Conditions for the induction of long-term potentiation and long-term depression by conjunctive pairing in the dentate gyrus in vitro.

Conditions for the induction of long-term potentiation and long-term depression by conjunctive pairing in the dentate gyrus in vitro. J. Neurophysiol. 78: 2569-2573, 1997. The conditions under which long-term potentiation (LTP) and long-term depression (LTD) of excitatory postsynaptic currents were induced by the conjunctive pairing-type protocol of afferent stimulation and postsynaptic depolarization were studied in the medial perforant pathway-granule cell synapse of the dentate gyrus in vitro. The conjunctive pairing of 1-Hz afferent stimulation and steady state postsynaptic depolarization to 0 mV did not induce LTP or LTD. Inhibition of LTD induction with a phosphatase inhibitor or ruthenium red resulted in induction of LTP after the conjunctive pairing. Such LTP induction was N-methyl--aspartate dependent. Conversely, inhibition of LTP induction with a kinase inhibitor resulted in LTD induction after the conjunctive pairing. Thus the failure to induce LTP or LTD with the pairing protocol involving depolarization to 0 mV membrane potential was due to simultaneous activation of intracellular processes that generate the induction of LTP and LTD. Increasing the frequency of afferent stimulation to 200 Hz, even for just eight stimuli, resulted in LTP induction. The studies show that two factors govern the induction of LTP/LTD, membrane potential and frequency of afferent stimulation, with either increased depolarization or increased afferent stimulation favoring LTP induction.