Voltage-gated Ca2+ influx and mitochondrial Ca2+ initiate secretion from Aplysia neuroendocrine cells.

Neuroendocrine secretion often requires prolonged voltage-gated Ca(2+) entry; however, the ability of Ca(2+) from intracellular stores, such as endoplasmic reticulum or mitochondria, to elicit secretion is less clear. We examined this using the bag cell neurons, which trigger ovulation in Aplysia by releasing egg-laying hormone (ELH) peptide. Secretion from cultured bag cell neurons was observed as an increase in plasma membrane capacitance following Ca(2+) influx evoked by a 5-Hz, 1-min train of depolarizing steps under voltage-clamp. The response was similar for step durations of ≥ 50 ms, but fell off sharply with shorter stimuli. The capacitance change was attenuated by replacing external Ca(2+) with Ba(2+), blocking Ca(2+) channels, buffering intracellular Ca(2+) with EGTA, disrupting synaptic protein recycling, or genetic knock-down of ELH. Regarding intracellular stores, liberating mitochondrial Ca(2+) with the protonophore, carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazone (FCCP), brought about an EGTA-sensitive elevation of capacitance. Conversely, no change was observed to Ca(2+) released from the endoplasmic reticulum or acidic stores. Prior exposure to FCCP lessened the train-induced capacitance increase, suggesting overlap in the pool of releasable vesicles. Employing GTP-γ-S to interfere with endocytosis delayed recovery (presumed membrane retrieval) of the capacitance change following FCCP, but not the train. Finally, secretion was correlated with reproductive behavior, in that neurons isolated from animals engaged in egg-laying presented a greater train-induced capacitance elevation vs quiescent animals. The bag cell neuron capacitance increase is consistent with peptide secretion requiring high Ca(2+), either from influx or stores, and may reflect the all-or-none nature of reproduction.

Role for protein kinase C in controlling Aplysia bag cell neuron excitability.

Targeting signalling molecules to ion channels can expedite regulation and assure the proper transition of changes to excitability. In the bag cell neurons of Aplysia, single-channel studies of excised patches have revealed that protein kinase C (PKC) gates a non-selective cation channel through a close, physical association. This channel drives a prolonged afterdischarge and concomitant neuropeptide secretion to provoke reproductive behaviour. However, it is not clear if PKC alters cation channel function and/or the membrane potential at the whole-cell level. Afterdischarge-like depolarizations can be evoked in cultured bag cell neurons by bath-application of Conus textile venom (CtVm), which triggers the cation channel through an apparent intracellular pathway. The present study shows that the CtVm-induced depolarization was reduced by nearly 50% compared to control following dialysis with the G-protein blocker, guanosine-5'-O-2-thiodiphosphate (GDP-ß-S), or treatment with either the phospholipase C inhibitor, 1-[6-[[(17ß)-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U-73122), or the PKC inhibitor, sphinganine. Neurons exposed to the PKC activator, phorbol 12-myristate 13-acetate (PMA), displayed depolarization with accompanying spiking, and were found to be far more responsive to depolarizing current injection versus control. Immunocytochemical staining for the two typical Aplysia PKC isoforms, Apl I and Apl II, revealed that both kinases were present in unstimulated cultured bag cell neurons. However, in CtVm-treated neurons, the staining intensity for PKC Apl I increased, peaking at 10 min post-application. Conversely, the intensity of PKC Apl II staining decreased over the duration of CtVm exposure. Our results suggest that the CtVm-induced depolarization involves PKC activation, and is consistent with prior work showing PKC closely-associating with the cation channel to produce the depolarization necessary for the afterdischarge and species propagation.

Ca2+ entry through a non-selective cation channel in Aplysia bag cell neurons.

Ca(2+) influx through voltage-gated Ca(2+) channels is a fundamental signaling event in neurons; however, non-traditional routes, such as non-selective cation channels, also permit Ca(2+) entry. The present study examines the Ca(2+) permeability of a cation channel that drives an afterdischarge in Aplysia bag cell neurons. The firing of these neurons induces peptide release and reproduction. Single channel-containing inside-out patches excised from cultured bag cell neurons, with the cytoplasmic face bathed in K(+)-aspartate and the extracellular face bathed in artificial seawater (11 mM Ca(2+)), had a reversal potential near +50 mV. In keeping with Ca(2+) permeability, this was right-shifted to approximately +60 mV in high Ca(2+) (substituted for Mg(2+)) and left-shifted to around +40 mV in zero Ca(2+) (replaced with Mg(2+)). The current showed inward rectification between +30 and +90 mV, and a conductance of 29 pS in normal Ca(2+), 30 pS in high Ca(2+), 32 pS in Ba(2+) (substituted for Ca(2+)), but only 21 pS in zero Ca(2+). Despite a greater conductance in Ba(2+), the channel did not display anomalous mol fraction in an equimolar Ca(2+)-Ba(2+) mix. Eliminating internal Mg(2+) lowered activity, but did not alter inward rectification, suggesting intracellular Mg(2+) is a fast, voltage-independent blocker. Imaging bag cell neurons in Mn(2+) saline (substituted for Ca(2+)) revealed enhanced fura-quench following cation channel activation, consistent with Mn(2+) permeating as a Ca(2+) surrogate. Finally, triggering the cation channel while tracking capacitance revealed a Ca(2+)-dependent increase in membrane surface area, consistent with vesicle fusion. Thus, the cation channel not only drives the afterdischarge, but also passes Ca(2+) to potentially initiate secretion. In general, this may represent an alternate means by which neurons elicit neuropeptide release.

Stability and variability of synapses in the adult molluskan CNS.

Synaptic transmission was examined between identified neurons in the central nervous system (CNS) of the freshwater mollusk, Lymnaea stagnalis. Four identified neurons were used: Right Pedal Dorsal one (RPeD1; a dopaminergic respiratory interneuron), Visceral Dorsal two and three (VD2/3), and Visceral Dorsal four (VD4; a cardiorespiratory interneuron). Neuron RPeD1 synapses onto both VD2/3 and VD4, while VD4 makes a reciprocal synapse onto RPeD1. When compared from animal to animal, the connections were variable in sign. Previously, we demonstrated that, in a given animal, the RPeD1 --> VD4 synapse could be either inhibitory, biphasic, or undetectable. The present study now expands this concept of variability by showing that the RPeD1 --> VD2/3 synapse was either excitatory or undetectable from animal to animal, while the synapse from VD4 to RPeD1 was observed as inhibitory, biphasic, depolarizing, excitatory, or undetectable. Next, we used 1-day organ culture to determine if the variability observed between animals is a product of ongoing change to the sign of these identified synapses and whether or not the extent of change could be influenced by the culture conditions. Changes to the sign of transmission occurred within minutes and, more commonly, after 24-h organ culture. All three synapses were investigated before and after 1-day organ culture, in either defined medium (DM) or brain-conditioned medium (CM). Regardless of culture conditions, the RPeD1 --> VD2/3 synapse showed no change of sign, i.e., it was relatively stable. However, the synapses between RPeD1 and VD4 did change sign, and when cultured in CM, the VD4 --> RPeD1 synapse changed significantly more than in DM. These data indicate that variability of some synapses reflects changes at these synapses. This is the first report that specific synapses in an adult CNS can change sign, and that the sign of transmission can be modulated by environmental conditions.

Activation of a Ca2+-permeable cation channel produces a prolonged attenuation of intracellular Ca2+ release in Aplysia bag cell neurones.

1. Brief synaptic stimulation, or exposure to Conus textile venom (CtVm), triggers an afterdischarge in the bag cell neurones of Aplysia. This is associated with an elevation of intracellular calcium ([Ca2+]i) through Ca2+ release from intracellular stores and Ca2+ entry through voltage-gated Ca2+ channels and a non-selective cation channel. The afterdischarge is followed by a prolonged (approximately 18 h) refractory period during which the ability of both electrical stimulation and CtVm to trigger afterdischarges or elevate [Ca2+]i is severely attenuated. By measuring the response of isolated cells to CtVm, we have now tested the contribution of different sources of Ca2+ elevation to the onset of the prolonged refractory period. CtVm induced an increase in [Ca2+]i in both normal and Ca2+-free saline, in part by liberating Ca2+ from a store sensitive to thapsigargin or cyclopiazonic acid, but not sensitive to heparin. 3. In the presence of extracellular Ca2+, the neurones became refractory to CtVm after a single application but recovered following approximately 24 h, when CtVm could again elevate [Ca2+]i. However, this refractoriness did not develop if CtVm was applied in Ca2+-free saline. Thus, elevation of [Ca2+]i alone does not induce refractoriness to CtVm-induced [Ca2+]i elevation, but Ca2+ influx triggers this refractory-like state. 4. CtVm produces a depolarization of isolated bag cell neurones. To determine if Ca2+ influx through voltage-gated Ca2+ channels, activated during this depolarization, caused refractoriness to CtVm-induced [Ca2+]i elevation, cells were depolarized with high external potassium (60 mM), which produced a large increase in [Ca2+]i. Nevertheless, subsequent exposure of the cells to CtVm produced a normal response, suggesting that Ca2+ influx through voltage-gated Ca2+ channels does not induce refractoriness. 5. As a second test for the role of voltage-gated Ca2+ channels, these channels were blocked with nifedipine. This drug failed to prevent the onset of refractoriness to CtVm-induced [Ca2+]i elevation, providing further evidence that Ca2+ entry through voltage-gated Ca2+ channels does not initiate refractoriness. 6. To examine if Ca2+ entry through the CtVm-activated, non-selective cation channel caused refractoriness, neurones were treated with a high concentration of TTX, which blocks the cation channel. TTX protected the neurones from the refractoriness to [Ca2+]i elevation produced by CtVm in Ca2+-containing medium. 7. Using clusters of bag cell neurones in intact abdominal ganglia, we compared the ability of nifedipine and TTX to protect the cells from refractoriness to electrical stimulation. Normal, long-lasting afterdischarges could be triggered by stimulation of an afferent input after a period of exposure to CtVm in the presence of TTX. In contrast, exposure to CtVm in the presence of nifedipine resulted in refractoriness. 8. Our data indicate that Ca2+ influx through the non-selective cation channel renders cultured bag cell neurones refractory to repeated stimulation with CtVm. Moreover, the refractory period of the afterdischarge itself may also be initiated by Ca2+ entry through this cation channel.

Dopamine activates two different receptors to produce variability in sign at an identified synapse.

Chemical synaptic transmission was investigated at a central synapse between identified neurons in the freshwater snail, Lymnaea stagnalis. The presynaptic neuron was the dopaminergic cell, Right Pedal Dorsal one (RPeD1). The postsynaptic neuron was Visceral Dorsal four (VD4). These neurons are components of the respiratory central pattern generator. The synapse from RPeD1 to VD4 showed variability of sign, i.e., it was either inhibitory (monophasic and hyperpolarizing), biphasic (depolarizing followed by hyperpolarizing phases), or undetectable. Both the inhibitory and biphasic synapse were eliminated by low Ca2+/high Mg2+ saline and maintained in high Ca2+/high Mg2+ saline, indicating that these two types of connections were chemical and monosynaptic. The latency of the inhibitory postsynaptic potential (IPSP) in high Ca2+/high Mg2+ saline was approximately 43 ms, whereas the biphasic postsynaptic potential (BPSP) had approximately 12-ms latency in either normal or high Ca2+/high Mg2+ saline. For a given preparation, when dopamine was pressured applied to the soma of VD4, it always elicited the same response as the synaptic input from RPeD1. Thus, for a VD4 neuron receiving an IPSP from RPeD1, pressure application of dopamine to the soma of VD4 produced an inhibitory response similar to the IPSP. The reversal potentials of the IPSP and the inhibitory dopamine response were both approximately -90 mV. For a VD4 neuron with a biphasic input from RPeD1, pressure-applied dopamine produced a biphasic response similar to the BPSP. The reversal potentials of the depolarizing phase of the BPSP and the biphasic dopamine response were both approximately -44 mV, whereas the reversal potentials for the hyperpolarizing phases were both approximately -90 mV. The hyperpolarizing but not the depolarizing phase of the BPSP and the biphasic dopamine response was blocked by the D-2 dopaminergic antagonist (+/-) sulpiride. Previously, our laboratory demonstrated that both IPSP and the inhibitory dopamine response are blocked by (+/-) sulpiride. Conversely, the depolarizing phase of both the BPSP and the biphasic dopamine response was blocked by the Cl- channel antagonist picrotoxin. Finally, both phases of the BPSP and the biphasic dopamine response were desensitized by continuous bath application of dopamine. These results indicate that the biphasic RPeD1 --> VD4 synapse is dopaminergic. Collectively, these data suggest that the variability in sign (inhibitory vs. biphasic) at the RPeD1 --> VD4 synapse is due to activation of two different dopamine receptors on the postsynaptic neuron VD4. This demonstrates that two populations of receptors can produce two different forms of transmission, i.e., the inhibitory and biphasic forms of the single RPeD1 --> VD4 synapse.

Modulation of a calcium-sensitive nonspecific cation channel by closely associated protein kinase and phosphatase activities.

Regulation of nonspecific cation channels often underlies neuronal bursting and other prolonged changes in neuronal activity. In bag cell neurons of Aplysia, it recently has been suggested that an intracellular messenger-induced increase in the activity of a nonspecific cation channel may underlie the onset of a 30-min period of spontaneous action potentials referred to as the "afterdischarge. " In patch clamp studies of the channel, we show that the open probability of the channel can be increased by an average of 10. 7-fold by application of ATP to the cytoplasmic side of patches. Duration histograms indicate that the increase is primarily a result of a reduction in the duration and percentage of channel closures described by the slowest time constant. The increase in open probability was not observed using 5'-adenylylimidodiphosphate, a nonhydrolyzable ATP analog, and was blocked in the presence of H7 or the more specific calcium/phospholipid-dependent protein kinase C (PKC) inhibitor peptide(19-36). Because the increase in activity observed in response to ATP occurred without application of protein kinase, our results indicate that a kinase endogenous to excised patches mediates the effect. The effect of ATP could be reversed by exogenously applied protein phosphatase 1 or by a microcystin-sensitive phosphatase also endogenous to excised patches. These results, together with work demonstrating the presence of a protein tyrosine phosphatase in these patches, suggest that the cation channel is part of a regulatory complex including at least three enzymes. This complex may act as a molecular switch to activate the cation channel and, thereby, trigger the afterdischarge.

Trophic and contact conditions modulate synapse formation between identified neurons.

We tested the ability of an identified interneuron from the mollusk, Lymnaea stagnalis, to reestablish appropriate synapses in vitro. In the CNS, the giant dopaminergic neuron, designated as right pedal dorsal one (RPeD1), makes an excitatory, chemical synapse with a pair of essentially identical postsynaptic cells known as visceral dorsal two and three (VD2/3). When the somata of the pre- and postsynaptic neurons were juxtaposed and cultured in vitro in defined medium, i.e. , a soma-soma synapse, only an inappropriate electrical synapse was observed. The postsynaptic cell still responded to applied dopamine, the presynaptic transmitter, indicating that the lack of chemical synapse formation was not due to lack of dopamine receptors. When the somata were cultured apart in conditioned medium (medium previously incubated with Lymnaea CNS, thereby deriving trophic factors), the cells exhibited overlapping neurite outgrowth that resulted in an appropriate excitatory, chemical synapse from RPeD1 to VD2/3. On the other hand, when the cell pair was cultured in a soma-soma configuration, but in conditioned medium, a mixed chemical-electrical synapse was observed. Because conditioned medium could partially overcome the limitations of the soma-soma configuration and initiate chemical synapse formation, this data suggests that conditioned medium contains a factor(s) that supports synaptogenesis.

Localization, physiology, and modulation of a molluskan dopaminergic synapse.

We investigated the location, physiology, and modulation of an identified synapse from the central nervous system (CNS) of the mollusk Lymnaea stagnalis. Specifically, the excitatory synapse from interneuron right pedal dorsal one (RPeD1) to neurons visceral dorsal two and three (VD2/3) was examined. The gross and fine morphology of these neurons was determined by staining with Lucifer yellow or sulforhodamine. In preparations where RPeD1 was stained with Lucifer yellow and VD2/3 with sulforhodamine, the axon collaterals occupied similar regions, suggesting that these neurons make physical contact in the CNS. Digital confocal microscopy of these preparations revealed that presynaptic varicosities made apparent contact (synapses) with smooth postsynaptic axon collaterals. The number of putative synapses per preparation was about five to 10. Regarding physiology, the synaptic latency was moderately rapid at 24.1 +/- 5.2 ms. Previous work indicated that RPeD1 uses dopamine as a neurotransmitter. The RPeD1 --> VD2/3 excitatory postsynaptic potential (EPSP) and the VD2/3 bath-applied dopamine (100-microM) response displayed a similar decrease in input resistance and a similar predicted reversal potential (-31 vs. -26 mV), indicating that the synapse and exogenous dopamine activate the same conductance. Finally, bath-applied serotonin (10 microM) rapidly and reversibly depressed the RPeD1 --> VD2/3 synapse but did not affect the VD2/3 bath-applied dopamine (100-microM) response, suggesting a presynaptic locus of action for serotonin. The effect of serotonin was not associated with any changes to the pre- or postsynaptic membrane potential and input resistance, or the presynaptic action potential half-width. The RPeD1 --> VD2/3 synapse provides an opportunity to examine the anatomy and physiology of transmission, and is amenable to the study of neuromodulation.

Glutamate as a putative neurotransmitter in the mollusc, Lymnaea stagnalis.

Bath-applied glutamate (10-1000 microM) produced excitatory and inhibitory responses on numerous identified neurons of the mollusc Lymnaea stagnalis. Using both in situ and in vitro preparations, glutamate or glutamate agonists produced a depolarization in identified neurons right pedal dorsal 1 and right pedal dorsal 2 and 3. However, attempts to block glutamate-evoked responses with glutamate antagonists were unsuccessful. We examined a potential glutamatergic neuron, visceral dorsal 4. Exogenous application of the peptides (GDPFLRFamide and SDPFLRFamide) could mimic the inhibitory, but not the excitatory effects of visceral dorsal 4 on its postsynaptic cells, implying the presence of a second transmitter. We tested the possibility that glutamate is this second neurotransmitter by using excitatory synapses between visceral dorsal 4 and postsynaptic cells right pedal dorsal 2 and 3, right pedal dorsal 1, visceral F group and right parietal B group neurons. Of all the putative neurotransmitters tested, only glutamate had consistent excitatory effects on these postsynaptic cells. Also, the amplitude of the right pedal dorsal 2 and 3 excitatory postsynaptic potentials was reduced in the presence of N-methyl-D-aspartate and other glutamate agonists, suggesting desensitization of the endogenous transmitter receptor. In conclusion, some identified Lymnaea neurons respond to glutamate via a receptor with novel pharmacological properties. Furthermore, a Lymnaea interneuron may employ glutamate as a transmitter at excitatory synapses.

Dopaminergic transmission between identified neurons from the mollusk, Lymnaea stagnalis.

1. Dopaminergic transmission was investigated in the central nervous system (CNS) of the freshwater snail, Lymnaea stagnalis. 2. The giant pedal neuron, designated as right pedal dorsal one (RPeD1), makes chemical, monosynaptic connections with a number of identified follower cells in the CNS. Previous work has shown that RPeD1 is an interneuron and a important component of the Lymnaea respiratory central pattern generator. In this study, the hypothesis that RPeD1 uses dopamine as its neurotransmitter was tested by chromatographic, pharmacological, and electrophysiological methods. Characterization of RPeD1's transmitter pharmacology is essential to clearly understand its role in Lymnaea. 3. Earlier studies demonstrated that the soma of RPeD1 contains dopamine. This was quantitated in the present study by high-performance liquid chromatography (with electrochemical detection) of isolated RPeD1 somata and growth cones, which yielded 0.8 +/- 0.3 and 0.10 +/- 0.08 pmol of dopamine per soma and growth cone, respectively. 4. Bath or pressure application of dopamine to follower cells of RPeD1, in situ, mimicked the effects of RPeD1 stimulation. Dose-response curves were constructed for the excitatory effect of dopamine on follower cells, visceral dorsal two and three (VD2/3) (ED50 = 39 microM; Hill coefficient = 1.03), and the inhibitory effect of dopamine on follower cell, visceral dorsal four (ED50 = 33 microM; Hill coefficient = 0.92). 5. The following dopamine agonists (100 microM) were tested by bath application: 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (ADTN), apopmorphine, 2-bromo-alpha-ergocryptine, deoxyepinephrine (DE), mesulergine, (-) quinpirole, SKF 38393, and tyramine. Only the general dopamine agonists, ADTN and DE, mimicked RPeD1's effects on its follower cells. 6. When VD2/3 was isolated and plated in vitro, it maintained a depolarizing response to dopamine. This response was reduced by intracellular injection of the G-protein blocker, GDP-beta-S (2 mM in electrode). Similarly, incubation of VD2/3, in vitro for approximately 18 h, with pertussis toxin (PTX; 5 micrograms/ml), the G-protein inactivating exotoxin, also reduced the dopamine response. Injecting GDP or incubating in heat-inactivated PTX did not effect the response. 7. Several dopamine antagonists were used in an attempt to block RPeD1's synapses: chlorpromazine, ergonovine, fluphenazine, haloperidol, 6-hydroxydopamine, SCH 23390, (+/-) sulpiride, and tubocurarine. Only the D-2 dopamine receptor antagonist, (+/-) sulpiride, reversibly blocked synaptic transmission from RPeD1 to its follower cells. Both the (+) and the (-) enantiomer of sulpiride also antagonized synaptic transmission. A dose-inhibition curve for (+/-) sulpiride was constructed (IC50 = 47 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

A neuronal network from the mollusc Lymnaea stagnalis.

The morphology, electrophysiology, and synaptic inputs of a ventrally located neuronal network from the CNS of the pond snail Lymnaea stagnalis was investigated. Three large, previously identified neurons [55] known as right parietal ventral one, two, and three (RPV1,2,&3) were found to be electrically coupled to one another. Coupling between either RPV1&2 or RPV1&3 was weak while coupling between RPV2&3 was strong. Consistent bursting activity was observed in neuron RPV1 while neurons RPV2&3 were either silent or fired tonically. When isolated in vitro, similar patterns of activity could be elicited in neurons RPV1-3. Lucifer yellow staining revealed that these cells send axons through nerves innervating musculature involved in locomotion, whole-body withdrawal, and cardio-respiratory function. Neurons RPV1-3 were found to be inhibited by an identified interneuron, visceral dorsal four, known to be directly involved in cardio-respiratory behavior [43]. Furthermore, neurons RPV1-3 were also inhibited by a wide-acting synaptic input, known as Input three [9], which is associated with respiratory pattern generation [43]. An interneuron, identified as right pedal dorsal eleven (RPeD11), which coordinates locomotory and withdrawal behavior [44], was found to excite neuron RPV1. When neurons RPeD11 and RPV1 were isolated in vitro and allowed to extend neurites, they formed a synaptic connection similar to that observed in the isolated brain. In vitro work on these neurons may make them an attractive model to study synapse formation and bursting activity.

Identification of mouse type-2-like astrocytes: demonstration of glutamate and GABA transmitter activated responses.

We have identified mouse type-2-like astrocytes and examined some of their electrophysiological properties. Cultures were prepared from P4 mouse neopallia. We demonstrate that mouse type-2-like astrocytes can be identified using the following criteria: presence of glial fibrillary acidic protein (GFAP), presence of chondroitin sulfate polysaccharide, and presence of gamma-aminobutyric acid (GABA). A2B5-binding is not a sufficient criterion to identify O2A lineage cells in mouse neopallial glial cultures since the monoclonal antibody A2B5 binds not only to O2A lineage cells but also to a subpopulation of large, flat type-1-like astrocytes. Mouse type-2-like astrocytes have resting membrane potentials of -76.2 +/- 2.1 mV-i.e., similar to that of mouse type-1-like astrocytes. The input resistance of 44.2 +/- 0.5 M omega is an order of magnitude greater than that of type-1-like astrocytes suggesting the type-2-like astrocytes are not extensively electrically coupled either to each other or to type-1-like astrocytes. Glutamate application caused an 8.8 +/- 1.7 mV depolarization of type-2-like astrocytes. Application of glutamate to barium treated astrocytes caused a fast depolarization with a peak amplitude of 21.4 +/- 1.8 mV; the cells repolarized from this peak by about 10 mV and upon removal of glutamate returned to its pre-glutamate value. Application of GABA caused a transient depolarization of 14.0 +/- 1.7 mV. The presence of barium resulted in a steady-state GABA-induced depolarization of 10.3 +/- 2.0 mV. Neither SITS nor beta-alanine interfered with the amplitude of the glutamate and GABA responses.

Ionic dependence of a P2-purinoceptor mediated depolarization of cultured astrocytes.

The membrane potential of cultured mouse astrocytes was recorded to assess the effects of extracellular adenosine 5'-triphosphate (ATP) and related H purines on astrocyte electrophysiology. The purines were applied with or without the presence of barium, which blocks the high resting K+ conductance in astrocytes. The response to ATP alone was a moderate depolarization; however, the response to ATP in the presence of barium was a large, dose dependent depolarization. The ED50 was approximately 10 microM. The effect of adenosine 5'-diphosphate (ADP) or adenosine 5'-monophosphate (AMP), in the presence of barium, on membrane potential was less than that of ATP. Adenosine, with or without barium, had no effect on membrane potential; furthermore, adenosine agonists in barium produced no response. The results of applying various ATP analogues indicate that the response is mediated via a P2-purinoceptor. Ion replacement studies reveal a complicated response to ATP that has several components and involves Na+ and K+. These results show that astrocytes respond with ionic changes to very small, physiological concentrations of extracellular ATP. We suggest that ATP plays a role in interactions between neurons/endothelial cells and glial cells.

Short-circuiting effects of K+ currents on electrical responses of type-1-like astrocytes from mouse cerebral cortex.

The membrane potential and membrane input resistance of cortical astrocytes from newborn mice were recorded with and without exposure to 1 mM barium. Barium treatment drastically decreased the membrane response to 0 and 35 mM K+. It also revealed an electrogenic component of the Na+,K(+)-ATPase as evident by a biphasic depolarization as a response to ouabain, which was monophasic without barium presence. Untreated mouse astrocytes reacted with small monophasic depolarizations to GABA and glutamate exposure. Barium-treated astrocytes exhibited additional transient responses to both transmitters, similar to those responses of rat astrocytes as found in the literature. The transmitter responses were not changed by exposure to uptake blockers for both transmitter substances. Thus, this electrophysiological study confirms earlier studies with radioactive K+ fluxes in showing that astrocytes derived from mouse brain are capable of short-circuiting electrogenic components and transmitter responses. This extreme high K+ permeability resembles the one reported for endfeet of retinal Muller cells and dissociated astrocytes from optic nerve.