Carbonic anhydrases and their functional differences in human and mouse sperm physiology.

Fertilization is a key reproductive event in which sperm and egg fuse to generate a new individual. Proper regulation of certain parameters (such as intracellular pH) is crucial for this process. Carbonic anhydrases (CAs) are among the molecular entities that control intracellular pH dynamics in most cells. Unfortunately, little is known about the function of CAs in mammalian sperm physiology. For this reason, we re-explored the expression of CAI, II, IV and XIII in human and mouse sperm. We also measured the level of CA activity, determined by mass spectrometry, and found that it is similar in non-capacitated and capacitated mouse sperm. Importantly, we found that CAII activity accounts for half of the total CA activity in capacitated mouse sperm. Using the general CA inhibitor ethoxyzolamide, we studied how CAs participate in fundamental sperm physiological processes such as motility and acrosome reaction in both species. We found that capacitated human sperm depend strongly on CA activity to support normal motility, while capacitated mouse sperm do not. Finally, we found that CA inhibition increases the acrosome reaction in capacitated human sperm, but not in capacitated mouse sperm.

Purkinje cell loss and motor coordination defects in profilin1 mutant mice.

Profilin1 is an actin monomer-binding protein, essential for cytoskeletal dynamics. Based on its broad expression in the brain and the localization at excitatory synapses (hippocampal CA3-CA1 synapse, cerebellar parallel fiber (PF)-Purkinje cell (PC) synapse), an important role for profilin1 in brain development and synapse physiology has been postulated. We recently showed normal physiology of hippocampal CA3-CA1 synapses in the absence of profilin1, but impaired glial cell binding and radial migration of cerebellar granule neurons (CGNs). Consequently, brain-specific inactivation of profilin1 by exploiting conditional mutants and Nestin-mediated cre expression resulted in a cerebellar hypoplasia, aberrant organization of cerebellar cortex layers, and ectopic CGNs. Apart from these findings we noted a loss of PCs and an irregularly shaped PC layer in adult mutants. In this study, we show that PC migration and development are not affected in profilin1 mutants, suggesting cell type-specific functions for profilin1 in PCs and CGNs. PC loss begins during the second postnatal week and progresses until adulthood with no further impairment in aged mutants. In Nestin-cre profilin1 mutants, defects in cerebellar cortex cytoarchitecture are associated with impaired motor coordination. However, in L7-cre mutants, lacking profilin1 specifically in PCs, the cerebellar cortex cytoarchitecture is unchanged. Thereby, our results demonstrate that the loss of PCs is not caused by cell-autonomous defects, but presumably by impaired CGN migration. Finally, we show normal functionality of PF-PC synapses in the absence of profilin1. In summary, we conclude that profilin1 is crucially important for brain development, but dispensable for the physiology of excitatory synapses.

Suppression of GABA input by A1 adenosine receptor activation in rat cerebellar granule cells.

Synaptic transmission has been shown to be modulated by purinergic receptors. In the cerebellum, spontaneous inhibitory input to Purkinje neurons is enhanced by ATP via P2 receptors, while evoked excitatory input via the granule cell parallel fibers is reduced by presynaptic P1 (A1) adenosine receptors. We have now studied the modulation of the complex GABAergic input to granule cells by the purinergic receptor agonists ATP and adenosine in acute rat cerebellar tissue slices using the whole-cell patch-clamp technique. Our experiments indicate that ATP and adenosine substantially reduce the bicuculline- and gabazine-sensitive GABAergic input to granule cells. Both phasic and tonic inhibitory components were reduced leading to an increased excitability of granule cells. The effect of ATP and adenosine could be blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), but not by other P1 and P2 receptor antagonists, indicating that it was mediated by activation of A1 adenosine receptors. Our results suggest that, in the cerebellar network, A1 receptor activation, known to decrease the excitatory output of granule cells, also increases their excitability by reducing their complex GABAergic input. These findings extend our knowledge on purinergic receptors, mediating multiple modulations at both inhibitory and excitatory input and output sites in the cerebellar network.

Preferential transport and metabolism of glucose in Bergmann glia over Purkinje cells: a multiphoton study of cerebellar slices.

Knowing how different cell types handle glucose should help to decipher how energy supply is adjusted to energy demand in the brain. Previously, the uptake of glucose by cultured brain cells was studied in real-time using fluorescent tracers and confocal microscopy. Here, we have adapted this technique to acute slices prepared from the rat cerebellum by means of multiphoton microscopy. The transport of the fluorescent glucose analogs 2NBDG and 6NBDG was several-fold faster in the molecular layer of the cerebellar cortex than in Purkinje cell somata and granule cells. After washout of free tracer, it became apparent that most phosphorylated tracer was located in Bergmann glia, which was confirmed by counterstaining with the glial marker sulforhodamine 101. The effective recovery of fluorescence after photobleaching showed that 2NBDG-P can diffuse horizontally across the molecular layer, presumably through gap junctions between Bergmann glial cells. Our main conclusion is that in acute cerebellar slices, the glucose transport capacity and glycolytic rate of Bergmann glia are several-fold higher than those of Purkinje cells. Given that the cerebellum is largely fueled by glucose and Purkinje neurons are estimated to spend more energy than Bergmann glial cells, these results suggest substantial shuttling of an energy-rich metabolite like lactate between glial cells and neurons.

Calcium influx into dendrites of the leech Retzius neuron evoked by 5-hydroxytryptamine.

5-Hydroxytryptamine (5-HT) is a ubiquitous neurotransmitter and neuromodulator that affects neural circuits and behaviours in vertebrates and invertebrates. In the present study, we have investigated 5-HT-induced Ca(2+) transients in subcellular compartments of Retzius neurons in the leech central nervous system using confocal laser scanning microscopy, and studied the effect of 5-HT on the electrical coupling between the Retzius neurons. Bath application of 5-HT (50mM) induced a Ca(2+) transient in axon, dendrites and cell body of the Retzius neuron. This Ca(2+) transient was significantly faster and larger in dendrites than in axon and cell body, and was half-maximal at a 5-HT concentration of 5-12mM. The Ca(2+) transient was suppressed in the absence of extracellular Ca(2+) and by methysergide (100mM), a non-specific antagonist of metabotropic 5-HT receptors, and was strongly reduced by bath application of the Ca(2+) channel blocker Co(2+) (2mM). Injection of the non-hydrolysable GTP analogue GTPgammaS increased and prolonged the dendritic 5-HT-induced Ca(2+) transient. The non-selective protein kinase inhibitor H7 (100mM) and the adenylate cyclase inhibitor SQ22536 (500 mM) did not affect the Ca(2+) transient, and the membrane-permeable cAMP analogue dibutyryl-cAMP (500 mM) did not mimic the effect of 5-HT application. 5-HT reduced the apparent electrical coupling between the two Retzius neurons, whereas suppression of the Ca(2+) influx by removal of external Ca(2+) improved the transmission of action potentials at the electrical synapses which are located between the dendrites of the adjacent Retzius neurons. The results indicate that 5-HT induces a Ca(2+) influx through calcium channels located primarily in the dendrites, and presumably activated by a G protein-coupled 5-HT receptor. The dendritic Ca(2+) increase appears to modulate the excitability of, and the synchronization between, the two Retzius neurons.

Activity-dependent accumulation of Ca2+ in axon and dendrites of the leech Leydig neuron.

We have investigated Ca2+ changes evoked by single action potentials (APs) in axon and dendrites of leech Leydig neurons. Dendritic Ca2+ transients induced by an AP were twice as large as in the axon, and Ca2+ recovery was significantly faster in the dendrites as compared to the axon. The AP-induced Ca2+ transients were blocked by Co2+ and suppressed in Ca2+-free saline, indicating Ca2+ influx through voltage-activated channels. During a train of APs, Ca2+ accumulated significantly more in the axon than in the dendrites. Suppression of the Ca2+ influx changed the shape of the action potential and increased the firing frequency. The results suggest a functional role of Ca2+ influx and Ca2+ accumulation during electrical activity in different neuronal subcompartments.

Strategies for metabolic exchange between glial cells and neurons.

The brain is a major energy consumer and dependent on carbohydrate and oxygen supply. Electrical and synaptic activity of neurons can only be sustained given sufficient availability of ATP. Glial cells, which have long been assigned trophic functions, seem to play a pivotal role in meeting the energy requirements of active neurons. Under conditions of high neuronal activity, a number of glial functions, such as the maintenance of ion homeostasis, neurotransmitter clearance from synaptic domains, the supply of energetic compounds and calcium signalling, are challenged. In the vertebrate brain, astrocytes may increase glucose utilization and release lactate, which is taken up and consumed by neurons to generate ATP by oxidative metabolism. The CO(2) produced is processed primarily in astrocytes, which display the major activity of carboanhydrase in the brain. Protons and bicarbonate in turn may contribute to drive acid/base-coupled transporters. In the present article a scenario is discussed which couples the transfer of energy and the conversion of CO(2) with the high-affinity glutamate uptake and other transport processes at glial and neuronal cell membranes. The transporters can be linked to glial signalling and may cooperate with each other at the cellular level. This could save energy, and would render energy exchange processes between glial cells and neurons more effective. Functions implications and physiological responses, in particular in chemosensitive brain areas, are discussed.

Calcium transients in subcompartments of the leech Retzius neuron as induced by single action potentials.

Regional Ca(2+) influx into neurons plays an essential role for fast signal processing, yet it is little understood. We have investigated intracellular Ca(2+) transients induced by a single action potential (AP) in Retzius neurons in situ of isolated ganglia of the leech Hirudo medicinalis using confocal laser scanning microscopy in the cell body, in different axonal branches, and in dendrites. In the cell body, a single AP induced a Ca(2+) transient in submembrane regions, while in central regions no fluorescence change was detected. Burst activity evoked a much larger Ca(2+) influx, which elicited Ca(2+) signals in central somatic regions, including the cell nucleus. A single AP induced a Ca(2+) transient in distal branches of the axon and in dendrites that was significantly larger than in the proximal axon and in the cell body (p <.05), and the recovery of the Ca(2+) transient was significantly faster in axonal branches than in dendrites (p <.01). The AP-induced Ca(2+) transient was inhibited by Co(2+) (2 mM). The P/Q-type Ca(2+) channel blocker omega-agatoxin TK (500 nM) and the L-type Ca(2+) channel blocker nifedipine (20 microM) had no effect on the Ca(2+) transient, whereas the L-type Ca(2+) channel blocker methoxyverapamil (D600, 0.5-1 mM) irreversibly reduced the Ca(2+) transient by 37% in axons and by 42% in dendrites. Depletion of intracellular Ca(2+) stores following inhibition of endoplasmic Ca(2+)-ATPases by cyclopiazonic acid (10 microM) decreased the AP-induced Ca(2+) transient in the dendrites by 21% (p <.01), but not in axons, and increased the Ca(2+) recovery time constant (tau) in the axonal branches by 129% (p <.01), but not in dendrites. The results indicate that an AP evokes a voltage-gated Ca(2+) influx into all subcompartments of the Retzius neuron, where it produces a Ca(2+) signal of different size and/or kinetics. This may contribute to the modulation of electrical excitation and propagation of APs, and to different modes of synaptic and nonsynaptic processes.

Bursting activity in leech Retzius neurons induced by low external chloride.

We have investigated the bursting activity of Retzius neurons in the central nervous system of the leech Hirudo medicinalis as induced in Cl(-)-free saline by measuring membrane potential, membrane current and the intracellular calcium concentration ([Ca2+]i), using fura-2 or Oregon-Green488-Bapta-1. The Retzius neurons changed their low tonic firing to rhythmical bursting activity when the extracellular Cl- concentration ([Cl-]o) was lowered to 1 mM or less. In Cl(-)-free saline (Cl- exchanged by gluconate), bursting was accompanied by a rise in intracellular Ca2+ in both cell body and axon, which oscillated in synchrony with the bursts. The Ca2+ transients depended on the amplitude and duration of the depolarization underlying the burst, and were presumably due to Ca2+ influx through voltage-dependent Ca2+ channels. In Ca(2+)-free, EGTA-buffered saline or in the presence of Ca2+ channel blockers verapamil (1 mM) or diltiazem (500 microM) the depolarizations underlying the bursts in Cl(-)-free saline were enhanced in amplitude and duration. Bursting was not affected by depleting the intracellular Ca2+ stores with cyclopiazonic acid. The depolarization in Cl(-)- and Ca(2+)-free saline did not evoke intracellular Ca2+ changes. The burst-underlying membrane depolarization induced by Cl- removal was found to be due to a Na(+)-dependent persistent inward current and could be inhibited by saxitoxin (25-50 microM). The results suggest that a persistent Na+ current is generated in Cl(-)-free saline and induces the depolarization underlying rhythmic activity, and that presumably Ca(2+)-induced K+ currents modulate the bursting behaviour.

Developmental downregulation of ATP-sensitive potassium conductance in astrocytes in situ.

In many neural and non-neural cells, ATP-sensitive potassium (K(ATP)) channels couple the membrane potential to energy metabolism. We investigated the activation of K(ATP) currents in astrocytes of different brain regions (hippocampus, cerebellum, dorsal vagal nucleus) by recording whole-cell currents with the patch-clamp technique in acute rat brain slices. Pharmacological tools, hypoglycemia and specific compounds in the pipette solution (cAMP, UDP), were used to modulate putative K(ATP) currents. The highest rate of K(ATP) specific currents was observed with a pipette solution containing cAMP and external stimulation with diazoxide (0.3 mM). The diazoxide-activated current had a reversal potential negative to -80 mV and was inhibited by tolbutamide (0.2 mM). We found that not all cells activated a K(ATP) current, and that the portion of cells with functional K(ATP) channel expression was developmentally downregulated. Whereas diazoxide activated K(ATP) currents in 57% of the astrocytes in rats aged 8-11 days (n = 21), the rate decreased to 38% at 12-15 days (n = 29) and to 8% at 16-19 days (n = 12). No significant difference was observed for the three brain regions. In recordings without cAMP in the internal solution, only 21% (12-15 days; n = 19) or none (16-19 days; n = 7), respectively, showed a potassium current upon diazoxide application. This metabolically regulated potassium conductance may be of importance, particularly in immature astrocytes with a complex current pattern, which have a relatively high input resistance: K(ATP) currents activated by energy depletion may hyperpolarize the cells, or stabilize a negative resting potential during depolarizing stimuli mediated, e.g., by glutamate receptors and/or uptake carriers.

A novel barium-sensitive calcium influx into rat astrocytes at low external potassium.

Cultured rat cerebellar astrocytes, loaded with the Ca2+-sensitive fluorescent dyes Fura-2 or Fluo-3, responded with cytoplasmic Ca2+ transients, when the external K+ concentration was reduced from 5 mM to below 1 mM. Ca2+ transients were generated after changing to a saline containing 0.2 mM K+ in 82% of the cells (n =303) with a delay of up to 4 min. Cultured rat cortical neurones, which responded in high-K+ saline (50 mM) with Ca2+ transients, showed no Ca2+ responses in low K+ (n =22). In acute rat hippocampal brain slices, presumed glial cells responded with Ca2+ transients in low K+ similar to astrocytes in culture (88%, n =17). The Ca2+ transients were observed both in somatic and dendritic regions of cultured astrocytes, as examined with confocal laser scanning microscopy. Patch-clamped astrocytes hyperpolarized in 0.2 mM K+ from an average resting potential of -65 +/- 4 mV to -98 +/- 20 mV (n =15). The Ca2+ transients in low K+ were suppressed in Ca2+-free saline, buffered with 0.5 mM EGTA, but not after depletion of intracellular Ca2+ stores by thapsigargin, cyclopiazonic acid or by Ruthenium Red, indicating that they were due to Ca2+ influx into the cells, and not caused by intracellular Ca2+ release. The addition of different divalent cations revealed that Ba2+, but not Ni2+, Cd2+, Sr2+ or Mg2+, reversibly blocked the Ca2+ transients in low K+. There was a significant reduction of the Ca2+ responses at micromolar Ba2+ concentrations (Ki = 3.8 microM). The application of different K+ channel blockers, tetraethylammonium, dequalinium, tolbutamide, clotrimazole, or quinidine had no effect on the Ca2+ responses. Removal of external Na+, or intracellular acidification by the addition of 40 mM propionate to the saline, had also no influence on the generation of the Ca2+ transients. The results suggest that reducing the external K+ concentration elicits a Ca2+ influx into rat astrocytes which is highly sensitive to Ba2+. It is discussed that this Ca2+ influx might occur through K+ inward rectifier channels, which become Ca2+-permeable when the extracellular K+ concentration decreases to 1 mM or below.

Histamine-induced calcium entry in rat cerebellar astrocytes: evidence for capacitative and non-capacitative mechanisms.

We have investigated the effects of histamine on the intracellular calcium concentration ([Ca2+]i) of cultured rat cerebellar astrocytes using fura-2-based Ca2+ imaging microscopy. Most of the cells responded to the application of histamine with an increase in [Ca2+]i which was antagonized by the H1 receptor blocker mepyramine. When histamine was applied for several minutes, the majority of the cells displayed a biphasic Ca2+ response consisting of an initial transient peak and a sustained component. In contrast to the initial transient [Ca2+]i response, the sustained, receptor-activated increase in [Ca2+]i was rapidly abolished by chelation of extracellular Ca2+ or addition of Ni2+, Mn2+, Co2+ and Zn2+, but was unaffected by nifedipine, an antagonist of L-type voltage-activated Ca2+ channels. These data indicate that the sustained increase in [Ca2+]i was dependent on Ca2+ influx. When intracellular Ca2+ stores were emptied by prolonged application of histamine in Ca2+-free conditions, Ca2+ re-addition after removal of the agonist did not lead to an 'overshoot' of [Ca2+]i indicative of store-operated Ca2+ influx. However, Ca2+ stores were refilled despite the absence of any substantial change in the fura-2 signal. Depletion of intracellular Ca2+ stores using cyclopiazonic acid in Ca2+-free saline and subsequent re-addition of Ca2+ to the saline resulted in an increase in [Ca2+]i that was significantly enhanced in the presence of histamine. The results suggest that besides capacitative mechanisms, a non-capacitative, voltage-independent pathway is involved in histamine-induced Ca2+ entry into cultured rat cerebellar astrocytes.

The low-affinity monocarboxylate transporter MCT4 is adapted to the export of lactate in highly glycolytic cells.

Transport of lactate and other monocarboxylates in mammalian cells is mediated by a family of transporters, designated monocarboxylate transporters (MCTs). The MCT4 member of this family has recently been identified as the major isoform of white muscle cells, mediating lactate efflux out of glycolytically active myocytes [Wilson, Jackson, Heddle, Price, Pilegaard, Juel, Bonen, Montgomery, Hutter and Halestrap (1998) J. Biol. Chem. 273, 15920-15926]. To analyse the functional properties of this transporter, rat MCT4 was expressed in Xenopus laevis oocytes and transport activity was monitored by flux measurements with radioactive tracers and by changes of the cytosolic pH using pH-sensitive microelectrodes. Similar to other members of this family, monocarboxylate transport via MCT4 is accompanied by the transport of H(+) across the plasma membrane. Uptake of lactate strongly increased with decreasing extracellular pH, which resulted from a concomitant drop in the K(m) value. MCT4 could be distinguished from the other isoforms mainly in two respects. First, MCT4 is a low-affinity MCT: for L-lactate K(m) values of 17+/-3 mM (pH-electrode) and 34+/-5 mM (flux measurements with L-[U-(14)C]lactate) were determined. Secondly, lactate is the preferred substrate of MCT4. K(m) values of other monocarboxylates were either similar to the K(m) value for lactate (pyruvate, 2-oxoisohexanoate, 2-oxoisopentanoate, acetoacetate) or displayed much lower affinity for the transporter (beta-hydroxybutyrate and short-chain fatty acids). Under physiological conditions, rat MCT will therefore preferentially transport lactate. Monocarboxylate transport via MCT4 could be competitively inhibited by alpha-cyano-4-hydroxycinnamate, phloretin and partly by 4, 4'-di-isothiocyanostilbene-2,2'-disulphonic acid. Similar to MCT1, monocarboxylate transport via MCT4 was sensitive to inhibition by the thiol reagent p-chloromercuribenzoesulphonic acid.

Glial strategy for metabolic shuttling and neuronal function.

Glial cells serve a variety of functions in nervous systems, some of which are activated by neurotransmitters released from neurons. Glial cells respond to these neurotransmitters via receptors, but also take up some of the transmitters to help terminate the synaptic process. Among these, glutamate uptake by glial cells is pivotal to avoid transmitter-mediated excitotoxicity. Here, a new model is proposed in which glutamate uptake via the excitatory amino acid transporter (EAAT) is functionally coupled to other glial transporters, in particular the sodium-bicarbonate cotransporter (NBC) and the monocarboxylate transporter (MCT), as well as other glial functions, such as calcium signalling, a high potassium conductance and CO(2) consumption. The central issue of this hypothesis is that the shuttling of sodium ions and acid/base equivalents, which drive the metabolite transport across the glial membrane, co-operate with each other, and hence save energy. As a result, glutamate removal from synaptic domains and lactate secretion serving the energy supply to neurons would be facilitated and could operate with greater capacity.

Enhancement of glutamate uptake transport by CO(2)/bicarbonate in the leech giant glial cell.

Glutamate uptake into glial cells via the excitatory amino acid transporter (EAAT) is accompanied by an influx of sodium and acid equivalents into the cells. The sodium-bicarbonate cotransport (NBC) in glial cells moves sodium and base equivalents across the glial membrane in both directions. We have studied possible interactions between these two electrogenic transporters in the giant glial cell of isolated ganglia of the leech Hirudo medicinalis. Changes in membrane potential, membrane current, intracellular sodium, and intracellular pH evoked by aspartate (1 mM), an EAAT agonist, were measured both in the absence and in the presence of CO(2)/bicarbonate. When 5% CO(2) and 24 mM bicarbonate was added to the saline (at constant pH 7.4), the aspartate-induced membrane current was increased, while the change in intracellular sodium was decreased. The acid influx evoked by aspartate was enhanced by CO(2)/bicarbonate but, because of the increased intracellular CO(2)/bicarbonate-dependent buffering power, the change in intracellular pH was decreased. 4,4'-Diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS, 0.5 mM), which inhibits the NBC, reversed the effects of CO(2)/bicarbonate on the aspartate-induced current and pH change. Our results suggest that the NBC helps counteract dissipation of the sodium and the acid-base gradients induced by the EAAT, enhancing the rate and capacity of glutamate uptake by glial cells.

Leech giant glial cell: functional role in a simple nervous system.

The giant glial cell in the central nervous system of the leech Hirudo medicinalis has been the subject of a series of studies trying to link its physiological properties with its role in neuron-glia interactions. Isolated ventral cord ganglia of this annelid offer several advantages for these studies. First, single giant glial cells can easily be identified and are quite accessible to electrophysiological and microfluorometric studies. Second, only two giant macroglial cells are located in the neuropil of each ganglion, rendering them well suited for studying neuron-glia interactions. Third, many neurons can be identified and are well known with respect to their physiology and their roles in controlling simple behaviors in the leech. This review briefly outlines the major recent findings gained by studying this preparation and its contributions to our knowledge of the functional role of glia in nervous systems. Emphasis is directed to glial responses during neuronal activity and to the analysis of intracellular Ca(2+) and H(+) transients mediated by neurotransmitter receptors and ion-driven carriers. Among its numerous properties, the leech giant glial cell prominently expresses a large K(+) conductance, voltage-dependent Ca(2+) channels, ionotropic non-NMDA glutamate receptors, and an electrogenic, reversible Na(+)-HCO(3)(-) cotransporter.

Peptide-mediated glial responses to leydig neuron activity in the leech central nervous system.

Neuronal activity may lead to a variety of responses in neighbouring glial cells; in general, an ensemble of neurons needs to be active to evoke a K+- and/or neurotransmitter-induced glial membrane potential change. We have now detected a signal transfer from a single neuromodulatory Leydig neuron to the giant neuropil glial cells in the central nervous system of the leech Hirudo medicinalis. Activation of a Leydig neuron, two of which are located in each segmental ganglion, elicits a hyperpolarization in the giant neuropil glial cells. This hyperpolarization could be mimicked by bath application of the peptide myomodulin A (1 nM-1.0 microM). Myomodulin-like immunoreactivity has recently been found to be present in a set of leech neurons, including Leydig neurons (Keating & Sahley 1996, J. Neurobiol., 30, 374-384). The glial responses to Leydig neuron stimulation persisted in a high-divalent cation saline, when polysynaptic pathways are suppressed, indicating that the effects on the glial cell were direct. The glial responses to myomodulin A application persisted in high-Mg2+/low-Ca2+ saline, when chemical synaptic transmission is suppressed, indicating a direct effect of myomodulin A on the glial membrane. The glial hyperpolarization evoked by myomodulin A was dose dependent (EC50 = 50 nM) and accompanied by a membrane conductance increase of approximately 25%. Ion substitution experiments indicated that myomodulin A triggered a Ca2+-independent K+ conductance. Thus, our results suggest, for the first time, direct signal transmission from an identified modulatory neuron to an identified glial cell using a myomodulin-like peptide.

Helix 8 and helix 10 are involved in substrate recognition in the rat monocarboxylate transporter MCT1.

Transport of lactate, pyruvate, and the ketone bodies, acetoacetate and beta-hydroxybutyrate, is mediated in many mammalian cells by the monocarboxylate transporter MCT1. To be accepted as a substrate, a carboxyl group and an unpolar side chain are necessary. Site-directed mutagenesis of the rat MCT1 was used to identify residues which are involved in substrate recognition. Helices 8 and 10 but not helix 9 were found to contain critical residues for substrate recognition. Mutation of arginine 306 to threonine in helix 8 resulted in strongly reduced transport activity. Concomitantly, saturable transport was transformed into a nonsaturable dependence of transport activity on lactate concentration, suggesting that binding of the substrate was strongly impaired. Furthermore, proton translocation in the mutant was partially uncoupled from monocarboxylate transport. Mutation of phenylalanine 360 to cysteine in helix 10 resulted in an altered substrate side chain recognition. In contrast to the wild-type transporter, monocarboxylates with more bulky and polar side chains were recognized by the mutated MCT1. Mutation of selected residues in helix 9 and helix 11 (C336A, H337Q, and E391Q) did not cause alterations of the transport properties of MCT1. It is suggested that substrate binding occurs in the carboxy-terminal half of MCT1 and that helices 8 and 10 are involved in the recognition of different parts of the substrate.

Intracellular chloride and calcium transients evoked by gamma-aminobutyric acid and glycine in neurons of the rat inferior colliculus.

Microfluorometric recordings showed that the inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine activated transient increases in the intracellular Cl- concentration in neurons of the inferior colliculus (IC) from acutely isolated slices of the rat auditory midbrain. Current recordings in gramicidin-perforated patch mode disclosed that GABA and glycine mainly evoked inward or biphasic currents. These currents were dependent on HCO3- and characterized by a continuous shift of their reversal potential (E(GABA/gly)) in the positive direction. In HCO3- -buffered saline, GABA and glycine could also evoke an increase in the intracellular Ca2+ concentration. Ca2+ transients occurred only with large depolarizations and were blocked by Cd2+, suggesting an activation of voltage-gated Ca2+ channels. However, in the absence of HCO3-, only a small rise, if any, in the intracellular Ca2+ concentration could be evoked by GABA or glycine. We suggest that the activation of GABAA or glycine receptors results in an acute accumulation of Cl- that is enhanced by the depolarization owing to HCO3- efflux, thus shifting E(GABA/gly) to more positive values. A subsequent activation of these receptors would result in a strenghtened depolarization and an enlarged Ca2+ influx that might play a role in the stabilization of inhibitory synapses in the auditory pathway.