Glial H+ transport and control of pH

Propionic acid induced behavioural effects of relevance to autism spectrum disorder evaluated in the hole board test with rats

Autism spectrum disorders (ASD) are a set of neurodevelopmental disorders characterized by abnormal social interactions, impaired language, and stereotypic and repetitive behaviours. Among genetically susceptible subpopulations, gut and dietary influences may play a role in etiology. Propionic acid (PPA), produced by enteric gut bacteria, crosses both the gut-blood and the blood-brain barrier. Previous research has demonstrated that repeated intracerebroventricular (ICV) infusions of PPA in adult rats produce behavioural and neuropathological changes similar to those seen in ASD patients, including hyperactivity, stereotypy, and repetitive movements. The current study examined dose and time related changes of exploratory and repetitive behaviours with the use of the hole-board task. Adult male Long-Evans rats received ICV infusions twice a day, 4 h apart, of either buffered PPA (low dose 0.052 M or high dose 0.26 M, pH 7.5, 4 μL/infusion) or phosphate buffered saline (PBS, 0.1 M) for 7 consecutive days. Locomotor activity and hole-poke behaviour were recorded daily in an automated open field apparatus (Versamax), equipped with 16 open wells, for 30 min immediately after the second infusion. In a dose dependent manner PPA infused rats displayed significantly more locomotor activity, stereotypic behaviour and nose-pokes than PBS infused rats. Low-dose PPA animals showed locomotor activity levels similar to those of PBS animals at the start of the infusion schedule, but gradually increased to levels comparable to those of high-dose PPA animals by the end of the infusion schedule, demonstrating a dose and time dependent effect of the PPA treatments.

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.

Mechanism of the kainate-induced intracellular acidification in leech Retzius neurons

We examined the effect of the glutamatergic agonist kainate on the membrane potential, the intracellular Na+ concentration ([Na+]i), the intracellular-free Ca2+ concentration, and on the intracellular pH of Retzius neurons of the medicinal leech, Hirudo medicinalis, in order to investigate the mechanism responsible for the intracellular acidification caused by glutamatergic stimulation. The recordings were made with Na+- and pH-sensitive microelectrodes and iontophoretically injected Fura-2. Bath application of kainate evoked a marked membrane depolarization, a [Na+]i increase, and an intracellular acidification. The intracellular acidification was unaffected by reversal of the electromotive force for H+, suggesting that an influx of H+ from the interstitial space does not contribute to the acidification. While the Ca2+ channel blockers La3+ and Co2+ had no effect on the kainate-induced intracellular acidification, suggesting that a Ca2+ influx via voltage-dependent Ca2+ channels was not relevant, the acidification was reduced in Ca2+-free saline solution. In Na+-free saline solution the kainate-induced intracellular acidification was absent, suggesting the involvement of Na+ influx in generating the acidification. When injected iontophoretically Na+ induced an intracellular acidification but Li+, K+, Rb+ or Cs+ did not. Furthermore, a [Na+]i increase induced by blocking the Na+/K+ pump also led to an intracellular acidification. We conclude that the [Na+]i increase is the crucial event underlying the kainate-induced intracellular acidification. Possible mechanisms linking the [Na+]i increase to the intracellular acidification are discussed.

Morphological organization of neuropile glial cells in the central nervous system of the medicinal leech (Hirudo medicinalis)

Neuropile glial (NPG) cells in the central nervous system of the medicinal leech, Hirudo medicinalis, were studied by histological, histochemical and immunocytochemical techniques. The NPG cells are often surrounded by electron-dense microglial cells. The central cytoplasm of NPG cells shows a significant zonation. The zone around the nucleus contains mitochondria, glycogen and vesicles. The cytoplasm also contains many ribosomes, a few dictyosomes and distinct inclusions up to 2 microns in diameter. A second zone around the perinuclear region is marked by the occurrence of bundles of intermediate filaments that correspond in thickness to glial filaments of vertebrates. We found a positive reaction with polyclonal antibodies against human glial fibrillary acidic protein (GFAP), and the areas of intense fluorescence correspond to the regions where intermediate filaments were found to be abundant. The peripheral zone contains numerous membrane stacks that could not be contrasted by lanthane nitrate or tannic acid. Therefore, the membrane stacks could be part of an extensive smooth endoplasmic reticulum, which is characteristic of cells with active lipid metabolism.

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

In patch-clamp experiments we characterized four Cl- channels (42 pS, 70 pS, 80 pS and 229 pS) underlying the large Cl- conductance of leech neuropile glial cells. They differed with respect to their gating, their rectification and their activity in the cell-attached configuration, showed the selectivity sequence I->Cl->/=Br->F- and were impermeable to SO42-. The four channels were blocked by NPPB, DPC, niflumic acid and DIDS and exhibited either three or four sublevel states. The outward rectifying 42 pS, 70 pS and 80 pS Cl- channels were classified as intermediate conductance Cl- channels and they could contribute to the high Cl- conductance of the glial membrane, which stabilizes the glial membrane potential. The inward rectifying 229 pS Cl- channel is very similar to vertebrate high conductance Cl- channels, which are assumed to be part of an emergency system that is activated under pathophysiological conditions. In voltage-clamp experiments we calculated that the Cl- conductance amounts to one-third of the total membrane conductance. Reduction of this Cl- conductance by Cl- channel inhibitors markedly depolarized the glial cell membrane. These prominent depolarizations depended on Na+ influx and in most cases the glial cells failed to regulate their membrane potential following wash-out of the inhibitors.

Ca2+ influx into leech glial cells and neurones caused by pharmacologically distinct glutamate receptors

The effect of glutamatergic agonists on the intracellular free Ca2+ concentration ([Ca2+]i) of neuropile glial cells and Retzius neurones in intact segmental ganglia of the medicinal leech Hirudo medicinalis was investigated by using iontophoretically injected fura-2. In physiological Ringer solution the [Ca2+]i levels of both cell types were almost the same (glial cells: 58 +/- 30 nM, n = 51; Retzius neurones: 61 +/- 27 nM, n = 64). In both cell types glutamate, kainate, and quisqualate induced an increase in [Ca2+]i which was inhibited by 6,7-dinitroquinoxaline-2,3-dione (DNQX). This increase was caused by a Ca2+ influx from the extracellular space because the response was greatly diminished upon removal of extracellular Ca2+. The glutamate receptors of neuropile glial cells and Retzius neurones differed with respect to the relative effectiveness of the agonists used, as well as with regard to the inhibitory strength of DNQX. In Retzius neurones the agonist-induced [Ca2+]i increase was abolished after replacing extracellular Na+ by organic cations or by mM amounts of Ni2+, whereas in glial cells the [Ca2+]i increase was largely preserved under both conditions. It is concluded that in Retzius neurones the Ca2+ influx is predominantly mediated by voltage-dependent Ca2+ channels, whereas in neuropile glial cells the major influx occurs via the ion channels that are associated with the glutamate receptors.

Self-sustained spreading depressions in the chicken retina and short-term neuronal-glial interactions within the gray matter neuropil

The chicken retina is an accessible piece of intact gray matter in which a self-sustained form of the 'Spreading Depression' (SD) wave can be easily elicited and recorded for many hours with double barrel ion-sensitive electrodes in the extracellular space. The blockade of glial (Müller) cell potassium channels with barium chloride added to the perfusing Ringer depressed both the negative potential shift typical of SDs and the velocity of spread. Moreover, there was separation of the extracellular increase of potassium and the drop in the extracellular potential: the peak of the potassium wave was increased, as well as its duration whereas the potential wave could be depressed to zero or even inverted to positive. By contrast the transient extracellular calcium drop could not be separated from the extracellular potential wave but appeared related to it: no transient calcium drop was observed when the negative potential was completely depressed or inverted. Both, the amplitude of the extracellular potential and extracellular calcium activity appeared to be important factors controlling the velocity of spread.