Influence of Na/Ca exchange and mobilization of intracellular calcium on the time course of the slow afterhyperpolarization current (IAHP) in bullfrog sympathetic ganglion neurons

Ryanodine-sensitive stores regulate the excitability of AH neurons in the myenteric plexus of guinea-pig ileum

Myenteric afterhyperpolarizing (AH) neurons are primary afferent neurons within the gastrointestinal tract. Stimulation of the intestinal mucosa evokes action potentials (AP) that are followed by a slow afterhyperpolarization (AHP(slow)) in the soma. The role of intracellular Ca(2+) ([Ca(2+)](i)) and ryanodine-sensitive Ca(2+) stores in modulating the electrical activity of myenteric AH neurons was investigated by recording membrane potential and bis-fura-2 fluorescence from 34 AH neurons. Mean resting [Ca(2+)](i) was approximately 200 nM. Depolarizing current pulses that elicited APs evoked AHP(slow) and an increase in [Ca(2+)](i), with similar time courses. The amplitudes and durations of AHP(slow) and the Ca(2+) transient were proportional to the number of evoked APs, with each AP increasing [Ca(2+)](i) by approximately 50 nM. Ryanodine (10 microM) significantly reduced both the amplitude and duration (by 60%) of the evoked Ca(2+) transient and AHP(slow) over the range of APs tested (1-15). Calcium-induced calcium release (CICR) was graded and proportional to the number of APs, with each AP triggering a rise in [Ca(2+)](i) of approximately 30 nM Ca(2+) via CICR. This indicates that CICR amplifies Ca(2+) influx. Similar changes in [Ca(2+)](i) and AHP(slow) were evoked by two APs in control and six APs in ryanodine. Thus, the magnitude of the change in bulk [Ca(2+)](i) and not the source of the Ca(2+) is the determinant of the magnitude of AHP(slow). Furthermore, lowering of free [Ca(2+)](i), either by reducing extracellular Ca(2+) or injecting high concentrations of Ca(2+) buffer, induced depolarization, increased excitability, and abolition of AHP(slow). In addition, activation of synaptic input to AH neurons elicited a slow excitatory postsynaptic potential (sEPSP) that was completely blocked in ryanodine. These results demonstrate the importance of [Ca(2+)](i) and CICR in sensory processing in AH neurons. Activity-dependent CICR may be a mechanism to grade the output of AH neurons according to the intensity of sensory input.

Localization of the plasma membrane Ca(2+)-ATPase isoform PMCA3 in rat cerebellum, choroid plexus and hippocampus

mRNA encoding rat plasma membrane Ca(2+)-ATPase isoform PMCA3 was localized in the granule cell layer of the cerebellum and in choroid plexus by in situ hybridization with an 35S-labelled oligodeoxynucleotide probe. In order to examine whether this isoform is expressed as a protein in brain, polyclonal antibodies were raised against a peptide corresponding to a C-terminal 18 amino acid sequence of PMCA3 which had been conjugated to bovine serum albumin. Using immunoblot analysis with affinity-purified antibodies, PMCA3 protein was found in rat brain microsomes and cultured neurons. The translated protein had an observed molecular mass of approximately 135 kDa, as predicted from molecular cloning studies. The pattern of localization of PMCA3 in brain using anti-peptide antibodies was consistent with findings from in situ hybridization. PMCA3-like immunoreactive sites were found in the granule cell and molecular layers of rat cerebellum and in choroid plexus, and the pattern of staining suggests that immunoreactive sites are associated with granule cell processes. This conclusion was supported by the finding that growth-associated protein-43, a protein known to be present in axons and nerve terminals, had a pattern of distribution similar to PMCA3 in the molecular layer of cerebellum. Very low levels of PMCA3-like immunoreactivity were associated with Purkinje cell soma or processes, consistent with the low levels of PMCA3 mRNA found in these neurons. PMCA3-like immunoreactivity was lower in hippocampus than in cerebellum; hippocampal CA1 region immunoreactivity was primarily associated with dendritic fields rather than with pyramidal cell bodies. The results demonstrate that a PMCA3-like protein is expressed in neurons of rat brain and is localized primarily in cell processes.

Changes in potassium channel activity following axotomy of B-cells in bullfrog sympathetic ganglion

1. Whole-cell and microelectrode voltage-clamp techniques were used to investigate the changes in ionic currents and action potential shape that follow axotomy of bullfrog paravertebral sympathetic ganglion B-cells. 2. Axotomy increased M-conductance (gM; muscarine-sensitive, voltage- and time-dependent K+ conductance) by 35% at -30 mV and slowed its deactivation kinetics. 3. The delayed rectifier K+ current (IK; at +50 mV) was reduced in axotomized neurones to 61% of control without any change in activation or deactivation kinetics. Steady-state intracellular Ca2+ levels and leak conductance were unchanged. 4. The fast, voltage-sensitive, Ca(2+)-activated K+ current (IC), evoked from -40 mV, was decreased to about 71% of control (at +30 mV) in axotomized neurones, whereas that evoked from -80 mV was largely unaffected. IC kinetics were also similar in control and axotomized neurones. This suggests that IC channels are not changed after axotomy. 5. In axotomized neurones, commands to +10 from -40 mV had to be extended by 16 ms to evoke voltage-insensitive Ca(2+)-dependent K+ current (IAHP) responses that were similar in magnitude to those observed in control cells. 6. The previously documented, axotomy-induced decrease in Ca2+ current (ICa) due to increased resting inactivation can account for the reduction in IC and IAHP and for the change in the shape of the action potential.

Calcium induced calcium release is involved in the afterhyperpolarization in one class of guinea pig sympathetic neurone

The mechanisms underlying two potassium conductances which are activated by Ca2+ influx during the action potential in sympathetic prevertebral neurones of guinea pigs have been investigated pharmacologically. One Ca-activated K+ conductance, which is present in all mammalian sympathetic postganglionic neurones, is maximal after the action potential and decays exponentially with a time constant of about 130 ms; this conductance was inhibited by apamin (50-100 nM) consistent with the involvement of SK channels. A second Ca-activated K+ conductance with much slower kinetics is present in a large subpopulation of coeliac neurones. This conductance was resistant to apamin but markedly inhibited by application of ryanodine (5-20 microM), suggesting that Ca2+ influx during the action potential triggers release of Ca2+ from intracellular stores which in turn activates a different class of K+ channel. Noradrenaline (100 microM) depressed the second K+ conductance selectively.