Abstracts

RATIONALE: In the hippocampal slice preparation, episodic population bursts in area CA3 terminate due to depletion of releasable glutamate at recurrent collateral terminals. The next burst occurs only after reaccumulation of releasable neurotransmitter. The rate of depletion and possibly reaccumulation of releasable neurotransmitter can be altered by extracellular calcium concentration ([Ca2+]o). We investigated the effect of [Ca2+]o on the rate of reaccumulation of releasable neurotransmitter. METHODS: Hippocampal slices were prepared in standard fashion. Spontaneous bursting in the CA3 pyramidal cell layer was induced by tetanic stimulation. The extracellular ACSF contained a [Ca2+] of 0.8, 1.3, or 1.8 mM. The extracellular magnesium concentration was also manipulated to maintain a constant divalent cation ratio. Throughout the experiment the ACSF also contained Picrotoxin, CGP and Nor-epinephrine or Apamin. For each [Ca2+]o, evoked bursts were induced at random time intervals (50 - 3000 ms) following a spontaneous burst. The evoked burst lengths vs the stimulation delays were determined and the analyzed data were fit to an exponential equation with one free variable, , the time constant for neurotransmitter reaccumulation. RESULTS: The at the high [Ca2+]o was shorter than the at the initial low baseline [Ca2+]. The average decrease was 56%. When the [Ca2+]o was changed back to the baseline value the remained shorter despite the equivalent concentrations. The average decrease was 67% from baseline. CONCLUSIONS: A higher [Ca2+]o resulted in a more rapid reaccumulation of releasable neurotransmitter. This change persisted after changing the [Ca2+] back to the original concentration. There appear to be long-term changes in neurotransmitter reaccumulation that occur with an increase in the [Ca2+]o.