l-Homocysteate Preferentially Activates N-methyl-D-aspartate Receptors to CA1 Rat Hippocampal Neurons

Intracellular recordings and current and single-electrode voltage-clamp techniques were used to study the membrane responses of CA1 pyramidal neurons to bath application of l-homocysteic acid (l-HC) in the rat hippocampal slice preparation. In control artificial cerebrospinal fluid (ACSF), l-HC (25 - 250 microM) depolarized the membrane and induced a burst-like firing pattern. Both the membrane depolarization and the burst firing were blocked by the N-methyl-d-aspartic acid (NMDA) receptor antagonists d-(-)-2-amino-5-phosphonovaleric acid (AP-5, 50 microM), d-(-)-2-amino-7-phosphonoheptanoic acid (AP-7, 50 microM) and (+/-)-3-(2-carboxy-piperazin-4-yl)-propyl-1-phosphonic acid (CPP, 20 microM). In ACSF containing tetrodotoxin (1 microM), l-HC (100 - 300 microM) induced at resting membrane potential a depolarization which was associated with a small increase in input conductance. These effects were unaffected by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 - 20 microM) but were fully blocked by AP-5, AP-7 (50 microM) and CPP (10 - 20 microM). In voltage-clamp experiments, l-HC induced slow inward currents which were voltage-dependent between - 70 and - 30 mV and reversed polarity near 0 mV. The l-HC-induced inward current was unaffected by CNQX (10 - 20 microM) but was strongly reduced by AP-5 or AP-7 (50 microM). The l-HC-induced inward current was temperature-dependent. Between - 60 and - 70 mV, its amplitude increased by 320% when the temperature was lowered from 33 to 22 degrees C. The l-HC-induced current was also potentiated by the specific l-HC uptake blocker beta-p-chlorophenylglutamate (Chlorpheg, 0.5 - 2 mM). These data suggest that l-HC preferentially activates NMDA receptors in CA1 hippocampal neurons.