Mechanism and generality of long-term potentiation

It has been proposed that long-term potentiation (LTP), a strong candidate for a substrate of recognition memory, is expressed by changes in dendritic spine dimensions that reduce spine neck resistance. This hypothesis predicts that the difference between potentiated synapses and unpotentiated synapses must be reduced if synaptic conductance is lowered. However, when synaptic conductance was lowered by either partially blocking postsynaptic receptors or reducing transmitter release, potentiated and control responses were reduced by about the same extent in the field of CA1 of hippocampus. This result provides evidence against the hypothesis that LTP results from a reduction of spine neck resistance.; Receptor pharmacology of the piriform cortex was studied as a first step to investigating the extent to which characteristics of hippocampal LTP generalize to other cortical regions. Synaptic responses of the lateral olfactory tract (LOT) and intrinsic feedback associational (ASSN) systems were completely blocked by twenty micromolar 6,7-dinitro-quinoxaline-2,3-dione, a non-NMDA receptor antagonist, but not affected by fifty micromolar D-amino-phosphonopentanoate, an NMDA receptor antagonist, in medium containing 2.5 mM Mg{dollar}sp{lcub}++{rcub}{dollar}. However, lowering Mg{dollar}sp{lcub}++{rcub}{dollar} concentration exposed a D-amino-phosphonopentanoate-sensitive component, indicating that there are NMDA receptors in those synapses which are quiescent at high Mg{dollar}sp{lcub}++{rcub}{dollar} concentration as in the hippocampus.; LTP was induced with low probability at LOT synapses and with higher probability at ASSN synapses by burst stimulation patterned after the brain EEG theta rhythm. However LTP was always induced at LOT synapses when the Mg{dollar}sp{lcub}++{rcub}{dollar} concentration was lowered. The LTP induction process was cooperative, reversibly blocked by D-amino-phosphonopentanoate, and facilitated by suppression of IPSPs by priming stimulation or GABA antagonists. LTP was stable throughout the recording periods (hours), increased stepwise, expressed mainly through non-NMDA responses, and expressed without decremental short-term potentiation. These results suggest that LTP in piriform cortex is similar to LTP in hippocampal CAI field except for the absence of short-term potentiation.; ASSN synapses did not show paired-pulse facilitation while LOT synapses did. However NMDA responses of ASSN synapses showed paired-pulse facilitation. Further analysis indicates that there is a Ca{dollar}sp{lcub}++{rcub}{dollar}-insensitive depression in ASSN synapses which could be due to either presynaptic or non-NMDA receptor-specific depression.