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Department of Pharmacological and Physiological Sciences, University of Chicago, IL 60637.

1. Current through Ca2+ channels was studied in acutely isolated guinea-pig pyramidal neurones from the CA3 region of the hippocampus. Both the whole-cell and single-channel patch-clamp configuration were used. 2. Both whole-cell and single-channel currents displayed holding potential sensitivity indicative of two high-threshold currents similar to L- and N-type Ca2+ currents. 3. A low-threshold whole-cell current, similar to T-type current seen in dorsal root ganglion (DRG) neurones, activated at -60 to -50 mV and was blocked by nickel (100 microM) and amiloride (500 microM). Exposure to 50 microM-cadmium left a fraction of the T-type current intact but blocked N- and L-type current. This T-like component needed extremely negative holding potentials to be completely reprimed. 4. Whole-cell N-type Ca2+ channel current was blocked by omega-conotoxin (1 microM). From a holding potential of -90 mV, omega-conotoxin decreased the peak whole-cell current by 33%. 5. A slowly inactivating high-threshold Ca2+ current (L-type) that was present at depolarized holding potentials, displayed dihydropyridine sensitivity. From a holding potential of -50 mV, addition of the dihydropyridine Ca2+ channel antagonist nimodipine (2 microM) to the bath decreased whole-cell peak current by 45%. Interestingly, at negative holding potentials nimodipine worked as an agonist. From a holding potential of -90 mV, nimodipine (2 microM) increased peak current at test potentials from -50 to -20 mV and shifted the peak of the current-voltage relationship in the hyperpolarizing direction similar to the effect of Ca2+ channel agonist Bay K 8644. Exposure to Bay K 8644 (2 microM) increased peak current and single channel open probability independent of holding potential while shifting the peak of the whole-cell current-voltage relationship 11 mV in the hyperpolarizing direction. Our experiments suggest that there are approximately the same number of L-type as omega-conotoxin sensitive N-type Ca2+ channels in CA3 neurones. 6. A high-voltage-activated whole-cell current was still present in cells exposed to both nimodipine and omega-conotoxin (2 and 1 microM, respectively) suggesting the existence of a fourth type of Ca2+ channel in these neurones or that a population of either L-type or N-type Ca2+ channels did not respond to dihydropyridine antagonists or omega-conotoxin, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

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