A NOVEL FORM OF LONG-TERM DEPRESSION IN THE CA1 AREA OF THE ADULT-RATHIPPOCAMPUS INDEPENDENT OF GLUTAMATE RECEPTORS ACTIVATION

In young rats, low frequency (1-2 Hz) stimulation of the Schaffer coll aterals for 15 min induces in the CA1 area of the hippocampus a homosy naptic and N-methyl-D-aspartate receptor-dependent form of long-term d epression (LTD) of synaptic efficacy. In the adults, while a similar s timulation paradigm is able to depress previously potentiated synapses , it leads to conflicting results when applied to naive synapses. In t he present experiments, different stimulation paradigms have been used to induce LTD in the CA1 area of the adult rat hippocampus in vitro. Thus, stimulation of the afferent pathway at frequencies higher than t hose used to produce LTD in young animals (5-10 Hz, for 15 min) reliab ly induced a homosynaptic form of LTD. This form of LTD was associated with a significant increase in paired-pulse facilitation ratio and wa s insensitive to ionotropic (CNQX, 10 mu M and CPP, 20 mu M) and metab otropic (S-MCPG, 1 mM) glutamate receptors antagonists, suggesting a p resynaptic mechanism for both LTD induction and expression. In conclus ion, our experiments clearly show that LTD is not a purely development al phenomenon but is present also in mature rats, which possess the wh ole machinery for LTD induction and this will greatly enhance the flex ibility and the storing capacity of neuronal circuits.