Evidence for multiple types of Ca2+ channels in acutely isolated hippocampal CA3 neurones of the guinea‐pig.

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Abstract
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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