MEMBRANE CURRENTS INDUCED BY L-HOMOCYSTEIC ACID IN MOUSE CULTURED HIPPOCAMPAL-NEURONS

The concentration-response relationship of membrane currents induced by L-homocysteic acid was studied on mouse embryonic hippocampal neurons in culture (n = 56). In the majority of neurons two phases in the dose-response relationship could be distinguished. The first was characterized by responses to 3-100-mu-M L-homocysteic acid which desensitized with a time-constant > 1 s in a concentration-dependent manner and were antagonized by 30-mu-M D-L-2-amino-5-phosphonovaleric acid indicating activation of the N-methyl-D-aspartate receptors. At higher concentrations of L-homocysteic acid this component was strongly depressed. The second phase was characterized by sustained responses that were concentration-dependent (1 mM L-homocysteic acid maximum concentration tested) and were not blocked by D-L-2-amino-5-phosphonovaleric acid indicating activation of non-N-methyl-D-aspartate receptors. Eight neurons did not exhibit these two-phase characteristics in the concentration-response relationship at the beginning of the recording. The magnitude of responses to L-homocysteic acid was positively related to concentration and the responses were partially blocked by D-L-2-amino-5-phosphonovaleric acid. In these neurons, however, repeated applications of L-homocysteic acid at concentrations 30-mu-M up to 300-mu-M resulted in a long-lasting, three- to four-fold increase of the membrane current. This increase was completely blocked by D-L-2-amino-5-phosphonovaleric acid (50-100-mu-M) suggesting that it was produced by activation of receptors. It is suggested that the long-lasting increase of the membrane current after repeated L-homocysteic acid application reflects long-term changes in the mechanisms involved in controlling N-methyl-D-aspartate receptors from an intracellular site in which Ca2+ plays an important but not an exclusive role.