Macroscopic and single-channel chloride currents in neuropile glial cells of the leech central nervous system

Ion Homeostasis in Rhythmogenesis: The Interplay Between Neurons and Astroglia

Proper function of all excitable cells depends on ion homeostasis. Nowhere is this more critical than in the brain where the extracellular concentration of some ions determines neurons' firing pattern and ability to encode information. Several neuronal functions depend on the ability of neurons to change their firing pattern to a rhythmic bursting pattern, whereas, in some circuits, rhythmic firing is, on the contrary, associated to pathologies like epilepsy or Parkinson's disease. In this review, we focus on the four main ions known to fluctuate during rhythmic firing: calcium, potassium, sodium, and chloride. We discuss the synergistic interactions between these elements to promote an oscillatory activity. We also review evidence supporting an important role for astrocytes in the homeostasis of each of these ions and describe mechanisms by which astrocytes may regulate neuronal firing by altering their extracellular concentrations. A particular emphasis is put on the mechanisms underlying rhythmogenesis in the circuit forming the central pattern generator (CPG) for mastication and other CPG systems. Finally, we discuss how an impairment in the ability of glial cells to maintain such homeostasis may result in pathologies like epilepsy and Parkinson's disease.

Calcium signaling in invertebrate glial cells

Calcium signaling studies in invertebrate glial cells have been performed mainly in the nervous systems of the medicinal leech (Hirudo medicinalis) and the sphinx moth Manduca sexta. The main advantages of studing glial cells in invertebrate nervous systems are the large size of invertebrate glial cells and their easy accessibility for optical and electrophysiological recordings. Glial cells in both insects and annelids express voltage-gated calcium channels and, in the case of leech glial cells, calcium-permeable neurotransmitter receptors, which allow calcium influx as one major source for cytosolic calcium transients. Calcium release from intracellular stores can be induced by metabotropic receptor activation in leech glial cells, but appears to play a minor role in calcium signaling. In glial cells of the antennal lobe of Manduca, voltage-gated calcium signaling changes during postembryonic development and is essential for the migration of the glial cells, a key step in axon guidance and in stabilization of the glomerular structures that are characteristic of primary olfactory centers.

Inwardly rectifying chloride channel activity in intestinal pacemaker cells

Cl(-) channels are proposed to play a role in gut pacemaker activity, but little is known about the characteristics of Cl(-) channels in interstitial cells of Cajal (ICC), the intestinal pacemaker cells. The objective of the present study was to identify whole cell Cl(-) currents in ICC associated with previously observed single-channel activity and to characterize its inward rectification. Whole cell patch-clamp studies showed that ICC express an inwardly rectifying Cl(-) current that was not sensitive to changes in cation composition of the extracellular solutions. Currents were not affected by replacing all cations with N-methyl-d-glucamine (NMDG(+)). Whole cell currents followed the Cl(-) equilibrium potential and were inhibited by DIDS and 9-anthracene carboxylic acid. Ramp protocols of single-channel activity showed that inward rectification was due to reduction in single-channel open probability, not a reduction in single-channel conductance. Single-channel data led to the hypothesis that strong cooperation exists between 30-pS channels that show less cooperation at potentials positive to the reversal potential. Hence, an inwardly rectifying Cl(-) channel plays a prominent role in determining pacemaker activity in the gut.

Mechanosensitive cation channels in arterial smooth muscle cells are activated by diacylglycerol and inhibited by phospholipase C inhibitor

Mechanosensitive cation channels may be involved in the development of the myogenic tone of arteries. The molecular identity of these channels is not clear, but transient receptor potential channels (TRPCs) are good candidates. In the present study, we searched for mechanosensitive channels at the single-channel level in arterial smooth muscle cells using the patch-clamp technique and investigated the channel properties in the light of properties of TRPCs. With 140 mmol/L CsCl in the pipette solution, application of negative pressures to the back of the pipette induced the activation of channels the open probability of which increased with the amount of negative pressure. The current-voltage relationship was linear in symmetrical ionic conditions, and the single-channel conductances for Cs+, K+, and Na+ were 30, 36, and 27 pS, respectively. When NMDG+ was substituted for Cs+ in the pipette solution, inward currents were abolished, whereas outward currents remained active, indicating that the channels were nonselective to cations. The channel activity was blocked by intracellular Gd3+ and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and increased by diacylglycerol and by cyclopiazonic acid. Phospholipase C inhibitor (U73122) inhibited not only channel activity but also the development of myogenic tone induced by stretching of the basilar arteries. These results suggest that the ion channel responsible for the development of myogenic tone is the 30-pS mechanosensitive cation channel that exhibits properties similar to those of TRPCs.

Effects of ATP and derivatives on neuropile glial cells of the leech central nervous system

We investigated the effects of ATP (adenosine 5'-triphosphate) and derivatives on leech neuropile glial cells, focusing on exposed glial cells. ATP dose-dependently depolarized or hyperpolarized neuropile glial cells in situ as well as exposed neuropile glial cells. These potential shifts varied among cells and repetitive ATP application did not change their amplitude, duration or direction. In exposed neuropile glial cells, ATP most frequently induced a Na(+)-dependent depolarization and decreased the input resistance. The agonist potency ATP > ADP (adenosine 5'-diphosphate) > AMP (adenosine 5'-monophosphate) > adenosine indicates that P2 purinoceptors mediate this depolarization. The P2Y agonist 2-methylthio-ATP mimicked the ATP-induced depolarization, whereas the P2Y antagonist PPADS (pyridoxal-phosphate-6-azophenyl-2', 4'-disulphonic acid) reduced it. P2X agonists were without effect. Because the P1 antagonist 8-SPT (8-(p-sulphophenyl)-theophylline) also depressed ATP-induced depolarizations and some ATP-insensitive glial cells responded to adenosine, we suggest coexpression of metabotropic P2Y and P1 purinoceptors. The ATP-induced depolarization requires activation of Na(+) channels or nonselective cation channels, whereas the ATP-induced hyperpolarization indicates activation of K(+) channels. ATP also increased the intracellular Ca(2+) concentration ([Ca(2+)](i)), that is independent of Ca(2+) influx but reflects intracellular Ca(2+) release possibly triggered by IP(3) formation. ADP and AMP also increased [Ca(2+)](i), but were less efficient than ATP; adenosine and 2-methylthio-ATP did not affect [Ca(2+)](i). In view of the mobilization of intracellular Ca(2+), ATP is clearly different from other leech neurotransmitters, because it enables intracellular Ca(2+) signaling without causing prominent changes in glial membrane potential. Thus disturbance of the extracellular microenvironment and the demand for metabolic energy are minimized.

Cl- transport in the lobster stretch receptor neurone

Experiments were performed to identify mechanisms underlying non-leakage and non-H+/HCO3--linked transmembrane Cl- transports in the slowly adapting stretch receptor neurone of the European lobster, using intracellular microelectrode and pharmacological techniques. In methodological tests, it was established that direct estimates of intracellular Cl- with ion-sensitive microelectrodes are statistically identical with indirect estimates by means of a GABA method, where 1-2 mM GABA is transforming the cell's membrane voltage into its Cl- equilibrium voltage from which the Cl- concentration is inferred by the Nernst equation. From experiments using sodium orthovanadate and ethacrynic acid, supposed to block primary Cl- pumps, and bumetanide, supposed to block Na-K-Cl co-transporters, it appeared that neither of the two Cl- transport systems exists in the stretch receptor neurone. It could be shown, however, that the cell is equipped with an electroneutral K-Cl co-transporter that (a) is blockable by furosemide in high (Km approximately 350 microM), by 4-acetamido-4'-isothiocyanato-stilbene-2,2-disulphonic acid (SITS) in medium-high (Km approximately 35 microM), and by 4, 4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) in low (Km approximately 15 microM) doses, (b) is (transiently) activatable by (1 mM) n-ethylmaleimide, (c) is not suppressed by extracellular Rb+ or NH4+, and (d) is not directly coupled to any transmembrane transports of Na+, H+ or HCO3-. From functional tests, with varying transmembrane K+ and Cl- gradients, evidence obtained that the K-Cl co-transporter is able to reverse its transport direction and to adjust its transport rate in a considerable range. As a whole, the results speak in favour of the K-Cl co-transporter being responsible (a) for normally keeping the intracellular Cl- concentration at low levels, for an optimization of the cell's inhibitory system, and (b) for achieving fast transmembrane shifts of K+ (and Cl-), as a means of stabilizing the cell's membrane excitability in conditions of varying extracellular K+ concentrations.

Modulation of burst frequency, duration, and amplitude in the zero-Ca(2+) model of epileptiform activity

Incubation of hippocampal slices in zero-Ca(2+) medium blocks synaptic transmission and results in spontaneous burst discharges. This seizure-like activity is characterized by negative shifts (bursts) in the extracellular field potential and a K(+) wave that propagates across the hippocampus. To isolate factors related to seizure initiation, propagation, and termination, a number of pharmacological agents were tested. K(+) influx and efflux mechanisms where blocked with cesium, barium, tetraethylammonium (TEA), and 4-aminopyridine (4-AP). The effect of the gap junction blockers, heptanol and octanol, on zero-Ca(2+) bursting was evaluated. Neuronal excitability was modulated with tetrodotoxin (TTX), charge screening, and applied electric fields. Glial cell function was examined with a metabolism antagonist (fluroacetate). Neuronal hyperpolarization by cation screening or applied fields decreased burst frequency but did not affect burst amplitude or duration. Heptanol attenuated burst amplitude and duration at low concentration (0.2 mM), and blocked bursting at higher concentration (0.5 mM). CsCl(2) (1 mM) had no effect, whereas high concentrations (1 mM) of BaCl(2) blocked bursting. TEA (25 mM) and low concentration of BaCl(2) (300 microM) resulted in a two- to sixfold increase in burst duration. Fluroacetate also blocked burst activity but only during prolonged application (>3 h). Our results demonstrate that burst frequency, amplitude, and duration can be independently modulated and suggest that neuronal excitability plays a central role in burst initiation, whereas potassium dynamics establish burst amplitude and duration.

Ionic mechanism of 4-aminopyridine action on leech neuropile glial cells

Extracellular 4-aminopyridine (4-AP), tetraethylammonium chloride (TEA) and quinine depolarized the neuropile glial cell membrane and decreased its input resistance. As 4-AP induced the most pronounced effects, we focused on the action of 4-AP and clarified the ionic mechanisms involved. 4-AP did not only block glial K+ channels, but also induced Na+ and Ca2+ influx via other than voltage-gated channels. The reversal potential of the 4-AP-induced current was -5 mV. Application of 5 mM Ni2+ or 0.1 mM d-tubocurarine reduced the 4-AP-induced depolarization and the associated decrease in input resistance. We therefore suggest that 4-AP mediates neuronal acetylcholine release, apparently by a presynaptic mechanism. Activation of glial nicotinic acetylcholine receptors contributes to the depolarization, the decrease in input resistance, and the 4-AP-induced inward current. Furthermore, the 4-AP-induced depolarization activates additional voltage-sensitive K+ and Cl- channels and 4-AP-induced Ca2+ influx could activate Ca2+-sensitive K+ and Cl- channels. Together these effects compensate and even exceed the 4-AP-mediated reduction in K+ conductance. Therefore, the 4-AP-induced depolarization was paralleled by a decreasing input resistance.