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

Abstract

SUMMARY1. 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 /M) and amiloride (500 #M). Exposure to 50 /M-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 wo-conotoxin (1 ,tM). From a holding potential of -90 mV, wo-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 /UM) 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 ,UM) 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,M) 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 wt)-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 w)-conotoxin (2 and 1 AM, 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 wi-conotoxin, respectively. From a holding potential of -50 mV, the combination of MS 8476 9-2 D. J. MOGUL AND A. P. FOX nimodipine and wo-conotoxin substantially blocked the Ca2" channel currents of some cells while others showed little effect. These results indicate that the multiple types of Ca2" channels were differentially distributed rather than incompletely blocked by one of the drugs.7. Single-channel current in cell-attached patches from CA3 neuronal somas showed three different unitary conductance levels of 7, 12 and 23 pS with 90 mmBa2+ as the external charge carrier. Ensemble average currents resembled currents recorded in the whole-cell configuration. No evidence for a fourth channel type was seen at the unitary level, sugg...

GABAB receptor‐mediated inhibition of Ca2+ currents and synaptic transmission in cultured rat hippocampal neurones.

“…It is also interesting to note that a significant portion of ICa remained after application of both w-CgTX and nimodipine (see also Regan et al 1991;Mogul & Fox, 1991), although all of the inward current recorded under the conditions of these experiments was blocked by Cd2-(not shown). It is apparent from the experiments described above that this remaining current is not strongly inhibited by baclofen.…”

“…In contrast, there is a relative scarcity of information on the subtypes of Ca2± channels present in central neurones. While single-channel studies indicate that multiple Ca2+ channel types exist in hippocampal neurones (Fisher, Gray & Johnston, 1990;Mogul & Fox, 1991), it is unclear whether the distinctions between whole-cell Ca2+ current subtypes that have been developed for peripheral neurones are appropriate for central neurones. We have chosen to use pharmacological antagonists of Ca21 channels, alone and in combination, to characterize the Ca2+ current subtypes in cultured pyramidal neurones and to determine which are susceptible to inhibition by baclofen.…”

“…These channels are believed to be the primary target of dihydropyridine ligands (Tsien, Lipscombe, Madison, Bley & Fox, 1988;Plummer et al 1989;Mogul & Fox, 1991). Although some evidence suggests that dihydropyridines may effect other Ca2+ channels (Takahashi, Wakamori & Akaike, 1989) such effects are usually seen at higher concentrations than is needed to effect L-type current.…”

GABAB receptors inhibit low-voltage activated and high-voltage activated Ca2+ channels in sensory neurons via distinct mechanisms

“…Therefore, we investigated effects of GABAB agonist baclofen on voltage-gated Ca 2+ currents in cultured small-diameter DRG neurons. DRG neurons express variety of VGCC subtypes, including high-voltage activated (HVA) N, P/Q and L-type channels [14,15,16,17,18] as well as low-voltage activated (LVA) T-type Ca 2+ channels [19,20,21].…”

Calcium Channel Density and Hippocampal Cell Death with Age in Long-Term Culture

“…The hippocampus is important for memory and cognitive processing (Morris et al, 1982;Z ola-Morgan and Squire, 1990) and is highly vulnerable to aging and neurodegenerative influences (Wisniewski and Terry, 1973;Coleman and Flood, 1987;Barnes, 1991;deToledo-Morrell and Morrell, 1991;Landfield et al, 1992;Disterhoft et al, 1994;West et al, 1994;Geinisman et al, 1995). In addition, hippocampal neurons are characterized by a high density of voltage-activated Ca 2ϩ currents (Wong and Prince, 1981;Fisher et al, 1990;Brown and Jaffe, 1994) and contain the multiple types of high voltageactivated (HVA) Ca 2ϩ channels that are present in many other neurons Fisher et al, 1990;Mogul and Fox, 1991;Llinás et al, 1992;Mintz et al, 1992;Eliot and Johnston, 1994;Dunlap et al, 1995;Elliot et al, 1995).…”