1. Mouse hippocampal neurons grown in dissociated cell culture were patch clamped using a whole cell voltage clamp (discontinuous switching clamp) technique. The currents generated by pressure applications of excitatory amino acids were studied over a wide range of holding potentials, and current-voltage curves were plotted. Excitatory amino acids that activated the N-methyl-D-aspartic acid (NMDA) receptor demonstrated some degree of desensitization with repeated applications, whereas the currents observed in response to kainic acid (KAI) did not. Desensitization could be minimized by keeping the frequency of application sufficiently low (i.e., less than 0.1 Hz). 2. The short-acting dissociative anaesthetic, ketamine (2–50 microM), selectively blocked L-aspartic acid (L-Asp), NMDA, and L-glutamic acid (L-Glu) currents while sparing those in response to KAI. Therefore, ketamine is a relatively selective blocker of the NMDA response versus that (those) activated by KAI. 3. The block by ketamine of excitatory amino acid currents is highly voltage dependent. Concentrations of ketamine that had little effect on outward current responses at depolarized potentials were quite effective at blocking inward current responses at hyperpolarized potentials. In contrast, DL-2-amino-5-phosphonovaleric acid (APV) was equally effective at blocking both inward and outward currents (voltage independent). The voltage dependence of ketamine (a positively charged molecule) could be accounted for if ketamine blocked the NMDA response by binding to a site that experienced 55% of the membrane field. 4. In the presence of ketamine, peak inward currents evoked by repeated applications of NMDA, L-Asp, or L-Glu progressively declined to a steady-state level of block (use-dependent block). This decrement occurred at frequencies much lower than those that were employed to demonstrate desensitization (in the absence of ketamine). Moving the membrane potential to depolarized values did not, in itself, relieve the ketamine block. However, if the appropriate excitatory amino acid (L-Asp, NMDA, L-Glu) was applied during the period of depolarization, a relief of the block could be demonstrated. No recovery from the blockade occurred with periods of rest (no amino acid application) as long as 5 min. Furthermore, no recovery was observed even when ketamine was washed out of the bathing solution until the appropriate agonist was applied. Thus recovery from blockade, like development of blockade, was use dependent.(ABSTRACT TRUNCATED AT 400 WORDS)
Use of AMS 14C Analysis in the Study of Problems in Aspartic Acid Racemization-Deduced Age Estimates on Bone
