Slow frequency-dependence of action potential afterhyperpolarization in bullfrog sympathetic ganglion neurones

Ca(2+)-induced Ca2+ release and its activation in response to a single action potential in rabbit otic ganglion cells

1. Ryanodine-sensitive intracellular Ca2+ release activated by Ca2+ entry was studied with fura-2 fluorescence and intracellular voltage recording techniques in rabbit otic ganglion cells. 2. The removal of extracellular Ca2+ reduced sustained, transient or oscillatory rises in intracellular Ca2+ ([Ca2+]i) induced at high extracellular K+ and abolished the [Ca2+]i oscillation in cultured neurones. 3. Ryanodine (10 microM) transiently increased [Ca2+]i and reduced the amplitude and rate of rise of the high-K(+)-induced rise in [Ca2+]i, while caffeine (5 mM) produced a few transient rises in [Ca2+]i in most cultured cells and [Ca2+]i oscillation only in one cell. 4. The two components of the slow after-hyperpolarization (AHP) of an action potential in neurones of freshly isolated ganglia were dependent on extracellular Ca2+ and abolished by Ca2+ channel blockers, Cd2+ or Co2+. 5. The late component of AHP (LAHP), but not the initial component, in 'fresh' neurones increased in area with an increase in the preceding interval, was abolished by ryanodine (10 microM) and intracellularly injected EGTA, and mimicked by intracellular injection of Ca2+. 6. A ryanodine-sensitive Ca(2+)-induced Ca2+ release thus exists, operates in response to an action potential-induced Ca2+ entry and underlies LAHP in rabbit otic ganglion cells.

Cyclic ADP-ribose modulates Ca2+ release channels for activation by physiological Ca2+ entry in bullfrog sympathetic neurons

Although Ca(2+)-induced Ca2+ release (CICR) via ryanodine receptors has been found to occur in intact neurons, little is known about the physiological processes that regulate it. We studied the effects of cyclic ADP-ribose (cADPR) on CICR in cultured bullfrog sympathetic neurons by fura-2 fluorescence recording and patch-clamp techniques. cADPR applied through a patch pipette augmented action potential- or depolarizing pulse-induced rises in intracellular Ca2+ without a change in Ca2+ entry initiating the responses, but not in the presence of ryanodine. Likewise, cADPR enhanced a single or oscillatory rise(s) in intracellular Ca2+ induced by caffeine. These results strongly suggest that cADPR can be an endogenous modulator of ryanodine receptors in neurons.

Kindling-induced long-lasting enhancement of calcium current in hippocampal CA1 area of the rat: relation to calcium-dependent inactivation

Daily tetanization of the Schaffer collaterals (kindling) in the rat hippocampus induces a persistent epileptogenic focus in area CA1. Neurons were enzymatically isolated from the focal region one day or six weeks after seven class V generalized seizures had been evoked. Calcium currents were measured under voltage-clamp conditions in the whole-cell patch configuration. One day after kindling, as well as six weeks later, the amplitudes of a slow-inactivating (tau = 90 ms) and a non-inactivating calcium current component were, in comparison to controls, enhanced by 30 and 40%, respectively. This enhancement was therefore related to the kindled state of enhanced excitability. The enhancement of the calcium current was independent of the steady-state intracellular calcium concentration. Fast calcium-dependent inactivation was provoked with double-pulse protocols that conditioned the neuron with a defined calcium-influx in the first pulse. Despite the larger calcium current during the conditioning pulse, the relative calcium-dependent inactivation of the sustained current component was reduced in neurons from the kindled focus. Repetitive depolarizations, once every second, evoked a cumulative calcium-dependent inactivation. Nothwithstanding the larger calcium current, kindling also persistently reduced this slow inactivation of both transient and sustained high threshold calcium current. The reduction in calcium-dependent inactivation cannot be responsible for the increased current, but can certainly enhance the calcium influx during prolonged activation or seizures. The changes can be explained by assuming that additional calcium channels are recruited at a location that prevents calcium-dependent inactivation.

Changes in sodium and calcium channel activity following axotomy of B-cells in bullfrog sympathetic ganglion

1. Currents mediated by Ca2+ channels using Ba2+ as a charge carrier (IBa), Na+ currents (INa) and voltage- and Ca(2+)-dependent K+ currents (IC) were recorded from bullfrog paravertebral sympathetic ganglion B-cells using whole-cell patch-clamp recording techniques. Currents recorded from control cells were compared with those from axotomized cells 13-15 days after transection of the postganglionic nerve. 2. Axotomy reduced peak IBa at -10 mV (holding potential = -80 mV) from 3.3 +/- 0.3 nA (n = 42) to 1.7 +/- 0.1 nA (n = 39, P < 0.001). Tail IBa at -40 mV following a step to +70 mV from a holding potential of -80 mV was also reduced in axotomized neurones (9.7 +/- 0.6 nA for forty-two control neurones and 5.2 +/- 0.3 nA for thirty-nine axotomized neurones; P < 0.001). Minimal changes were observed in the kinetics of activation and deactivation. 3. Pharmacological experiments using 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2- trifluoromethylphenyl)-pyridine-5-carboxylic acid methyl ester (BayK 8644), nifedipine and omega-conotoxin showed that axotomy predominantly affected the N-type Ca2+ channels which carry the majority of ICa in these neurones. L-type Ca2+ current was little affected and T-type Ca2+ currents were not observed in control or axotomized cells. 4. Development of inactivation of 0 mV and recovery from inactivation of IBa at -80 mV exhibited three distinct components in both control and axotomized neurones: 'fast', 'intermediate' and 'slow'. The relative proportions of both the 'fast' and 'intermediate' components of inactivation at 0 mV were almost doubled after axotomy (fast component was 15% in control and 29% in axotomized neurones; intermediate component was 17% in control and 26% in axotomized neurones). 'Fast' and 'intermediate' inactivation tended to develop more rapidly and recover more slowly after axotomy. The rate of onset of 'slow' inactivation was unaffected by axotomy but the steady-state level at -40 mV was increased. Most of the change in IBa properties may be secondary to enhanced inactivation associated with axotomy. 5. Axotomy reduced IC (measured at the end of a 3 ms step from -40 to +20 mV) from 34.5 +/- 4.9 (n = 26) to 19.2 +/- 1.5 nA (n = 49, P < 0.005). This reduction may be secondary to the reduction in calcium channels available for activation from -40 mV following axotomy. 6. The TTX-sensitive and TTX-insensitive components of peak Na+ conductance (GNa) were both increased after axotomy. Total GNa was increased from 184.9 +/- 8.4 to 315.2 +/- 16.4 nS (n = 37 for both P < 0.001). Most of the kinetic and steady-state properties of INa were unchanged after axotomy.(ABSTRACT TRUNCATED AT 400 WORDS)