Alternating low frequency stimulation of medial septal and commissural fibers induces NMDA‐dependent, long‐lasting potentiation of hippocampal synapses in urethane‐anesthetized rats

Abstract

Recent evidence indicates that some synapses exhibit long-lasting synaptic potentiation in response to low frequency (1 Hz) stimulation, similar to long-term potentiation (LTP) following high frequency induction protocols. Here, the authors characterize a form of long-lasting synaptic potentiation in the hippocampal CA1 area following alternating, single pulse stimulation of the medial septum (MS) and hippocampal CA3 commissural fibers (MS-H LTP). In urethane-anesthetized rats, alternating single pulse stimulation of the MS and CA3 (50 pulses each at 0.5 Hz, 1,000 ms interstimulus interval [ISI]) produced gradual increases of field excitatory postsynaptic potential (fEPSP) amplitude in CA1 ( approximately 123% of baseline), while MS or CA3 stimulation alone was ineffective. The fEPSP enhancement was long-lasting (>4h) and repeated episodes of alternating MS-CA3 stimulation tended to result in greater levels of potentiation than those elicited by a single episode. Surprisingly, ISIs of 500, 750, and 1,500 ms did not result in significant changes in fEPSP amplitude, while an ISI of 100 ms produced synaptic depression. MS-H LTP was resistant to systemic administration of nicotinic and muscarinic receptor antagonists (scopolamine, mecamylamine), but abolished by systemic MK-801 (0.5 mg/kg, i.p.) or local CA1 application of AP-V (10 mM), indicative of a critical role of hippocampal NMDA receptors in this effect. Paired-pulse facilitation experiments revealed a gradually developing, significant inverse correlation between fEPSP enhancement and decrease in paired-pulse facilitation ratio, suggesting a role of changes in presynaptic transmitter release. Together, these data demonstrate a novel form of long-lasting synaptic enhancement in CA1 neurons in response to low frequency activity in separate afferent systems, an activity that might mimic some aspects of natural discharge patterns during the acquisition or consolidation of memory processes in hippocampal circuits.