Does intracellular release of Ca2+ participate in the afterhyperpolarization of a sympathetic neurone?

Mitochondrial modulation of Ca2+ -induced Ca2+ -release in rat sensory neurons

Ca2+ -induced Ca2+ -release (CICR) from ryanodine-sensitive Ca2+ stores provides a mechanism to amplify and propagate a transient increase in intracellular calcium concentration ([Ca2+]i). A subset of rat dorsal root ganglion neurons in culture exhibited regenerative CICR when sensitized by caffeine. [Ca2+]i oscillated in the maintained presence of 5 mM caffeine and 25 mM K+. Here, CICR oscillations were used to study the complex interplay between Ca2+ regulatory mechanisms at the cellular level. Oscillations depended on Ca2+ uptake and release from the endoplasmic reticulum (ER) and Ca2+ influx across the plasma membrane because cyclopiazonic acid, ryanodine, and removal of extracellular Ca2+ terminated oscillations. Increasing caffeine concentration decreased the threshold for action potential-evoked CICR and increased oscillation frequency. Mitochondria regulated CICR by providing ATP and buffering [Ca2+]i. Treatment with the ATP synthase inhibitor, oligomycin B, decreased oscillation frequency. When ATP concentration was held constant by recording in the whole cell patch-clamp configuration, oligomycin no longer affected oscillation frequency. Aerobically derived ATP modulated CICR by regulating the rate of Ca2+ sequestration by the ER Ca2+ pump. Neither CICR threshold nor Ca2+ clearance by the plasma membrane Ca2+ pump were affected by inhibition of aerobic metabolism. Uncoupling electron transport with carbonyl cyanide p-trifluoromethoxy-phenyl-hydrazone or inhibiting mitochondrial Na+/Ca2+ exchange with CGP37157 revealed that mitochondrial buffering of [Ca2+]i slowed oscillation frequency, decreased spike amplitude, and increased spike width. These findings illustrate the interdependence of energy metabolism and Ca2+ signaling that results from the complex interaction between the mitochondrion and the ER in sensory neurons.

Regulation of membrane excitability in the central nervous system by serotonin receptor subtypes

Serotonin exerts multiple electrophysiological effects on neurons of the central nervous system. It is now known that this diversity reflects at least in part the existence of multiple serotonin receptor subtypes. An example of this occurs in the CA1 region of the hippocampus where as many as ten different serotonin receptor subtypes appear to be expressed. Recent electrophysiological studies have been able to assign specific functional roles to at least 5 of these receptors. These receptors are differentially expressed in the two different cell types present in this region, pyramidal cells and GABAergic interneurons, and mediate different effects on membrane excitability. This distribution is consistent with the different functional roles played by these cells in hippocampus. Thus the differential expression of serotonin receptor subtypes in the CA1 region allows serotonin to modify the function of hippocampal neuronal networks in a manner that is both selective and precise.

Intracellular calcium dynamics in response to action potentials in bullfrog sympathetic ganglion cells

1. Dynamic changes in the intracellular free Ca2+ concentration ([Ca2+]i) following electrical membrane activity, were recorded from the neurone soma of the excised bullfrog sympathetic ganglion, using Fura-2 fluorescence and compared with the accompanying Ca(2+)-dependent electrical membrane responses. 2. The resting [Ca2+]i was about 100 nM, a value little changed by penetration with an intracellular electrode. 3. A net rise in fluorescence at a wavelength of 340 nm (Ca2+ transient) induced by a single action potential in Ringer solution rose almost in parallel with the initial decay phase of a slow Ca(2+)-dependent after-hyperpolarization; decayed in parallel with the late phase; and increased in amplitude and duration in the presence of tetraethylammonium (20 mM). 4. A Ca2+ transient induced by repetitive action potentials was increased asymptotically in amplitude and progressively in duration by increasing the number of spikes, and was slower in time course than the associated Ca(2+)-dependent K+ current. 5. Scanning a single horizontal line across the cytoplasm with an ultraviolet argon ion laser (351 nm) and recording Indo-1 fluorescence with a confocal microscope demonstrated an inward spread of a rise in [Ca2+]i following a tetanus. 6. Both single spike- and tetanus-induced Ca2+ transients were abolished in a Ca(2+)-free solution, while single or repetitive transient rises in [Ca2+]i induced by caffeine (5-10 mM) were generated under the same conditions. 7. Ryanodine (10-50 microM) did not affect tetanus-induced Ca2+ transients, whereas it blocked completely the caffeine-induced oscillation of [Ca2+]i. 8. Ca2+ transients induced by a tetanus in Ringer solution were independent of the interval from the preceding tetanus. The amplitude of Ca2+ transients induced by a tetanus in the presence of caffeine (5 mM) was equal to, or greater than, that generated in Ringer solution in any of the phases of [Ca2+]i oscillation. 9. It is suggested that under the physiological conditions here, the induction of action potentials does not cause the release of Ca2+ in the cells of the freshly excised bullfrog sympathetic ganglion, and that Ca(2+)-buffering systems contribute not only to lowering a transient rise in [Ca2+]i but also to sustaining an increased [Ca2+]i after a large Ca2+ load into the cell.

Multiple muscarinic responses directly evoked in isolated neurones dissociated from rabbit sympathetic ganglia

Single sympathetic neurones, enzymatically isolated from the superior cervical ganglia of adult rabbits and maintained in culture medium, responded directly to DL-muscarine with a variety of electrical responses. The falling phase of the action potential was markedly accelerated while the late portion of the after-spike hyperpolarization was depressed. These changes occurred before any detectable change in membrane potential. The slow depolarizing changes in membrane potential induced by muscarine were associated with different changes in membrane conductance: a voltage-independent increase, a decrease at the levels of membrane potential positive to about -70 mV and a voltage-independent decrease. Muscarine can produce these multiple membrane changes often within the same cell, suggesting that the different effects are not due to heterogeneous cell types.

Calcium-dependent after-potentials in visceral afferent neurones of the rabbit

Intracellular recordings were made from neurones in nodose ganglia excised from rabbits. In C neurones, 1-60 action potentials were followed by an after-hyperpolarization with a peak amplitude of 16 mV and a time constant of decay ranging from 3 to 10 s. In A neurones, the action potentials were followed only by a brief (up to 50 ms) after-hyperpolarization. The after-hyperpolarization was associated with an increase in the membrane conductance to potassium ions; it reversed polarity at the potassium equilibrium potential. The increase in conductance following the action potentials was blocked by removal of calcium ions, or addition of cobalt to the extracellular solution. Intracellular injection of ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) abolished the after-hyperpolarization; intracellular injection of calcium mimicked the after-hyperpolarization. It is concluded that calcium entry during the action potential leads to a long-lasting increase in potassium conductance in visceral afferent C neurones.

Calcium-dependent hyperpolarizations in bullfrog sympathetic neurons

Intracellular recordings were made from neurons in bullfrog sympathetic ganglia. Fast B and slow B neurons were identified and selected for the analysis [Dodd and Horn (1983) J. Physiol., Lond. 334, 255-269]. A single soma action potential was followed by a prolonged afterhyperpolarization lasting for several hundred ms up to 2 s. Part of the spike afterhyperpolarization was due to potassium conductance activation triggered by calcium entry during an action potential. Acetylcholine was directly applied onto the soma membrane by iontophoresis. A rapid nicotinic depolarization was followed by a slow muscarinic depolarization. The nicotinic depolarization was followed by a hyperpolarization when the muscarinic depolarization was blocked by scopolamine. This hyperpolarization was several mV in amplitude and from 1 to 10 s in duration. It disappeared when the preceding nicotinic depolarization was blocked by (+)-tubocurarine. A single fast excitatory postsynaptic potential was also followed by a hyperpolarization in the presence of scopolamine. The acetylcholine-induced hyperpolarization was due to potassium conductance activation triggered by calcium entry during the nicotinic depolarization. The present findings show that non-synaptic autoinhibition is operating in sympathetic neurons. In other words, a rapid nicotinic transmission leads to prolonged hyperpolarizations which are not mediated by any transmitters but are mediated by calcium.

(+)-Tubocurarine blocks the Ca2+-dependent K+-channel of the bullfrog sympathetic ganglion cell

(+)-Tubocurarine [+)-Tc: 10-100 microM) reduced the duration of the afterhyperpolarization, which was induced by the activation of Ca2+-dependent K+-conductance (GK,Ca) following an action potential in the bullfrog sympathetic ganglion cell, but did not affect the maximum rates of rise and fall of Na+- and Ca2+-dependent action potentials. The amplitudes of slow rhythmic membrane hyperpolarizations produced by rhythmic rises in the GK,Ca were also decreased by (+)-Tc without a change in their intervals. Thus, (+)-Tc appears to block the Ca2+-dependent K+-channel of the bullfrog sympathetic ganglion cell.

Muscarinic agonists depress calcium-dependent gK in bullfrog sympathetic neurons

Intracellular recordings were made from 'fast B' [9] neurons in bullfrog sympathetic ganglia. A single soma action potential was followed by a prolonged after-hyperpolarization lasting for several hundred milliseconds up to 2 s. The spike afterhyperpolarization, which is generated by calcium-dependent potassium conductance increase (gKCa) [3,20-24], was shortened by the muscarinic action of acetylcholine and oxotremorine (30-300 nM). These concentrations of muscarinic agonists were too low to cause any detectable changes in resting membrane potential, input resistance or action potential wave form. ACh released from presynaptic terminal under a physiological condition also caused the shortening of the calcium-dependent hyperpolarization. The results suggested that the shortening of calcium-dependent spike afterhyperpolarization may permit the neuron to pass the high frequency of discharge during the muscarinic excitation.

Calcium entry through acetylcholine-channels can activate potassium conductance in bullfrog sympathetic neurons

Fast B neurons in bullfrog sympathetic ganglia were voltage clamped with two microelectrodes. Acetylcholine (ACh) was applied onto the soma membrane by iontophoresis. A rapid nicotinic inward current was followed by a slow muscarinic inward current. After an addition of scopolamine to Ringer solution so as to block the muscarinic current, the nicotinic inward current was found to be followed by an outward current lasting for several hundred ms. It disappeared when the preceding nicotinic inward current was blocked by (+)-tubocurarine. The ACh-induced outward current was due to calcium entry through ACh-channels and subsequent opening of potassium channels. This may indicate that a rapid excitatory transmission leads to non-synaptic autoinhibition. The interaction between the calcium-dependent potassium conductance and the muscarinic action of ACh is proposed.

Origin of calcium ions involved in the generation of a slow afterhyperpolarization in bullfrog sympathetic neurones

The origin of Ca2+ that activates the Ca2+-dependent K+ conductance which is responsible for the slow afterhyperpolarization (a.h.p.) following an action potential was studied in bullfrog sympathetic ganglia. The decay phase of the a.h.p. was a graded function of the extracellular Ca2+, and showed a voltage sensitivity opposite to that of the Ca2+-dependent K+-current reported previously (Pallotta et al. 1981), indicating that it reflected the time course of an increase in intracellular free Ca2+. An a.h.p. of longer duration was generated in cells which showed more pronounced rhythmic hyperpolarizations induced by intracellular Ca2+ release. The duration of the a.h.p. recorded with electrodes filled with K3-citrate [a.h.p. (citrate)], which favors Ca2+ release, was longer than the a.h.p. recorded with KCl-filled electrodes [a.h.p. (C1)]. D-600 (50-100 microM) drastically reduced the a.h.p. (C1), but had less effect on the a.h.p. (citrate). Caffeine which facilitates Ca2+ release prolonged the a.h.p. (C1), but had less effect on the a.h.p. (citrate). The a.h.p. (citrate) showed a greater sensitivity to a low temperature than the a.h.p. (C1). Mn2+ (1-3 mM) depressed both types of a.h.ps to the same extent. These results suggest that the origin of intracellular Ca2+ for a.h.p. (C1) is mainly Ca2+ influx during an action potential, while that for the a.h.p. (citrate) is both Ca2+ entry and intracellular Ca2+ release, although the effect of Mn2+ is difficult to explain fully.