CA1 synapse has not been previously attempted by other researchers using freely behaving animals
Summary
Accumulated knowledge derived from behavioral and electrophysiological research has enabled more detailed insights into the role of the different hippocampal subfields in learning and memory (Kemp and Manahan-Vaughan, 2004, 2008; Vago et al, 2007). Information to the hippocampal subfields is conveyed via the superficial layers II and III of entorhinal cortex (EC) via the perforant path. In contrast to the trisynaptic input, which is diffusely widespread in terms of its connections (Ishizuka et al, 1990), the direct pathway from EC layer III to the CA1 is organized in a topographical way, forming almost a one-to-one connection. The two dendritic layers differ in terms of receptor composition (Nolan et al, 2004; Otmakhova et al, 2005; Ahmed and Siegelbaum, 2009) and molecular underpinnings of plasticity induction (Magee, 1999; Jarsky et al, 2005; Losonczy et al, 2008; Remy et al, 2009; Takahashi and Magee, 2009)
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