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Calcium regulation in long-term changes of neuronal excitability in the hippocampal formation

University of British Columbia

The regulation of calcium (Ca²⁺) was examined during long-term changes of neuronal excitability in the mammalian CNS. The preparations under investigation included the kindling model of epilepsy, a genetic form of epilepsy and long-term potentiation (LTP) of neuronal activity. The study also includes a discussion of the possible roles of a neuron-specific calcium-binding protein (CaBP). The findings are summarized as follows: 1) The distribution of CaBP was determined in cortical areas of the rat using a specific radioimmunoassay. The protein was found to have an unequal distribution in various cortical areas with preponderence in ventral structures. 2) Extending previous studies on the role of CaBP in kindling-induced epilepsy, its decline was correlated to the number of evoked afterdischarges (AD's) during the process of kindling. 3) Marked changes in CaBP levels were also found in the brains of the epileptic strain of mice (El). The hippocampal formation and the dorsal occipital cortex contained significantly lower CaBP than the control (CF-1) strain. The induction of seizures further decreased the levels of CaBP in the El mice. These findings are indicative of a possible genetic impairment of neuronal Ca²⁺ homeostasis in the El strain. 4) The levels of total hippocampal Ca²⁺ and Zn²⁺ were measured by atomic absorption spectrophotometry in control and commissural-kindled animals. While no change was found in the total Ca²⁺ content of the region, hippocampal Zn²⁺ of kindled preparations was found to be significantly elevated. 5) To measure Ca²⁺ -homeostasis, the kinetic analysis of ⁴⁵Ca uptake curves was undertaken in the in vitro hippocampus. This technique was found to be a valid method for assessment of Ca²⁺-regulation in the CNS under both physiological and pathophysiological conditions. The effect of various extracellular Ca²⁺ concentrations, 2,3-dinitrophenol (DNP), calcitonin, nifedipine and 3-isobutyl-1-methylxanthine (IBMX) on ⁴⁵Ca uptake curves was examined in order to identify the two exchangeable Ca²⁺ pools derived through kinetic analysis. 6) The kinetic analysis of ⁴⁵Ca uptake curves revealed that Ca²⁺-regulation of the hippocampus is impaired following amygdala- and commissural kindling. The changes reflect an enhancement of a Ca²⁺ pool that includes free cytosolic Ca²⁺ and a concomitant decrease in the amount of buffered calcium probably as a result in the decrease of hippocampal CaBP levels. 7) A novel form of long-term potentiation (LTP) of neuronal activity in the CA1 region of the hippocampus is described. Perfusion of 100 uM of IBMX in the hippocampal slice preparation induced a long lasting increase in the amplitude of the stratum radiatum evoked population spike and EPSP responses with changes in synaptic efficacy as indicated by the altered input/output relationships. Intracellular correlates of IBMX-induced LTP included lowering of synaptic threshold and enhancement of the rate of rise of the EPSP with no alterations in the passive membrane characteristics of CA1 pyramidal neurons. The fact that IBMX was able to exert its effect even in the presence of the calcium-blocker cation Co²⁺, taken together with the drug's action on hippocampal exchangeable Ca²⁺, raises the possibility that the Ca²⁺ necessary for induction of LTP may be derived from an intraneuronal storage site. These studies indicate the significance of intracellular Ca²⁺ -regulatory mechanisms in long-term changes of neuronal excitability which occur in experimental models of epilepsy and long-term potentiation.

Text

2010-06-23

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http://hdl.handle.net/2429/25929

Physiology

University of British Columbia

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